CS203916B2 - Austentic ferrous-chrom-nickel steels with high centent of silicon for the stress at temperature over 800 degree c - Google Patents

Abstract

For the production of structural components and machine components which are exposed to temperatures above 800 DEG C in air combustion atmospheres, high-silicon, austenitic iron-chromium-nickel alloys are proposed. Owing to increasing the silicon content to 3.5 to 5.0%, these alloys do not show any sulphur sensitivity nor any embrittlement above 850 DEG C in such atmospheres. The alloys consist of 0.01-0.25% of C, 3.5-5.0% of Si, at most 2.0% of Mn, 17.0 to 20.0% of Cr, 14.0-18.0% of Ni, at most 0.2% of N and 0 to 2.0% of Nb, the remainder being iron and impurities.

Description

The invention relates to the use of high silicon austenitic ferro-chromium alloys, which are widely used in the form of rolled and forged steels and in the form of a steel alloy at high temperatures, for example in industrial furnaces.

The steel components used in such furnaces are exposed to air-burning atmospheres which are produced by the combustion of various fuel gases with combustion air. The required furnace temperatures can be very high; temperatures of up to 1350 ° C can be used in the heat treatment of iron and steel alloys. Steel components for furnaces such as cover plates, backpacks, traveling wheels, protective tubes for thermocouples and the like must therefore be heat resistant, that is to say, they must in particular be resistant to oxidation at such high temperatures.

The heat-resistant steels used at this time for operating temperatures / above about 800 ° C can be austenitic chromium-nickel steels with increased silicon contents, the silicon being intended to promote the chromium effect. However, both regulating elements, both chromium and silicon, have the disadvantage that their excessively high contents lead to embrittlement and must therefore be avoided. For these reasons, the chromium contents are limited upwards to about 25% by weight, and the silicon contents to about 2.5% by weight.

So far, austenitic chromium-nickel alloys with higher silicon contents, for example about 4% by weight, have gained importance only as acid-resistant materials.

More recent, detailed investigations of such materials with silicon contents up to 6% by weight, with regard to oxidation resistance and embrittlement behavior, have produced surprising results. At the same time, the effect of alloying measures on the resistance to high temperatures was observed, which is naturally interesting under stresses at high temperatures.

The investigations were carried out on the following material groups, the percentages being by weight.

203918

1. 2. 3.

0,01 - 0.40% C 0,01 - 0.40% C 0,01 - 0.40% C 0.20 - 6.00% Si 0.20 - 6.00% Si 0.20 - 6.00% Si max. 2.00% Qty max. 2.00% Qty max. 2.00% Qty 18,00% Cr 20.00% Cr 22.00 O / o Cr 15.00 O / o Ni 20.00% Ni 25.00 θ / o Ni 0,03 - 0.30% N 0,03 - 0.30% N 0,03 - 0.30 «/ o N 0,00 - 3.00% Nb 0,00 - 3.00% Nb 0,00 - 3.00% Nb

Accordingly, three different chromium and nickel contents were the basis for the investigation, the percentages being 18% by weight of chromium and 15% of nickel, 20% of chromium and 20% of nickel, 22% of chromium and 25% of nickel. Within these three material groups, the contents of carbon, nitrogen and niobium were varied within the stated limits with respect to the hot elongation limit, respectively. high temperature resistance, and silicon contents especially with respect to oxidation resistance. The results of these investigations can be summarized as follows:

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

2. In an otherwise constant analysis, carbon, nitrogen and niobium do not have a noticeable effect on the oxygenation resistance within the concentration ranges investigated, but may be increased by a silicon content above 2,5% by weight, in particular from 3,0% by weight, instantaneously.

3. Against concerns, the embrittlement at high temperatures is exacerbated by increasing the silicon content more readily than by increasing the chromium content, and to about the same extent if the basic composition is austenitic.

To give an idea of the instantaneous improvement in high-silicon oxidation resistance, the results of the experiments obtained with seven experimental steels from the group containing about 18 wt.%, Chromium and 15 wt. . The weight loss in g / m ² at the test temperatures of 1050, 1100 and 1150 ° C was determined in dependence on the silicon content. The results were individually:

Steel No. Weight loss in g / m ² at temperatures.

Si% wt. 1100 ° C 1100 ° C

1 0.20 15.0 25.0 40.0 2 1.20 12.0 18.0 27.0 3 2.30 8.0 13.0 19.0 4 3.10 2,0 3.5 5.0 5 3.80 0.9 1.5 2,0 6 4.60 0.5 0.7 1.0 7 5.60 0.3 0.5 0.7

Accordingly, at all test temperatures it appears that as the silicon content increases from 2.3 to 3.1% by weight, the weight loss instantaneously decreases and thus the oxidation resistance is instantaneously increased.

The same results could also be achieved for steels from material groups with a content by weight of 20% chromium and% nickel, respectively. 22% chromium and 25% nickel, while naturally weight loss, especially for alloys containing 22% chromium, was correspondingly less.

The exact assemblies of test steels Nos. 1 to 7 are given in the following table; wherein the percentages are by weight.

