EP3088537A1 - Production method for hpi cast iron - Google Patents

Abstract

The invention relates to a method for producing and heat treating a cast workpiece (1) of nodular cast iron in a sand mold (2). The sand mold (2) contains at least one core (3) which has a coolant supply (4), preferably in the form of one or more tubes laid in or towards the core (3), by means of which the core (3) is provided with a coolant (5 ), preferably water, can be added. The process for the preparation and heat treatment contains several steps according to the invention.

Description

The present invention relates to a process for the preparation and subsequent heat treatment of a cast piece of nodular cast iron in a sand mold according to the preamble of claim 1 and a mold designed therefor according to the preamble of claim 4, as well as a cast workpiece produced therewith.

Cast iron castings made of spheroidal graphite cast iron (GJS for short) are widely used in industry and have become very important in global production. Among other things, this material is also used in the automotive industry for the production of various vehicle components (in particular for safety components in the chassis or the suspension). Spheroidal graphite cast iron, also called ductile iron, has particularly good mechanical material properties in terms of tensile strength and elongation at break for such applications. In spheroidal graphite cast iron, the carbon is predominantly in the form of spherical graphite particles. The shape of the graphite spheres is the distinguishing feature distinguishing GJS from other gray cast iron materials - lamellar graphite cast iron (GJL) or vermicular graphite cast iron (GJV). The shape which the carbon or graphite takes on its precipitation from the solidifying cast iron structure substantially influences the mechanical properties of the alloy. The graphite inclusions are crucial for the mechanical and technological properties. The different graphite forms weaken the iron matrix in different ways. The spherical form of graphite in GJS represents the most compact form in which the matrix experiences the least weakening. As a result, GJS is superior to the other two forms of GJL or GJV in terms of mechanical properties.

• Die Werkstoffzusammensetzung mit Legierungselementen wie z.B. Si, Mg, Cu, Ni, Cr oder Mo, welche einen wichtigen Einfluss auf die Gefügebildung und auf die resultierenden mechanischen Eigenschaften haben.
• Das Aufschmelzen des Rohmaterials (Roheisen, Stahlschrott) im Schmelzofen. (z.B. Induktionsofen, Kupol- oder Lichtbogenofen).
• Die Magnesiumbehandlung der Schmelze, z.B. mit dem bekannten Konverterverfahren.
• Das Impfen der Schmelze mit Si, Ca, Al, Bi, Zr, Fe oder seltene Erden, welche die Keimbildung bei der Erstarrung beeinflussen.
• Und schliesslich die Beeinflussung der Erstarrungs- und Abkühlgeschwindigkeit, wodurch das gewünschte Grundgefüge erzeugt und die Graphitausbildung gezielt gesteuert werden kann.

The production process for the production of spheroidal graphite cast iron (GJS) is well known. The most important points of the manufacturing process, which influence the quality of the produced GJS regarding material properties are the following:

• The material composition with alloying elements such as Si, Mg, Cu, Ni, Cr or Mo, which have an important influence on the structure formation and on the resulting mechanical properties.
• The melting of the raw material (pig iron, steel scrap) in the smelting furnace. (eg induction furnace, cupola or electric arc furnace).
• The magnesium treatment of the melt, for example with the known converter method.
• Inoculation of the melt with Si, Ca, Al, Bi, Zr, Fe or rare earths, which influence the nucleation during solidification.
• Finally, the influence of the solidification and cooling rate, whereby the desired basic structure can be generated and the graphite formation can be controlled in a targeted manner.

