AGING EMBRITTLEMENT AND GRAIN-BOUNDARY SEGREGATION IN A NICRMOV ROTOR STEEL

Clean 3.5NiCrMoV steels with limited contents in trace elements (P, Sn, As, Sb) are commonly provided for manufacturing big rotor shafts. The possible increase in temperature in future steam turbines has promoted the development of "superclean" steels characterized by an extra drastic decrease of manganese and silicon contents. Their higher cost in comparison to 'clean" steels leads to concern above which temperature they must be considered as mandatory for resisting aging embrittlement in operation. 3.5NiCrMoV "clean" steel samples (Mn = 0.30 pct; Si = 0.10 pct) were aged at 300-degrees-C, 350-degrees-C, and 400-degrees-C for 10,000 hours up to 30,000 hours. No embrittlement results from aging at 300-degrees-C and 350-degrees-C, but holding at 400-degrees-C is highly detrimental. Auger spectroscopy confirms that, when aging at 400-degrees-C, phosphorus is the main embrittling trace element. It is suggested that grain boundary embrittlement is associated with the building of a layer that contains, on the one hand, Ni and P and, on the other hand, Mo and Cr.