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<p>The effect of Ca addition on phase transformation and impact toughness of coarse-grained heat-affected zone (CGHAZ) during high heat input welding (HHIW) in Mg deoxidized shipbuilding steel plates is investigated. The Ca addition exacerbates TiN particles dissolution and coarsening by altering the equilibrium Ti concentration in austenite during HHIW, resulting in increasing the average size and decreasing the number density from 2.47 to 1.37 μm<sup>-2</sup> for TiN particles. The pinning effect of residual TiN particles on austenite grain boundaries is weakened after HHIW, which is consistent with the <i>in-situ</i> observation results that the growth rate of austenite grain in Mg-Ca steel is faster than that in Mg steel. This leads to the increase in average size of prior austenite grain (PAG) from 367 to 444 μm. <i>In-situ</i> observation shows that the Widmanstätten ferrites (WFs) will more preferentially form than the intragranular acicular ferrites (IAFs) due to a relatively smaller driving force required. The length of WFs in Mg-Ca steel is longer because of the sufficient growth space in larger PAGs in Mg-Ca steel. Meanwhile, the length of IAFs in Mg-Ca steel is also becomes longer due to the fewer effective inclusions inducing the formation of IAFs with chaotic orientations in Mg-Ca steel. These factors lead to a decrease in high-angle grain boundaries (HAGBs) fraction and an increase in the effective grain size (EGS). Therefore, the Ca addition results in the decreases in the number density of TiN particles and HAGBs fraction, the increases in PAG size, WFs content and EGS, which ultimately leads to the deterioration of mechanical properties of CGHAZ, especially with a decrease in the impact toughness at −&#xa0;20°&#xa0;C from 175 J of Mg steel to 23 J of Mg-Ca steel.</p>

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Insight into Ca Addition on Phase Transformation in CGHAZ of Mg Deoxidized Shipbuilding Steel Plates with In-situ Observation

  • 期刊论文

  • 发表日期：

    2025年5月12日

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  • Liang Wang,
  • Jian Yang,
  • Yinhui Zhang,
  • Yuqi Zhang,
  • Yanli Chen,
  • Zhipeng Dai,
  • Tingting Li

摘要

The effect of Ca addition on phase transformation and impact toughness of coarse-grained heat-affected zone (CGHAZ) during high heat input welding (HHIW) in Mg deoxidized shipbuilding steel plates is investigated. The Ca addition exacerbates TiN particles dissolution and coarsening by altering the equilibrium Ti concentration in austenite during HHIW, resulting in increasing the average size and decreasing the number density from 2.47 to 1.37 μm-2 for TiN particles. The pinning effect of residual TiN particles on austenite grain boundaries is weakened after HHIW, which is consistent with the in-situ observation results that the growth rate of austenite grain in Mg-Ca steel is faster than that in Mg steel. This leads to the increase in average size of prior austenite grain (PAG) from 367 to 444 μm. In-situ observation shows that the Widmanstätten ferrites (WFs) will more preferentially form than the intragranular acicular ferrites (IAFs) due to a relatively smaller driving force required. The length of WFs in Mg-Ca steel is longer because of the sufficient growth space in larger PAGs in Mg-Ca steel. Meanwhile, the length of IAFs in Mg-Ca steel is also becomes longer due to the fewer effective inclusions inducing the formation of IAFs with chaotic orientations in Mg-Ca steel. These factors lead to a decrease in high-angle grain boundaries (HAGBs) fraction and an increase in the effective grain size (EGS). Therefore, the Ca addition results in the decreases in the number density of TiN particles and HAGBs fraction, the increases in PAG size, WFs content and EGS, which ultimately leads to the deterioration of mechanical properties of CGHAZ, especially with a decrease in the impact toughness at − 20° C from 175 J of Mg steel to 23 J of Mg-Ca steel.