• Spannungsarmglühen
• Weichglühen
• Carbidzerfallsglühen
• Härten und Vergüten
• Perlitglühen (Normalisieren)
• ADI-Wärmebehandlung (ADI: Austempered Ductile Iron)

The mechanical properties of castings are significantly influenced by the shape which the carbon or graphite takes on its precipitation from the solidifying structure of cast iron. In order to further improve the mechanical properties of manufactured spheroidal graphite cast iron (GJS), however, the GJS material can also be further tempered by means of a downstream heat treatment process. One differentiates in the state of the art:

• Stress relief
• annealing
• Carbidzerfallsglühen
• Hardening and tempering
• Perlite annealing (normalizing)
• ADI heat treatment (ADI: Austempered Ductile Iron)

The ADI heat treatment process produces a nodular graphite cast iron which has improved mechanical properties through its heat treatment obtained from the starting material GJS. With higher strength values, with good toughness values, the so-called ADI material can be an alternative to forged steel. Thanks to its material properties, the material ADI offers many application possibilities. The FIG. 1 shows that the ADI material or the ausferritic spheroidal graphite cast iron has specific strengths that approximate steel or even the typical aluminum or magnesium based lightweight alloys. This opens up numerous application possibilities, especially in the automotive sector, especially as this industry is an important purchaser of castings and the automotive industry today attaches great importance to reducing vehicle weight.

1. 1. Austenitisierung des Bauteils (A-B),
2. 2. Abschrecken im Kühlmedium (C-D) und isothermes Halten (D-E),
3. 3. Abkühlen auf Raumtemperatur (E-F).

The production of ADI is known and is done by the thermal process of a heat treatment. The procedure for the production of ADI is based on the following steps, which are described in the FIG. 2 being represented:

1. 1. austenitizing the component (AB),
2. 2. quenching in the cooling medium (CD) and isothermal holding (DE),
3. 3. Cool to room temperature (EF).

For complete austenitization, the casting is heated from room temperature to an austenitizing temperature in the range of 850 ° to 950 ° C. During austenitization, the iron changes its lattice structure. The cubic-space-centered α-iron folds into the cubic-surface-centered γ-iron and accumulates with carbon up to the saturation line. The component is held at Austenitisierungstemperatur until a uniform carbon content has set in the entire component. In a coolant, the austenitized component is then quenched to the transformation temperature. These are usually oil or salt baths, which are heated to transformation temperature. During the rapid loss of temperature, the austenite works locally into ferrite. Since ferrite has only a low C solubility, locally high carbon concentrations that stabilize the austenite arise. The Austenitisierungszeit depends on the selected Austenitisierungstemperatur. The higher this Temperature is chosen, the faster the carbon saturation is reached. Other influencing factors which determine the duration of austenitisation include the graphite sphere size, the graphite sphere distance and the structure type of the cast piece of work (perlite or ferrite) prior to the ADI process.

Like from the FIG. 2 as can be seen, the austenitizing temperature quenches the component to the transformation temperature. This lies in the range between 230 ° and 450 ° C. The transformation temperature is a crucial parameter for the adjustment of the ADI matrix matrix and thus also for the properties of the ADI material. Quenching is often done by immersing the heat-treated casting workpiece in the tempered to the transition temperature oil or salt bath. At low holding temperatures in the range between 230 ° and 320 ° C of the bath, the austenite is strongly supercooled and thus promotes ferrite formation over the ferrite growth. Furthermore, the diffusion rate of the carbon from the ferrite distorted by the transformation krz lattice of ferrite is so low that it comes to carbide precipitations within the ferrite needles. Due to the diffusion barrier, only the adjacent austenite can be stabilized; the remaining, unstable austenite folds to martensite on cooling to room temperature. This mixed structure of ferrite, martensite and austenite produces the high-strength ADI grades. In contrast, tough ADI varieties are produced at higher aging temperatures. The austenite is completely stabilized and there is no martensite formation. The higher residual austenite content together with the ferrite allow good elongation at high strengths.

The conversion time fulfills two functions in the generation of ADI. On the one hand, it ensures that the ausritritic structure created by deterrence does not fall over when the martensite start line is exceeded, and on the other hand, it allows the carbon to diffuse or the ferrite needles to grow. The formation of the ferrite takes place in the first 30 minutes. The relaxation of the ferrite grid also takes place in a short time. The expansion of the austenite lattice through the uptake of carbon requires more time and is only after about completed one hour.

