Effect of Preheating Temperature on Microstructure and Performance of Overlay Layers on K360 Wear-Resistant Steel

Literature Overview

The paper by Deng Hanzhong, Meng Xiangfeng, Jia Yinghui, and Yang Sen, published in the Journal of the China Coal Society (Volume 37, Issue 8, 2012, pages 1412–1417), investigates the influence of preheating temperature on the overlay weld layer deposited on worn K360 wear-resistant steel plates using flux-cored wire under CO2 gas shielding. K360 is a high-carbon, high-chromium cast steel widely employed in coal mining equipment, conveyor chutes, and material handling systems where severe abrasive wear is a dominant failure mode. The study systematically examines overlay layers produced at preheat temperatures of 150 °C, 200 °C, and 250 °C, evaluating microstructure, XRD phase composition, hardness, impact toughness, and abrasion wear resistance. This work is highly relevant to engineers responsible for repair and refurbishment of mining and bulk-material handling equipment, where overlay welding is the most practical field repair method for restoring worn surfaces.

Core Technical Findings

All overlay layers, regardless of preheat temperature, exhibited a microstructure consisting of martensite plus carbides with a small amount of retained austenite. The XRD results confirmed the presence of M7C3 and M23C6 chromium carbides as the primary hard phases, consistent with the high carbon and high chromium composition of the flux-cored wire used. The key differentiating factor across the three preheat conditions was not the phase composition but rather the morphology, size, and distribution of the martensitic laths and carbide particles, which directly governed the mechanical and wear properties.

The following table summarizes the principal results reported in the paper:

The most striking observation is that a preheat temperature of 250 °C, while producing a layer with comparable hardness to the lower preheat conditions, caused a dramatic reduction in impact toughness and a corresponding drop in abrasion wear resistance. This result challenges the common assumption that higher preheat temperatures are always beneficial for reducing cracking susceptibility in high-carbon overlay welds, and it underscores the importance of finding an optimal preheat window rather than simply maximizing preheat.

Interpretation of Technical Mechanisms

The degradation in toughness and wear resistance at 250 °C preheat can be attributed to several metallurgical mechanisms. First, a higher preheat temperature slows the cooling rate through the austenite-to-martensite transformation range, which promotes the formation of coarser martensitic laths and larger retained austenite islands. Coarser martensite reduces the effective yield strength of the matrix and lowers resistance to plastic deformation under abrasive contact. Second, slower cooling allows more time for carbide coarsening and spheroidization, which reduces the density of fine, uniformly distributed hard particles that are essential for three-body abrasion resistance. Third, the increased retained austenite fraction at higher preheat temperatures can act as a soft phase in the microstructure, providing little contribution to wear resistance while also reducing the overall hardness of the matrix.

At 150 °C and 200 °C preheat, the cooling rate is sufficient to produce fine, acicular martensite with a high density of finely dispersed carbides. The retained austenite content is low enough to avoid significant softening of the matrix but is still present in small amounts to provide some strain-induced transformation toughening. This combination of fine hard phases in a relatively tough matrix yields the best balance of hardness, toughness, and abrasion resistance. The wear resistance improvement of 1.583× and 1.494× over the base metal at 150 °C and 200 °C respectively represents a substantial practical benefit for extending the service life of worn K360 components.

Engineering Practice Implications

For field repair of K360 wear plates in mining and coal handling applications, the following practical recommendations emerge from this study:

1. The optimal preheat temperature window for CO2 gas-shielded flux-cored wire overlay on K360 steel is 150–200 °C. Preheat temperatures above 250 °C should be avoided unless specifically required to prevent cold cracking in highly restrained joints, and even then the loss in wear performance must be weighed against the cracking risk.
2. The use of flux-cored wire under CO2 shielding is a practical and cost-effective approach for field overlay repair, as it does not require inert gas equipment and can be performed in open or semi-enclosed environments typical of mining sites.
3. Multi-pass overlay deposition should be employed to achieve sufficient overlay thickness for service, with each pass maintained within the optimal preheat window using interpass temperature monitoring.
4. Post-overlay hardness testing should be supplemented with impact testing or at minimum a toughness assessment method such as Charpy V-notch testing on representative test coupons, because hardness alone does not reliably predict wear resistance in this material system.

Key Questions and Reflections

One question that arises from this study is whether the observed toughness degradation at 250 °C preheat is specific to the particular flux-cored wire composition used or would generalize to other overlay consumables. The high carbon and chromium content of K360-type overlay wires inherently produce martensitic structures, and the sensitivity of martensite morphology to cooling rate is well established. However, overlay wires with different alloy compositions, such as those containing niobium or titanium as carbide-forming elements, may exhibit different sensitivity to preheat temperature. Another practical consideration is the effect of overlay thickness on the thermal cycle experienced by each pass. In multi-pass overlay, the first pass experiences the base metal thermal cycle, while subsequent passes experience a thermal cycle modified by the already-deposited overlay metal, which has different thermal properties. The preheat temperature optimization should therefore ideally be validated for the specific multi-pass sequence intended for the actual repair application.

The study also raises an important point about the relationship between impact toughness and abrasion wear resistance. In many field conditions, particularly in mining and bulk material handling, the overlay layer is subjected not only to abrasion but also to impact loading from falling materials. A layer that is extremely hard but brittle may spall or delaminate under impact-abrasion combined loading, even if its dry abrasion wear resistance is high. The finding that 150 °C preheat provides the best combination of toughness and wear resistance is therefore particularly valuable for applications involving impact-abrasion service.

Study Insights and Conclusions

This study provides clear, actionable guidance for the overlay repair of K360 wear-resistant steel components. The optimal preheat temperature of 150–200 °C represents a practical compromise between cold cracking prevention and maintenance of fine martensitic microstructure with high toughness and wear resistance. The dramatic degradation observed at 250 °C preheat serves as a cautionary example of how exceeding an optimal thermal window can undermine the very properties the overlay is intended to provide. For engineers in mining, coal handling, and bulk material processing industries, this work reinforces the principle that overlay welding is not merely a matter of depositing hard material but requires careful thermal management to achieve the desired microstructure and performance balance. The use of CO2 gas-shielded flux-cored wire as the overlay process is particularly attractive for field applications where equipment portability and operational simplicity are critical, and the demonstrated wear resistance improvement of nearly 1.6× over the base metal justifies the investment in proper preheat control and process qualification.