AGING EMBRITTLEMENT AND LATTICE IMAGE-ANALYSIS IN A FE-CR-NI DUPLEX STAINLESS-STEEL AGED AT 400-DEGREES-C

Aging embrittlement, normally observed in ferritic stainless steels, was found in a Fe-Cr-Ni duplex stainless steel thermally aged at 400-degrees-C for a long time. The ferrite content and composition of the duplex stainless steel was changed by varying the solution annealing temperature in order to investigate the effect of ferrite phase on the aging embrittlement. The material was aged at 400-degrees-C up to 10 000 h. Aging embrittlement was characterized by microhardness and other mechanical tests. The results show that the aging embrittlement of the duplex stainless steel was attributed to the degradation in ferrite phase. Microstructural studies in the ferrite phase indicated that spinodal decomposition and G-phase precipitation led to the degradation. High resolution electron microscopy (HREM) was utilized to analyze the lattice images of G-phase and ferrite matrix. The extent of embrittlement was found to be strongly dependent on the ferrite content and the composition in ferrite. The Ni content in ferrite seems to play an important role in aging embrittlement by promoting G-phase precipitation. As Ni content increased in the ferrite, the degradation of the material aged at 400-degrees-C became more severe.