CLADDING TECHNOLOGY SHANXI CO., LTDGMAW Cladding Layer Weld Pass Overlap and Mechanical Properties Research
Literature Overview
This 2017 study by Jiang Xiangsheng and colleagues from Xinjiang University investigates the influence of weld pass overlap ratio on the mechanical properties of gas metal arc welding overlay deposits. Funded by the National Natural Science Foundation of China, the research addresses a fundamental but often underappreciated aspect of multi-pass overlay welding: how the geometric relationship between adjacent weld passes affects the final mechanical performance of the overlay layer. The study is particularly relevant for industrial applications where GMAW overlay is used to build up thick corrosion-resistant or wear-resistant layers on carbon and low-alloy steel substrates.
Weld Pass Overlap Geometry and Process Parameters
In multi-pass GMAW overlay welding, the overlap ratio between adjacent weld passes is defined as the ratio of the overlap width to the weld bead width. Typical overlap ratios range from 20 to 60 percent, with values below 20 percent risking incomplete fusion between passes and values above 60 percent causing excessive re-melting of previously deposited metal. The study systematically varies the overlap ratio while maintaining constant welding parameters to isolate the effect of overlap on mechanical properties.
The welding parameters used in this study include wire diameter of 1.2 millimeters, current of 180 to 220 amperes, voltage of 22 to 26 volts, travel speed of 300 to 450 millimeters per minute, and shielding gas flow rate of 15 to 20 liters per minute using argon-carbon dioxide mixtures. The substrate material is Q345R low-carbon steel, and the overlay filler metal is ER308L stainless steel wire. The overlay thickness is built up to 3 to 5 millimeters over 4 to 6 passes, with interpass temperature controlled below 150 degrees Celsius.
| Overlap Ratio | Weld Bead Width | Overlap Width | Number of Passes | Overlay Thickness |
|---|---|---|---|---|
| 20% | 8 mm | 1.6 mm | 6 | 4.2 mm |
| 30% | 8 mm | 2.4 mm | 5 | 4.0 mm |
| 40% | 8 mm | 3.2 mm | 5 | 4.1 mm |
| 50% | 8 mm | 4.0 mm | 4 | 3.8 mm |
| 60% | 8 mm | 4.8 mm | 4 | 3.6 mm |
Mechanical Property Analysis
The mechanical properties evaluated include tensile strength, yield strength, elongation, hardness, and impact toughness. The results reveal a non-linear relationship between overlap ratio and mechanical performance. At low overlap ratios (20 to 30 percent), the tensile strength is relatively high due to limited re-melting of previously deposited metal, but the elongation and impact toughness are reduced due to incomplete fusion and potential lack of bond between adjacent passes. At high overlap ratios (50 to 60 percent), the tensile strength decreases slightly due to grain coarsening from repeated thermal cycling, but the elongation and toughness improve due to more uniform microstructure and reduced residual stress concentration.
The optimal overlap ratio for balanced mechanical properties is found to be approximately 35 to 45 percent. At this range, the tensile strength remains above 550 MPa, the elongation exceeds 30 percent, and the Charpy impact energy at room temperature is above 80 joules. The hardness profile across the overlay thickness shows a gradient from the substrate interface to the surface, with hardness increasing from approximately 200 HV near the interface to 250 HV at the surface due to the increasing dilution of carbon steel into the stainless steel deposit.
Microstructural Evolution with Overlap Ratio
The microstructure of the GMAW overlay deposit is strongly influenced by the overlap ratio through its effect on the thermal cycle experienced by each subsequent pass. Low overlap ratios result in each new pass being deposited on a cooler, previously solidified weld metal, leading to higher cooling rates and finer grain structures. However, the incomplete fusion between passes creates weak interfaces that act as crack initiation sites under cyclic loading.
High overlap ratios cause significant re-melting of previously deposited metal, which homogenizes the microstructure but also leads to grain coarsening and potential sensitization of austenitic stainless steel to intergranular corrosion. The grain size increases from approximately 50 micrometers at 20 percent overlap to 120 micrometers at 60 percent overlap, as measured at the weld centerline. This grain coarsening directly correlates with the observed decrease in tensile strength and increase in ductility.
The dilution ratio varies across the overlay thickness, with the highest dilution occurring at the first pass near the substrate and decreasing in subsequent passes. The average dilution ratio across the entire overlay thickness is approximately 8 to 12 percent for the 35 to 45 percent overlap condition, which is within the acceptable range for maintaining corrosion resistance of the 308L stainless steel overlay.
Engineering Practice Recommendations
Based on the research findings, the following practical recommendations are provided for GMAW overlay welding operations:
- For overlay thicknesses below 3 millimeters with 3 or fewer passes, an overlap ratio of 30 to 40 percent is recommended to maintain adequate bond strength between passes while avoiding excessive re-melting.
- For overlay thicknesses above 3 millimeters with more than 4 passes, an overlap ratio of 40 to 50 percent is advisable to ensure uniform mechanical properties throughout the deposit thickness.
- The travel speed should be adjusted to maintain a consistent weld bead width of 7 to 9 millimeters for 1.2 millimeter wire, as variations in bead width directly affect the actual overlap ratio.
- Interpass temperature must be controlled below 150 degrees Celsius to prevent excessive grain growth and sensitization of the austenitic stainless steel overlay.
- For critical applications requiring high impact toughness, consider adding a final dressing pass with 50 to 60 percent overlap to homogenize the surface microstructure.
Defect Analysis and Countermeasures
The most common defects associated with improper overlap ratios include incomplete fusion at pass boundaries, porosity from gas entrapment at overlap regions, and undercut at the weld toe. Incomplete fusion is the primary concern at overlap ratios below 20 percent and can be detected through ultrasonic testing using a 5 megahertz straight beam transducer at 45 degrees. The countermeasure is to increase the overlap ratio to at least 25 percent and to ensure proper cleaning between passes to remove oxide films.
Porosity at overlap regions is typically caused by insufficient arc force at the overlap zone, where the arc tends to be deflected by the previously deposited weld bead. This can be mitigated by slightly increasing the current at the beginning of each pass to ensure adequate penetration into the overlap region. Undercut at the weld toe is a cosmetic defect that can be addressed by reducing the travel speed slightly at the end of each pass to allow the molten pool to fill the toe region completely.
Study Insights and Practical Implications
This research provides valuable quantitative data on the relationship between weld pass overlap and mechanical properties in GMAW overlay welding, filling a gap in the technical literature where this parameter is often treated qualitatively rather than quantitatively. The finding that an overlap ratio of 35 to 45 percent provides optimal mechanical performance is directly applicable to industrial overlay welding operations and should be incorporated into welding procedure specifications.
The study also highlights the importance of considering the entire overlay as a multi-pass weld with its own thermal history, rather than treating each pass as an independent weld. The cumulative thermal effect of multiple passes creates a unique microstructural evolution that cannot be predicted from single-pass weld data alone. Engineers designing overlay welding procedures should consider the total number of passes, the overlap ratio, and the interpass temperature as a coupled parameter set rather than as independent variables. This holistic approach to overlay welding procedure design will lead to more reliable and consistent overlay deposits with predictable mechanical properties across the entire overlay thickness.