CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Post-Weld Tempering on Microstructure and Temper Brittleness of 9Cr13 Roll Steel Weld Overlay Layer
Literature Overview
Authored by Li Tao and Wang Xiangjie from Hubei Polytechnic University and published in "Foundry Technology" in 2018, this study examines how post-weld tempering treatment influences the microstructure and temper brittleness susceptibility of weld overlay layers applied to 9Cr13 roll steel. The research was supported by the Hubei Polytechnic University talent introduction project (Grant No. 14xjz02R).
Core Technical Content
9Cr13 Roll Steel Characteristics
9Cr13 is a high-carbon martensitic stainless steel widely used for rolling mill rolls due to its excellent wear resistance and moderate corrosion resistance. Key properties include:
| Property | Value |
|---|---|
| Carbon content | 0.90–1.05 wt% |
| Chromium content | 12.0–14.0 wt% |
| Hardness (as-quenched) | 55–60 HRC |
| Hardness (tempered 600 °C) | 40–45 HRC |
| Typical overlay application | Restoration of worn roll surfaces |
Overlay Microstructure Without Tempering
In the as-welded condition, the overlay layer exhibits a martensitic microstructure with retained austenite content typically ranging from 8% to 15%, depending on the specific welding process and filler metal composition. The high carbon content inherited from the substrate through dilution creates a highly stressed martensitic structure prone to temper embrittlement.
Effect of Tempering Temperature on Microstructure
The study systematically investigates tempering at various temperatures:
| Tempering Temperature | Microstructure | Hardness (HRC) | Embrittlement Susceptibility |
|---|---|---|---|
| 400 °C | Tempered martensite + fine carbides | 52–55 | Low |
| 500 °C | Tempered martensite + Cr7C3 + M7C3 | 47–50 | Medium |
| 550 °C | Tempered martensite + coarse carbides + retained austenite | 42–46 | High |
| 600 °C | Tempered martensite + M23C6 + significant retained austenite | 38–42 | Very high |
Temper Embrittlement Mechanism
The study identifies that temper embrittlement in the overlay layer is governed by the following mechanisms:
- Segregation of impurity elements – Sulfur, phosphorus, and tin segregate to prior austenite grain boundaries during tempering in the critical temperature range (470–600 °C), reducing intergranular fracture resistance.
- Carbide precipitation at grain boundaries – Chromium carbides preferentially nucleate at grain boundaries, depleting the boundary region of chromium and creating an embrittled zone.
- Retained austenite decomposition – Decomposition of retained austenite during tempering creates a complex microstructure with variable local properties.
Process Optimization and Recommendations
Recommended Tempering Protocol
Based on the study findings, the following tempering protocol is recommended for 9Cr13 roll steel overlay layers:
- Single tempering at 420–450 °C for 2 hours – This provides adequate stress relief while maintaining hardness above 50 HRC and minimizing embrittlement susceptibility.
- Double tempering (quench and re-temper) – For critical applications requiring maximum toughness, a double tempering cycle (quench from 820 °C followed by tempering at 400 °C × 2 hours) reduces retained austenite and stabilizes the microstructure.
- Avoid tempering in the 470–580 °C range – This is the peak embrittlement susceptibility range for this alloy system.
Welding Process Considerations
| Parameter | Recommendation | Rationale |
|---|---|---|
| Welding process | TIG or plasma arc | Low dilution, precise heat input control |
| Heat input | 0.8–1.5 kJ/mm | Limit substrate dilution, control microstructure |
| Interpass temperature | < 150 °C | Maintain hardenability, avoid carbide coarsening |
| Filler metal | Matching 9Cr13 or slightly lower carbon | Balance hardness with weldability |
| Preheat | 100–150 °C | Reduce cracking tendency without affecting temper response |
Engineering Practice Integration
In rolling mill maintenance operations, the restoration of worn roll surfaces through weld overlay is a routine but technically demanding activity. The findings of this study have direct practical significance:
- Roll reconditioning shops must ensure that post-weld tempering is performed outside the embrittlement temperature range to prevent premature roll failure during rolling operations.
- Quality control protocols should include Charpy V-notch testing at service temperature to verify that temper embrittlement has not developed.
- Welder qualification per NB/T 47014 should specifically address the tempering response of the overlay layer, not merely the weld metal tensile properties.
Key Reflections and Study Insights
This study provides valuable insights into a frequently overlooked aspect of roll restoration welding – the post-weld heat treatment response of the overlay layer. In practice, many shops focus on achieving the required surface hardness but neglect the toughness implications of the tempering cycle. The clear identification of the embrittlement temperature range (470–580 °C) provides a simple, actionable guideline for production engineers.
The study also highlights an important principle: in high-carbon martensitic stainless steel systems, the interaction between carbon content, tempering temperature, and impurity segregation creates a complex but predictable microstructural evolution that can be managed through careful process control. This principle extends to other high-carbon overlay applications such as hardfacing of mining equipment and cement mill rollers.