Overlay Welding Repair of Fiber Carbon Forming Machine Screw Shaft

Literature Overview

This paper, published in Welding Technology (Vol. 22, No. 6, 1993), authored by Li Jixian from the Hanzhong General Machinery Factory in Shaanxi Province, addresses the overlay welding repair of a screw shaft in a fiber carbon forming machine. The screw shaft, made of CrWMn tool steel, operates under severe conditions of high temperature (400-500°C), high friction, and high extrusion pressure. The original component failed to meet its design life of 600 hours, with severe wear occurring in just 2 hours of operation, resulting in 6-8 mm of material loss on the front section of the screw. The authors successfully applied manual overlay welding to extend the component life to at least 150 hours, demonstrating a practical repair solution in the absence of specialized overlay welding equipment.

Service Environment and Failure Analysis

The operating environment of the fiber carbon forming machine screw shaft is exceptionally severe. The combination of elevated temperature, abrasive wear from the carbon material being formed, and high mechanical loading creates a multi-mechanism wear regime that challenges even high-performance tool steels.

The failure analysis reveals that the original CrWMn material, combined with inappropriate heat treatment, was fundamentally unsuitable for the service conditions. CrWMn is a high-carbon tool steel containing chromium, tungsten, and manganese, designed for cold work applications where wear resistance and hardness are critical. However, at operating temperatures of 400-500°C, this material experiences significant thermal softening, as the tempering stability of the martensitic structure degrades at elevated temperatures.

The 6-8 mm wear depth in just 2 hours indicates an extremely high wear rate, suggesting that the material hardness at operating temperature was insufficient to resist the abrasive action of the carbon material. The combination of thermal softening and abrasive wear created a self-accelerating failure mechanism: as the surface softened, the wear rate increased, exposing fresh material that would then soften and wear at an even higher rate.

Manual Overlay Welding Solution

The repair was performed using manual arc welding (SMAW) without specialized overlay welding equipment. This constraint is significant because it reflects the practical realities of maintenance welding in industrial settings, where access to dedicated overlay welding equipment may be limited.

The selection of the overlay material is critical for this application. The overlay material must provide:

1. High hardness at elevated temperature: The overlay material must maintain adequate hardness at 400-500°C to resist abrasive wear. Materials such as high-speed steel alloys, cobalt-based alloys, or chromium-carbide-based alloys are candidates for this temperature range.
2. Good thermal fatigue resistance: The overlay material must withstand repeated thermal cycling without cracking or spalling. This requires a ductile matrix with properly dispersed hard phases.
3. Adequate bond strength to the base material: The overlay must maintain strong metallurgical bonding to the CrWMn base metal under thermal and mechanical loading.
4. Compatibility with manual welding: The overlay material must be weldable with SMAW, requiring appropriate filler metal availability and processability.

Performance Results and Validation

After overlay welding repair, the screw shaft was returned to service and monitored. After 150 hours of operation, inspection revealed that the wear was minimal, and the component was deemed suitable for continued use. This represents a substantial improvement over the original 2-hour failure life, achieving approximately 75 times the original service life.

The 150-hour result, while a dramatic improvement, still falls short of the original 600-hour design target. This gap may be attributable to several factors: the limitations of manual welding in achieving optimal overlay microstructure, the dilution effects of the CrWMn base metal on the overlay composition, and the inherent challenges of maintaining overlay integrity over extended service periods under severe conditions.

Engineering Practice Implications

This case study illustrates the value of welding repair technology in extending component life, even when the original component design or material selection was fundamentally flawed. The ability to perform effective overlay welding with basic SMAW equipment makes this technology accessible to a wide range of industrial applications, particularly in smaller facilities or remote locations where specialized equipment is unavailable.

The case also highlights the importance of proper material selection for high-temperature wear applications. CrWMn, while an excellent cold work tool steel, is not appropriate for sustained operation at 400-500°C. For such conditions, materials with higher tempering stability, such as austenitic stainless steels, cobalt-based alloys, or specialized high-temperature wear alloys, would be more appropriate for the base material or overlay material.

Key Technical Insights

The successful repair demonstrates that even in the absence of ideal equipment and materials, skilled welding practice can achieve meaningful results. The systematic approach of identifying the failure mode, selecting an appropriate overlay material, and executing the repair with available equipment reflects a practical engineering methodology that is directly applicable to maintenance and repair operations.

This paper also underscores the importance of understanding the service environment when specifying materials for wear-critical components. The mismatch between the CrWMn material properties and the 400-500°C operating temperature was the root cause of the premature failure, and this insight is valuable for future component design and material selection decisions.