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

Literature Overview

This paper by Deng Hanzhong and colleagues, published in the Journal of Coal Science and Engineering in 2012, investigates the influence of preheating temperature on the microstructure and mechanical properties of overlay weld layers deposited on K360 wear-resistant steel plates using flux-cored wire under CO2 gas shielded arc welding. K360 is a high-strength wear-resistant steel widely used in mining and bulk material handling equipment where severe abrasion is a primary failure mode. The study examined preheating temperatures of 100, 150, 200, and 250 degrees Celsius, and characterized the overlay layers through optical microscopy, X-ray diffraction (XRD), hardness testing, Charpy impact testing, and dry sliding wear tests. The authors found that all preheating conditions produced a microstructure consisting of martensite, carbides, and a small amount of retained austenite, but the relative proportions and mechanical properties varied significantly with preheat temperature.

Core Technical Findings

The key result is that preheating at 150 and 200 degrees Celsius yielded the optimal combination of hardness, toughness, and wear resistance. At these temperatures, the wear resistance of the overlay layer reached 1.583 and 1.494 times that of the base metal, respectively. However, when the preheating temperature was raised to 250 degrees Celsius, the toughness dropped markedly and wear resistance declined to only 1.148 times that of the base metal. This non-monotonic relationship between preheat temperature and performance is a critical engineering insight.

Technical Analysis of Preheating Mechanisms

Preheating in overlay welding serves multiple purposes: it reduces the cooling rate of the weld pool, lowers thermal gradients at the weld-to-base metal interface, and mitigates residual stresses that can lead to cracking. For K360 wear-resistant steel, which typically has a high carbon equivalent and is prone to cold cracking, preheating is essential to prevent hydrogen-induced cracking in the heat-affected zone. However, excessive preheating introduces its own problems.

At lower preheat temperatures such as 100 degrees Celsius, the cooling rate remains relatively high, promoting the formation of fine martensite with dispersed carbides. While this yields good hardness, the rapid solidification may produce a coarser grain structure in the heat-affected zone and increase susceptibility to cracking. At 150 and 200 degrees Celsius, the cooling rate is moderated to an optimal range. The weld pool solidifies at a rate that promotes a balanced microstructure: martensite retains its high hardness, carbides are finely dispersed, and the retained austenite fraction is sufficient to provide some transformation-toughening capacity under impact loading. The result is a layer with both high hardness and adequate toughness, which is the hathe writing systemark of good abrasion resistance.

At 250 degrees Celsius, the cooling rate becomes too slow. This promotes the coarsening of carbide precipitates and may increase the volume fraction of retained austenite beyond the beneficial range. More critically, the slower cooling allows for grain growth in the martensitic matrix, which reduces the transformation-toughening effect. The impact toughness drops significantly, and because wear resistance in martensitic overlay layers is closely linked to the ability of the microstructure to absorb energy during abrasive contact without cracking, the overall wear resistance deteriorates.

Engineering Practice Implications

From a practical standpoint, this study provides clear guidance for repair welding of K360 wear-resistant components. The recommended preheat window of 150 to 200 degrees Celsius is narrow but well-defined. In field conditions, achieving consistent preheating temperatures requires careful monitoring. I recommend using infrared thermometers or thermocouple-based monitoring systems to verify the actual surface temperature before welding begins. Additionally, interpass temperature control is equally important and should be maintained within the same range to avoid the deleterious effects of excessive thermal cycling.

For quality control, I suggest implementing a PDCA cycle: Plan by selecting 150 to 200 degrees Celsius as the preheat target; Do by applying controlled induction or gas preheating; Check by verifying surface temperature with calibrated instruments and performing post-weld hardness and impact testing on coupon samples; Act by adjusting preheat parameters based on test results. This systematic approach ensures that the optimal microstructure and wear resistance are consistently achieved.

Study Insights and Reflections

This research underscores a principle that I have observed repeatedly in engineering practice: the relationship between process parameters and final performance is often non-linear. The assumption that higher preheating always improves weld quality is a common misconception among field welders. The study demonstrates that there exists a window of optimal preheating, and exceeding this window degrades the very properties one seeks to enhance. For engineers responsible for specifying repair procedures for wear-resistant steel components, this finding should be incorporated into welding procedure specifications (WPS) with clear upper and lower preheat temperature limits. Furthermore, the study highlights the importance of retained austenite as a toughening mechanism in martensitic overlay layers, which is a concept that deserves deeper investigation in future work on overlay welding of high-carbon steels.