Effect of Tempering Temperature on Cladding Mold Wear Resistance - A Study Note

Research Background and Significance

Molds used in metal casting, forging, and plastic injection molding are subjected to severe thermal cycling, mechanical loading, and abrasive wear. Weld overlay cladding is widely employed to enhance the surface properties of molds by depositing wear-resistant and thermally stable alloy layers. The post-weld heat treatment, particularly the tempering temperature, plays a decisive role in determining the final microstructure, hardness, toughness, and wear resistance of the cladding layer. This study investigates the systematic effect of tempering temperature on the wear resistance of cladding layers deposited on mold steels.

Experimental Design and Materials

The study employed 45 steel (Q235) as the base material and deposited a multi-layer cladding of high-speed steel powder (W6Mo5Cr4V2 equivalent) using plasma transferred arc (PTA) welding. The cladding layer thickness was 3 mm, and the powder feed rate was 0.8 kg/h with an arc power of 4.5 kW. The cladding layers were tempered at five different temperatures: 500 °C, 550 °C, 600 °C, 650 °C, and 700 °C, each for 2 hours in a furnace.

Microstructural Evolution with Tempering Temperature

The microstructural evolution follows a predictable pattern: at lower tempering temperatures (500-550 °C), retained martensite is present along with fine secondary carbides, resulting in high hardness but moderate toughness. As the tempering temperature increases to 600 °C, the carbides coarsen and transform from M2C to M6C type, providing an optimal balance of hardness and toughness. Beyond 600 °C, the carbides continue to coarsen, and the risk of temper embrittlement increases, leading to reduced wear resistance.

Wear Testing and Performance Analysis

Wear testing was conducted using a pin-on-disk tribometer with alumina (Al2O3) balls as the counterface under a normal load of 20 N and a sliding distance of 1000 m. The wear volume loss was measured using a profilometer, and the wear coefficient was calculated.

The results showed a clear peak in wear resistance at 600 °C tempering:

• At 500 °C: Wear coefficient of 2.8 × 10^-6 mm³/N·m
• At 550 °C: Wear coefficient of 1.9 × 10^-6 mm³/N·m
• At 600 °C: Wear coefficient of 1.2 × 10^-6 mm³/N·m (optimal)
• At 650 °C: Wear coefficient of 1.8 × 10^-6 mm³/N·m
• At 700 °C: Wear coefficient of 3.2 × 10^-6 mm³/N·m

The wear mechanism transitioned from abrasive wear at lower tempering temperatures to a combination of abrasive and adhesive wear at higher temperatures. At 600 °C, the tempered martensite matrix with well-distributed M6C carbides provided the best resistance to both ploughing and material transfer.

Engineering Implications for Mold Design

The optimal tempering temperature of 600 °C has direct implications for mold design and manufacturing:

1. For hot work dies subjected to thermal cycling above 600 °C, a tempering temperature of 650 °C may be preferred to ensure dimensional stability, even at the cost of slightly reduced wear resistance
2. For cold work dies operating at room temperature, a tempering temperature of 550-600 °C provides the best balance of hardness and toughness
3. For plastic injection molds with high cycle counts, 600 °C tempering is recommended to maximize surface life
4. The tempering temperature must be coordinated with the base material's tempering temperature to avoid differential thermal expansion and residual stress buildup

Study Insights and Reflections

This study demonstrates that the tempering temperature is not merely a post-weld finishing step but a critical design parameter that determines the final performance of the cladding layer. The existence of an optimal tempering temperature window (550-600 °C for high-speed steel cladding) reflects the fundamental trade-off between hardness and toughness in tempered martensitic structures. The peak in wear resistance at 600 °C is attributed to the formation of M6C carbides, which are thermodynamically stable at this temperature and provide effective resistance to abrasive wear without the brittleness associated with M2C carbides at lower temperatures. This study reinforces the principle that post-weld heat treatment must be carefully designed based on the specific service conditions, and that a one-size-fits-all approach to tempering temperature selection is inappropriate for engineering applications. The findings also highlight the importance of microstructural characterization in understanding the relationship between processing parameters and final performance, which is essential for rational design of cladding systems for mold applications.