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EP1865088B1 - Method for tempering stainless steel and fused salt for performing the method - Google Patents

Method for tempering stainless steel and fused salt for performing the method Download PDF

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Publication number
EP1865088B1
EP1865088B1 EP07010534A EP07010534A EP1865088B1 EP 1865088 B1 EP1865088 B1 EP 1865088B1 EP 07010534 A EP07010534 A EP 07010534A EP 07010534 A EP07010534 A EP 07010534A EP 1865088 B1 EP1865088 B1 EP 1865088B1
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Prior art keywords
weight
molten salt
carbon
chloride
stainless steel
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German (de)
French (fr)
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EP1865088A1 (en
Inventor
Ulrich Dr. Baudis
Michael Niedermeyer
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Durferrit GmbH Thermotechnik
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Durferrit GmbH Thermotechnik
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/34—Methods of heating
    • C21D1/44—Methods of heating in heat-treatment baths
    • C21D1/46—Salt baths
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/40—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using liquids, e.g. salt baths, liquid suspensions
    • C23C8/42—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using liquids, e.g. salt baths, liquid suspensions only one element being applied
    • C23C8/44—Carburising
    • C23C8/46—Carburising of ferrous surfaces
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/40—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using liquids, e.g. salt baths, liquid suspensions
    • C23C8/42—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using liquids, e.g. salt baths, liquid suspensions only one element being applied
    • C23C8/48—Nitriding
    • C23C8/50—Nitriding of ferrous surfaces
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/40—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using liquids, e.g. salt baths, liquid suspensions
    • C23C8/52—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using liquids, e.g. salt baths, liquid suspensions more than one element being applied in one step
    • C23C8/54—Carbo-nitriding
    • C23C8/56—Carbo-nitriding of ferrous surfaces

Definitions

  • the invention relates to a method for hardening stainless steel and a molten salt for carrying out the method.
  • stainless steel Due to its excellent corrosion resistance, stainless steel is used in chemical apparatus engineering, in food technology, in the petrochemical industry, in the offshore sector, in shipbuilding and aircraft construction, in architecture, in building construction and equipment construction and in many other industrial sectors.
  • the steel is generally not sufficiently resistant to corrosion to be considered stainless steel.
  • the content of metallic chromium in the steel is thus an important criterion for the Corrosion resistance, as in P. Gümpel et al. Stainless Steels, Expert Verlag, Volume 349, Renningen Malmsheim 1998 is executed.
  • a major disadvantage of most common stainless steels such as 1.4301, 1.4441, 1.4541 or 1.4575 is that these steels are quite soft and thus susceptible to scratching the surface by hard particles such as dust or sand.
  • Most stainless steels - except for the very special martensitic stainless steels - are not hardenable by physical methods such as annealing and quenching. The low surface hardness often hinders the use of stainless steel.
  • Another disadvantage of most stainless steels is their strong tendency to seize, i. for welding the surface of two mutually sliding surfaces due to adhesion.
  • thermochemical treatment e.g. By nitriding or nitrocarburizing in gas (under ammonia atmosphere), in plasma (under nitrogen / argon) or in molten salt (in molten cyanates), the surface of stainless steel can be enriched with nitrogen to form iron and chromium nitrides.
  • the resulting layers form out of the material, so they are - unlike galvanic or physical layers - not applied from the outside and therefore extremely adherent.
  • hard layers of 5 to 50 ⁇ m thickness are formed. The hardness of such nitrided or nitrocarburized layers on stainless steel reaches over 1000 units on the Vickers hardness scale due to the high hardness of the resulting iron and chromium nitrides.
  • nitrided or nitrocarburized layers on stainless steel The problem with the practical use of such nitrided or nitrocarburized layers on stainless steel is that although these layers are hard, they lose their corrosion resistance.
  • the reason for this is the relatively high treatment temperature, which is around 580 ° C during nitriding or nitrocarburizing.
  • the diffusing elements form nitrogen and carbon with the chromium-stable chromium nitride (CrN) or chromium carbides (Cr 7 C 3 ) in the area of the component surface.
  • the corrosion resistance-free free chromium is removed from the stainless steel matrix to a depth of about 50 ⁇ m below the surface and converted into chromium nitride or chromium carbide.
  • the surface of the component becomes hard due to the formation of iron and chromium nitride, but it is susceptible to corrosion. In use, such layers are rapidly worn away due to corrosion.
  • a hard and at the same time corrosion-resistant layer can be thermochemically produced by the so-called Kolsterizing® on stainless steel.
  • This process is for example in Kolster by® - corrosion-resistant surface hardening of austenitic stainless steel - Information Sheet of Bodycote Hardiffbv, Parimariboweg 45, NL-7333 Apeldoorn, info@hardiff.de and M. Wägner Increasing the wear resistance of stainless steels. Steel STEEL No. 2 (2004) 40-43 mentioned.
  • the conditions of the process are described neither in the patent literature nor in the generally accessible scientific literature.
  • treated components have a hard, wear-resistant layer between 10 and 20 microns thick, the corrosion resistance of the base material remains.
  • Kolsterised® components must not be heated above 400 ° C, otherwise they will lose their corrosion resistance.
  • the workpiece to be cured is first surface activated by treatment with an acid and then treated in a heated fluidized bed containing active nitrogen and preferably also active carbon capable of diffusing into the workpiece.
  • a method for carburizing austenitic metal is described. According to this method, the metal is kept in a fluorine-containing or fluoride-containing guest atmosphere under heating prior to carburizing. The carburizing of the metal is then carried out at a temperature of at most 680 ° C.
  • US-A-19996269 discloses a melt for curing iron parts comprising the following components: 50 parts of barium chloride, 25 parts of sodium chloride, 25 parts of potassium chloride, 5-10 parts of sodium cyanide and 3-5% of a mixture of 1 part of strontium chloride and 7 parts of barium chloride.
  • JP-A-52123345 discloses a melt for nitriding iron at 690 ° C comprising the following components: 15% potassium chloride, 15% sodium chloride, 80% barium chloride, sodium titanate and potassium hexacyanoferrate.
  • the invention has for its object to provide a cost-effective rational method by means of which a hardening of stainless steel is made possible, in which the corrosion resistance of the stainless steel is maintained as much as possible.
  • the molten salt according to the invention comprises the following components: 30-60% by weight Potassium chloride (KCL) 20-40% by weight Lithium chloride (LiCl) 15-30% by weight an activator substance consisting of barium chloride (BaCl 2 ) and / or strontium chloride (SrCl 2 ) and / or magnesium chloride (MgCl 2 ) and / or calcium chloride (CaCl 2 ) 0.2-25% by weight a carbon donating substance consisting of a free cyanide and / or a complex cyanide.
  • KCL Potassium chloride
  • LiCl Lithium chloride
  • BaCl 2 barium chloride
  • SrCl 2 strontium chloride
  • MgCl 2 magnesium chloride
  • CaCl 2 calcium chloride
  • the present invention avoids high equipment and energy costs and makes use of a light, easy for less qualified personnel easily executable procedure.
  • the invention further enhances the tendency of the stainless steel to eat, i. For cold welding and thus the adhesive wear significantly reduced.
  • the hardness of the surface of the stainless steel is increased from 200 - 300 Vickers to values up to 1000 Vickers, which results in a high scratch resistance.
  • the use of the molten salt according to the invention makes it possible to harden stainless steel while maintaining its corrosion resistance.
  • the method according to the invention is based on the following principle
  • these processes using the molten salt of the invention are used as a reactive medium and as a heat transfer agent.
  • the molten salt according to the invention contains constituents from which diffusible carbon and / or nitrogen can be liberated and suitable activator substances which cause the release of diffusible nitrogen and / or carbon at low temperatures. It is essential that the treatment temperatures in the molten salt below 450 ° C and are particularly advantageous to lower than the formation temperature of chromium carbide (420 - 440 ° C) or chromium nitride (350 - 370 ° C) to the formation of nitrides and To avoid carbides in the steel matrix completely or as far as possible.
  • the concentration of the active carbon or nitrogen donating substances in the form of complex or free cyanides in the molten salt of the invention is very high compared with the concentration of corresponding substances (ammonia, methane, carbon monoxide) in gas atmospheres or in a plasma.
  • the relatively long treatment times required for the method according to the invention are based on the fact that the diffusion rate of C and N is a function of the temperature and decreases significantly at temperatures below 450 ° C. At the low temperatures necessary to avoid chromium carbide and chromium nitride formation, long diffusion times of 12 to 60 hours must be used.
  • Austenitic stainless steels or so-called duplex steels (ferritic-austenitic steels) are against such long heat treatment times very insensitive and change their other mechanical properties or the structure as good as not.
  • the molten salt consists of a salt mixture of potassium chloride, barium chloride and lithium chloride.
  • a melt of strontium chloride, potassium chloride and lithium chloride can be used.
  • magnesium chloride and / or calcium chloride instead of barium chloride or strontium chloride.
  • the melting points of the eutectic mixtures of these salts are from 320 ° C to 350 ° C.
  • the salt should be dried for at least 12 to 24 hours at 120-140 ° C before the addition and freed from the water of crystallization, since it contains 3 mol equivalents of water of crystallization in the form supplied.
  • the red potassium hexacyanoferrate (III) that is, K 3 Fe (CN) 6 , which contains no water of crystallization, can be added to the melt.
  • the amount of complex cyanide added is in the range of 2 to 10% by weight.
  • complex metal cyanides can also be used as carbon donors.
  • these are tetracyanoxide or tetracyanozinc compounds, for example Na 2 Ni (CN) 4 or Na 2 Zn (CN) 4 .
  • complex nontoxic iron cyanides or metal cyanides and sodium and / or potassium cyanide can be added in free form, in an amount of 0.1 to 25 wt.%, Preferably between 3 and 10 wt.%.
  • the results are similar to the use of complex cyanides, and mixtures of complex and free cyanides can also be used.
  • molten salts with complex cyanides is that it does not handle toxic substances because hexacyanoferrate is non-toxic per se.
  • the decay is very slow.
  • the carbon formed during the decomposition diffuses into the austenitic stainless steel to be hardened and remains there at temperatures below 420 ° C. in solid, saturated or supersaturated solution.
  • Austenite has a high solubility for carbon, a lower one for nitrogen.
  • Part of the resulting nitrogen also diffuses into the stainless steel surface. If the treatment temperature is below 350 - 370 ° C, so does the nitrogen - like the carbon - in solid solution, the temperature is between 370 ° C and 420 ° C, so the nitrogen forms with the alloying element chromium chromium nitride and thus potentially reduce the corrosion resistance of stainless steel on the surface. However, even in this temperature range, a formation of chromium carbide is still avoided, so that the alloy matrix of the stainless steel, despite the Chromnitridön given in this temperature range still little chromium is removed, so that reducing the corrosion resistance of the stainless steel may still be acceptable.
  • Cyanide ions which resulted from the decomposition of the complex metal salt, are oxidized to cyanate ions by atmospheric oxygen, which is omnipresent in the melt. These can decompose to form carbon monoxide and nitrogen. Cyanations are usually the source of diffusible nitrogen. However, cyanide ions can also be further oxidized to carbonate ions, resulting in carbon monoxide. Carbon monoxide can continue to react with carbon dioxide by releasing diffusible carbon.
  • cyanide can react with barium ions of the activator substance contained as barium chloride in the molten salt to barium cyanide Ba (CN) 2 , which turns into barium cyanamide BaNCN. This releases carbon, which can diffuse into the components.
  • the barium cyanamide further reacts with atmospheric oxygen to form barium carbonate and nitrogen which is released. Similar reactions are to be expected with strontium, calcium and magnesium, if strontium chloride, calcium chloride or magnesium chloride is used as the activator substance.
  • strontium, calcium and magnesium if strontium chloride, calcium chloride or magnesium chloride is used as the activator substance.
  • the alkaline earth metals in the form of their halides thus form in the process according to the invention activator substances which bring about the release of diffusible nitrogen and carbon in the temperature range of the process according to the invention. Without the participation of at least one alkaline earth element of the series magnesium, calcium, strontium and barium, the diffusion of the necessary carbon into the stainless steel surface is not possible.
  • the other alkali metals Na, K, Rb and Cs do not show this effect.
  • An analytical control of the molten salts according to the invention can be carried out as follows:
  • the change in the concentration of the active Ingredients can be monitored by potentiometric titration.
  • K 4 Fe (CN) 6 can be titrated with cerium (IV) sulfate solution.
  • Free cyanide can be determined very well with nickel (II) sulfate. Consumed cyanide or complex cyanide is added accordingly.
  • an inert gas such as argon, nitrogen or carbon dioxide may be introduced therein.
  • the molten salt can be operated in a closed retort using nitrogen, argon or carbon dioxide as a protective gas.
  • a crucible made of heat-resistant steel e.g. from the material 1.4828, 42 kg dry potassium chloride, 34 kg dry lithium chloride and 20 kg barium chloride siccum weighed and mixed loosely. All salts must have a residual moisture content of less than 0.3% by weight.
  • the mixture is heated to 400 ° C and gives a water-clear melt.
  • 4 kg of potassium hexacyanoferrate (II) are slowly added, which had previously been dried for 12 h at 140 ° C in a muffle furnace.
  • the potassium hexacyanoferrate (II) is introduced, a very small amount of carbon deposits on the crucible wall and on the surface of the melt. This carbon is skimmed off with a sifting spoon.
  • a water-clear melt is present, which is brought to an operating temperature of 400 ° C.
  • V2A stain is a mixture of 100 ml of water and conc. 100 ml of hydrochloric acid. (HCI, 30%) and 0.3% "bird's reagent".
  • Vogel's reagent is a mixture of 60% 2-methoxy-2-propanol (H3C-O-CH2Oh-CH3) 5% thiourea (H2N-CS-NH2) 5% nonylphenol ethoxylate residue ethanol.
  • the cross section is photographically in FIG. 1 shown in a 500-fold magnification.
  • the surface hardness of this layer is determined to be 642 - 715 HV (0.5) or 1100 - 1210 HV (0.025).
  • the element distribution within the layer can be determined by glow discharge spectroscopy (GDOES) and is exemplified in FIG. 2 shown.
  • GDOES glow discharge spectroscopy
  • FIG. 2 is the penetration depth of the elements N, C, Fe, Cr 2 , Ni, Mo shown in the surface of the cured with the molten salt workpiece, that is, the mass concentrations of these elements in percent, depending on the depth in the workpiece in microns.
  • FIG. 3 shows the hardness curve as a function of the depth (in ⁇ m) for this workpiece.
  • the hardness profile was measured by the Vickers method under a test load of 0.010 kp (10 grams).
  • the hardness of the workpiece is significantly increased.
  • the result of this treatment is a 10-25 ⁇ m thick diffusion layer on the surface of the treated components and samples. which can be made visible metallographically by a cross-section and etching with the etchant V2A-stain.
  • the result of this treatment is an approx. 10 ⁇ m thick diffusion layer on the surface of the treated components and samples, which can be made visible metallographically by cross-sectioning and etching with the etchant V2A-stain.
  • the hardness of this layer is determined to be 620 HV (0.5).

