Microstructure and Hardness of Stainless Steel Surface Overlay Weld Joints on Q235A Steel

Literature Overview

Wang Yan, from the School of Mechanical and Power Engineering at China Three Gorges University, published a study in Guangdong Chemical Industry (Volume 45, Issue 24, 2018, pages 81-83) examining the microstructure and hardness characteristics of overlay weld joints produced using A102 stainless steel electrodes on Q235A carbon steel substrate. The study employed SMAW (Shielded Metal Arc Welding) under different welding process parameters and investigated the effects of post-weld heat treatment (PWHT) on the microstructure and hardness distribution across the weld joint. This work addresses a common industrial requirement: providing corrosion-resistant surface protection on carbon steel components through overlay welding.

Welding Process and Materials

The A102 electrode is an E308L-type stainless steel electrode with a low-carbon (≤0.03% C) austenitic composition, typically containing approximately 18-20% Cr and 8-10% Ni. This composition is selected to produce weld metal with excellent resistance to intergranular corrosion, making it suitable for overlay applications where the weld metal must resist corrosion in the presence of the carbon steel substrate. The Q235A steel is a standard Chinese carbon steel grade equivalent to mild steel with a yield strength of approximately 235 MPa and a typical composition of ≤0.22% C, ≤0.60% Mn, and trace amounts of Si, S, and P.

Microstructural Analysis

The as-welded weld metal exhibited a dendritic austenitic microstructure, which is characteristic of rapidly solidified stainless steel welds. The dendritic morphology results from the high cooling rates typical of SMAW welding, where the solidification front grows in a columnar and then equiaxed dendritic pattern as the cooling rate decreases away from the fusion line. The grain size in both the weld metal and the overheated heat-affected zone (HAZ) was found to decrease with decreasing welding current, which is consistent with the fundamental relationship between heat input and grain growth.

The HAZ of the Q235A substrate underwent significant microstructural changes due to the thermal cycle of welding. The overheated zone, where temperatures exceeded approximately 1000°C, experienced austenite grain growth, while the intercritical zone (approximately 727-900°C) underwent partial austenitization and subsequent transformation. The weld fusion line represents a region of maximum dilution, where the carbon steel substrate and the austenitic stainless steel fill metal mix, potentially producing a mixed ferrite-austenite microstructure depending on the dilution ratio.

After post-weld heat treatment, the weld metal retained its austenitic character but exhibited a more uniform distribution of phases. The PWHT likely promoted homogenization of the microstructure by allowing diffusion of alloying elements and dissolution of any segregated phases that formed during rapid solidification. The heat treatment significantly improved the hardness of both the base metal and the HAZ, which is somewhat counterintuitive and warrants careful interpretation.

Hardness Distribution and PWHT Effects

The hardness distribution across the weld joint showed that the weld metal consistently exhibited the highest hardness both before and after PWHT. This is expected because the weld metal is a high-alloy austenitic stainless steel with solid solution strengthening from Cr and Ni, while the Q235A base metal is a relatively soft carbon steel. The hardness differential between the weld metal and base metal can range from approximately 200-250 HV for the weld to 120-150 HV for the base metal in the as-welded condition.

The observation that PWHT significantly increased the hardness of the base metal and HAZ is an important finding. This could be attributed to several mechanisms: (1) precipitation of carbides in the HAZ due to the carbon content of the Q235A steel migrating toward the weld metal during the heat treatment cycle; (2) formation of martensite in regions of the HAZ that experienced sufficient austenitization followed by slow cooling during PWHT; or (3) strain aging effects. The increase in HAZ hardness is a concern from a structural integrity perspective, as it may indicate the formation of brittle phases that could reduce the toughness of the joint.

Engineering Practice and Standards Considerations

From an engineering practice standpoint, overlay welding of stainless steel on carbon steel is a widely used technique for providing corrosion resistance to carbon steel components that are exposed to corrosive environments. Common applications include chemical processing equipment, marine structures, and pipeline components in sour service. The selection of A102 (E308L) electrodes is appropriate for this application because the low carbon content minimizes the risk of intergranular corrosion in the weld metal.

However, several quality concerns must be addressed in practice. The high dilution rate typical of SMAW overlay welding (often 30-50% for single-pass overlays) can significantly alter the composition of the weld metal, potentially reducing its corrosion resistance. Multiple overlay passes may be necessary to achieve a weld metal composition that meets the required corrosion resistance specifications. According to ASME B31.3 and API 5L, overlay welds in service must be qualified through appropriate weld procedure qualification and qualification testing.

The PWHT requirements for overlay welds should follow the applicable code requirements. For carbon steel base materials, PWHT is typically required when the base metal thickness exceeds specified limits (e.g., 19 mm for carbon steel per ASME B31.3). The heat treatment temperature and duration must be carefully controlled to avoid sensitization of the austenitic stainless steel overlay, which can occur at temperatures between 450-850°C where chromium carbides precipitate at grain boundaries.

Key Questions and Reflections

This study raises several important questions for practicing engineers. First, the PWHT temperature and duration are not explicitly stated in the abstract, which limits the ability to fully evaluate the heat treatment effects. The choice of PWHT parameters is critical: temperatures too high may cause sensitization of the stainless steel overlay, while temperatures too low may not adequately relieve residual stresses. A typical PWHT for this type of joint might involve heating to 600-650°C for a duration proportional to the thickness, but this must be balanced against the sensitization risk for the austenitic overlay.

Second, the study focuses primarily on microstructure and hardness but does not address other critical properties such as toughness, corrosion resistance, or residual stress. In practical applications, the toughness of the weld joint is often the limiting factor, particularly in low-temperature service or cyclic loading applications. The formation of brittle phases in the HAZ due to PWHT could significantly reduce the fracture toughness of the joint.

Third, the study does not discuss the residual stress state of the weld joint. SMAW welding produces significant residual stresses, particularly in the transverse direction, which can be detrimental to the fatigue life and dimensional stability of the component. Post-weld stress relief is important but must be carefully controlled to avoid adverse effects on the overlay layer.

The practical value of this research lies in its demonstration that A102 stainless steel overlay welding on Q235A steel produces joints with acceptable microstructural characteristics and hardness profiles, both in the as-welded and post-weld heat treated conditions. Engineers should use this information as a starting point for developing qualified welding procedures, supplementing with additional testing for toughness, corrosion resistance, and residual stress as required by the applicable design codes.