CLADDING TECHNOLOGY SHANXI CO., LTDHydrogen-Induced Cold Cracking Sensitivity in Transverse MIG Welds of 12Ni5CrMoV Medium-Alloy High-Strength Steel
Literature Overview
This study, published in the Journal of Iron and Steel Research in 1990 by Zhu Mingsheng and Zhang Youwei, addresses one of the most critical fracture mechanisms in medium-alloy high-strength steel welding — hydrogen-induced cold cracking (HICC) in the transverse direction of MIG welds of 12Ni5CrMoV steel. The 12Ni5CrMoV alloy is a 12% nickel-chromium-molybdenum-vanadium martensitic steel widely used in cryogenic pressure vessels, hydrogenation reactors, and other low-temperature service equipment where both toughness and corrosion resistance are demanded simultaneously. Understanding the transverse cracking susceptibility is particularly important because transverse cracks propagate perpendicular to the welding direction, often initiated at the fusion boundary or in the heat-affected zone (HAZ), and are far more difficult to detect during in-process inspection than longitudinal cracks.
Core Technical Content
The study investigates the hydrogen-induced cold cracking behavior in the transverse direction of MIG (gas metal arc welding) welds in 12Ni5CrMoV steel. The key technical focus lies in three interrelated factors: the susceptibility of the weld metal and HAZ to hydrogen embrittlement, the residual stress state in the transverse direction, and the microstructural transformation during cooling that creates susceptible phases.
The 12Ni5CrMoV alloy has a high hardenability due to the combined effects of 12% Ni, Cr, Mo, and V. The high nickel content stabilizes austenite and delays the martensitic transformation start temperature (Ms), which paradoxically increases the cooling rate sensitivity and the volume fraction of retained austenite at certain cooling rates. This retained austenite can transform to martensite during post-weld cooling or during subsequent service, introducing additional transformation-induced residual stresses that promote transverse cracking.
The MIG process parameters studied likely include wire feed speed, travel speed, arc voltage, shielding gas composition (typically Ar + CO₂ or Ar + O₂ mixtures), and preheat temperature. The hydrogen content in the weld metal, derived from moisture in the flux or shielding gas, diffusion through the base metal, and absorption from the atmosphere, is a primary driver of HICC. The study likely employed standard hydrogen extraction methods such as gas chromatography or palladium membrane diffusion to quantify dissolved hydrogen levels in the weld metal and HAZ.
Technical Parameters and Process Windows
| Parameter | Typical Range for 12Ni5CrMoV MIG | Critical Threshold |
|---|---|---|
| Preheat temperature | 150–250 °C | Below 150 °C: cracking risk increases sharply |
| Interpass temperature | ≤ 250 °C | Above 250 °C: coarse grain growth in HAZ |
| Shielding gas | Ar + 5% CO₂ or Ar + 2% O₂ | Pure Ar: excessive penetration, dilution issues |
| Wire feed speed | 4–8 m/min | Depends on wire diameter (1.2–1.6 mm) |
| Travel speed | 200–400 mm/min | Lower speed: higher heat input, coarser microstructure |
| Heat input | 0.8–2.5 kJ/mm | Above 2.5 kJ/mm: retained austenite instability |
| Hydrogen content | < 5 mL/100 g | Above 8 mL/100 g: cracking risk unacceptable |
| Diffusible hydrogen | < 3 mL/100 g | Critical for HICC susceptibility |
Microstructural Analysis and Defect Mechanisms
The transverse cracking mechanism in 12Ni5CrMoV MIG welds involves a complex interplay of hydrogen diffusion, phase transformation, and residual stress. During welding, the rapid heating and cooling cycle produces a martensitic microstructure in both the weld metal and the coarse-grained HAZ. The martensite in this alloy is characterized by high hardness (typically 400–550 HV), which increases the susceptibility to hydrogen trapping at martensite lath boundaries and carbide-matrix interfaces.
The transverse residual stress state is particularly critical. In a single-pass or multi-pass MIG weld, the transverse contraction during solidification and subsequent cooling generates tensile stresses perpendicular to the weld axis. These stresses, combined with the high hardness of the martensitic microstructure and the presence of diffusible hydrogen, create the necessary conditions for transverse cracking. The cracking typically initiates at the fusion boundary where the cooling rate is highest and the microstructure is most susceptible.
The study likely employed delayed cracking tests, such as the modified Charpy V-notch (CVN) test with delayed fracture observation, or the wedge-type cracking test, to evaluate the cracking susceptibility under different hydrogen levels and thermal cycle conditions. The results would have shown that the cracking sensitivity increases with hydrogen content, hardness, and cooling rate, and that the critical hydrogen content for cracking decreases as the hardness of the HAZ increases.
Engineering Practice Implications
For engineers fabricating pressure vessels or structural components from 12Ni5CrMoV steel, this study provides several actionable guidelines. First, strict control of hydrogen sources is essential: the welding consumables must be stored in dry conditions, the shielding gas must be free of moisture, and the base metal surface must be cleaned to remove oxide and moisture. Second, adequate preheat (typically 150–250 °C) and interpass temperature control are necessary to reduce the cooling rate and allow hydrogen to diffuse out of the weld zone before the microstructure becomes brittle. Third, post-weld heat treatment (PWHT) is strongly recommended to relieve residual stresses and temper the martensitic microstructure, reducing hardness to below 350 HV.
From a quality control perspective, the transverse direction of MIG welds in 12Ni5CrMoV steel should be subjected to rigorous non-destructive testing, including ultrasonic testing (UT) with transverse probe orientation and possibly phased array ultrasonic testing (PAUT) for improved sensitivity to transverse defects. Magnetic particle testing (MT) is also applicable for surface and near-surface transverse cracks.
Key Reflections
The 1990 publication date places this study in the early period of systematic research on hydrogen-induced cracking in high-nickel martensitic steels. Despite the passage of time, the fundamental mechanisms described remain highly relevant to modern engineering practice. The study's emphasis on the transverse direction is particularly valuable because many fabrication codes and procedures focus primarily on longitudinal cracking, potentially overlooking the transverse cracking risk. Engineers working with 12Ni5CrMoV or similar high-nickel alloys should treat transverse cracking as a primary failure mode and incorporate appropriate preventive measures into their welding procedures and inspection protocols.