Microstructure Properties and Residual Stress in Overlay Welding Repair of Locomotive Wheel Hubs

Literature Overview

This paper by You Xinyu, Jiao Yang, Liu Minglei, Liu Fang, and Zhang Yong, published in Hot Working Technology (2018, Vol. 47, No. 19, pp. 58-61), investigates the overlay welding repair of locomotive wheel hubs using CO₂ gas shielded arc welding (GMAW) with JQ MG50-6 flux-cored wire. The work was supported by the Liaoning Provincial Natural Science Foundation (Grant No. 201602123). The authors applied ultrasonic vibration treatment to the weld overlay and systematically evaluated microstructure, hardness, tensile strength, and residual stress distribution before and after post-weld treatment.

Core Technical Findings

The study establishes several critical quantitative benchmarks for overlay repair of locomotive wheel hubs:

The overlay layer exhibits a dense, crack-free microstructure dominated by bainite, which is consistent with the cooling rates expected from CO₂-shielded GMAW on a thick steel substrate. The JQ MG50-6 wire, a manganese-silicon type flux-cored wire, provides sufficient carbon and alloy content to promote bainitic transformation during rapid solidification and subsequent cooling.

Residual Stress Analysis and Ultrasonic Vibration Treatment

A key contribution of this paper is the systematic characterization of residual stress in both the inner bore and outer circumference of the wheel hub after overlay welding, followed by evaluation of ultrasonic vibration stress relief (UVSR). The results demonstrate that:

1. Pre-treatment: Significant tensile residual stresses exist in both the inner bore and outer overlay regions.
2. Post UVSR treatment: All tensile stresses are eliminated and converted to compressive stresses.

This finding is of considerable practical significance. Tensile residual stresses at the inner bore of a wheel hub are particularly detrimental because they superimpose on operational bending and contact stresses, accelerating fatigue crack initiation. The conversion to compressive stress through ultrasonic vibration introduces beneficial surface compressive stresses that can significantly extend fatigue life.

Engineering Practice Implications

From a quality control standpoint, this work highlights several critical considerations for overlay repair of railway wheel components:

• Wire selection: JQ MG50-6 provides adequate hardenability without excessive carbon content that could cause cracking. Engineers should verify the wire composition conforms to GB/T 17493 or equivalent flux-cored wire standards.
• Post-weld treatment: Ultrasonic vibration stress relief should be considered a mandatory step for high-fatigue-critical applications such as locomotive wheel hubs. This aligns with practices described in EN ISO 18812 for shot peening and surface treatment of railway components.
• Microstructural verification: Metallographic examination per GB/T 13298 should confirm the absence of cracks, porosity, and unmelted inclusions in the overlay layer.
• Residual stress verification: Strain gauge or X-ray diffraction methods per GB/T 3075 should be employed to confirm the residual stress state after UVSR treatment.

Key Reflections

The 1.8× hardness ratio between overlay and base material is substantial. While this provides excellent wear resistance for the hub bore surface, it introduces a significant hardness gradient at the weld interface. In engineering practice, this gradient can create stress concentrations if the base material is not preheated adequately. The paper does not explicitly report preheating temperatures, which is a gap that practitioners should address. For locomotive wheel hubs typically made from medium-carbon or low-alloy steel, preheating to 150-200°C is advisable to reduce cooling rates and minimize the hardness differential.

Furthermore, the tensile strength of 640-650 MPa in the overlay layer is notably higher than the base material's 435 MPa. This strength mismatch, combined with the hardness gradient, means that during overload events, the base material will yield before the overlay layer fails. While this provides a degree of damage tolerance, it also means that the overlay layer may not participate in plastic deformation during overload, potentially leading to brittle fracture of the overlay under impact loading. Practitioners should therefore limit the use of this repair method to wear-damage scenarios rather than impact or overload damage.

Study Insights

This paper effectively demonstrates that a relatively straightforward GMAW overlay repair process, combined with ultrasonic vibration stress relief, can produce a serviceable repair on locomotive wheel hubs. The crack-free, dense overlay with bainitic microstructure and improved mechanical properties represents a reliable repair technology. However, the work would benefit from fatigue life testing of the repaired component under simulated wheel loading conditions, and from a more detailed discussion of the dilution rate and its influence on overlay properties. For engineers in railway maintenance, this paper provides a practical foundation for developing overlay repair procedures, provided that residual stress management is treated as a critical quality control step.