The subject of this investigation is a low interstitial 29Cr-4Mo ferritic stainless steel developed and patented by E.I. duPont de Nemours and Co., Inc. The alloy, commonly known as 29-4, is characterized by exceptionally high resistance to chloride pitting and stress corrosion cracking and by comparatively good room temperature toughness and ductility.
Sheet specimens from a commercial heat were exposed for times up to 100 h at temperatures from 371 to 982°C (700 to 1800°F). Metallographic changes that were observed were correlated to changes in impact and tensile properties, crevice corrosion in ferric chloride, and intergranular corrosion resistance in ferric sulfate-sulfuric acid. Excess phase identifications and studies of failure modes were conducted on selected samples.
Two embrittlement regions were found centered at about 482°C (900°F) and 760°C (1400°F). The lower temperature embrittlement region is quite narrow and was ascribed to 475°C (885°F) embrittlement. No deterioration in corrosion resistance was detected in this embrittlement region. Embrittlement occurred much more rapidly in the higher temperature region and existed over a wide temperature range. Crevice corrosion resistance and corrosion resistance to ferric sulfate- sulfuric acid deteriorated rapidly in this embrittlement region. Changes in mechanical properties and corrosion resistance upon heat treating in the region were ascribed primarily to the precipitation of sigma and chi phases.