高强管线钢焊接临界再热粗晶区中逆转奥氏体的逆相变晶体学

Crystallography of Reverted Austenite in the Intercritically Reheated Coarse-Grained Heat-Affected Zone of High Strength Pipeline Steel

利用Gleeble热模拟以及电子背散射衍射(EBSD)技术,研究了高强管线钢焊接临界再热粗晶区(ICCGHAZ)中逆转奥氏体(γ_(r))在不同第二道次峰值温度(760、800和840℃)下的逆相变规律及其晶体学关系。结果表明,在3个峰值温度下形成的γ_(r)体积分数依次为4.1%、8.9%和25.2%,γ_(r)优先在第一道次粗晶区的原奥氏体晶界(PAGB)处形核,其次是原奥氏体晶粒(γ_(p))内板条束(block)的交界处。在极快的焊接加热速率下,γ_(r)倾向于以块状形式长大。晶体学研究表明,γ_(r)在PAGB处的逆相变不是自由形核,而是依托于PAGB一侧γ_(p)的晶体学取向按照近似K-S关系通过逆相变而形成,而与另一侧的γ_(p)没有确定的晶体学关系。第二道次峰值温度较低(760℃)时,γ_(r)在PAGB处形核后向非K-S关系一侧的晶粒内部长大,γ_(r)呈链状分布于PAGB处。随峰值温度的升高(800和840℃),γ_(r)向PAGB两侧同时生长。分析表明,γ_(r)在第二道次加热过程中的逆相变行为对其在后续冷却过程中的相变过程、终态组织及其韧性等有重要影响。

Low carbon microalloyed high-strength pipeline steels processed by the thermomechanical controlled process have a good strength-toughness combination.However,after welding,the microstructure and mechanical properties of the heat-affected zone(HAZ)become deteriorated.Previous studies show that martensite-austenite(M-A)constituent formed in the HAZ is a key factor that lowers the toughness,especially necklace-type M-A constituent formed in the intercritically reheated coarse-grained HAZ(ICCGHAZ).However,the phase transformation mechanism of necklace-type M-A constituent in the ICCGHAZ is unclear.In this study,the crystallography of reverted austenite(γ_(r))during the reversion phase transformation upon heating the ICCGHAZ of a high-strength pipeline steel was studied using Gleeble thermal simulation and electron backscatter diffraction(EBSD)technique.Two thermal cycles with peak temperatures of 1300℃ and 760℃/800℃/840℃ were conducted to simulate the phase transformation process in the ICCGHAZ.The samples were directly quenched to room temperature after been reheated to the second peak temperature,and the reversion behavior,distribution,and crystallography of γ_(r) were studied.The results showed that the volume fraction of γ_(r) formed at 760,800,and 840℃ was 4.1%,8.9%,and 25.2%,respectively. γ_(r) preferred to nucleate along prior austenite grain boundaries(PAGB),and posterior the block boundaries within the prior austenite grains. γ_(r) preferred to grow to blocky type along the PAGB,and the acicular type γ_(r) between bainite laths was suppressed.The crystallographic study showed that the formation of γ_(r) at PAGB was not free nucleation.However, γ_(r) was formed based on the crystallographic orientation of PAGB alongside prior austenite grain complying with Kurdjuov-Sachs(K-S)relationship,while having non K-S relationship with the prior austenite grain on the other side.After nucleation at PAGB,and at a low second peak temperature(760℃), γ_(r) transformed to prior austenite grain with non K-S relationship,and the γ_(r) formed in blocky and necklace-type along the PAGB.With an increase in the second peak temperature(800-840℃), γ_(r) transformed to the prior austenite grains on both sides.The analysis showed that the reversion behavior and crystallography of γ_(r) during the second pass reheating have a big impact on the phase transformation upon cooling and the corresponding microstructure and mechanical properties.