Continuous cooling transformation behavior in the weld coarse grained heat affected zone and mechanical properties of Nb-microalloyed and HY85 steels. (15th January 2016)

• Record Type: Journal Article
• Authors: Kumar, Sanjeev; Nath, S.K.; Kumar, Vinod
• Is Part Of: Materials & design. Volume 90(2016)
• Page Start: 177
• Page End: 184
• Publication Date: 2016-01-15
• Subjects: Weldability -- Steels -- CCT diagram -- Toughness -- Weld HAZ simulation -- Phase transformations; Materials -- Periodicals; Engineering design -- Periodicals; Matériaux -- Périodiques; Conception technique -- Périodiques; Electronic journals; 620.11
• DOI: 10.1016/j.matdes.2015.10.071
• Languages: English
• ISSNs: 0264-1275

Abstract

Abstract: Continuous cooling transformation (CCT) diagrams were determined for weld simulated coarse grained heat affected zone (CGHAZ) of Nb-microalloyed and HY 85 steels using a Gleeble®3800 thermo-mechanical simulator. Samples were heated to a peak temperature of 1573 K (1300 °C) and cooled at different cooling rates from 0.5 K/s to 120 K/s. Microstructures, hardness, and impact toughness at 223 K were determined corresponding to simulated samples of weld CGHAZ thermal cycles. Transformation temperatures AC3, AC1, Bs, Bf, Ms, and Mf were also determined using dilatometric analysis. At slow cooling rates pro-eutectoid ferrite and pearlite were observed in Nb-microalloyed steel and a mixture of bainitic ferrite and granular bainite was observed in HY 85 steel but no pro-eutectoid ferrite was observed. At moderate cooling rates, only bainite was observed in both the steels. At high cooling rates, lath martensite was observed for both the steels. A strong dependence of prior austenite grain size on mechanical properties has been found. A prior austenite grain size of approximately 30 μm (80 K/s) has shown impact toughness more than 100 J and 198 J for Nb-microalloyed steel and HY85 steel samples respectively. Based on microstructures, hardness, impact toughness, and transformation temperatures, CCT diagrams for the weld CGHAZ region were determined. Graphical abstract: Highlights: Based on microstructures, hardness, impact toughness, and dilatometric studies using thermo-mechanical simulator, CCT diagrams for Nb-microalloyed and HY 85 steels have been constructed for weld CGHAZ region. In HY 85 steel, no proeutecoid ferrite and pearlite have been observed at slower cooling rates and for higher cooling rates bainite, bainite & martensite, and martensite have been observed. For Nb-microalloyed and HY 85 steels, a cooling rate of approximately 35 K/s and 70 K/s respectively will ensure a hardness value of 350VHN in the weld CGHAZ region which is considered safe from the point of view of cold cracking. HY 85 steel shows higher impact toughness as compared to Nb-microalloyed steel at all test temperatures. HY 85 steel shows impact toughness greater than 78 J at cryogenic temperature (223 K) for cooling rate between 40 K/s and 70 K/s. Prior austenite grain size (PAGS) values have shown a strong influence on impact toughness and hardness. The higher the PAGS the lower the impact toughness and hardness value are.