In this study, welding thermal cycle experiments of X80 line pipe steel were conducted using a Gleeble-3500 thermo-mechanical simulator, with primary peak temperatures of 1150 °C, 1250 °C, and 1350 °C, followed by secondary peak temperatures ranging from 720 °C to 820 °C. Combined with microstructural characterization, microhardness measurements, and electrochemical hydrogen permeation tests, the morphology, size, distribution, and hydrogen trapping behavior of martensite/austenite (M/A) constituents were systematically investigated. The results demonstrate that the intercritically reheated coarse-grained heat-affected zone (ICCGHAZ) exhibits a lower effective hydrogen diffusion coefficients, along with higher hydrogen concentrations (C0) and hydrogen trap density (Nt) compared with the the coarse-grained heat-affected zone (CGHAZ), indicating a greater susceptibility to hydrogen embrittlement. The microstructural evolution of M/A constituents is synergistically governed by the first peak temperature (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">$T_{{{\text{p}}_{{1}} }}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>T</mi> <msub> <mtext>p</mtext> <mn>1</mn> </msub> </msub> </math></EquationSource> </InlineEquation>) and the second peak temperature (<InlineEquation ID="IEq2"> <EquationSource Format="TEX">$T_{{{\text{p}}_{{2}} }}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>T</mi> <msub> <mtext>p</mtext> <mn>2</mn> </msub> </msub> </math></EquationSource> </InlineEquation>), with <InlineEquation ID="IEq3"> <EquationSource Format="TEX">$T_{{{\text{p}}_{{2}} }}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>T</mi> <msub> <mtext>p</mtext> <mn>2</mn> </msub> </msub> </math></EquationSource> </InlineEquation> playing a dominant role. Within the temperature range of 740 °C to 780 °C, M/A constituents in the ICCGHAZ coarsen significantly and form chain-like structures distributed quasi-continuously along the prior austenite grain boundaries, thereby establishing a contiguous hydrogen-trapping network. Under these conditions, Deff reaches its minimum value, while C0 and Nt attain their maximum levels. In contrast, when <InlineEquation ID="IEq4"> <EquationSource Format="TEX">$T_{{{\text{p}}_{{2}} }}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>T</mi> <msub> <mtext>p</mtext> <mn>2</mn> </msub> </msub> </math></EquationSource> </InlineEquation> approaches Ac1 or Ac3, the M/A constituents are more finely dispersed, which disrupts the trapping network and weakens their ability to the retardation of hydrogen diffusion, resulting in increased Deff and decreased C0 and Nt. Multivariate regression analysis further reveals that the spatial fraction of M/A constituents has a 2 to 3 times greater influence on hydrogen permeation parameters than their average size, identifying it as the dominant factor. Additionally, although the ICCGHAZ exhibits higher hardness than the CGHAZ, the peak hardness does not align with the maximum M/A volume fraction, indicating that the carbon segregation contributes to matrix softening.

Evolution of M/A Constituents in the Intercritical Coarse-Grained Heat Affect Zone of X80 Line Pipe Steel and Their Effect on Hydrogen Permeation Behavior

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发表日期：

2026年5月6日

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Yunwu Li,
Wei Zhao,
Kai Chen,
Hui Zhang,
Yuexia Lv

摘要

In this study, welding thermal cycle experiments of X80 line pipe steel were conducted using a Gleeble-3500 thermo-mechanical simulator, with primary peak temperatures of 1150 °C, 1250 °C, and 1350 °C, followed by secondary peak temperatures ranging from 720 °C to 820 °C. Combined with microstructural characterization, microhardness measurements, and electrochemical hydrogen permeation tests, the morphology, size, distribution, and hydrogen trapping behavior of martensite/austenite (M/A) constituents were systematically investigated. The results demonstrate that the intercritically reheated coarse-grained heat-affected zone (ICCGHAZ) exhibits a lower effective hydrogen diffusion coefficients, along with higher hydrogen concentrations (C₀) and hydrogen trap density (N_t) compared with the the coarse-grained heat-affected zone (CGHAZ), indicating a greater susceptibility to hydrogen embrittlement. The microstructural evolution of M/A constituents is synergistically governed by the first peak temperature ( $T_{{{\text{p}}_{{1}} }}$ $T_{p_{1}}$ ) and the second peak temperature ( $T_{{{\text{p}}_{{2}} }}$ $T_{p_{2}}$ ), with $T_{{{\text{p}}_{{2}} }}$ $T_{p_{2}}$ playing a dominant role. Within the temperature range of 740 °C to 780 °C, M/A constituents in the ICCGHAZ coarsen significantly and form chain-like structures distributed quasi-continuously along the prior austenite grain boundaries, thereby establishing a contiguous hydrogen-trapping network. Under these conditions, D_eff reaches its minimum value, while C₀ and N_t attain their maximum levels. In contrast, when $T_{{{\text{p}}_{{2}} }}$ $T_{p_{2}}$ approaches Ac₁ or Ac₃, the M/A constituents are more finely dispersed, which disrupts the trapping network and weakens their ability to the retardation of hydrogen diffusion, resulting in increased D_eff and decreased C₀ and N_t. Multivariate regression analysis further reveals that the spatial fraction of M/A constituents has a 2 to 3 times greater influence on hydrogen permeation parameters than their average size, identifying it as the dominant factor. Additionally, although the ICCGHAZ exhibits higher hardness than the CGHAZ, the peak hardness does not align with the maximum M/A volume fraction, indicating that the carbon segregation contributes to matrix softening.