TYPE IV CREEP RUPTURE CHARACTERISTICS OF P92 STEEL WELDMENT

Abstract

Creep tests at 600—650 with applied stresses in the range 100—240 MPa and microstructural observations by means of OM, SEM, TEM were conducted on welded joints of P92 steel prepared by submerged arc welding (SAW) process to investigated its characteristics of type IV creep rupture. The results showed that type IV failure tack place at higher temperature and lower stress and tends to have a critical condition expressed by Larson-Miller parameter (LMP) and stress level which values are 35.5 and 120 MPa, respectively. Type IV failure showed a lack of ductility and located in the fine grained heat affected zone (FGHAZ) (heated to just above AC3)close to intercritical heat affected zone (HAZ), where microstructural changes are obviously different from those in the base metal, including formation of equiaxed sub-grain structure, mass precipitation and rapid growth of Laves phases on the grain boundaries during creep exposure, which lead to the type IV failure. The size of M23C6 carbide in the FGHAZ was almost the same as that in the base metal, which has little effect on the failure. Type IV rupture is a brittle intergranular fracture due to cavity coalescence, which was nucleated at coarse precipitates of Laves phase. The fraction of grain boundary facets cavitated and the area fraction of voids can be employed to quantify grain boundary damage and evaluate type IV failure of P92 steel welded joints, and when their values were about 1.2% and 0.5%, type IV failure would occur.