Cobalt-Based Hardfacing on Hot Upsetting Machine Molds

Literature Overview

This technical paper by Xiong Xuehui and Fu Haijun, published in 1994 from Harbin Bearing Factory, addresses the application of cobalt-based hardfacing technology to molds used in hot upsetting machines. Hot upsetting is a forging process in which a heated workpiece is compressed axially to produce a bulge or upset, commonly used in the manufacture of bearing rings, fasteners, and other axisymmetric components. The molds used in hot upsetting face extreme conditions including high temperatures (the workpiece is heated to 900 to 1200 °C), high contact pressures, thermal cycling, and abrasive wear from scale and oxide layers.

Technical Background and Mold Requirements

Hot upsetting molds are typically made of hot work tool steels such as H13 (4Cr5MoSiV1) or H21 (3Cr2W8V), which provide good hot hardness and thermal fatigue resistance. However, even these high-performance tool steels have limited service life in hot upsetting applications due to the severity of the operating conditions. Surface hardfacing with cobalt-based alloys can significantly extend mold life by providing a wear-resistant, thermally stable overlay on the mold cavity surfaces.

The cobalt-based alloys most commonly used for hot upsetting mold hardfacing include:

The selection of the specific cobalt-based alloy depends on the upset temperature, the material being upset, the frequency of cycles, and the required mold life. For high-temperature upsetting of bearing steel rings, Stellite 21 is often preferred due to its superior thermal fatigue resistance compared to Stellite 6.

Hardfacing Process and Welding Strategy

The hardfacing of hot upsetting molds is typically performed using one of the following processes:

For hot upsetting molds, the welding strategy typically involves the following steps:

1. Surface preparation: The mold cavity is machined to remove any damaged surface layer, and the surface is cleaned to remove scale and contamination. A shallow groove may be prepared to improve overlay adhesion.
2. Transition layer: A single pass of a medium-carbon steel electrode may be applied to create a metallurgical transition between the tool steel base and the cobalt-based overlay. This layer reduces dilution and improves bond strength.
3. Cobalt-based overlay: Two to four passes of the cobalt-based hardfacing material are applied to achieve the target overlay thickness, typically 3 to 8 mm for hot upsetting molds.
4. Post-weld grinding: The overlay surface is ground to the required dimensional tolerance and surface finish, typically Ra 0.4 to 1.6 μm for mold cavity surfaces.

The welding parameters for cobalt-based hardfacing are carefully controlled to minimize dilution and maintain the overlay composition. A typical SAW process uses a current of 300 to 500 A, a travel speed of 100 to 200 mm/min, and a flux with controlled moisture content. The preheat temperature is typically 200 to 350 °C to reduce residual stress and prevent cracking.

Microstructure and Performance

The microstructure of cobalt-based hardfacing deposits typically consists of:

• Face-centered cubic (FCC) cobalt matrix: Provides toughness and thermal stability
• M7C3 carbides: Chromium-rich carbides providing wear resistance
• M23C6 carbides: Chromium carbides contributing to hardness
• W-rich carbides: Tungsten carbides providing hot hardness
• η phase (Co₃W): Tungsten-rich intermetallic contributing to wear resistance

The relative proportion of these phases is influenced by the cooling rate and the alloy composition. A slower cooling rate promotes the formation of larger, more coherent carbides and reduces the amount of brittle phases, while a faster cooling rate produces finer microstructures with potentially higher hardness but reduced toughness.

The performance of cobalt-based hardfacing on hot upsetting molds is evaluated through several key criteria:

Engineering Applications and Case Studies

The application of cobalt-based hardfacing to hot upsetting molds has demonstrated significant performance improvements in bearing manufacturing applications. In the production of bearing rings, where the upsetting process is repeated thousands of times, hardfaced molds have shown 3 to 5 times longer life compared to unclad molds made of H13 tool steel. The economic benefit is substantial given the high cost of mold replacement and the production downtime associated with mold changes.

The paper's contribution to the field lies in providing practical experience with cobalt-based hardfacing on hot upsetting molds, including specific process parameters, material selections, and performance data. The Harbin Bearing Factory's experience with this technology provided valuable insights into the long-term reliability of hardfaced molds in production conditions.

Defect Analysis and Quality Control

Common defects in cobalt-based hardfacing of hot upsetting molds include:

Quality control includes visual inspection for surface quality, hardness survey for uniformity, magnetic particle testing for cracks, ultrasonic testing for subsurface defects, and a bond strength test on a witness coupon. The overlay must maintain its hardness after the post-weld stress relief treatment, which is why materials with good temper stability are preferred.

Key Reflections and Study Insights

A key insight from this study is the importance of the transition layer in cobalt-based hardfacing of tool steel molds. The metallurgical compatibility between the cobalt-based overlay and the iron-based tool steel base is not always optimal, and a transition layer of medium-carbon steel can significantly improve bond strength and reduce dilution. This approach has been widely adopted in subsequent practice and is now considered a best practice in mold hardfacing.

The study also highlights the challenges of maintaining dimensional accuracy during the hardfacing process. The thermal input from welding can cause distortion of the mold cavity, which must be corrected through subsequent grinding. The grinding allowance must be carefully planned to ensure that the final cavity dimensions are within tolerance after both hardfacing and grinding operations.

Another important consideration is the post-weld stress relief treatment. The residual stress from welding can contribute to cracking during subsequent thermal cycling in service. A stress relief treatment at 600 to 650 °C for 2 to 4 hours is typically sufficient to reduce residual stresses to acceptable levels without significantly affecting the overlay hardness.

Summary

This 1994 paper by Xiong Xuehui and Fu Haijun provides valuable technical insights into the application of cobalt-based hardfacing to hot upsetting machine molds. The paper demonstrates that cobalt-based overlays can significantly extend mold life in the severe conditions of hot upsetting, with life extensions of 3 to 5 times over unclad molds. The key success factors identified include careful alloy selection, proper transition layer design, precise process parameter control, and rigorous quality control. These principles continue to form the basis of modern mold hardfacing practice, and the engineering approach of tailoring the overlay solution to the specific service conditions remains the most effective strategy for mold surface engineering.