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Abstract

In the hardening of a stainless steel workpiece, the workpiece surface is exposed to an incoming diffused atmosphere of carbon and/or nitrogen. The workpiece is submerged in a hot molten salt solution of KCl (30-60 wt.%), LiCl (20-40 wt.%), BaCl 2 and or SrCl 2 and or MgCl 2 and or CaCl 2 (15-30 wt.%), and free or complex cyanide (0.2-25 wt.%). The bath solution is maintained at 450[deg]C and the workpiece is maintained in a submerged condition for 15 minutes to 240 hours.

Description

Die Erfindung betrifft ein Verfahren zum Härten von Edelstahl und eine Salzschmelze zur Durchführung des Verfahrens.The invention relates to a method for hardening stainless steel and a molten salt for carrying out the method.

Edelstahl wird aufgrund seiner ausgezeichneten Korrosionsbeständigkeit im chemischen Apparatebau, in der Lebensmitteltechnologie, in der petrochemischen Industrie, im Offshorebereich, im Schiffs- und Flugzeugbau, in der Architektur, im Hausbau und Gerätebau und in vielen weiteren Industriebereichen verwendet.Due to its excellent corrosion resistance, stainless steel is used in chemical apparatus engineering, in food technology, in the petrochemical industry, in the offshore sector, in shipbuilding and aircraft construction, in architecture, in building construction and equipment construction and in many other industrial sectors.

Von korrosionsbeständigem Edelstahl spricht man, wenn einem Eisenwerkstoff mindestens 13 Gew. % Chrom zulegiert sind. In den meisten Fällen ist zusätzlich noch Nickel, Titan und Molybdän in der Eisenlegierung enthalten, wie beispielsweise in Stahl Merkblatt 821 Edelstahl Rostfrei - Eigenschaften Informationsstelle Edelstahl, PF 102205, 40013 Düsseldorf www.edelstahlrostfrei.de und in P. Gümpel et al. Rostfreie Stähle, Expert Verlag, Band 349, Renningen Malmsheim 1998 ausgeführt ist. Typische austenitische Edelstähle sind die Legierungen der Stähle 1.4301 oder 1.4571 mit folgenden Zusammensetzungen:

  • 1.4301 : C 0,05 Si 0,5 Mn 1,4 Cr 18,5 Ni 9,5 Gew. %
  • 1.4571 : C 0,03 Si 0,5 Mn1,7 Cr 17,0 Ni 11,2 Mo 2,2 Ti 0,1 Gew. %
Corrosion-resistant stainless steel is used when at least 13% by weight chromium is alloyed to an iron material. In most cases nickel, titanium and molybdenum are additionally contained in the iron alloy, as for example in Stahl Merkblatt 821 Stainless Steel - Properties Information Office Stainless Steel, PF 102205, 40013 Düsseldorf www.edelstahlrostfrei.de and in P. Gümpel et al. Stainless Steels, Expert Verlag, Volume 349, Renningen Malmsheim 1998 is executed. Typical austenitic stainless steels are the alloys of steels 1.4301 or 1.4571 having the following compositions:
  • 1.4301: C 0.05 Si 0.5 Mn 1.4 Cr 18.5 Ni 9.5 wt%
  • 1.4571: C 0.03 Si 0.5 Mn 1.7 Cr 17.0 Ni 11.2 Mo 2.2 Ti 0.1% by weight

Beträgt der Chromgehalt weniger als 13 Gew. %, so ist der Stahl im Allgemeinen nicht ausreichend korrosionsbeständig um als Edelstahl zu gelten. Der Gehalt an metallischem Chrom im Stahl ist somit ein wichtiges Kriterium für die Korrosionsbeständigkeit, wie in P. Gümpel et al. Rostfreie Stähle, Expert Verlag, Band 349, Renningen Malmsheim 1998 ausgeführt ist.If the chromium content is less than 13% by weight, the steel is generally not sufficiently resistant to corrosion to be considered stainless steel. The content of metallic chromium in the steel is thus an important criterion for the Corrosion resistance, as in P. Gümpel et al. Stainless Steels, Expert Verlag, Volume 349, Renningen Malmsheim 1998 is executed.

Ein großer Nachteil der meisten gebräuchlichen Edelstähle wie 1.4301, 1.4441, 1.4541 oder 1.4575 besteht darin, dass diese Stähle ziemlich weich sind und somit anfällig gegen Verkratzen der Oberfläche durch harte Partikel wie Staub oder Sand sind. Die meisten Edelstähle - abgesehen von den sehr speziellen martensitischen Edelstählen - sind nicht durch physikalische Methoden wie Glühen und Abschrecken härtbar. Die geringe Oberflächenhärte steht der Verwendung des Edelstahls häufig im Wege. Ein weiterer Nachteil der meisten Edelstähle ist ihre starke Neigung zum Fressen, d.h. zum Verschweißen der Oberfläche zweier gegeneinander gleitender Flächen aufgrund von Adhäsion.A major disadvantage of most common stainless steels such as 1.4301, 1.4441, 1.4541 or 1.4575 is that these steels are quite soft and thus susceptible to scratching the surface by hard particles such as dust or sand. Most stainless steels - except for the very special martensitic stainless steels - are not hardenable by physical methods such as annealing and quenching. The low surface hardness often hinders the use of stainless steel. Another disadvantage of most stainless steels is their strong tendency to seize, i. for welding the surface of two mutually sliding surfaces due to adhesion.

