CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Process Parameters on the Microstructure and Properties of Weld Overlay Layers
Literature Overview
This 2009 study by Liu Zhengjun, Ci Honggang, Su Yunhai, and Liu Changjun from the School of Materials Science and Engineering at Shenyang University of Technology represents a systematic investigation into the influence of welding process parameters on the microstructure and mechanical properties of weld overlay layers. Published in the Welding Journal, this research addressed a fundamental question in cladding technology: how do variations in welding parameters affect the quality and performance of the overlay layer? The study is particularly relevant to engineering practice because the selection of appropriate welding parameters is often the most practical lever available to the fabricator for optimizing overlay performance, especially when the alloy composition is fixed by design requirements.
Process Parameters and Their Effects
The authors investigated several key welding process parameters including welding current, voltage, travel speed, wire feed rate, shielding gas flow rate, and inter-pass temperature, and systematically correlated their effects with overlay microstructure, dilution ratio, hardness distribution, and mechanical properties. The welding process studied was submerged arc welding (SAW) overlay, which is widely used for heavy-duty cladding applications due to its high deposition rate and deep penetration characteristics.
The welding current is one of the most influential parameters, as it directly affects the heat input, penetration depth, bead geometry, and solidification rate of the overlay. Higher welding current increases the heat input and penetration depth, which leads to higher dilution ratios and coarser microstructures. The study found that for typical overlay applications, welding currents in the range of 300–500 A produced acceptable dilution ratios of 15–30%, while currents above 550 A resulted in excessive dilution and reduced overlay hardness.
| Process Parameter | Typical Range | Effect on Dilution | Effect on Microstructure | Effect on Hardness |
|---|---|---|---|---|
| Welding current | 300–500 A | Higher current increases dilution | Coarser grains at higher current | Lower hardness at higher current |
| Travel speed | 200–600 mm/min | Higher speed reduces dilution | Finer grains at higher speed | Higher hardness at higher speed |
| Wire feed rate | 6–12 m/min | Higher feed rate reduces dilution | Affects bead geometry | Moderate effect |
| Shielding gas flow | 15–30 L/min | Minimal effect on dilution | Affects oxide inclusions | Minor effect |
| Inter-pass temperature | 100–300°C | Higher temperature increases dilution | Coarser grains at higher temperature | Lower hardness at higher temperature |
Heat Input and Solidification Behavior
The linear heat input, calculated as the product of voltage, current, and time divided by travel speed, is a critical parameter that governs the solidification behavior of the overlay. The study demonstrated that heat input has a direct and predictable effect on the solidification microstructure. At low heat input values (below 15 kJ/mm), the overlay exhibits a fine columnar dendritic structure with high hardness due to rapid solidification and fine carbide precipitation. At moderate heat input values (15–30 kJ/mm), the microstructure transitions to a coarser dendritic structure with reduced hardness. At high heat input values (above 30 kJ/mm), the microstructure becomes equiaxed with coarse dendrites and significantly reduced hardness.
The solidification rate, which is inversely related to heat input, also plays a crucial role in determining the overlay microstructure. Higher solidification rates promote the formation of finer microstructures and more uniform carbide distributions, which generally improve wear resistance and mechanical properties. However, excessively high solidification rates can lead to hot cracking, particularly in high-carbon and high-sulfur compositions.
Dilution and Its Management
Dilution, defined as the percentage of base metal incorporated into the overlay layer, was identified as the primary factor governing overlay performance. The study quantified dilution ratios for various parameter combinations and demonstrated that dilution could be effectively controlled through the coordinated adjustment of welding current, travel speed, and wire feed rate. The key finding was that the ratio of wire feed rate to travel speed (deposition rate per unit length) was the most effective parameter for controlling dilution. Higher deposition rates per unit length result in lower dilution ratios because more overlay material is deposited per unit of base metal melted.
The study also examined the effect of the number of overlay passes on dilution and microstructure uniformity. Multi-pass overlay with 2–4 passes produced significantly more uniform hardness profiles and lower overall dilution compared to single-pass overlay. The first pass typically exhibited the highest dilution due to the absence of previously deposited overlay material, while subsequent passes showed progressively lower dilution as the overlay material from previous passes became incorporated into the melt pool.
Mechanical Properties and Hardness Distribution
The hardness distribution within the overlay layer was found to be strongly dependent on the welding parameters. At the substrate-overlay interface, the hardness was typically 200–300 HV lower than the bulk overlay hardness due to dilution effects. The hardness gradient from the interface to the surface was steeper at higher heat inputs and shallower at lower heat inputs. The bulk overlay hardness ranged from 350 to 600 HV depending on the composition and parameters used, with the highest hardness achieved at low heat input conditions that promoted fine martensitic structures with high carbide volume fractions.
Tensile testing of the overlay-substrate system revealed that the tensile strength of the overlay layer was generally lower than the base metal due to the dilution effects at the interface. The fracture typically occurred in the dilution zone, confirming that this region is the weakest link in the cladding system. The study recommended that the dilution zone hardness should be maintained above a minimum threshold value to ensure adequate mechanical integrity of the overlay system.
Engineering Practice and Parameter Optimization
For practical cladding applications, the study provided a framework for parameter optimization based on the specific performance requirements of the application. For wear-resistant applications requiring maximum hardness, low heat input parameters with high travel speed and moderate current are recommended. For applications requiring good toughness and resistance to cracking, moderate heat input with controlled inter-pass temperatures is preferred. For applications requiring uniform hardness throughout the overlay thickness, multi-pass cladding with controlled parameters for each pass is essential.
The study also addressed the practical issue of parameter stability during production cladding operations. Maintaining consistent welding parameters is critical for achieving uniform overlay properties. The authors recommended the use of automated welding equipment with closed-loop control of current, voltage, travel speed, and wire feed rate to ensure parameter stability and overlay quality consistency.
Key Questions and Reflections
The study raises important questions about the interaction between multiple process parameters and their combined effects on overlay performance. While the study systematically varied individual parameters, the real-world optimization of welding parameters requires consideration of multiple parameters simultaneously, and the interactions between parameters can be complex and non-linear. Additionally, the study focused on SAW overlay, but the principles of parameter optimization apply to other cladding processes such as GMAW, FCAW, PTA, and laser cladding, each of which has its own specific parameter ranges and sensitivities.
Study Insights and Implications
This study provides a comprehensive and practical framework for understanding and controlling the effects of welding process parameters on overlay microstructure and properties. The systematic approach of correlating parameter variations with dilution, microstructure, and mechanical properties offers valuable guidance for engineers seeking to optimize cladding processes for specific applications. The emphasis on dilution control through coordinated parameter adjustment and multi-pass strategies is directly applicable to modern cladding practice, regardless of the specific welding process used. The study also highlights the importance of understanding the dilution zone as the critical region in the cladding system, where the weakest mechanical properties and potential failure sites are located.