Austenitic stainless steel excellent in corrosion resistance and brazeability

Abstract

The present invention provides austenitic stainless steel which is not only excellent in brazeability, but is also excellent in corrosion resistance in an environment where condensation of combustion exhaust gas causes formation of condensed water which contains nitric acid ions or sulfuric acid ions and which is low in pH or in an environment of an aqueous solution which contains chloride ions, which contains, by mass %, C: 0.080% or less, Si: 1.2 to 3.0%, Mn: 0.4 to 2.0%, P: 0.03% or less, S: 0.003% or less, Ni: 6.0 to 12.0%, Cr: 16.0 to 20.0%, Cu: 0.2 to 3.0%, Al: 0.002 to 0.10%, N: 0.030 to 0.150%, and Mo: 0.1 to 1.0%, has a balance of Fe and unavoidable impurities, and satisfies Formula (A): 1.6≦[Cu]×[Si≦]4.4 and Formula (B): 0.16≦2[N]+[Mo≦]1.0.

Description

TECHNICAL FIELD[0001]The present invention relates to austenitic stainless steel which is used for a structure which is joined by a nickel braze filler metal, copper braze filler metal, or other braze filler metal. In particular, the present invention relates to austenitic stainless steel which is not only excellent in brazeability, but is also excellent in corrosion resistance in an environment in which condensation of combustion exhaust gas causes formation of condensed water which contains nitric acid ions and sulfuric acid ions and which is low in pH and in corrosion resistance in an environment of an aqueous solution which contains chloride ions.BACKGROUND ART[0002]Brazing is the art of joining materials by using a braze filler metal with a lower melting point than a structural material and performing heat treatment at a temperature somewhat higher than the melting point of the braze filler metal. Brazing is a joining method which is widely used even for stainless steel. The br...

AI Technical Summary

Benefits of technology

The present invention provides an excellent stainless steel material that has good resistance to corrosion and brazing. By controlling the content of copper, silicon, nitrogen, and molybdenum, this material can be used for manufacturing various heat recovery devices and structures that are fuelled by gas and oil. This helps to improve the performance of the devices and reduce their maintenance costs.

Problems solved by technology

In brazing of stainless steel, the passivation film of the stainless steel impedes the brazeability.

On the other hand, if the braze filler metal is too good in wettability, the braze filler metal flows out from the clearances between joined materials, the clearances cannot be filled by the braze filler metal, and the joint strength is lowered.

However, a SUS316-based material and SUS316-based material have the problem of being inferior in stress corrosion cracking resistance.

Stress corrosion cracking occurs when tensile stress remains in a material which is exposed to an environment where corrosion occurs and which is highly susceptible to stress corrosion cracking.

In this case, tensile stress occurs at the parts after assembly and stress corrosion cracking is liable to be caused.

Under such an environment, copper is corroded.

Therefore, as materials which form the exhaust system parts of automobiles and secondary heat exchangers of hot water heaters which are equipped with latent heat recovery devices, copper cannot be used.

Further, when used for exhaust system parts of automobiles or secondary heat exchangers of hot water heaters which are equipped with latent heat recovery devices, since the atmosphere which is taken in includes chlorides, in particular when used in high salt damage regions near the coast, the corrosion resistance in environments which contain chloride ions also becomes an issue.

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