Ultrafine Microstructure and Wear Resistance of High Chromium Stainless Steel Overlay Alloys

Microstructure Control and Grain Refinement Mechanisms

The wear resistance of high chromium stainless steel overlay alloys is fundamentally governed by the microstructure of the carbide network, the matrix composition, and the grain size of the austenitic or martensitic phase. This study demonstrates that ultrafine grain structures, with grain sizes below 5 micrometers, can be achieved through rapid solidification induced by high cooling rates during overlay welding. The grain refinement is attributed to the high nucleation density created by the rapid temperature gradient at the solidification front, combined with the presence of fine TiC or NbC particles that act as heterogeneous nucleation sites.

The following table compares the microstructural characteristics and mechanical properties of conventionally solidified and ultrafine-grained overlay alloys.

The ultrafine grain structure exhibits approximately 50% higher hardness and 60% improved abrasion resistance compared to the conventional structure, at the expense of a moderate reduction in impact toughness. This trade-off is acceptable for most wear-resistant applications where the operating temperature does not exceed 600 °C.

Carbide Morphology and Distribution

The type, size, and distribution of carbides in high chromium stainless steel overlays are critical determinants of wear performance. The following carbide phases are commonly observed:

In the ultrafine-grained alloy, the M7C3 carbides are predominantly fine and uniformly distributed, with an average size of 1.2 micrometers. In contrast, the conventional alloy exhibits coarse, blocky M23C6 carbides along grain boundaries, which serve as crack initiation sites during abrasive wear. The refinement of the carbide network through rapid solidification not only increases the hardness of individual carbides but also improves their bonding with the matrix, reducing the likelihood of carbide pull-out during sliding contact.

Wear Testing and Performance Evaluation

The wear performance was evaluated using a pin-on-disk tribometer with alumina (Al2O3) pins under a normal load of 10 N and a sliding speed of 0.5 m/s. The following results were obtained after 1000 meters of sliding distance.

The addition of titanium to the ultrafine-grained alloy further refines the microstructure and promotes the formation of a protective oxide film during sliding, reducing the wear volume loss by an additional 26% compared to the ultrafine alloy without titanium.

Engineering Application Considerations

The ultrafine-grained high chromium stainless steel overlay alloy is particularly suitable for applications involving abrasive wear in corrosive environments, such as pump impellers, valve seats, and mining equipment. However, the reduced impact toughness necessitates careful evaluation for applications involving impact loading or thermal cycling. The following guidelines are recommended for engineering application:

Study Insights and Conclusions

The study demonstrates that microstructural refinement through rapid solidification is a powerful tool for enhancing the wear resistance of high chromium stainless steel overlay alloys. The key mechanism is the combined effect of grain boundary strengthening, carbide refinement, and improved matrix-carbide bonding. For engineers designing overlay systems for severe wear applications, the ultrafine-grained approach offers a significant performance improvement without requiring exotic alloy compositions. The challenge lies in achieving consistent grain refinement across large production welds, which requires careful control of heat input, travel speed, and preheating. Future research should focus on developing multi-pass overlay strategies that maintain ultrafine grains through subsequent thermal cycles, potentially through the use of interpass cooling or cryogenic treatment between passes.