The ferritic structure of the ADI material produced by the heat treatment is characterized by a good combination of strength and toughness. The strength of ADI can be nearly twice that of GJS. These mechanical properties allow application in areas previously reserved for forging steels. The high strengths result from the micro-internal stresses in the microstructure, which are created by the coercively dissolved carbon in the austenite. Sliding dislocations cause further solidification of the material under load, which improves wear resistance during use. In addition to the high strengths, ADI also has high toughness and wear resistance, and the fracture toughness KIC, which describes the resistance to critical crack propagation, is comparable to the values of steels in ADI. However, nodular graphite makes ADI easier to machine than high strength steels of the same strength. The carbon provides a self-lubricating effect and thus reduces the load on the tools. The ADI method is described, inter alia, in the patents U.S. 4,880,477 and US Pat. No. 7,497,915 B2 described.

Unfortunately, the ADI method also has its disadvantages. The quenching of the heat-treated casting in a salt bath is environmentally problematic (salt vapors, disposal, highly corrosive environment for equipment and machinery). In the Canadian patent CA 2 218 788 It is proposed to use water with an aqueous polymer solution instead of salt baths.

A further disadvantage of the ADI process is its high energy consumption: the GJS casting must be heated again from room temperature to the austenitizing temperature for the application of the ADI process so that the heat treatment process can be carried out.

The patent application US 2005/0189043 A1 suggests a heat treatment of a GJS material, in which the solidified but still very hot GJS blank removed in a temperature range of 980 to 950 ° C from the mold, a plastic deformation and then subjected to a heat treatment. The patent application US 2005/0189043 A1 Therefore, the casting heat remaining in the casting is used for its heat treatment.

In an article from 1984 ( Janovak, JF and Gundlach, RB, "Development of a ductile cast iron suitable for interstage tempering"; Foundry practice, pp. 317-330, volume 19, 1984; Specialist publisher Schiele & Schön GmbH, Berl in) a method is proposed for intermediate tempering of GJS in which the casting undergoes ADI heat treatment directly from the mold and without the use of salt or oil baths. See the time-temperature diagram in FIG. 3 : The cast casting is cooled in the mold to a defined temperature in the austenitic region (the structure is therefore austenitic) and is then evacuated from the mold or knocked out and cooled in the air (so-called hot emptying). For example, once the casting has cooled to the desired intermediate stage tempering temperature (about 370 ° C), it is placed in an air circulation oven and allowed to sit for a reasonable amount of time so that the intermediate stage tempering, a bainitic reaction, can occur at the desired intermediate stage tempering temperature. Thereafter, the casting is removed from the oven and cooled in air to room temperature. This creates a cast structure with martensite and retained austenite. The method further provides for a two-stage tempering by tempering the intermediate tempered GJS cast part in the ausferritic transformation range, so that austenite is formed into an ausritritic ferrite structure.

The invention is therefore an object of the invention to provide a more economical heat treatment process, which requires less energy over the prior art, low demands on the technical equipment and handling, environmentally unproblematic, while a GJS cast piece with improved microstructure and thus improved mechanical Component properties achieved.

This object is achieved by the method and the mold for the production and heat treatment of castings from GJS cast iron according to the features of claim 1 and claim 4, respectively.

Thanks to the production and heat treatment process according to the invention of cast iron castings with nodular cast iron, targeted cooling of the castings in the casting mold and immediately after casting gives the cast workpieces a locally improved mechanical properties which normally could only be adjusted by subsequent heat treatment.

According to the invention, the cast workpiece or a partial cast workpiece region is cooled specifically with water or steam at a defined temperature above the eutectoid transformation. As a result, the eutectoid conversion region is passed through more rapidly or the cooling takes place past the pearlite nose without reaching the martensite start temperature. Subsequently, in the sand mold, the cast workpiece or even only the partial, heat-treated casting work piece area is kept within a defined temperature range for a certain period of time. This is done by regulating the amount of refrigerant supplied (e.g., water or steam, respectively). As a result, the formation of the inventive structure and thus the improved component properties. The material produced by this process is called High Performance Iron (HPI).

By means of the method according to the invention, cast workpieces can be produced which are distinguished by improved component properties on the whole or only on local regions of the cast workpiece. Thus, the component can be loaded locally or higher overall, or at the same load, the geometry of the cast workpiece can be made slimmer. With the application of this inventive method and the production of the inventive HPI (high-performance iron) is the possibility for the production of lightweight cast iron components with high demands on the component strength or lifetime granted.

