Overlay Welding Technology for Left and Right Turntables of EBZ125 Roadheader

Literature Overview

This study addresses the overlay welding technology applied to the left and right turntables of the EBZ125 roadheader, a medium-sized mining tunneling machine used in underground coal mining and tunnel excavation. The turntables are critical structural components that support the cutting head and rotate it to follow the tunnel profile. They are subjected to severe impact loading, abrasion from rock and coal, and vibration fatigue. The literature examines the selection of overlay materials, welding process optimization, and performance evaluation for this specific application.

Core Technical Analysis

The turntables of the EBZ125 roadheader are typically fabricated from medium-carbon alloy steel such as Q345B or 40CrNiMoA, with a base thickness of 40 to 80 millimeters. The wear areas on the turntables include the bearing raceways, the pinion gear contact surfaces, and the edges that contact the tunnel walls. These areas are subjected to combined sliding wear, impact wear, and fretting wear, requiring overlay layers that provide high hardness and good fatigue resistance.

The overlay welding process for the turntables is typically performed using GTAW (gas tungsten arc welding) for the transition layer and SAW (submerged arc welding) for the build-up passes. The GTAW process provides excellent control over the heat input and is suitable for the initial bond layer, while the SAW process offers a high deposition rate for the subsequent wear-resistant layers.

Process Design and Welding Procedure

The welding procedure for the turntable overlay involves several critical steps. First, the wear areas are prepared by machining to a uniform profile, removing any existing wear damage, and cleaning the surface to ensure good bonding. The base metal is preheated to 200 to 300 degrees Celsius to reduce the risk of cracking, particularly for the alloy steel substrates.

The overlay is applied in a multi-pass sequence. The first pass is a transition layer using a low-alloy steel electrode or wire to dilute the base metal and prevent cracking. Subsequent passes use the wear-resistant overlay material to build up the required thickness. The total overlay thickness is typically 3 to 8 millimeters, depending on the severity of the service conditions.

The interpass temperature should be maintained below 250 degrees Celsius to promote the formation of fine martensite and to control the grain size. The welding sequence should be designed to minimize distortion, typically using a symmetric pattern that balances the heat input on both sides of the turntable.

Post-Weld Heat Treatment

Post-weld heat treatment is essential for the Mn-Mo overlay layers on the turntables. Tempering at 250 to 350 degrees Celsius for 2 to 4 hours reduces the residual stresses and improves the toughness of the overlay layer. The tempering temperature and duration must be carefully controlled to avoid excessive softening of the hardfacing layer.

The PWHT procedure should also address the base metal HAZ. For alloy steel substrates such as 40CrNiMoA, a tempering treatment at 580 to 620 degrees Celsius may be required to relieve the residual stresses in the base metal HAZ. However, this temperature may soften the overlay layer, so a compromise temperature of 300 to 350 degrees Celsius is often used, which provides adequate stress relief without significantly affecting the overlay hardness.

Defect Analysis and Quality Assurance

The turntable overlay welds are susceptible to several defect types. Cracking is the most critical concern, particularly in the transition layer between the base steel and the overlay. The high carbon content of the hardfacing material, combined with the alloy steel base, creates a high carbon equivalent at the bond line that promotes cold cracking.

Quality assurance should include visual inspection, magnetic particle inspection for surface and near-surface cracks, and ultrasonic testing for subsurface defects. Hardness testing should be performed at multiple locations across the overlay surface to verify uniformity. The overlay thickness should be measured by ultrasonic thickness gauge or by sectioning.

Engineering Practice Integration

In mining operations, the turntable overlay welding is typically performed as a preventive maintenance activity or as a repair after significant wear has occurred. The overlay should be applied before the wear reaches the bearing raceway or the pinion contact surface, as these areas are critical for the structural integrity of the turntable.

The economic analysis of overlay repair versus replacement shows that overlay welding can extend the service life of the turntables by 2 to 3 times, with a cost savings of 70 to 85 percent compared to complete replacement. The overlay welding can be performed in the field or in a workshop, depending on the size of the turntable and the available equipment.

Study Insights and Summary

The overlay welding technology for the EBZ125 roadheader turntables demonstrates the importance of process optimization and material selection for heavy mining equipment repair. The key insight is that the transition layer design is critical for preventing bond line cracking, and that the welding sequence must be carefully planned to minimize distortion in the large, heavy turntable structure. Engineers should prioritize the use of low-hydrogen welding consumables, adequate preheating, and controlled cooling rates to ensure the reliability of the overlay. The combination of proper material selection, process control, and quality assurance can significantly extend the service life of these critical components and reduce the overall maintenance costs of the mining operation.