Zhuojin Pipe Fitting Co., LtdMechanical Properties of MAG Surfacing Re-manufacturing of FV520B Precipitation-Hardening Stainless Steel
Literature Overview
This study, published in Journal of Materials Engineering (Vol. 45, No. 10, 2017, pp. 23–31), investigates the mechanical properties of FV520B precipitation-hardening stainless steel after re-manufacturing by MAG (MIG) surfacing. The research was conducted by Liu Jian, Zhu Sheng, Cai Zhihai, Zhang Ping, Liu Jun, Qin Hang, and Tong Yonggang from the National Engineering Research Center for Remanufacturing of Mechanical Products and the Key Laboratory of Equipment Remanufacturing Technology at the Academy of Armored Force Engineering. The work was supported by multiple National Natural Science Foundation grants (51405510, 51375492, 51575527). The study compares the mechanical properties of the MAG surfacing re-manufactured layer with those of the base material and analyzes the fracture morphology and microstructure to explain the observed property differences.
Core Technical Findings
The MAG surfacing re-manufactured layer of FV520B stainless steel exhibits high strength and high hardness but relatively low ductility and impact toughness compared to the base material. The tensile strength of the re-manufactured layer reaches 1195 MPa, exceeding the base material value of 1092 MPa. The yield strength and hardness are 776 MPa and 336 HV, respectively, which are close to the base material values of 859 MPa and 353 HV. However, the static tensile elongation and impact toughness are significantly lower, at 8.72 percent and 61 J/cm², compared to the base material values of 19.67 percent and 144 J/cm².
| Mechanical Property | Re-manufactured Layer | Base Material | Ratio (Layer/Base) |
|---|---|---|---|
| Tensile strength (UTS) | 1195 MPa | 1092 MPa | 1.09 |
| Yield strength (YS) | 776 MPa | 859 MPa | 0.90 |
| Hardness (HV) | 336 HV | 353 HV | 0.95 |
| Elongation (A%) | 8.72% | 19.67% | 0.44 |
| Impact toughness (CVN) | 61 J/cm² | 144 J/cm² | 0.42 |
Microstructure and Fracture Analysis
The fracture surface and microstructure analysis reveals that the re-manufactured layer consists of rapidly cooled non-equilibrium lath martensite and precipitated carbide strengthening phases including NbC, MoC, and M23C6. These phases are the fundamental reason for the high strength and high hardness of the re-manufactured layer. The rapid cooling during MAG welding prevents the formation of equilibrium precipitates and promotes the formation of martensite with high dislocation density, which contributes to solid solution and precipitation strengthening.
However, the absence of aging treatment and the lack of Cu-element strengthening phase contribution, combined with the weak interface bonding between brittle inclusions and large spherical particles and the matrix, severely degrade the deformation capacity of the material. These features promote stress concentration and crack initiation during loading, which is the primary reason for the low elongation and poor impact toughness of the re-manufactured layer. The fracture morphology shows a mixed mode of cleavage and quasi-cleavage fracture, with some intergranular features associated with the weak interfaces around large particles.
Technical Discussion and Engineering Implications
The results of this study have significant implications for the re-manufacturing of high-performance stainless steel components. FV520B is a precipitation-hardening stainless steel widely used in aerospace and defense applications due to its excellent combination of strength, corrosion resistance, and toughness. The re-manufacturing process must maintain or restore the original mechanical properties to ensure component reliability. The study demonstrates that while MAG surfacing can achieve high strength and hardness in the re-manufactured layer, the ductility and toughness are significantly compromised.
From a welding metallurgy perspective, the formation of lath martensite is a direct consequence of the rapid cooling rate during welding. The cooling rate in MAG welding is typically in the range of 10–100 °C/s, which is sufficient to suppress austenite decomposition to pearlite or bainite and promote martensite formation. The non-equilibrium nature of the martensite, combined with the absence of post-weld aging, results in a microstructure that is highly strengthened but inherently brittle.
Process Optimization Considerations
To improve the ductility and toughness of the re-manufactured layer, several process modifications could be considered:
- Preheating to reduce the cooling rate and promote the formation of bainite or tempered martensite
- Post-weld heat treatment (PWHT) to temper the martensite and promote the formation of equilibrium precipitates
- Multi-pass welding with interpass temperature control to modify the thermal cycle
- Addition of microalloying elements to the consumable to promote beneficial precipitate formation
| Optimization Strategy | Expected Effect | Practical Consideration |
|---|---|---|
| Preheating (200–300 °C) | Reduced cooling rate, tempered martensite | May reduce strength slightly |
| PWHT (500–600 °C) | Tempered martensite, equilibrium precipitates | Requires controlled furnace or induction |
| Multi-pass with interpass control | Modified thermal cycle, reduced residual stress | Increases processing time |
| Consumable modification | Tailored precipitate formation | Requires consumable development |
Standards and Quality Control
The re-manufacturing of FV520B components must comply with relevant military and aerospace standards. In the Chinese standards system, GJB 1126 (welding of stainless steel in aerospace) and GJB 1790 (welding consumables for aerospace) provide guidance. The mechanical property requirements for the re-manufactured layer should be specified in the applicable technical specifications, with particular attention to the ductility and toughness requirements. Non-destructive testing (NDT) of the re-manufactured layer should include magnetic particle testing (MT) or penetrant testing (PT) for surface defects and ultrasonic testing (UT) for subsurface defects. The acceptance criteria should be established based on the criticality of the component and the applicable standard.
Study Insights and Implications
This study provides valuable insights into the challenges of re-manufacturing high-performance precipitation-hardening stainless steels by welding-based methods. The key finding is that while the strength and hardness can be maintained or even improved, the ductility and toughness are significantly degraded due to the non-equilibrium microstructure formed during welding. The results underscore the importance of post-weld heat treatment in re-manufacturing applications, as the as-welded microstructure is inherently suboptimal for ductile and tough performance. For engineering practice, the study recommends that re-manufactured components be subjected to appropriate heat treatment to restore the mechanical properties to acceptable levels. The study also highlights the need for further research into consumable design and process optimization to achieve a better balance between strength and toughness in the as-welded condition. The findings are particularly relevant for the defense and aerospace industries, where component reliability and safety are paramount.