Weld Overlay Repair of Sintering Car Body A Technical Study Note

Background and Operational Context

Sintering cars are critical components in the iron and steel industry, used to transport sinter mix through the sintering process from the mixing stage to the sintering machine and then to the cooling section. These cars are subjected to extreme thermal cycling, mechanical impact from material loading and unloading, and abrasive wear from the movement of hot sinter material. The car body, typically fabricated from carbon steel or low-alloy steel, experiences severe degradation over time due to the combined effects of thermal fatigue, impact damage, and abrasive wear. Weld overlay repair of worn or damaged areas is a widely adopted maintenance strategy that extends the service life of sintering cars while reducing the need for complete replacement, which is both costly and time-consuming.

Wear Mechanisms and Damage Assessment

The wear mechanisms affecting sintering car bodies are complex and often involve multiple simultaneous mechanisms. Abrasive wear from the sliding of hot sinter particles (typically 1000-1200°C) is the dominant mechanism on the floor and side walls of the car body. Thermal fatigue cracking occurs at the surface due to the repeated heating and cooling cycles during the sintering process. Impact damage from the loading and unloading of sinter material causes localized deformation and cracking. Oxidation and corrosion at elevated temperatures contribute to surface degradation and material loss.

Before proceeding with overlay repair, a thorough damage assessment is conducted to determine the extent of material loss, identify areas requiring repair, and evaluate the structural integrity of the car body. The assessment involves measuring the remaining thickness of the base material, identifying areas of thermal fatigue cracking, and evaluating the extent of impact damage. Areas with material loss exceeding 30% of the original thickness or with extensive thermal fatigue cracking may require structural repair or replacement rather than overlay repair.

Overlay Process Selection and Parameters

The selection of the overlay process for sintering car body repair must account for the large surface area to be covered, the need for high deposition rates, and the requirement for good metallurgical bonding with the carbon steel base material. Submerged Arc Welding (SAW) is the preferred process for large-area overlay due to its high deposition rate, low spatter, and excellent penetration. Manual Metal Arc Welding (MMAW) is used for localized repairs and for areas that are difficult to access with SAW equipment.

The overlay alloy selection depends on the specific wear mechanism and service conditions. For abrasive wear dominant areas, Cr-C based hardfacing alloys with hardness values of 55-65 HRC are preferred. For areas subject to combined impact and abrasion, Ni-Cr based alloys with hardness values of 40-50 HRC provide better toughness. A graded overlay approach with a Ni-Cr transition layer followed by a Cr-C hardfacing layer is recommended for areas subject to both impact and abrasion, providing a tough transition to the base material while maintaining high surface hardness.

Microstructural Analysis and Performance

The microstructure of the overlay layer on carbon steel sintering car bodies is influenced by the dilution rate, cooling rate, and post-weld heat treatment. The dilution rate, typically ranging from 15% to 30% for SAW overlay, is higher than for MMAW overlay due to the higher heat input and deeper penetration of SAW. High dilution can lead to the formation of softer phases such as ferrite and pearlite in the overlay layer, reducing hardness and wear resistance. The cooling rate, which is lower for SAW due to the higher heat input, promotes the formation of coarser carbides and retained austenite, reducing hardness but improving toughness.

The performance of the overlay layer is evaluated through hardness profiling, microstructural examination, and wear testing. A typical acceptable hardness profile for a Cr-C overlay on carbon steel shows a gradual transition from the base material hardness (approximately 20-30 HRC) to the overlay hardness (55-65 HRC), with no abrupt changes exceeding 15 HRC over a distance of 1 mm. The wear resistance is evaluated using standard methods such as ASTM G65 (dry sand-rubber wheel) or ASTM G98 (slurry erosion), with wear rates typically in the range of 10-30 mg/N·m for Cr-C overlays.

Defect Control and Quality Assurance

The primary defects encountered in overlay welding on sintering car bodies are cracking, porosity, and lack of fusion. Cracking is the most serious defect, as it can lead to spalling of the overlay layer under thermal and mechanical loading. Hot cracking is associated with the wide solidification range of Cr-C alloys and the segregation of low-melting-point eutectics, while cold cracking is associated with hydrogen diffusion and the high carbon equivalent of the weld metal. Porosity is caused by inadequate shielding, wet flux, or contamination of the base surface. Lack of fusion is caused by insufficient current, excessive travel speed, or inadequate joint preparation.

Quality assurance of overlay repairs involves a combination of non-destructive testing and destructive testing. Visual examination (VT) is the first step, followed by magnetic particle testing (MT) for surface and near-surface defects. Ultrasonic testing (UT) is used for subsurface defect detection, although the coarse-grained structure of the overlay can complicate signal interpretation. Hardness testing across the overlay thickness is mandatory to verify the hardness distribution and dilution gradient. Metallographic examination of cross-sections provides detailed information on the microstructure, carbide distribution, and dilution zone characteristics.

Engineering Practice and Maintenance Strategy

The overlay repair of sintering car bodies is typically performed during scheduled maintenance intervals, when the cars are removed from service for inspection and repair. The repair process involves grinding back the worn surface to sound material, removing all decarburized and work-hardened layers, and machining into a suitable groove profile. The base surface should be cleaned to a minimum Sa 2.5 surface roughness to ensure adequate wetting and adhesion. The overlay is applied in multiple passes, with the first pass using a transition alloy if the dilution rate is expected to be high, followed by subsequent passes of the hardfacing alloy.

A comprehensive maintenance strategy for sintering cars includes regular inspection of the car body for material loss, thermal fatigue cracking, and impact damage. The inspection interval should be based on the service conditions and the rate of material loss, with more frequent inspections for cars operating in more severe conditions. The overlay repair should be performed when the material loss reaches a predetermined threshold, typically 20-30% of the original thickness, to ensure that the remaining base material has sufficient structural integrity to support the overlay.

Summary and Conclusions

The weld overlay repair of sintering car bodies is a critical maintenance practice in the iron and steel industry that extends the service life of these components while reducing replacement costs and downtime. The selection of overlay process, alloy composition, and process parameters must be carefully matched to the specific wear mechanisms and service conditions. SAW is preferred for large-area overlay due to its high deposition rate, while MMAW is suitable for localized repairs. Cr-C based hardfacing alloys provide excellent abrasion resistance for pure abrasion environments, while graded overlay systems combining Ni-Cr transition layers with Cr-C hardfacing layers provide better toughness for combined impact-abrasion environments. Rigorous quality control, including hardness profiling, microstructural analysis, and field performance tracking, is essential to ensure that overlay repairs deliver the intended service life and reliability in the demanding operating conditions of sintering plants.