The Assessment of Creep-Fatigue Crack Initiation in Carburized Austenitic Stainless Steel

Abstract

It has been identified that in-service carburization of austenitic stainless steels in the UK Advanced Gas Cooled Reactor (AGR) fleet can impact upon creep-fatigue crack initiation, which is assessed using the R5 Volume 2/3 assessment methodology.Material properties for the carburized layer have been derived through testing of preconditioned specimens. These properties have been used to propose a simplified assessment methodology for the treatment of creep-fatigue crack initiation in carburized specimens. The methodology accounts for the negative impacts of carburization, such as the reduced creep ductility, but does not benefit from the potentially positive impacts, such as the increased creep deformation resistance.At relatively low strain ranges, typically seen by plant components, these simplifications generally result in a very conservative creep-fatigue lifetime prediction. However, at short dwell times (in load control) at high strain ranges (in strain control) the methodology has the potential to be nonconservative when the stress mismatch between the carburised layer and bulk material is severe due to differing cyclic stressstrain properties. Therefore pragmatic limits of application must be applied to the simplified approach. This paper explores these limitations of the simplified assessment advice, the impact on plant assessments and how current research is looking at how less conservative assessment methodology can be developed.

Creep performance of carburized 316H stainless steel at 550 °C

“…The pronounced primary creep rate observed in 'High Mn' material pre-conditioned for 3000 h at 600 °C (Fig. 13) has been suggested to be as a result of the increase in tensile strength [3]. This increase is solely because of the significantly modified carburized layer microstructure with the inner non-carburized bulk possessing similar tensile properties to the as-received material.…”

“…During pre-conditioning, a duplex surface oxide layer developed on both materials at both 550 and 600 °C. This duplex oxide comprises an outer magnetite (Fe3O4) layer, and an inner spinel (MCr3O4) layer where M = Ni, Cr [2,3,19]. Beneath the duplex oxide, a carburized layer develops during pre-conditioning, where the excess carbon mainly precipitates as inter and intra-granular Cr-rich M23C6 carbides [19].…”

“…4.14 MPa) and contains minor additions of CO, CH4, H2O, and H2, herein referred to as AGR primary gas coolant. The environmental degradation of these boiler components causes simultaneous external surface oxidation and sub-surface carburization, and those time-dependant (sub-)surface processes may impact creep ductility and crack initiation under creep and creepfatigue conditions [2,3]. Tolerance to creep deformation becomes a life limiting factor for some structural components in high-temperature nuclear power plants [4].…”

“…Such environmental effects are not explicitly accounted for within any international design codes. Therefore, understanding the impact of carburization on the creep behaviour of 316H steel is crucial, to aid both the accuracy and level of conservatism in assessments of component remnant life [2,3] and design codes.…”

“…Only a limited number of studies to date have examined the effect of a sub-surface carburized layer, that has developed beneath a duplex oxide layer, on the creep response of 316H steels. When 316H stainless steel specimens from only a single cast were exposed to an AGR primary gas coolant for 3000 h at 600 °C, their subsequent creep response at 550 °C and stresses larger than 300 MPa, were characterised by higher minimum creep rates and reduced rupture times as compared to equivalent non-carburized material [2,3,18]. This creep behaviour in carburized specimens was attributed to grain boundary embrittlement and cracking throughout the carburized layer, and a consequential loss of load-bearing capacity [18].…”