Microstructure and Properties of Stainless Steel Overlay on 35 Steel

Literature Overview

This study note examines the microstructural evolution and mechanical performance of stainless steel weld overlay deposits applied onto 35 steel (a medium-carbon structural steel with approximately 0.35 wt% carbon). The core objective of the research is to understand how the overlay process affects the bond line integrity, the dilution behavior at the interface, and the resulting wear resistance and corrosion resistance of the composite structure. The work is particularly relevant to engineers designing components that require a tough carbon steel substrate combined with a corrosion-resistant or wear-resistant surface layer, such as pump casings, valve bodies, and chemical processing equipment.

Core Technical Findings

The microstructure of the overlay deposits is governed by the cooling rate, the composition of the welding consumable, and the number of overlay passes. A single-pass overlay typically produces a columnar dendritic structure with significant dilution from the 35 steel substrate, often resulting in a dilution ratio exceeding 25 percent. This high dilution can compromise the austenitic stability of the overlay layer, introducing ferrite or martensite phases that reduce corrosion resistance. Multi-pass overlay, however, progressively reduces dilution because each subsequent pass dilutes the previously deposited material rather than the substrate. After three to four passes, the dilution from the base metal typically drops below 10 percent, and the overlay microstructure approaches that of the intended stainless steel composition.

The bond line between the 35 steel substrate and the stainless steel overlay is the critical region for mechanical integrity. Microstructural analysis reveals that the transition zone often contains a mixture of ferrite, martensite, and austenite phases depending on the carbon and alloy element partitioning. The hardness gradient across the bond line is a key indicator of potential cracking susceptibility, particularly under cyclic loading or thermal cycling conditions. The study demonstrates that controlling the interpass temperature between 150 and 250 degrees Celsius is essential for minimizing residual stress and preventing interpass cracking in the overlay layers.

Process Considerations and Defect Analysis

The choice of welding process significantly influences the overlay quality. Submerged arc welding (SAW) offers high deposition rates but produces wide, shallow welds with greater dilution. Gas tungsten arc welding (GTAW) provides precise control over heat input and dilution but is limited in deposition rate. Gas metal arc welding (GMAW) with a stainless steel electrode such as ER308 or ER316L provides a good balance of deposition rate and dilution control for most industrial applications.

Common defects observed in stainless steel overlay on 35 steel include hot cracking in the overlay layer due to sulfur and phosphor segregation, cold cracking at the bond line caused by hydrogen diffusion from the substrate, and undercut at the overlay edges which creates stress concentration sites. The study recommends using low-hydrogen fluxes and preheating the 35 steel to 100-150 degrees Celsius to mitigate hydrogen-induced cracking. Post-weld heat treatment at 700-750 degrees Celsius for stress relief is also beneficial for improving the toughness of the bond line region.

Engineering Practice Implications

In engineering practice, the findings of this study directly inform the design of overlay specifications for pressure vessels and chemical equipment. When applying stainless steel overlay to 35 steel components, engineers should specify a minimum of three overlay passes to ensure adequate dilution control. The consumable selection should consider the service environment; ER308L or ER316L electrodes are preferred for their low carbon content which reduces sensitization risk. For applications requiring higher wear resistance, a two-layer approach with a transition layer of 309L followed by a 316L top layer is recommended. This layered approach creates a compositional gradient that accommodates the thermal expansion mismatch between the carbon steel substrate and the austenitic overlay, reducing the risk of spalling under thermal cycling.

The study also highlights the importance of non-destructive testing at the bond line. Magnetic particle testing (MT) is effective for detecting surface and near-surface cracks, while ultrasonic testing (UT) with contact probes can identify delamination at the substrate-overlay interface. Engineers should incorporate these inspection requirements into fabrication procedures to ensure long-term reliability of overlay-clad components.

Summary and Reflections

The research on stainless steel overlay on 35 steel provides valuable insights into the microstructural control and mechanical performance optimization of bimetallic composite structures. The key takeaway for practitioners is that dilution management through multi-pass welding, careful control of interpass temperatures, and appropriate consumable selection are the three pillars of achieving a high-quality overlay. The bond line remains the weakest link in the system, and its integrity must be verified through both destructive and non-destructive testing methods. Understanding the phase transformations at the interface enables engineers to predict and prevent failure modes in service, ultimately extending the operational life of critical components in corrosive and abrasive environments.