Zhuojin Pipe Fitting Co., LtdEffects of Different Post-Weld Treatments on Microstructure and Wear Resistance of High-Alloy Overlay Welds on 9CrMoV Roll Steel
Literature Overview
This paper by Wu Taiyong and Li Guoxue from Chongqing Huayu Electric Group Co., Ltd., published in Hot Working Technology (Vol. 44, No. 15, 2015, pp. 219–221), investigates the influence of different post-weld cooling and heat treatment conditions on the microstructure and wear resistance of high-alloy flux-cored wire overlay welds deposited on 9CrMoV roll steel substrate.
Core Technical Context
9CrMoV is a high-strength, high-temperature alloy steel widely used for rolling mill rolls, particularly in hot strip mills and heavy plate mills. The steel contains 9% Cr, 1% Mo, and 0.5–0.8% V, providing excellent strength retention at elevated temperatures (up to 600 °C) and good resistance to thermal fatigue. However, the base steel surface is susceptible to severe abrasive and adhesive wear during hot rolling operations, necessitating surface protection through overlay welding.
The high-alloy flux-cored wire used in this study is designed to produce a martensitic overlay with high hardness and wear resistance. The post-weld treatment significantly affects the final microstructure and properties of the overlay layer, making it a critical process variable.
Experimental Design
Test Matrix
| Treatment Condition | Cooling Method | Heat Treatment | Target Microstructure |
|---|---|---|---|
| Air cooling | Natural air cooling | None | Coarse lamellar martensite |
| Sand cooling | Buried in sand | None | Refined martensite |
| Quench and temper | Oil quench | Temper at 500–550 °C | Fine tempered martensite + bainite |
Welding Parameters
| Parameter | Specification |
|---|---|
| Base material | 9CrMoV roll steel |
| Welding process | FCAW (flux-cored arc welding) |
| Electrode | High-alloy flux-cored wire (Cr-Mo-V type) |
| Wire diameter | 1.6 mm |
| Current | 220–260 A |
| Voltage | 24–28 V |
| Shielding gas | CO₂ (20–25 L/min) |
| Preheat | 200–250 °C |
| Interpass temperature | ≤ 250 °C |
| Overlay thickness | 3–4 mm |
Microstructural Analysis
Air Cooling Condition
Under air cooling, the overlay layer develops a coarse lamellar martensite structure with interlamellar spacing of 0.5–1.5 μm. The high cooling rate from the welding thermal cycle, combined with the absence of post-weld treatment, results in a hard but brittle microstructure. Retained austenite content is relatively high (15–25%) due to the rapid cooling preventing complete transformation.
The wear resistance under air cooling is relatively poor despite high hardness because:
- Coarse lamellar structure provides fewer obstacles to wear debris penetration
- High retained austenite content (soft phase) reduces effective wear resistance
- Residual stresses promote microcracking under wear loading
Sand Cooling Condition
Sand cooling provides a moderate cooling rate (slower than air, faster than furnace cooling), resulting in a refined martensite structure with interlamellar spacing of 0.2–0.5 μm. The reduced cooling rate allows for more complete austenite transformation, reducing retained austenite to 8–12%.
The microstructure shows a mixed morphology:
- Predominantly fine lamellar martensite
- Partial lath martensite in regions with slower cooling
- Reduced carbide coarsening compared to air cooling
The wear resistance improves significantly due to:
- Finer martensite lamellae providing more slip system obstacles
- Reduced retained austenite increasing overall hardness uniformity
- Lower residual stress reducing microcrack initiation
Quench and Temper Condition
The quench and temper treatment produces the finest and most uniform microstructure:
- Predominantly fine lath martensite (tempered) with lath spacing of 0.1–0.3 μm
- Partial bainite in regions with slower cooling during welding
- Dispersed fine carbides (Cr₇C₃, Mo₂C, VC) within the martensite matrix
- Minimal retained austenite (3–8%)
The tempering process:
- Dissolves coarse carbides and redistributes them as fine particles
- Reduces residual stresses through recovery and recrystallization
- Optimizes the hardness-toughness balance
- Improves wear resistance through fine carbide dispersion
Wear Resistance Comparison
| Treatment Condition | Hardness (HRC) | Wear Volume (mm³) | Relative Wear Resistance |
|---|---|---|---|
| Air cooling | 58–62 | 120–150 | 1.0 (reference) |
| Sand cooling | 55–59 | 80–100 | 1.3–1.5 |
| Quench and temper | 52–56 | 50–70 | 1.7–2.0 |
The results demonstrate that wear resistance is not solely determined by hardness. The quench and temper condition, despite having lower hardness than air cooling, exhibits significantly better wear resistance due to the finer, more uniform microstructure and reduced residual stresses.
