CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Tempering Temperature on Microstructure and Wear Resistance of Iron-Based Multicomponent Alloy Overlay
Literature Overview
This 2006 study by researchers at Hebei Institute of Building Materials examines the post-weld thermal treatment effects on iron-based multicomponent alloy overlay deposits. The research focuses on how tempering temperature influences the microstructural evolution and resulting tribological performance of overlay layers containing multiple alloying elements designed for enhanced wear resistance. This work addresses a critical practical question: how to optimize the post-deposition heat treatment to achieve the best balance between hardness, toughness, and wear resistance in complex alloy overlay systems.
Core Technical Analysis
Microstructural Evolution with Tempering
The iron-based multicomponent alloy overlay typically contains a combination of Cr, Mo, V, W, and sometimes Ni to achieve specific property targets. The as-welded microstructure generally consists of martensite with dispersed carbides (M₇C₃, M₂C, or MC type depending on composition). Tempering induces several sequential transformations:
| Tempering Temperature | Primary Microstructural Changes | Hardness Trend |
|---|---|---|
| 200–300 °C | Carbide nucleation on dislocations; stress relief | Slight decrease |
| 300–400 °C | Tempered martensite formation; coarsening of fine carbides | Moderate decrease |
| 400–500 °C | Precipitation of secondary carbides (M₆C, M₂₃C₆); carbide coarsening | Significant decrease |
| 500–600 °C | Peak secondary hardening (Mo, W, V carbides); spheroidization | Possible increase then decrease |
| >600 °C | Coarse carbide precipitation; austenite recovery | Sharp decrease |
Wear Mechanism Analysis
The wear resistance of the overlay deposit is governed by the interaction between the matrix and the dispersed carbides. The primary wear mechanisms in iron-based overlay alloys include:
- Abrasive wear: Governed by the hardness of both the matrix and carbides, as well as the volume fraction and distribution of carbides. The Hall-Petch relationship applies to the matrix, while the carbide hardness (typically 1500–2200 HV for M₇C₃) provides the primary wear resistance.
- Adhesive wear: Reduced by surface oxide layers formed during tempering and by increasing the hardness-to-elasticity ratio of the matrix.
- Fatigue wear: Related to the subsurface crack initiation and propagation, which is influenced by the tempering-induced changes in microhardness gradient and residual stress state.
Key Findings on Optimal Tempering
The research demonstrates that the optimal tempering temperature for maximum wear resistance is not necessarily at the peak hardness temperature. Instead, the optimal condition often corresponds to a tempering temperature that produces:
- A tempered martensite matrix with moderate hardness (50–60 HRC) that provides sufficient toughness to resist crack initiation.
- Well-dispersed, fine secondary carbides (0.2–1.0 μm) that provide effective abrasive resistance without creating stress concentration sites.
- Minimal retained austenite, which can be detrimental to wear performance due to its lower hardness and tendency to transform under deformation.
Process Optimization and Engineering Application
Recommended Heat Treatment Schedule
| Parameter | Specification | Rationale |
|---|---|---|
| Tempering temperature | 400–550 °C (composition-dependent) | Balances hardness and toughness |
| Holding time | 1–2 hours per 25 mm thickness | Ensures uniform transformation |
| Heating rate | ≤150 °C/h | Prevents thermal cracking in high-carbon deposits |
| Cooling method | Air cooling or furnace cooling | Avoids quench cracking; controls residual stress |
| Number of cycles | 1–2 cycles for thick sections | Achieves uniform microstructure through thickness |
Dilution Effects on Tempering Response
An important practical consideration is that the dilution ratio between the overlay deposit and the base metal significantly affects the tempering response. Higher dilution introduces more base metal alloying elements (typically Mn, Si, P, S from carbon steel) into the deposit, which can:
- Alter the carbide precipitation sequence and morphology.
- Shift the tempering transformation temperatures.
- Introduce impurity elements that promote intergranular cracking during tempering.
Study Insights and Reflections
This research underscores a fundamental principle in overlay engineering: the as-welded microstructure is merely a starting point, and the final properties depend critically on the post-weld thermal history. Engineers must approach overlay design as a two-stage process: first, selecting a consumable that produces an appropriate as-welded microstructure; second, applying a heat treatment that optimizes the microstructure for the specific service condition.
The concept of "peak secondary hardening" is particularly important for overlays containing Mo, W, and V. At temperatures of 450–550 °C, these elements precipitate as fine, coherent carbides that can actually increase hardness above the as-quenched value. However, this benefit is transient, and excessive tempering temperature or time leads to carbide coarsening and rapid hardness loss.
From a practical standpoint, the most significant finding is that wear resistance does not correlate linearly with hardness. An overlay deposit tempered to 55 HRC with well-dispersed carbides may exhibit superior wear resistance to one tempered to 65 HRC with coarse, sparsely distributed carbides. This insight challenges the common engineering practice of maximizing hardness as a proxy for wear performance and emphasizes the need for holistic microstructural evaluation.