In the welding process of wear plates for mining machinery, a decrease in hardness in the heat-affected zone (HAZ) is a common problem. This is mainly due to changes in the microstructure of the material caused by the high welding temperature, such as martensite decomposition, carbide precipitation, or grain coarsening. These changes weaken the wear resistance and strength of the material, affecting the service life and safety of the mining machinery. Therefore, optimizing the welding process requires addressing key aspects such as controlling heat input, adjusting cooling rate, selecting matching welding materials, optimizing welding sequence, and post-weld treatment to suppress the softening tendency of the HAZ.
Controlling heat input is the core measure to avoid a decrease in HAZ hardness. Excessive heat input will expand the HAZ and exacerbate microstructure softening, while insufficient heat input may lead to incomplete fusion or cracking. In actual welding, low heat input welding methods, such as pulsed MIG/MAG welding, should be prioritized. By adjusting the current, voltage, and welding speed, the heat input per unit length of weld can be precisely controlled within a reasonable range. Simultaneously, using narrow-gap welding technology to reduce the amount of deposited metal can also effectively reduce heat input and shrink the HAZ size.
Adjusting the cooling rate is crucial for the microstructure transformation of the HAZ. Rapid cooling can suppress carbide precipitation and martensite decomposition, preserving the material's original hardness, but excessively rapid cooling may induce cracks; slow cooling, on the other hand, easily leads to microstructure coarsening and reduced hardness. Therefore, an appropriate cooling method must be selected based on the material's characteristics. For example, for high-carbon, high-chromium wear plates, insulation cotton or forced air cooling can be used immediately after welding to control interpass temperature and avoid localized overheating; for thick plate welding, a combination of preheating and slow post-weld cooling can be used to mitigate the temperature gradient and reduce residual stress.
Selecting welding consumables that match the base metal is fundamental to ensuring weld quality. The chemical composition and mechanical properties of the welding consumables should be similar to those of the wear plates to reduce dilution in the fusion zone and avoid microstructure segregation due to compositional differences. For example, when welding high-chromium alloy wear plates, welding wire or electrodes with comparable chromium content should be selected to ensure that the hardness and wear resistance of the weld metal are not lower than those of the base metal. Furthermore, low-hydrogen welding consumables can reduce the risk of hydrogen-induced cracking and improve joint toughness.
Optimizing the welding sequence and direction can disperse heat accumulation and reduce repeated heating in the heat-affected zone. For example, symmetrical welding, segmented back welding, or skip welding can ensure uniform heating of the weldment and avoid localized overheating. For complex structures, areas with less shrinkage should be welded first, followed by areas with greater shrinkage, to reduce the secondary impact of welding deformation on the heat-affected zone. Simultaneously, a reasonable arrangement of welding layers, such as multi-layer, multi-pass welding, allows the heat treatment of subsequent weld layers to refine the microstructure of the preceding weld layers, improving overall performance.
Post-weld heat treatment is an effective means of improving the properties of the heat-affected zone. Stress-relief annealing can eliminate residual welding stress and reduce cracking tendency; while quenching and tempering can adjust the microstructure and restore hardness. For example, for welded high-manganese steel wear plates, water quenching can be used, heating the material above the critical temperature, holding it at that temperature, and then rapidly cooling it to obtain a uniform austenitic microstructure, improving toughness and wear resistance. However, it is important to note that heat treatment parameters must be strictly set according to the material type and thickness to avoid overheating or burning, which can lead to performance degradation.
Operational details during the welding process also affect the quality of the heat-affected zone. For example, maintaining appropriate arc length and welding angle ensures a stable molten pool, reducing porosity and slag inclusions; using anti-spatter agents reduces surface defects in the weld; and regularly cleaning the welding nozzle and contact tip prevents arc instability due to poor conductivity. These details reduce the welding defect rate and indirectly reduce performance fluctuations in the heat-affected zone.
Optimizing the welding process for wear plates in mining machinery requires a comprehensive approach, encompassing heat input control, cooling rate adjustment, welding consumable matching, welding sequence design, post-weld treatment, and operational details. Through systematic process adjustments, the hardness decrease in the heat-affected zone can be effectively suppressed, improving the wear resistance and fatigue resistance of the welded joint, thereby extending the service life of the mining machinery, reducing maintenance costs, and ensuring production safety.
