The Effect of Arc Current on Microstructure and Mechanical Properties of Hybrid LasTIG Welds of High-Strength Low-Alloy
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HIGH-STRENGTH steels are widely used in cars and trucks chassis, cranes, earthmoving machines and bridge construction, the shipbuilding industry and other branches designing structures handling large amounts of stress or where a good strength-to-weight ratio is required. Thinner sections of high-strength steels can be used to assure the same mechanical properties related to buckling criteria.[1] Conventional high-strength steels obtain their strength through the high alloying content. Unfortunately, this results in a high carbon equivalent, reducing its weldability. Modern fine-grained high-strength low-alloy (HSLA) steels reach their strength properties by a different mechanism, and the risk of undesirable brittle structure generation is minimal; thus, preheating is usually not necessary.[1] Nevertheless, conventional welding procedures can lead to excessive grain growth and subsequent softening in the heat-affected zone (HAZ) of fine-grained HSLA steels as was demonstrated for the ASTM A945 Grade 65, which is the American
HANA SˇEBESTOVA´, PETR HORNI´K, LIBOR MRNˇA, and ELISˇKA MIKMEKOVA´ are with the Institute of Scientific Instruments of the Czech Academy of Sciences, Kra´lovopolska´ 147, 612 64 Brno, Czech Republic. Contact e-mail: [email protected] PAVEL DOLEZˇAL is with the Faculty of Mechanical Engineering, Institute of Materials Science and Engineering, Brno University of Technology, Technicka´ 2896/2, 616 69 Brno, Czech Republic. Manuscript submitted April 17, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
equivalent of the S460MC steel examined in our research.[2] Welding techniques with minimal heat input are recommended to maintain the fine structure providing high-strength properties and to reduce heat-induced distortions.[1,3] S460MC and S700MC are HSLA structural steels with minimum yield strength of 460 and 700 MPa, respectively. Sheets made of these steels combine good toughness properties of the thermo-mechanically rolled fine-grained steels according to EN 10025-4 with excellent properties of cold-forming steels according to EN 10149-2.[4–6] Their strength is given by a very fine microstructure resulting from precise microalloying. Thanks to the low content of carbon, these steels have a low tendency to cold cracking, but their microstructure and thus mechanical properties are highly impacted by the welding operation.[4] Fine-grained HSLA steels have generally good weldability using conventional welding methods, but undesirable hardness changes in both fused metal and HAZ can appear when high-power-density heat sources such as lasers are used.[2,7,8] The hardness increase[7] refers to the formation of quenching microconstituents as a result of very high cooling rates in orders of 102 to 103 °C s1 present during laser welding.[9] The minimal cooling rate for the martensite formation in steels equivalent to S460MC is 100 °C s1.[10] Martensite is a hard but a brittle phase, and its presence in the steel microstructure can lead to ductility reduction and an increase of susceptibility to cold c
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