The invention requires no additional, energy-consuming heating of the cast workpiece. The heat treatment is carried out exclusively by using the heat energy already contained in the directly cast component. Due to the heat treatment directly in the mold, the invention makes it possible to dispense with additional technical equipment (for example heat treatment furnaces). The handling is also substantially simplified: The inventive heat treatment process requires no hot emptying and manual repackaging of the cast workpiece, the heat treatment is indeed directly in the mold. Manual and thus comparatively expensive handling steps are eliminated. In addition, the heat treatment process according to the invention can also be automated and used e.g. integrate in automated manufacturing facilities. The invention can also be implemented relatively easily, without major technical investments.

In the present invention, the temperature balance of the component in the mold is controlled so that a very rapid cooling takes place after solidification from the austenite without martensite formation can take place. For this purpose, it is helpful to ensure the supply of water to a defined distance in front of the component, on the component itself, the steam generated by the heat is used for cooling. The water vapor must be discharged accordingly. If the component or the region with the target HPI properties has reached a certain temperature, the temperature in a window is kept between 350 and 600 ° C. for a defined time in order to obtain the HPI microstructure according to the invention. The holding time depends on the component volume, ie the wall thickness to be treated. Subsequently, the component is cooled and unpacked from the mold.

A method comparable to the heat treatment method according to the invention is the heat treatment for producing ADI cast materials (Ausferritic Ductile Iron), as described above. The production of the ADI material However, has the disadvantages mentioned above. However, the invention allows the very economical production of the new HPI material according to the invention, which very closely approximates the mechanical properties of the ADI nodular cast iron and avoids the disadvantages of the ADI treatment. See the tensile stress-strain diagram in FIG. 4 ,

• Bereitstellung einer Gussform 7 mit einer Sandform 2.
• Bereitstellung mindestens eines Kernes 3 aus organisch oder anorganisch gebundenem Sand mit integrierter Kühlmittelzuführung 4, z.B. durch Integration eines oder mehrerer im oder zum Kern oder Kernpaket 3 hin verlegter Rohre, welche der Kühlmittelführung dienen und zu gegebener Zeit während der Wärmebehandlung die Versetzung des Kernes oder Kernpaketes 3 mit dem Kühlmittel 5 (z.B. Wasser) gewährleisten können.
• Je nach Bedarf eventuelle Integration einer Wasserdampfabführung in der Gussform 7, z.B. in der Form einer dampfdurchlässigen Kernstütze 12 oder als zusätzlich anzufertigende Bohrung 12 (sog. Luftpfeife) oder auch als eingelegte Schnur.

In the production and heat treatment process of an HPI cast workpiece according to the invention, the production takes place, for example, according to the following steps (cf. Figures 5 and 6 ):

• Provision of a mold 7 with a sand mold 2.
• Provision of at least one core 3 of organically or inorganically bound sand with integrated coolant supply 4, for example by integration of one or more laid in or to the core or core package 3 down pipes, which serve the coolant flow and at any time during the heat treatment, the displacement of the core or core package 3 with the coolant 5 (eg water) can ensure.
• Depending on requirements, any integration of a steam discharge in the mold 7, for example in the form of a vapor-permeable core support 12 or as an additional bore 12 to be made (so-called air pipe) or as an inserted cord.

1. 1. Giessen der flüssigen Gusseisenschmelze 6 in die Sandform 2,
2. 2. Abkühlung der Gusseisenschmelze 6 in der Sandform 2 auf eine Temperatur T_A, vorzugsweise in einem Bereich von 800 - 1000 °C,
3. 3. Beaufschlagung der Kühlmittelzuführung 4 mit Kühlmittel 5 und Abschreckung des erstarrenden Gusswerkstückes 1 auf eine Temperatur T_B, vorzugsweise in einem Bereich von 350 - 600 °C, besonders bevorzugt über eine Abschreckgeschwindigkeit von ≥ 5 [K/s],
4. 4. Halten der Temperatur T_B über eine Zeitspanne t_S, vorzugsweise über eine Zeitspanne t_S von 15 bis 120 min., besonders bevorzugt durch Regulierung der über die Kühlmittelzuführung 4 zugeführten Kühlmittelmenge 5,
5. 5. weitere Abkühlung des erstarrten Gusswerkstückes 1, vorzugsweise durch Verbleib in der Sandgussform 2, z.B. bis auf Raumtemperatur oder Auspacktemperatur.

