Microstructure and Properties of 310 Stainless Steel Overlay on Q235 Carbon Steel

Literature Overview

This study investigates the microstructural evolution, mechanical properties, and corrosion behavior of a 310 stainless steel (UNS S31008) overlay deposited on Q235 carbon steel substrate. The research addresses a common industrial scenario where a low-cost carbon steel component requires localized corrosion resistance enhancement without the expense of using austenitic stainless steel throughout the entire component. The authors systematically examine how dilution, carbon diffusion, and welding parameters influence the overlay's phase composition, hardness, and resistance to intergranular corrosion.

Core Technical Points

Material Characterization

Q235 is a plain carbon structural steel with a minimum yield strength of 235 MPa and a carbon content of approximately 0.12–0.20%. Its microstructure consists of ferrite and pearlite with a typical hardness of 120–150 HBW. The low carbon content and absence of alloying elements make Q235 highly susceptible to rapid cooling-induced hardening during welding, but also means that dilution into the overlay primarily introduces carbon and manganese rather than beneficial alloying elements.

310 stainless steel is a high-temperature austenitic alloy containing 24–26% Cr and 19–22% Ni. Its defining characteristic is exceptional resistance to oxidation and scaling at temperatures up to 1150°C. The high Cr and Ni content stabilizes the austenitic phase, and the alloy exhibits excellent resistance to intergranular corrosion even without stabilizing additions because the high nickel content promotes chromium redistribution after sensitization.

Welding Process and Parameters

The overlay was deposited using gas tungsten arc welding (GTAW/TIG) with a 310L stainless steel electrode (lower carbon variant to reduce sensitization risk).

The heat input range of 0.9–1.5 kJ/mm is moderate, balancing the need for adequate penetration to achieve metallurgical bonding with the requirement to limit dilution. The preheat temperature of 100–150°C is relatively low because Q235 has a low carbon equivalent and is not prone to hydrogen-induced cracking.

Dilution and Phase Evolution

Dilution is the primary factor governing the overlay's microstructure and properties. The study characterizes dilution by measuring the Cr and Ni content at various depths below the overlay surface using optical emission spectroscopy (OES).

The dilution gradient demonstrates that the first 0.6 mm of the overlay maintains Cr and Ni content above 22% and 17.5% respectively, which is sufficient for full austenitic stability and good corrosion resistance. Beyond 0.9 mm, the dilution exceeds 20%, and the microstructure begins to transition from austenite to a mixture of austenite and ferrite, with increasing amounts of martensite appearing near the interface.

Microstructural Analysis

Overlay surface (low dilution zone): The microstructure is fully austenitic with a dendritic growth pattern. The dendrite arm spacing is approximately 15–25 μm. No carbide precipitation is observed at the grain boundaries because the low carbon content of 310L and the high nickel content prevent sensitization. The grain size is fine (ASTM 8–10) due to the rapid cooling rate from the welding thermal cycle.

Intermediate zone (moderate dilution): The microstructure transitions to a duplex of austenite (γ) and delta ferrite (δ). The ferrite fraction increases with depth, reaching approximately 15–25% at 0.9 mm. The ferrite forms preferentially at the dendrite boundaries because the local chromium enrichment from dilution promotes ferrite stabilization. This ferrite is beneficial for crack resistance but detrimental to corrosion resistance if it exceeds 10%.

Interface zone (high dilution): Near the fusion line, the microstructure is dominated by martensite (α') with some retained austenite (γ). The high carbon content from the Q235 base metal, combined with the chromium enrichment from the overlay, produces a high-hardness martensitic phase. The hardness in this zone reaches 380–420 HV, which is approximately 2.5 times the base metal hardness. This hard zone is susceptible to cracking under thermal cycling or mechanical loading.

Carbon Diffusion

One of the most significant findings is the observation of a carbon-enriched band at the overlay-base interface. During welding, carbon from the Q235 base diffuses into the molten overlay pool, and during subsequent cooling, carbon redistributes between the phases. The resulting carbon profile shows:

• Carbon peak of 0.35–0.50 wt% at the interface
• Carbon content decreasing to 0.10–0.15 wt% at 0.5 mm into the overlay
• Carbon content returning to 0.12–0.15 wt% in the base metal beyond the HAZ

This carbon enrichment at the interface is responsible for the formation of martensite and the associated high hardness. It also creates a zone of potential cracking susceptibility during thermal cycling.

Corrosion Behavior and Performance

Intergranular Corrosion Resistance

The overlay was tested for intergranular corrosion using ASTM A262 Practice E (65°C oxalic acid test) and Practice B (65°C CuSO4/H2SO4 test).

The results confirm that the overlay maintains acceptable intergranular corrosion resistance up to a depth of 0.6 mm, corresponding to a dilution of approximately 12%. Beyond this depth, the combination of carbon enrichment and chromium depletion at grain boundaries leads to sensitization and intergranular attack.

Corrosion Rate Measurement

The corrosion rate was measured using potentiodynamic polarization in 3.5% NaCl solution at room temperature.

The surface of the overlay exhibits a corrosion current density of 0.5 μA/cm², which is approximately 90 times lower than the Q235 base metal. Even at 0.6 mm depth, where dilution is 12%, the corrosion rate is still 20 times lower than the base. This confirms that the overlay provides effective corrosion protection within the functional surface zone.

Key Reflections and Study Insights

The study provides a clear demonstration that the performance of a weld overlay is governed by the dilution profile, which in turn is controlled by welding parameters, number of passes, and post-weld treatment. The practical implication is that overlay design must be approached as a dilution management problem, with the overlay thickness and pass sequence optimized to ensure that the functional surface zone maintains the required alloy composition.

The observation of carbon enrichment at the interface is particularly important for engineering practice. It means that the interface zone, despite being only 0.2–0.5 mm thick, can be a critical location for cracking and corrosion initiation. In applications where the overlay is subjected to thermal cycling or mechanical fatigue, this interface zone requires special attention.

The solution annealing treatment at 1050°C × 1 h is effective for dissolving any sensitized carbides and restoring chromium to grain boundaries. However, this treatment is only practical for small components or when the overlay is applied to a component that can withstand such a high temperature. For large components or when the base metal cannot tolerate solution annealing, the dilution control strategy becomes even more critical.

From a standards perspective, the study highlights the importance of verifying overlay composition and properties according to ASTM A263/A263M for weld overlay cladding plate. The requirement for a minimum chromium content and a maximum ferrite content in the overlay must be verified through chemical analysis and ferrite number measurement.