Durch thermochemische Behandlung - z.B. durch Nitrieren oder Nitrocarburieren im Gas (unter Ammoniakatmosphäre), im Plasma (unter Stickstoff / Argon) oder in der Salzschmelze (in geschmolzenen Cyanaten) kann die Oberfläche von Edelstahl mit Stickstoff angereichert werden, wobei sich Eisen- und Chromnitride bilden. Die dabei entstehenden Schichten bilden sich aus dem Werkstoff heraus, sie sind also - anders als bei galvanischen oder physikalischen Schichten - nicht von außen aufgetragen und deshalb extrem haftfest. Je nach Behandlungsdauer bilden sich harte Schichten von 5 bis 50 µm Dicke. Die Härte solcher nitrierter oder nitrocarburierter Schichten auf Edelstahl erreicht wegen der hohen Härte der dabei entstandenen Eisen- und Chromnitride Werte über 1000 Einheiten auf der Härteskala nach Vickers.By thermochemical treatment - e.g. By nitriding or nitrocarburizing in gas (under ammonia atmosphere), in plasma (under nitrogen / argon) or in molten salt (in molten cyanates), the surface of stainless steel can be enriched with nitrogen to form iron and chromium nitrides. The resulting layers form out of the material, so they are - unlike galvanic or physical layers - not applied from the outside and therefore extremely adherent. Depending on the duration of treatment, hard layers of 5 to 50 μm thickness are formed. The hardness of such nitrided or nitrocarburized layers on stainless steel reaches over 1000 units on the Vickers hardness scale due to the high hardness of the resulting iron and chromium nitrides.

Das Problem beim praktischen Einsatz solcher nitrierter oder nitrocarburierter Schichten auf Edelstahl besteht darin, dass diese Schichten zwar hart sind, jedoch ihre Korrosionsbeständigkeit verlieren. Die Ursache dafür ist die relativ hohe Behandlungstemperatur, die beim Nitrieren oder Nitrocarburieren im Bereich um 580°C liegt. Bei dieser Temperatur bilden die eindiffundierenden Elemente Stickstoff und Kohlenstoff mit dem Chrom stabile Chromnitride (CrN) bzw. Chromcarbide (Cr7C3) im Bereich der Bauteiloberfläche. Auf diese Weise wird das für die Korrosionsbeständigkeit unabdingbare freie Chrom aus der Edelstahlmatrix bis in eine Tiefe von ungefähr 50 µm unter der Oberfläche entfernt und in Chromnitrid oder Chromcarbid umgewandelt. Die Bauteiloberfläche wird aufgrund der Bildung von Eisen- und Chromnitrid zwar hart, aber korrosionsanfällig. Im Gebrauch werden solche Schichten aufgrund von Korrosion rasch abgenutzt bzw. abgetragen.The problem with the practical use of such nitrided or nitrocarburized layers on stainless steel is that although these layers are hard, they lose their corrosion resistance. The reason for this is the relatively high treatment temperature, which is around 580 ° C during nitriding or nitrocarburizing. At this temperature, the diffusing elements form nitrogen and carbon with the chromium-stable chromium nitride (CrN) or chromium carbides (Cr 7 C 3 ) in the area of the component surface. In this way, the corrosion resistance-free free chromium is removed from the stainless steel matrix to a depth of about 50 μm below the surface and converted into chromium nitride or chromium carbide. The surface of the component becomes hard due to the formation of iron and chromium nitride, but it is susceptible to corrosion. In use, such layers are rapidly worn away due to corrosion.

Um dieses Problem zu vermeiden, existieren folgende Verfahrensweisen.To avoid this problem, the following procedures exist.

Es ist bekannt, dass die Oberflächenhärte auf Edelstahl durch galvanische Beschichtungen, z.B. durch Vernickeln, oder physikalische Beschichtungen, z.B. mittels PVD-Beschichtung (Physical Vapor Deposition) verbessert werden kann. Dabei wird jedoch ein artfremder Stoff auf die Oberfläche des Stahls aufgebracht. Die mit dem verschleißenden oder korrosiven Medium in Kontakt stehende Oberfläche ist nicht mehr die Stahloberfläche selbst. Es ergeben sich Probleme der Haftung und der Korrosionsbeständigkeit. Diese Verfahren sind daher zur Verbesserung der Härte und des Verschleissverhaltens von Edelstahl nicht sehr verbreitet.It is known that the surface hardness on stainless steel is achieved by galvanic coatings, e.g. by nickel plating, or physical coatings, e.g. can be improved by PVD (Physical Vapor Deposition) coating. However, a foreign substance is applied to the surface of the steel. The surface in contact with the abrasive or corrosive medium is no longer the steel surface itself. Problems of adhesion and corrosion resistance result. These methods are therefore not very common for improving the hardness and wear behavior of stainless steel.

Eine harte und gleichzeitig korrosionsbeständige Schicht kann man durch das so genannte Kolsterisieren® auf Edelstahl thermochemisch erzeugen. Dieses Verfahren ist beispielsweise in Kolsterisieren® - korrosionsfestes Oberflächenhärten von austenitischem rostfreiem Stahl - Informationsblatt der Bodycote Hardiffbv, Parimariboweg 45, NL-7333 Apeldoorn, info@hardiff.de sowie M. Wägner Steigerung der Verschleißfestigkeit nichtrostender aust. Stähle STAHL Nr. 2 (2004) 40-43 erwähnt. Die Bedingungen des Prozesses sind weder in der Patentliteratur noch in der allgemein zugänglichen wissenschaftlichen Literatur beschrieben. So behandelte Bauteile weisen eine harte, verschleissfeste Schicht zwischen 10 und 20 µm Dicke auf, die Korrosionsbeständigkeit des Grundwerkstoffs bleibt erhalten. Kolsterisierte® Bauteile dürfen nicht über 400°C erhitzt werden, da sie sonst ihre Korrosionsbeständigkeit verlieren.A hard and at the same time corrosion-resistant layer can be thermochemically produced by the so-called Kolsterizing® on stainless steel. This process is for example in Kolsterisieren® - corrosion-resistant surface hardening of austenitic stainless steel - Information Sheet of Bodycote Hardiffbv, Parimariboweg 45, NL-7333 Apeldoorn, info@hardiff.de and M. Wägner Increasing the wear resistance of stainless steels. Steel STEEL No. 2 (2004) 40-43 mentioned. The conditions of the process are described neither in the patent literature nor in the generally accessible scientific literature. Thus treated components have a hard, wear-resistant layer between 10 and 20 microns thick, the corrosion resistance of the base material remains. Kolsterised® components must not be heated above 400 ° C, otherwise they will lose their corrosion resistance.

Durch Plasmanitrieren, welches beispielsweise in H.-J. Spies et al. Mat.-Wiss. u. Werkstofftechnik 30 (1999) 457-464 , Y. Sun, T. Bell et al. The Response of Austenitic Stainless Steel to Low Temp. Plasma Nitriding Heat Treatment of Metals Nr. 1 (1999) 9-16 beschrieben ist, oder durch Unterdruckaufkohlung, welches beispielsweise in D. Günther, F. Hoffmann, M. Jung, P. Mayr Oberflächenhärtung von austenitischen Stählen unter Beibehaltung der Korrosionsbeständigkeit Härterei-Techn. Mitt. 56 (2001) 74-83 beschrieben ist, kann bei niedrigen Temperaturen eine übersättigte Lösung von Stickstoff und/oder Kohlenstoff in der Oberfläche von Bauteilen aus Edelstahl erzeugt werden, die die gewünschten Eigenschaften, d.h. höhere Härte bei unveränderter Korrosionsfestigkeit, aufweist.By plasma nitriding, which, for example, in H.-J. Spies et al. Mat.-Wiss. u. Materials Engineering 30 (1999) 457-464 . Y. Sun, T. Bell et al. The Response of Austenitic Stainless Steel to Low Temp. Plasma Nitriding Heat Treatment of Metals No. 1 (1999) 9-16 or by Unterdruckaufkohlung, which, for example, in D. Günther, F. Hoffmann, M. Jung, P. Mayr Surface hardening of austenitic steels while maintaining the corrosion resistance Hardening Techn. Mitt. 56 (2001) 74-83 at low temperatures, a supersaturated solution of nitrogen and / or carbon may be produced in the surface of stainless steel components having the desired properties, ie higher hardness with unchanged corrosion resistance.

Beide Verfahren erfordern jedoch einen hohen apparativen Aufwand und hohe Investitions- und Energiekosten, zur Bedienung der Anlagen ist besonders geschultes, meist sogar wissenschaftlich ausgebildetes Personal erforderlich.However, both methods require a high expenditure on equipment and high investment and energy costs, to operate the equipment is particularly trained, usually even scientifically trained staff required.

Aus der DE 35 01 409 A1 ist ein Verfahren zum Einsatzhärten von rostfreiem Stahl bekannt. Bei diesem Verfahren wird das zu härtende Werkstück zunächst durch Behandlung mit einer Säure oberflächenaktiviert und dann in einem erhitzten Fließbett behandelt, das aktiven Stickstoff und vorzugsweise auch aktiven Kohlenstoff enthält, die in der Lage sind in das Werkstück zu diffundieren.From the DE 35 01 409 A1 For example, a method of case-hardening stainless steel is known. In this process, the workpiece to be cured is first surface activated by treatment with an acid and then treated in a heated fluidized bed containing active nitrogen and preferably also active carbon capable of diffusing into the workpiece.

