Microstructure and Hardness of Stainless Steel Welding Rod Surface Overlay Weld Joints

Literature Overview

This 2018 study published in Guangdong Chemical Industry by Wang Yan from China Three Gorges University examines the microstructure evolution and hardness distribution in surface overlay weld joints produced using stainless steel welding rods (SMAW process). The research focuses on the practical aspects of manual arc welding overlay processes commonly employed in industrial maintenance and repair of chemical processing equipment, where accessibility constraints and on-site repair conditions favor the use of coated electrodes over mechanized processes.

Technical Context and Process Description

SMAW Overlay Application Background

Manual metal arc welding (SMAW) with stainless steel electrodes remains the most widely used overlay process in industrial maintenance settings due to:

• Portability and flexibility for on-site repair
• Low equipment investment compared to mechanized processes
• Applicability to complex geometries and confined spaces
• Availability of qualified welders in most industrial regions

Common stainless steel electrode types used for overlay include:

Study Objectives

The research investigates:

1. The effect of electrode type on overlay deposit microstructure
2. Hardness distribution across the overlay cross-section
3. Dilution characteristics and their influence on properties
4. Bond strength between overlay and substrate
5. Practical recommendations for field application

Microstructure Analysis

Overlay Deposit Microstructure

The study examines the microstructure of single-pass and multi-pass SMAW overlay deposits on carbon steel substrates. Key observations include:

Single-pass deposits:

• Columnar dendritic structure growing from the substrate interface
• Ferrite-austenite duplex structure in 309L and 316L deposits
• Widmanstätten ferrite in 310 deposits due to high alloy content
• Martensitic transformation in dilution zones where carbon and Cr combine

Multi-pass deposits:

• Recrystallized columnar grains in lower passes
• Equiaxed grain structure in upper passes due to grain refinement
• Progressive reduction of dilution effect with increasing number of passes
• Possible hot cracking in high-alloy deposits (E310) due to Laves phase formation

Dilution Zone Analysis

The dilution zone at the overlay-substrate interface is the critical region for mechanical and corrosion performance. The study quantifies dilution ratios of 20–40% for single-pass and 10–25% for multi-pass deposits. The dilution zone exhibits:

• Elevated carbon content from substrate diffusion
• Reduced Cr/Ni ratio leading to ferrite enrichment
• Possible martensite formation in dilution zones of 304/316 deposits
• Hardness peaks of 400–500 HV in dilution zones compared to 200–250 HV in the bulk deposit

Phase Identification

Hardness Distribution and Mechanical Properties

Hardness Profiling

The study presents detailed hardness profiles across the overlay cross-section:

Bond Strength Testing

The study includes bond strength testing per ASTM A959 or equivalent, showing:

• Single-pass bond strength: 250–350 MPa
• Multi-pass bond strength: 300–420 MPa
• Effect of interpass temperature: Strength decreases above 250 °C interpass temperature
• Effect of electrode coating condition: Properly stored electrodes show 15–20% higher bond strength

Practical Recommendations for Field Application

Welding Procedure Optimization

1. Preheat control: 50–100 °C for carbon steel substrates to reduce hydrogen cracking risk
2. Interpass temperature: Maintain below 250 °C to prevent excessive grain growth
3. Arc travel technique: Use weeping arc or drag technique to minimize dilution
4. Pass thickness: Limit individual pass thickness to 3–4 mm for optimal bonding
5. Cleaning between passes: Remove all slag and spatter to prevent inclusions

Quality Assurance Requirements

Study Insights and Implications

This research provides valuable practical guidance for engineers and welders performing stainless steel overlay repairs in industrial settings. The key insight is that SMAW overlay, while less controllable than mechanized processes, can produce acceptable results when proper procedures are followed. The dilution zone remains the critical region requiring the most attention, as it governs both mechanical integrity and corrosion performance. For engineers specifying overlay repairs, the study emphasizes the importance of electrode selection based on the specific service environment, the need for proper welder qualification, and the necessity of post-overlay inspection to verify bond integrity and microstructural quality.