DE2331099C3 - Use of austenitic iron-chromium-nickel alloys in a nitrogenous atmosphere at temperatures above 400 ° - Google Patents

Description

The present invention is concerned with austenitic iron-chromium-nickel alloys which are used for Manufacture of construction and machine parts are suitable, which at temperatures above 400 ° C against nitrogenization must be constant.

The usual austenitic chromium-nickel steels and cast alloys tend at temperatures above about 400 ° C in some nitrogen containing atmospheres, e.g. B. in an ammonia cracked gas atmosphere, noticeable for embroidery. Also in the production of MeI-aminsäure z. B. split off nitrogen, which leads to the embroidery of the system parts. The extent of nitrogen uptake generally increases with increasing temperature. It can both lead to the formation of a steadily growing nitride layer come on the surface; but coarse chromium nitrides can also be used, especially at higher temperatures be excreted at the grain boundaries and in the grain interior. With increasing amount of excretion of the brittle chromium nitrides, the resistance to oxidation of the base material is due to the associated chromium removal and also the ductility of the material is reduced. Above all In the case of thermal cycling, this leads to the formation of cracks after a relatively short period of operation and component failure.

About the premature failure of austenitic chromium-nickel steels in atmospheres with a sticky effect To prevent it, materials with a higher nickel content, especially those according to the two, were used Basic types with 25% Cr and 20% Ni (see for example W-No. 4841 in the steel-iron material sheet 470-60) and with 35% Ni and 20% Cr are used, which at the same time also have silicon contents of up to about 2.5%. For steels of the former type are used in the standard range information in accordance with the »steel key« (VIg. steel key Wengst KG., 9th ed., 1971, pages 220/223) for silicon areas from 2.00 to 3.00%, but without any indication of improved nitrogen resistance connected is.

Austenitic chrome-nickel steels with even higher Silicon content, e.g. B. those with about 4% Si, have been used successfully as acid-resistant materials have been, but have been used for stresses at elevated temperatures above about 400 ° C not used. The reason for this was the fear that excessively high silicon contents would be too

_{] 0} unbearable warm embrittlement.

Investigations of such materials with silicon contents up to 6% with regard to nitrogen resistance and with regard to the embrittlement behavior however surprising results. At the same time, the influence of the alloying measures observed on the hot yield strength, which naturally occurs under stresses at elevated temperatures is of interest. The investigations carried out extended to those listed below Material groups.

b) 0.01-0.40% C 0.20-6.00% Si max 2.00% Mn -20.00% Cr -20.00% Ni 0.03-0.30% N 0.0-3.00% Nb

a) 0.01-0.40% C
0.20-6.00% Si
max 2.00% min
-18.00% Cr

- 15.00% Ni
0.03-0.30% N
0.0-3.00% Nb

c) 0.01-0.40% C
0.20-6.00% Si
max 2.00% min
-22.00% Cr
-25.00% Ni
0.03-0.30% N
0.0-3.00% Nb

Accordingly, three different chromium and nickel contents were found, namely those of 18% Cr and 15% Ni, 20% Cr and 20% Ni and 22% Cr and 25% Ni are used as the basis for the investigations. Within of these material groups were the carbon, nitrogen and niobium contents with regard to the hot yield point or high temperature strength and the silicon content, especially with regard to the nitrogen resistance varies within the specified limits. The results of these investigations can can be summarized as follows:

1. If the analysis is otherwise the same, it increases in a known manner with an increase in the carbon, Nitrogen and niobium content, the hot yield strength and the high temperature strength, but also with an increase the silicon content can be increased.

With otherwise the same analysis, carbon, nitrogen and niobium have within the examined Concentration ranges no noticeable influence on the nitrogen resistance, but this increases sharply at silicon contents above 2.5%, especially from 3.0% will.

Contrary to previous fears, increasing the silicon content makes high-temperature embrittlement less deteriorated than by increasing the chromium content in approximately the same size, provided the basic structure is austenitic.

The nitrogen resistance was tested in an ammonia atmosphere? investigated in the temperature range between 400 and 1100 ^{0 C.} Surface nitride layers are formed on austenitic chromium-nickel steels at temperatures of up to about 75O ^{1- C.} At higher temperatures, the nitrogen increasingly penetrates the interior of the material and is deposited there in the form of a coarse chromium-nitride network, as metallographic studies have shown.

The resistance against these phenomena, i.e. against the growth of the surface nitride layer lower temperatures and against the penetration of nitrogen at higher temperatures Silicon content over 2.5%, but above all from 3.0% increased by leaps and bounds, as shown by metallographic examinations and additionally by determining the total nitrogen content on round samples with 8 mm Diameter could be determined after the same treatment times.

