Effect of Welding Process on Compatibility Between 1Cr16Ni4Mo2Cu2W1VN Steel and Co6B Alloy Overlay Layer

Literature Overview

This 2024 paper published in Heat Treatment of Metals (Vol. 49, No. 7, pp. 326–330) by Wu Weijian, Liu Chengmiao, and Li Quan from Shanghai No. 1 Machine Tool Works and the Special Steel Research Institute of the Iron and Steel Research Institute investigates the compatibility between a high-alloy steel substrate (1Cr16Ni4Mo2Cu2W1VN) and a Co6B overlay alloy under two different welding processes: oxyacetylene welding and argon arc welding (GTAW). This work addresses a critical practical question in overlay welding: how does the choice of welding process affect the metallurgical compatibility and resulting performance of the overlay system?

Substrate and Overlay Material Characteristics

The substrate steel 1Cr16Ni4Mo2Cu2W1VN is a high-alloy austenitic-martensitic stainless steel containing significant amounts of Cr, Ni, Mo, Cu, W, V, and N. This composition provides excellent combination of strength, wear resistance, and corrosion resistance, making it suitable for demanding machine tool applications. The Co6B overlay alloy is a cobalt-based hardfacing alloy known for its outstanding high-temperature hardness retention, wear resistance, and corrosion resistance.

The metallurgical challenge in overlaying Co6B onto this steel substrate lies in achieving a strong metallurgical bond without excessive dilution that would degrade the Co6B overlay properties, and without creating a brittle intermetallic compound layer at the interface.

Comparative Analysis of Welding Processes

Oxyacetylene Welding (OAW)

Oxyacetylene welding provides a lower energy density and slower cooling rate compared to GTAW. The flame-based heat source produces a wider, shallower weld pool. Key characteristics observed in this study:

• Wider fusion transition zone: The broader heat-affected zone in OAW results in a more gradual compositional gradient between the base steel and the overlay alloy.
• Higher overlay hardness: The OAW overlay layer exhibits hardness approximately 7.3 HRC higher than the GTAW overlay layer. This is attributed to the slower cooling rate promoting different carbide precipitation patterns and potentially different matrix microstructure.
• Higher dilution: The wider weld pool in OAW allows more base metal to mix into the overlay, which can be detrimental if the base metal composition is significantly different from the overlay alloy.

Argon Arc Welding (GTAW)

GTAW provides a higher energy density and faster cooling rate with a narrower, deeper weld pool. Key characteristics:

• Narrower fusion transition zone: The focused heat input creates a sharper compositional boundary between base and overlay.
• Lower overlay hardness: The faster cooling rate may result in different carbide morphology and distribution.
• Lower dilution: The concentrated heat input minimizes base metal mixing, better preserving the overlay alloy composition.

Effect of Heat Treatment

The study demonstrates that heat treatment significantly improves the performance of both overlay systems, but with different optimal parameters:

The solution treatment + deep freeze + tempering sequence is particularly effective for cobalt-based alloys because:

1. Solution treatment: Dissolves carbides and homogenizes the microstructure at elevated temperatures.
2. Deep freeze treatment (typically −78 °C to −196 °C): Promotes the precipitation of fine, uniformly distributed carbides during subsequent tempering, and stabilizes the retained austenite if present.
3. Tempering: Relieves residual stresses, promotes carbide precipitation for strengthening, and improves toughness.

Metallurgical Compatibility Assessment

The study concludes that 1Cr16Ni4Mo2Cu2W1VN steel and Co6B alloy exhibit good metallurgical compatibility. This finding is significant because cobalt-based overlays on high-alloy steels can sometimes produce brittle intermetallic phases at the interface. The good compatibility is likely due to:

• The high Ni content in the base steel, which promotes austenite stability and reduces the tendency for brittle phase formation.
• The presence of Mo and W in the base steel, which are also present in many cobalt-based alloys and promote solid solubility.
• The careful selection of welding parameters to control dilution and cooling rate.

Engineering Practice Integration

For engineers working with high-alloy steel machine tool components that require hardfacing overlay, this study provides several practical guidelines:

1. Process selection: Oxyacetylene welding is preferred when maximum overlay hardness and toughness are required, provided that the wider dilution zone does not compromise the overlay composition beyond acceptable limits.
2. Post-weld heat treatment is essential: The dramatic improvement in properties after solution + deep freeze + tempering treatment underscores the importance of post-weld heat treatment in cobalt-based overlay systems.
3. Microstructural verification: Engineers should perform microsection examination of the overlay interface to verify the absence of brittle intermetallic phases and to confirm the intended microstructure.

Study Insights and Reflections

This paper addresses a practical question that is often overlooked in overlay welding practice: the influence of welding process on the metallurgical compatibility between substrate and overlay. In many industrial settings, the choice between oxyacetylene and arc welding is driven by equipment availability and operator skill rather than metallurgical considerations. This study demonstrates that the welding process choice has a measurable impact on overlay performance, and that this impact can be optimized through appropriate post-weld heat treatment.

The finding that OAW produces a harder and tougher overlay after heat treatment is somewhat counterintuitive, as OAW typically produces more dilution. However, the wider fusion zone may create a more gradual compositional transition that reduces residual stresses at the interface, and the slower cooling rate may promote more favorable carbide precipitation. This suggests that the relationship between dilution and overlay performance is not always straightforward and depends on the specific material system and heat treatment.

The use of deep freeze treatment in the heat treatment sequence is a sophisticated approach that is not commonly employed in industrial overlay welding. The additional equipment and processing time required for deep freeze treatment may not be justified for all applications, but for high-value machine tool components where overlay performance is critical, this approach provides measurable benefits.

Reference Value and Outlook

This study provides valuable guidance for engineers selecting welding processes and heat treatment sequences for cobalt-based overlay applications on high-alloy steel substrates. The systematic comparison of two welding processes with and without heat treatment creates a comprehensive performance matrix that can inform process selection decisions. Future work could extend this investigation to include additional welding processes such as laser cladding and HVOF spraying, and to evaluate the overlay performance under actual service conditions involving wear, corrosion, or erosion.