Study Of Mechanical Properties And Microstructure Transformation At Heat Affected Zone With High Heat Input Welding In Micro-Zr Steel

In recent years, welding has been paid more attention as an important part of the secondary processing of steel structures, with the large-scale and large-span. In order to save working time and reduce component manufacturing costs, high heat input welding technology has been widely used in shipbuilding, marine engineering, large storage tanks, high-rise building and other large equipment manufacturing field. During high heat input welding thermal cycle, the temperature at coarse grain heat affected zone (CGHAZ) near fusion line is usually around 1400℃ or higher, esily leading to autenite grain coarsening and presence of upper bainite (UB), side plate ferrite (SPF) and other brittle microstructure, resulting in the deterioration of the strength and toughness at welded joints, the components safty and service performance are seriously affected. In this context, the microstructure and mechanical properties contronl technology based on CGHAZ with high heat input welding are widely concerned. According to process characteristics and action principle of high heat input welding steel, microstructure transformation and mechanical properties at CGHAZ with high heat input welding were studied and analyzed in micro-Zr steel, based on the choice of alloying elements, optimization of physical metallurgy process, behavior of continuous cooling HAZ phase transformation, secondary phase particles pinning mechanisms and acicular ferrite (AF) nucleation mechanism inside austenite. The main research work and results in this paper is as follows:(1) According to the basic principle of oxide metallurgy technonlogy, chemical composition of high heat input welding steel was designed; austenite grain growth was inhibited, the nucleation of AF was promoted and process route of improvement HAZ with high heat input welding was developed.(2) Behaviours of microstruture transformation at HAZ have been systematically studied, continuous cooling transformation SH-CCT diagram was plotted in micro-Zr steel by use of Formastor-Ⅱ Automatic phase transition instrument.(3) Effect of Zr on inclusions in steel has been analyzed. Results show that appropriate amount of Zr elements addition into steel can make MnS in steel balling, because crystal structure of ZrOx is similar to MnS, elongated MnS changes into spherical composite inclusions as ZrOx core, the composite inclusions with good thermal stability can effectively pin austenite grain boundaries, prevent excessive growth of austenite grains.(4) Effect of prior austenite grain size on AF transformation has been researched with different treatment process. Results show that the number of AF increases with austenite grain size increases, because increase the austenite grain size, dereasing the austenite grain boundaries area, reducing the grain boundaries ferrite potential nucleation sites, promoting AF nucleation on inclusions; while at the same cooling conditions, when the austenite grain size is less than the critical austenite grain size, the transformation at grain boundaries is dominate, while the austenite grain size is larger than the critical austenite grain size, the main phase transition is AF transformation inside grain.(5) Researches of M-A constituent at CGHAZ show that M-A constituent size in steel containing Zr is smaller than in Zr-free steel. When the average size of M-A constituent is reduced from 5μm to 2.2μm, the critical stress from 1941Mpa to 2926Mpa, M-A constituent critical stress is larger, M-A constituent is more difficult to crack, it is an effective way for improving toughness at HAZ with high heat input welding to decrease M-A constituent size at HAZ.(6) Inclusions assisting AF nucleation at CGHAZ have been analyzed by use of electron probe micro analysis (EPMA) in micro-Zr steel. Results show that formation of Mn-depleted zone (MDZ) around an oxide particle caused by the absorption of Mn into the Zr oxide phase, because the radius of Zr ion is close to Mn, increasing the chemical driving force for the austenite to ferrite transformation around the particle, promoting nucleation of AF on the multi-component inclusion. Therefore, appropriate amount Zr element addition into steel is favorouble to AF transformation at CGHAZ, improving toughness at CGHAZ with high heat input welding thermal cycle.(7) In heat input range from 100 kJ/cm to 1000 kJ/cm, the relationship beteewn impact toughness and heat input at CGHAZ in micro-Zr steel has been studied. Results show that impact energy of experimental steel is always greater than 123J at -20℃ in range from 100 kJ/cm to 1000 kJ/cm, micro-Zr steel exhibited excellent impact toughness of CGHAZ at low temperature. It is found that impat toughness firstly increases with heat input and changes downward trend at low temperature impacting test, the reason is that toughness at CGHAZ is not only related with the number, size and distribution of AF inside grain, but also with prior austenite grain size. When the austenite grain size increases, dereasing the total area of austenite grain boundaries, reducing the grain boundaries ferrite potential nucleation sites, promoting nucleation on inclusions and growth of AF, improving toughness at CGHAZ with high heat input welding.(8) Nuleation and growth of AF mechanisms at CGHAZ with high heat input welding have been studied. Results show that primary AF firstly nuleated on inclusions and grew up in the early phase transformation, and in the subsequent phase transformation process, presence of large number of dislocations at primary AF provided carbon diffusion channel, accelerating the diffusion of carbon around the interface, resulting in a carbon concentration gradient interface of primary AF, providing driving force for secondary AF nucleation at the interface. The number of secondary AF is much more than primary AF, therefore, secondary AF play a major role in preventing crack propagation and improving toughness at CGHAZ.(9) Results of gas electric welding (GEW) experiments show that absorbed energy at distance of 1mm from fusion line is more than 100J or more, at HAZ with heat input of 120 kJ/cm in micro-Zr steel. It is found that welding thermal cycle conditions on the thermal simulation machine is more harsh than the real welding conditions, therefore, a large number of experimental data and conclusions by welding thermal simulation, obtained for microstructure transformation laws and mechanical properties at CGHAZ, is of important reference value for high heat input welding steel research and development.