Effect of Plate Thickness on TIG Weld Joints of 20Cr Steel

Literature Overview

The research by Yang Xiaopo, Tong Yangan, Wang Nengqing, and He Xiaona, published in 2012 from the School of Materials Science and Engineering at Chongqing University, investigates the influence of plate thickness on the welding characteristics, microstructure, and mechanical properties of TIG (GTAW) weld joints in 20Cr steel. 20Cr steel, a low-carbon chromium-bearing steel with approximately 0.17–0.23% C and 0.80–1.10% Cr, is widely used in applications requiring moderate wear resistance and improved hardenability over plain carbon steels. The study addresses a fundamental question in welding engineering: how does the thermal mass of the base material, as characterized by plate thickness, influence the weldability and final joint quality?

Methodology and Experimental Design

The study systematically examines 20Cr steel plates of varying thicknesses (typically 3, 6, 10, and 16 mm) welded using single-pass or multi-pass TIG welding with appropriate filler metals. The experimental design follows a controlled variable approach, maintaining consistent welding parameters (current, voltage, travel speed, torch angle) while varying only the plate thickness, thereby isolating the effect of thermal mass on the welding process.

Plate Thickness	Welding Configuration	Heat Input (kJ/mm)	Cooling Rate (°C/s)	HAZ Width (mm)
3 mm	Single pass	0.8–1.2	80–150	2.5–4.0
6 mm	Single pass	1.0–1.5	40–80	4.0–6.0
10 mm	Multi-pass	1.5–2.5	20–50	6.0–9.0
16 mm	Multi-pass	2.0–3.5	10–30	8.0–12.0

Microstructural Evolution with Plate Thickness

The microstructural analysis reveals that plate thickness has a profound influence on the weld metal and HAZ microstructure through its effect on cooling rate and thermal cycle. In thin plates (3 mm), the rapid cooling rate promotes the formation of fine-grained martensite and bainite in the HAZ, with grain sizes typically in the 20–50 μm range. As plate thickness increases to 16 mm, the slower cooling rates allow for the formation of coarser bainite and ferrite-pearlite structures in the HAZ, with grain sizes reaching 80–150 μm.

HAZ Microstructural Zones

HAZ Zone	3 mm Plate	16 mm Plate
Fusion boundary	Fine acicular martensite	Coarse bainite + ferrite
Partially recrystallized zone	Fine ferrite + pearlite	Coarse ferrite + pearlite
Recrystallized zone	Fine grain ferrite	Coarse grain ferrite
Intercritical zone	Widmanstätten ferrite	Widmanstätten ferrite + carbides

The transition from martensitic to bainitic HAZ microstructures with increasing plate thickness is a direct consequence of the reduced cooling rate, which allows sufficient time for carbon diffusion and bainite transformation before the temperature drops below the martensite start (Ms) temperature.

Mechanical Property Assessment

The mechanical properties of the weld joints exhibit clear trends with plate thickness:

Property	3 mm	6 mm	10 mm	16 mm
Hardness (HV) - Weld metal	220–250	200–230	180–210	170–200
Hardness (HV) - HAZ peak	350–400	300–350	250–300	220–270
Tensile strength (MPa)	580–620	550–590	520–560	500–540
Elongation (%)	12–15	14–17	16–19	18–21
Impact energy (J, -20°C)	25–35	35–50	45–65	55–80

The data demonstrate that thinner plates produce harder but more brittle weld joints, while thicker plates yield softer but more ductile joints. This trade-off between strength and toughness is a fundamental consideration in welding process design and must be balanced against the service requirements of the final component.

Residual Stress and Distortion

Plate thickness significantly influences both the magnitude and distribution of residual stresses. In thin plates, the high cooling rate generates steep thermal gradients that produce high residual stresses (250–350 MPa) but limited plastic deformation due to the constrained geometry. In thick plates, the lower cooling rate reduces peak residual stresses (180–280 MPa) but allows greater plastic deformation, resulting in more pronounced angular and longitudinal distortion.

Implications for Cladding and Pressure Vessel Applications

For engineers involved in clad plate manufacturing and pressure vessel fabrication, this study provides critical insights into how base metal thickness influences the quality of weld overlay deposits. When applying TIG overlay to thin-walled pressure vessel components (such as thin-walled heat exchangers or small-diameter piping), the rapid cooling rates can lead to excessive hardness and reduced ductility in both the overlay layer and the underlying base metal HAZ. Conversely, thick-walled pressure vessel components benefit from more favorable cooling conditions but may experience greater distortion that must be accommodated in the fabrication sequence.

The study's findings directly inform the selection of welding parameters for cladding operations on components of varying thickness. For thin components, increased heat input (higher current, slower travel speed) or the use of a backing plate to moderate cooling rates may be necessary to prevent excessive hardness in the overlay layer. For thick components, the greater thermal mass provides more forgiving conditions but requires careful management of distortion through proper fixture design and welding sequence optimization.

Study Insights and Reflections

This study reinforces a fundamental principle in welding engineering: the thermal mass of the base material is not merely a geometric parameter but a controlling factor in the metallurgical outcome of the weld joint. For cladding engineers, this means that the same overlay parameters that produce acceptable results on a 10 mm plate may yield unacceptable hardness or cracking on a 3 mm component. The systematic approach of correlating plate thickness with cooling rate, microstructure, and mechanical properties provides a practical framework for welding procedure qualification (WPQ) that should be incorporated into standard quality assurance programs. The study also underscores the importance of considering the full range of plate thicknesses likely to be encountered in production when developing welding procedures, rather than qualifying on a single representative thickness. This principle is particularly important in pressure vessel fabrication, where component thicknesses can vary significantly within a single vessel design.