CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Fiber Laser-Arc Composite Welding 20Mn2 Pipe Welds
Literature Overview
This study, conducted by He Lijia, Zhao Xiaojie, Wang Han, and Gao Rui from the School of Materials Science and Engineering at Liaoning Technical University, was published in "Thermal Processing Technology" in 2016. It was supported by the Liaoning Provincial Natural Science Foundation (grant 2013020086) and the Central Support for Local Universities Development Fund (grant 20138921319). The research investigates the microstructure and mechanical properties of welds produced by fiber laser-arc composite welding on 20Mn2 steel pipes.
Core Technical Points
20Mn2 is a medium-carbon low-alloy steel commonly used in pressure vessels, pipelines, and structural applications. It contains approximately 0.17–0.23% C and 1.20–1.60% Mn, providing good strength and weldability. However, welding 20Mn2 steel presents challenges related to hydrogen-induced cracking (HIC), hot cracking, and the formation of hard, brittle phases in the heat-affected zone (HAZ).
The fiber laser-arc composite welding process combines the deep, narrow penetration of fiber laser welding with the stable arc and higher deposition rate of arc welding (typically MIG/MAG or TIG). This hybrid approach offers several advantages:
- Reduced risk of burn-through in thin-walled pipes
- Improved weld geometry with reduced spatter
- Enhanced mechanical properties due to refined microstructure
- Higher productivity compared to laser-only or arc-only welding
Welding Process Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Laser power | 2–6 kW | Fiber laser, 1.07 µm wavelength |
| Arc current | 150–250 A | MIG/MAG or TIG |
| Arc voltage | 18–25 V | Depends on process |
| Travel speed | 300–800 mm/min | Affects heat input |
| Filler wire | ER70S-6 or equivalent | Low-carbon, low-sulfur |
| Shielding gas | Ar + 5–20% CO₂ | Balances penetration and spatter |
| Wire diameter | 1.0–1.2 mm | For MIG/MAG |
| Preheat temperature | 50–150 °C | Depends on thickness |
Interpretation of Technical Findings
The microstructure of the weld metal in laser-arc composite welds typically consists of acicular ferrite (AF), granular ferrite (GF), and proeutectoid ferrite (PF) in a matrix of pearlite. The laser component promotes rapid solidification, leading to finer grain structures and reduced inter-dendritic spacing. The arc component provides additional heat input, which can slow the cooling rate and promote the formation of acicular ferrite—a beneficial phase that improves toughness.
The HAZ microstructure is critically important for the overall weld performance. In 20Mn2 steel, the HAZ can develop a coarse-grained zone (CGHAZ) with martensite or bainite phases, which are susceptible to hydrogen-induced cracking. The laser-arc composite process can mitigate this risk by controlling the cooling rate through parameter optimization.
Mechanical Properties Comparison
| Property | Base Metal | Laser-Arc Composite Weld | Arc-Only Weld |
|---|---|---|---|
| Tensile strength (MPa) | 420–500 | 450–550 | 430–520 |
| Yield strength (MPa) | 250–320 | 280–350 | 260–330 |
| Elongation (%) | 22–28 | 20–25 | 22–27 |
| Impact energy (J, 20°C) | 60–80 | 50–70 | 45–65 |
| Hardness (HV) | 150–180 | 180–220 | 170–210 |
Note: Values are representative and should be verified against the original publication.
Integration with Engineering Practice
In pipeline and pressure vessel fabrication, the laser-arc composite welding process offers a compelling alternative to conventional arc welding, particularly for medium-thickness pipes (6–25 mm). The improved mechanical properties and refined microstructure can lead to longer service life and reduced maintenance requirements.
From a standards perspective, the welding procedure must be qualified in accordance with applicable codes such as ASME Section IX, AWS D1.1, or EN ISO 15614. The qualification procedure should include:
- Weld procedure qualification (WPQ) with mechanical testing
- Welder qualification (WQQ) demonstrating operator proficiency
- Non-destructive testing (NDT) to verify weld integrity
- Metallographic examination to assess microstructure
FMEA for Weld Defects
| Defect | Cause | Detection | Prevention |
|---|---|---|---|
| Porosity | Moisture in flux, inadequate shielding | RT, UT | Dry flux, improve shielding |
| Cracking | High cooling rate, hydrogen embrittlement | MT, PT, UT | Preheat, low-hydrogen filler |
| Lack of fusion | Low heat input, poor technique | RT, UT | Increase heat input, train operators |
| Burn-through | Excessive heat input | Visual, RT | Reduce laser power, increase speed |
Study Insights and Reflections
This research demonstrates the practical advantages of hybrid welding processes in industrial applications. The fiber laser-arc composite welding process provides a balance between productivity, weld quality, and cost-effectiveness that is difficult to achieve with single-process welding.
For practitioners, the key insight is that the laser-arc composite process requires careful coordination between the laser and arc parameters. The relative positioning of the laser beam and arc, the timing of their interaction, and the overall heat input must be optimized to achieve the desired weld geometry and properties. This optimization requires a systematic approach, combining process parameter studies with microstructural and mechanical evaluation.