CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Subcritical Quenching on Wear Resistance of NiCrMo-3 Cladding Layer
Literature Overview
This study examines the influence of subcritical quenching on the wear resistance of NiCrMo-3 (UNS S64200) cladding layers applied to carbon steel substrates. NiCrMo-3 is a high-nickel martensitic alloy cladding material designed for severe wear applications, particularly in mining, cement, and heavy industry equipment. The base cladding deposit, as-welded, consists of retained austenite and martensite with varying carbon content. Subcritical quenching—a heat treatment involving austenitization below the Ac1 temperature (approximately 820°C for NiCrMo-3) followed by rapid cooling—is investigated as a method to optimize the microstructure and enhance wear resistance beyond the as-welded condition.
Core Technical Findings
The as-welded NiCrMo-3 cladding layer exhibits a microstructure consisting of approximately 30–40% retained austenite and 60–70% martensite, with hardness values of 45–50 HRC. The retained austenite, stabilized by the high nickel content (approximately 10 wt%), provides some toughness but contributes limited wear resistance. The wear test results (dry sliding against a 62 mm diameter 52100 bearing steel ball under 20 N normal load) show a specific wear rate of approximately 4.5 × 10⁻⁴ mm³/N·m for the as-welded condition.
After subcritical quenching at 780°C for 60 minutes followed by oil quenching, the microstructure undergoes significant transformation. The retained austenite content decreases to approximately 10–15%, as the reduced austenite stability (due to the subcritical temperature) allows partial transformation to martensite during quenching. Simultaneously, fine carbide precipitation occurs within the martensite matrix. The resulting hardness increases to 52–55 HRC, and the specific wear rate decreases to approximately 2.1 × 10⁻⁴ mm³/N·m—a 53% improvement in wear resistance.
| Heat Treatment Condition | Hardness (HRC) | Retained Austenite (%) | Specific Wear Rate (10⁻⁴ mm³/N·m) | Wear Life Improvement |
|---|---|---|---|---|
| As-welded | 45–50 | 30–40 | 4.5 | Baseline |
| Subcritical quench 780°C/60min | 52–55 | 10–15 | 2.1 | 2.1× |
| Subcritical quench 780°C/120min | 53–56 | 8–12 | 1.8 | 2.5× |
| Full austenitize 870°C/60min + oil quench | 55–58 | 5–8 | 1.5 | 3.0× |
| Subcritical quench 780°C/60min + temper 200°C | 50–53 | 12–18 | 2.8 | 1.6× |
Microstructural Analysis and Mechanism Interpretation
The enhancement in wear resistance after subcritical quenching is attributed to three primary mechanisms. First, the reduction in retained austenite content increases the overall hardness of the cladding layer, as martensite is inherently harder than austenite. Second, the subcritical austenitization temperature promotes the precipitation of fine, uniformly distributed carbides (primarily M₇C₃ and M₂₃C₆ type) within the martensite matrix, providing effective resistance to abrasive wear. Third, the reduced retained austenite content minimizes the risk of stress-induced martensitic transformation during wear, which can lead to surface cracking and material loss.
However, the study also identifies a critical limitation. Subcritical quenching at temperatures too close to Ac1 (above 800°C) begins to dissolve some of the beneficial carbides, reducing the carbide reinforcement effect. Conversely, temperatures significantly below Ac1 (below 750°C) provide insufficient austenite stabilization change, resulting in minimal microstructural modification. The optimal window appears to be 770–790°C for 60–120 minutes, balancing carbide precipitation with austenite transformation.
An important observation is that the bond strength between the NiCrMo-3 cladding layer and the carbon steel substrate is not adversely affected by subcritical quenching. This is because the thermal expansion mismatch between the cladding and substrate at subcritical temperatures (770–790°C) is relatively small, and the resulting thermal stresses remain within the elastic range of the substrate material. This finding is significant for engineering applications where the cladding layer must withstand both wear and mechanical loading.
Process Considerations and Defect Analysis
From a manufacturing perspective, subcritical quenching of NiCrMo-3 cladding layers presents several process challenges that must be carefully managed:
- Thermal distortion: The differential thermal expansion between the Ni-rich cladding layer and the carbon steel substrate can cause warping during heating and quenching. Preheating the substrate to 200–250°C before applying the cladding deposit reduces residual stresses and minimizes distortion risk.
- Cracking susceptibility: Rapid quenching of the high-nickel martensitic cladding can induce cracking, particularly at the cladding-substrate interface. Oil quenching (rather than water quenching) provides a more controlled cooling rate and significantly reduces cracking risk.
- Interpass temperature control: For multi-pass NiCrMo-3 cladding, maintaining interpass temperatures below 200°C is essential to prevent excessive grain growth and carbide coarsening in previously deposited layers.
- Quenching medium selection: Oil quenching is preferred over air cooling (which may be too slow for adequate martensite formation) and water quenching (which creates excessive thermal shock).
Common defects observed in NiCrMo-3 cladding layers include porosity (from gas entrapment during welding), lack of fusion at the cladding-substrate interface (from insufficient heat input), and microcracks (from excessive cooling rates). The subcritical quenching process can actually heal some of these defects—specifically, fine porosity can be partially closed through plastic deformation during heating—but cannot repair lack of fusion or gross cracking.
Engineering Practice Integration
In practical applications, NiCrMo-3 cladding is commonly used on equipment such as ball mill liners, conveyor rollers, crusher jaws, and pump impellers. The subcritical quenching treatment, when applied correctly, can extend service life by 2–3 times compared to the as-welded condition. For example, a ball mill liner cladded with NiCrMo-3 and subcritical quenched may achieve a service life of 18–24 months compared to 8–12 months for the as-welded condition, representing significant economic benefit.
The welding procedure for NiCrMo-3 cladding typically employs submerged arc welding (SAW) or flux-cored arc welding (FCAW) for thick deposits (greater than 3 mm), with gas metal arc welding (GMAW) or gas tungsten arc welding (GTAW) for thin deposits or repair work. The welding parameters must be carefully controlled to ensure adequate penetration into the substrate (typically 0.5–1.0 mm) and sufficient dilution control to maintain the NiCrMo-3 composition in the final deposit.
Summary and Outlook
The study demonstrates that subcritical quenching is an effective and practical method for enhancing the wear resistance of NiCrMo-3 cladding layers, with optimal results achieved at 780°C for 60–120 minutes followed by oil quenching. The improvement is primarily driven by the reduction in retained austenite and the precipitation of fine carbides within the martensite matrix. For engineering practice, this finding provides a straightforward post-weld heat treatment option that does not require specialized equipment beyond a conventional furnace and oil quenching facility. The key challenge lies in managing thermal distortion and cracking during the quenching process, which requires careful control of heating rates, preheating temperatures, and quenching medium selection. Future investigations should explore the effects of subcritical quenching on other high-nickel cladding alloys such as NiCrMo-4 and NiCrMo-6, as well as the long-term stability of the quenched microstructure under cyclic thermal loading conditions typical of mining and cement industry applications.