Abstract
Because of its outstanding strength at elevated temperaturs and resistivity to intergranular corrosion, stabilized austenitic stainless steel has been widely used throughout the petroleum and petrochemical industries. Recently, however, authors have experienced the embrittlement of commercial stabilized austenitic stainless steel after long time service at temperatures between 600°C and 800°C. It was found through an examination that the embrittlement was remarkable in a zone affected by the welding heat cracking occasionally occurring by tensile stress such as thermal stress in the embrittled heat affected zone. Authors studied on the properties of embrittled material and the cause of cracking. The results obtained were as follows;
1) The crack takes place in the base metal within 0.5-1mm distance from the welding fusion line, and is mainly intergranular cracking.
2) It is found that a let of fine needle-shaped carbide precipitated near the crack, and it seems that there is some correlation between needle-shaped carbide and cracking.
3) A high temperature tensile test at 650°C shows that the embrittled zone has ductility large enough for normal austenitic stainless steel type AISI 321.
4) Judging from the above fact, the cracking is caused by coincidence of low ductility of the material at lower temperatures and arrest of creep phenomenon accompanied by precipitation of carbide, and thermal stress. Accordingly, the cracking may be prevented by relief of concentration of stress and thermal stress.
5) And then, the embrittlement after long time service at elevated temperatures may be recovered by solution heat treatment.