Aus der DE 695 10 719 T2 ist ein Verfahren zum Aufkohlen von austenitischem Metall beschrieben. Gemäß diesem Verfahren wird das Metall in einer fluor- oder flouridhaltigen Gastatmosphäre unter Erhitzung vor dem Aufkohlen gehalten. Das Aufkohlen des Metalls erfolgt dann bei einer Temperatur von maximal 680°C.From the DE 695 10 719 T2 a method for carburizing austenitic metal is described. According to this method, the metal is kept in a fluorine-containing or fluoride-containing guest atmosphere under heating prior to carburizing. The carburizing of the metal is then carried out at a temperature of at most 680 ° C.

US-A-19996269 offenbart eine Schmelze zum Härten von Eisenteilen umfassend folgende Komponenten: 50 Teile Bariumchlorid, 25 Teile Natriumchlorid, 25 Teile Kaliumchlorid, 5-10 Teile Natriumcyanid und 3-5% einer Mischung aus 1 Teil Strontiumchlorid und 7 Teile Bariumchlorid. US-A-19996269 discloses a melt for curing iron parts comprising the following components: 50 parts of barium chloride, 25 parts of sodium chloride, 25 parts of potassium chloride, 5-10 parts of sodium cyanide and 3-5% of a mixture of 1 part of strontium chloride and 7 parts of barium chloride.

JP-A-52123345 offenbart eine Schmelze zum Nitrieren von Eisenteilen bei 690°C umfassend folgende Komponenten: 15% Kaliumchlorid, 15% Natriumchlorid, 80% Bariumchlorid, Natriumtitanat und Kaliumhexacyanoferrat. JP-A-52123345 discloses a melt for nitriding iron at 690 ° C comprising the following components: 15% potassium chloride, 15% sodium chloride, 80% barium chloride, sodium titanate and potassium hexacyanoferrate.

Der Erfindung liegt die Aufgabe zugrunde ein kostengünstiges rationelles Verfahren bereitzustellen, mittels dessen ein Härten von Edelstahl ermöglicht wird, bei welchem die Korrosionsbeständigkeit des Edelstahls möglichst weitgehend erhalten bleibt.The invention has for its object to provide a cost-effective rational method by means of which a hardening of stainless steel is made possible, in which the corrosion resistance of the stainless steel is maintained as much as possible.

Zur Lösung dieser Aufgabe sind die Merkmale der Ansprüche 1 und 12 vorgesehen. Vorteilhafte Ausführungsformen und zweckmäßige Weiterbildungen der Erfindung sind in den Unteransprüchen beschrieben.To achieve this object, the features of claims 1 and 12 are provided. Advantageous embodiments and expedient developments of the invention are described in the subclaims.

Mittels des erfindungsgemäßen Verfahrens erfolgt ein Härten von aus Edelstahl bestehenden Werkstücken durch Eindiffundieren der Elemente Kohlenstoff und/oder Stickstoff in die Werkstückoberflächen, in dem die Werkstücke in eine Salzschmelze eingetaucht werden und dieser bei Temperaturen unterhalb von 450°C für einen Zeitraum von 15 Minuten bis 240 Stunden ausgesetzt werden.By means of the method according to the invention is carried out hardening of stainless steel workpieces by diffusing the elements carbon and / or nitrogen in the workpiece surfaces in which the workpieces are immersed in a molten salt and this at temperatures below 450 ° C for a period of 15 minutes Be exposed for 240 hours.

Die erfindungsgemäße Salzschmelze umfasst dabei folgende Komponenten: 30 - 60 Gew. % Kaliumchlorid (KCL) 20 - 40 Gew. % Lithiumchlorid (LiCl) 15 - 30 Gew. % einer Aktivatorsubstanz bestehend aus Bariumchlorid (BaCl2) und/oder Strontiumchlorid (SrCl2) und/oder Magnesiumchlorid (MgCl2) und/oder Calciumchlorid (CaCl2) 0,2 - 25 Gew. % einer kohlenstoffspendenden Substanz bestehend aus einem freien Cyanid und/oder einem komplexen Cyanid. The molten salt according to the invention comprises the following components: 30-60% by weight Potassium chloride (KCL) 20-40% by weight Lithium chloride (LiCl) 15-30% by weight an activator substance consisting of barium chloride (BaCl 2 ) and / or strontium chloride (SrCl 2 ) and / or magnesium chloride (MgCl 2 ) and / or calcium chloride (CaCl 2 ) 0.2-25% by weight a carbon donating substance consisting of a free cyanide and / or a complex cyanide.

Die vorliegende Erfindung vermeidet hohen apparativen und energetischen Aufwand und bedient sich einer leichten, auch für weniger qualifiziertes Personal leicht ausführbaren Verfahrensweise.The present invention avoids high equipment and energy costs and makes use of a light, easy for less qualified personnel easily executable procedure.

Durch die Erfindung wird weiterhin die Neigung des Edelstahls zum Fressen, d.h. zum Kaltverschweissen und damit auch der adhäsive Verschleiss wesentlich reduziert. Die Härte der Oberfläche des Edelstahls wird von 200 - 300 Vickers auf Werte bis zu 1000 Vickers gesteigert, wodurch eine hohe Kratzfestigkeit entsteht.The invention further enhances the tendency of the stainless steel to eat, i. For cold welding and thus the adhesive wear significantly reduced. The hardness of the surface of the stainless steel is increased from 200 - 300 Vickers to values up to 1000 Vickers, which results in a high scratch resistance.

Durch die erfindungsgemäße Verwendung der Salzschmelze wird ein Härten von Edelstahl unter Erhaltung dessen Korrosionsbeständigkeit ermöglicht.The use of the molten salt according to the invention makes it possible to harden stainless steel while maintaining its corrosion resistance.

Dem erfindungsgemäßen Verfahren liegt dabei folgendes Prinzip zugrundeThe method according to the invention is based on the following principle

Edelstahl liegt typischerweise in der Form eines austenitischen Stahls vor, d.h. die Eisenmatrix hat die Struktur des Austenits, ein kubisch flächenzentriertes Gitter. In diesem Gitter können sich nichtmetallische Elemente wie Stickstoff und Kohlenstoff in fester Lösung aufhalten. Gelingt es, Kohlenstoff oder Stickstoff oder beide Elemente in die Oberfläche eines austenitischen Edelstahls einzubringen und dort in fester gesättigter oder sogar übersättigter Lösung zu halten, so treten zwei Effekte ein:

  1. (a) Wenn Kohlenstoff unterhalb der Bildungstemperatur des Chromcarbids (420 - 440°C) und Stickstoff unterhalb der Bildungstemperatur von Chromnitrid (350 - 370°C) eindiffundiert, bilden sich keine Carbide oder Nitride des Chroms. Demzufolge wird der Legierungsmatrix im Bereich der Diffusionsschicht kein Chrom entzogen und die Korrosionsbeständigkeit des Edelstahls bleibt erhalten.
  2. (b) Die eindiffundierten Elemente dehnen das austenitische Gitter und führen zu einer starken Druckspannung im Bereich der Diffusionszone. Dies wiederum führt zu einer beträchtlichen Härtesteigerung. In der wissenschaftlichen Literatur spricht man von expandiertem Austenit oder einer so bezeichneten S - Phase, die eine Härte bis 1000 auf der Vickers Skala annehmen kann. Der Begriff der S-Phase ist beispielsweise in Y. Sun, T. Bell et al. The Response of Austenitic Stainless Steel to Low Temp. Plasma Nitriding Heat Treatment of Metals Nr. 1 (1999) 9-16 erläutert.
Stainless steel is typically in the form of an austenitic steel, that is, the iron matrix has the structure of austenite, a cubic face centered lattice. In this grid, non-metallic elements such as nitrogen and carbon can be in solid solution. If it is possible to introduce carbon or nitrogen or both elements into the surface of an austenitic stainless steel and keep it in a solid saturated or even supersaturated solution, two effects occur:
  1. (a) When carbon diffuses below the formation temperature of the chromium carbide (420-440 ° C) and nitrogen below the formation temperature of chromium nitride (350-370 ° C), no carbides or nitrides of chromium are formed. As a result, no chromium is removed from the alloy matrix in the region of the diffusion layer and the corrosion resistance of the stainless steel is retained.
  2. (b) The diffused elements stretch the austenitic lattice and lead to a strong compressive stress in the area of the diffusion zone. This in turn leads to a considerable increase in hardness. In the scientific literature one speaks of expanded austenite or a so-called S - Phase that can take a hardness up to 1000 on the Vickers scale. The term S-phase is for example in Y. Sun, T. Bell et al. The Response of Austenitic Stainless Steel to Low Temp. Plasma Nitriding Heat Treatment of Metals No. 1 (1999) 9-16 explained.

In der vorliegenden Erfindung werden diese Vorgänge unter Verwendung der erfindungsgemäßen Salzschmelze als reaktives Medium und als Wärmeüberträger genutzt.In the present invention, these processes using the molten salt of the invention are used as a reactive medium and as a heat transfer agent.

Die erfindungsgemäße Salzschmelze enthält Bestandteile, aus denen diffusionsfähiger Kohlenstoff und / oder Stickstoff frei gesetzt werden kann und geeignete Aktivatorsubstanzen, die die Freisetzung von diffusionsfähigem Stickstoff und / oder Kohlenstoff bei niedrigen Temperaturen bewirken. Wesentlich hierbei ist, dass die Behandlungstemperaturen in der Salzschmelze unterhalb von 450°C liegen und besonders vorteilhaft auf Werte unterhalb der Bildungstemperatur von Chromcarbid (420 - 440°C) oder Chromnitrid (350 - 370°C) abgesenkt werden um die Bildung von Nitriden und Carbiden in der Stahlmatrix vollständig oder möglichst weitgehend zu vermeiden.The molten salt according to the invention contains constituents from which diffusible carbon and / or nitrogen can be liberated and suitable activator substances which cause the release of diffusible nitrogen and / or carbon at low temperatures. It is essential that the treatment temperatures in the molten salt below 450 ° C and are particularly advantageous to lower than the formation temperature of chromium carbide (420 - 440 ° C) or chromium nitride (350 - 370 ° C) to the formation of nitrides and To avoid carbides in the steel matrix completely or as far as possible.

