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接的物理冶金。短期研究目标是建立X70管线钢热循环至临界温度上下的非平衡相变理论，并通过控制多道焊焊接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，长期以来，粗晶区被认为是韧性较差的局部脆性区的位置。最近，焊接过程中的最高温度落在奥氏体形成的下限(AC1)和上限(AC3)临界温度之间的临界热影响区与焊接锅炉管的IV类损伤的“软区”相关联。冷却速度效应通过CCT图得到了很好的理解。然而，对于焊接加热速度、峰值温度和各种初始组织的影响，对相变的定量预测是不可用的。提出了以下研究主题： 主题1：多层焊接的热历史 进行焊接试验，并确定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：非常规有条件现金流程图 使用Gleeble和淬火膨胀仪重现X70钢的热循环。模拟和测量预先存在的组织、加热/冷却速度、奥氏体晶尺寸、碳化物溶解和均匀化对后续过冷转变的影响。 主题3：多道焊接和附加制造的模型 通过表征Gleeble样品的显微组织，颗粒溶解时贝氏体峰的出现，或颗粒未完全溶解时异形铁素体峰的出现，我们将建立一个微观组织演变模型，通过量化奥氏体形成和碳氮化物溶解(开加热)和铁素体类型转变(开冷却)的峰值温度来优化冲击韧性。 这些科学发现将有助于材料工程和物理冶金，为微合金钢的先进焊接提供组织-加工-性能关系的新知识。首次对多道焊复杂热影响区组织的形成机理进行了系统、定量的研究。该研究项目也是培养众多高质量物理冶金研究生的理想平台，以提升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.