‘‘885°F’’ (474°C) Embrittlement

One of the limitations of ferritic stainless steels (those alloys of iron possessing greater than about 14 pecent chromium) has been the loss of toughness at room temperature that occurs after these materials are exposed for long times to temperatures in the range of 610 to 1000_F (320 to 538_C). This is commonly referred to as 885_F (474_C) embrittlement, corresponding approximately to the temperature at which many of the alloys degrade the fastest.

FIGURE A3.20 The classic Nelson diagram indicating the choice of steel warranted to avoid
hydrogen attack as a function of operating temperature and partial pressure of hydrogen. Austenitic
materials are satisfactory at all temperatures and pressure from hydrogen damage

 

The compositional effects in commercial alloys on 885_F (474_C) embrittlement have not been systematically investigated. However, it is clear that the degree of embrittlement increases as chromium content increases. The effects that other elements may have is not clear. Of these, most important is carbon, and it has been reported as having from no effect to a retarding effect on embrittlement.

This phenomenon results in increased hardness and strength, with a corresponding decrease in ductility, fracture toughness, and a decrease in corrosion resistance. Loss of toughness can be particularly severe, and in fact has tended to relegate the use of this class of alloy to temperature regimes below which significant embrittlement can occur.

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