Hydrogen-Induced Cracking Initiation and Propagation in 10CrNi3MoV Steel During GMAW Welding

Literature Overview

The study by Wei Jinshan, Zhang Tianhong, and Fang Hongyuan (Welding Journal, Vol. 23, No. 1, 2002, pp. 53-56) investigates the hydrogen-induced cracking (HIC) behavior of 10CrNi3MoV steel, a 590 MPa-grade marine high-strength steel, during gas metal arc welding (GMAW). The research was conducted at Harbin Institute of Technology and the Luoyang Ship Material Research Institute, two institutions with deep expertise in shipbuilding materials and welding metallurgy. The work is particularly significant because 10CrNi3MoV steel belongs to the class of martensitic and martensite-ferrite steels widely used in marine applications where high strength and toughness at low temperatures are required. The cracking susceptibility of such steels during welding has always been a critical concern for shipbuilders and designers.

Core Technical Findings

The investigation employed the small iron rod (Xiao Tie Yan) cracking resistance test method under controlled conditions of -5 °C ambient temperature and 80% relative humidity. The welding consumable was WM960-S flux-cored wire with a shielding gas mixture of Ar + 20% CO₂, and no preheating was applied. The authors compared single-pass and double-pass welding configurations and systematically analyzed crack initiation sites, crack rates, and crack propagation modes.

The following table summarizes the key experimental conditions and comparative results:

The most striking finding is the fundamental difference in crack propagation mode between single-pass and double-pass welding. In single-pass welds, hydrogen-induced cracks propagated in an intergranular fashion, following grain boundaries in the coarse-grained heat-affected zone (CGHAZ). In double-pass welds, the cracks propagated transgranularly, cutting through grains rather than along their boundaries. This shift in propagation mode is directly linked to the microstructural refinement that occurs in the CGHAZ of the first pass when subjected to the thermal cycle of the second pass.

Interpretation of Microstructural Mechanisms

The authors attributed the superior cracking resistance of double-pass welding to the microstructural refinement occurring in the HAZ of the first pass. During the first welding pass, the CGHAZ experiences a peak temperature well above the A₃ temperature, resulting in significant grain coarsening. The coarse austenite grains in this region provide long, continuous grain boundaries that serve as preferential paths for hydrogen embrittlement and crack propagation. When the second pass is deposited, the thermal cycle experienced by the first-pass CGHAZ is equivalent to a tempering and partial recrystallization event. The peak temperature of the second-pass thermal cycle in the first-pass CGHAZ falls in the range of approximately 700-900 °C, which is sufficient to initiate recrystallization and grain refinement at the grain boundaries of the originally coarse grains.

The resulting fine grains flanking the original coarse grain boundaries create a barrier to intergranular crack propagation. Hydrogen atoms, which normally accumulate at grain boundaries due to lower binding energy and higher diffusion mobility, are now distributed across a much higher density of grain boundaries. This dispersion effect reduces the local hydrogen concentration at any single grain boundary below the critical threshold required for crack initiation. Furthermore, the transgranular crack propagation mode observed in double-pass welds indicates that the hydrogen concentration at grain boundaries has been sufficiently reduced, forcing cracks to propagate through the grain interiors, which requires significantly higher energy input.

From a metallurgical perspective, the martensitic microstructure of 10CrNi3MoV steel in the HAZ is inherently susceptible to hydrogen embrittlement. The retained austenite, carbide precipitates, and dislocation density variations within the martensite laths create trapping sites for hydrogen atoms. The combination of high hydrogen diffusivity in the martensitic matrix and the coarse grain structure of the CGHAZ creates a particularly dangerous scenario for HIC. The double-pass approach effectively mitigates this risk through microstructural modification rather than through hydrogen control alone, which is a fundamentally different and more robust strategy.

Engineering Practice Implications

The findings of this study have direct practical implications for the welding of high-strength marine steels. In shipbuilding practice, the use of multi-pass welding for thick sections is already standard, but this study provides a metallurgical justification for the practice specifically in terms of hydrogen cracking resistance. For sections where single-pass welding is feasible (typically thin sections), the risk of HIC is elevated and additional precautions such as preheating, low-hydrogen consumables, and post-weld baking become essential.

The following table presents recommended countermeasures for HIC prevention in 590 MPa-grade steels:

A critical observation from this work is that the Ar + 20% CO₂ shielding gas mixture, while providing good weld metal properties and arc stability, may introduce slightly higher hydrogen levels compared to pure argon due to the dissociation of CO₂ at the arc. In the context of HIC-sensitive steels, this trade-off must be carefully evaluated. The WM960-S flux-cored wire, being a low-hydrogen type, partially compensates for this effect, but the overall hydrogen content in the weld metal still requires monitoring through diffusion hydrogen measurements.

Key Questions and Reflections

One question that arises from this study is the quantitative threshold for CGHAZ grain size that transitions the crack propagation mode from intergranular to transgranular. The study describes the phenomenon qualitatively but does not provide a precise grain size criterion. Future work should employ quantitative metallographic analysis to establish a clear relationship between CGHAZ grain size, hydrogen concentration, and crack propagation mode. This would enable engineers to predict cracking behavior based on measured grain sizes in production welds.

Another consideration is the interaction between hydrogen-induced cracking and cold cracking (delayed cracking) in this steel grade. 10CrNi3MoV steel is susceptible to both types of cracking, and the mechanisms are related but distinct. The study focuses specifically on HIC, but in practice, the two mechanisms often overlap, particularly in the CGHAZ where both coarse grains and high hardness martensite coexist. A comprehensive cracking resistance assessment should consider both mechanisms simultaneously.

The study also raises the question of whether the observed microstructural refinement in double-pass welding is sufficient to prevent HIC under more severe service conditions, such as very low temperatures or high hydrogen environments. The test conditions of -5 °C and 80% humidity are representative of cold and humid environments but may not capture the full range of conditions encountered in Arctic or offshore applications.

Study Insights and Implications

This study exemplifies the value of fundamental metallurgical research in informing practical welding procedures. The discovery that double-pass welding improves HIC resistance through CGHAZ grain refinement, rather than through hydrogen reduction, represents an important conceptual advancement. It suggests that multi-pass welding provides a dual benefit: it reduces hydrogen concentration through thermal tempering of the first-pass weld metal, and it refines the HAZ microstructure to create barriers against crack propagation. For engineers designing welding procedures for high-strength marine steels, this work reinforces the importance of multi-pass welding not merely as a means to achieve full penetration but as a metallurgical strategy to enhance cracking resistance.