Zhuojin Pipe Fitting Co., LtdMicrostructure Evolution in Heat-Affected Zone During Multi-Layer Overlay Welding of 15CrMo Steel
Literature Overview
The research by Feng Linjie et al. (2021), published in Hot Working Technology, examines the microstructural evolution in the heat-affected zone (HAZ) of 15CrMo steel during multi-layer overlay welding. This study is particularly significant for power plant and petrochemical engineers who routinely encounter 15CrMo steel components requiring surface repair or hardfacing without post-weld heat treatment (PWHT). The findings provide a quantitative basis for determining the minimum number of overlay layers required to eliminate the high-hardness zone in the HAZ.
Multi-Layer Thermal Cycle Effects on HAZ Microstructure
The study reveals a systematic evolution of the HAZ microstructure as the number of overlay layers increases. The following table summarizes the key observations:
| Overlay Layer | HAZ Microstructure | Peak Hardness (HV) | Hardness Differential vs. Base Metal |
|---|---|---|---|
| 1 layer | Coarse Widmanstätten ferrite in over-tempered zone | ~280+200 = 480 HV | +200 HV above base metal |
| 2-3 layers | Transition zone, partial Widmanstätten dissolution | ~350-400 HV | +150-170 HV above base metal |
| 4 layers | Fine equiaxed ferrite beginning to form | ~300-340 HV | +120-140 HV above base metal |
| ≥5 layers | Fully equiaxed ferrite, no Widmanstätten | ~270-280 HV | <100 HV above base metal |
The mechanism driving this evolution is the progressive tempering and recrystallization of the Widmanstätten ferrite structure formed during the first weld pass. The coarse Widmanstätten ferrite, characterized by plate-like ferrite grains growing from prior austenite grain boundaries, exhibits high hardness due to its fine cementite lamellae and high dislocation density. Subsequent heat input from additional weld passes provides the thermal energy necessary to dissolve the cementite lamellae and recrystallize the ferrite into equiaxed grains.
Engineering Significance for Post-Weld Heat Treatment Exemption
The critical engineering finding is that after five or more overlay layers, the entire weld joint hardness remains below 280 HV, which is within the acceptable range for 15CrMo steel components operating at elevated temperatures. This finding has direct implications for maintenance scheduling in power plants and refineries, where PWHT of repaired components can be time-consuming and expensive. If the overlay repair can be designed to achieve sufficient layer count for HAZ softening, the PWHT step may be eliminated, significantly reducing repair turnaround time.
The hardness threshold of 280 HV is not arbitrary—it corresponds to the maximum allowable hardness specified in many code standards for 15CrMo steel components, ensuring adequate ductility and toughness for pressure-containing service. The reduction of the hardness differential to less than 100 HV between the HAZ and base metal also minimizes the risk of stress concentration and crack initiation at the interface during thermal cycling.
Process Optimization Recommendations
Based on the findings of this study, the following process recommendations emerge for overlay welding of 15CrMo steel components:
- Layer count planning: A minimum of five layers should be applied to ensure complete Widmanstätten dissolution and HAZ softening
- Heat input control: Each subsequent layer should provide sufficient heat input to temper the previous layer's HAZ without causing excessive grain growth
- Interpass temperature management: Maintaining interpass temperature between 200-300°C facilitates gradual microstructural refinement without thermal shock
- Post-repair verification: Hardness mapping of the completed overlay should confirm that the maximum hardness does not exceed 280 HV throughout the joint
The study also highlights the importance of using homogenous consumables (same composition as base metal) for overlay applications where the primary objective is HAZ conditioning rather than surface hardening. This approach ensures that the deposit composition does not introduce additional hardness peaks or embrittlement risks.
Study Insights and Practical Implications
This research provides a rare quantitative analysis of the microstructural evolution during multi-layer welding, moving beyond the qualitative understanding that "more layers soften the HAZ" to precise predictions of hardness reduction as a function of layer count. For engineers responsible for repair qualification and procedure development, this data enables the design of overlay repair procedures that are specifically optimized for HAZ conditioning.
The findings also connect to broader principles of welding metallurgy. The Widmanstätten-to-equiaxed transformation is a well-documented phenomenon in low-alloy steel welding, but the specific layer count required for complete transformation in 15CrMo steel had not been previously established. This gap in knowledge has been filled by the present study, providing a solid foundation for procedure qualification and quality assurance in industrial repair operations.
The practical impact of this research extends to maintenance cost reduction, as eliminating PWHT for overlay repairs of 15CrMo components can save significant downtime and energy costs in power generation and petrochemical facilities.