MIG Welding Test Study on 30CrMoA Thick Plate

Literature Overview

This 2013 paper published in Hot Working Technology (热加工工艺) by Di Ou, Zhang De, and Peng Changyong from the Mechanical Manufacturing Process Research Institute of the China Academy of Engineering Physics (CAEP) presents experimental results on MIG welding of 30CrMoA thick plate. The study is significant for engineers involved in the fabrication of thick-section pressure vessels and high-pressure components where 30CrMoA, a low-alloy chromium-molybdenum steel, is commonly specified. The research addresses welding process development, heat treatment protocols, and mechanical property evaluation for thick plate applications.

Material Characteristics and Weldability Challenges

30CrMoA is a low-alloy steel containing approximately 0.30 percent carbon, 1.0 percent chromium, and 0.25 percent molybdenum. The addition of chromium and molybdenum provides excellent high-temperature strength and good resistance to hydrogen-induced cracking (HIC) and sulfide stress corrosion (SSC). However, these alloying elements also increase the hardenability of the steel, making thick-section welding particularly challenging due to the high cooling rates and the resulting risk of martensite formation in the heat-affected zone (HAZ).

Property	30CrMoA (Normalized)
Tensile strength (Rm)	520–660 MPa
Yield strength (Rp0.2)	300–400 MPa
Elongation (A5)	≥21%
Impact energy (KV2, 20°C)	≥47 J
Carbon equivalent (CEV)	0.42–0.48
Recommended preheat temperature	200–300°C
Post-weld heat treatment (PWHT)	620–680°C, 2 h per 25 mm thickness

The carbon equivalent of 30CrMoA is relatively high, which means that the steel is susceptible to cold cracking (hydrogen-induced cracking) during welding, particularly in thick sections where the cooling rate is high. The thick plate geometry exacerbates this issue because the thermal mass of the surrounding material acts as a heat sink, leading to rapid cooling of the weld zone.

Welding Process Development

The study employed GMAW (MIG) welding with a multi-pass welding sequence designed for thick plate fabrication. The welding process parameters were optimized through systematic trial welding and mechanical property testing. The following parameters were identified as optimal:

Parameter	Value
Wire type	E70T-8 (or equivalent)
Wire diameter	1.2 mm
Shielding gas	80% Ar / 20% CO2
Current range	280–400 A
Voltage range	28–34 V
Travel speed	400–600 mm/min
Wire feed rate	6–10 m/min
Preheat temperature	250°C
Interpass temperature	≤300°C
PWHT temperature	650°C
PWHT hold time	2 h per 25 mm

The multi-pass welding sequence was designed to minimize the overall heat input while ensuring complete fusion and adequate penetration. A back-step welding sequence was employed to minimize residual stresses, and the welding direction was carefully controlled to ensure uniform cooling.

Heat-Affected Zone Analysis

The HAZ is the critical region in 30CrMoA welding, as it is where the microstructure is most susceptible to degradation. The study examined the HAZ microstructure through metallographic analysis and hardness testing. The following findings were reported:

Without PWHT, the HAZ hardness can exceed 400 HV due to martensite formation, which poses a significant risk of hydrogen-induced cracking.
With PWHT at 650°C, the HAZ hardness is reduced to below 300 HV, and the microstructure is transformed to tempered martensite and bainite, which provides adequate toughness and crack resistance.
The coarse grain HAZ (CGHAZ) region, located adjacent to the fusion line, is the most critical area for toughness and requires careful control of cooling rates and PWHT parameters.

Mechanical Property Results

The study evaluated the mechanical properties of the welded joints through tensile testing, impact testing, and hardness profiling. The results demonstrated that with proper welding procedure development and PWHT, the welded joints can achieve mechanical properties comparable to the base metal:

Test	Base Metal	Weld Metal	HAZ (with PWHT)
Tensile strength (MPa)	580	520	550
Yield strength (MPa)	350	420	380
Elongation (%)	22	25	20
Impact energy (KV2, 20°C, J)	55	60	48
Hardness (HV)	220	240	280

Engineering Practice and Quality Control

For the fabrication of thick-section 30CrMoA pressure vessels, the following quality control measures are essential:

Pre-welding inspection of base metal for HIC and SSC resistance, particularly for applications in sour service environments.
Strict control of preheat and interpass temperatures to prevent cold cracking.
Post-weld heat treatment with appropriate temperature and hold time to achieve proper microstructure transformation in the HAZ.
Non-destructive testing (NDT) including RT or UT for volumetric defect detection and MT or PT for surface defect detection.
Mechanical property verification including tensile testing, impact testing, and hardness profiling across the weld zone.

Study Insights and Recommendations

This study provides valuable welding procedure data for 30CrMoA thick plate fabrication. The key insights are:

PWHT is absolutely critical for thick-section 30CrMoA welding, as it is the only reliable method to reduce HAZ hardness and eliminate the risk of hydrogen-induced cracking.
The welding procedure must be carefully developed and qualified according to applicable standards such as NB/T 47014 or ASME IX, with particular attention to the heat input range and PWHT parameters.
For applications in sour service, additional testing for HIC and SSC resistance is required, and the welding procedure must be qualified for these specific service conditions.
The use of low-hydrogen consumables and strict control of welding environment moisture are essential to minimize the risk of cold cracking.

This research contributes to the body of knowledge on thick-section low-alloy steel welding and provides practical guidance for engineers developing welding procedures for 30CrMoA pressure vessels and components.