Phase transformations in the heat-affected zone of microalloyed steels

该计划的长期目标是开发用于化石燃料和可再生能源的下一代钢焊缝的物理冶金。短期研究目标是建立X70管线钢在低或高临界温度热循环过程中的非平衡相变理论，并通过控制多道焊热影响区组织来优化低温韧性。 焊接产生热影响区，其中最关键的是粗晶区，这一直被认为是“局部脆性区”的位置，韧性差。最近，在焊接期间峰值温度福尔斯在奥氏体形成的下临界温度（Ac 1）和上临界温度（Ac 3）之间的临界间热影响区已经与焊接锅炉管中的IV型损伤的“软区”相关。冷却速率效应已通过CCT图很好地理解。然而，相变的定量预测是不可用的焊接加热速率，峰值温度，和各种起始微观结构的影响。建议的研究主题如下： 主题1：多道焊的热历史 进行焊接试验，并表征电弧焊临界热影响区的典型热循环。将进行数值热模拟，以了解焊接几何形状和热输入对温度场的影响。 主题2：非常规CCT图 使用Gleeble和淬火温度计再现X70钢的热循环。模拟和测量预先存在的显微组织、加热/冷却速率、奥氏体晶粒尺寸、碳化物溶解和均匀化对后续冷却转变的影响。 主题3：多道焊和增材制造模型 通过表征Gleeble样品的最终显微组织，如果颗粒溶解，则出现贝氏体峰，或者如果颗粒未完全溶解，则出现他形铁素体峰，我们将通过量化奥氏体形成和碳氮化物溶解的峰值温度来开发用于优化冲击韧性的显微组织演变模型（加热时）和铁素体型转变（冷却时）。 这些研究成果将为材料工程和物理冶金学提供新的知识，为先进的微合金钢焊接提供组织-工艺-性能关系。首次对多道焊热影响区复杂组织的形成机理进行了系统的定量研究。该研究项目也是培养众多高素质物理冶金研究生的理想平台，以推进他们的学术或职业前景。

The long-term objective of this program is to develop the physical metallurgy for next generation steel welds for fossil-fuel and renewable energy. The short-term research objectives are to develop a theory for non-equilibrium phase transformations in X70 pipeline steel thermally cycled to lower or upper critical temperatures, and to optimize the low-temperature toughness by controlling the multipass weld heat-affected zone microstructure. Welding generates the heat- affected zone, the most critical of which is the coarse- grained zone, which has long been identified as the location for “local brittle zone” for poor toughness. More recently the inter-critical heat -affected zone, where the peak temperature during welding falls between the lower (Ac1) and upper (Ac3) critical temperatures for austenite formation, has been correlated with the “soft zone” for Type IV damage in welded boiler tubes. The cooling rate effect has been well understood through the CCT diagrams. However, a quantitative prediction of phase transformations is not available for the effects of weld heating rates, peak temperature, and various starting microstructure. The following research themes are proposed: Theme 1: Thermal History of Multipass Welding Conduct welding tests, and characterize the thermal cycles typical for the intercritical heat-affected zone for arc welding. Numerical thermal simulations will be conducted to understand the temperature fields as influenced by the weld geometry and heat input. Theme 2: Non-Conventional CCT Diagrams Reproduce the thermal cycles on X70 steel using the Gleeble and a quench dilatometer. Simulate and measure the effects of pre-existing microstructure, heating/cooling rates, austenite grain sizes, dissolution of carbides, and homogenization on subsequent on-cooling transformations. Theme 3: A Model for Multipass Welds and Additive Manufacturing By characterizing the resulting microstructure of the Gleeble samples, the appearance of bainite peak if particles have dissolved, or the appearance of allotriomorphic ferrite peak if particles have not completely dissolved, we will develop a microstructural evolution model for optimizing the impact toughness by quantifying the peak temperatures on austenite formation and carbonitride dissolution (on--heating), and ferritic- type transformations (on--cooling). The scientific findings will contribute to materials engineering and physical metallurgy by providing new knowledge on microstructure--processing--properties relationship for advanced welding of microalloyed steels. For the first time, the mechanism for the complicated heated-affected zone microstructure of multipass welds will be studied systematically and quantitatively. This research program also serves as an ideal platform to train numerous high -quality graduate students in physical metallurgy to advance their academic or career prospects.