CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Tempering Temperature on Microstructure and Wear Resistance of Iron-Based Multi-Component Alloy Cladding Layer
Literature Overview
This study investigates the influence of tempering temperature on the microstructure evolution and wear resistance of iron-based multi-component alloy cladding layers deposited on low-carbon steel substrates. The multi-component alloy system typically includes elements such as Cr, Mo, V, Nb, and W, which form complex carbide networks responsible for wear resistance. The study systematically examines tempering temperatures ranging from 400°C to 800°C in 100°C increments, correlating the resulting microstructural changes with pin-on-disk wear test results. The findings are particularly relevant to cladding applications in the mining, cement, and power generation sectors where post-weld heat treatment is routinely performed for stress relief or mechanical property optimization.
Core Technical Points
The microstructure of as-welded iron-based multi-component alloy cladding layers typically consists of martensite, retained austenite, and various carbide phases including M₇C₃, M₂C, and MC. The tempering process fundamentally alters this assemblage through several mechanisms:
- Carbide precipitation and coarsening: At lower tempering temperatures (400–500°C), fine secondary carbides precipitate within the martensite, increasing hardness through precipitation strengthening. As temperature increases to 600–700°C, these carbides coarsen and aggregate, reducing hardness but improving toughness.
- Retained austenite decomposition: Retained austenite, which acts as a toughening phase, begins to decompose into ferrite and cementite above 550°C, reducing the overall ductility of the overlay.
- Matrix softening: The martensitic matrix undergoes temper softening above 500°C as dislocations are annihilated and subgrain structures develop.
Microstructural Evolution with Tempering Temperature
| Tempering Temp (°C) | Dominant Carbide Phases | Matrix Structure | Hardness (HV) | Wear Resistance (mg loss) |
|---|---|---|---|---|
| 400 | Fine M₇C₃, M₂C | Tempered martensite + RA | 850–900 | 12–15 |
| 500 | M₇C₃, M₂C | Tempered martensite + RA | 780–830 | 18–22 |
| 600 | Coarsened M₇C₃, M₂C | Ferrite + pearlite + carbides | 620–680 | 28–35 |
| 700 | Aggregated M₂C, M₇C₃ | Ferrite + coarse carbides | 520–580 | 40–50 |
| 800 | Coarse M₂C | Ferrite + coarse carbides | 450–500 | 55–70 |
Process and Standards Analysis
The study employs a systematic approach that aligns with the PDCA cycle: the Plan phase involves selecting tempering temperatures based on prior literature and thermodynamic calculations; the Do phase involves actual heat treatment and microstructural characterization; the Check phase involves wear testing and property evaluation; and the Act phase involves recommending optimal tempering windows for specific service conditions.
From a standards perspective, the post-weld heat treatment of overlay welds must comply with ASME VIII Div.1 UW-40 and NB/T 47014 requirements. The tempering temperature selected must not exceed the maximum allowable temperature for the base metal to avoid compromising the substrate's mechanical properties. For low-carbon steel substrates (e.g., SA-516 Gr.70), tempering temperatures above 700°C risk reducing the base metal yield strength below code minimums.
A critical observation from the study is that the optimal tempering temperature for wear resistance is not necessarily the same as the optimal temperature for stress relief. The study finds that 500°C tempering provides the best balance between wear resistance (hardness ~800 HV) and acceptable toughness, while also providing effective stress relief. Temperatures above 600°C significantly degrade wear resistance due to carbide coarsening and matrix softening.
Engineering Practice Implications
In practical cladding applications, the tempering temperature is often dictated by the need to relieve welding residual stresses rather than by wear performance optimization. This creates a conflict that engineers must navigate carefully. The study provides valuable guidance: for components where wear resistance is the primary concern (e.g., crusher liners, grinding media), tempering should be limited to 450–550°C to preserve the fine carbide distribution. For components where stress relief is critical (e.g., thick-section pressure vessels with overlay welds), tempering at 600–650°C may be necessary, accepting a moderate reduction in overlay hardness.
The study also highlights the importance of cooling rate after tempering. Air cooling from tempering temperatures above 500°C can cause re-hardening due to the formation of new martensite during cooling, particularly in high-carbon, high-chromium overlays. This phenomenon, known as temper embrittlement in some alloy systems, can negate the benefits of the tempering treatment. Furnace cooling or controlled-rate cooling is recommended for overlays with temper embrittlement susceptibility.
Key Questions and Reflections
An important question raised by this study is the effect of tempering on the bond strength between the overlay and the base metal. While the study focuses on overlay microstructure and wear properties, the interface region is also affected by tempering. Excessive tempering temperatures can cause intergranular cracking at the fusion line if the dilution zone contains brittle phases. The study does not extensively address this, but engineering experience suggests that bond strength testing (per ASTM A263 or EN 10028-7) should be performed on specimens that have undergone the same tempering treatment as the production component.
Another reflection is the role of alloying elements in controlling carbide stability during tempering. The study notes that adding 0.5–1.0 wt% Nb or V significantly improves carbide retention at higher tempering temperatures. This is because NbC and VC have higher thermodynamic stability than Cr₇C₃ or Cr₂₃C₆, and they act as nucleation sites that inhibit the coarsening of less stable carbides. This insight opens possibilities for developing temper-resistant cladding compositions for applications requiring high-temperature wear resistance.
Study Insights and Summary
This study provides a comprehensive understanding of how tempering temperature governs the microstructure-property relationship in iron-based multi-component alloy cladding layers. The key takeaway is that tempering is a double-edged sword: it is essential for stress relief and toughness improvement, but it inevitably reduces wear resistance through carbide coarsening and matrix softening. The optimal tempering window of 450–550°C represents a practical compromise that preserves most of the as-welded hardness while providing adequate stress relief. For engineers involved in specifying post-weld heat treatment for cladded components, this study reinforces the importance of tailoring the PWHT cycle to the specific performance requirements of the application rather than applying a one-size-fits-all approach. The broader implication is that overlay design must consider the entire thermal history of the component, from welding through post-weld treatment to in-service thermal cycling.