Microstructure and Properties of Overlay Welds on Quenched 42Cr2Mo Steel Using Bainitic Electrodes

Literature Overview

This study by Gao Bingyi from Nanchong Vocational and Technical College, published in Welding Technology (Volume 39, Issue 2, 2010), investigates the welding characteristics and overlay weld properties when using bainitic welding electrodes to repair quenched 42Cr2Mo heat-resistant steel. The research focuses on the influence of welding current and interpass temperature on the microstructure and mechanical properties of the overlay layer. The study is motivated by the need to repair worn surfaces on heat-resistant steel components in power generation and industrial heating applications, where 42Cr2Mo steel is commonly used due to its excellent high-temperature strength and creep resistance.

Core Technical Findings

The primary finding of this study is that bainitic welding electrodes provide good weldability and satisfactory service properties when used for overlay repair of quenched 42Cr2Mo steel. The weld metal microstructure is characterized by a predominantly granular bainite structure, which offers a favorable combination of strength, toughness, and wear resistance. The welding current and interpass temperature were identified as the key process parameters influencing the overlay microstructure and properties.

The observation that higher welding current and higher interpass temperature produce finer microstructures is counterintuitive from a conventional welding metallurgy perspective, where increased heat input typically leads to coarser microstructures. This finding warrants careful analysis and may be attributed to several factors:

1. Increased dilution: Higher current and interpass temperature increase the dilution of the base metal into the weld metal. The quenched 42Cr2Mo base metal contains alloying elements (Cr, Mo) that promote bainitic transformation and refine the microstructure. Increased dilution introduces more of these alloying elements into the weld metal, promoting finer bainite formation.
2. Modified cooling rate: The interaction between increased heat input and increased dilution may result in a complex cooling rate profile. While the peak temperature is higher, the cooling rate from the austenitization temperature may be affected by the increased alloy content in the weld metal, which can shift the transformation temperature range and influence the bainite formation kinetics.
3. Interpass temperature effect: Higher interpass temperature allows for more complete recovery and recrystallization of the previous pass before the next pass is deposited. This can lead to a more uniform austenite grain structure in the previously deposited material, which in turn affects the transformation behavior during subsequent solidification and cooling.

Metallurgical Analysis of Bainitic Microstructure

The granular bainite microstructure observed in the overlay welds is metallurgically significant. Granular bainite consists of ferrite grains nucleating at austenite grain boundaries and austenite inclusions, with the ferrite grains growing to impinge upon each other. This microstructure is characterized by:

• Good toughness: The equiaxed ferrite grains provide good resistance to crack propagation, particularly at low temperatures.
• Moderate strength: The strength is lower than martensitic microstructures but higher than ferrite-pearlite microstructures, making it suitable for repair applications where toughness is important.
• Adequate wear resistance: The bainitic microstructure provides sufficient hardness for wear applications, particularly when combined with the alloying elements from the base metal dilution.
• Thermal stability: Bainitic microstructures are generally more thermally stable than martensitic microstructures, making them suitable for heat-resistant steel repair applications where the component may be exposed to elevated temperatures during service.

The maximum hardness observed in the heat-affected zone (HAZ) and fusion zone is a critical finding. In quenched 42Cr2Mo steel, the HAZ is subject to a thermal cycle that may produce a hard and brittle microstructure, particularly if the cooling rate is rapid enough to form martensite. The high hardness in the HAZ indicates that the thermal cycle imposed by the welding process is sufficient to produce a martensitic or martensitic-bainitic microstructure in this region. This is a potential concern for crack susceptibility and should be addressed through appropriate welding procedure design.

Engineering Practice Considerations

For engineers involved in the repair of heat-resistant steel components, the following practical considerations emerge from this study:

1. Electrode selection: Bainitic electrodes are suitable for overlay repair of quenched 42Cr2Mo steel, providing good weldability and satisfactory service properties. The electrode selection should be based on the specific service conditions, including temperature, pressure, and wear environment.
2. Current and interpass temperature control: The study demonstrates that both welding current and interpass temperature significantly influence the overlay microstructure and properties. A systematic approach to parameter selection is recommended, with lower current and interpass temperature used for applications requiring higher toughness, and higher values used for applications requiring finer microstructure and potentially higher hardness.
3. HAZ management: The high hardness in the HAZ and fusion zone indicates a potential crack susceptibility issue. To mitigate this risk, the following measures should be considered:

• Preheating the base material to reduce thermal gradients and cooling rates in the HAZ
• Using low-hydrogen electrodes to minimize hydrogen-induced cracking risk
• Applying post-weld heat treatment to relieve residual stresses and soften the HAZ microstructure
• Limiting the interpass temperature to prevent excessive HAZ softening

1. Multi-pass deposition: For thick overlay repairs, the deposition of multiple passes is necessary. The interpass temperature should be carefully controlled to ensure that each pass contributes to a uniform microstructure without excessive softening of the previously deposited material.
2. Post-weld inspection: The overlay weld should be inspected for defects (cracks, porosity, lack of fusion) using appropriate NDT methods (magnetic particle testing for surface defects, ultrasonic testing for internal defects). The HAZ hardness should be measured to verify that it is within acceptable limits for the application.

Study Insights and Reflections

The study by Gao Bingyi provides valuable practical guidance for the repair of heat-resistant steel components using bainitic welding electrodes. The demonstration that bainitic electrodes produce satisfactory weld metal properties on quenched 42Cr2Mo steel expands the range of available repair options for this important alloy. The finding that higher welding current and interpass temperature produce finer microstructures is particularly interesting and warrants further investigation to fully understand the underlying metallurgical mechanisms.

One area for further research is the long-term creep behavior of the overlay welds at elevated temperatures. Heat-resistant steel components are often subjected to sustained loads at temperatures approaching the creep regime, and the overlay weld must maintain its mechanical integrity under these conditions. The bainitic microstructure, while offering good short-term properties, may be susceptible to creep degradation over extended service life. Accelerated creep testing would provide valuable data on the long-term reliability of the overlay repair.

Additionally, the study could be extended to include impact testing at various temperatures to evaluate the toughness of the overlay welds under different service conditions. The HAZ, which exhibits the highest hardness, should be particularly scrutinized for low-temperature toughness, as hard and brittle microstructures are susceptible to brittle fracture at low temperatures.

The practical significance of this work lies in its potential to extend the service life of heat-resistant steel components through reliable overlay repair. By providing clear guidance on electrode selection and welding parameter optimization, the study supports engineers in making informed decisions that balance repair cost, component availability, and service reliability.

Summary

The five studies reviewed in this document collectively address critical aspects of overlay welding technology across diverse applications: hot work die repair with nitrogen-alloyed martensitic stainless steel electrodes, hearth roller overlay repair with hot crack-resistant flux-cored wires, non-destructive inspection of granulation die overlay layers, hot isostatic pressing of nickel-based alloy overlay welds, and repair of quenched heat-resistant steel with bainitic electrodes. Each study contributes unique insights into the metallurgical behavior, process optimization, and quality assurance of overlay welding systems. The common thread across all studies is the fundamental challenge of balancing competing requirements—hardness versus toughness, defect elimination versus property retention, process simplicity versus microstructural control. Engineers must adopt a holistic approach that considers the specific service conditions, failure modes, and quality requirements of each application when selecting and optimizing overlay welding procedures. The integration of metallurgical understanding, process engineering, and quality assurance practices is essential for achieving reliable and cost-effective overlay repair solutions across the spectrum of industrial applications.