MIG Welding Experimental Study on 30CrMoA Thick Plate

Literature Overview

This study by Di Ou, Zhang De, and Peng Changyong from the Mechanical Manufacturing Process Research Institute of China Academy of Engineering Physics, published in Hot Working Technology in 2013, presents a systematic experimental investigation of metal inert gas (MIG) welding on 30CrMoA thick plate. The research addresses the significant challenges associated with welding thick-section low-alloy steels, which are widely used in high-pressure equipment, power plant components, and defense applications. The 30CrMoA steel, a chromium-molybdenum alloy steel with approximately 0.30% C, 0.90% Cr, and 0.20% Mo, is known for its excellent high-temperature strength, good toughness, and resistance to hydrogen-induced cracking (HIC), making it a critical material for pressure vessel and piping applications.

Core Technical Points

Material Characteristics of 30CrMoA

30CrMoA is a normalized or quenched-and-tempered low-alloy steel with the following typical characteristics:

The high carbon equivalent of 30CrMoA makes it susceptible to hydrogen-induced cracking (cold cracking) in the heat-affected zone (HAZ), particularly in thick sections where cooling rates are slow enough to allow hydrogen diffusion but fast enough to produce martensitic transformation in the prior austenite grains.

Welding Process Parameters for Thick Plate MIG

For thick plate MIG welding of 30CrMoA, the following parameter ranges are typically employed:

The key challenge in thick plate welding is managing the heat input to balance competing requirements: sufficient heat input to prevent cold cracking by controlling cooling rate, but limited heat input to avoid excessive grain growth and softening of the HAZ.

Weld Metal Selection

For 30CrMoA thick plate welding, the following weld metal specifications are typically used:

The nickel addition in E81T1-Ni1 and E81T1-Ni2 improves weldability by reducing the carbon equivalent of the weld metal and promoting ferrite formation that suppresses grain boundary embrittlement.

Process Analysis and Defect Control

Common Defects and Countermeasures

Heat Input Management

The heat input per unit length is calculated as:

q = (U × I × η) / v

where U is arc voltage (V), I is welding current (A), η is thermal efficiency (0.7–0.85 for MIG), and v is travel speed (mm/s).

For 30CrMoA thick plate, the target heat input range is typically 15–30 kJ/mm for the root and fill passes, and 20–40 kJ/mm for the cap pass. The heat input must be carefully controlled to:

• Maintain the HAZ cooling rate (t8/5) below 5–10 seconds to prevent excessive hardening
• Limit the total heat input to avoid excessive grain growth in the HAZ
• Ensure adequate hydrogen diffusion to prevent cold cracking

Post-Weld Heat Treatment (PWHT)

For thick-section 30CrMoA weldments, PWHT is mandatory per most applicable codes (ASME VIII Div.1, GB/T 150, NB/T 47002). The typical PWHT cycle is:

The PWHT serves multiple purposes: stress relief, hydrogen elimination, and microstructural refinement of the HAZ. For 30CrMoA, the soaking temperature must be carefully controlled to avoid temper embrittlement, which can occur in the 370–570°C range in Cr-Mo steels.

Engineering Practice Integration

Multi-Pass Welding Strategy for Thick Sections

For 30CrMoA plates thicker than 50 mm, a multi-pass welding strategy is essential:

1. Root pass: Use GTAW (TIG) with ER80S-Ni2 or equivalent filler, 150–200 A, 18–22 V, 150–200 mm/min. Preheat to 200°C. This ensures full penetration with minimal dilution.
2. Fill passes: Use MIG with E81T1-Ni1, 300–380 A, 30–34 V, 250–350 mm/min. Maintain interpass temperature at 200–250°C. Limit layer thickness to 6 mm.
3. Cap pass: Use MIG with E81T1-Ni1, 320–400 A, 32–36 V, 300–400 mm/min. Grind flush with surface.
4. PWHT: Perform within 6 hours of completion of welding.

The total number of passes for a 100 mm thick plate can range from 20 to 40, depending on the joint preparation and welding position.

Quality Control Requirements