Analysis of Cracking in Narrow-Gap GMAW Circumferential Welds of 27SiMn Steel

Problem Description and Background

This study addresses the cracking phenomenon observed in narrow-gap gas metal arc welding (GMAW) circumferential welds of 27SiMn steel, a low-alloy steel widely used in mechanical drive shafts, hydraulic cylinders, and pressure vessel components. 27SiMn steel contains approximately 0.27 percent carbon, 1.5 to 2.0 percent manganese, and 0.6 to 1.0 percent silicon, giving it good strength and hardenability. However, the combination of relatively high carbon equivalent and the constrained geometry of narrow-gap welding creates conditions favorable for hydrogen-induced cracking and low-temperature cracking. The study systematically analyzes the root causes of cracking using metallurgical examination, chemical analysis, and process parameter evaluation.

Material Properties and Welding Characteristics

The 27SiMn steel has a carbon equivalent (CE) value calculated per IIW formula of approximately 0.48 to 0.52, which places it in the high-cracking-susceptibility category. The material is typically supplied in the normalized or quenched-and-tempered condition, with a yield strength of 600 to 700 megapascals and an ultimate tensile strength of 750 to 850 megapascals.

The narrow-gap GMAW process was employed to reduce the weld cross-sectional area and heat input compared to conventional single-pass welding. The typical gap width was 6 to 10 millimeters with a plate thickness of 25 to 40 millimeters. The narrow-gap configuration results in a high aspect ratio (depth-to-width ratio of 3:1 to 5:1), which creates significant constraint on the weld metal during solidification and cooling.

Crack Formation Mechanism

The cracking observed in the narrow-gap welds was identified as low-temperature cracking (cold cracking) occurring in the heat-affected zone and near the fusion line. The three essential factors for cold cracking are: susceptible microstructure (hardened martensite), sufficient diffusible hydrogen, and excessive residual stress.

The HAZ microstructure near the fusion line exhibited a high proportion of martensite and bainite, with hardness values reaching 380 to 420 HV. This hardened microstructure was attributed to the high cooling rate resulting from the narrow-gap configuration and the high carbon equivalent of the base steel. The cooling rate in the HAZ was estimated to be 20 to 40 degrees Celsius per second, well above the critical cooling rate for martensite formation.

The diffusible hydrogen content in the weld metal was measured at 5 to 8 milliliters per 100 grams, which exceeds the recommended maximum of 5 milliliters per 100 grams for steels with CE above 0.45. The hydrogen source was traced to moisture in the welding flux and contamination on the base metal surface. The residual stress in the narrow-gap weld was estimated to be 350 to 450 megapascals in the transverse direction, driven by the high constraint of the narrow geometry and the differential thermal contraction between the weld metal and the base steel.

Root Cause Analysis Using 5W2H Method

A systematic root cause analysis was conducted to identify the contributing factors:

Corrective Measures and Verification

The following corrective measures were implemented and verified through re-welding and testing:

1. Increase the preheat temperature from 100 degrees Celsius to 250 degrees Celsius, maintaining the interpass temperature between 200 and 300 degrees Celsius.
2. Dry the welding flux at 300 degrees Celsius for four hours prior to use, and store in a heated cabinet at 150 degrees Celsius.
3. Clean the base metal surface with wire brush and solvent to remove rust, oil, and moisture.
4. Reduce the travel speed by 15 to 20 percent to increase the heat input and reduce the cooling rate.
5. Apply post-weld heat treatment at 600 to 650 degrees Celsius for two hours to relieve residual stresses and reduce HAZ hardness.
6. Implement a hydrogen control program with mandatory hydrogen measurement after each weld.

After implementing these measures, the diffusible hydrogen content was reduced to 2 to 3 milliliters per 100 grams, the HAZ hardness was reduced to 300 to 330 HV, and the residual stress was reduced to 180 to 250 megapascals. No cracks were observed in the re-welded specimens after 72 hours of aging.

Quality Control Recommendations

For future narrow-gap GMAW welding of 27SiMn steel, the following quality control measures should be enforced:

• Mandatory preheat of 250 degrees Celsius for plate thicknesses above 20 millimeters.
• Hydrogen measurement on every production weld, with acceptance limit of 3 milliliters per 100 grams.
• Hardness testing of the HAZ, with acceptance limit of 350 HV maximum.
• Magnetic particle testing of the weld and HAZ after 24 hours of aging.
• Weld procedure qualification per NB/T 47014 with crack testing per ASTM A403.

In conclusion, the cracking in narrow-gap GMAW welds of 27SiMn steel is primarily caused by the synergistic effect of high carbon equivalent, excessive diffusible hydrogen, and high residual stress under constrained narrow-gap geometry. The corrective measures centered on preheat control, hydrogen management, and post-weld heat treatment effectively eliminated the cracking problem. Engineers should recognize that narrow-gap welding of high-carbon-equivalent steels demands more rigorous process control than conventional welding, and the quality assurance program must be tailored accordingly.