Zhuojin Pipe Fitting Co., LtdMicrostructure and Erosion Resistance of Martensitic Stainless Steel Overlay Layers with Microalloying
Literature Overview
This paper by Gu Ruichao, Bao Yefeng, and Yang Ke, published in Electric Welder (2012, Vol. 42, Issue 2, pp. 16–19), investigates the microstructure and erosion resistance of martensitic stainless steel overlay layers produced by submerged arc surfacing. The study compares conventional 2Cr13 martensitic stainless steel with a microalloyed variant of 1Cr13 modified with V, Ti, Nb, and N, examining the effects of microalloying elements and heat treatment on overlay properties. The work is supported by the National Natural Science Foundation of China (51101050) and Jiangsu Provincial Natural Science Foundation (BK2011257).
Background and Technical Context
Martensitic stainless steels are widely used for erosion and wear protection in applications such as hydroelectric penstocks, pump impellers, and valve components. The conventional 1Cr13 and 2Cr13 grades offer moderate corrosion resistance and good mechanical properties, but their erosion resistance can be limited by the coarse microstructure and relatively low hardness achievable through conventional heat treatment. Microalloying with elements such as V, Ti, Nb, and N is a well-established strategy for enhancing the properties of martensitic steels through grain refinement and precipitation hardening.
Material Comparison
| Property | 2Cr13 (Conventional) | 1Cr13 + V, Ti, Nb, N (Microalloyed) |
|---|---|---|
| C content | 0.16–0.25 wt% | 0.08–0.15 wt% |
| Cr content | 12.0–14.0 wt% | 11.5–13.5 wt% |
| Microalloying elements | None | V, Ti, Nb, N |
| Expected microstructure | Coarse martensite | Fine martensite with dispersed carbides |
| Expected hardness (tempered) | 35–45 HRC | 40–55 HRC |
| Expected toughness | Moderate | Improved |
Microalloying Mechanisms
Grain Refinement During Solidification
The microalloying elements V, Ti, Nb, and N interact with carbon during the early stages of austenite nucleation to form fine, stable compound particles. These particles serve as heterogeneous nucleation sites, significantly increasing the nucleation rate and resulting in a finer austenite grain structure. The key reactions include:
- Ti + N → TiN (extremely stable, forms early)
- Nb + N → NbN (stable, forms early)
- V + C → VC (forms during cooling)
- Nb + C → NbC (forms during cooling)
The formation of these fine particles during solidification is critical because:
- They refine the austenite grain size, which in turn refines the martensite lath structure after transformation.
- They remain stable during welding heat cycles, providing persistent nucleation sites.
- They contribute to precipitation strengthening in the final tempered condition.
Precipitation Strengthening During Tempering
After the austenite-to-martensite transformation, the microalloying carbides and nitrides that formed during solidification remain dispersed in the martensitic matrix. During tempering:
- Some carbon compounds precipitate between martensite laths, providing secondary strengthening.
- The carbon content in the martensite matrix is reduced, improving toughness.
- The retained carbide particles continue to provide precipitation strengthening.
This dual mechanism — grain refinement during solidification and precipitation strengthening during tempering — results in a synergistic improvement in both strength and toughness.
Erosion Resistance Analysis
Strengthening Mechanisms and Their Contribution to Erosion Resistance
| Strengthening Mechanism | Source | Effect on Erosion Resistance |
|---|---|---|
| Grain refinement | V, Ti, Nb, N during austenite nucleation | Increases resistance to crack initiation and propagation |
| Precipitation strengthening | Fine carbides/nitrides in matrix | Increases hardness and resistance to micro-cutting |
| Secondary strengthening | Carbon compounds between martensite laths after tempering | Additional hardness contribution |
| Matrix toughness improvement | Reduced carbon content in tempered martensite | Reduces spalling and chipping under impact erosion |
Erosion Resistance Performance
The study demonstrates that the microalloyed 1Cr13 overlay layers exhibit superior erosion resistance compared to conventional 2Cr13 overlays. The improved performance is attributed to:
- Finer martensitic structure: The refined lath martensite structure provides more uniform stress distribution and higher resistance to localized plastic deformation.
- Dispersed carbide particles: The fine, uniformly distributed carbides resist abrasive particle cutting and ploughing.
- Enhanced toughness: The improved toughness of the tempered martensitic matrix reduces the tendency for spalling and delamination under erosive impact loading.
- Optimized hardness-toughness balance: The combination of high hardness from precipitation strengthening and improved toughness from reduced matrix carbon content results in a material that resists both cutting and chipping wear mechanisms.
Heat Treatment Optimization
The tempering process plays a critical role in optimizing the properties of the martensitic overlay layers:
| Tempering Temperature | Effect on Microstructure | Effect on Properties |
|---|---|---|
| 500–550°C | Carbide coarsening begins, matrix softening | Moderate hardness, good toughness |
| 550–600°C | Optimal carbide precipitation between laths, reduced matrix carbon | Best hardness-toughness balance |
| 600–650°C | Excessive carbide coarsening, significant matrix softening | Reduced hardness, improved ductility |
The optimal tempering temperature for the microalloyed overlay is in the range of 550–600°C, where the secondary strengthening from inter-lath carbide precipitation is maximized while maintaining adequate matrix hardness.
Engineering Practice Implications
Application Suitability
The microalloyed martensitic stainless steel overlay layers are particularly suitable for:
- Hydroelectric applications: Penstock linings, turbine components, and water conveyance systems subject to solid particle erosion.
- Pump and valve components: Impellers, wear rings, and valve seats in slurry service.
- Mining equipment: Crusher components, conveyor chutes, and slurry handling equipment.
- Power generation: Boiler tubes, ash handling systems, and flue gas ducts.
Process Selection
Submerged arc surfacing was selected for this study due to its advantages for overlay applications:
- High deposition rate (5–10 kg/h).
- Deep penetration, which can be controlled by adjusting parameters.
- Good shielding from the flux, reducing oxidation and porosity.
- Suitable for large surface areas requiring multi-pass buildup.
However, engineers should note that submerged arc surfacing produces higher heat input compared to processes such as plasma arc or laser surfacing, which can lead to higher dilution and coarser microstructure. The microalloying strategy helps mitigate these effects by providing grain refinement even under high-heat-input conditions.