CS199253B2 - Use of austenitic iron-chromium-nickel alloys having high silicon content,available for heat stress above 800 c - Google Patents

Abstract

The alloys serve as a material for producing structural components and machine components intended for use in a temperature range above 800 DEG C in an atmosphere causing carburisation. The alloys consist of 0.01-0.40% of C, more than 3.0 to 6.0% of Si, at most 2.0% of Mn, 15.0 to 22.0% of Cr, 12.0 to 25.0% of Ni, at most 0.3% of N and 0 to 3.0% of Nb, the remainder being iron and impurities. Owing to the increased silicon content, a substantially higher resistance to carburisation is achieved.

Description

The invention relates to the use of high silicon austenitic iron-chromonic alloys which, by their composition, are suitable for the production of articles, such as machine or structural parts, which must be resistant to carburization at high temperatures.

At temperatures above 700 ° C, austenitic chromium-nickel and foundry cast irons are noticeably prone to carburization in a carbon-emitting atmosphere.

The extent of carburization generally increases with increasing temperature. The carbon diffusing into the steel is excreted in the form of chromium carbides when the solubility limit is exceeded, especially at the grain boundaries. This removes chromium from the steel matrix and reduces its oxidation resistance. As chromium carbide excretion increases, the ductility of the material decreases. Particularly under the stress of temperature changes to which many high-temperature treated components are exposed, after relatively short periods of use, cracks are formed and thus the components are discontinued.

These processes are qualitatively the same for rolled and forged steels as well as for cast materials.

In order to prevent premature failure of austenitic chromium-nickel materials when deployed in a carburizing atmosphere, materials having a higher nickel content, derived from both basic compositions by weight of 25 percent chromium and 20% nickel, respectively, have hitherto been used. 35% nickel and 20% chromium. A chromium content of 20% or more guarantees sufficient oxidation resistance for most purposes, while an increased nickel content of 20% or more greatly improves the carburization resistance.

This action of nickel results in a decrease in the solubility of carbon with increasing proportions. Thus, only a marginal carbon concentration can be generated. However, in order for the carbon to diffuse from the edge into the material, there must be a concentration gradient between the edge and the interior of the material. If this concentration gradient is small, as with materials with a higher nickel content, the diffusion rate is low and hence the carburization is reasonably slow.

Carburisation resistance of refractory materials. Thus, high nickel steels are based on the fact that nickel decreases the solubility of carbon or, as a result, increases the activity of carbon. Logically, therefore, all alloying elements that are effective in the same way would also increase the resistance to carburization. This applies in particular to silicon, as is already known in principle.

The conventional heat-resistant rolled and forged steels and refractory cast alloys of today have an increased silicon content, but not exceeding 2.5% by weight. These silicon additives serve primarily to improve the oxidation resistance and are also provided for this purpose.

Austenitic chromonic nickel alloys with a higher silicon content, for example about 4 wt.

Surveys referred to in the brief report were intended to elucidate the effect of up to 6% silicon content on oxidation resistance and, for practical purposes, the very important embrittlement behavior of austenitic chromium-nickel steels. In this case, the materials in the range of the following percentages were weighted.

and.

2.

0.01-0.40% C 0.20-6.00% Si max 2.00% Mn oo 18.00% Cr oo 15.0% Ni 0.03—0.30% N 0.00— 3.00% Nb

3.

0.01-0.25 ° / o C 0.20-6.00% Si max 2.00% Mn ^ 22.00% Cr oo 25.00% Ni

N, 0.03-0.30

0.00—3.00% Nb

Steel no.

0.01-0.40% C

0.20—6.00% Si max · 2.00% Mn oo 20.00% Cr oo 20.00% Ni 0.03—0.30% N

0.00-3.00% Nb i% w / w

These three material groups are always characterized by a constant chromium and nickel content, in percent by weight, i.e. 18% Cr and 15% Ni, 20% Cr and 20% Ni, 22% Cr and 25% Ni.

Within each material group, the content of silicon, but also of carbon, nitrogen and niobium was now varied within these limits.

Taken together, the results of the experiments suggest that:

1. in an otherwise constant assay in a known manner with increasing carbon, nitrogen and niobium content, the hot yield limit increases, but is also increased by the co-acquisition of the silicon content,

2. In an otherwise constant assay, carbon, nitrogen and niobium do not affect the oxidation resistance, which is however instantly improved by additives of more than 2,5 ° / o silicon for all material groups, and that

3. Contrary to the general concept, increasing the silicon content while maintaining an austenitic base composition is less conducive to high-temperature embrittlement than by increasing the chromium content equally.

