Microstructure and Properties of H3Cr5WMoV Submerged Arc Surfacing Alloy Layer

Literature Overview

The paper by Liu Shuofeng (2002), published in the Journal of Anhui University of Technology (Natural Science Edition) (Vol. 19, No. 4, pp. 263-265), investigates the microstructure and mechanical properties of an H3Cr5WMoV alloy layer deposited by automatic submerged arc surfacing on Q235 carbon steel substrate. The author is affiliated with the Technical Center of Nanjing Iron & Steel Co., Ltd. The work is classified under TG455 (surfacing welding) and addresses the development of wear-resistant and thermally fatigue-resistant overlay layers for industrial applications.

Core Technical Context

H3Cr5WMoV is a hot work tool steel containing chromium, tungsten, molybdenum, and vanadium, known for its excellent combination of high-temperature strength, wear resistance, and thermal fatigue resistance. The deposition of this alloy on a lower-cost Q235 carbon steel substrate provides an economical solution for applications requiring a wear-resistant surface without the cost of a full H3Cr5WMoV component.

Application Scenarios

• Hot work dies and molds: Forging dies, extrusion dies, and stamping dies operating at elevated temperatures.
• Rolling mill rolls: Wear-resistant surfacing on roll surfaces exposed to hot metal.
• Industrial components: Parts subjected to abrasive wear and thermal cycling, such as furnace fixtures, conveyor components, and casting molds.

Surfacing Process and Heat Treatment

The automatic submerged arc welding (SAW) process was used to deposit the H3Cr5WMoV alloy layer on the Q235 substrate. Submerged arc welding is particularly suitable for surfacing applications due to its high deposition rate, deep penetration, and consistent weld quality. The flux provides both shielding and alloying effects, contributing to the final composition of the deposit.

Process Configuration

The post-weld tempering treatment is critical for optimizing the mechanical properties of the surfacing layer. The as-welded microstructure contains retained austenite and possibly untempered martensite, which can be detrimental to toughness and thermal fatigue resistance.

Microstructural Analysis

As-Welded Microstructure

The as-welded surfacing layer exhibits a microstructure consisting of:

• Lath martensite: The primary phase, formed during the rapid cooling of the weld metal. The lath morphology is characteristic of medium-carbon to high-carbon martensitic steels.
• Retained austenite: Residual austenite that did not transform during cooling, stabilized by alloying elements such as carbon, chromium, and nickel (if present).

Tempered Microstructure

After tempering treatment, the microstructure evolves as follows:

• Tempered martensite: The lath martensite transforms to tempered martensite with fine carbide precipitation.
• Reduced retained austenite: Some retained austenite may transform during tempering, depending on the tempering temperature and time.
• Carbide precipitation: Secondary carbides of chromium, tungsten, and vanadium precipitate during tempering, contributing to secondary hardening.

Mechanical Property Comparison

Hardness Analysis

The counterintuitive finding that the tempered state exhibits higher hardness than the as-welded state can be explained by the secondary hardening effect of H3Cr5WMoV. During tempering at appropriate temperatures (typically 500-650°C), fine carbides of tungsten and vanadium precipitate within the martensitic matrix, providing a significant hardening effect that can exceed the as-quenched hardness. This secondary hardening is a hathe writing systemark of high-speed steels and hot work tool steels.

Wear Resistance

The improved wear resistance in the tempered state is attributed to:

• Higher hardness from secondary carbide precipitation.
• More uniform microstructure with reduced retained austenite.
• Reduced residual stresses that could promote crack initiation during wear.

Thermal Fatigue Resistance

Thermal fatigue resistance is enhanced in the tempered state due to:

• Improved ductility and toughness from tempering.
• Reduced residual stresses that would otherwise contribute to thermal fatigue crack initiation.
• More stable microstructure under thermal cycling conditions.

Engineering Practice Implications

The findings of this study have direct implications for the design and application of surfaced components:

1. Heat treatment is mandatory: The as-welded state is not acceptable for service. Tempering treatment is essential to achieve optimal mechanical properties.
2. Tempering temperature optimization: The tempering temperature must be carefully selected to maximize secondary hardening while avoiding over-tempering. Typical tempering temperatures for H3Cr5WMoV range from 500°C to 650°C.
3. Multi-pass surfacing: For thick surfacing layers, multi-pass deposition with interpass temperature control is necessary to manage the thermal history and microstructure.
4. Flux selection: The composition of the submerged arc flux influences the final alloy composition of the surfacing layer. Flux with appropriate alloy content can supplement the wire composition to achieve the desired H3Cr5WMoV chemistry.

Key Questions and Reflections

Several important questions arise from this study:

1. Dilution effects: The dilution of the surfacing layer by the Q235 substrate affects the final composition and properties. The degree of dilution must be controlled to ensure the surfacing layer meets the required mechanical properties. Multi-pass surfacing with a high first-pass dilution and subsequent passes with lower dilution is a common strategy.
2. Bond strength: The adhesion strength between the surfacing layer and the Q235 substrate is critical for service life. The thermal mismatch between the high-alloy surfacing layer and the low-carbon substrate can lead to cracking at the interface during cooling or thermal cycling.
3. Residual stress: The residual stress state in the surfacing layer and substrate is not discussed. Residual stresses can be tensile or compressive and significantly affect the service performance of the surfaced component.
4. Long-term thermal fatigue: The thermal fatigue testing methodology and the number of thermal cycles applied are not detailed. Long-term thermal fatigue behavior under industrial service conditions may differ from laboratory test results.

Study Insights and Implications

This study demonstrates that submerged arc surfacing of H3Cr5WMoV on Q235 steel, combined with appropriate tempering treatment, produces a surfacing layer with superior hardness, wear resistance, and thermal fatigue resistance compared to the as-welded state. The secondary hardening effect of the high-alloy composition is the key mechanism driving the property improvement.

For practicing engineers, the critical takeaway is that the combination of process parameters, heat treatment, and material selection must be optimized as a system. The submerged arc process provides the deposition, the H3Cr5WMoV composition provides the alloying basis, and the tempering treatment activates the secondary hardening mechanism. Any deviation from the optimized combination can result in suboptimal properties.

The economic advantage of surfacing a high-alloy layer on a low-cost substrate is substantial. A full H3Cr5WMoV component would be significantly more expensive than a Q235 substrate with a thin H3Cr5WMoV surfacing layer. This approach is particularly attractive for large components where the volume of material is significant.