Massive Ferrite Causes Toughness Decrease in Low Cr-Mo Steel Weld
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Massive transformation occurs when one alloy phase transforms into a new phase by blocky or massive grain growth, such that the entire material volume may transform. Massive transformation is described by the following characteristics: massive patch features with planar and curving boundaries; composition-invariance; interface-controlled growth by short-distance atomic jumps; and rapid orientation-free growth.[1] Massive transformations were discussed in detail in 2000 at ‘‘The Mechanism of the Massive Transformation’’ symposium (Fall 2000 TMS Meeting, St. Louis). However, to date, few papers have discussed the massive ferrite that is produced in weld metals, and its brittle property and harmful effect on the weldment toughness. Toughness that is measured in the transition temperature range is determined from the length of the ductile crack, which is initiated and propagated in front of the
JIANHONG CHEN and GANG LIU are with the State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, China Contact e-mail: [email protected]. XINYU LI, YONG JIANG, and XINGGUI MAO are with the Atlantic China Welding Consumables, Inc., Zigong, 643000, China. RUI CAO is with the State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, School of Materials Science and Engineering, Lanzhou University of Technology Contact e-mail: [email protected]. Maunscript submitted June 20, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
notch and is stopped by cleavage cracking.[2] The low toughness is caused by the early triggering of cleavage cracking, which is induced by coarse grains, brittle phases or defects.[3] We studied brittle belts that were formed by lath massive ferrite blocks, which initiated cleavage cracking and provided a path for its propagation. The low toughness region in the weldment was caused by massive ferrite belts. The factors that affect the formation of the massive ferrite belts were analyzed. Tables I, II, and III show the weld metal compositions; the regimes of the submerged arc welding (SAW) and the heat-treatment; and the strength and toughness of the weld metal. Table I shows that the material is a low Cr-Mo weld steel. The most interesting observation is that in the three sequential weldment layers (A), (B), and (C) in Figures 1(a) and (b), the measured toughness values differ. The toughness in top layer (A) was highest in the range of 232 to 296 J; however in bottom layer (C), the toughness was lowest in the range of 32 to 69 J. We wanted to understand why the toughness drops more than four times in one weldment from the top to the bottom layer. The microfeatures were studied. The macroscopic weldment features are shown in Figure 1(a), and a sampling schematic of the impact specimens is shown in Figure 1(b). The 33-mm-thick weldment was divided into three layers (A), (B), and (C). The macrostructures of each layer are shown in Figures 1(f) through (h). Black belts exist in eac
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