CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Plate Thickness on TIG Welded Joints of 20Cr Steel
Literature Overview
This 2012 research paper by Yang Xiaopo, Tong Yangang, Wang Nengqing, and He Xiaona from Chongqing University's School of Materials Science and Engineering investigates how plate thickness influences the metallurgical behavior and mechanical performance of TIG (GTAW) welded joints in 20Cr steel. The study addresses a fundamental question in welding engineering: how does the geometric constraint of plate thickness alter the thermal cycle, microstructure evolution, and final weld quality?
Core Technical Points
20Cr steel is a low-carbon, low-alloy steel containing approximately 0.17–0.23% C and 0.80–1.10% Cr, commonly used in automotive components, machinery parts, and pressure equipment due to its good hardenability and wear resistance. The chromium content provides moderate atmospheric corrosion resistance and enhances the hardenability of the HAZ. When welding 20Cr steel with TIG, the primary concerns are:
| Parameter | Thin Plate (2–4 mm) | Medium Plate (4–8 mm) | Thick Plate (8–16 mm) |
|---|---|---|---|
| Typical current (A) | 40–80 | 80–150 | 150–250 |
| Travel speed (mm/min) | 200–400 | 100–250 | 50–150 |
| Number of passes | 1–2 | 2–4 | 4–8 |
| Heat input (kJ/mm) | 0.3–0.8 | 0.8–2.0 | 2.0–5.0 |
| Primary concern | Burn-through, LOF | Porosity, undercut | Cracking, distortion |
Microstructural Evolution with Plate Thickness
The study demonstrates that plate thickness directly governs the cooling rate at the weld centerline and HAZ, which in turn controls the microstructural transformation:
- Thin plates exhibit rapid cooling rates (>100°C/s in the HAZ), leading to the formation of martensite and bainite with high hardness (potentially exceeding 350 HV), which increases susceptibility to cold cracking.
- Medium plates show intermediate cooling rates (10–100°C/s), producing a mixed microstructure of upper bainite and acicular ferrite with moderate hardness.
- Thick plates experience slower cooling due to greater heat capacity, resulting in coarser grain structures in the HAZ and lower hardness but potentially reduced toughness if the grain size exceeds acceptable limits.
The chromium content in 20Cr steel promotes carbide precipitation at grain boundaries during slow cooling, which can reduce intergranular toughness in thick-section welds.
Process Optimization Strategies
The research proposes differentiated process approaches based on plate thickness:
- For thin plates (≤4 mm): Use pulsed TIG welding with a base current of 30–50 A and peak current of 80–120 A to control heat input and prevent burn-through. Preheating is generally unnecessary but joint fit-up must be precise (gap ≤0.5 mm).
- For medium plates (4–8 mm): Single or double-pass welding with 100–150 A at 150–250 mm/min. Preheating to 100–150°C is recommended to reduce hydrogen-induced cracking risk.
- For thick plates (≥8 mm): Multi-pass welding with preheat of 150–200°C. Interpass temperature should be maintained below 250°C. Consider using low-hydrogen filler metals (E309L or ER308L for overlay applications) to minimize cold cracking susceptibility.
Welding Procedure Qualification Considerations
From a standards perspective, the thickness effect must be addressed in WPS qualification per NB/T 47014 or ASME IX. The qualification thickness range for GTAW typically allows the qualified thickness to cover up to 1.5 times the qualified thickness for plates exceeding 19 mm, but for thinner plates, the qualified thickness must be within the actual range. This means that a procedure qualified on 4 mm plate cannot automatically be applied to 12 mm plate without re-qualification.
Study Insights and Implications
This research provides a systematic framework for understanding how geometric variables influence weld quality in low-alloy steel. The findings reinforce the principle that welding is not merely about arc parameters but about managing the thermal-metallurgical interaction between the process and the base material geometry. For engineers designing welding procedures for variable-thickness components (such as pressure vessel nozzles or flanges), this work highlights the necessity of establishing thickness-specific parameter windows rather than relying on a single universal procedure. The emphasis on cooling rate as the governing parameter for microstructure and properties is consistent with fundamental welding metallurgy and provides a rational basis for process optimization beyond empirical trial-and-error approaches.