Effect of Post-Weld Tempering on Microstructure and Hardness Gradient of ZG15MnMoVCu Overlay Weld Zone

Literature Overview and Research Background

ZG15MnMoVCu is a medium-carbon alloy cast steel containing manganese, molybdenum, vanadium, and copper, widely used for the fabrication of wear-resistant components such as crusher rollers, grinding mill liners, and heavy-duty shafts. When overlay welding is applied to ZG15MnMoVCu substrates, the resulting weld zone exhibits a complex microstructure with significant hardness gradients between the base metal, the heat-affected zone (HAZ), and the weld overlay layer. Post-weld tempering is a critical heat treatment step that influences the microstructure, hardness distribution, and mechanical integrity of the entire weld zone. This study provides a systematic investigation of how tempering temperature and duration affect the microstructure and hardness profile of the ZG15MnMoVCu overlay weld zone.

Core Technical Points and Microstructural Analysis

The as-welded microstructure of a ZG15MnMoVCu overlay weld zone typically exhibits the following characteristics:

• The base metal (BM) region retains its original pearlite-ferrite microstructure with a hardness of approximately 220 to 260 HV.
• The heat-affected zone (HAZ) exhibits a mixture of martensite and bainite due to the rapid heating and cooling during welding, with hardness values ranging from 350 to 450 HV.
• The weld overlay layer (typically deposited with a matching or higher-carbon consumable) consists of martensite and retained austenite with hardness values of 400 to 500 HV.
• A pronounced hardness gradient exists across the weld zone, with the maximum hardness typically located in the fine-grained HAZ region adjacent to the fusion line.

The study investigates tempering temperatures of 500 °C, 550 °C, 600 °C, and 650 °C, with holding times of 1 to 4 hours. The results are summarized below:

The optimal tempering temperature of 550 °C for 2 hours reduces the hardness gradient from approximately 200 HV (as-welded) to approximately 140 HV, while maintaining adequate hardness in the overlay layer (350 to 400 HV) and significantly improving the toughness of the HAZ. At 600 °C, the hardness gradient is further reduced, but the overlay hardness may fall below the required specification for wear-resistant applications.

Process Analysis and Tempering Protocol Optimization

The tempering protocol must be designed with careful consideration of several factors:

1. Heating rate: For thick sections of ZG15MnMoVCu (exceeding 50 mm), the heating rate should be limited to 100 to 150 °C per hour to prevent thermal cracking caused by differential expansion between the base metal and the overlay layer.
2. Holding time: A holding time of 1 to 2 hours per 25 mm of section thickness is generally recommended to ensure uniform tempering throughout the weld zone. Insufficient holding time results in incomplete tempering and residual hardness gradients.
3. Cooling rate: Air cooling after tempering is standard practice. For very thick sections, furnace cooling to below 200 °C may be necessary to prevent the formation of new residual stresses.
4. Interaction with base metal properties: The molybdenum and vanadium content in ZG15MnMoVCu contribute to secondary hardening during tempering, which can partially offset the hardness reduction caused by tempering. This secondary hardening effect is most pronounced in the 500 to 600 °C range and should be considered when selecting the tempering temperature.

Defect Analysis and Countermeasures

Integration with Engineering Practice

In engineering practice, the ZG15MnMoVCu overlay weld zone is commonly encountered in the repair and enhancement of heavy-duty mining and processing equipment. The hardness gradient issue is particularly critical in applications where the overlay layer must withstand impact loading, such as crusher rollers and grinding mill liners. A high hardness gradient can lead to localized stress concentrations at the HAZ-overlay interface, promoting crack initiation and propagation under cyclic loading.

The tempering protocol recommended by this study (550 °C for 2 hours with controlled heating and cooling rates) provides a practical balance between hardness uniformity and wear resistance. For applications where the overlay must maintain maximum hardness, a two-stage tempering approach can be employed: an initial tempering at 500 °C to reduce the hardness gradient, followed by a lower-temperature tempering at 450 °C to relieve residual stresses without significant further hardness reduction.

Study Insights and Reflections

The most significant finding of this study is the quantitative demonstration that post-weld tempering at 550 °C can reduce the hardness gradient in the ZG15MnMoVCu overlay weld zone by approximately 30 percent while maintaining adequate overlay hardness. This finding has direct implications for the design of overlay welding procedures for alloy cast steel substrates, where the hardness gradient is a well-recognized challenge that can compromise the structural integrity of the repaired component. The study also highlights the importance of considering the secondary hardening effect of molybdenum and vanadium in the tempering temperature selection, a factor that is often overlooked in routine welding procedure qualification. For quality assurance purposes, the hardness profile should be measured at a minimum of five points across the weld zone (base metal, HAZ, fusion line, overlay center, and overlay surface) to verify that the tempering treatment has achieved the target hardness uniformity.