Zhuojin Pipe Fitting Co., LtdCCT Diagram and Transformation Microstructure of Surface Hardfacing Layer Metals
Literature Overview
This paper by Wang J. G., Wang G., Liu X. G., and Hou C. Q. from Baotou Iron and Steel Institute and Baotou Iron and Steel Group Corporation, published in Ordnance Materials Science and Engineering (1999, Vol. 22, No. 3, pp. 37-40), investigates the continuous cooling transformation (CCT) behavior of hardfacing deposit metals. Funded by the Inner Mongolia Autonomous Region Key Research Program, this work uses a Formastor-Digital fully automatic transformation recorder to simulate hardfacing thermal cycles and determine critical cooling rates and transformation microstructures.
Core Technical Content
The study addresses a fundamental metallurgical question in hardfacing technology: how does the cooling rate during welding affect the microstructure and properties of the deposited metal? The CCT diagram provides the definitive answer to this question by mapping the relationship between cooling rate, transformation temperature, and resulting microstructure.
Key Findings from CCT Analysis
| Parameter | Value | Engineering Significance |
|---|---|---|
| Critical quenching cooling rate | ~300°C/min | Below this rate, transformation occurs; above it, martensite forms |
| A→P transformation | Strongly delayed | Pearlite formation is suppressed |
| A→B transformation | Significantly delayed | Bainite formation is suppressed |
| Hardening capacity | Very high | Deposition can achieve high hardness |
CCT Diagram Interpretation
The CCT diagram reveals that the hardfacing deposit metal exhibits extremely high hardenability, characterized by:
- Strong suppression of austenite-to-pearlite (A→P) transformation: The nose of the pearlite transformation curve is shifted significantly to the right (lower cooling rates), indicating that pearlite formation is difficult to achieve under normal welding cooling conditions.
- Significant suppression of austenite-to-Bainite (A→B) transformation: The bainite transformation curve is also shifted to the right, though not as dramatically as the pearlite curve.
- High critical cooling rate (~300°C/min): Cooling rates above 300°C/min will result in martensitic transformation, while rates below this threshold will allow some diffusion-controlled transformation to occur.
Microstructural Heterogeneity in Multi-Pass Hardfacing
A critical finding of this study is the severe microstructural non-uniformity that exists under hardfacing thermal cycling conditions. In multi-pass hardfacing:
- First pass: Cools directly from the melting temperature through the base metal, experiencing the fastest cooling rate and producing predominantly martensitic microstructure.
- Subsequent passes: Experience a thermal cycle that includes re-austenitization followed by cooling. The starting temperature for cooling is the interpass temperature (typically 200-350°C), not the melting temperature. This results in:
- Lower peak temperatures in the HAZ of previous passes
- Slower effective cooling rates for the newly deposited metal
- Potential for mixed microstructures (martensite + bainite + retained austenite)
Cooling Rate Distribution in Multi-Pass Deposits
| Layer Position | Cooling Rate | Dominant Microstructure | Hardness |
|---|---|---|---|
| Top pass | Highest | Predominantly martensite | Highest |
| Middle passes | Medium | Martensite + bainite mixture | Medium-high |
| Bottom pass (near base) | Lowest | Martensite + bainite + some pearlite | Lower |
Engineering Implications
The high hardenability and hardening capacity of hardfacing metals have direct implications for process design:
1. Preheating Requirements
Given the critical cooling rate of ~300°C/min, preheating the base metal to 200-350°C is essential for:
- Reducing the effective cooling rate below the critical value
- Preventing hydrogen-induced cracking in martensitic microstructures
- Controlling HAZ hardness to acceptable levels
2. Interpass Temperature Control
Maintaining interpass temperatures between 200-350°C serves multiple purposes:
- Ensures sufficient thermal mass for the next pass to cool at an acceptable rate
- Prevents the formation of coarse grain structures from excessive peak temperatures
- Allows some stress relief through thermal cycling
3. Post-Weld Heat Treatment
For applications requiring toughness rather than maximum hardness, PWHT is necessary to:
- Temper the martensitic microstructure to tempered martensite
- Reduce residual stresses
- Improve ductility and fracture resistance
Typical PWHT parameters for hardfacing deposits:
- Temperature: 500-650°C (for martensitic hardfacing metals)
- Hold time: 1-2 hours per 25 mm of thickness
- Cooling rate: Controlled to prevent re-hardening
Study Reflection
This 1999 study, while focused on a specific hardfacing metal composition, establishes fundamental principles that apply broadly to surfacing technology. The determination of CCT diagrams for hardfacing metals is a critical step in welding procedure development, as it provides the metallurgical basis for setting preheating, interpass temperature, and PWHT parameters. The finding of severe microstructural non-uniformity in multi-pass deposits underscores the importance of considering the entire thermal history of the deposit, not just the final pass conditions.
The high hardenability reported in this study is characteristic of many alloy hardfacing metals, particularly those designed for wear resistance applications such as chrome carbide overlays, high-speed steel deposits, and cobalt-based hardfacing alloys. Engineers working with these materials must always consider the cooling rate implications when designing welding procedures.