DE2117030B2 - Use of a nickel-chromium steel of a certain composition as a high-temperature material - Google Patents

Description

30 C% + Ni% + 0.5 Mn% + 16 N% = Cr% + 0.5 (Nb + 0.5 Ta)% + 3.5 Ti% + 1.5 Si% + Mo% + 11 ± 2

is sufficient as a material for external parts of reforming or cracking furnaces, which ^{have to be highly resistant to temperature changes at temperatures of 800 to 1200 0} C and highly resistant to intergranular corrosion for applications in the temperature range between 600 and 1250 "C.

2. Use of a steel according to claim 1 with the proviso that the composition of the steel the condition

30 C% = (2 Ti% + 6 (Nb + 0.5 Ta)%) ± 20%

fulfilled, for the purpose according to claim 1.

3. Use of a steel according to claim 1 with the proviso that in the steel the weight percentage The proportion of (Nb + 0.5 Ta) corresponds to at least six times the carbon content for which

Purpose according to claim 1.

4. Use of a steel according to claim 1 with the proviso that the titanium content is twice that Carbon content for the purpose of claim 1.

5. Use of a steel according to claim 1, consisting of

0.10 to 0.15% C, 25 to 28% Cr, 33 to 40% Ni, 03 to 1.2% Mn, 1.2 to 1.5% Si, 0.6 to 1.2% (Nb + 0.5 Ta), 02 to 0.8% Ti, 0 to 030% Mo, 0.05 to 0.15% N, at most 0.05% (S + P), less than 0.05% Al, less than 0.01% Pb, less than 0.01% Sn, less than 0.01% Zn, less than 0.10% Cu, balance iron and common impurities for the purpose of claim 1.

The invention relates to the use of a nickel-chromium steel consisting of 0.05 to 0.20% C, 30 to 40% Ni, 23 to 30% Cr, 0.5 to 13% (Nb + 0.5 Ta), 0.04 to 0.2% N, 0.6 up to 1.5% Mn, 1.0 up to 1.8% Si, 0 to 1.00% Ti, 0 to 0.30% Mo, at most J5 0.05% (S + P), remainder iron and usual impurities, which of the relationship

30 C% + Ni% + 0.5 Mn% + 16 N% = Cr% + 0.5 (Nb + 0.5 Ta)% + 3.5 Ti% + 1.5 Si% + Mo% + 11 ± 2

is sufficient as a material for external parts of reforming or cracking furnaces that operate at temperatures of 800 to 1200 ° C highly resistant to temperature changes and must be highly resistant to intergranular corrosion for applications in the temperature range between 600 and 1250 ° C.

The invention thus relates to specific compositions in which the respective ratios of Follow various elements of a voting rule that improve at least some of the corresponds to the properties listed below: creep resistance, thermal shock resistance, Corrosion resistance at high temperatures to oxygen, carbon, sulfur or any other etchants that can come into contact with the alloy, especially resistance against intergranular corrosion, weldability, resistance to aging, especially resistance against heat fatigue, good hot formability, easy machining made possible by bending, forging, etc.; these properties give this alloy an optimal one Resistance to the combustion gases generated due to the excess air that is used for the Combustion of the fuel used to heat the stoves is inevitably oxidizing.

If the respective contents of the various constituents correspond to the voting rules given at the beginning, relative, for the austenitic and ferritic phase maintain suitable proportions. The one in the left part of the relationship The components listed are austenite-forming components, i.e. components that cause the gamma phase favor, while the constituents of the second part of the formula favor ferrite-forming constituents are, i.e. components that promote the formation of the alpha phase. What the titan and that As for molybdenum, the corresponding levels can be zero, as indicated above.

