Method of mitigating stress corrosion cracking in austenitic solid solution strengthened stainless steels

Abstract

A method of providing resistance to intergranular stress corrosion cracking in an alloy material, the method comprising sensitizing the alloy to form carbides, allowing the carbides to precipitate, and applying a heat treatment to replenish a chromium-depleted zone.

Description

BACKGROUND[0001]The present disclosure is generally directed to methods for mitigation stress corrosion cracking in austenitic solid solution strengthened stainless steels.[0002]In applications such as nuclear reactors, steam driven turbines, or water deaerators, high-temperatures in high purity water or water / steam systems can become aggressive environments that can adversely affect structural materials either by general corrosion or by stress corrosion cracking (SCC). For example, high temperature water may cause stress corrosion SCC in materials such as stainless steels, nickel-iron base alloys, and nickel base alloys. SCC develops when the material is subjected to an applied or residual tensile stress in the presence of some corrosive environment, especially chloride or sulfate-containing environments at higher temperatures. These stresses can result or originate from differences in thermal expansion or contraction between components, relatively high or varying operating pressur...

AI Technical Summary

Benefits of technology

The patent text describes methods for improving resistance to intergranular stress corrosion cracking in a Fe—Ni—Cr alloy material. The methods involve sensitizing the material to form carbide precipitates at grain boundary interfaces and chromium-depleted zones, and then heating the material to a temperature and time effective to diffuse chromium into the chromium-depleted zone. The heat treatment results in increased resistance to intergranular stress corrosion cracking relative to the material without the heat treatment.

Problems solved by technology

In applications such as nuclear reactors, steam driven turbines, or water deaerators, high-temperatures in high purity water or water / steam systems can become aggressive environments that can adversely affect structural materials either by general corrosion or by stress corrosion cracking (SCC).

For example, high temperature water may cause stress corrosion SCC in materials such as stainless steels, nickel-iron base alloys, and nickel base alloys.

In addition to stresses, residual plastic strains produced during the manufacture or assembly of the components or system can make a material more susceptible to SCC.

For example, SCC can result from residual stresses caused by welding, cold working, and other thermomechanical metal treatments.

Intergranular slip step oxidation reactions weaken the grain boundaries, which then open under an applied load and physical cracks are formed.

The cracks propagate with little or no evidence of plastic deformation, and failure of the component is likely.

The precipitation of carbides depletes the adjacent grain boundaries of chromium to an extent that they are no longer corrosion resistant.

Unfortunately, the hydrogen water chemistry technique can require large quantities of hydrogen to effectively reduce the stress corrosion cracking susceptibility to acceptable levels in the various components.

Stainless steels, higher chromium super stainless steels, Fe—Ni-base and Ni-based alloys are alloyed with chromium to achieve general corrosion resistance but this approach can fail to mitigate SCC if the amount of Cr added is insufficient to maintain a stable oxide film in a corrosive environment or if microstructural changes create regions of lower chromium concentration.

One problem is that these materials may not be microstructurally stable at their intended operating temperatures or may have been heat treated or welded to produce SCC susceptible microstructures.

One problem with this approach is that it requires a commitment by the end user to actively maintain the SCC mitigation strategy.

Additionally, it will incur additional operating costs over a component or plant lifetime.

Images