Microstructure and Fretting Wear Performance of SMAW Weld Overlay Joints

Literature Overview and Core Focus

This study investigates the microstructural evolution and fretting wear behavior of weld overlay joints produced by Shielded Metal Arc Welding (SMAW), also known as manual arc welding. The work addresses a frequently encountered practical challenge in the fabrication of cladded components where SMAW is employed for repair or localized overlay applications, particularly on large-scale equipment such as pressure vessels, heat exchanger shells, and piping systems. The core contribution lies in correlating the weld microstructure—specifically grain morphology, phase distribution, and residual stress state—with the resulting fretting wear resistance under cyclic contact loading.

Microstructural Characteristics of the SMAW Overlay Layer

SMAW overlay deposits exhibit a columnar dendritic structure growing epitaxially from the base metal. The microstructure is highly sensitive to the interpass temperature, travel speed, and welding current. In this study, the overlay layer was deposited on Q345R carbon steel substrate using E309L-type electrodes, producing a transition zone with a pronounced diffusion layer.

Parameter	Typical Range	Influence on Microstructure
Welding current	90–130 A	Higher current widens the fusion zone and promotes coarser grains
Travel speed	30–60 mm/min	Slower speed increases heat input and grain growth
Interpass temperature	50–200 °C	Controlled below 150 °C to limit grain coarsening
Number of passes	2–4	Multi-pass welding refines the surface layer grain structure
Heat input	0.8–2.5 kJ/mm	Excessive input promotes carbide precipitation and brittleness

The study reveals that the first pass of the overlay exhibits the most severe dilution from the base metal, with carbon content in the weld metal potentially exceeding 0.25 wt%, leading to martensite formation in the transition zone. Subsequent passes progressively reduce dilution as the preceding weld metal acts as a dilution barrier. The final surface layer typically achieves a duplex microstructure of austenite and ferrite, with carbide particles (primarily Cr₂₃C₆ and Cr₇C₃) dispersed at grain boundaries.

Fretting Wear Mechanism Analysis

Fretting wear, characterized by small-amplitude oscillatory sliding, differs fundamentally from gross sliding wear in both mechanism and damage morphology. The study identifies three distinct fretting damage regimes:

Stick-slip regime (displacement amplitude < 10 μm): Surface oxidation dominates, producing fine oxide debris with limited material loss.
Partial slip regime (displacement amplitude 10–50 μm): Abrasive wear becomes significant as oxide debris is entrained between contacting surfaces.
Full slip regime (displacement amplitude > 50 μm): Adhesive and abrasive wear mechanisms combine, leading to rapid material degradation.

The overlay layer demonstrates superior fretting wear resistance compared to the bare base metal, primarily attributed to:

Higher microhardness (HV 280–350 in the overlay versus HV 150–180 in the base metal)
Compressive residual stresses at the weld surface from solidification shrinkage
Hard carbide particles providing micro-ploughing resistance
Lower coefficient of friction due to the chromium-rich surface chemistry

Key Technical Findings and Engineering Implications

The most significant finding is that the fretting wear volume loss follows a logarithmic relationship with the number of cycles, with the wear rate stabilizing after approximately 50,000 cycles as a protective oxide layer establishes itself. However, the transition zone remains the weakest link, where microcracks initiate preferentially under cyclic fretting loads. The study recommends a minimum overlay thickness of 3.0 mm to ensure the transition zone falls below the maximum deformation depth experienced during fretting contact.

From a practical standpoint, this research validates the use of SMAW overlay for protecting pressure vessel flanges, pump shafts, and valve stems from fretting corrosion in high-vibration environments. The recommended electrode selection follows a dilution control strategy: E309L for austenitic overlay on carbon steel, with interpass temperature maintained below 150 °C and a maximum of four passes to optimize the hardness-wear resistance balance.

Study Insights and Conclusions

This literature provides valuable insight into the fundamental relationship between weld microstructure and fretting wear performance. The key engineering takeaway is that SMAW overlay, while less precise than automated processes, remains a viable and economical solution for fretting protection when proper weld procedure control is maintained. The emphasis on dilution management, interpass temperature control, and adequate overlay thickness provides a clear procedural framework that can be directly incorporated into welding procedure specifications for pressure vessel repair and maintenance operations.