CS199254B2 - Austenitic iron-chromium-nickel alloys having high silicon content,available for heat stress above 400 centigrad - Google Patents

Abstract

The alloys serve as a material for producing structural components and machine components intended for use in the temperature range above 400 DEG C in an atmosphere which causes nitriding. The alloys consist of 0.01-0.40% of C, more than 3.0-6.0% of Si, at most 2.0% of Mn, 15.0-22.0% of Cr, 12.0-25.0% 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 nitriding is achieved.

Description

The invention relates to the use of high silicon austenitic iron-chromonic nickel alloys which, by their composition, are suitable for the manufacture of structural alloys. and machine parts to be resistant to degradation at elevated temperatures.

Common austenitic chromium - nickel steels and. - Cast iron is at temperatures. above about 40 ° C in many nitrogen-containing atmospheres, such as ammonia fission gas, noticeably susceptible to degradation. Also in the production of melamine acid, for example, nitrogen is cleaved, thereby degrading the components of the plant. nitrogen capture - generally increases with increasing temperature. There may be an ever-increasing layer of nitridone surface. but they can also, especially when. - higher temperatures, coarse chromnitrides are deposited at the grain boundaries and inside. As the amount of brittle chromnitrides secreted increases, the resistance of the matrix to oxidation decreases due to the associated removal of chromium and the ductility of the material. Especially under stress under temperature changes after ´ relatively short. working hours to create. cracks and component parts.

In order to prevent premature failure of austenitic chromium-nickel steels in atmospheres causing degradation, materials having a higher nickel content, in particular both of the two basic grades containing 25% chromium and 20% nickel, as well as 35, % of nickel and 20% of chromium, which also have high silicon contents, up to about 2.5%, the percentages being by weight.

Austenitic chromium-nickel steels - with even higher silicon contents, eg about 4 why, by weight, have been used successfully as acid-resistant materials, but were not picked up for stress at elevated temperatures above about 400 ° C. - This was caused by the concern that excessively high silicon contents would lead to unbearable heat embrittlement.

However, the examination of such materials with a silicon content of up to 6% by weight with regard to the resistance to disintegration and to the embrittlement behavior yielded surprising results. At the same time, the effect of the alloying measures on the yield strength was observed, which is of natural interest for the stresses at elevated temperatures. The surveys were based on the following material groups, the percentages being by weight:

1. · л '· ..

0.011-0.40% C 0.20-6.00% Si max 2.00% Mn ~ 18.00% Cr ~ 15.00 '% Ni 0.03-0.30%' N 0.0— 3.00 '% Nb

30.01-0.40% C 0.20-6.00 '% Si max 2.00:% Μη ~ 22.00% Cr ~ 25.00% Ni 0.03-0.30% N 0, 0—3.00% Nb

According to you

2.

0.01 - 0.40 '% C 0.20 - 6.00% Si max 2.00 ·% Mn ~ 2000% Cit ~ 20.00 · «/ o Ni 0.03—0.30% N 0 , 0-3.00% Nb was the basis for the investigation of three different contents of chromium and nickel, in amounts by weight of 18% chromium and 15% nickel, 20% chromium and 20% nickel, 22% chromium and 25% nickel. Within these material groups, the contents of carbon, nitrogen and niobium were varied with respect to the hot elongation limit, respectively. resistance to high temperatures, and silicon contents, in particular with regard to resistance to degradation within the stated limits. The results of these surveys can be summarized as follows:

1. In an otherwise constant assay, the hot elongation and high temperature resistance increase in a known manner as the content of carbon, nitrogen and niobium increases, but these are also increased by the increase in silicon content.

2. With an otherwise constant analysis, carbon, nitrogen and niobium do not have a noticeable effect on the resistance to degradation within the concentration areas studied, but may, for contents of silicon above 2.5 o / o by mass, in particular from 3.0 % · Mass, instantaneous increase.

3. Against current concerns about increasing the silicon content, the brittleness at high temperatures worsens more weakly than by increasing the chromium content to the same extent if the basic composition is austenitic.

The degradation resistance was investigated in an ammonia atmosphere in the temperature range between 400 and 1100 ° C. At temperatures up to about 750 ° C, nitride surface layers are formed on austenitic chromium-nickel steels. At higher temperatures, nitrogen is increasingly pushed into the interior of the material and excreted there in the form of coarse chromnitrld lattice, as shown by metallographic investigations.

Resistance to these phenomena, i.e. to the growth of the nitride surface layer at lower temperatures and to the ingress of nitrogen at higher temperatures, is silicon contents above 2.5 wt.%, In particular from 3.0 wt. determined by calculating the total nitrogen content of circular test specimens · 8 mm in diameter, always after the same processing times.

The aforementioned drawbacks are eliminated by the use of austenitic iron-chromonic nickel alloys with a high silicon content and a composition by weight of 0.01 to 0.40% carbon, above 2.5 to 6.0 o / o silicon, max. 2.0% manganese, 15.0 to 22.0% chromium, 12.0 to 25.0% nickel, max. 0.3% nitrogen, max. 3.0 why, niobium, iron residue and unavoidable impurities, for the manufacture of and components designed to operate in a temperature range above 400 ° C in a humidifying atmosphere.