The composition of the steel preferably fulfills the condition:

30 C% = (2 Ti% + 6 (Nb + 0.5 Ta)%) ± 20%

Observance of this condition makes it possible to achieve maximum stabilization of the austenitic and ferritic phase and, moreover, to largely avoid ^{intergranular corrosion at high temperatures between 900 and 1250 ° C.}

According to a feature of the invention, the silicon content of the steel to be used is between 1.0 and 1.8 wt%. If this content is below 1.0%, If the oxidation resistance proves to be insufficient and is only from a content of about 1.2% excellent; if the silicon content exceeds 1.8%, the weldability is poor, and to achieve a

55

60

perfect weldability, this content will preferably be at most 1.5%. To achieve The chromium content should provide the best resistance to oxidation without impairing the other properties ideally between 23 and 30%, advantageously between 25 and 2½%.

The titanium content should preferably be about twice the carbon content, while the niobium content (if no tantalum is present) at least six times the carbon content to maintain adequate creep resistance should, as well as the carbon content; in the aforementioned range (0.05 to 0.20%, preferably 0.10 to 0.15% carbon).

As the aforementioned voting rules, which correspond to the conditions of maximum stabilization, show, the niobium can partially penetrate Tantalum can be replaced, namely in the ratio of between; i Part tantalum for part niobium. It will be seen that tantalum is generally included as the technical niobium used to form the niobium alloys generally contains a certain proportion of tantalum contains as an impurity

The molybdenum does not form any harmful impurity, provided its content does not exceed 0.30%

According to the invention it was also found that in addition to the phosphorus and sulfur, which together a total content of 0.05% should be restricted to certain elements among the impurities of the Iron adversely affect some properties of the alloy used according to the invention. That's why the contents of possible impurities, such as aluminum, lead, tin, zinc and copper, in each case 0.05%, 0.01%, 0.01%, 0.01% and 0.10% limited. These elements lead to decreased warm or KaHdeformbarkeit a difficulty in hot forming or reduced creep resistance or Thermal fatigue resistance.

The alloys used according to the invention can be seen when viewing cast carbon through an ordinary microscope or an electron microscope an austenitic structure, in which carbide precipitates, e.g. B. Precipitations of CarbHen M23C6, in the form of rectangular platelets, e.g. B. in the order of 800 A ₁ or finely distributed in the form of herringbones, this structure having areas in which a micro-dislocation has taken place around the precipitation particles, which appears in the form of long stripes. These observations reveal neither ferrite nor M ₇ C3 carbides.

A steel with the following composition, which is highly resistant to temperature changes, is preferably used for the stated purpose: 0.10 to 0.15% C, 25 to 28% Cr, 33 to 40% Ni, 0.8 to 1.2% Mn, 1.2 to 1.5% Si, 0.6 to 1.2% (Nb + 0.5 Ta), 0 , 2 to 0.8% Ti, 0 to 0.30% Mo, 0.05 to 0.15% N, at most 0.05% (S + P), less than 0.05% Al, less than 0.01% Pb, less than 0.01% Sn, less than 0.01% Zn, less than 0.10% Cu, the remainder iron.

In relation to the preferred embodiments described in DE-OS 20 47 700 is the Chromium content of the alloys preferably used according to the invention is somewhat increased, and in the more specific ones In alloys there is also some titanium; in relation to the alloys which the embodiments correspond to DE-OS 20 47 700, the alloys used according to the invention, depending on the

special intended composition increased weldability and better creep resistance as well as a slightly better oxidation resistance at a temperature range of 1100 ° C have 1,150 ⁰ C.

The invention is illustrated below by examples illustrating the extraordinary properties of the alloys used according to the invention:

Example one for the very claimed use advantageous composition:

C. 0.12% by weight Cr 25% by weight Ni 35% by weight Mn 1% by weight Si 1.5% by weight (Nb + 0.5 Ta) 0.7 wt% N 0.08% by weight Ti 0.3% by weight Mon 0.20% by weight S + P 0.05 wt% Fe and usual

Impurities: residual percentage.

The alloys used according to the invention, such as. As the alloy of the example have currently the best Oxydationsbesiändigkeit in the temperature range of 1100 ⁰ C to 115o C ⁰ thanks to the relatively high content of Cr and Si, and this high level is consistent with a good weldability.

