Zhuojin Pipe Fitting Co., LtdEffect of Heat Treatment on D618 Overlay Layer Microstructure and Properties
Literature Overview
This paper by Peng Jixiang, Wang Shunxing, and Liu Yong from Luoyang Institute of Technology (Metal Heat Treatment, 2002, Vol. 27, No. 5, pp. 48–50), funded by the Henan Provincial Department of Education, investigates the effect of heat treatment on the high-carbon, high-chromium cast iron type D618 hard alloy overlay layer. The study addresses a practical industrial need for improving the corrosion-resistant wear performance of overlay welds through post-weld thermal processing.
Technical Parameters and Heat Treatment Conditions
The D618 overlay alloy is a high-carbon, high-chromium cast iron composition, typically containing approximately 2–4% C and 20–30% Cr. The study evaluated three heat treatment conditions:
| Heat Treatment Condition | Process Description | Purpose |
|---|---|---|
| Condition A | 850°C heating + air cooling | Controlled cooling for carbide precipitation |
| Condition B | 850°C heating + oil quenching + 400°C tempering | Quench-and-temper for hardness optimization |
| Reference | 35CrMo low-alloy steel, carbonitriding (850°C oil quench + 200°C temper) | Baseline comparison |
Microstructural Evolution
The D618 overlay layer, being a high-carbon, high-chromium cast iron system, contains primary carbides (predominantly Cr₇C₃ type) in a martensitic or austenitic matrix depending on the cooling rate. The heat treatment at 850°C facilitates carbide redistribution and matrix transformation:
- Air cooling from 850°C: Produces a tempered martensite matrix with refined secondary carbides, providing a balance of hardness and toughness.
- Oil quenching + 400°C tempering: Achieves maximum hardness through rapid cooling followed by controlled tempering to relieve residual stresses while maintaining carbide integrity.
Performance Results
The key finding is that both the 850°C air-cooled and 850°C oil-quenched + 400°C tempered conditions yield significantly higher hardness and corrosion-resistant wear performance compared to the reference 35CrMo carbonitrided condition. The improvement in corrosion-resistant wear performance is quantified as a factor of 3 to 12 times.
| Treatment | Relative Hardness | Corrosion-Wear Resistance Improvement |
|---|---|---|
| D618 + 850°C air cool | High | 3–12× vs. 35CrMo carbonitrided |
| D618 + 850°C oil quench + 400°C temper | Highest | 3–12× vs. 35CrMo carbonitrided |
| 35CrMo carbonitrided (reference) | Baseline | Baseline |
Engineering Practice Applications
This research has direct relevance to overlay repair of pipe fittings and industrial components exposed to combined corrosive and abrasive environments:
- Post-weld heat treatment (PWHT) necessity — The significant performance improvement from heat treatment demonstrates that as-welded D618 overlays may not achieve their full potential. In engineering practice, PWHT should be specified as part of the overlay qualification procedure, particularly for critical components such as valve seats, pump impellers, and pipe elbows in slurry service.
- Cooling rate sensitivity — The comparison between air cooling and oil quenching reveals that cooling rate is a critical parameter. In large pipe fitting overlays, achieving uniform cooling rates across the overlay surface may be challenging, and localized variations could lead to inconsistent hardness and wear performance.
- Comparison with surface hardening alternatives — The 3–12× improvement over 35CrMo carbonitriding demonstrates that overlay welding with high-Cr cast iron alloys is a superior alternative to surface hardening for combined corrosion-wear protection, particularly where the component geometry makes uniform surface treatment impractical.
Key Reflections
The 850°C heat treatment temperature is significant because it is near the A₁ temperature for high-carbon, high-chromium systems, where carbide dissolution and redistribution occur without excessive grain growth. This temperature window is narrow, and precise control is essential — deviation of even 20–30°C could result in either incomplete carbide redistribution or excessive softening.
The 400°C tempering temperature for the oil-quenched condition is chosen to balance hardness retention with residual stress relief. In overlay applications, residual stress management is critical because excessive tensile residual stress at the overlay-base interface can lead to delamination or cracking under service loads.
Study Insights
This work reinforces the principle that overlay weld performance is not determined solely by consumable selection but is significantly influenced by post-weld thermal processing. For pipe and fitting manufacturers, this implies that overlay qualification procedures must include standardized PWHT protocols, not merely consumable specification. The 3–12× improvement in corrosion-wear resistance represents a compelling economic case for overlay repair over component replacement, particularly for large-diameter pipe fittings where replacement costs are prohibitive.