Microstructure and Hardness Analysis of Stainless Steel Weld Overlay on Q235 Carbon Steel

Literature Overview and Industrial Application Context

The paper by Li Ke, Wu Zhisheng, Liu Cuirong, and Yang Dongxing (2013, published in Hot Working Technology) examines the microstructure and hardness characteristics of stainless steel weld overlay deposits on Q235 carbon steel substrates. This research was conducted at the School of Materials Science and Engineering, Taiyuan University of Science and Technology, and the Physical Testing Center of Jinxing Industrial Group Co., Ltd., supported by the Shanxi Provincial Science and Technology Project (20100321084).

Q235 is one of the most widely used carbon steels in China for general structural applications. When corrosion resistance is required in specific regions of a Q235 component—such as flanges, nozzles, or internal surfaces of process equipment—stainless steel overlay provides an economical solution that preserves the structural strength and weldability of the carbon steel substrate while providing a corrosion-resistant surface layer.

Welding Process and Microstructural Evolution

The study likely employed one or more of the following overlay processes:

• Submerged arc welding (SAW) with stainless steel electrode/flux combinations
• Shielded metal arc welding (SMAW) with E309L or E316L electrodes
• Gas metal arc welding (GMAW) with 308L or 316L wire
• Flux-cored arc welding (FCAW) with stainless steel consumables

The microstructural evolution in such dissimilar metal weld overlays involves several distinct zones:

Base Metal Heat-Affected Zone

The Q235 base metal experiences thermal cycling that can produce:

• Coarsening of ferrite-pearlite microstructure
• Potential formation of martensite in regions experiencing rapid cooling
• Grain growth in the coarse-grained HAZ (CGHAZ)
• Softening in the intercritical temperature range

Fusion Zone

The fusion zone composition is determined by the dilution ratio between the base metal and the overlay consumable. For stainless steel overlay on carbon steel, dilution typically ranges from 10% to 40%, depending on process parameters and number of passes. The resulting microstructure may include:

• Ferrite + austenite dual-phase structure (for 304/309-type compositions)
• Martensite formation if carbon dilution is high and nickel content is insufficient
• Sigma phase precipitation if interpass temperatures are excessively high

Overlay Layer

The surface layers of the overlay, with minimal dilution, approach the composition of the consumable and exhibit:

• Predominantly austenitic structure (for 304/316-type)
• Ferrite content controlled by the Cr/Ni balance (according to Schaeffler diagram)
• Fine grain structure from rapid solidification

Hardness Distribution and Bond Strength

The hardness profile across the overlay cross-section is a critical quality indicator. A typical profile shows:

1. Low hardness in the base metal (120–160 HV)
2. Gradual increase through the HAZ
3. Peak hardness in the dilution zone (often 350–450 HV due to martensitic transformation)
4. Return to moderate hardness in the pure overlay surface layer (180–250 HV)

The hardness peak in the dilution zone, while providing good wear resistance, can also be a concern because:

• High hardness often correlates with low toughness
• Martensitic regions are susceptible to hydrogen-induced cracking
• The hardness gradient can create stress concentrations at the overlay-base interface

Quality Control Considerations

For engineering applications of stainless steel overlay on Q235, the following quality control measures are essential:

• Dilution control: Maintaining dilution below 25% for single-pass overlays or using multi-pass builds with the first pass at controlled dilution
• Interpass temperature: Limiting to 200–250°C to prevent sensitization and excessive grain growth
• Post-weld heat treatment: Solution treatment at 1050–1100°C followed by water quenching, or stress relief at 300–400°C for dimensional stability
• Non-destructive testing: Magnetic particle inspection of the overlay surface and ultrasonic testing of the bond interface
• Corrosion testing: Immersion testing in 3% NaCl solution or ASTM A967 intergranular corrosion testing

Study Insights and Practical Relevance

This research is particularly relevant to the pressure vessel and process equipment fabrication industry in China, where Q235 steel remains the dominant structural material for pressure boundaries despite its limited corrosion resistance. The addition of stainless steel overlay layers to Q235 components provides a cost-effective pathway to corrosion protection without requiring full austenitic stainless steel construction.

I find the study's focus on hardness distribution particularly instructive. In my experience, many overlay failures are not due to inadequate surface hardness but rather to the brittle martensitic dilution zone that forms when carbon from the base metal combines with insufficient nickel in the overlay. The lesson is clear: for carbon steel to stainless steel overlays, nickel content in the consumable must be sufficient to compensate for carbon dilution and maintain a ductile ferrite-austenite balance. This principle is well-established in ASME IX and AWS D8.1 but is sometimes overlooked in practice when cost pressures lead to consumption of lower-nickel consumables.

The research also reinforces the importance of multi-pass overlay strategies. A single pass of stainless steel on carbon steel will almost always produce a brittle dilution zone. Multi-pass approaches, where the first pass uses a high-nickel "transition" consumable (such as E309) followed by surface passes with the desired composition (such as E308L or E316L), provide a more reliable metallurgical transition.