Steel C Si Mn Cr Ni N Nb C. % % % % % % %

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 0.05 7 0.030 5.60 0.81 17.9 14.9 0,038 0.06

The abovementioned drawbacks are eliminated by the use of austenitic iron-nickel alloys with a high silicon content of 0.01 to 0.40% carbon, above 2.5 to 6.0% silicon, max. 2.0% manganese, 15.0 to 22% 0% chromium, 12.0 to 25.0% nickel, max. 0.3% nitrogen, max. 3.0% niobium, iron residue and unavoidable impurities that are provided for the manufacture of machine and structural components for use in the temperature range above 800 ° C, in particular above 850 ° C in air combustion atmospheres.

The discovery of a new property of austenitic iron-nickel alloys is that it has been found that these alloys are particularly resistant to combustion at temperatures above 800 ° C.

Comparative investigation on both standardized X 15 CrNiSi 25 20 steels containing 0.14% carbon, 2.1% silicon, 1.08% manganese, 25.3% chromium and 20.6% nickel as well as X 12 NiCrSi 36 16 containing 0.12 o / o carbon, 1.95% / silicon, 1.36% manganese, 16.3% chromium and 35.2% nickel, the percentages shown by weight have shown that oxidation resistance of these high-alloy steels about as good as steels Nos. 5, 6 and 7 with more than 3.8 wt% silicon of the present invention.

When selecting the alloy composition for the purpose of the present invention, it is essential to align the alloying elements with each other so as to maintain the austenitic composition. If the alloys contain ferrite, the tendency to embrace is so great that they are useless for many purposes in practice.

In the case of molded castings, it is advantageous to allow the deltaferrite content - max. 10% - to prevent hot cracks. With these proportions, the inevitable conversion of ferrite into a brittle sigmaphase is not yet detrimental to the behavior of the castings in use.

However, even with the austenitic composition of the recommended steels and alloys, the tendency to become brittle should be expected as the silicon content increases. Prolonged processing of pre-machined test bars with notched steel has shown that the embrittlement region appears at 750 ° C, which extends up to about 850 ° C. Above this temperature, embrittlement no longer occurs.

Since the field of application for high-oxygen-oxidizing steels with high silicon contents is primarily in the high temperature range, it hardly limits the application possibilities of these steels to the uptake range bounded by 850 ° C. Because embrittlement at. temperature of 750 ° C occurs only after several hundred hours of holding time, there is no danger of embrittlement during extremely slow heating to the use temperature or during extremely slow cooling from this temperature.

In comparison, the known and already mentioned heat-resistant 15 CrNiSi 25 20 steel, which thus contains, besides about 25% by weight, chromium and 20% by weight, nickel, also 2% by weight, silicon, has an embrittlement range which extends up to about 1000 Deň: 32 ° C.

In particular, the carbon content of the alloys recommended according to the invention is at most 0.25% by weight. Higher carbon contents are often of interest in molded castings. In order to achieve effective oxidation resistance on the one hand and on the other hand to reduce the embrittlement region as much as possible, the preferred silicon content of the alloys recommended according to the invention appears to be between 3.5 and 5% by weight. At this silicon content, the chromium content is suitably chosen between 17 and 20% by weight, and the nickel content is between 14 and 18% by weight. These nickel contents are sufficient to guarantee an austenitic composition. If an increased resistance to high temperatures is required, a nitrogen addition of up to 0.2% by weight is generally sufficient. Also, by the addition of niobium - suitably between 1.0 to 2.0% by weight, the high temperature resistance can be increased sufficiently.

The high silicon alloys recommended according to the invention have been tested to a large extent in heat treatment plants in a metallurgical plant in the form of rolled and forged steel articles and in the form of shaped castings. The furnace temperatures were in the range between 900 and 1250 ° C. The materials to be tested were used in the form of cover plates which were directly exposed to the flames of the burners, in the form of crossbands, which during heat treatment by melting were quenched in water or air, and in the form of protective sheaths for thermocouples and the like. The materials used for this were both basic steel grades containing 18% chromium, 15% nickel and 4% silicon; 20% chromium, 20% nickel and 4% silicon, as well as 22% chromium, 25% nickel with 3% and 5% silicon, the percentages being by weight.

In all cases, the service life was achieved, which was several times that of the standard steel X part

CrNiSi 25 20, which has been widely used to produce such components.

Air furnaces containing smaller or larger amounts of sulfur compounds may be generated in heat treatment furnaces in metallurgical plants, in waste incineration plants, as well as in all furnaces in which sulfur-containing fuels are heated. In such cases, the known sulfur sensitivity of steels having high nickel contents may be unpleasant, thereby shortening the service life of components made of such steels considerably.

Surprisingly, this sulfur sensitivity is not observed when using the high silicon steels recommended by the present invention. This is obviously due to the high resistance to scale, which is achieved by high silicon contents.

Claims (1)

Use of high silicon austenitic ferro-chromium alloys having a composition of 0.01 to 0.25 wt. % carbon, 3.5 to 5.0 wt. % silicon, traces up to 2.0 wt. % manganese, 17.0 to 20.0 wt. % chromium, 14.0 to 18.0 wt. % nickel, traces up to 0.2 wt. ^% Nitrogen, optionally 1.0 to 2.0 wt. % niobium, the remainder iron and unavoidable impurities, for the manufacture of structural and machine parts intended to operate over a temperature range above 800 ° C, in particular above 850 ° C, in an atmosphere of combustion with excess air.