Zhuojin Pipe Fitting Co., LtdCCT Diagram and Transformation Behavior of Surface Surfacing Layer Metal
Research Background and Methodology
This paper by Wang Jianguo and colleagues from Baotou Iron and Steel Institute (Armament Materials Science and Engineering, 1999, Vol. 22, No. 3, pp. 37-40) presents a fundamental materials science investigation into the continuous cooling transformation (CCT) behavior of surfacing layer metal. The research was supported by the Inner Mongolia Autonomous Region Key Research Project and employed a Formaster-Digital automatic phase transformation recorder (Japanese manufacture) to simulate surfacing thermal cycles and measure the CCT diagram of the surfacing metal.
Understanding the CCT behavior of surfacing metal is of paramount importance for predicting the microstructure and mechanical properties of weld overlays under various cooling conditions. The surfacing process involves rapid heating and cooling cycles that are fundamentally different from equilibrium conditions, and the resulting microstructure is highly sensitive to cooling rate. This research provides the foundational data needed to optimize surfacing process parameters for specific microstructural and property targets.
CCT Diagram Analysis and Transformation Kinetics
The key findings from the CCT analysis are summarized below:
| Transformation | Behavior | Critical Cooling Rate | Engineering Implication |
|---|---|---|---|
| Austenite → Pearlite (A→P) | Strongly delayed, difficult to occur | Very high (effectively suppressed in typical surfacing cooling rates) | Pearlite formation is unlikely under normal surfacing conditions |
| Austenite → Bainite (A→B) | Significantly delayed | Approximately 300°C/min | Bainite formation is possible but requires relatively slow cooling |
| Austenite → Martensite (A→M) | Occurs at cooling rates above ~300°C/min | Below 300°C/min | Martensitic transformation dominates under typical surfacing cooling conditions |
The critical finding is the exceptionally high hardenability of the surfacing metal, with a critical quenching cooling rate of approximately 300°C/min. This means that under typical surfacing thermal cycles, where cooling rates often exceed 500°C/min, the overlay microstructure will predominantly consist of martensite and bainite, with little or no pearlite formation.
Microstructural Inhomogeneity and Its Consequences
The paper highlights a critical issue: the severe inhomogeneity of microstructural transformation under surfacing thermal cycles. This inhomogeneity arises from several factors:
- Thermal gradient within the overlay: The top surface of the overlay cools much faster than the root, creating a gradient in cooling rate and consequently in microstructure.
- Multi-pass effects: In multi-pass surfacing, the heat input from subsequent passes reheats previously deposited layers, causing partial or complete tempering of martensite in the lower passes. This creates a microstructural gradient from the root (tempered martensite or bainite) to the surface (untempered martensite).
- Base metal interaction: The heat sink effect of the base metal influences the cooling rate at the weld root, potentially causing a different microstructure in the dilution zone compared to the bulk overlay.
- Cooling rate sensitivity: Even small variations in cooling rate can cause significant changes in the phase transformation path, leading to localized microstructural differences within the same overlay layer.
Engineering Implications for Surfacing Process Design
The findings from this research have direct implications for surfacing process optimization:
- Preheating control: Preheating the base metal reduces the cooling rate, which can shift the transformation from martensite to bainite. This may be desirable for reducing cracking susceptibility but must be balanced against the desired hardness and wear resistance.
- Interpass temperature management: Maintaining appropriate interpass temperatures is critical for controlling the microstructure of multi-pass overlays. Excessive interpass temperatures can cause undesirable grain growth and phase transformation in previously deposited layers.
- Cooling rate prediction: Engineers must have the ability to predict cooling rates for specific surfacing configurations to anticipate the resulting microstructure. This requires understanding the thermal properties of both the overlay metal and the base metal, as well as the geometry of the surfacing operation.
- Post-weld heat treatment: For applications where tempered martensite is desired (rather than untempered martensite), post-weld tempering may be necessary. However, the tempering temperature must be carefully controlled to avoid over-tempering or undesirable phase transformations.
Study Insights and Implications
This research, while published in 1999, remains highly relevant to modern surfacing practice. The fundamental principles of CCT behavior and transformation kinetics are universal and apply regardless of the specific surfacing process or material system. The emphasis on microstructural inhomogeneity is particularly important, as it reminds engineers that surfacing overlays are not homogeneous materials; their properties vary with depth and position. Understanding and controlling this inhomogeneity is essential for achieving the desired performance in service. For engineers designing surfacing procedures for critical applications such as wear-resistant overlays on mining equipment, hardfacing on cutting tools, or corrosion-resistant overlays in chemical processing, the CCT data presented in this paper provides a valuable foundation for process optimization and quality assurance.