Zhuojin Pipe Fitting Co., LtdEffect of Post-Weld Tempering Treatment on Microstructure and Temper Brittleness of 9Cr13 Rolling Mill Steel Overlay Layer
Literature Overview
This 2018 paper by Li Tao and Wang Xiangjie from Hubei Polytechnic University, published in Casting Technology, investigates the influence of post-weld tempering temperature on the microstructure, hardness, and toughness of overlay layers deposited on 9Cr13 rolling mill steel using Cr-Ni-Mn system flux-cored wire. The research is particularly significant for the steel industry where rolling mill rolls require periodic restoration through overlay welding, and proper post-weld heat treatment is critical to achieving optimal service performance.
Background and Technical Challenge
The 9Cr13 steel is a widely used bearing steel for rolling mill work rolls, valued for its high hardness and wear resistance. However, after prolonged service, the roll surface becomes worn and must be restored through overlay welding. The overlay layer, deposited using Cr-Ni-Mn system flux-cored wire, develops a microstructure dominated by martensite due to the rapid cooling rates inherent in the welding process. Without proper tempering, this martensitic microstructure presents several problems:
- Excessive hardness leading to poor toughness and susceptibility to cracking
- High residual stresses that can cause dimensional instability
- Retained austenite that may transform during service, causing dimensional changes
- Tempering brittleness in certain temperature ranges that severely compromises toughness
Systematic Tempering Study Results
The authors conducted a comprehensive study of tempering temperatures ranging from 50°C to 600°C, examining microstructural evolution, hardness changes, and impact toughness behavior.
Microstructural Evolution with Tempering Temperature
| Tempering Temperature | Dominant Microstructure | Hardness Trend | Toughness Trend |
|---|---|---|---|
| 50-200°C | Coarse plate-like tempered martensite | High (minimal change) | Very low (brittle) |
| 200-300°C | Transition from plate-like to finer tempered martensite | Slight decrease | Gradual improvement |
| 300-450°C | Reducing coarse plate-like martensite, increasing fine lenticular martensite | Progressive decrease | Progressive improvement |
| 450-550°C | Fine lenticular tempered martensite with dispersed fine granular structure | Moderate decrease | Significant improvement |
| 550°C | Fine lenticular tempered martensite + dispersed fine granular mixed structure | Moderate | Optimal (high toughness with adequate hardness) |
| 600°C | Larger granular structure (tempered carbide coarsening) | Further decrease | Decrease (over-tempered) |
Tempering Brittleness Analysis
The study identified a critical tempering brittleness temperature range of 150-200°C where impact toughness values dropped significantly. This corresponds to Type I temper brittleness (low-temperature temper embrittlement), which is associated with:
- Precipitation of unstable carbides at grain boundaries
- Segregation of sulfur and phosphorus to prior austenite grain boundaries
- Formation of continuous grain boundary networks that facilitate crack propagation
The fracture surface analysis confirmed these findings:
- At 50-200°C: Typical brittle fracture with flat cleavage facets and river patterns
- At 300-450°C: Transition from brittle to mixed fracture mode
- At 550°C: Ductile fracture characterized by numerous fracture dimples indicating good toughness
- At 600°C: Coarsened microstructure with reduced dimple density
Process Recommendations and Engineering Application
Based on the comprehensive study, the following process recommendations were established for overlay welding of 9Cr13 rolling mill steel:
- Optimal tempering temperature: 550°C provides the best balance of hardness and toughness, with a fine mixed microstructure of lenticular tempered martensite and dispersed granular carbides
- Avoid tempering in the 150-200°C range to prevent Type I temper brittleness
- Controlled cooling from tempering to avoid secondary quenching effects
- Post-tempering hardness verification to confirm adequate wear resistance for service requirements
The practical implications for rolling mill operations are significant. Proper tempering of overlay layers ensures:
- Extended service life through improved resistance to thermal fatigue and wear
- Reduced risk of catastrophic roll failure during high-stress rolling operations
- Maintained dimensional stability during the thermal cycles of hot rolling
- Cost savings through extended intervals between roll restoration cycles
Study Insights and Reflections
This research exemplifies the critical importance of post-weld heat treatment in overlay welding applications. The systematic approach of examining microstructure, hardness, and toughness across a wide temperature range provides valuable data for process development. The identification of the tempering brittleness range at 150-200°C is particularly important from a safety perspective, as operation in this temperature range could lead to unexpected brittle failure. The finding that 550°C tempering produces the optimal microstructure with fine lenticular tempered martensite and dispersed granular carbides provides a clear target for process specification. For engineers working on rolling mill roll restoration, this study provides both the scientific understanding and practical parameters needed to develop reliable overlay welding procedures that deliver consistent performance in demanding service conditions.