Effect of Preheating Temperature on Microstructure and Properties of Stellite 6 Overlay Coating on 5CrNiMo Steel

Literature Overview

This paper by Zhang Peng, Cui Mingliang, Fu Qiang, and Yuan Wuhua, published in Hot Working Technology in 2020, investigates the influence of preheating temperature on the microstructure and mechanical properties of Stellite 6 overlay coatings applied to 5CrNiMo die steel via plasma arc overlay welding. The study was conducted jointly by China Erzhong Heavy Machinery Group and Hunan University, combining industrial expertise with academic research capabilities. The work provides valuable insights into the optimization of preheating parameters for overlay welding of cobalt-based hardfacing alloys on high-alloy steel substrates, a process widely used in the refurbishment and protection of die and mold components.

Substrate and Coating Materials

5CrNiMo is a widely used hot work die steel with a nominal composition of 0.5% C, 1.5% Cr, 0.5% Ni, and 0.3% Mo. It is characterized by high strength, good hot hardness, and excellent resistance to thermal fatigue. Stellite 6 is a cobalt-chromium-tungsten alloy with a nominal composition of 55% Co, 25% Cr, 5% W, 5% Fe, and balance C and other elements. It is renowned for its exceptional wear resistance, corrosion resistance, and hot hardness, making it ideal for overlay applications in severe service environments.

The significant difference in thermal properties between the base metal and the overlay alloy creates a challenging welding scenario. The mismatch in thermal conductivity and thermal expansion coefficient leads to high thermal stresses at the weld interface, which can cause cracking, delamination, or distortion if not properly managed.

Effect of Preheating Temperature on Microstructure

The study examined preheating temperatures of 200°C, 300°C, and 400°C, and the results demonstrated a clear trend in microstructural evolution. The overlay layer microstructure consisted primarily of dendritic Co solid solution with interdendritic eutectic colonies of Co-Cr solid solution and carbides. The carbide morphology and distribution were strongly influenced by the preheating temperature.

At 200°C preheat, the rapid cooling rate resulted in fine dendritic structures with small, irregularly shaped carbides. The high cooling rate also promoted the formation of fine martensitic structures in the HAZ. At 300°C preheat, the microstructure exhibited the most favorable characteristics: fine dendritic cells, uniform carbide distribution, and minimal segregation. At 400°C preheat, the slower cooling rate led to coarsening of the dendritic structure and increased carbide size, which reduced the overall hardness and wear resistance of the overlay layer.

The HAZ microstructure also showed significant dependence on preheating temperature. At 200°C, the HAZ contained coarse needle-like martensite with high hardness values exceeding 60 HRC. At 300°C, the martensite was finer and more tempered, resulting in a more uniform hardness distribution. At 400°C, the HAZ exhibited significant tempering of the martensitic structure, which reduced the hardness but improved the toughness.

Mechanical Properties and Wear Performance

The mechanical properties of the overlay layer and HAZ were evaluated through microhardness measurements and wear testing. The results showed that the overlay layer hardness ranged from 50 to 55 HRC across all preheat conditions, with the highest values achieved at 200°C preheat. However, the HAZ hardness showed a more pronounced dependence on preheat temperature.

The wear resistance of the overlay layer was evaluated using a pin-on-disk wear test. The results showed that the 300°C preheat condition provided the best balance of wear resistance and toughness. The finer carbide distribution and more uniform microstructure at 300°C preheat contributed to improved wear performance through a combination of abrasive resistance and fatigue resistance.

Process Optimization Recommendations

Based on the study results, the following process recommendations are provided for plasma arc overlay welding of Stellite 6 on 5CrNiMo steel:

• Optimal preheat temperature: 300°C
• Heat input range: 1.2-1.8 kJ/mm
• Travel speed: 30-50 mm/min
• Shielding gas: Argon with 2-5% hydrogen
• Wire feed rate: 4-6 m/min
• Post-weld stress relief: 750°C for 2 hours

The 300°C preheat temperature is recommended as the optimal condition because it provides the best combination of overlay layer microstructure quality, HAZ hardness uniformity, and overall wear performance. This temperature is high enough to reduce the cooling rate and prevent HAZ cracking, but not so high as to cause excessive grain growth and carbide coarsening in the overlay layer.

Study Insights and Engineering Practice

This study provides a clear demonstration of the importance of preheating temperature control in overlay welding operations. The results show that a seemingly simple parameter, preheat temperature, can have a profound effect on the microstructure, mechanical properties, and service performance of the overlay weld. For engineers working with cobalt-based overlay coatings on high-alloy steel substrates, the study provides specific, actionable recommendations for process optimization.

The findings are directly applicable to the refurbishment of die and mold components in industrial settings, where the goal is to extend service life while maintaining dimensional accuracy and surface quality. The emphasis on HAZ hardness uniformity as a key performance indicator is particularly relevant for engineers who need to ensure that the overlay weld does not compromise the mechanical integrity of the base component. The study also highlights the value of systematic parameter optimization studies in developing reliable welding procedures for specialized applications.