Investigation of Microstructural Features Determining the Toughness of 980 MPa Bainitic Weld Metal
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WITH sustained improvement of the properties of high-strength steels, the impact toughness has reached around 200J at lower temperatures of 223 K (50 °C) for 980 MPa grade steels.[1] However, the toughness of the weld metal with matching strength is appreciably lower than this value. How to increase the toughness of weld metal through the modification of the microstructure has been attracting a the attention of welding metallurgists. Before the 1980s, the microstructural feature that controlled the impact toughness was identified as the ferrite grain size.[2] Reference 3 illustrated that both ends of fracture facets coincided with the packet boundaries in the lath structure or the grain boundaries in blocky transformation products. The effective grain size, which controls the toughness, was determined by the packet width in lath structure and ferrite grain size in the blocky transformation products. In tempered martensite, the effective grain size was dependent on the austenite grain size, which explained the austenite size dependency of the toughness of tempered martensite steels. From the 1980s, the critical event controlling brittle fracture at low temperatures has been generally considered R. CAO and J.H. CHEN, Professors, and Z. WANG, Master, are with the State Key Laboratory of Gansu Advanced Non-ferrous Metallic Materials and Key Laboratory of Non-ferrous Metal Alloys of the Ministry of Education, Lanzhou University of Technology, Lanzhou 730050, P.R. China. Contact e-mail: [email protected] X.B. ZHANG, Doctor, is with the Department of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai 200240, P.R. China. Y. PENG, W.S. DU, and Z.L. TIAN, Professors, are with the Central Iron and Steel Research Institute, Beijing 100081, P.R. China. Manuscript submitted February 5, 2013. Article published online October 16, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
as the propagation of a particle crack, and the presence of brittle particle alone would be determinant for the brittle fracture process.[4] Reference 5 indicated that in the transition region, the critical fracture stress rf defined the cleavage fracture in reactor pressure vessel (RPV) steels, and its value obtained from the mean diameter of the carbides only above some critical size, quantitatively 1 pct of the total population, were in good agreement with the values from the finite-element method (FEM) calculations. However, Reference 6 pointed out that in their study it appears that grain size, as well as the surrounding microstructure, had a significant influence. The susceptibility to cleavage fracture did not depend on the size of the fracture initiating particle but additionally on the local ferrite grain size. The higher sensitivity to cleavage fracture of coarse-grained microstructures demonstrated the modulating influence of the grain size in microstructures with a substantial presence of cleavage inducing brittle particles. In bainitic steels, Reference 7 revealed that when the austenite grain size was large, each grain was t
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