After the casting mold has been made available, the actual heat treatment process is carried out by means of the following steps (cf. Figures 5 and 6 ):

1. 1. pouring the molten cast iron melt 6 into the sand mold 2,
2. 2. Cooling of the cast iron melt 6 in the sand mold 2 to a temperature T _A , preferably in a range of 800-1000 ° C,
3. 3. charging of the coolant supply 4 with coolant 5 and quenching of the solidifying cast workpiece 1 to a temperature T _B , preferably in a range of 350-600 ° C., particularly preferably over a quenching rate of ≥ 5 [K / s],
4. 4. holding the temperature T _B over a period of time t _S , preferably over a period of time t _S of 15 to 120 min., Particularly preferably by regulating the amount of coolant 5 supplied via the coolant supply 4,
5. 5. further cooling of the solidified cast workpiece 1, preferably by remaining in the sand mold 2, for example, up to room temperature or unpacking temperature.

In the following, the invention and the inventive concept will be explained by means of various examples (see FIGS. 6 to 8 ). It should be expressly pointed out at this point that they show only conceivable embodiments, wherein the invention and the inventive concept are not limited to these forms.

Thus, according to FIG. 8 the coolant supply 9, for example, lead the coolant 10 directly to the surface of the cast workpiece 1. In such a direct coolant application, a gaseous coolant 10 is preferably used, for example water vapor.

For the production and heat treatment process according to the invention, a molten cast iron having a carbon content of 2.8 to 3.8 percent by weight, a silicon fraction of 2.1 to 4.5 percent by weight and a magnesium content of 0.025 to 0.05 percent by weight is preferably used.

Like in the FIG. 6 is shown, for the inventive manufacturing and heat treatment process hereinafter preferably a mold 7 is used with a molding box 8 in which a sand mold 2, wherein in the sand mold 2 at least one core 3 or a core package of organic or inorganic bonded sand is used. The sand mold can be made in one, two or more parts. According to the invention, the casting mold 7 has one or more coolant feeds 4 which supply the coolant 5 to the core or cores 3. Preferably, the coolant supply 4 is designed such that the core 3 or the core package can be mixed with a coolant 5.

For the displacement of the core or the core package with a coolant, these may have a porous structure (coolant is located in the pores of the core or core package). Like in the FIG. 6 is shown, the coolant supply 4 for this end at the core surface. In the FIG. 7 In contrast, the coolant supply 4 ends in the core, ie the coolant 5 is conveyed directly into the core. As already mentioned, according to the invention there is also the possibility that via the coolant supply 9, the coolant 10 leads directly to the surface of the cast workpiece (see FIG. 8 ), this is particularly suitable for vaporous coolant (eg water vapor 10).

The coolant supply 4 may be in the form of one or more laid pipes. Preferably, the tubes extend as shown FIGS. 6 to 8 up to or to the coolant 3 to be displaced core 3 or to the surface of the cast workpiece 1. Preferably, the tubes extend in this case also by the sand mold 2. In a further preferred variant, the coolant supply contains pipes made of metal.

Since the coolant evaporates, the casting molds according to the invention preferably have a vapor removal. The vapor removal can be provided in the sand mold and / or in the core or core package. The steam discharge for the coolant 5 to be evaporated can be ensured by means of a (porous) core support 11, via an inserted cord or by means of a bore 12.

In a further preferred embodiment, 3 temperature sensors are inserted in the sand mold 2 or in the core. These serve to detect the temperature in the core 3, on the core surface or on the casting surface or in the casting itself. This embodiment is particularly suitable for experiments and measurements or for adjusting a mold according to the invention.

The present invention is not limited to the explicitly stated possibilities and embodiments. These variants are intended rather as a suggestion for the expert to implement the idea of the invention as low as possible.

