Simulation of Microstructure Evolution and Fluid Flow in Welding of Ultra-fine Grain Steels
The evolution of grain structure in the weld HAZ (heat affected zone) of ultra fine grain steel was simulated. An integrated 3D Monte Carlo (MC) simulation methodology was employed to simulate grain growth in welding thermal cycle. The results indicate that MC simulation is an effective way to investigate the grain growth in weld HAZ. The method not only can simulates the non-isothermal dynamics process of the grain growth in the weld HAZ, but also the thermal pinning effect can be easily included. The grain sizes of the CGHAZ (coarse grain heat affected zone) obtained from MC simulation are basically in agreement with that of the real welded joints under various heat input. Furthermore, the simulation shown that the grain growth degree is higher for ultra fine grain steel comparing to the conventional steel. With the increase in the heat input, the grain growth of the CGHAZ rapidly increases. As the activation energy of the grain growth is lower for ultra fine grain steel, austenite grain can grow even in the rather lower temperature, consequently, the range of the CGHAZ becomes wider. In addition to the simulation of grain growth in weld HAZ, a 3D mathematical model was established to calculate the temperature and velocity distribution in a moving GTA (gas tungsten arc) weld pool with various active elements, especially sulfur and oxygen, concentrations. The results shown that，the weld size could be changed remarkably with increasing active elements content. Actually, when sulfur content increased beyond 80ppm, the increase of sulfur content does not have an appreciable difference on the weld pool size and shape. Sulfur changes the temperature dependence of surface tension coefficient from a negative value to a positive value and cause significant changes on flow patterns. The increase of sulfur content and the decrease of free surface temperature can extend the region of positive surface tension coefficient. As sulfur content exceeds 125ppm, the sign of surface tension coefficient is positive. Depending upon the sulfur concentrations, three, one or two vortexes that have different positions, strength and directions may be found in the weld pool. The contrary vortexes can efficiently transfer the thermal energy from the arc, creating a deep weld pool. The addition of oxygen, however, showed the similar results as that of the addition of sulfur. Oxygen can also cause significant changes in the weld shape by varying the sign of the surface tension coefficient. The situation with the maximum surface tension moves from the edge to the center with increasing oxygen content. As oxygen content exceeds a critical value, positive surface tension coefficient dominates the flow patterns.
ultra fine grain steel grain growth Monte Carlo simulation surface active elements fluid flow weld penetration
Xiaoyan Li Yaowu Shi Dong Chen Yuzhen Zhao
School of Materials Science and Engineering, Beijing University of Technology, 100 Pingleyuan, Chaoyang District, Beijing 100124, PR China
国际会议
济南
英文
1-18
2012-08-24（万方平台首次上网日期，不代表论文的发表时间）