Continuous cooling transformation diagrams applicable to the heat-affected zone of HSLA-80 and HSLA-100 steels
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00 are two contemporary lowalloy steels certified by the United States Navy. These steels are used for naval and structural applications and exhibit an excellent combination of high strength and low-temperature fracture toughness. The alloy design of the steels was primarily aimed at minimizing cold cracking in the heataffected zone (HAZ) following welding. The extension numbers 80 and 100 stand for minimum yield strength of 80 ksi (560 MPa) and 100 ksi (700 MPa), respectively. The steels have a low carbon content along with solid solution and precipitation strengthening elements such as nickel, chromium, molybdenum, copper, and niobium.[1] Typically, the microstructure of HSLA-80 steel consists of polygonal ferrite, acicular ferrite (AF), lath martensite (LM), and fine copper and niobium-carbonitride [Nb(CN)] precipitates. The microstructure in HSLA-100 steel contains tempered LM and AF with a uniform distribution of copper and Nb(CN) precipitates, typically in the size range of 0.013 to 0.02 mm.[2–5] The additional strengthening effect in HSLA-100 steel is provided by the increased copper and nickel content. For critical applications, these steels are fusion welded by the gas metal arc and shielded metal arc welding processes. These processes are preferred as they can deliver low heat inputs of 10 to 40 kJ/cm, which ensures a HAZ microstructure with acceptable notch toughness properties.[6] The austenite grain size changes along the width M. SHOME, Researcher, is with the R&D and Scientific Services Division, Tata Steel, Jamshedpur 831007, India. Contact e-mail: mshome@ tatasteel.com O.N. MOHANTY, Tata Research Professor, formerly with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India, is Vice-Chancellor, the Biju Patnaik University of Technology, Bhubaneswar 751003, India. Manuscript submitted July 5, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
of the HAZ due to varying thermal cycles the steel plate experiences during welding.[7] In HSLA-80 and HSLA-100 steels, fine grains are produced within a region away from the weld centerline that gets exposed to low peak temperature (Tp , 1100 °C) thermal cycles and is termed as the fine-grain heat-affected zone (FGHAZ).[8] Coarse austenite grains are formed at location close to the fusion line that experience peak temperatures above 1100 °C and is called the coarse-grain heat-affected zone (CGHAZs). The transient condition and the short dwell time of the thermal cycle in the austenite regime do not allow the grains to attain an equilibrium phase composition and thereby a distinct shape at low peak temperatures.[7] On the contrary, the longer dwell time in a high Tp thermal cycle allows diffusion to take place and produces regular shaped polygonal grains. In these steels, niobium-carbonitride precipitates prevent the austenite grains growing during heating as long as they are stable. Austenite grain growth as a result of precipitate dissolution has been extensively investigated under isothermal conditions.[9–
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