Thermal Simulation Study On Microstructure And Toughness Of Heat Affected Zone Of S690QL Steel

Nowadays it is a great trend to promote deep-sea strategy for oil exploitation anddevelopment. Therefore, high strength steels get broader application space at thebackground of high parametric design requirement for deep-sea drilling equipment. HSLAS690QL researched in this article will be designed as the base metal of main steel structureof jack-up in the subsequent of marine engineering project. In order to assess the toughnessof S690QL welding heat-affected zone microstructure, the effect on microstructure andtoughness of S690QL welding HAZ by welding thermal cycle was investigated usingwelding thermal simulation. The relationship between microstructure of CGHAZ under firstand second thermal cycle and toughness was analyzed emphatically.The experimental results show that the toughness of S690QL simulated CGHAZ at the firstpeak temperature (Tp1)1150℃and1350℃during the first thermal cycle deteriorate sharply,whose impact energy are at low range between28J and41J with various of cooling time t8/5.The lowest value of range is close to minimum toughness requirement (27J) of base metalS690QL. The smaller variation of impact energy with t8/5different indicated the effect oflimited welding heat input variation on toughness of S690QL CGHAZ is less. Except forCGHAZ, the other simulated region with different Tp1and t8/5doesn’t show significantbrittle, the average impact energy of which exceed100J.Microstructure of S690QL CGHAZ start as coarsen lath martensite, gradually transformedinto organic combination with lath martensite and lath baintie, eventually transformed intowhole upper baintie. The microstructure type at Tp1=1150℃is basically same as what atTp1=1350℃, but the prior γ grain is refiner at Tp1=1150℃.The coarsen lath martensite, lathbaintie which take parallel growth as main crystal orientation and M-A constituents formedbetween laths and in prior γ grain boundary are main factors deteriorating toughness whent8/5=10s and20s. The upper baintie together with carbides precipitated and gathered at γgrain boundaries and between BF laths both promote brittle when t8/5=40s. Additionally, theeffect on CGHAZ toughness by difference of prior austenitic grain size in CGHAZ withdifferent peak temperature is less.The low temperature toughness of microstructure was not significantly improved afterCGHAZ with first thermal cycle experience second thermal cycle with different Tp2.Coarsen lath of martensite together with carbides further precipitated and gathered betweenlaths both deteriorate toughness in simulated UACGHAZ at Tp2=1350℃. Embrittlement of the region at Tp2=1150℃is mainly related to formation of upper baintie. Refinement ofprior γgrain together with formation in a systematic way of lower bainite and lathmartensite improve toughness a little in simulated SCCGHAZ at Tp2=950℃.