HAZ Microstructure Evolution During Multi-Layer Overlay on 15CrMo and Hardness Effects

Literature Overview

This study examines the microstructural evolution in the heat-affected zone (HAZ) of 15CrMo steel during multi-layer overlay welding and its direct impact on joint hardness. The research provides a detailed understanding of how repeated thermal cycles from successive overlay passes affect the base metal microstructure, which is critical for ensuring the long-term integrity of clad pressure vessel components.

Core Technical Points

The HAZ in multi-layer overlay welding is subjected to a unique thermal history that differs from single-pass welding. Each subsequent overlay pass subjects the HAZ to additional thermal cycles, progressively altering the microstructure through recrystallization, grain growth, and phase transformations. For 15CrMo steel, which contains 0.9-1.1% Cr and 0.4-0.6% Mo, the microstructural response is particularly complex due to the precipitation behavior of chromium and molybdenum carbides.

Microstructural Zones in the HAZ

The HAZ can be divided into several distinct zones based on peak temperature and cooling rate:

Multi-Pass Thermal Cycle Effects

The study demonstrates that the number of thermal cycles experienced by the HAZ has a cumulative effect on microstructure and properties. After the first overlay pass, the HAZ exhibits a typical single-pass thermal cycle response. However, with each subsequent pass, the recrystallized zone grows, grain size increases, and carbide coarsening becomes more pronounced. By the fourth or fifth pass, the HAZ near the cladding interface may exhibit grain sizes 2-3 times larger than the base metal, with corresponding reductions in toughness.

Hardness Profile Analysis

The hardness distribution across the HAZ and overlay interface shows a characteristic pattern:

• The base metal hardness is relatively uniform at 260-310 HV
• The HAZ shows a hardness dip in the recrystallized zone (200-240 HV) due to grain refinement and tempering
• A hardness peak occurs at the fusion line due to rapid solidification and martensite formation
• The overlay layer hardness depends on the filler material and welding parameters

The study identifies that the hardness dip in the recrystallized zone is a potential concern for fatigue and creep performance, as this zone represents a region of reduced strength adjacent to the high-stress cladding interface.

Defect Analysis and Countermeasures

The microstructural evolution in the HAZ can lead to several defects and performance issues:

• Soft zone formation: The recrystallized zone with reduced hardness may become a preferential location for deformation under high-temperature service conditions
• Grain boundary segregation: Repeated thermal cycling can promote segregation of sulfur, phosphorus, and other impurities to grain boundaries, increasing susceptibility to temper embrittlement
• Cracking susceptibility: The combination of residual stresses and microstructural heterogeneity can promote cracking, particularly in the temper-embrittled zone

Countermeasures include:

1. Limiting the number of overlay passes to the minimum required for the specified thickness
2. Using low-heat-input processes for the final passes to reduce HAZ thermal exposure
3. Applying post-weld heat treatment to homogenize the microstructure and relieve residual stresses
4. Conducting Charpy V-notch testing on the HAZ region to verify toughness

Engineering Practice Integration

For clad pressure vessel fabrication, this study's findings have direct implications for procedure qualification and quality control. The WPS qualification should include HAZ hardness mapping as a mandatory acceptance criterion, with defined limits for the minimum hardness in the recrystallized zone. Additionally, the number of overlay passes should be justified in the design documentation, with consideration given to the cumulative thermal effects on the base metal.

Study Insights

The most significant contribution of this research is the quantitative correlation between the number of thermal cycles, HAZ microstructure, and hardness. The study provides a predictive framework that can be used to optimize overlay procedures for specific applications. For 15CrMo clad pressure vessels operating at elevated temperatures, I recommend that the HAZ hardness profile be evaluated as part of the fitness-for-service assessment, particularly for components subject to cyclic loading or thermal fatigue. This approach ensures that the microstructural effects of multi-pass overlay welding are properly accounted for in the long-term integrity assessment.