Microstructure and Properties of Stainless Steel Wear-Resistant Overlay Layer on 35 Steel

Literature Overview

Published in 2013 in the journal of Hot Working Technology, this paper by Li Ke, Wu Zhisheng, Liu Cuirong, and Song Xu from the School of Materials at Taiyuan University of Science and Technology investigates the microstructure and properties of a stainless steel overlay layer deposited on 35 steel (a medium carbon steel). Supported by the Shanxi Province Science and Technology Key Project (20100321084), the Taiyuan University of Science and Technology Young Fund (20113001), and other institutional grants, this work addresses a practical and common engineering need: providing corrosion and wear resistance to carbon steel components through stainless steel overlay cladding.

Technical Background and Application Context

35 steel is a widely used medium carbon structural steel with good mechanical properties and weldability, commonly employed in shafts, gears, bolts, and structural components. However, when exposed to corrosive environments or abrasive wear, 35 steel degrades rapidly due to its lack of corrosion resistance and relatively low hardness. Stainless steel overlay cladding offers an economical solution by depositing a thin layer of corrosion-resistant and wear-resistant stainless steel onto the carbon steel substrate, combining the strength of the base material with the surface properties of the overlay.

This approach is particularly relevant in industries such as chemical processing, food processing, marine engineering, and environmental protection, where carbon steel components are exposed to corrosive media and mechanical wear simultaneously. The overlay layer must therefore provide both corrosion resistance and wear resistance, which imposes specific requirements on the microstructure and composition of the deposited layer.

Overlay System and Welding Parameters

The dilution ratio is the critical parameter in this system. Excessive dilution introduces carbon and manganese from the 35 steel into the overlay, reducing the chromium equivalent and potentially forming a ferritic or martensitic microstructure instead of the desired austenitic structure. This can significantly degrade both corrosion resistance and wear resistance.

Microstructural Analysis

Overlay Layer Microstructure

The overlay layer microstructure was examined at different depths from the top surface to the dilution interface. At the top of the overlay, a fully austenitic microstructure was observed, consisting of equiaxed austenite grains with dispersed carbides (primarily Cr23C6). This region exhibited the highest corrosion resistance and good wear resistance.

Moving toward the dilution interface, the microstructure transitioned through a region of mixed austenite and ferrite, to a predominantly ferritic or martensitic structure at the very bottom of the overlay. This transition is a direct consequence of the increasing dilution with the carbon-rich 35 steel base metal. The ferrite and martensite regions have lower corrosion resistance and may exhibit different wear behavior compared to the austenitic region.

Dilution Zone Characteristics

The dilution zone, typically 0.5-1.5 mm thick, was found to be the most critical region for overall performance. The authors observed that the dilution zone exhibited a gradient in carbon content from approximately 0.05% at the top of the dilution zone to 0.35% at the base metal interface. This carbon gradient directly influenced the phase composition and properties.

The hardness distribution across the overlay and dilution zone showed a characteristic profile: highest hardness in the dilution zone (due to martensite formation from carbon enrichment), moderate hardness in the upper overlay (austenitic matrix with carbides), and lower hardness in the base metal. This hardness mismatch can be a source of stress concentration and potential failure initiation.

Wear Performance

Abrasive wear tests were conducted using a pin-on-disk apparatus with alumina ceramic balls as counterfaces. The stainless steel overlay exhibited lower wear resistance compared to the hardened base metal (quenched and tempered 35 steel), which is expected given the lower hardness of austenitic stainless steel. However, the overlay provided significantly better wear resistance than the as-received (annealed) 35 steel.

The wear mechanism of the austenitic overlay was predominantly abrasive ploughing, with the carbide particles providing some resistance to material removal. The wear rate increased with increasing load, consistent with general abrasive wear behavior. The presence of Cr23C6 carbides in the austenitic matrix contributed to wear resistance by acting as hard obstacles to abrasive particle ploughing.

Corrosion Performance

Electrochemical corrosion tests were conducted in 3.5% NaCl solution to evaluate the corrosion resistance of the overlay layer. The top region of the overlay with a fully austenitic microstructure exhibited excellent corrosion resistance with a corrosion potential shifted positively and a low corrosion current density. The dilution zone, with its mixed austenite-ferrite-martensite microstructure, showed reduced corrosion resistance due to the presence of corrosion-sensitive phases such as martensite and ferrite.

The galvanic couple between the stainless steel overlay and the 35 steel base metal was also evaluated. The potential difference between the two materials was found to be within acceptable limits, and no significant galvanic corrosion was observed at the interface under normal exposure conditions. However, the authors noted that in aggressive chloride environments, the dilution zone could be susceptible to intergranular corrosion if sensitization occurred during welding.

Quality Control and Defect Prevention

The study identified several quality control measures essential for achieving consistent overlay performance: (1) pre-heating the base metal to 150-200°C to reduce hydrogen-induced cracking risk; (2) using low-hydrogen welding consumables and controlling interpass temperature below 250°C; (3) ensuring adequate heat input to promote full austenitization of the overlay; and (4) performing visual and magnetic particle inspection of the overlay surface for cracks and lack of fusion.

The FMEA (Failure Mode and Effects Analysis) approach was applied to identify critical failure modes: dilution-related corrosion failure (severity 8, occurrence 5, detection 4, RPN 160), intergranular corrosion at the dilution zone (severity 9, occurrence 3, detection 5, RPN 135), and overlay cracking due to hydrogen (severity 8, occurrence 4, detection 3, RPN 96). The highest RPN values indicated that dilution control and sensitization prevention were the most critical quality factors.

Engineering Practice Implications

This study provides practical guidance for engineers applying stainless steel overlay to carbon steel components in corrosive and mildly abrasive environments. The key recommendations are: (1) control the dilution ratio below 20% to maintain acceptable corrosion resistance; (2) consider post-weld heat treatment to relieve residual stresses and reduce cracking risk; (3) design the overlay thickness to ensure that the fully austenitic region is thick enough to withstand the expected wear before reaching the dilution zone; and (4) perform corrosion testing on the as-welded overlay to verify that the specific welding parameters used in production yield the required corrosion performance.

Study Insights and Reflections

This paper, while addressing a seemingly straightforward application, reveals the complexity inherent in overlay design. The challenge is not merely depositing stainless steel onto carbon steel but ensuring that the dilution zone does not become the weakest link in the system. The finding that the dilution zone exhibits the highest hardness but potentially the lowest corrosion resistance highlights the inherent trade-off in overlay design between wear and corrosion performance. For engineers working on chemical equipment, food processing equipment, or marine components, this study reinforces the principle that overlay design must be driven by the specific service environment, and that standard overlay specifications may not be suitable for all applications. The systematic approach to quality control, incorporating FMEA and non-destructive testing, provides a practical framework that can be directly applied to production environments.

Concluding Remarks

These five studies collectively illustrate the breadth and depth of knowledge required in the field of weld overlay and cladding technology. From the composition optimization of PTA coatings for wear resistance to the thermal fatigue behavior of overlay layers on hot work dies, from the historical foundations of hot rolling mill roll overlay to the load-dependent wear mechanisms of niobium-containing alloys and the practical challenges of stainless steel overlay on carbon steel, each paper contributes a unique perspective to the engineering challenge of surface protection. The common thread running through all five studies is the recognition that overlay performance is governed by the intricate interplay between composition, microstructure, welding parameters, and service conditions. Engineers who master these interrelationships will be best positioned to select, design, and implement overlay solutions that deliver reliable, long-term performance in demanding industrial applications.