Effect of TiN particles and microstructure on fracture toughness in simulated heat-affected zones of

Effect of TiN particles and microstructure on fracture toughness in simulated heat-affected zones of

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Effect of TiN Particles and Microstructure on Fracture Toughness in Simulated Heat-Affected Zones of a Structural Steel L.P. ZHANG, C.L. DAVIS, and M. STRANGWOOD Thermally stable TiN particles can effectively pin austenite grain boundaries in weld heat-affected zones (HAZs), thereby improving toughness, but can also act as cleavage initiators. The HAZs simulated in a GLEEBLE 1500 TCS using two peak temperatures ( T p ) and three cooling times ( D t 8/ 5 ) have determined the effects of matrix microstructure and TiN particle distribution on the fracture toughness (crack tip opening displacement (CTOD)) of three steels microalloyed with 0.006, 0.045, and 0.1 wt pct Ti. Coarse TiN (0.5 to 6 m m) particles are identified in steels with the two higher levels of Ti, and fine Ti(C, N) (35 to 500 nm) particles were present in all three steels. Large prior austenite grain size caused by higher T p decreased fracture toughness considerably in steels containing coarse TiN particles but had little effect in their absence. Fracture toughness was largely independent of matrix microstructure in the presence of coarse particles. Cleavage fracture initiation was observed to occur at coarse TiN particles in the samples with a large prior austenite grain size. Alloy thermody- namics have been used to rationalize the influence of Ti content on TiN formation and its size. I. INTRODUCTION needs to be solved; hence, Ti content control is very important. In addition to the coarse grain size in the CGHAZ T HE problem of sudden brittle fracture at low tempera- region, the balance of microstructural constituents, such as tures is still a concern in structural material applications, martensite, bainite, and ferrite with various morphologies, particularly in welded structures where failure is often found also affects the fracture toughness. to initiate in the heat-affected zone (HAZ). The thermal In order to investigate the relationship between micro- cycle experienced during single-pass welding results in four structure and fracture toughness in the HAZ, thermal cycle characteristic regions in the HAZ: a coarse-grained region simulation techniques can be used with material toughness (CGHAZ), a fine-grained region (FGHAZ), an intercritical being determined by crack tip opening displacement region (ICHAZ), and a subcritical region (SCHAZ). The (CTOD) tests. CGHAZ is generally the location of initiation for brittle fracture. This is due to its large prior austenite grain size, which increases the ductile-to-brittle transition temperature II. EXPERIMENTAL PROCEDURE (DBTT), and the presence of low toughness microstructures in this region, causing it to have the lowest toughness in Three steels with a similar nominal composition, except for Ti additions, were used in this study, although changes the HAZ. [1,2,3] In order to improve HAZ toughness, finely dispersed particles of a thermally stable second phase can in Ti level were also accompanied by changes in other ele- ments, particularly Ni and Si. The compositions of the steels be used to restrict austenite grain coarsening.
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Effect of TiN particles and microstructure on fracture toughness in simulated heat-affected zones of

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