DE1558656C3 - Rust and corrosion resistant cast steel - Google Patents

Description

G-X 8 CrNi 26.7 G-X 20 CrNi 24.8 C. max. 0.08 about 0.20 Cr about 26.0 about 24.0 ³⁰ Ni about 6.5 about 7.50 Stretch limit at least 45 at least 30 (kg / mm ² ) tensile strenght 65 to 80 50 to 65 35 (kg / mm ² ) Elongation f ⁰ Z) at least 15 at least 15

In the standards for stainless and corrosion-resistant cast steel (e.g. steel iron material sheet 410/60) a distinction is made between the following groups of stainless steels (see table 1 there).

1. Heat-treatable chromium steels with 12.0 to 18.0% Cr, due to their carbon and possibly nickel content Heat-treatable steels are for lower corrosion stress.

2. Ferritic-carbidic chromium (molybdenum) steels with 27.0 to 29.0 Cr, 0.50 to 1.30% C (and 2.0 to 2.5% Mo). These steels are so-called "chrome cast iron" with high hardness and low hardness Toughness properties and good wear resistance.

3. Ferritic-austenitic-carbidic steels with 25.0 to 28.0% Cr, 3.5 to 5.5% Ni and 0.30 to 0.50% C. This cast steel is predominantly ferritic and has somewhat better toughness properties than the cast steel after group 2.

4. Austenitic chromium-nickel (molybdenum) steels with 16.5 to 20.0% Cr, 8.0 to 12.5% Ni (and 2.0 to 2.5% Mo) in the form of unstabilized steel grit and in the form of stabilized steel cast, with the Niobium content is 8 χ% C. These steels may also contain ferrite (see note 3 in table 5 of steel sheet 410-60). The cast steel of this group is characterized by good Corrosion resistance. The corrosion resistance is better than that of steels in groups 1 to 3.

5. Austenitic chromium-nickel-molybdenum special

The increased yield strength and strength of this

• 40 steels are due to a considerably increased ferrite content, which is around 50% for GX 8 CrNi 26.7 steel and around 30% for GX 20 CrNi 24.8 steel. .
The disadvantage of these ferritic-austenitic cast steel alloys is that they - especially if they also contain molybdenum - are extremely brittle in the as-cast state because of the sigma phase formation that occurs during cooling; In addition to the sigma phase embrittlement, they are also subject to 475 ° embrittlement, so that after major welding work, post-heat treatment is used in normal cases to eliminate the embrittlement phenomena. With regard to corrosion resistance, ferritic-austenitic steels can have disadvantages compared to austenitic steels, especially in the case of strongly reducing attack media.

The object of the present invention is to use the cast steel grades of group 4 in to improve their mechanical properties so that a higher mechanical load capacity at least the same good corrosion resistance is achieved. The cast steel should also be weldable; after The welding should be resistant to intergranular corrosion and good toughness against intergranular corrosion and good toughness properties can be present in the transition zone without that post-heat treatment is required.

3 4

The invention consists in the use of a

Austenitic-ferritic steel alloy, consisting of compositions and the mechanical values for

three steels from the aforementioned areas are listed: a maximum of 0.07% carbon,

maximum 2.0% silicon,
1.0 to 5.0% manganese,
17.0 to 26.0% chromium,
8.0 to 11.0% nickel,
0.15 to 0.30% nitrogen,
0.05 to 0.25% niobium,

Remaining iron and production-related impurities as a material for the production of rust-resistant and corrosion-resistant cast steel that can be welded without post-heat treatment 15 Ni, in which the ferrite content of the structure is reduced to 2 to 7 or 7 to 15 or 15 to 25% due to the level of manganese and chromium content and thus the minimum yield strength value is set to 35, 40 or 45 kp / mm ² .

The invention further provides that the above-mentioned austenitic-ferritic steel alloy instead of 8 to 11% nickel, 10.0 to 12.5% nickel, and contains 2.0 to 3.0% molybdenum and that at temperatures down to -200 ⁰ C. is used.

