低合金高强钢焊接临界再热粗晶区中M-A组元的相变机制研究

Toughness deterioration of intercritically reheated coarse-grained heat-affected zone (ICCGHAZ) formed during multi-pass welding is one of key factors which notably influence the safety and life span of the weldments of thick gauge high-strength low-alloyed (HSLA) steels. Necklace-type M-A constituent along the prior austenite grain boundaries in the ICCGHAZ is the main reason for the deterioration of toughness in ICCGHAZ. However, the transformation mechanism of the M-A constituent in ICCGHAZ was not well understood. The present project aims to study the transformation process of the M-A constituent in ICCGHAZ of high strength pipeline steel via combination of Gleeble simulation instrument, modern materials characterization technique and thermodynamic/kinetic calculation. The fraction, distribution and the enrichment of carbon and alloy elements in the partly reverted austenite formed at different peak temperatures of the second welding thermal cycle will be studied. And the decomposition products of the partly reverted austenite under different cooling rates and their crystallographic relationship, the redistribution of alloying elements will be studied using ‘interrupted quenching’ method. The existing status, specific structure, the alloy distribution and the crystallographic relationship of M-A constituent formed at different peak temperature or under different cooling rates will also be studied. The phase transformation mechanism will be elucidated based on the abovementioned results. The findings of the present project could provide guidance for how to control the existing status of M-A constituent in ICCGHAZ and thus to elevate the HAZ toughness, which is of significant.

低合金高强钢厚板多层多道焊时形成的临界再热粗晶区（ICCGHAZ）韧性的恶化是影响焊接结构件服役安全的关键因素之一。沿原奥晶界连续分布的M-A组元是导致ICCGHAZ韧性骤降的主要原因，因此对其相变机制进行研究对于完善低合金高强钢HAZ韧性调控理论具有重要的科学意义。本项目拟通过Gleeble热模拟并结合热、动力学计算，对不同第二道次临界峰值温度下逆转奥氏体的含量、分布、元素富集程度等进行研究，揭示再加热过程中奥氏体的逆相变规律；采用“中途淬火法”对部分逆转奥氏体在不同冷速下的分解过程、分解产物类型及其晶体学关系、元素再分配等进行研究，揭示其冷却分解规律；并对不同二次峰值温度、冷速下最终形成的M-A组元的分布状态、精细结构及其与周边产物的晶体学关系等进行研究，最终阐明ICCGHAZ中M-A组元的相变机制，建立相变模型，为发展调控ICCGHAZ中M-A组元、提高其韧性的新技术提供理论指导。

HSLA钢厚板多层多道焊时形成的ICCGHAZ导致的韧性恶化是影响焊接结构件服役安全的关键因素之一，沿原奥晶界连续分布的M-A组元是导致其韧性骤降的主要原因，因此对ICCGHAZ中M-A组元的相变机制进行研究对于完善低合金高强钢HAZ韧性调控理论具有重要的科学意义。本项目以X100 管线钢为母材，通过 Gleeble 热模拟配合“中途淬火法”，对 X100 管线钢ICCGHAZ中 M-A 组元形成过程中的相变机制进行了研究。实验结果表明，再加热过程中，C 向逆转奥氏体（γr）中第一次富集，逆相变由 NPLE 控制，且二道次峰值温度越高，相变速度越快，形成γr的体积分数越高，C 在γr中的富集程度逐渐降低；不同峰值温度下形成的粒状γr大都只与其一侧γp的晶内组织具有 K-S/N-W 位向关系，且温度越高，粒状γr的比例越大。冷却过程发生C 向γr中的第二次富集，由PLE、NPLE混合控制。随着温度的降低，二道次峰值温度为760℃时， C 在γr中的富集程度增加，少量γr分解为F、B 组织，大部分γr转化为 M-A 组元，在PAGB处呈链状分布；二道次峰值温度为800℃和840℃时， C 在γr中的富集程度先降低后增加，大部分γr分解为F、AF、B 组织，剩余γr转化为 M-A 组元，分别呈断续链状分布和细小离散分布；840℃下发生分解的γr比例更多，碳在M-A组元中的含量更低。γr的分解产物中，F与该γr无位向关系，而AF、B及M-A组元与该γr具有 K-S/N-W位向关系，随着第二道次峰值温度的升高，ICCGHAZ中有效大角晶界的密度变大。对ICCGHAZ终态组织的分析结果表明，随着第二道次峰值温度的升高，M-A组元的分布越来越离散，ICCGHAZ的韧性也越来越高。本项目研究成果可为发展调控ICCGHAZ中M-A组元、提高HAZ韧性的新技术提供理论依据。