The micrographic examination shows that the intergranular corrosion is much weaker than with the known refractory alloys, which are usually considered to be sufficiently resistant to oxidation at high temperature.

The forgeability, the aU ability of the metal to Shaping is determined by plastic deformation in a warm state without errors occurring, can be of great value in rolling or forging one in the case of high temperature resistant steels Ingots for the production of wires and for the processing by bending of extruded tubes.

The steels used according to the invention have the best forgeability by rolling or forging under the following conditions:

- Maximum temperature during heating 1200 ° C

- Temperature at the end of the forging process:> 950 ° C.

They have the best ability to deform by bending under the following conditions:

- Heating temperature: 1050 to UOO ⁰ C

- bending i> ei> 950 ° C, preferably in the temperature range between 1050 and 1100 ° C.

The results of these experiments have been confirmed in practice by the satisfactory behavior of compounds of the workpieces produced with the alloys according to the invention in particularly harsh Verhätnissen (UOO to ⁰ C for a long time under an oxidizing atmosphere).

Although DE-OS 18 17 254 discloses steels whose composition overlaps with steels used according to the invention in border areas, tests have shown that of the in

this reference steels specifically mentioned is not one used in the temperature range between 900 ° and 1250 ° C This is probably also the reason that fatigue tests were carried out with these known steels, only up to a temperature of 760 ⁰ C.

This different behavior of the respective steels is probably due to the apparently minor, but in reality influential differences: So are the inventiveB steels used usually contain the chromium content

despite a slight overlap, higher than at the well-known steel, which also did not use the voting rule of the invention Steels meet, as Table 1 with information on steels A to H of the mentioned publication and to The steels used according to the invention can be recognized even if, for example, the known steel B with almost 23% chromium, which puts this steel in the range of the lower limit of the Chromium content of the steels used according to the invention is obtained with

P = 30 C% + Ni% + 0.5 Mn% + 16 N% and Q = Cr% + 0.5 (Nb + 0.5 Ta)% + 3.5 Ti% + 1.5 Si% + Mo %

for this known steel a value for (P-Q) of 3.1 instead of, as required according to the invention, a value between 9 and 13 (11 ± 2).

Similar values result for the other known steels, such as the table ^! refer to. However, this means that all of these known steels A to H are ferritic steels, possibly with austenitic components, but by no means purely austenitic steels.

From "Der Praktiker" 12/1972, page 240/242, it is true from the Schaeffler diagram shown and explained here known how austenite- and ferrite-forming components are to be coordinated with one another, this one However, even in combination with the first-mentioned document, the document is not a suggestion for one Selection of a very special range from the large austenitic range to produce very special ones Components to be removed. The person skilled in the art knows that the multipliers in the diagram are by no means may be adopted schematically. Rather, it requires a careful one for each individual steel Adaptation of these factors, which takes into account the interaction between the individual alloy components. The inventive achievement is therefore to use steels for the claimed use specific selection of a narrow range from the austenitic field and, when used, special Modified Schaeffler factors found on the steels in this selected austenitic range to have.

These are the properties that the steels used according to the invention have for their special purpose can be used, due to the known steels, could not be expected and could not be derived, shows the further table H. In this table, in addition to the compositions, are compared with one another Steels hot creep resistance, thermal fatigue resistance and durability shown against intergranular corrosion. In particular, steel B according to the prior art is shown been used because its chromium content is close to the lower limit of the chromium content of the invention steels used. The steels used according to the invention are about twice as large

J5 creep resistance and thermal fatigue resistance.