As an example of the instantaneous improvement, which achieves silicon contents above 2.5 wt.%, And in particular 3 wt.%, The results of surveys of seven steels of the material group containing about 18 wt.%, Chromium and 15 wt. of which 8 mm in diameter were suspended in a carbon monoxide atmosphere at 1050 ° C for up to 8 weeks. To assess the carburization resistance, the total carbon contents of the samples were added according to different residence times.

The results of these surveys are shown in the following table:

Total carbon content in% by weight after weeks

6 8

1 0.20 0,029 1,25 1.35 1.42 1.45 2 1,20 ' 0,035 1.00 0.97 1.12 1,25 3 2.30 0,028 0.84 0.90 0.98 1.05 4 3.10 0,032 0.30 0.32 0.36 0.37 5 3.80 0,028 0.21 0.21 0.23 0.25 6 4.60 0,027 0.10 0.12 0.12 0.17 7 5.60 0.030 0.07 0.06 0.08 0.11

The compositions by weight of steels 1 to 7 were individually as follows:

Leg. C. C Si% Mn% Cr% Ni% N% Nb ° / o- 1 0,029 0.20 0.58 18.5 15.4 0,038 0.06. 2 0,035 1.20 0.75 18.3 15.1 0,041 0.03 3 0,028 2.30 0,70 ' 18.0 15.2 . 0,045 0.04 4 0,032 3.10 0.64 17.7 14.9 0,035 0.03 5 0,028 3.80 0.64 18.1 14.5 0,037 0.04 6 0,027 4.60 0.78 18.3 14.5 0.040 O, O5 7 0.030 5.60 0.81 17.9 14.9 0,038 0.06

As the results of the carburization experiments show, more or less strong carburization occurs in the first two weeks, depending on the silicon content, which only increases with further decay. For alloys 4 to 7 recommended according to the invention, however, carbon capture is strongly inhibited by comparison.

The aforementioned drawbacks are eliminated by the use of austenitic iron-chromonic nickel alloys having a high silicon content of 0.01 to 0.40% by weight of carbon, over 2.5 to 6.0% of silicon, max. 2.0% of manganese, 15.0 up to 22.0% chromium, 12.0 to 25.0 why, nickel, max. 0.3% nitrogen, max. 3.0; % niobium, iron residue and unavoidable impurities for the manufacture of machinery and components provided for use in a temperature range above 800 ° C, in particular above 850 ° C, in a carburizing atmosphere.

The discovery of a new feature of the use of high silicon austenitic ferro-chromium alloys is that said alloys are resistant to carburization over a temperature range above 800 ° C.

In particular, the carbon content of the alloys recommended according to the invention is at most 0.25%. Higher carbon contents are often of interest in molded castings. As with conventional austenitic chromium-nickel steel casting, it is expedient when using the alloys recommended according to the present invention to align the proportions of alloying additives so that the composition contains something of deltaferrite - maximum 10% - to prevent hot cracking. By using this, it is inevitable to convert ferrite into a brittle sigmaphase without having a detrimental effect on the behavior of the castings in use.

In the case of rolled and forged steels, on the other hand, it is advisable to use entirely austenitic alloys.

However, even with a completely austenitic composition, increasing silicon contents should also be expected with some tendency to become brittle. Long-term heat treatments on pre-machined notch impact bars showed that there is an embrittlement region at 750 ° C that extends up to about 850 ° C. Above this temperature, embrittlement no longer occurs.

Since the field of application of the alloys recommended according to the invention is primarily in the high temperature range, i.e. above 850 ° C, with the 850 ° C upward range the embrittlement range does not limit the possibility of use. In addition, embrittlement at temperature occurs only after holding for several hundred hours, so that even then there is no risk of embrittlement if the components are slowly heated or cooled.

In comparison, the known refractory steel X 15 CrNiSi 25 20, containing about 25% by weight, has a chromium region of embrittlement which extends up to about 1000 ° C, whereby the weight of this steel is greatly reduced.

Comparative experiments with such steel containing 0.18 o / o carbon, 2.25 θ / ο silicon, 1.85 why, manganese, 25.3 ° / o chromium, 20.63% nickel and X 12 NiCrSi steels 36 16 containing 0.12% carbon, 1.95% silicon, 0.96% manganese, 16.4% chromium, 36.2% nickel, the percentages being by weight, showed that alloys 4 to 7 recommended according to this of the invention, i.e. alloys having a content of 3.8% by weight, silicon and more, show, by comparison, low nickel contents as good as carburizing resistance as a standard steel.