Die Konzentration der aktiven kohlenstoff- oder stickstoffabgebenden Stoffe in Form von komplexen oder freien Cyaniden ist in der erfindungsgemäßen Salzschmelze sehr hoch verglichen mit der Konzentration entsprechender Stoffe (Ammoniak, Methan, Kohlenoxid) in Gasatmosphären oder in einem Plasma. Die für das erfindungsgemäße Verfahren erforderlichen relativ langen Behandlungsdauern beruhen darauf, dass die Diffusionsgeschwindigkeit von C und N eine Funktion der Temperatur ist und bei Temperaturen unter 450°C signifikant sinkt. Bei den notwendigen niedrigen Temperaturen zur Vermeidung der Chromcarbid- und Chromnitridbildung müssen lange Diffusionszeiten von 12 bis 60 h angewendet werden. Austenitische rostfreie Stähle oder sog. Duplex-Stähle (ferritisch - austenitische Stähle) sind gegen solch lange Wärmebehandlungsdauern sehr unempfindlich und verändern ihre sonstigen mechanischen Eigenschaften oder das Gefüge so gut wie nicht.The concentration of the active carbon or nitrogen donating substances in the form of complex or free cyanides in the molten salt of the invention is very high compared with the concentration of corresponding substances (ammonia, methane, carbon monoxide) in gas atmospheres or in a plasma. The relatively long treatment times required for the method according to the invention are based on the fact that the diffusion rate of C and N is a function of the temperature and decreases significantly at temperatures below 450 ° C. At the low temperatures necessary to avoid chromium carbide and chromium nitride formation, long diffusion times of 12 to 60 hours must be used. Austenitic stainless steels or so-called duplex steels (ferritic-austenitic steels) are against such long heat treatment times very insensitive and change their other mechanical properties or the structure as good as not.

Die Salzschmelze besteht aus einem Salzgemisch aus Kaliumchlorid, Bariumchlorid und Lithiumchlorid. Alternativ kann eine Schmelze aus Strontiumchlorid, Kaliumchlorid und Lithiumchlorid eingesetzt werden. Alternativ oder zusätzlich kann anstelle von Bariumchlorid oder Strontiumchlorid auch Magnesiumchlorid und/oder Calciumchlorid eingesetzt werden. Die Schmelzpunkte der eutektischen Gemische dieser Salze liegen bei 320°C bis 350°C. Diesen Salzen wird als kohlenstoffabgebende Substanz das gelbe Kaliumhexacyanoferrat (II), das heißt K4Fe(CN)6 in einer Menge 0,2 bis 25 Gew. %, insbesondere von 1 bis 25 Gew. % zugegeben. Das Salz sollte vor der Zugabe mind. 12 - 24 h bei 120-140°C getrocknet und vom Kristallwasser befreit werden, da es in der Lieferform 3 Mol-Äquivalente Kristallwasser enthält. Alternativ kann der Schmelze das rote Kaliumhexacyanoferrat (III), das heißt K3Fe(CN)6 zugegeben werden, das kein Kristallwasser enthält. Vorzugsweise liegt die Menge des zugegebenen komplexen Cyanids im Bereich von 2 bis 10 Gew. %.The molten salt consists of a salt mixture of potassium chloride, barium chloride and lithium chloride. Alternatively, a melt of strontium chloride, potassium chloride and lithium chloride can be used. Alternatively or additionally, it is also possible to use magnesium chloride and / or calcium chloride instead of barium chloride or strontium chloride. The melting points of the eutectic mixtures of these salts are from 320 ° C to 350 ° C. The yellow potassium hexacyanoferrate (II), that is to say K 4 Fe (CN) 6, in an amount of 0.2 to 25% by weight, in particular from 1 to 25% by weight, is added to these salts as the carbon-emitting substance. The salt should be dried for at least 12 to 24 hours at 120-140 ° C before the addition and freed from the water of crystallization, since it contains 3 mol equivalents of water of crystallization in the form supplied. Alternatively, the red potassium hexacyanoferrate (III), that is, K 3 Fe (CN) 6 , which contains no water of crystallization, can be added to the melt. Preferably, the amount of complex cyanide added is in the range of 2 to 10% by weight.

Alternativ oder zusätzlich zu den genannten komplexen Eisencyaniden können auch andere komplexe Metallcyanide als kohlenstoffspendende Substanzen verwendet werden. Beispiele hierfür sind Tetracyanonickel- oder Tetracyanozink-Verbindungen wie z.B. Na2Ni (CN)4 oder Na2Zn (CN)4.Alternatively or in addition to the complex iron cyanides mentioned, other complex metal cyanides can also be used as carbon donors. Examples of these are tetracyanoxide or tetracyanozinc compounds, for example Na 2 Ni (CN) 4 or Na 2 Zn (CN) 4 .

Anstelle der komplexen ungiftigen Eisencyanide oder Metallcyanide kann auch Natrium- und/oder Kaliumcyanid in freier Form zugegeben werden, in einer Menge von 0,1 bis 25 Gew. %, vorzugsweise zwischen 3 und 10 Gew. %. Die Ergebnisse sind ähnlich wie bei Verwendung von komplexen Cyaniden, auch Gemische aus komplexen und freien Cyaniden können eingesetzt werden.Instead of the complex nontoxic iron cyanides or metal cyanides and sodium and / or potassium cyanide can be added in free form, in an amount of 0.1 to 25 wt.%, Preferably between 3 and 10 wt.%. The results are similar to the use of complex cyanides, and mixtures of complex and free cyanides can also be used.

Der Vorteil der Salzschmelzen mit komplexen Cyaniden liegt darin, dass nicht mit giftigen Stoffen umgegangen wird, da Hexacyanoferrat per se ungiftig ist.The advantage of molten salts with complex cyanides is that it does not handle toxic substances because hexacyanoferrate is non-toxic per se.

Der Vorteil der freien Cyanide ist der geringere Preis, wenn eine Abwasserentgiftungsanlage für Cyanide vorhanden ist, bietet diese Verfahrensweise Vorteile.The advantage of the free cyanides is the lower price, if a sewage detoxification plant for cyanides is present, this procedure offers advantages.

Im Folgenden wird anhand einer Salzschmelze mit Eisencyaniden als kohlenstoffspendenden Substanzen exemplarisch der Ablauf der Einduffusion von Kohlenstoff und Stickstoff aus der Salzschmelze in den Edelstahl und die hierbei von den Aktivatorsubstanzen übernommene Funktion erläutert. Die Betriebstemperatur der Salzschmelze wird in diesem Beispiel auf 350 bis 420°C gesetzt. Bei dieser Temperatur zerfallen die komplexen Eisencyanide gemäß folgender Beziehungen:

         K4Fe(CN)6 => Fe + 2 C + 4 KCN + N2

         K3Fe(CN)6 => Fe + 3 C + 3 KCN + 3/2 N2

In the following, the course of the Einduffusion of carbon and nitrogen from the molten salt in the stainless steel and the case taken over by the activator substances function is explained using a molten salt with iron cyanides as carbon donating substances. The operating temperature of the molten salt is set at 350 to 420 ° C in this example. At this temperature the complex iron cyanides decompose according to the following relationships:

K 4 Fe (CN) 6 => Fe + 2 C + 4 KCN + N 2

K 3 Fe (CN) 6 => Fe + 3 C + 3 KCN + 3/2 N 2

Der Zerfall ist jedoch sehr langsam. Der bei dem Zerfall entstehende Kohlenstoff diffundiert in den zu härtenden, austenitischen Edelstahl ein und bleibt dort bei Temperaturen unterhalb von 420°C in fester, gesättigter oder übersättigter Lösung. Austenit hat ein hohes Lösungsvermögen für Kohlenstoff, ein geringeres für Stickstoff.The decay is very slow. The carbon formed during the decomposition diffuses into the austenitic stainless steel to be hardened and remains there at temperatures below 420 ° C. in solid, saturated or supersaturated solution. Austenite has a high solubility for carbon, a lower one for nitrogen.

Auch ein Teil des entstehenden Stickstoffs diffundiert in die Edelstahloberfläche ein. Ist die Behandlungstemperatur unterhalb von 350 - 370°C, so bleibt auch der Stickstoff - wie der Kohlenstoff - in fester Lösung, liegt die Temperatur zwischen 370°C und 420°C, so bildet der Stickstoff mit dem Legierungselement Chrom Chromnitrid und kann dadurch potentiell die Korrosionsbeständigkeit des Edelstahls an der Oberfläche vermindern. Jedoch wird auch in diesem Temperaturbereich eine Bildung von Chromcarbid noch vermieden, so dass der Legierungsmatrix des Edelstahls trotz der in diesem Temperaturbereich gegebenen Chromnitridbildung noch wenig Chrom entzogen wird, so dass die Reduzierung der Korrosionsbeständigkeit des Edelstahls noch akzeptabel sein kann. Um die Korrosionsbeständigkeit in diesem Temperaturbereich weiter zu verbessern, ist die Eindiffusion von Stickstoff zu vermeiden und nur Kohlenstoff in fester Lösung in die Bauteiloberfläche zu bringen, wobei dann Temperaturen bis zu 440°C angewendet werden können. Bei Temperaturen unterhalb von 370°C können dagegen Stickstoff und Kohlenstoff gemeinsam in fester Lösung einzudiffundieren, ohne dass sich Chromnitrid oder Chromcarbid bildet.Part of the resulting nitrogen also diffuses into the stainless steel surface. If the treatment temperature is below 350 - 370 ° C, so does the nitrogen - like the carbon - in solid solution, the temperature is between 370 ° C and 420 ° C, so the nitrogen forms with the alloying element chromium chromium nitride and thus potentially reduce the corrosion resistance of stainless steel on the surface. However, even in this temperature range, a formation of chromium carbide is still avoided, so that the alloy matrix of the stainless steel, despite the Chromnitridbildung given in this temperature range still little chromium is removed, so that reducing the corrosion resistance of the stainless steel may still be acceptable. In order to further improve the corrosion resistance in this temperature range, the diffusion of nitrogen is to be avoided and only bring carbon in solid solution in the component surface, then temperatures up to 440 ° C can be applied. At temperatures below 370 ° C, on the other hand, nitrogen and carbon can diffuse together in solid solution without forming chromium nitride or chromium carbide.

