Embrittlement in the Weld of High Purity 30Cr-2Mo Steel

Study on Welding Metallurgy in High Purity Ferritic Stainless Steels (Report 1)

Abstract

The factors which influence the toughness in the weld metal of high purity 30Cr-2Mo steel were investigated. On the basis of the results, the mechanism of the deterioration in the toughness of the weld metal was discussed. The results obtained in this study are as follows:
(1) The remarkable deterioration in the toughness is occurral in the TIG weld metal when the shielding for the welds is insufficient.
(2) Grain size and 475°C embrittlement do not significantly influence the toughness of the weld metal in as-welded condition.
(3) The main cause of the deterioration in the toughness of the weld metal is picking-up of nitrogen during welding.
(4) When picking-up of nitrogen is slight and nitrogen in the weld metal is less than about 100 ppm, no nitride precipitates. In this case, interstitial nitrogen atoms cause the embrittlement due to forming Cottrell atomosphere or distorting the lattice. However, when picking-up of nitrogen is heavier and the content of nitrogen in the weld metal grows up more than about 100 ppm, disc-like Cr₂N precipitates on (100) plane in the matrix. Consequently, in addition to the cause mentioned above, the precipitation hardening by Cr₂N and the lowering the energy for the cleavage fracture due to the fact that disc-like Cr₂N precipitates on (100) plane cause the embrittlement of the weld metal.