The subject of the invention is therefore the use of the alloy according to claim I for the purpose indicated there.

When selecting the alloy composition for the purpose of the present invention, it is essential to match the alloying elements to one another in such a way that the austenitic structure is retained. If the alloys contain ferrite, the embrittlement is so great that they are useless for many practical purposes. In the case of castings, however, in order to avoid hot cracks, it is advisable to allow a delta ferrite content - max. 10%. Up to these proportions, the unavoidable conversion of the ferrite into the brittle sigma phase does not yet have a detrimental effect on the performance of the castings. ^J5

Even with a fully austenitic structure of the recommended steels and alloys, an increase in the tendency to embrittlement is to be expected with increasing silicon content. Long-term treatments of pre-machined impact specimens from the steels examined showed that there is an embrittlement area at 750 ³ C, which extends to 850 ° C. Above this temperature there is no longer any embrittlement.

If the steels recommended according to the invention are used in the critical range, which is within the range of possible Temperature range, however, it is an advantage that the embrittlement only after holding times of several 100 h occurs and can therefore generally be avoided by operational measures.

An extremely slow heating up to operating temperatures temperatures above 850 C or extremely slow cooling from such high temperatures are safe.

There is no risk of embrittlement at an operating temperature below approx. 700 C.

In comparison, the well-known heat-resistant standard steel X 15 CrNiSi 2520 (W. Nr. 1.4841), an embrittlement area that extends up to about 1000 ° C extends.

The C content of the alloys recommended according to the invention is preferably a maximum of 0.25%. Higher C contents are sometimes of interest in molded castings.

In order to achieve an effective resistance to nitrogenization on the one hand and the area of embrittlement on the other As much as possible, the preferred Si content is that recommended according to the invention Alloys between 3.5 and 5%. With these Si contents, the Cr content is expediently between 17.0 and 20% and the Ni content between 14.0 and 18.0%. These Ni contents are sufficient to produce a to ensure austenitic structure. If an increased heat resistance is required, an N addition is required up to 0.2% is generally sufficient. Also through an addition of Nb - expediently between 1.0 to 2.0% - the heat resistance can be increased sufficiently for most purposes.

For practical testing of the high silicon contents recommended according to the invention, tubes were made of a steel with 0.045% C, 4.2% Si, 0.8% Mn, 18.5% Cr, 15.3% Ni, 0.038% N and 0.03% Nb into one with Ammonia cracked gas operated system installed for heat treatment of stainless wires serves. In order to avoid the oxidation of such wires during the heat treatment, electrical heated tube furnaces are used, in the tubes of which cracked gas is introduced. Such ovens work generally in the temperature range between 1020 and 1100 C. The furnace pipes that are used here not only have to be resistant to oxidation, but also, in particular, to be resistant to nitrogen absorption because the cracked gas consists of hydrogen and nitrogen.

So far, a steel with about 25% Cr, 20% Ni and 2% Si has been used for these pipes. A risk of embrittlement does not exist when using this steel, as the operating temperature is over 1000'C.

The pipes built in parallel to these conventional pipes from aem above, according to the invention Recommended steel with the comparatively lower chromium and nickel contents, but with the high silicon content, produced the same service life as the tubes from the higher in all cases alloy steel.

The metallographic examinations of the removed pipes showed that the nitrogen uptake, judged on the amount of chromium nitrides formed, the same for steel with the higher silicon content long exposure was lower.

The pipes in this system were not subject to any significant mechanical stresses, so that increased nitrogen or niobium content to increase the heat resistance during practical testing could be dispensed with.

Claims (2)

Patent claims: 1. Use of austenitic iron-chromium-nickel alloys with a high silicon content, consisting of 0.01 to 0.40% C, over 2.5 to 6.0% Si, max 2.0 ⁰ Z ₀ Mn, 15.0 to 22 % Cr, 12.0 to 25.0 «/ ο Ni, max 0.3» / ο N, 0 to 3.0 <Vo Nb, remainder iron and unavoidable impurities that are resistant to nitrogenization between 400 and HOO ⁰ C in an ammonia atmosphere and not embrittled even at temperatures between 750 and 850 ° C, as a material for construction and machine parts that are used in a temperature range above 400 ° C in an atmosphere with an embrittling effect. 2. Use of austenitic iron-chromium-nickel alloys with a high silicon content, consisting of from 0.01 to 0.25%> C, 3.5 to 5.0% Si, max 0.2%> Mn, 17.0 to 20.0% Cr, 14.0 to 18.0% Ni, max 0.2 Vo N, 1.0 to 2.0 VoNb, remainder iron and inevitable impurities as a material for construction and machine parts for the purpose according to claim 1.