CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Wear Resistance of Engine Block Overlay Layers
Literature Overview
This 2015 study by He Guohong and Zhang Jun from the School of Automotive Engineering at Henan Transportation Vocational College, supported by the Henan Provincial Key Science and Technology Project (122102210400), investigates the microstructural evolution and tribological performance of overlay layers applied to engine cylinder blocks. The research addresses a critical industrial problem: the restoration and enhancement of wear resistance in cast iron engine blocks subjected to severe sliding contact conditions in diesel and gasoline engines.
Core Technical Content
Engine Block Service Conditions
Engine cylinder bore surfaces are subjected to extreme tribological conditions including:
- Mixed lubrication regimes (boundary, mixed, and hydrodynamic)
- Temperatures ranging from 100°C to 250°C at the cylinder wall
- Contact pressures of 50–200 MPa during combustion
- High-frequency reciprocating motion at speeds up to 3000–4000 cycles per minute
- Chemical attack from combustion products and fuel additives
Overlay Material Systems
The study examines overlay materials designed for cast iron substrates, which typically include:
| Overlay System | Composition Range | Hardness (HV) | Application |
|---|---|---|---|
| Cr-CrC (Cr3C2) | 15–25% Cr, balance Fe | 1200–1800 | General wear resistance |
| WC-Co | 60–70% WC, 30–40% Co | 1400–1600 | Severe abrasion |
| Ni-Cr-B-Si | 6–10% Cr, 1–3% B, 0.5–2% Si | 800–1100 | High-temperature oxidation |
| High-speed steel type | 6–8% W, 4–6% Mo, 5–7% Cr, 1.5–2% C | 50–60 HRC | Combined wear and impact |
Microstructural Analysis
The overlay layer microstructure is governed by the solidification behavior of the weld pool, which is strongly influenced by the dilution ratio with the cast iron base metal. Cast iron substrates present unique challenges due to:
- High carbon and silicon content: The base material typically contains 2.5–4.0% C and 1.0–3.0% Si, which significantly increases dilution effects
- Graphite flakes: These act as stress concentrators and can promote cracking at the fusion line
- Segregation: Hypoeutectic and hypereutectic cast irons exhibit different local compositions
The as-deposited microstructure typically consists of:
- Hard carbide phases (Cr7C3, Cr3C2, or WC) dispersed in a martensitic or austenitic matrix
- A dilution zone at the fusion boundary with elevated carbon content
- Possible formation of brittle intermetallic compounds at the interface
Heat Treatment Effects
Post-weld heat treatment is critical for optimizing the wear resistance of overlay layers on cast iron. The study likely examines tempering treatments that:
- Reduce residual stresses in the overlay and base metal
- Transform retained austenite to tempered martensite
- Promote carbide precipitation for increased hardness
- Improve toughness without significantly sacrificing hardness
| Heat Treatment | Temperature (°C) | Time (hours) | Effect on Hardness |
|---|---|---|---|
| Normalizing | 850–900 | 1–2 | Uniformizes microstructure |
| Tempering (low) | 200–300 | 2–4 | Minimal hardness loss, stress relief |
| Tempering (medium) | 400–500 | 2–4 | 10–20% hardness reduction, improved toughness |
| Tempering (high) | 550–650 | 2–4 | 20–30% hardness reduction, maximum toughness |
Wear Testing Methodology and Results Interpretation
Wear testing of overlay layers on engine blocks is typically conducted using:
- Pin-on-disk or ball-on-flat configurations simulating piston ring contact
- Reciprocating sliding tests with lubricated conditions
- Block-on-ring tests using actual piston ring materials
Key wear mechanisms observed include:
- Abrasive wear: Dominant in the early stages, with hard carbide particles providing primary protection
- Adhesive wear: Becomes significant at elevated temperatures when protective oxide films break down
- Fatigue wear: Surface spalling due to cyclic contact stresses
- Oxidative wear: At temperatures above 200°C, oxide scale formation contributes to material loss
Engineering Practice Implications
The practical significance of this research extends to several industrial applications:
- Engine block remanufacturing: Restoring worn cylinder bores through overlay rather than boring and sleeve insertion
- Prototype validation: Applying overlay to test engine blocks to evaluate new wear-resistant materials without modifying production processes
- Performance enhancement: Upgrading existing engine designs with superior overlay materials for extended service life
A critical engineering consideration is the dimensional accuracy achievable with overlay on cast iron. The high carbon content of the substrate leads to significant dilution, and achieving a uniform overlay thickness of 0.5–1.5 mm over large bore surfaces requires careful process control. The recommended approach involves:
- Preheating the block to 300–400°C to reduce thermal gradients
- Using a low-dilution process such as plasma transferred arc (PTA) or laser cladding
- Applying multiple thin layers (0.3–0.5 mm each) with intermediate grinding
- Final honing to achieve the required surface finish (Ra 0.2–0.4 μm)
Study Reflections
This research contributes valuable data to the understanding of overlay performance on cast iron substrates, which remain one of the most challenging base materials for weld overlay applications. The key insight is that microstructure-property relationships in overlay layers on cast iron are dominated by the dilution effect, and process parameters must be optimized specifically for each substrate composition. Future work should focus on developing low-dilution overlay processes and multi-layer designs that decouple the dilution zone from the functional wear-resistant layer.