In der Salzschmelze sind weiterhin folgende Reaktionen möglich:

         2 KCN + O2 => 2 KOCN

         4 KOCN => K2CO3 + 2 KCN + CO + 2 <N>

         2 KCN + 2 O2 => K2CO3 + CO + N2

         2 CO + Fe => Fe3C + CO2

In the molten salt, the following reactions are also possible:

2 KCN + O 2 => 2 KOCN

4 KOCN => K 2 CO 3 + 2 KCN + CO + 2 <N>

2 KCN + 2 O 2 => K 2 CO 3 + CO + N 2

2 CO + Fe => Fe 3 C + CO 2

Cyanidionen, die aus dem Zerfall des komplexen Metallsalzes entstanden sind, werden durch Luftsauerstoff, der in der Schmelze allgegenwärtig ist, zu Cyanationen oxidiert. Diese können unter Bildung von Kohlenmonoxid und Stickstoff zerfallen. Cyanationen sind meistens die Quelle diffusionsfähigen Stickstoffs. Cyanidionen können aber auch weiter oxidiert werden zu Carbonationen, wobei Kohlenmonoxid entsteht. Kohlenmonoxid kann unter Abgabe von diffusionsfähigem Kohlenstoff weiter zu Kohlendioxid reagieren.Cyanide ions, which resulted from the decomposition of the complex metal salt, are oxidized to cyanate ions by atmospheric oxygen, which is omnipresent in the melt. These can decompose to form carbon monoxide and nitrogen. Cyanations are usually the source of diffusible nitrogen. However, cyanide ions can also be further oxidized to carbonate ions, resulting in carbon monoxide. Carbon monoxide can continue to react with carbon dioxide by releasing diffusible carbon.

Daneben kann Cyanid mit Bariumionen der als Bariumchlorid in der Salzschmelze enthaltenen Aktivatorsubstanz zu Bariumcyanid Ba(CN)2 reagieren, das sich in Bariumcyanamid BaNCN umwandelt. Dabei wird Kohlenstoff frei, der in die Bauteile diffundieren kann.

         BaCl2 + 2 KCN => Ba(CN)2 + 2 KCl

        Ba(CN)2 => BaNCN + <C>

         BaNCN + 3/2 O2 => BaCO3 + N2

In addition, cyanide can react with barium ions of the activator substance contained as barium chloride in the molten salt to barium cyanide Ba (CN) 2 , which turns into barium cyanamide BaNCN. This releases carbon, which can diffuse into the components.

BaCl 2 + 2 KCN => Ba (CN) 2 + 2 KCl

Ba (CN) 2 => BaNCN + <C>

BaNCN + 3/2 O 2 => BaCO 3 + N 2

Das Bariumcyanamid reagiert weiter mit Luftsauerstoff zu Bariumcarbonat und Stickstoff, der freigesetzt wird. Ähnliche Reaktionen sind mit Strontium, Calcium und Magnesium zu erwarten, falls als Aktivatorsubstanz Strontiumchlorid, Calciumchlorid beziehungsweise Magnesiumchlorid verwendet wird. Die Erdalkalimetalle in Form ihrer Halogenide bilden somit bei dem erfindungsgemäßen Verfahren Aktivatorsubstanzen, die die Freisetzung von diffusionsfähigem Stickstoff und Kohlenstoff in dem Temperaturbereich des erfindungsgemäßen Verfahrens bewirken. Ohne Beteiligung wenigstens eines Erdalkalielements der Reihe Magnesium, Calcium, Strontium und Barium ist die Eindiffusion des notwendigen Kohlenstoffs in die Edelstahloberfläche nicht möglich. Eine ähnliche Rolle spielt das Element Lithium, das in ähnlicher Weise wie die Erdalakalimetalle ebenfalls als Aktivator für die Diffusion von Kohlenstoff wirkt:

         2 LiCl + 2 KCN => 2 LiCN + 2 KCl

         2 LiCN => Li2NCN + <C>

         Li2NCN + 3/2 O2 => Li2CO3 + N2

The barium cyanamide further reacts with atmospheric oxygen to form barium carbonate and nitrogen which is released. Similar reactions are to be expected with strontium, calcium and magnesium, if strontium chloride, calcium chloride or magnesium chloride is used as the activator substance. The alkaline earth metals in the form of their halides thus form in the process according to the invention activator substances which bring about the release of diffusible nitrogen and carbon in the temperature range of the process according to the invention. Without the participation of at least one alkaline earth element of the series magnesium, calcium, strontium and barium, the diffusion of the necessary carbon into the stainless steel surface is not possible. A similar role is played by the element lithium, which functions similarly as the alkaline earth metals as an activator for the diffusion of carbon:

2 LiCl + 2 KCN => 2 LiCN + 2 KCl

2 LiCN => Li 2 NCN + <C>

Li 2 NCN + 3/2 O 2 => Li 2 CO 3 + N 2

Die übrigen Alkalimetalle Na, K, Rb und Cs zeigen diese Wirkung nicht.The other alkali metals Na, K, Rb and Cs do not show this effect.

Die angeführten Reaktionen erklären den Mechanismus der Übertragung von Kohlenstoff und Stickstoff auf die behandelten Bauteile aus Edelstahl in eutektischen Salzschmelzen aus Erdalaklichloriden und Lithiumsalzen. Sie erklären auch das Auftreten kleiner Mengen von Cyanat- und Carbonationen nach einer gewissen Betriebsdauer der Schmelzen aufgrund der Oxidationsvorgänge.The reported reactions explain the mechanism of transfer of carbon and nitrogen to the treated stainless steel components in eutectic molten salts of alkaline earth chlorides and lithium salts. They also explain the occurrence of small amounts of cyanate and carbonate ions after a certain period of operation of the melts due to the oxidation processes.

Eine analytische Kontrolle der erfindungsgemäßen Salzschmelzen kann wie folgt durchgeführt werden: Die Änderung der Konzentration der wirksamen Bestandteile (komplexe Cyanide oder freies Cyanid) kann durch potentiometrische Titration überwacht werden. Im Falle des K4Fe(CN)6 kann mit Cer(IV) Sulfat-Lösung titriert werden. Freies Cyanid kann mit Nickel(II)Sulfat sehr gut bestimmt werden. Verbrauchtes Cyanid oder komplexes Cyanid wird dementsprechend ergänzt.An analytical control of the molten salts according to the invention can be carried out as follows: The change in the concentration of the active Ingredients (complex cyanide or free cyanide) can be monitored by potentiometric titration. In the case of K 4 Fe (CN) 6 can be titrated with cerium (IV) sulfate solution. Free cyanide can be determined very well with nickel (II) sulfate. Consumed cyanide or complex cyanide is added accordingly.

Zur Verdrängung von Luft und zur Verhinderung der Oxidation des freien und/oder komplexen Cyanids in der erfindungsgemäßen Salzschmelze kann in dieser ein inertes Gas wie zum Beispiel Argon, Stickstoff oder Kohlendioxid eingeleitet werden. Besonders vorteilhaft kann zur Verdrängung von Luft und zur Verhinderung der Oxidation des freien und komplexen Cyanids die Salzschmelze in einer abgeschlossenen Retorte unter Verwendung von Stickstoff, Argon oder Kohlendioxid als Schutzgas betrieben werden.In order to displace air and to prevent the oxidation of the free and / or complex cyanide in the molten salt of the invention, an inert gas such as argon, nitrogen or carbon dioxide may be introduced therein. Particularly advantageous for displacing air and preventing the oxidation of the free and complex cyanide, the molten salt can be operated in a closed retort using nitrogen, argon or carbon dioxide as a protective gas.

Die Erfindung wird im Folgenden anhand von Beispielen und Darstellungen erläutert. Dabei zeigen:

Figur 1:
Darstellung eines Querschnitts einer mit einer erfindungsgemäβen Salzschmelze gehärteten Probe aus Edelstahl 1.4571.
Figur 2:
Element-Tiefenprofilanalyse für einen mit einer erfindungsgemäßen Salzschmelze gehärteten Edelstahl 1.4541.
Figur 3:
Härteverlauf in Abhängigkeit der Eindringtiefe im Oberflächenbereich eines mit einer erfindungsgemäßen Salzschmelze behandelten Edelstahls 1.4541.
The invention is explained below with reference to examples and illustrations. Showing:
FIG. 1:
Representation of a cross section of a hardened with a erfindungsgemäβen molten salt sample of stainless steel 1.4571.
FIG. 2:
Element depth profile analysis for a 1.4541 stainless steel hardened with a molten salt according to the invention.
FIG. 3:
Hardness curve as a function of the penetration depth in the surface region of a stainless steel 1.4541 treated with a molten salt according to the invention.

