Microstructure and Erosion Resistance of Martensitic Stainless Steel Overlay Layers with Microalloying

Literature Overview

Published in Welding & Cutting (2012, Vol. 42, No. 2, pp. 16–19) and supported by the National Natural Science Foundation of China (51101050), the Jiangsu Provincial Natural Science Foundation (BK2011257), and the Jiangsu University Provincial Key Laboratory of Advanced Welding Technology, this paper by Gu Ruichao, Bao Yefeng, and Yang Ke from Hohai University and Jiangsu University investigates the effect of microalloying (V, Ti, Nb, N) and heat treatment on the microstructure and erosion resistance of martensitic stainless steel overlay layers. The study compares unalloyed 2Cr13 with microalloyed 1Cr13 martensitic stainless steels used for submerged arc overlay welding.

Material Design and Microalloying Strategy

Martensitic stainless steels are widely used in erosion-corrosion applications such as hydroelectric turbine components, pump impellers, and slurry handling equipment. The base materials studied are:

The microalloying strategy employs four elements that serve distinct metallurgical functions:

• V (Vanadium): Forms stable carbides and nitrides, promotes grain refinement, and provides precipitation strengthening.
• Ti (Titanium): Forms very stable carbides and nitrides, acts as a grain refiner, and scavenges sulfur and nitrogen.
• Nb (Niobium): Forms stable carbides and nitrides, promotes grain refinement, and provides solid solution strengthening.
• N (Nitrogen): Forms nitrides with V, Ti, and Nb, provides solid solution strengthening, and promotes austenite stability.

Microstructural Evolution and Strengthening Mechanisms

The study identifies three distinct strengthening mechanisms that operate at different stages of the solidification and heat treatment sequence:

1. Grain Refinement (During Austenite Nucleation)

During the initial stages of austenite nucleation from the liquid, V, Ti, Nb combine with C and N to form fine, uniformly distributed stable compound particles. These particles act as heterogeneous nucleation sites, promoting the formation of finer austenite grains. The resulting fine-grained microstructure provides:

• Increased grain boundary area, which impedes dislocation motion
• More uniform distribution of secondary phases
• Improved toughness through smaller grain size

2. Precipitation Strengthening (After Austenite Grain Stabilization)

Once the austenite grains have stabilized, the same V, Ti, Nb compounds continue to exert a strengthening effect through precipitation strengthening. The fine, dispersed particles impede dislocation motion within the austenite grains, increasing the yield strength.

3. Secondary Strengthening (After Tempering)

During tempering, some carbon compounds precipitate between the martensite laths. This secondary precipitation:

• Provides additional strengthening of the tempered martensite
• Reduces the carbon content in the martensite, which improves ductility and toughness
• Creates a tempered martensite microstructure with improved strength-toughness balance

Erosion Resistance Performance

The erosion resistance of the overlay layers is the primary performance indicator for applications such as hydroelectric components and slurry handling equipment. The study demonstrates that the combination of base matrix strength-toughness and the strengthening mechanisms described above leads to improved erosion resistance.

Key Findings on Erosion Resistance

1. Microalloyed 1Cr13 outperforms 2Cr13: The microalloyed variant exhibits superior erosion resistance compared to the conventional 2Cr13, demonstrating the effectiveness of the microalloying strategy.
2. Heat treatment is critical: The tempering treatment significantly improves erosion resistance by optimizing the strength-toughness balance. Over-tempered materials may have reduced hardness and erosion resistance, while under-tempered materials may be too brittle.
3. Fine-grained microstructure contributes to erosion resistance: The grain refinement achieved through microalloying not only improves strength but also improves the material's ability to absorb erosive impact energy through plastic deformation rather than fracture.

Engineering Practice Integration

For engineers working with martensitic stainless steel overlay applications, this study provides several practical guidelines:

1. Microalloying is beneficial: Adding V, Ti, Nb, and N to martensitic stainless steel overlay consumables improves both microstructure and erosion resistance. The microalloying elements are relatively inexpensive and can be added in small quantities (typically 0.05–0.30% each).
2. Heat treatment optimization: The tempering temperature and duration must be carefully optimized to achieve the desired balance of hardness and toughness. Typical tempering temperatures for martensitic stainless steels range from 400–600 °C, with lower temperatures providing higher hardness and higher temperatures providing better toughness.
3. Application suitability: Martensitic stainless steel overlays are particularly suitable for applications involving:

• Clean water erosion (hydroelectric turbine components)
• Mildly corrosive slurry erosion
• Low-velocity solid particle erosion
• Applications where moderate corrosion resistance is required

1. Limitations: Martensitic stainless steels have limited resistance to highly corrosive environments compared to austenitic or duplex stainless steels. For severe corrosion-erosion environments, duplex stainless steel or super austenitic overlays may be more appropriate.

Study Insights and Reflections

This paper demonstrates a sophisticated understanding of the interplay between microalloying, heat treatment, and mechanical performance in martensitic stainless steel overlay systems. The identification of three distinct strengthening mechanisms operating at different stages of processing is a valuable conceptual framework that can be applied to other alloy systems.

The use of submerged arc welding (SAW) as the overlay process is noteworthy. SAW provides high deposition rates and good process stability, making it suitable for thick overlay layers. However, the lack of external shielding gas (the flux provides shielding) means that the microstructure is influenced by both the welding parameters and the flux chemistry. Engineers should be aware that changes in flux composition can affect the microalloying effectiveness.

The emphasis on the strength-toughness balance for erosion resistance is an important insight. Erosion resistance is not simply a function of hardness; it depends on the material's ability to resist both deformation and fracture under erosive impact. A very hard but brittle material may suffer from rapid erosion through fracture, while a tough but soft material may suffer from rapid erosion through deformation. The optimal erosion resistance is achieved at an intermediate hardness-toughness combination, which is precisely what the microalloying and tempering strategy provides.

Reference Value and Outlook

This study provides a solid foundation for the development of microalloyed martensitic stainless steel overlay consumables for erosion-resistant applications. The systematic investigation of microalloying elements and heat treatment effects offers practical guidance for consumable development and process optimization. Future research could explore the combined effects of microalloying with additional elements such as Mo, Cu, and W, and could investigate the performance of microalloyed overlays under more severe erosion-corrosion conditions. The principles of grain refinement, precipitation strengthening, and secondary strengthening identified in this study are broadly applicable to other overlay welding systems and could inform the development of next-generation overlay materials.