LIST OF REFERENCE NUMBERS

1

Cast workpiece

2

sandbox

3

core

4

Coolant supply

5

coolant

6

Cast iron melt

7

mold

8th

molding box

9

Coolant supply directly to the surface of the cast workpiece

10

Water vapor as a coolant

11

core support

12

Bore or so-called air pipe

Claims (11)

Process for the production and heat treatment of a cast piece (1) of nodular cast iron in a sand mold (2), the sand mold containing at least one core (3) of organically or inorganically bound sand having a coolant supply (4), preferably in the form of a or a plurality of tubes laid in or to the core (3), by means of which the core (3) can be mixed with a coolant (5), preferably water, the process for the preparation and heat treatment comprising the following steps: Pouring the molten cast iron melt (6) into the sand mold (2), Cooling the cast iron melt (6) in the sand mold (2) to a temperature T _A , preferably in a range from 800 to 1000 ° C, - Applying to the coolant supply (4) with coolant (5) and quenching the solidifying casting workpiece (1) to a temperature T _B , preferably in a range of 350 - 600 ° C, more preferably over a quenching rate of ≥ 5 [K / s] . Maintaining the temperature T _B over a period of time t _S , preferably over a period of time t _S of 15 to 120 min., Particularly preferably by regulating the amount of coolant (5) supplied via the coolant supply (4), - Wherein subsequently the solidified cast workpiece (1) is further cooled, preferably by remaining in the sand mold (2). Method for producing and heat treating a cast piece (1) of ductile iron according to claim 1, characterized in that the coolant supply (9) directs the coolant (10) directly to the surface of the cast workpiece (1), water vapor being used as the coolant (10). is used. Process for producing and heat treating a cast piece (1) of ductile iron according to claim 1 or 2, characterized in that the molten cast iron (6) has a carbon content of 2.8 to 3.8 weight percent, a silicon content of 2.1 to 4.5 weight percent and a magnesium content of 0.025 to 0.05 weight percent. Casting mold (7) for producing and heat treating a cast piece (1) of nodular cast iron, the casting mold (7) having a molding box (8) in which a sand mold (2) lies, wherein in the sand mold (2) at least one core (3) made of organically or inorganically bound sand, characterized in that the casting mold (7) has one or more coolant feeds (4) which supply coolant (5) to the core (s) (3), preferably the coolant feed (4 ) such that the core or cores (3) are mixed with coolant (5). Casting mold (7) for producing and heat treating a cast piece (1) of nodular cast iron according to claim 4, characterized in that the coolant supply (9) directs the coolant (10) directly to the surface of the cast work piece (1), wherein water vapor is used as the coolant (10) is used. Casting mold (7) for producing and heat treating a cast piece (1) of ductile iron according to claim 4 or 5, characterized in that the coolant supply (4) is designed in the form of one or more laid pipes, preferably the pipes extend into or on the core (3) to be displaced with coolant (5) or up to the surface of the cast workpiece (1), particularly preferably the tubes extend through the sand mold (2). Casting mold (7) for the production and heat treatment of a cast workpiece (1) made of spheroidal graphite cast iron according to claim 4, characterized in that the coolant supply (4) ends in the interior of the core (3). Casting mold (7) for the production and heat treatment of a cast workpiece (1) made of spheroidal graphite cast iron according to one of the preceding claims, characterized in that the coolant supply (4) contain tubes made of metal. Casting mold (7) for producing and heat treating a cast piece (1) of ductile iron according to one of the preceding claims, characterized in that in the sand mold (2) and / or in the core (3) a steam discharge for the evaporating coolant (5) is provided, preferably the steam discharge via a core support (11), via an inserted cord or via a bore (12) is ensured. Casting mold (7) for producing and heat treating a cast piece (1) of nodular cast iron according to one of the preceding claims, characterized in that temperature sensors are inserted in the sand mold (2) or in the core (3) for detecting the temperature in the core (3 ), on the core surface, on the casting surface or in the casting itself. Cast workpiece (1) produced by a method or with a casting mold (7) according to one of the preceding claims.

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