Micromechanical Testing of Fracture Initiation Sites in Welded High-Strength Low-Alloy Steel
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IN today’s load bearing steel structures, there are strict limitations on the use of high-strength materials to avoid brittle fracture. Their concern increases when the materials are considered for use in Arctic regions where the temperatures can fall as low as 213 K (60 °C), and the loads from strong winds and the ice are of concern. Understanding how brittle fracture initiates and propagates under these environmental conditions can help prevent dangerous failures to occur and allow for the development of both strong and cost-efficient materials. Welding is, in many cases, unavoidable in modern steel structures. To make production as cost efficient, buildings are constructed by joining together steel plates, pipes, and forged components. The treatments used during onsite installation and the joining together of subassemblies, as well as subsequent repairs, places great challenges on steel materials. Novel high-strength low-alloy (HSLA) steel processed by modern metallurgical methods has excellent tensile strength and ductile to brittle transition (DBT) properties.[1] However, its strength and toughness can be deteriorated by welding thermal cycles, producing local brittle zones (LBZ) with localized weakness at welded joints.[2] The heat-affected zone (HAZ) is defined as the area most critically affected by welding. In particular, two regions within the HAZ have been closely investigated due to their unfavorable microstructure. During single pass welding, formation of a coarse-grained HAZ
BJØRN RUNESØRA˚S ROGNE, Ph.D. Candidate, CHRISTIAN THAULOW, Professor, and AFROOZ BARNOUSH, Post-Doctoral Research Fellow, are with the Department of Engineering Design and Materials, Norwegian University of Science and Technology, 7491, Trondheim, Norway. Contact e-mail: [email protected] Manuscript submitted April 24, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
(CG HAZ), consisting of mainly upper bainite and Widmansta¨tten ferrite, results in the degradation of the fracture toughness.[3] After a second weld cycle, the CG HAZ is reheated to the intercritical (IC) region where the austenite nucleates along the prior austenite grain boundaries and between the bainite laths.[4] The high diffusivity of the carbon in ferrite and the high solubility of the carbon in austenite cause the carbon to accumulate in these small austenite grains.[5] Depending on the carbon content of these austenite grains and the cooling rate, after the cooling they may contain martensite (M) and various fractions of retained austenite (A) that were stabilized by the high carbon content and did not transform, and therefore, they are usually called M–A constituents.[6] Currently, the reduction of fracture mechanical properties caused by the M–A constituents is not fully understood. In addition to the observed reduction in fracture properties from Charpy impact tests and fracture mechanics tests, fractographic analysis often tracks the initiation site of fracture back to the second phase constituents that either have cracked or debounded from the sur
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