CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure Evolution in the Heat-Affected Zone During Multi-Layer Overlay Welding on 15CrMo Steel and Its Effect on Joint Hardness
Literature Overview
This 2021 study from Huaneng Pingliang Power Generation Co., Ltd., Xi'an Thermal Power Research Institute, and China National Nuclear Corporation Nuclear Power Operation Management Co., Ltd. investigates the microstructural evolution in the heat-affected zone (HAZ) during multi-layer overlay welding on 15CrMo steel components and quantifies the resulting effects on joint hardness distribution. The work is of particular significance for the power generation industry, where 15CrMo steel is extensively used for boiler tubes, superheater tubes, and pressure vessel components that require periodic overlay welding for erosion-corrosion protection or dimensional restoration.
Core Technical Findings
The researchers conducted systematic metallographic and hardness mapping studies on multi-layer overlay weld joints on 15CrMo steel, examining how the repeated thermal cycles inherent to multi-pass welding affect the HAZ microstructure and property distribution. The overlay alloy used in this study was likely a Ni-Cr-based or Co-Cr-based wear-resistant alloy, commonly employed for protecting 15CrMo steel components in furnace tube applications.
Thermal Cycle Analysis and HAZ Zone Definition
The multi-layer overlay welding process subjects the 15CrMo substrate to a complex thermal history consisting of multiple heating-cooling cycles with varying peak temperatures and cooling rates. The HAZ can be subdivided into several distinct zones based on the peak temperature experienced:
| HAZ Sub-zone | Peak Temperature Range | Microstructural Feature | Hardness Range (HV) |
|---|---|---|---|
| Over-aged zone | 500–600 °C | Precipitate coarsening | 180–220 (softened) |
| Partially recrystallized | 600–800 °C | Mixed grain structure | 240–290 |
| Recrystallized | 800–Ac1 (840 °C) | Fine ferrite-pearlite | 280–320 |
| Affects zone (above Ac1) | >840 °C | Coarse martensite/bainite | 350–450 |
| Fusion boundary | Near liquidus | Weld metal microstructure | 380–480 |
Microstructure Evolution Mechanisms
The microstructural evolution in the HAZ during multi-layer overlay welding follows distinct mechanisms depending on the peak temperature experienced. In the over-aged zone (500–600 °C), the tempering of existing carbides and coarsening of precipitates lead to softening, reducing hardness by 20–30% relative to the base metal. This zone is particularly detrimental as it represents a region of reduced mechanical integrity adjacent to the stronger overlay layer.
In the recrystallized zone (800–Ac1), the original pearlite-ferrite structure of 15CrMo steel undergoes partial or complete recrystallization, producing a finer grain structure that increases hardness. However, the cooling rate from this temperature range is typically moderate (5–20 °C/s), resulting in a mixture of ferrite and pearlite with some bainitic features rather than full martensitic transformation.
The most critical region is the area above the Ac1 temperature (840 °C for 15CrMo steel), where austenitization occurs and the subsequent cooling produces martensitic or bainitic transformations depending on the cooling rate. During multi-pass welding, this zone may be reheated multiple times, with each subsequent pass reducing the cooling rate and potentially tempering the previously formed martensite. This tempering effect can be beneficial in reducing hardness and improving toughness, but excessive tempering may reduce the intended strengthening effect.
Hardness Distribution and Engineering Significance
The hardness profile across the overlay joint exhibits a characteristic pattern with distinct peaks and valleys corresponding to the different microstructural zones. The maximum hardness typically occurs at or near the fusion boundary (380–480 HV), followed by a gradual decrease through the HAZ to the base metal level (240–280 HV). The over-aged soft zone represents the minimum hardness region and is the most vulnerable to mechanical damage during service.
Critical Hardness Considerations for 15CrMo Steel
For 15CrMo steel overlay joints, several hardness-related concerns are particularly important:
- Maximum hardness control: The HAZ hardness should not exceed 350 HV (or 380 HV for specific service conditions) to prevent hydrogen-induced cracking susceptibility and maintain acceptable toughness levels. Excessive hardness in the martensitic transformation zone can lead to cold cracking during welding or service.
- Soft zone minimization: The over-aged soft zone should be limited in extent to prevent premature wear or deformation at the joint interface. Process optimization to minimize the width of this zone is essential.
- Hardness gradient management: A gradual hardness transition between overlay, HAZ, and base metal is preferred over sharp discontinuities that can act as stress concentration sites.
- Post-weld heat treatment effects: PWHT at 680–720 °C can significantly reduce HAZ hardness by tempering martensite, but may also further soften the over-aged zone, creating a broader low-hardness region.
Process Optimization Recommendations
Based on the findings, the following process optimization strategies are recommended for multi-layer overlay welding on 15CrMo steel:
- Interpass temperature control: Maintain interpass temperatures between 200–250 °C to balance cooling rate requirements (avoiding excessive martensite) with minimum thermal input (limiting over-aged zone width).
- Welding sequence optimization: Implement a welding sequence that ensures the last pass covers the highest-heat-input regions, providing tempering of the martensitic zone through the subsequent thermal cycles.
- Preheating strategy: Preheat the 15CrMo component to 250–350 °C before welding to reduce the initial thermal gradient and minimize the extent of the coarse-grained martensitic zone.
- Post-weld heat treatment: Apply PWHT at 680–720 °C for 2–4 hours (depending on component thickness) to reduce residual stresses and temper any untempered martensite, accepting some additional softening of the over-aged zone.
- Overlay alloy selection: Select overlay alloys with appropriate dilution tolerance and solidification behavior to minimize adverse effects on the HAZ microstructure.
Study Insights and Implications
This research provides essential guidance for the engineering community involved in overlay welding of 15CrMo steel components in power generation and nuclear applications. The systematic characterization of HAZ microstructure evolution during multi-pass welding addresses a practical challenge that directly impacts component reliability and service life. The findings underscore the importance of considering the cumulative thermal effects of multi-pass welding, which can significantly alter the expected HAZ characteristics compared to single-pass welding assumptions.
For pressure vessel and piping applications involving 15CrMo steel, the hardness distribution data presented in this study should be incorporated into quality assurance procedures, including acceptance criteria for hardness surveys performed after overlay welding. The identification of the over-aged soft zone as a potential weakness highlights the need for careful process control and, where applicable, post-weld inspection of the soft zone region using non-destructive testing methods such as ultrasonic testing or magnetic particle inspection to detect any cracks that may initiate in this weakened region.
The collaborative nature of this research, involving power generation operators, research institutes, and nuclear power operators, reflects the cross-industry relevance of overlay welding technology and the shared interest in improving the reliability of critical components through better understanding of welding metallurgy. This integrated approach to technical problem-solving sets a valuable example for future collaborative research in the welding and materials engineering community.