The steels to be used according to the invention have a ferrite content of at least 2%. By choosing the chrome and / or

Manganese content can be the ferrite content and thus the heat treatment for these steels

the yield strength can be further increased. For example, 30 1050 ⁰ C with subsequent cooling in water,
with a chromium content of 21.0 to 23.5% The examples show the increase in the mechanical

a manganese content of 2.0 to 4.0% a ferrite niche properties in steels, of which steel 1 of about 7 to 15% can be achieved. With this ferrite content only about 4%, steel 2 about 11% and steel 3 about content, a minimum yield strength of 40 kg / mm ^{2 can} contain 18% ferrite.

guaranteed. 35 The steel alloy to be used according to the invention

With a chromium content of around 24 to 26% and a significantly higher yield point, a manganese content of 1 to 3%, it is even possible and other mechanical values higher than that of having a ferrite content of around 15 to 25% - Steel alloys mentioned at the outset. The corro rich. Such a steel has a minimum elongation resistance of 45 kg / mm ^{2, the} limit to be used according to the invention. The other mechanical steels are at least as good as the CrNi values result from the following table (minimum or CrNiMo steels of group 4. The values of the invention): according to the steels to be used are according to the

Welding resistant to intergranular corrosion

and have good toughness properties in the transition area of weld metal base material without

post-heat treatment is required.
In normal cases, a cast material is used with

about 10% ferrite aimed, in special cases - ζ. Β.

in the case of nitric acid exposure - however, the chromium content should be selected at the lower limit in order to to get the lowest possible ferrite content.

Steel 1 Steel 2 *) Steel 3 0.04 0.04 Chemical composition 0.65 0.78 in % 2.80 1.24 C. 0.05 23.10 25.80 Si 0.69 10.30 10.60 Mn 3.35 2.60 0.01 Cr 19.15 0.28 0.25 Ni 10.20 0.10 0.06 Mon 2.13 42.7 46.5 N 0.23 67.6 68.8 Nb 0.15 32.5 35.0 Yield point (kg / mm ² ) 38.9 17.1 / 15.2 / Tensile strength (kg / mm ² ) 60.3 18.6 21.7 Strain (%) 27.0 Notched impact strength 1.6 / (mkg / cm ² ) 18.7 *) Example brought up.

Ferrite content 2 to 7.1 to 15.1 to 7% 15% 25 ° / Yield point (kg / mm ² ) 35 40 45 Tensile strength (kg / mm ² ) 55 60 65 Strain (%) 25th 25th 25th Notched impact strength 10 10 10 values (mkg / cm ² )

Claims (3)

Patent claims: 1. Use of an austenitic-ferritic steel alloy, consisting of maximum 0.07% maximum 2.0% 1.0 to 5.0% 17.0 to 26.0% 8.0 to 11.0% 0.15 to 6.30% 0.05 to 0.25% Carbon, Silicon, Manganese, Chrome, Nickel, Nitrogen, Niobium, The remainder iron and production-related impurities as a material for the production of rust and corrosion-resistant cast steel that can be welded without post-heat treatment, in which the ferrite content of the structure is 2 to 7 or 7 to 15 or 15 to 25% and thus due to the level of the manganese and chromium content the minimum yield strength value is set to 35, 40 or 45 kp / mm ² . 2. Use of an austenitic-ferritic steel alloy according to claim 1 with the proviso, that it contains 10.0 to 12.5% nickel and 2.0 to 3.0% molybdenum instead of 8 to 11% nickel for the purpose of claim 1. 3. Use of an austenitic-ferritic steel alloy according to claim 1 or 2 at temperatures to -200 ° C for the purpose of claim 1. In special cases, steels with higher molybdenum and nickel contents as well as the addition of copper bring about a further improvement in the corrosion resistance compared to steels in group 4.
The foreign standards for rust-resistant and corrosion-resistant cast steel contain the same steels with, in some cases, only slightly modified alloy content.
For the chemical industry, the chromium-nickel (molybdenum) steels of group 4 are of particular importance because they have good corrosion resistance against the various media. The only disadvantage of this steel group is the relatively low mechanical load capacity, since the steels only have a yield point of at least 20 or 21 kg / mm ² with a strength of at least 45 kg / mm ² . If a corrosion-resistant cast steel should have higher strength values, it was taken up in the meantime further developed steels of group 3 (ferritic-austenitic steels), of which z. B. the following two variants have become known.