In order to achieve effective carburization resistance on the one hand, and on the other hand to reduce the embrittlement region as much as possible, the preferred silicon content in the alloys recommended according to the invention is between 3.5 and 5% by weight. With this silicon content, the chromium content is suitably chosen between 17 and 20% by weight and the nickel content between 14 and 18% by weight. These nickel contents are sufficient to guarantee austenitic compositions. If an increased resistance to higher temperatures is desired, the addition of nitrogen up to 0.28 masses is generally sufficient. Also, niobium admixture - suitably between 1 and 2 wt. —- the resistance to high temperatures can be sufficiently increased for most purposes.

Steel alloys recommended according to the invention with a composition in percent by weight:

1. 0.035% carbon, 4.2% silicon, 1.20% manganese, 18.3% chromium, 15.4% / nickel and 0.045% nitrogen,

2. 0,12% carbon, 3,9% silicon, 0,87% manganese, 17,9% chromium, 14,8% nickel and 0,18% nitrogen,

3. 0,029% carbon, 5,20% silicon, 1,23% manganese, 18,9% chromium, 15,6% nickel, 0,037% nitrogen and 2,00% niobium, with the remainder in all cases of iron and unavoidable impurities, have been tested in gas cementing plants of the automotive industry. In such devices, large amounts of annealing hearths, glow gratings and retorts are required. Such components are predominantly made of a material with a content of about 35% by weight, nickel and 20% by weight, chromium. the carbon potential caused by easily decomposable organic substances is usually between 1.2 and 1.5 wt. It is therefore a very carburizing atmosphere.

Annealing hearths and heating gratings of steel alloys 1, 2, 3 were used for testing compared to the alloy parts used so far containing about 35% by weight, nickel and about 20, θ / ο by weight, chromium and showed at least just as good behavior as components of this known alloy. In most cases of concurrent deployments, it has even been shown that the alloys recommended according to the present invention have survived three times as long as the cracks are formed by comparison. Additional investigations have shown that the final carbon content of the known high nickel alloy was somewhat higher than the high silicon alloys recommended in the present invention.

For a high nickel alloy, the oxidation resistance also decreases more strongly during carburization. This loss is explained by the fact that by carburizing chromium is bound in the form of chromium carbides and is therefore no longer available for resistance to oxidation. However, in alloys with a high silicon content, silicon is fully retained in maitica, so that even with a certain loss of chromium by chromium carbide depletion, the oxidation resistance by the presence of a high proportion of silicon in the matrix is largely maintained. In the known high-nickel alloys, with increasing carburization, increased grain boundary oxidation occurs, which in conjunction with brittle chromium carbides promotes the formation of cracks.

If the annealing hearths and heating grates, as in melting operation, are not subjected to special mechanical stresses, the resistance of the tested alloy 1 to higher temperatures is in all cases sufficient.

In the newer, continuously operating devices, the heating grates equipped with the annealed material are transported by the device after collision. Under such normal stress, it is necessary to compare greater resistance to higher temperatures. In the case of the alloy gratings 1, they were slightly deformed during the time. On the other hand, grids of alloys 2 and 3 did not

In addition, the test piece of alloy 1 tube was deployed for testing in a gasoline cracker, where 25% by weight of chromium and 20% by weight of nickel are mainly used for cracked tubes. The operating temperature is around 950 ° C. This tube cut showed perfect behavior after 12 months of use.

Claims (2)

OBJECT OF THE INVENTION The use of austenitic iron-chromium-nickel alloys having a high silicon content of 0.01 to 0.40% / o, preferably 0Ό1 to 0.25% by weight. % carbon, 3.0 to 6.0, preferably 3.5 to 5.0 wt. % silicon, 2.0 wt. % Manganese, 15.0 to 22.0 ^R preferably 17.0 to 20.0 wt. % Of chromium, 12.0 to 25.0, preferably 14.0 to 18.0 wt. % nickel, traces up to 0.3, preferably 0.2 wt. % nitrogen, traces up to 3.0, preferably 1.0 to 2.0 wt. % niobium, the remainder iron and unavoidable impurities, for the manufacture of machinery and components intended to be used in a temperature range above 800 ° C, in particular above 850 ° C, in a carburizing atmosphere.