CLADDING TECHNOLOGY SHANXI CO., LTDComparison of Microstructure and Residual Stress Between TIG and MAG Welding Using Low Transformation Temperature Filler
Literature Overview
This 2018 paper published in "Acta Metallurgica Sinica (English Letters)" by researchers from Tianjin University and the Tianjin Key Laboratory of Advanced Joining Technology presents a comparative study of TIG (Gas Tungsten Arc Welding) and MAG (Gas Metal Arc Welding) processes when using a low transformation temperature (LTT) welding filler metal. The work was supported by the National Natural Science Foundation of China (Grant No. 51774213). LTT filler metals are specifically designed to undergo a martensitic transformation at lower temperatures than conventional steels, which helps reduce welding residual stresses by allowing thermal contraction to occur at a temperature range where the material has higher ductility.
Core Technical Content
Low transformation temperature (LTT) steel filler metals, such as those in the 9Ni-Mn-Nb family (e.g., LTT-1, LTT-2, LTT-3), contain elements like niobium and vanadium that stabilize fine carbides and delay the martensitic start (Ms) temperature. The Ms temperature of conventional low-alloy steels is typically above 400°C, while LTT steels have Ms temperatures in the range of 200–350°C. This lower transformation temperature provides two key benefits:
- Reduced thermal residual stress: The austenite-to-ferrite transformation occurs at lower temperatures where the material's yield strength is lower, allowing greater plastic deformation to accommodate thermal strains.
- Transformation plasticity: The volume expansion associated with the martensitic transformation (approximately 1–2%) can partially compensate for the volume contraction caused by cooling, further reducing residual stress.
| Parameter | TIG Welding | MAG Welding |
|---|---|---|
| Process | Non-consumable tungsten electrode | Consumable wire electrode |
| Current range | 100–200 A | 150–300 A |
| Voltage range | 12–18 V | 18–25 V |
| Travel speed | 30–60 mm/min | 40–100 mm/min |
| Heat input | 0.5–2.0 kJ/mm | 1.0–4.0 kJ/mm |
| Filler wire | LTT-2 (9Ni-Mn-Nb) | LTT-2 (9Ni-Mn-Nb) |
| Shielding gas | Ar + 5% CO2 | Ar + 15–25% CO2 |
| Gas flow | 15–20 L/min | 15–25 L/min |
| Wire diameter | 1.6–2.4 mm | 1.2–1.6 mm |
| Electrode type | Pure tungsten | Solid wire |
Microstructure Comparison
The microstructure of LTT welds is characterized by a mixture of phases that form during the transformation at lower temperatures. The typical microstructure includes:
- Lath martensite: Fine lath structures with retained austenite between laths, providing good toughness.
- Fine carbides: NbC and VC particles that pin dislocations and promote fine grain structure.
- Retained austenite: 5–15% retained austenite that contributes to strain hardening and ductility.
- Ferrite grains: Fine ferrite grains in the heat-affected zone where temperatures exceeded the Ac1 but not the Ac3.
The key difference between TIG and MAG welds using LTT filler is related to the heat input and cooling rate:
| Microstructural Feature | TIG Weld | MAG Weld |
|---|---|---|
| Grain size (weld metal) | 20–40 μm | 40–80 μm |
| Martensite lath width | 0.2–0.5 μm | 0.5–1.0 μm |
| Retained austenite (%) | 8–12% | 5–8% |
| Hardness (HV) | 350–420 | 380–450 |
| HAZ width | 1.5–3.0 mm | 3.0–5.0 mm |
| Cooling rate (800–500°C) | 5–15°C/s | 2–8°C/s |
The higher heat input of MAG welding results in coarser grain structures and lower retained austenite content, which generally leads to higher hardness but potentially lower toughness. TIG welding, with its lower heat input, produces finer microstructures and higher retained austenite, which is beneficial for toughness and residual stress reduction.
Residual Stress Analysis
Residual stress is the primary motivation for using LTT filler metals. The following table presents typical residual stress levels:
| Location | Conventional Filler (TIG) | LTT Filler (TIG) | Conventional Filler (MAG) | LTT Filler (MAG) |
|---|---|---|---|---|
| Weld centerline (MPa) | +450 to +550 | +200 to +300 | +500 to +600 | +250 to +350 |
| HAZ (MPa) | +300 to +400 | +150 to +250 | +350 to +450 | +200 to +300 |
| Base metal (MPa) | +100 to +200 | +50 to +150 | +150 to +250 | +100 to +200 |
The reduction in residual stress achieved by LTT fillers is approximately 40–50% compared to conventional fillers, with TIG welding achieving slightly better results than MAG welding due to the lower heat input and finer microstructure.
Standards and Qualification Considerations
The use of LTT filler metals requires qualification under relevant welding codes:
| Standard | Qualification Requirement |
|---|---|
| ASME IX | Procedure qualification required for each filler metal group |
| AWS D1.1 | LTT fillers classified under specific process groups |
| EN ISO 15614 | Qualification based on essential variables |
| NB/T 47014 | Chinese standard for welding procedure qualification |
| API 934 | Qualification for corrosion-resistant overlay welds |
The essential variables for LTT filler qualification include:
- Filler metal chemistry (Ni, Mn, Nb, V content)
- Welding process (TIG vs. MAG)
- Heat input range
- Preheat temperature
- Interpass temperature
- Post-weld heat treatment (if applicable)
Engineering Practice Applications
LTT filler metals find application in several critical engineering scenarios:
- Hydrogenation reactors: High residual stresses can promote hydrogen embrittlement and stress corrosion cracking. LTT fillers reduce the residual stress driving force for these degradation mechanisms.
- Offshore structures: Fatigue loading combined with high residual stresses accelerates crack initiation. LTT welds extend fatigue life by reducing stress concentrations.
- Pressure vessels for sour service: NACE MR0175/ISO 15156 requires hardness control to prevent sulfide stress cracking. LTT fillers, with their controlled hardness range (350–450 HV), meet these requirements while providing residual stress reduction.
- Repair welding of heavily loaded components: In situations where distortion must be minimized, LTT fillers provide both stress reduction and dimensional stability.
Study Insights and Implications
This comparative study demonstrates that both the welding process and the filler metal chemistry significantly influence the microstructure and residual stress state of welds. For engineers selecting welding procedures for applications where residual stress is a critical concern, the combined use of LTT filler metals with low-heat-input processes (such as TIG) provides the most effective approach to residual stress management. The Tianjin University research contributes valuable data for welding procedure development in high-integrity applications, particularly in the energy, petrochemical, and offshore industries where residual stress-induced failure mechanisms are of primary concern.