Microstructure and Tensile Properties of Pulse TIG Welded 304 Stainless Steel Thin-Walled Pipe Under Different Process Parameters

Literature Overview

The paper authored by Li Ning, Liu Shaolong, and colleagues from Huadian International Shuozhou Thermal Power Branch and Xi'an University of Technology was published in Materials for Mechanical Engineering (Vol. 46, No. 2, 2022, pp. 58–62). The study investigates the weld microstructure and tensile properties of Φ14 mm × 2 mm 0Cr18Ni9 austenitic stainless steel thin-walled pipes fabricated by Pulse TIG (P-TIG) welding. The research systematically examines the influence of base current (20–30 A), peak current (40–60 A), and pulse frequency (0.3, 0.5 Hz) at a fixed 50% duty ratio on the weld zone and heat-affected zone (HAZ) microstructure, as well as the mechanical performance of the joint.

Core Technical Content and Parameter Analysis

The experimental design follows a structured approach where the pulse duty ratio is held constant at 50%, while three independent variables are varied. The base current range of 20–30 A governs the inter-pulse cooling and solidification behavior, the peak current of 40–60 A determines the maximum heat input and penetration depth, and the pulse frequency of 0.3–0.5 Hz controls the thermal cycling rate. The combination of these parameters creates distinct thermal cycles in the weld pool, which directly influence the solidification microstructure and the subsequent phase evolution.

Microstructure Interpretation

The weld zone microstructure under all tested conditions consists of austenite (γ) matrix with δ-ferrite, which is consistent with the Schaeffler diagram prediction for 304 stainless steel welds. However, the morphology and volume fraction of δ-ferrite vary significantly with process parameters. At the optimal condition (base current 20 A, peak current 40 A, frequency 0.3 Hz), δ-ferrite exhibits a worm-like (vermicular) morphology with relatively higher content. This is attributed to the lower base current and peak current creating a reduced average heat input, which slows the cooling rate and promotes δ-ferrite nucleation at the austenite grain boundaries during solidification. The lower pulse frequency of 0.3 Hz extends the inter-pulse cooling interval, allowing more complete thermal equilibration between successive pulses.

In the HAZ, austenite grain coarsening is observed under all conditions, but the degree of coarsening is minimized at the optimal parameter set. The lower peak current and base current at the optimal condition reduce the peak temperature in the HAZ, thereby limiting grain growth. The resulting HAZ grain size is notably smaller compared to conditions with higher current settings, which is critical for maintaining toughness and corrosion resistance in thin-walled pipe applications.

Tensile Performance and Fracture Analysis

The optimal welding condition yields superior mechanical properties with a yield strength of 401.38 MPa, tensile strength of 701.51 MPa, and a yield-to-tensile ratio (Y/T ratio) of 0.57. The Y/T ratio of 0.57 indicates a favorable balance between strength and ductility, which is essential for thin-walled pipe applications where plastic deformation capacity is required to accommodate service loads and thermal stresses. The fracture surface at the optimal condition is characterized by uniform, small equiaxed dimples, indicating a ductile fracture mechanism with good microvoid coalescence behavior.

Engineering Practice Implications

For thin-walled austenitic stainless steel pipe welding in power generation and process industries, the findings carry significant practical value. The optimal parameter window of 20 A base / 40 A peak / 0.3 Hz frequency provides a reproducible process recipe that engineers can directly apply to similar pipe geometries. The reduced heat input strategy minimizes HAZ grain coarsening, which is particularly important for pipes operating in high-temperature or corrosive environments where grain boundary corrosion susceptibility must be controlled. The worm-like δ-ferrite morphology, while potentially raising concerns about intergranular corrosion in certain service conditions, also provides beneficial crack resistance during solidification. Engineers should note that the δ-ferrite content should be verified against service-specific requirements, particularly for chloride-containing environments where 430-type ferrite corrosion may be a concern.

Key Reflections and Study Insights

The most valuable insight from this study is the demonstration that pulse TIG welding offers a powerful means to independently control the thermal cycle parameters of austenitic stainless steel welds. Unlike conventional TIG welding where heat input is primarily governed by a single current setting, P-TIG allows decoupling of peak energy delivery from average heat input through the combination of base current, peak current, and frequency. This decoupling is the key to achieving both fine grain refinement in the HAZ and controlled δ-ferrite formation in the weld zone simultaneously. The study also highlights that for thin-walled pipes, the lower current settings are not merely conservative choices but are actively beneficial for microstructure control. In engineering practice, this means that welders should not assume that higher currents always produce stronger welds; for thin sections, the optimal approach may well be to reduce energy input and rely on pulse modulation to maintain adequate penetration while preserving microstructural integrity.