CLADDING TECHNOLOGY SHANXI CO., LTDPractice of High-Strength Wear-Resistant Alloy Overlay on Sintering Machine Tail Scraper
Background and Application Context
Sintering machines are critical equipment in iron and steel production, where the tail scraper (also known as the tail discharge scraper) removes sintered material from the conveyor bed at the discharge end. The scraper is subjected to severe abrasive wear from the hot sinter cake, which contains hard mineral particles at temperatures ranging from 800°C to 1200°C. The service life of the scraper is typically limited by wear of the cutting edge and the contact surface, and conventional steel scrapers may require replacement every 3–6 months, resulting in significant downtime and material costs. This study note examines the application of high-strength wear-resistant alloy overlay welding to extend the service life of sintering machine tail scrapers.
Wear Mechanism Analysis
The wear mechanism on the sintering machine tail scraper is predominantly abrasive, with a secondary contribution from thermal fatigue due to the cyclic heating and cooling experienced during operation. The sinter cake contains hard quartz and other mineral particles that act as abrasive agents, causing material removal from the scraper surface through micro-ploughing and micro-cutting mechanisms. The high operating temperature further reduces the hardness of the scraper material, accelerating the wear rate.
Metallographic examination of worn scrapers revealed deep wear grooves with a depth of 2–5 mm, accompanied by surface cracking and material spalling at the cutting edge. The base material, typically Q235 carbon steel, had a hardness of only 120–150 HB, which was insufficient to resist the abrasive action of the sinter cake. The study identified the need for a high-hardness overlay layer with adequate toughness to resist both abrasive wear and thermal fatigue cracking.
| Parameter | Original Scraper | Requirement for Overlay |
|---|---|---|
| Material | Q235 carbon steel | High-carbon alloy overlay |
| Hardness | 120–150 HB | ≥ 55 HRC |
| Service life | 3–6 months | ≥ 18 months |
| Wear depth | 2–5 mm per cycle | ≤ 0.5 mm per cycle |
| Operating temperature | 800–1200°C | Stable to 1000°C |
Overlay Material Selection and Process Design
The overlay material selected for the scraper was a high-carbon, high-chromium alloy with a composition of approximately 3.5% C, 20% Cr, 4% Mo, and 2% V. This composition provides a high volume fraction of hard carbides (Cr7C3 and Cr3C) in a tough martensitic matrix, offering excellent resistance to abrasive wear while maintaining adequate toughness to resist cracking under thermal cycling.
The overlay process was multi-pass submerged arc welding (SAW) using a specialized flux-cored wire. The base plate was preheated to 250°C to reduce residual stress and minimize the risk of cracking in the high-carbon overlay layer. The welding was performed in three passes, with the first pass serving as a transition layer between the carbon steel base and the high-carbon overlay, and the subsequent two passes providing the full hardness and wear resistance.
The transition layer was deposited using a medium-carbon alloy wire with 1.0% C and 10% Cr, which provided a gradual increase in hardness and carbon content from the base material to the final overlay layer. This transition layer strategy is critical for preventing cracking at the overlay-to-base metal interface, which is a common failure mode in high-carbon overlay applications on low-carbon steel substrates.
| Pass | Filler Composition | Hardness (HRC) | Thickness (mm) | Purpose |
|---|---|---|---|---|
| Base | Q235 | 120–150 HB | — | Substrate |
| Pass 1 | 1.0% C, 10% Cr | 40–45 | 2.0 | Transition layer |
| Pass 2 | 3.5% C, 20% Cr | 55–60 | 2.0 | Wear-resistant layer |
| Pass 3 | 3.5% C, 20% Cr | 55–60 | 2.0 | Wear-resistant layer |
Welding Process Parameters and Quality Control
The SAW overlay process was performed with a current of 450–500 A, a voltage of 32–35 V, and a travel speed of 250–300 mm/min. The flux was a low-hydrogen type specifically designed for high-carbon alloy overlay welding, which provided adequate slag coverage and gas shielding while minimizing hydrogen pickup. The interpass temperature was maintained below 250°C to prevent excessive grain growth and to maintain the martensitic microstructure of the overlay layer.
Quality control included visual inspection, magnetic particle testing, and hardness profiling at each pass. The overlay surface was inspected for cracks, porosity, and undercut, and any defects were repaired before proceeding to the next pass. The hardness profile was measured across the overlay layer and into the base material to verify the hardness gradient and confirm that the transition layer effectively bridged the hardness difference between the base and the overlay.
| Inspection Stage | Method | Acceptance Criteria | Result |
|---|---|---|---|
| After Pass 1 | Visual + MT | No cracks, no undercut | Pass |
| After Pass 1 | Hardness profile | 40–45 HRC | Pass |
| After Pass 2 | Visual + MT | No cracks, no porosity | Pass |
| After Pass 2 | Hardness profile | 55–60 HRC | Pass |
| After Pass 3 | Visual + MT | No defects | Pass |
| Final | Hardness profile | 55–60 HRC (surface) | Pass |
Performance Results and Service Life Extension
The overlay-treated scrapers were installed on the sintering machine and monitored for wear over an extended service period. The results demonstrated a significant improvement in service life compared to the original carbon steel scrapers. The overlay layer exhibited a wear rate of approximately 0.3 mm per 1000 operating hours, compared to 3.5 mm per 1000 hours for the original steel. This represents a tenfold improvement in wear resistance and extends the service life from 3–6 months to over 18 months.
The overlay layer maintained its hardness and microstructural integrity throughout the service period, with no evidence of thermal fatigue cracking or spalling. The transition layer effectively prevented cracking at the overlay-to-base metal interface, and the overall bond strength between the overlay and the base material remained adequate throughout the service life.
| Performance Metric | Original Scraper | Overlay Scraper | Improvement |
|---|---|---|---|
| Wear rate (mm/1000h) | 3.5 | 0.3 | 11.7x |
| Service life (months) | 3–6 | 18–24 | 4–8x |
| Surface hardness (HRC) | 120–150 HB | 55–60 | Significant |
| Cracking resistance | Poor | Excellent | Qualitative |
| Cost per operating hour | High | Low | 60% reduction |
Engineering Practice and Process Optimization
The study highlights several important considerations for the practical application of high-strength wear-resistant alloy overlay on sintering machine scrapers. First, the transition layer strategy is essential for preventing cracking at the overlay-to-base metal interface. The gradual increase in carbon and chromium content from the base material to the final overlay layer reduces the residual stress and thermal mismatch at the interface, which is the primary cause of interface cracking.
Second, the preheat temperature must be carefully controlled. Insufficient preheat leads to high cooling rates and excessive residual stress, which can cause cracking in the high-carbon overlay layer. Excessive preheat, on the other hand, can reduce the hardness of the overlay layer and increase the dilution ratio. The optimal preheat temperature of 250°C represents a balance between these competing requirements.
Third, the welding sequence must be designed to minimize thermal distortion. Scrapers are typically thin-walled components, and excessive thermal input can cause warping that affects the scraper's contact with the conveyor bed. The study recommends a symmetric welding sequence with alternating directions to balance the thermal input and minimize distortion.
In conclusion, the application of high-strength wear-resistant alloy overlay welding to sintering machine tail scrapers represents a highly effective and cost-saving maintenance strategy that significantly extends component service life while reducing downtime and material costs. The systematic approach documented in the study, encompassing careful material selection, process optimization, and rigorous quality control, provides a reliable framework for similar wear-resistant overlay applications in the iron and steel industry. The key to success lies in the integration of metallurgical understanding with practical process control, ensuring that the overlay layer delivers consistent performance under the demanding operating conditions of sintering machines.