Microstructure and Hardness Analysis of Q235 Steel Stainless Steel Surfacing Joint

Literature Overview

This 2013 paper in Hot Working Technology by Li Ke, Wu Zhisheng, Liu Cuirong, and Yang Dongxing from Taiyuan University of Science and Technology and Jinxi Industrial Group investigates the microstructure and hardness distribution of a surfacing joint produced by automatic submerged arc welding of 3Cr13 martensitic stainless steel onto Q235 carbon steel substrates. The research was supported by Shanxi Provincial Science and Technology Project (20100321084) and Taiyuan University of Science and Technology Youth Fund (20113001).

Technical Background and Significance

The combination of carbon steel substrates with martensitic stainless steel surfacing layers represents a common engineering challenge in pipeline and equipment repair. The fundamental metallurgical incompatibility between the two materials creates several challenges:

• Dilution: The carbon steel substrate dilutes the surfacing metal, reducing the chromium and carbon content below levels required for martensitic transformation and corrosion resistance.
• Microstructural incompatibility: The ferritic structure of Q235 steel is fundamentally different from the martensitic structure of 3Cr13 stainless steel, creating a sharp interface with potential for cracking.
• Thermal expansion mismatch: The coefficient of thermal expansion differs between the two materials, generating residual stresses during cooling.
• Carbon migration: Carbon can diffuse from the carbon steel into the stainless steel during high-temperature exposure, potentially causing intergranular corrosion in the heat-affected zone.

Welding Process Parameters

The automatic submerged arc welding process was selected for this application because:

• High deposition rate: Suitable for building up thick surfacing layers efficiently.
• Excellent slag protection: The flux provides comprehensive shielding, minimizing atmospheric contamination.
• Deep penetration: Effective fusion with the carbon steel substrate ensures good joint integrity.
• Repeatability: Automated process ensures consistent weld quality across multiple passes.

Typical process parameters for this type of surfacing include:

Microstructural Analysis

The joint microstructure exhibits a characteristic gradient from substrate to surfacing layer:

Substrate Region (Q235 Steel)

• Average hardness: 132 HV
• Microstructure: Ferrite-pearlite with potential grain growth in the heat-affected zone
• The heat-affected zone may show a tempered martensite-like structure due to carbon diffusion and rapid cooling

Interface Region

• Transition zone where substrate dilution is most significant
• Possible formation of mixed microstructure with both ferritic and martensitic constituents
• This region is most susceptible to cracking due to the combination of high hardness differential and residual stress concentration

Surfacing Layer

• Average hardness: 432 HV
• Maximum hardness: 453 HV (located in the second layer)
• Microstructure: Martensite with possible retained austenite and carbide precipitates
• The hardness distribution within the surfacing layer reflects the dilution gradient, with the first layer showing lower hardness due to higher substrate dilution

Hardness Distribution Analysis

The observation that maximum hardness occurs in the second layer rather than the uppermost layers is a critical finding. This indicates that:

1. The first layer experiences excessive dilution, preventing full martensitic transformation.
2. The second layer achieves an optimal dilution level where sufficient carbon and chromium remain for martensitic transformation while maintaining adequate toughness.
3. Upper layers, while having lower dilution, may experience slightly reduced hardness due to retained austenite formation or carbide coarsening from inter-pass reheating.

Engineering Considerations for Pipeline Applications

For pipeline and equipment repair applications, the following considerations are essential:

Crack Prevention Strategies

• Preheating: 200-300°C preheat reduces thermal gradients and hydrogen-induced cracking risk.
• Inter-pass temperature control: Maintained between 250-350°C to avoid excessive grain growth while minimizing residual stress.
• Post-weld heat treatment: Stress relief at 550-650°C for 1-2 hours per 25 mm thickness.
• Layer thickness control: Individual pass thickness limited to 3-5 mm to minimize residual stress per pass.

Performance Requirements

• Wear resistance: The 432 HV average hardness provides excellent resistance to abrasive and erosive wear.
• Corrosion resistance: 3Cr13 offers moderate corrosion resistance in oxidizing environments but requires proper heat treatment for optimal performance.
• Toughness: The hardness-toughness balance must be evaluated for impact loading applications.

Quality Control Requirements

• Visual inspection: Each pass inspected for surface defects, undercut, and porosity.
• Hardness testing: Vickers hardness survey across the joint cross-section to verify microstructural integrity.
• Metallographic examination: Transverse section analysis to verify dilution levels and microstructure distribution.
• Non-destructive testing: UT or MT for subsurface crack detection, particularly at the interface region.

Study Insights and Practical Recommendations

The research demonstrates that the hardness distribution in multi-pass surfacing joints follows a predictable pattern governed by dilution behavior. The first layer always experiences the highest dilution and consequently the lowest hardness, while subsequent layers progressively approach the as-deposited composition. This understanding enables process optimization:

1. Strategic first pass: Consider using a transition filler material for the first pass to bridge the metallurgical gap between substrate and final surfacing alloy.
2. Layer sequencing: The second layer often represents the optimal performance layer, suggesting that the total surfacing thickness should be designed to position this layer at the critical wear surface.
3. Dilution management: For critical applications, consider using a consumable insert or pre-deposited alloy layer to reduce substrate dilution of subsequent passes.

The hardness differential of approximately 300 HV between substrate and surfacing layer creates a significant stress concentration at the interface. For cyclic loading applications, this interface represents a potential fatigue crack initiation site. Engineering practice should incorporate fatigue analysis and potentially include a gradual hardness transition zone through strategic filler material selection and process parameter control. The combination of high hardness surfacing with the relatively soft substrate creates a composite structure that can be highly effective for wear resistance, provided the interface integrity is maintained through proper process control and quality assurance.