Microstructures relevant to brittle fracture initiation at the heat-affected zone of weldment of a low carbon steel
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INTRODUCTION
THE heat-affected zone (HAZ) of a weldment is generally the weakest part crucial for the failure of steel structures. In low carbon steels, the microstructure of HAZ is complicated varying sensitively with the thermal cycle experienced during welding. The area intercritically reheated during multipass welding is known as the local brittle zone, with poor toughness associated with coarse grain size (or intercritically reheated coarse grained (ICCG) HAZ). The major microstructure is upper bainite (Bu) with large prior austenite grain size. The Bu, which is transformed between 600 ~ to 500 ~ at proper cooling rates, is characterized by the presence of stringers of martensite and martensiteaustenite (M-A) constituent along bainitic ferrite lath boundaries. [q In ICCG HAZ, M-A phase is formed at the second weld heating from reversely transformed austenite along the prior austenite grain boundaries and the bainitic ferrite lath boundaries, occasionally taking blocky morphology. The poor toughness of ICCG HAZ has been discussed with respect to the role of M-A constituent. Correlation of the fracture transition temperature at Charpy impact tests with the volume fraction of M-A constituent is a well-recognized fact.t2,3JIt can be intuitively accepted that the cracking of hard M-A particles or debonding of the interface with the matrix acts as the Griffith crack for the brittle fracture initiation. The coincidence of the stationary crack size with the M-A particle size, together with the fractographic observation revealing M-A particles at the brittle fracture initiation site, was reported by Aihara and Haze. [4] Okada et al. have systematically examined the role of M-A particles on HAZ toughness revealing the detrimental effect of grobular M-A particles35,6,71 Tagawa et al. estimated the stress for the cleavage fracture from M-A particles in terms of the Griffith or Petch model3s] However, the estimated fracture stress was not so satisfactory as to the incipient crack KENJI OHYA, JONGSEOP K/M, and KEN'ICHI YOKOYAMA, Graduate Students, and MICHIHIKO NAGUMO, Professor, are with the Department of Materials Science and Engineering, Waseda University, Tokyo 169, Japan. Manuscript submitted July 26, 1995. 2574---VOLUME 27A, SEPTEMBER 1996
formation at M-A particles. Presence of a definite particle at the fracture initiation site is not always the case. [91 Alternately, the volume fraction of slender M-A particles with aspect ratios of over 4 has been correlated with the ductilebrittle transition temperature (DBTT) at Charpy test.rio] The effects of morphology and distribution of M-A constituent have been discussed with respect to the stress and strain concentrations at M-A particles and also at the neighboring ferrite matrix. An elongated M-A particle can act as a local hard zone that produces a large mismatch of strain with the matrix, thus inducing cracking at the local hard zone.t1~] Recent discussion by Davis and King about the significance of networks of blocky M-A particles also noticed the matrix stress
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