THE SUSCEPTIBILITY TO SULFIDE STRESS CRACKING OF LOW-ALLOY STEELS

The susceptibility to sulphide stress cracking (SSC) of low alloy Mn-V and Cr-Mo-Nb steels in standard NACE solution was determined. The cri tical stress (S-c) and threshold stress (sigma(th)) of steels were det ermined in both as-received and heat treated condition, In the as-rece ived state the steel shows a highly susceptibility to SSC dominantly d ue to its microstucture. By quenching and tempering of steels at high temperature the SSC resistance was increased. It was found that the su sceptibility to SSC for Cr-Mo-Nb steel is lower than that of Mn-V stee l at a comparable yield strength. It was explained by the presence of different second phases in the ferrite matrix and spheroidized carbide particles resulting from the higher tempering temperature. In additio n to Fe3C, the presence of Cr3C2, Mo2C and NbC in the ferrite matrix o f Cr-Mo-Nb steel play an important role in their resistance to SSC. In all cases tested was S-c >sigma(th) which suggests of the importance of plastic deformation during the test. The microfractographic analysi s of specimens before and after SSC testing have been done. Occurrence of cracks in the near outer surface of non-failed tensile specimens d uring 720 hours test suggested that SSC could be explained as a combin ation of hydrogen-induced cracking (HIC) and stress corrosion cracking (SCC).