Delamination Fracture Related to Tempering in a High-Strength Low-Alloy Steel
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TRODUCTION
HIGH-STRENGTH low-alloy (HSLA) plate steels are widely used throughout the world for various structural applications. These steels combine particularly high strengths with good toughness, which are especially developed by the controlled hot-rolling process.[1,2] However, due to possible applications in severe service environments, the low-temperature impact toughness is being given more attention and becomes significant to the application of such steels. Therefore, the toughness at a certain low temperature is always one of the most important properties specified for HSLA steels. As to the HSLA steel in the present work, its impact toughness at –30 °C should be high enough to meet service demands. However, when the strength is very high, it becomes more difficult to achieve excellent toughness, especially at low temperatures, due to the conflict of the commonly used mechanisms for improving strength and toughness and the limited capability of manufacturing facilities. In addition, delamination, i.e., splitting in the form of single or multiple secondary cracks perpendicular to the main crack and parallel to the plate surface, is often encountered during Charpy impact tests of hot-rolled high-strength steels,[3–7] and the density of delaminations usually initially increases with the decreasing testing temperature[8–10] and then goes through a maximum and decreases.
WEI YAN, Research Associate, LIN ZHU and WEI WANG, Doctoral Students, and YI-YIN SHAN and KE YANG, Professors, are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. WEI SHA, Professor, is with the School of Planning Architecture & Civil Engineering, Queen’s University of Belfast, Belfast, United Kingdom. Contact e-mail: [email protected] Manuscript submitted March 17, 2009. Article published online November 3, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
The effect of delamination on impact toughness has been widely investigated in many HSLA steels, such as X60, X70, and X80 pipeline steels.[11–14] It has often been reported that the delaminations reduced the notch impact value in the upper shelf region.[8–10,15–19] On the other hand, some other publications[5,20–22] showed that the delaminations can improve toughness due to the delamination toughening effect. It will be shown that in the present steel, delamination did not seem to have much influence on the low-temperature toughness at –30 °C, similar to the results in Reference 4. This point should be the subject of further study. The initiation of the delamination in either dynamic (e.g., Charpy V-notch (CVN), impact test) or static (tensile test) tests has not yet been sufficiently explained. From previous studies, it seems that such features as the bent ferrite-pearlite microstructure,[5] elongated grain shape,[15,16,23] strip microstructure,[3,14] certain texture characteristics,[8,18,21,23,24] decohesion of grain boundaries,[25] segregation of impurity atoms,[26,27] aligned particles,[17,28] and inclusions[3,14,15,23,26,29] could lead to delaminatio
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