Microstructure and Hardness Analysis of Q235 Steel Surfacing Stainless Steel Joint

Literature Overview

This study by Li Ke, Wu Zhisheng, Liu Cuirong, and Yang Dongxing from Taiyuan University of Science and Technology and Jinci Industrial Group Co., Ltd. was published in "Hot Working Technology" in 2013 (Volume 42, Issue 17, pages 155-156). Funded by Shanxi Provincial Science and Technology Key Project, Taiyuan University Youth Fund, and Graduate Student Innovation Project, the research investigates the microstructure and hardness distribution in a surfacing joint produced by automatic submerged arc welding of 3Cr13 stainless steel onto Q235 carbon steel. The work addresses a common engineering practice of applying corrosion-resistant or wear-resistant stainless steel overlays on carbon steel substrates for enhanced service performance.

Technical Approach and Process Description

The researchers employed automatic submerged arc welding (SAW) to deposit 3Cr13 martensitic stainless steel onto Q235 carbon steel test plates. Submerged arc welding is a highly productive process with deep penetration, excellent weld quality, and good process repeatability, making it well-suited for surfacing applications where consistent deposit properties are required.

3Cr13 is a martensitic stainless steel with approximately 13% chromium content. When properly heat-treated, it achieves high hardness and good corrosion resistance, making it suitable for applications requiring both wear resistance and moderate corrosion resistance. The automatic SAW process ensures consistent welding parameters and reproducible deposit properties, which is advantageous for systematic microstructural study.

The study focuses on two primary characterization methods: metallographic analysis of the joint microstructure and microhardness testing across the joint. These methods provide complementary information about the relationship between microstructure and mechanical properties in the surfacing joint.

Key Findings: Hardness Distribution and Microstructure

The study reveals a clear correlation between hardness distribution and microstructural changes across the surfacing joint. The hardness values demonstrate a significant gradient from the substrate through the heat-affected zone to the surfacing deposit.

The most striking finding is the large difference between the substrate hardness (132 HV) and the surfacing deposit hardness (432 HV average, 453 HV maximum). This represents more than a threefold increase in hardness, which directly translates to enhanced wear resistance and potentially improved corrosion resistance in the surface region.

The maximum hardness occurring in the second layer of the surfacing deposit is an important observation. This suggests that the first layer may experience higher dilution from the Q235 substrate, resulting in a slightly lower chromium content and consequently lower hardness. The second layer, being deposited on the first layer of stainless steel, experiences less dilution and achieves a composition closer to the intended 3Cr13 composition, resulting in higher hardness.

Microstructural Analysis and Phase Evolution

The microstructural evolution across the joint follows the expected pattern for a dissimilar metal surfacing joint:

• The Q235 substrate retains its original ferrite-pearlite structure, with minimal microstructural change away from the weld interface.
• The heat-affected zone in the Q235 substrate shows evidence of thermal effects, including possible grain growth and transformation to finer pearlite or partial martensite formation depending on the cooling rate.
• The first surfacing layer shows a microstructure influenced by dilution from the carbon steel substrate, with a chromium content intermediate between 3Cr13 and Q235.
• The second and subsequent surfacing layers approach the full martensitic structure of 3Cr13 stainless steel, with fine carbide precipitation.

The hardness distribution corresponds directly to these microstructural changes. The ferrite-pearlite structure of Q235 provides relatively low hardness, while the martensitic structure of the stainless steel deposit provides significantly higher hardness. The transition zone between these two structures represents a region of property gradient that is critical for joint integrity.

Engineering Significance for Pipeline and Component Applications

The surfacing of stainless steel onto carbon steel is a widely used technique in pipeline and pressure vessel engineering for several important reasons:

1. Corrosion protection: The stainless steel overlay provides a corrosion-resistant barrier while maintaining the structural integrity of the carbon steel substrate.
2. Wear resistance: The high hardness of the martensitic stainless steel deposit provides excellent resistance to abrasive and erosive wear.
3. Cost effectiveness: Using a carbon steel substrate with a thin stainless steel overlay is significantly less expensive than using a full stainless steel component.
4. Repair capability: Worn or corroded carbon steel components can be restored to service by applying a stainless steel overlay.

For pipeline applications specifically, this technology is relevant for:

• Pump impellers and valve components in corrosive service
• Pipeline sections experiencing erosion-corrosion
• Heat exchanger tubes requiring enhanced corrosion resistance
• Pump casing repairs where both structural integrity and corrosion resistance are required

Critical Analysis and Practical Considerations

The large hardness difference between the substrate and the surfacing deposit raises important engineering considerations. While the high hardness of the deposit provides excellent wear and corrosion resistance, the mismatch between the hard, brittle deposit and the softer, more ductile substrate creates potential concerns:

• Thermal stress cracking: The coefficient of thermal expansion mismatch and the large temperature gradients during welding and cooling can induce significant residual stresses at the interface.
• Peel-off failure: Under severe impact or cyclic loading, the hard deposit may peel off the softer substrate if the interface strength is insufficient.
• Intermetallic formation: The carbon steel substrate may promote the formation of brittle intermetallic phases at the interface, reducing joint toughness.

The fact that the maximum hardness occurs in the second layer rather than the first layer is a practical consideration for multi-pass surfacing. It suggests that at least two passes are needed to achieve the full intended properties of the 3Cr13 deposit, with the first pass serving primarily to establish a dilution-reduced transition layer.

The submerged arc welding process used in this study offers excellent consistency and repeatability, which is advantageous for producing uniform surfacing deposits. However, the relatively low productivity of manual processes compared to automatic SAW means that this technology is most suitable for production environments rather than field repair applications.

Study Insights and Conclusions

This study provides clear, quantitative data on the microstructure and hardness characteristics of 3Cr13 stainless steel surfacing on Q235 carbon steel using automatic submerged arc welding. The threefold increase in hardness from substrate to deposit demonstrates the effectiveness of this technology for enhancing surface properties while maintaining structural integrity.

The finding that the maximum hardness occurs in the second layer highlights the importance of multi-pass surfacing for achieving full performance. Engineers should design surfacing procedures with at least two passes to ensure adequate dilution reduction and property achievement. The correlation between hardness and microstructure provides a reliable basis for quality control, as hardness testing can serve as a non-destructive indicator of deposit quality.

For pipeline and pressure vessel engineering, this technology offers a practical solution for extending the service life of carbon steel components in corrosive and abrasive environments. The systematic characterization of the joint microstructure and hardness provides the data needed for proper procedure qualification and quality assurance. This study contributes valuable baseline data for the engineering community working with dissimilar metal surfacing joints, and the methodology employed can be adapted for other stainless steel and substrate combinations commonly encountered in pipeline and process equipment applications.