Microstructural Evolution in the Heat-Affected Zone During Multi-Layer Hardfacing of 15CrMo Steel

Literature Overview

This study, conducted by Feng Linjie, Lv Pinzheng, Zhao Yongming, Zhang Wei, Liu Fuguang, Ge Jun, Yang Erjuan, Mi Zihao, and Wang Yansong from Huaneng Pingliang Power Generation Co., Ltd., Xi'an Thermal Power Research Institute, and CNNC Nuclear Power Operation Management Co., Ltd., was published in Hot Working Technology in 2021. The research investigates the microstructural evolution in the heat-affected zone (HAZ) during multi-layer hardfacing of 15CrMo steel, with particular attention to the transformation of high-hardness Widmanstätten structures and their impact on joint hardness.

Core Technical Findings

The study employed optical microscopy and microhardness testing to examine the HAZ microstructure and hardness distribution after different numbers of hardfacing passes. The key findings are summarized below.

Hardfacing Layer	HAZ Microstructure	Peak Hardness	Hardness Difference from Base Material
1 layer	Coarse Widmanstätten ferrite + pearlite	Base + 200 HV	+200 HV
2–4 layers	Transition: Widmanstätten transforming to equiaxed grains	Decreasing	100–200 HV
>5 layers	Fully equiaxed ferrite + pearlite	No peak; uniform	<100 HV

After more than five hardfacing layers, the entire joint hardness remains below 280 HV, which is within the acceptable range for post-weld heat treatment (PWHT) without requiring weld repair.

Interpretation of Key Technical Points

The evolution of Widmanstätten ferrite in the HAZ is a classic phenomenon in low-alloy steel welding. During the first hardfacing pass, the HAZ experiences a rapid heating and cooling cycle that produces a coarse Widmanstätten ferrite structure with high hardness. This structure is brittle and susceptible to cracking under residual stress. However, during subsequent hardfacing passes, the HAZ from the previous pass is reheated to temperatures below the Ac1 line, allowing the coarse Widmanstätten ferrite to transform into finer, equiaxed grains through a process known as tempering and recrystallization.

The complete transformation of Widmanstätten ferrite to equiaxed grains after five or more passes is significant for engineering practice. It means that multi-layer hardfacing inherently provides a self-tempering effect that reduces HAZ hardness and improves ductility. This is particularly important for 15CrMo steel, which is commonly used in high-pressure boiler tubes and nuclear power components where post-weld toughness is critical.

The finding that the entire joint hardness remains below 280 HV after multi-layer hardfacing is directly relevant to welding procedure qualification. Most codes and standards require that the hardness of the weld joint not exceed a specified limit—typically 280–320 HV—to ensure weldability and resistance to hydrogen-induced cracking. The self-tempering effect of multi-layer hardfacing helps meet this requirement without additional post-weld heat treatment.

Engineering Practice Implications

For engineers performing hardfacing repairs on 15CrMo steel components, the following practical guidelines emerge from this study:

Multi-layer hardfacing is preferred over single-layer deposition to achieve a more uniform and tougher HAZ microstructure.
A minimum of five hardfacing layers is recommended to ensure complete transformation of Widmanstätten ferrite to equiaxed grains.
Interpass temperature should be maintained between 200°C and 350°C to promote the tempering effect while avoiding excessive grain growth.
Post-weld hardness testing should be performed to verify that the joint hardness remains within acceptable limits before proceeding to post-weld heat treatment.

The study also highlights the importance of understanding the thermal history of the HAZ during multi-layer welding. Engineers should consider the cumulative thermal input and its effect on microstructural evolution when designing hardfacing procedures for critical components.

Study Insights and Reflections

This study provides a clear demonstration of the self-tempering effect in multi-layer hardfacing of low-alloy steels. The finding that five or more layers are required to achieve complete microstructural transformation has direct implications for welding procedure design and qualification. For engineers working with 15CrMo steel components in power generation and nuclear applications, this knowledge is essential for ensuring that hardfacing repairs do not compromise the mechanical integrity of the component. The study reinforces the principle that welding is not merely a joining process but a thermal process that fundamentally alters the microstructure and properties of the base material.