Microstructure and Property Analysis of SMAW Overlay Metal in Cladding Applications

Literature Overview

This study by Zhong Yu, Qu Jinshan, Chen Wenjing, Pan Quanxi, and Luo Chaoyu from Xihua University's School of Materials Science and Engineering, published in Welding Technology (2007), examines the microstructure and mechanical properties of overlay metal deposited by shielded metal arc welding (SMAW). SMAW remains one of the most widely used cladding methods in industrial practice due to its portability, low equipment cost, and applicability to field repair and maintenance operations.

Core Technical Content

The study systematically investigates how SMAW process parameters and welding conditions influence the microstructural evolution and resulting properties of the overlay layer. SMAW cladding is characterized by relatively low deposition rates, high dilution with the base metal, and significant heat input concentration—all of which affect the final overlay quality.

The microstructural analysis typically reveals a columnar grain structure growing from the fusion boundary toward the surface, with possible equiaxed grain regions near the top of each pass. The grain morphology depends on the cooling rate, which in turn is governed by the heat input, plate thickness, interpass temperature, and number of passes.

Microstructural Analysis and Interpretation

The overlay metal microstructure in SMAW cladding is typically characterized by several distinct regions: the heat-affected zone (HAZ) in the base metal, the transition zone at the interface, and the weld overlay layer itself. Each region exhibits different microstructural features and mechanical properties, creating a gradient in properties across the cladding thickness.

In the overlay layer, the microstructure is predominantly columnar dendritic, with dendrite arm spacing influenced by the cooling rate. Higher cooling rates produce finer dendrites and potentially finer intradendritic precipitates, which generally improve hardness and strength but may reduce toughness. The composition of the overlay metal is significantly affected by dilution from the base metal, which can alter the phase balance and properties of the final deposit.

For stainless steel overlay applications (such as 304, 316, or 321), the dilution of carbon steel or low-alloy steel into the overlay layer introduces ferrite formation in what should be an austenitic microstructure. This ferrite content must be controlled—typically maintained below 10% for single-phase austenitic applications—to ensure adequate corrosion resistance and ductility. The ferrite content can be estimated using the Ferrite Number (FN) method per ASTM E1245.

The mechanical properties of the overlay layer—including hardness, tensile strength, and elongation—are directly related to the microstructure and composition. Hardness is influenced by phase composition, grain size, and precipitate distribution. In martensitic overlay deposits, hardness can range from 30 to 60 HRC depending on carbon content and cooling rate. Austenitic overlays typically exhibit lower hardness (15–25 HRC) but superior corrosion resistance and ductility.

Engineering Practice Considerations

SMAW cladding is extensively used in field repair of pressure vessels, pipelines, and heat exchangers where access is limited or equipment cannot be removed from service. The portability of SMAW equipment makes it the method of choice for in-situ overlay applications. However, the lower deposition rates (typically 1–3 kg/h) and higher dilution rates (20–40%) compared to mechanized methods must be considered in cost-benefit analyses.

Quality control of SMAW overlays requires attention to several factors: electrode storage and conditioning, interpass temperature control, proper cleaning between passes, and verification of overlay thickness and composition. The dilution rate should be measured by chemical analysis of the overlay layer, and the results compared against acceptance criteria specified in the applicable welding procedure specification (WPS).

For pressure vessel applications governed by NB/T 47002 or ASME VIII Div.1, the SMAW overlay process must be qualified per NB/T 47014 or ASME IX, with testing of dilution, mechanical properties, and (where required) corrosion resistance. The qualified WPS must specify all essential variables including electrode type, current range, travel speed, interpass temperature, and number of passes.

Key Questions and Reflections

The primary concern with SMAW overlay is the reproducibility of results. Unlike mechanized processes, SMAW is operator-dependent, and variations in technique can significantly affect overlay quality. This variability must be addressed through rigorous operator qualification, detailed WPS documentation, and enhanced quality control procedures.

Another consideration is the residual stress state in SMAW overlays. The sequential heating and cooling of each pass creates complex residual stress patterns that can affect the dimensional stability and fatigue performance of the cladded component. Post-weld stress relief treatment may be required for critical applications, though this must be carefully controlled to avoid sensitization of stainless steel overlays.

Study Insights and Conclusion

This work provides valuable insight into the microstructural evolution and property development in SMAW overlay deposits. The understanding gained from systematic microstructural analysis enables engineers to predict and control overlay properties through appropriate process parameter selection. For practitioners, the key message is that SMAW cladding, while operationally flexible, requires careful process control and quality assurance to achieve consistent results. The microstructural characterization techniques described—optical microscopy, SEM, XRD, and hardness mapping—should be incorporated into routine quality verification for critical cladding applications.