Tabel I. 0.05-0.20 0-1.00 stole P- DE-OS 0.019 18 17 254 0.13 C. 0.07 D. 0.17 E. 0.15 F. 0.14 G 0.11 Ii 0.11 (brought up examples) 23-30 0-0.30 Q- A. 21.2 B. 22.9 19.0 22.3 16.2 16.0 19.3 19.3 according to the invention 30-40 0.04-0.2 0.05-0.20 (0.08-0.14) (P-QH 31.3 36.8 29.2 31.2 35.3 35.0 33.7 33.2 used steel 0.6-1.5 0.05 15-25 (18-22) 1.0 0.97 0.61 0.97 0.81 0.68 0.64 1.00 C. 1.0-1.8 QX13 25-40 (30-34) 0.61 0.64 0.70 0.41 1.29 1.44 1.22 0.72 Cr (Nb + V ₂ Ta) 0.5-1.3 <2.0% 1.11 1.50 0.75 0.90 1.0 1.14 0.76 0.74 Ni Ti 0.3-1.5 Mn Mon 0.60-1.5 Si N <0.15 <0.15 <0.15 <0.15 <0.15 <0.15 <0.15 <0.15 S + P 9 <(P- <0.15 ■ 17.22 23.27 17.31 21.00 22.96 22.38 20.79 20.90 SS0.05 PS0.04 ■ 22.66 24.61 20.42 23.36 18.59 18.73 21.51 20.77 ► -5.44 -1.34 -3.11 -2.36 +4.37 3.65 -0.72 0.13

P = 30 (% C) + Ni + 0.5 (% Mn) + 16 ■ (% N)

O ■ - = % Cr + 0.5 * (% Nb + 0.5 * (% Ta)) + 3.5 * (% Ti) + 1.5 * (% Si) +% Mo

Table U

steels used according to the invention A. 0.19 B. 0.105 C. 0.17 Steels according to 0.13 - DE-OS 18 0.17 17 254 0.14 H 0.11 28.7 23.5 25.7 B. 22.9 - D. 22.3 F. 16.0 19.3 C. 36.80 32.2 31.2 36.8 <0.15 31.2 35.0 33.2 Cr 0.95 1.01 1.12 0.97 P <0.04 0.97 0.68 1.00 Ni 1.35 1.10 1.24 0.64 S <0.04 0.41 1.44 0.72 Mn 0.88 0.53 1.00 1.50 -1.34 0.90 1.14 0.74 Si 0.75 0.25 0.15 - - - (Nb + ViTa) 0 O 0 - - - Ti 0.05 0.13 0.10 <0.15 <0.15 <0.15 Mon <0.05 <0.05 <0.05 P <0.04 P <0.04 P <0.04 N S <0.04 S <0.04 S <0.04 S + P -2.36 3.65 0.13 Table U (continued) (P-O)

Steels used according to the invention
A B

Creep resistance at 750 C under a load of 744 kg / cm ² 588 kg / cm ²

Thermal fatigue resistance ¹ ) Number of cycles until the onset of crack occurrence. Crack propagation

Resistance to intergranular corrosion ² )

2250 4660

590

0.7 after 0.7 after

2740 cycles 3050 cycles

no intergranular corrosion fracture completely transgranular

2540 5130

560

0.7 after 3000 cycles

Remarks:

To test the thermal fatigue resistance, specimens of triangular cross-section were heated and rapidly cooled; the number of cycles which caused a crack to occur and the rate of propagation of the crack were measured.

² ) The intergranular corrosion was determined by microscopic observation of the fracture zone of the samples broken under the above conditions of creep strength. A completely transgranular break corresponds to an undetectable absence of intergranular corrosion. A partially intergranular partially transgranular fracture means that certain parts of the rupture zone are broken intergranular corresponding to the shape of the grains, while other parts are broken trans granular (through the grains); Such a type of fracture corresponds to the presence of intergranular corrosion.

Claims (1)

Patent claims: 1. Use of a nickel-chromium steel, consisting of 0.05 to 0.20% C, 30 to 40% Ni, 23 to 30% Cr, 0.5 to 1.3% (Nb + 0.5 Ta), 0.04 to 0.2% N, 0.6 to 14% Mn, 1.0 to 1.8% Si, 0 to 1.00% Ti, 0 to 030% Mo, at most 0.05% (S + P), the remainder iron and usual Impurities, whichever of the relationship