Effect of TiN particles and microstructure on fracture toughness in simulated heat-affected zones of a structural steel
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I. INTRODUCTION
THE problem of sudden brittle fracture at low temperatures is still a concern in structural material applications, particularly in welded structures where failure is often found to initiate in the heat-affected zone (HAZ). The thermal cycle experienced during single-pass welding results in four characteristic regions in the HAZ: a coarse-grained region (CGHAZ), a fine-grained region (FGHAZ), an intercritical region (ICHAZ), and a subcritical region (SCHAZ). The 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 (DBTT), and the presence of low toughness microstructures in this region, causing it to have the lowest toughness in the HAZ.[1,2,3] In order to improve HAZ toughness, finely dispersed particles of a thermally stable second phase can be used to restrict austenite grain coarsening. One of the most effective precipitates in pinning the grain boundaries is titanium nitride (TiN). Compared with the nitrides of Al, Nb, or V, TiN precipitates have a higher thermodynamic stability, i.e., low solubility in the matrix and high resistance to particle coarsening.[4,5,6] In order to pin the grain boundaries more effectively, it is emphasized that the TiN precipitate distribution should have the largest volume fraction and smallest particle size possible. According to recent research work,[7,8,9] however, a high Ti content promotes the formation of coarse TiN particles with sizes over 0.5 mm. These coarse TiN particles are ineffective in pinning the grain boundaries and can act as cleavage nucleation sites. Therefore, the problem of conflict between the beneficial effect of fine particles inhibiting grain growth and the possibility of coarse particles impairing toughness L.P. ZHANG, Research Fellow, C.L. DAVIS, Lecturer, and M. STRANGWOOD, Senior Lecturer, are with the School of Metallurgy and Materials, The University of Birmingham, Birmingham, B15 2TT, United Kingdom. Manuscript submitted August 17, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
needs to be solved; hence, Ti content control is very important. In addition to the coarse grain size in the CGHAZ region, the balance of microstructural constituents, such as martensite, bainite, and ferrite with various morphologies, also affects the fracture toughness. In order to investigate the relationship between microstructure and fracture toughness in the HAZ, thermal cycle simulation techniques can be used with material toughness being determined by crack tip opening displacement (CTOD) tests. II. EXPERIMENTAL PROCEDURE Three steels with a similar nominal composition, except for Ti additions, were used in this study, although changes in Ti level were also accompanied by changes in other elements, particularly Ni and Si. The compositions of the steels are shown in Table I. Steels 1, 2, and 3 contain 0.1, 0.045, and 0.006 wt pct Ti, respectively. The microstructures were observed after etching the samples in 2 pct nital,
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