Beispiel 1:Example 1:

In einen Tiegel aus hitzefestem Stahl, z.B. aus dem Werkstoff 1.4828, werden 42 kg trockenes Kaliumchlorid, 34 kg trockenes Lithiumchlorid und 20 kg Bariumchlorid siccum eingewogen und locker vermischt. Alle Salze müssen eine Restfeuchte von weniger als 0,3 Gew. % aufweisen. Die Mischung wird auf 400°C aufgeheizt und ergibt eine wasserklare Schmelze. In diese werden 4 kg Kaliumhexacyanoferrat (II) langsam eingetragen, welches vorher 12 h bei 140°C in einem Muffelofen getrocknet worden war. Beim Eintragen des Kaliumhexacyanoferrats (II) scheidet sich eine sehr kleine Menge Kohlenstoff an der Tiegelwand und an der Oberfläche der Schmelze ab. Dieser Kohlenstoff wird mit einem Sieblöffel abgeschöpft. Danach liegt eine wasserklare Schmelze vor, die auf eine Betriebstemperatur von 400°C gebracht wird. In diese Schmelze werden 10 kg Werkstücke aus dem Edelstahl 1.4571 (Werkstoff X6CrNiMoTil7-12,2), an Stahldrähten befestigt, eingetaucht und über einen Zeitraum von 48 h dem Einfluss der Schmelze ausgesetzt.In a crucible made of heat-resistant steel, e.g. from the material 1.4828, 42 kg dry potassium chloride, 34 kg dry lithium chloride and 20 kg barium chloride siccum weighed and mixed loosely. All salts must have a residual moisture content of less than 0.3% by weight. The mixture is heated to 400 ° C and gives a water-clear melt. In this 4 kg of potassium hexacyanoferrate (II) are slowly added, which had previously been dried for 12 h at 140 ° C in a muffle furnace. When the potassium hexacyanoferrate (II) is introduced, a very small amount of carbon deposits on the crucible wall and on the surface of the melt. This carbon is skimmed off with a sifting spoon. Thereafter, a water-clear melt is present, which is brought to an operating temperature of 400 ° C. In this melt 10 kg of workpieces made of stainless steel 1.4571 (material X6CrNiMoTil7-12,2), attached to steel wires, immersed and exposed to the influence of the melt over a period of 48 h.

Das Resultat dieser Behandlung ist eine 20 -22 µm dicke Diffusionsschicht auf der Oberfläche der behandelten Bauteile und Proben, die metallographisch durch einen Querschliff und anätzen mit dem Ätzmittel V2A-Beize sichtbar gemacht werden kann. V2A Beize ist ein Gemisch aus 100 ml Wasser und 100 ml Salzsäure conc. (HCI, 30 %) und 0,3 % "Vogels Reagenz". Vogels Reagenz ist ein Gemisch aus 60 % 2 - Methoxy- 2 - propanol (H3C-O-CH2Oh-CH3) 5 % Thioharnstoff (H2N-CS-NH2) 5 % Nonyl-phenol-ethoxylat Rest Ethanol. Der Querschnitt ist photographisch in Figur 1 in einer 500-fachen Vergrößerung dargestellt. Die Oberflächenhärte dieser Schicht wird zu 642 - 715 HV (0,5) beziehungsweise 1100 -1210 HV (0,025) ermittelt. Die Elementverteilung innerhalb der Schicht kann mit der Glimmentladungsspektroskopie (GDOES) ermittelt werden und ist beispielhaft in Figur 2 dargestellt. In Figur 2 ist die Eindringtiefe der Elemente N, C, Fe, Cr2, Ni, Mo in die Oberfläche des mit der Salzschmelze gehärteten Werkstücks dargestellt, das heißt es sind die Massekonzentrationen dieser Elemente in Prozent in Abhängigkeit der Tiefe im Werkstück in µm aufgetragen. Die in Figur 2 dargestellten Kurvenverläufe von Fe, O, Cr2 und Ni sind jeweils auf Massekonzentrationen von 100 % bezogen, während die Kurvenverläufe von C, Mo auf Massekonzentrationen von 10 % und der Kurvenverlauf von N auf eine Massekonzentration von 25 % bezogen sind. Wie aus Figur 2 ersichtlich beträgt die erreichte Diffusionstiefe für Kohlenstoff etwa 25 - 27 µm, die Diffusionstiefe für Stickstoff ist etwas geringer. Die in der Randzone des Werkstücks festgestellten Mengen an Stickstoff und Kohlenstoff liegen nicht als Nitride oder Carbide vor, sondern größtenteils in der Form von Stickstoff und Kohlenstoff in fester, übersättigter Lösung.The result of this treatment is a 20-22 μm thick diffusion layer on the surface of the treated components and samples, which can be made visible metallographically by cross-sectioning and etching with the etchant V2A-stain. V2A stain is a mixture of 100 ml of water and conc. 100 ml of hydrochloric acid. (HCI, 30%) and 0.3% "bird's reagent". Vogel's reagent is a mixture of 60% 2-methoxy-2-propanol (H3C-O-CH2Oh-CH3) 5% thiourea (H2N-CS-NH2) 5% nonylphenol ethoxylate residue ethanol. The cross section is photographically in FIG. 1 shown in a 500-fold magnification. The surface hardness of this layer is determined to be 642 - 715 HV (0.5) or 1100 - 1210 HV (0.025). The element distribution within the layer can be determined by glow discharge spectroscopy (GDOES) and is exemplified in FIG. 2 shown. In FIG. 2 is the penetration depth of the elements N, C, Fe, Cr 2 , Ni, Mo shown in the surface of the cured with the molten salt workpiece, that is, the mass concentrations of these elements in percent, depending on the depth in the workpiece in microns. In the FIG. 2 illustrated curves of Fe, O, Cr 2 and Ni are each referenced to mass concentrations of 100%, while the curves of C, Mo are related to mass concentrations of 10% and the graph of N to a mass concentration of 25%. How out FIG. 2 it can be seen that the achieved diffusion depth for carbon is about 25 - 27 μm, the diffusion depth for nitrogen is somewhat lower. The amounts of nitrogen and carbon found in the periphery of the workpiece are not present as nitrides or carbides, but mostly in the form of nitrogen and carbon in a solid, supersaturated solution.

Figur 3 zeigt für dieses Werkstück den Härteverlauf in Abhängigkeit der Tiefe (in µm). Der Härteverlauf wurde gemessen mit der Vickers Methode unter einer Prüflast von 0,010 kp (10 Gramm). Wie aus dem Vergleich der Figuren 2 und 3 ersichtlich, ist in der Randzone des Werkstücks, in welche mittels der Salzschmelze Stickstoff und Kohlenstoff eindiffundiert wurde, die Härte des Werkstücks signifikant erhöht. FIG. 3 shows the hardness curve as a function of the depth (in μm) for this workpiece. The hardness profile was measured by the Vickers method under a test load of 0.010 kp (10 grams). As from the comparison of Figures 2 and 3 As can be seen, in the edge zone of the workpiece into which nitrogen and carbon have been diffused by means of the molten salt, the hardness of the workpiece is significantly increased.

Beispiel 2:Example 2:

In einen Tiegel aus hitzefestem Stahl werden 43 kg trockenes Kaliumchlorid, 30 kg trockenes Lithiumchlorid, 17 kg Strontiumchlorid siccum und 3 kg Bariumchlorid siccum, eingewogen und locker vermischt. Alle Salze müssen eine Restfeuchte von weniger als 0,3 Gew. % aufweisen. Die Mischung wird auf 400°C aufgeheizt und ergibt eine wasserklare Schmelze. In diese wird 7 kg Kaliumhexacyanoferrat (II) langsam eingetragen welches vorher 12 h bei 140°C in einem Muffelofen getrocknet worden war. Danach liegt eine wasserklare Schmelze vor, die auf eine Betriebstemperatur von 370°C abgesenkt wird. In diese Schmelze werden 10 kg Werkstücke aus dem Edelstahl 1.4301, an Stahldrähten befestigt, eingetaucht und über einen Zeitraum von 24 - 48 h dem Einfluss der Schmelze ausgesetzt.43 kg of dry potassium chloride, 30 kg of dry lithium chloride, 17 kg of strontium chloride siccum and 3 kg of barium chloride siccum are weighed into a crucible made of heat-resistant steel and mixed loosely. All salts must have a residual moisture content of less than 0.3% by weight. The mixture is heated to 400 ° C and gives a water-clear melt. In this 7 kg potassium hexacyanoferrate (II) is added slowly which had been previously dried for 12 h at 140 ° C in a muffle furnace. Thereafter, a water-clear melt is present, which is lowered to an operating temperature of 370 ° C. In this melt 10 kg of workpieces made of stainless steel 1.4301, attached to steel wires, immersed and exposed for a period of 24 - 48 h the influence of the melt.

Das Resultat dieser Behandlung ist je nach Behandlungsdauer eine 10 - 25 µm dicke Diffusionsschicht auf der Oberfläche der behandelten Bauteile und Proben, die metallographisch durch einen Querschliff und Anätzen mit dem Ätzmittel V2A-Beize sichtbar gemacht werden kann.Depending on the treatment duration, the result of this treatment is a 10-25 μm thick diffusion layer on the surface of the treated components and samples. which can be made visible metallographically by a cross-section and etching with the etchant V2A-stain.

Beispiel 3:Example 3:

In einen Tiegel aus hitzefestem Stahl werden 37 kg trockenes Kaliumchlorid, 26 kg trockenes Lithiumchlorid und 17 kg Strontiumchlorid siccum eingewogen und locker vermischt. Alle Salze müssen eine Restfeuchte von weniger als 0,3 Gew. % aufweisen. Die Mischung wird auf 400°C aufgeheizt und ergibt eine wasserklare Schmelze. In diese werden 10 kg KCN und 10 kg NaCN langsam eingetragen. Die entstandene Schmelze wird auf eine Betriebstemperatur von 400 - 410°C gebracht. In diese Schmelze werden 10 kg Werkstücke aus dem Edelstahl 1.4301, an Stahldrähten befestigt, eingetaucht und über einen Zeitraum von 24 h dem Einfluss der Schmelze ausgesetzt.37 kg of dry potassium chloride, 26 kg of dry lithium chloride and 17 kg of strontium chloride siccum are weighed into a crucible made of heat-resistant steel and mixed loosely. All salts must have a residual moisture content of less than 0.3% by weight. The mixture is heated to 400 ° C and gives a water-clear melt. 10 kg of KCN and 10 kg of NaCN are slowly added to these. The resulting melt is brought to an operating temperature of 400 - 410 ° C. In this melt 10 kg of workpieces made of stainless steel 1.4301, attached to steel wires, immersed and exposed to the influence of the melt over a period of 24 h.

