Sources of Variability and Lower Values in Toughness Measurements of Weld Metals
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JMEPEG (2017) 26:2472–2483 DOI: 10.1007/s11665-017-2686-7
Sources of Variability and Lower Values in Toughness Measurements of Weld Metals R. Cao, J.J. Yuan, Z.G. Xiao, J.Y. Ma, G.J. Mao, X.B. Zhang, and J.H. Chen (Submitted August 29, 2016; in revised form March 2, 2017; published online April 20, 2017) In this paper, the sources of variability and lower values in toughness measurements of a high-strength low alloy weld metal are investigated by detailed observations of fracture surfaces and the microstructures at crack initiation. The results reveal that the variability of the notch Charpy toughness is related to the location of the cleavage initiation origins. The three factors jointly contribute to give rise to variability and lower value of toughness. That is, (1) the location that the notch is sampled at deposited weld metal or reheated weld metal, (2) the location that a large grain region is appeared on the path ahead of the notch root, (3) the distributed location of the defect or the brittle second-phase particle. When three factors were simultaneously satisfied, the lower value of the Charpy toughness is appeared. The notch is sampled at the largest grain region at deposited weld metal, and the defect or the brittle second-phase particle is close to the notch root and sampled by its high stress field, the lowest Charpy toughness is obtained. Keywords
fracture, HSLA weld metal, toughness, variability
1. Introduction High-strength low alloy API 70 steels with excellent mechanical properties have been widely used in the construction of long-distance oil and gas transportation systems (Ref 1). However, their mechanical properties become worse after welding. The failure probability occurred at weld metal is much higher than that of any other regions within the pipes (Ref 2). For a weld metal with certain welding procedure and certain chemical composition, the tensile strength of the steel weld metal is stable, but the low temperature impact toughness has a large variation and low values. Moreover, it is also very difficult to predict the variation (Ref 1). Thus, it is crucial to improve the low-temperature impact toughness of the weld metal. The toughness of the weld metal is determined by chemical composition, heat input (cooling rate) and microstructure. And, the microstructure depends on the alloy elements and cooling rate (Ref 3). It is well accepted that the addition of appropriate amount and combination proportion ratios of alloying elements, i.e., Ni, Mn, Mo, Ti (Ref 4-6), can improve the impact toughness of the weld metal. These alloy elements have some effects on phase transformation and microstructure and contribute to refine and homogenize the microstructure of the weld metal. Moreover, the heat input also affects the prior austenite grain size and the impact toughness (Ref 2, 7) and found that the increase in heat input caused softening of the WM and HAZ and degreasing of toughness due to the coarsening of the microstructure (Ref 8, 9). The microstructure of R. Cao, J.J. Yuan, J.Y. Ma, G.J. Mao, X.
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