CLADDING TECHNOLOGY SHANXI CO., LTDMechanical Properties of MAG Weld Overlay Remanufacturing on FV520B Precipitation-Hardening Stainless Steel
Literature Overview
This 2017 study published in Materials Engineering by Liu Jian, Zhu Sheng, Cai Zhihai, Zhang Ping, Liu Jun, Qin Hang, and Tong Yonggang, funded by the National Natural Science Foundation of China (grants 51405510, 51375492, and 51575527), investigates the mechanical properties of FV520B precipitation-hardening stainless steel after gas metal arc welding (MAG) weld overlay remanufacturing. Conducted at the State Key Laboratory of Equipment Remanufacturing Technology and the National Engineering Research Center for Mechanical Product Remanufacturing, both affiliated with the Academy of Armored Force Engineering, this research addresses a critical challenge in the remanufacturing of high-performance structural components.
Background and Technical Challenges
FV520B is a precipitation-hardening stainless steel widely used in high-strength structural applications, including military vehicles, aerospace components, and heavy machinery. Its excellent strength-to-weight ratio and corrosion resistance make it a preferred material for demanding applications. However, when these components require repair or restoration of worn surfaces, the weld overlay process can significantly alter the microstructure and mechanical properties of the material, particularly in the heat-affected zone (HAZ).
The primary challenges in remanufacturing FV520B with MAG weld overlay include:
- Precipitation dissolution: The high temperatures during welding can dissolve the precipitates that provide the strength of FV520B, leading to a significant loss of strength in the HAZ.
- Phase transformation: The rapid heating and cooling cycles can cause undesirable phase transformations, including the formation of brittle phases or excessive grain growth.
- Residual stress: The thermal mismatch between the weld metal and base material can generate high residual stresses, potentially leading to cracking or reduced fatigue life.
- Microstructural heterogeneity: The weld overlay creates a complex microstructure with distinct regions (weld metal, fusion line, HAZ, and unaffected base material), each with different mechanical properties.
Welding Process Parameters
The study examined MAG welding with different process parameters to optimize the mechanical properties of the weld overlay. The following table summarizes the welding parameters investigated:
| Parameter | Range Investigated | Optimal Value |
|---|---|---|
| Shielding gas | Ar, CO2, Ar+CO2 mixtures | 80% Ar + 20% CO2 |
| Wire diameter | 1.0-1.6 mm | 1.2 mm |
| Current | 150-300 A | 200-250 A |
| Voltage | 22-30 V | 25-27 V |
| Travel speed | 300-800 mm/min | 500-600 mm/min |
| Preheat temperature | 0-200°C | 100-150°C |
| Interpass temperature | 0-250°C | <150°C |
Microstructural Analysis
Metallographic examination revealed that the weld overlay created a distinct microstructural gradient from the weld metal to the unaffected base material. In the weld metal, a mixture of austenite and ferrite was observed, with the relative proportions depending on the welding consumable composition and process parameters. The fusion line region exhibited a fine-grained microstructure due to the rapid cooling rates.
In the HAZ, the most critical region for mechanical property degradation, the original precipitation-hardened microstructure of FV520B was significantly altered. The precipitates responsible for the strength of FV520B were partially or completely dissolved, leading to a softening of the material. The extent of this softening depended on the peak temperature reached during welding, which in turn was determined by the heat input.
The following table summarizes the microstructural changes observed in different regions:
| Region | Microstructure | Hardness (HV) | Strength (MPa) |
|---|---|---|---|
| Unaffected base material | Precipitation-hardened martensite | 350-400 | 1200-1400 |
| Coarse-grained HAZ | Softened martensite + precipitates | 250-300 | 800-1000 |
| Fine-grained HAZ | Fine martensite + partial precipitates | 300-350 | 1000-1200 |
| Fusion line | Fine austenite + ferrite | 320-370 | 1100-1300 |
| Weld metal | Austenite + ferrite + carbides | 300-350 | 900-1100 |
Mechanical Property Evaluation
The mechanical properties were evaluated through tensile testing, hardness mapping, and impact testing. The results showed that the weld overlay significantly reduced the strength and hardness of the base material in the HAZ. The maximum hardness drop in the HAZ was approximately 25-35% compared to the unaffected base material, depending on the welding parameters used.
To mitigate this property degradation, the authors investigated post-weld heat treatment (PWHT) to restore the precipitation-hardened microstructure. A solution treatment followed by aging treatment was found to be effective in restoring the hardness and strength of the HAZ. The optimal PWHT cycle consisted of a solution treatment at 1050°C for 1 hour followed by an aging treatment at 480°C for 4 hours. This cycle restored the hardness in the HAZ to approximately 80-90% of the original value, while maintaining acceptable mechanical properties in the weld metal.
Engineering Practice Implications
For engineers involved in the remanufacturing of FV520B components, this study provides critical guidance on welding process selection and post-weld treatment. The following recommendations are derived from the research findings:
- Preheat and interpass temperature control: Preheating to 100-150°C and maintaining interpass temperatures below 150°C helps reduce thermal stresses and minimize the extent of HAZ softening.
- Heat input minimization: Using lower heat input parameters (current, voltage, travel speed) reduces the peak temperature in the HAZ, thereby preserving more of the original precipitation-hardened microstructure.
- Post-weld heat treatment: A properly designed PWHT cycle is essential for restoring the mechanical properties of the HAZ. The solution treatment and aging parameters must be carefully controlled to avoid over-aging or under-aging.
- Welding consumable selection: The composition of the welding consumable should be carefully selected to ensure good metallurgical compatibility with FV520B and to minimize the formation of brittle phases.
Key Questions and Reflections
One important question is whether the PWHT cycle recommended in this study is universally applicable to all FV520B components, or whether it needs to be adjusted based on component geometry, thickness, and residual stress state. Another consideration is the effect of repeated weld overlay and PWHT cycles on the long-term mechanical properties of the component, as is common in maintenance and repair scenarios.
The study also raises the question of whether alternative welding processes, such as laser welding or electron beam welding, might offer better control over heat input and thus better preservation of the original microstructure. However, these processes are often more expensive and less practical for large-scale remanufacturing operations.
Study Insights and Implications
This research provides valuable insights into the challenges and solutions for remanufacturing FV520B components using MAG weld overlay. The comprehensive analysis of microstructural changes, mechanical property degradation, and post-weld treatment options offers practical guidance for engineers involved in component repair and restoration. The emphasis on the importance of process parameter control and post-weld heat treatment underscores the need for a systematic approach to weld overlay remanufacturing. Engineers should use this study as a reference for developing welding procedures and PWHT cycles that balance the need for surface restoration with the preservation of the base material's mechanical properties.