The impact of the iron pipe pallet welding process on its overall structural strength stems from the complex interaction of material microstructure changes, defect generation, and stress distribution during welding. The core mechanisms can be analyzed through three perspectives: welding thermal cycle, defect control, and stress management.
The welding thermal cycle alters the material structure at the connection between the iron pipe and the pallet. Under high temperatures, the metal grains in the weld area coarsen, forming columnar crystals or widmanstattenite. This microstructure reduces the material's yield strength and toughness. If a multi-pass welding process is used, the repeated heating of the subsequent weld layer over the previous layer further refines the grains, partially offsetting the coarsening effect. However, excessive heating can cause softening in the heat-affected zone (HAZ), creating localized weak zones. For example, when the welding current is too high or the welding speed is too slow, the increased heat input expands the HAZ, exacerbating the softening and reducing the pallet's ability to resist deformation under load.
Welding defects are a key factor affecting structural strength. Common defects such as porosity, slag inclusions, and lack of fusion directly reduce the effective load-bearing area of the weld. As three-dimensional defects, pores have a much higher stress concentration factor than spherical inclusions. Especially under dynamic loads, cracks are more likely to initiate and propagate at their edges. Lack of fusion defects, due to their planar nature, accelerate fatigue crack initiation under alternating stresses. For example, when a corner joint is used between an iron pipe and a pallet, if the weld root is not fully penetrated, the lack of fusion can become a stress concentration point under vertical load, leading to a sudden drop in local strength. Furthermore, undercut defects caused by poor weld formation can simultaneously reduce the load-bearing area and cause stress concentration, a dual effect that exacerbates strength loss.
Welding stress has a long-term impact on structural strength. Residual tensile stress generated during welding can be superimposed on external loads. When local stress exceeds the material's yield strength, plastic deformation can accumulate even before fracture occurs. During long-term load-bearing on a pallet, residual stress can promote fatigue crack propagation, reducing the structural service life. For example, while rigid fixing welding can reduce welding distortion, it can also increase residual stress levels, necessitating post-weld heat treatment for stress relief. If the heat treatment process is omitted, the pallet may experience premature fatigue fracture under dynamic loads.
The selection of welding process parameters directly impacts structural strength and stability. Parameters such as welding current, voltage, and speed must be matched to the iron pipe wall thickness and pallet material. Improper parameter settings can easily lead to surface defects such as spatter and arc craters. While these defects do not directly reduce static load strength, they can serve as pathways for corrosive media to penetrate, accelerating localized corrosion in humid environments and indirectly weakening structural strength. Furthermore, the welding sequence significantly influences the distribution of residual stress. Symmetrical welding can reduce structural distortion, while skip welding can disperse heat input and reduce localized stress concentrations.
Joint design is a key factor in controlling structural strength. Butt joints offer excellent geometric continuity and minimize stress concentration factors, making them suitable for areas with high load-bearing requirements. Although corner joints are simple to fabricate, stress concentration is prone to occur at the weld toe, requiring improvement through grinding or beveling. T-joints, due to their sudden cross-sectional changes, require reinforcement plates or plug welding to enhance strength. For example, using full-penetration butt joints at key support points on iron pipe pallets significantly improves bending stiffness, while using corner joints on standard rack pallets balances cost and strength requirements.
The impact of welding processes on the overall structural strength of iron pipe pallets spans the entire process, from material organization and defect control to stress management, parameter optimization, and joint design. By rationally selecting welding methods, strictly controlling process parameters, optimizing joint configurations, and implementing post-weld treatment, a balance between structural strength and manufacturing costs can be achieved, meeting the load-bearing requirements of various application scenarios.