Metallurgical Mechanisms
The wear resistance improvement with quench and temper treatment can be attributed to several mechanisms:
- Microstructural refinement: Finer martensite laths provide more obstacles to dislocation motion and wear debris penetration
- Carbide dispersion: Tempering promotes the formation of fine, uniformly distributed carbides that act as wear-resistant second phases
- Stress relief: Reduction of tensile residual stresses eliminates microcrack initiation sites
- Phase stability: Reduced retained austenite eliminates the soft phase that preferentially wears
- Work hardening capacity: Tempered martensite retains good work hardening capacity, providing sustained resistance during wear
Engineering Practice Implications
For industrial applications involving overlay welding on 9CrMoV or similar alloy steels, the following recommendations emerge:
- Post-weld heat treatment is essential: Air cooling alone produces unacceptable wear resistance despite high hardness
- Quench and temper is the preferred treatment: Despite requiring additional processing time and cost, the wear resistance improvement justifies the investment
- Tempering temperature optimization: 500–550 °C provides the best balance of hardness and toughness for rolling mill applications
- Cooling rate control: Sand cooling can serve as an interim measure when full quench and temper is not feasible, providing intermediate wear resistance
Defect Considerations
| Defect | Risk Factor | Prevention |
|---|---|---|
| Cracking during quenching | High carbon, high alloy content | Preheat before quench, use warm oil quench |
| Excessive distortion | Thermal mismatch during quench | Controlled cooling, symmetrical quenching |
| Soft spots | Incomplete transformation | Ensure adequate austenitization temperature |
| Carbide coarsening | Over-tempering | Limit tempering time, control temperature |
| Intergranular cracking | Sensitization | Avoid prolonged exposure at 450–600 °C |
Study Insights and Reflections
This study provides valuable insights into the relationship between post-weld treatment, microstructure, and wear performance in high-alloy overlay welds. The key finding—that wear resistance is not simply a function of hardness but depends critically on microstructure refinement and phase composition—has broad implications for overlay welding practice.
The results challenge the common engineering assumption that higher hardness always translates to better wear resistance. In reality, the microstructural characteristics that determine wear resistance include:
- Carbide size and distribution
- Matrix microstructure (lamellar vs. lath martensite)
- Retained austenite content
- Residual stress state
- Phase stability under wear loading
For engineers designing overlay welding procedures for rolling mill rolls, the practical implications are significant. The quench and temper treatment, while requiring additional processing steps, provides a 70–100% improvement in wear resistance compared to air cooling alone. This translates directly to extended roll life, reduced maintenance frequency, and lower total cost of ownership.
The study also highlights the importance of systematic experimental design in welding process development. By varying only the post-weld treatment condition while keeping all other parameters constant, the authors were able to isolate the effect of treatment on microstructure and properties—a methodology that should be applied to all welding process development work.
Reference Value and Outlook
This work contributes to the growing body of knowledge on overlay welding for wear-resistant applications. The findings are particularly relevant for engineers working on rolling mill roll maintenance, where the cost of roll replacement and downtime is substantial.
Future research should explore:
- The effect of tempering temperature on wear resistance (systematic study from 400 to 600 °C)
- Long-term wear performance under actual rolling conditions
- The effect of overlay thickness on wear resistance
- Comparison with other post-weld treatments (e.g., cryogenic treatment, austempering)
- Application to other alloy steels (e.g., 5CrMoV, 4Cr5MoSiV)
The methodology established in this work—systematic variation of post-weld treatment with comprehensive microstructural and mechanical characterization—provides a template for similar studies in other alloy systems and applications.