Das Resultat dieser Behandlung ist eine ca. 10 µm dicke Diffusionsschicht auf der Oberfläche der behandelten Bauteile und Proben, die metallographisch durch einen Querschliff und anätzen mit dem Ätzmittel V2A-Beize sichtbar gemacht werden kann. Die Härte dieser Schicht wird zu 620 HV (0,5) ermittelt.The result of this treatment is an approx. 10 μm thick diffusion layer on the surface of the treated components and samples, which can be made visible metallographically by cross-sectioning and etching with the etchant V2A-stain. The hardness of this layer is determined to be 620 HV (0.5).

Beispiel 4:Example 4:

In einen Tiegel aus hitzefestem Stahl werden 42 kg trockenes Kaliumchlorid, 34 kg trockenes Lithiumchlorid, 10 kg Bariumchlorid siccum und 10 kg Strontiumchlorid siccum eingewogen und locker vermischt. Alle Salze müssen eine Restfeuchte von weniger als 0,3 Gew. % aufweisen. Die Mischung wird auf 400°C aufgeheizt und ergibt eine wasserklare Schmelze. In diese wird 4 kg K3Fe(CN)6 langsam eingetragen. Es bildet sich eine wasserklare Schmelze, die auf eine Betriebstemperatur von 400 - 410°C gebracht wird. In diese Schmelze werden 10 kg Werkstücke aus dem Edelstahl 1.4301 und 14541, an Stahldrähten befestigt, eingetaucht und über einen Zeitraum von 24 h dem Einfluss der Schmelze ausgesetzt.42 kg of dry potassium chloride, 34 kg of dry lithium chloride, 10 kg of barium chloride siccum and 10 kg of strontium chloride siccum are weighed into a crucible made of heat-resistant steel and loosely mixed. All salts must have a residual moisture content of less than 0.3% by weight. The mixture is heated to 400 ° C and gives a water-clear melt. In this 4 kg K 3 Fe (CN) 6 is slowly added. It forms a water-clear melt, which is brought to an operating temperature of 400 - 410 ° C. In this melt 10 kg of workpieces made of stainless steel 1.4301 and 14541, attached to steel wires, immersed and exposed to the influence of the melt over a period of 24 h.

Beispiel 5:Example 5:

In einen Tiegel aus hitzefestem Stahl werden 42 kg trockenes Kaliumchlorid, 34 kg trockenes Lithiumchlorid, 10 kg Bariumchlorid siccum und 2 kg Strontiumchlorid siccum eingewogen und locker vermischt. Alle Salze müssen eine Restfeuchte von weniger als 0,3 Gew. % aufweisen. Die Mischung wird auf 400°C aufgeheizt und ergibt eine wasserklare Schmelze. In diese werden 4 kg K3Fe(CN)6 sowie 4 kg KCN und 4 kg NaCN langsam eingetragen. Es bildet sich eine klare Schmelze, die auf eine Betriebstemperatur von 400 - 410°C gebracht wird. In diese Schmelze werden 10 kg Werkstücke aus dem Edelstahl 1.4301 und 1.4541, an Stahldrähten befestigt, eingetaucht und über einen Zeitraum von 24 h dem Einfluss der Schmelze ausgesetzt.42 kg of dry potassium chloride, 34 kg of dry lithium chloride, 10 kg of barium chloride siccum and 2 kg of strontium chloride siccum are weighed into a crucible made of heat-resistant steel and mixed loosely. All salts must have a residual moisture content of less than 0.3% by weight. The mixture is heated to 400 ° C and gives a water-clear melt. 4 kg of K 3 Fe (CN) 6 and 4 kg of KCN and 4 kg of NaCN are added slowly to this mixture. It forms a clear melt, which is brought to an operating temperature of 400 - 410 ° C. In this melt 10 kg workpieces made of stainless steel 1.4301 and 1.4541, attached to steel wires, immersed and exposed to the influence of the melt over a period of 24 h.

Claims (17)

  1. Molten salt for hardening surfaces of stainless steel, comprising the following components: 30 - 60 weight % potassium chloride (KCl) 20 - 40 weight % lithium chloride (liCl) 15 - 30 weight % of an activator substance consisting of barium chloride (BaCl2) and/or strontium chloride (SrCl2) and/or magnesium chloride (MgCl2) and/or calcium chloride (CaCl2) 0.2 - 25 weight % of a carbon-donating substance consisting of a free cyanide and/or a complex cyanide.
  2. Molten salt according to claim 1, characterised in that this contains as activator substance additionally to barium chloride and/or strontium chloride also magnesium chloride and/or calcium chloride in an amount of 0.1 to 10 weight %.
  3. Molten salt according to one of claims 1 and 2, characterised in that this contains potassium hexacyanoferrate (II) and/or potassium hexacyanoferrate (III) as carbon-donating substance.
  4. Molten salt according to claim 3, characterised in that this contains the following components: 42 weight % KCL 34 weight % LiCl 20 weight % BaCl2 as activator substance 4 weight % potassium hexacyanoferrate (II) as carbon-donating substance.
  5. Molten salt according to claim 3, characterised in that this contains the following components: 40 weight % KCl 33 weight % LiCl 2 weight % BaCl2 and 20 weight % SrCl2 as activator substances 5 weight % potassium hexacyanoferrate (II) as carbon-donating substance.
  6. Molten salt according to one of claims 1 and 2, characterised in that this contains a tetracyano nickel compound or a tetracyano zinc compound as carbon-donating substance.
  7. Molten salt according to claim 6, characterised in that this contains Na2Ni(CN)4 or Na2Zn(Cn)4 as carbon-donating substance.
  8. Molten sale according to any one of claims 1 to 7, characterised in that it contains the alkali metals Li, Na and/or K in an amount of 0.1 to 25 weight % as carbon-donating substance of free cyanide.
  9. Molten salt according to claim 8, characterised in that this contains the following components: 44 weight % KCl 30 weight % LiCl 5 weight % BaCl2 and 15 weight % SrCl2 as activator substances 3 weight % potassium hexacyanoferrate (II), 2 weight % NaCN and 1 weight % KCN as carbon-donating substances.
  10. Molten salt according to claim 8, characterised in that this contains the following components: 37 weight % KCl 26 weight % LiCl 17 weight % SrCl2 as activator substance 10 weight % NaCN and 10 weight % KCN as carbon-donating substances.
  11. Molten salt according to any one of claims 1 to 10, characterised in that this contains cyanate ions (NCO-) in an amount of 0.1 weight % to 10 weight % and carbonate ions (CO3)2- in a concentration of 0.1 to 10 weight % as additional components.
  12. Method of hardening workpieces consisting of stainless steel by diffusion of the elements carbon and/or nitrogen into the workpiece surfaces, in that the workpieces are immersed in a molten salt according to any one of claims 1 to 11 and these are exposed at temperatures below 450° C for a time period of 15 minutes to 240 hours.
  13. Method according to claim 12, characterised in that the workpieces are exposed to the molten salt at a temperature in the range of 350° C to 410° C.
  14. Method according to one of claims 12 and 13, characterised in that the workpieces are exposed for a time period of 48 hours to a molten salt with the following composition: 42 weight % KCl 34 weight % LiCl 20 weight % BaCl2 4 weight % potassium hexacyanoferrate (II).
  15. Method according to any one of claims 12 to 14, characterised in that an inert gas is conducted through the molten salt for displacing air and preventing oxidation of the free and complex cyanide.
  16. Method according to claim 15, characterised in that argon, nitrogen or carbon dioxide is used as inert gas.
  17. Method according to any one of claims 12 to 16, characterised in that for the displacement of air and prevention of oxidation of the free and complex cyanide the molten salt is used in a closed retort with use of nitrogen, argon or carbon dioxide as protective gas.
EP07010534A 2006-06-09 2007-05-26 Method for tempering stainless steel and fused salt for performing the method Active EP1865088B1 (en)

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DE102007059229A1 (en) * 2007-11-19 2009-05-20 Konrad Damasko Method for producing a component with a hardened surface
US9101896B2 (en) * 2010-07-09 2015-08-11 Sri International High temperature decomposition of complex precursor salts in a molten salt
FR2972459B1 (en) * 2011-03-11 2013-04-12 Hydromecanique & Frottement FOUNDED SALT BATHS FOR NITRIDING STEEL MECHANICAL PARTS, AND METHOD FOR IMPLEMENTING THE SAME
DE102011087960A1 (en) * 2011-12-08 2013-06-13 Witzenmann Gmbh Flexible metal element used for manufacture of automobile, is made of stainless steel and has partially nitrided surface
EP2680090A1 (en) * 2012-06-28 2014-01-01 Nivarox-FAR S.A. Mainspring for a clock piece
DE102012018551A1 (en) * 2012-09-20 2014-03-20 Wika Alexander Wiegand Se & Co. Kg Stainless steel element, useful in pressure measurement system and as disk-shaped membrane, where element is made of an austenitic chrome-nickel steel and material edge zone of element has increased strength and hardness
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JP7178832B2 (en) * 2018-08-31 2022-11-28 日本パーカライジング株式会社 Method for manufacturing surface hardening material
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US7909943B2 (en) 2011-03-22
US20080099108A1 (en) 2008-05-01
BRPI0702568B1 (en) 2015-10-13
DE102006026883B8 (en) 2007-10-04
ZA200704591B (en) 2008-08-27
PL1865088T3 (en) 2010-01-29
BRPI0702568A (en) 2008-02-19
MX2007006969A (en) 2008-10-30
ATE443163T1 (en) 2009-10-15
CN101235477A (en) 2008-08-06
JP5101173B2 (en) 2012-12-19
DE102006026883B3 (en) 2007-08-16
CN101235477B (en) 2011-05-18
ES2331383T3 (en) 2009-12-30
KR20070118008A (en) 2007-12-13
CA2591244A1 (en) 2007-12-09
JP2007332459A (en) 2007-12-27
DE502007001522D1 (en) 2009-10-29
EP1865088A1 (en) 2007-12-12

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