Microstructure and Hardness Analysis of Q235 Steel Clad with Stainless Steel

Overview and Research Background

This study by Li Ke, Wu Zhisheng, Liu Cuirong, and Yang Dongxing from Taiyuan University of Science and Technology and Jinxí Industrial Group investigates the microstructural evolution and hardness distribution at the interface between Q235 carbon steel and a stainless steel cladding layer. The work was supported by Shanxi Provincial Science and Technology Program (20100321084) and published in the journal Hot Working Technology in 2013. The research addresses a fundamental engineering challenge in bimetal manufacturing: understanding the metallurgical bonding mechanism and property gradient at dissimilar metal interfaces.

Core Technical Points

Interface Microstructure Analysis

The study examines the weld overlay zone where stainless steel is deposited onto Q235 steel substrates. Key metallurgical phenomena at the interface include:

• Diffusion zone formation: Carbon diffusion from the Q235 base metal into the austenitic cladding layer creates a gradient in carbon content, leading to localized ferrite precipitation in what would otherwise be an austenitic matrix.
• Dendritic solidification pattern: The cladding layer exhibits columnar dendrites growing from the interface into the deposited material, with interdendritic regions rich in delta ferrite.
• Intermetallic compound formation: At the fusion boundary, thin layers of intermetallic phases (FeCr, FeCr₂) may form, particularly if the welding heat input is excessive.

Hardness Distribution Characteristics

The hardness profile across the joint typically follows a characteristic pattern:

Process Parameters and Their Influence

Welding Heat Input Effects

The heat input per pass is the single most critical parameter governing interface quality. The study demonstrates that:

1. Low heat input (< 1.5 kJ/mm): Results in incomplete melting of the base metal, creating a cold crack-prone interface with potential lack of fusion. The dilution rate is low, but the bonding strength is compromised.
2. Optimal heat input (2.0–3.5 kJ/mm): Achieves proper melting of the Q235 surface layer, promoting metallurgical bonding while limiting excessive dilution. The stainless cladding retains its corrosion-resistant composition.
3. High heat input (> 4.0 kJ/mm): Excessive dilution introduces carbon into the cladding layer, forming chromium carbides (Cr₂₃C₆, Cr₇C₃) that deplete the matrix of chromium and reduce corrosion resistance.

Dilution Rate Control

The dilution rate—the fraction of base metal melted into the cladding—directly affects the final composition of the near-interface region. For a single-pass overlay:

• Dilution rate of 10–20% is generally acceptable for maintaining 304/316 stainless properties
• Dilution above 25% risks sensitization and reduced pitting resistance
• Multi-pass welding with a transition layer (e.g., 309L) is recommended when dilution exceeds 20%

Engineering Practice Integration

Defect Identification and Countermeasures

Practical Recommendations for Fabrication

Based on the findings, the following engineering guidelines are proposed for Q235/stainless steel cladding applications in pressure vessel manufacturing:

• Use E309L or ER309L filler material for the first pass to tolerate dilution
• Maintain interpass temperature below 200°C for austenitic stainless cladding
• Apply a minimum of two passes: a build-up pass with high dilution tolerance, followed by a cap pass with low dilution to restore corrosion resistance
• Perform intergranular corrosion testing (ASTM A923 Practice E) on coupon welds to verify that sensitization has not occurred
• Conduct hardness traverse testing perpendicular to the interface to confirm no brittle intermetallic zones exceed 250 HV

Study Insights and Reflections

This research provides valuable fundamental data on the Q235/stainless steel interface, which remains one of the most common cladding configurations in Chinese pressure vessel manufacturing. The hardness gradient analysis confirms that the fusion boundary region consistently exhibits peak hardness values, indicating potential stress concentration sites under cyclic loading. From a pressure vessel design perspective, this implies that the fatigue strength of clad components must be evaluated conservatively at the interface region, particularly in hydrogenation reactor applications where hydrogen embrittlement compounds the problem.

The work also highlights that Q235, while economical, presents challenges due to its relatively high carbon content (up to 0.22%) and potential for sulfur segregation. In modern practice, using Q345R or 16MnR as the base material with controlled sulfur content (<0.015%) would significantly improve interface quality and reduce the risk of sulfide stress corrosion cracking in downstream service.

The study's methodology—combining optical microscopy, SEM-EDS elemental mapping, and microhardness traverses—sets a standard approach for interface characterization that should be replicated in any production qualification procedure. Engineers working on clad plate pressure vessels should ensure that their welding procedure qualification (WPQ) includes full interface metallographic examination at a minimum of 500x magnification to detect intermetallic compound formation.