Application of Cobalt-Based Overlay Welding on Hot Heading Machine Dies

Literature Overview

This paper, published in "Bearings" (1994, Issue 4, pp. 46-47) by Xiong Xuehui and Fu Haijun from Harbin Bearing Factory, describes the application of cobalt-based overlay welding to extend the service life of hot heading machine dies. The work is classified under both TG315.2 (forging dies) and TG455 (overlay welding), reflecting its dual focus on tool life improvement and surface engineering.

Problem Definition and Background

Hot heading machines are used in the production of bearings, fasteners, and other precision metal components. The dies in these machines are subjected to extreme conditions:

• High temperature: Contact with hot workpieces at 800-1100°C
• High pressure: Forging pressures often exceeding 1000 MPa
• Severe wear: Combined abrasive, adhesive, and fatigue wear mechanisms
• Thermal cycling: Repeated heating and cooling during the forging cycle

The base material specified is H13 (equivalent to AISI H13 or 4Cr5MoSiV1), a hot-work tool steel known for its excellent hot hardness and thermal fatigue resistance. However, even H13 exhibits limited life under severe hot forging conditions, necessitating surface hardening through overlay welding.

Overlay Material Selection and Properties

The paper specifies the use of cobalt-based welding rod 112 (likely equivalent to a Stellite-type or Hastelloy-type cobalt alloy). Cobalt-based alloys are particularly well-suited for hot forging die applications due to:

The cobalt-based overlay provides a hard, wear-resistant surface that maintains its properties at elevated temperatures, while the H13 base material provides the necessary structural strength and thermal fatigue resistance for the die body.

Process Design and Execution

The overlay welding process for hot heading dies requires careful consideration of several factors:

1. Preheating: The die must be preheated to 300-400°C to reduce thermal shock and minimize the risk of cracking in the H13 base material
2. Heat input control: Excessive heat input can soften the H13 base material, reducing its hot hardness and thermal fatigue resistance
3. Overlay thickness: Typically 2-5 mm, sufficient to provide wear resistance while maintaining the structural integrity of the die
4. Welding sequence: The welding pattern should be designed to minimize distortion, particularly for complex die geometries
5. Post-weld treatment: Stress relief at 550-650°C to reduce residual stresses without softening the H13 base material

The use of SMAW (shielded metal arc welding) with rod 112 suggests a manual or semi-automatic process, which is appropriate for the complex geometries of forging dies and allows for precise control over the deposit shape and thickness.

Metallurgical Considerations

The HAZ of H13 during overlay welding is a critical concern. H13 is a high-alloy tool steel containing approximately 5% Cr, 1% Mo, 0.5% V, and 1% Si. These alloying elements contribute to the hot hardness and thermal fatigue resistance of the base material, but they also make the steel susceptible to cracking during welding.

The key metallurgical challenges include:

• Quench cracking: Rapid cooling can form hard martensite in the HAZ, leading to cracking
• Tempered martensite embrittlement: If the HAZ is exposed to temperatures in the 350-500°C range during cooling, embrittlement can occur
• Interfacial cracking: Thermal expansion mismatch between the cobalt overlay and H13 base can generate interfacial stresses

Mitigation strategies include:

• Preheating to 300-400°C to reduce cooling rates
• Low heat input welding (using small electrode diameters and low currents)
• Post-weld stress relief at 550-650°C for 2-4 hours
• Multi-pass welding with interpass temperature control

Engineering Practice and Life Extension

The primary objective of this work is to extend the service life of hot heading machine dies. In practice, the life improvement is measured by the number of forging cycles before the die requires resurfacing or replacement. The cobalt-based overlay can typically extend die life by 2-5 times compared to the uncoated H13 die, depending on the severity of the forging conditions.

The economic justification for overlay welding is based on the following factors:

• Cost of overlay material and welding labor
• Cost of die replacement (material and fabrication)
• Production downtime due to die failure
• Maintenance and resurfacing frequency

For high-volume production operations, the life extension provided by cobalt-based overlay welding often results in significant cost savings, even considering the higher material and processing costs.

Study Insights

This work demonstrates the effective use of surface engineering to solve a practical industrial problem. The combination of H13 base material and cobalt-based overlay provides an optimal balance of structural integrity and surface wear resistance. The selection of SMAW with rod 112 is pragmatic, considering the availability of the process and the ability to weld complex die geometries.

The key insight is that the overlay layer must be thick enough to provide wear resistance but thin enough to avoid excessive cost and potential delamination. The HAZ must be carefully controlled to prevent cracking while maintaining the hot hardness of the H13 base material.