The discovery of a new property of high-silicon ferro-chromium alloys is that they have been found to exhibit extremely high resistance over a temperature range above 400 ° C in a nitriding atmosphere. This phenomenon is very important for the manufacture of components and machine parts intended to be used in a temperature range above 400 ° C in a nitriding atmosphere.

When selecting alloy compositions for the purpose of the present invention, it is essential that the alloying elements be coordinated with each other so as to maintain the austenitic composition. If the alloys contain ferrite, the tendency to embrittlement is so great that they cannot be used in practice for many purposes. However, in the case of molded castings, it is advisable to allow a certain proportion of deltaferrite - max. 10 ·% - to prevent hot cracks. Up to these proportions, the inevitable conversion of ferrite into a brittle sigmaphase is not detrimental to the behavior of castings in use.

However, even with the austenitic composition of the recommended steels and alloys, the tendency towards embrittlement is to be expected with increasing silicon content. The long-term processing of the pre-machined notched impact bars of the investigated steels showed that there is an embrittlement region at 750 ° C that extends up to 850 ° C. Above this temperature, embrittlement no longer occurs.

However, when using the steels recommended by the present invention in the critical temperature range, it is advantageous that the embrittlement only takes place over a period of several hundred hours and can therefore generally be prevented by operating measures.

Extremely slow heating to operating temperatures above 850 ° C or extremely slow cooling from such high temperatures is safe.

Thus, there is no risk of embrittlement at an operating temperature below about 700 ° C.

In comparison, the known refractory standardized steel X 15 CrNiSi 25 20, which in addition to about 25 wt%, chromium and 20 wt%, nickel contains about 2 wt%, silicon, an embrittlement region that extends up to about

1000 ° C.

In particular, the carbon content of the alloys recommended according to the invention is at most 0.25% by weight. Higher carbon contents of molded castings are often interesting.

Now, on the one hand, on the one hand to achieve an effective resistance to disintegration and on the other hand to reduce the embrittlement region as much as possible, the silicon content in the alloys recommended according to the invention seems to be between 3.5. % to 5 wt. With this silicon content, the chromium content is suitably chosen between 17.0 and 20% by weight, and the nickel content is between 14.0 and 18.0% by weight. These nickel contents are sufficient to guarantee an austenitic composition. If an increased resistance to high temperatures is desired, an admixture of nickel of up to 0.2% by weight is generally sufficient. Also, niobium admixtures - suitably between 1.0 and 2.0% by weight. - high temperature resistance can be sufficiently increased in most cases.

For practical testing of the invention, the recommended high silicon contents were steel tubes with a weight content of 0.045% carbon, 4.2% silicon, 0.8% manganese, 18.5% chromium, 15.3 why, nickel, 0.038 % · Nitrogen and 0.03% niobium are installed in a fission gas-ammonia plant for the heat treatment of stainless steel wires. In order to prevent oxidation of such wires during heat treatment,

Claims (1)

The use of austenitic iron-chromium-nickel alloys having a high silicon content of 0.01 to 0.40% by weight moles, preferably 0.1 to 0.40% by weight ^. 0.01 to 0.25 wt.%, Carbon, above 3.0 wt.%, Up to 6.0 wt.%, Preferably 3.5 to 5 wt.%, Silicon, max. 2.0 wt.%, Manganese 15.0 to 22.0% by weight, preferably 17.0 to 20.0% by weight. %, chromium, 12.0 to 25.0 wt.%, to use electrically heated tube furnaces into whose tubes fission gas is introduced. Such furnaces generally operate in a temperature range between 1020 and 1100 ° C. The furnace tubes used here must be resistant not only to oxidation but also to nitrogen scavenging because the fission gas consists of hydrogen and nitrogen. So far, steels having a weight content of about 25%, chromium, 20% nickel and 2% silicon have been used for these tubes. Danger There is no danger of embrittlement when using this steel as the operating temperature is above 1000 ^q C. In parallel to these conventional tubes, the steel tubes of the above-mentioned steel recommended according to the invention, compared with the low chromium and nickel contents but with a high silicon content, yielded in all cases the same life as the steel tubes. Metallographic examinations of the dismantled tubes resulted in the fact that nitrogen scavenging, judged by the amount of chromium nitride formed, was less for steel with a higher silicon content at the same stress. The tubes were not subjected to any remarkable mechanical stresses in the apparatus, so that increased nitrogen and respiratory contents could be dispensed with. niobium in practical tests to increase resistance to high temperatures. Preferably, 14.0 to 18.0 wt.%, Nickel, max. 0.3 wt.%, Preferably 0.2 wt.%, Nitrogen, max. 3.0 wt.%, Preferably 1.0 to 2 wt. , 0% by weight, niobium, iron residue and unavoidable impurities, for the manufacture of structural and machine parts intended to operate over a temperature range above 400 ° C in a nitriding atmosphere.