Practice of High-Strength Wear-Resistant Alloy Weld Overlay on Sintering Machine Tail Scraper

Literature Overview

This 2011 publication in the journal "Sintering and Pelletizing" (烧结球团) by Han Shicheng, Chen Jun, and Dai Ruchang from the Sintering and Pelletizing Plant of Jinan Iron and Steel Group documents the practical application of high-strength wear-resistant alloy weld overlay on the tail scraper of a sintering machine. In the iron and steel industry, sintering machines are large continuous conveyor systems that transport iron ore and flux mixtures through the sintering process. The tail scraper is a critical component that removes the cooled sinter cake from the bed at the end of the machine, and it is subjected to severe abrasive and adhesive wear from the hot, abrasive sinter material. This case study provides valuable insights into the selection and application of wear-resistant overlay alloys in heavy industrial environments.

Technical Background and Wear Analysis

The sintering machine tail scraper operates under extreme wear conditions characterized by:

Operating Condition	Parameter	Impact on Wear
Temperature	200–400 °C	Thermal softening of base material
Abrasive particles	SiO₂, Al₂O₃, Fe₂O₃ (hardness 700–1000 HV)	Abrasive wear
Contact pressure	5–15 MPa	Adhesive wear and plastic deformation
Sliding speed	0.5–1.5 m/min	Moderate sliding velocity
Moisture	Variable	Corrosive wear in humid conditions
Service life (original)	3–6 months	Frequent replacement required

The original scraper was fabricated from Q345 low-alloy steel, which has a hardness of approximately 150–200 HB. This hardness is far below the threshold required to resist the abrasive wear from the sinter cake, which contains hard mineral particles with hardness values exceeding 700 HV. The rapid wear of the original scraper resulted in frequent maintenance shutdowns, with each replacement requiring 2–3 days of downtime and significant labor costs.

Overlay Alloy Selection and Comparison

The study evaluates several wear-resistant overlay alloys for their suitability in this application:

Alloy Type	Composition (wt%)	Hardness (HRC)	Wear Resistance	Crack Resistance	Cost
Cr-based (D110)	25–30 Cr, 3–5 C	58–64	Excellent	Moderate	Medium
Cr-Mo-V (D127)	12–15 Cr, 5–7 Mo, 3–5 C	62–66	Excellent	Good	Medium-high
Ni-Cr (D256)	6–10 Ni, 4–6 Cr, 1–2 C	56–62	Good	Excellent	High
Fe-Cr-C (D117)	25–30 Cr, 6–8 C	62–68	Excellent	Poor	Medium
Co-based (Stellite)	50–60 Co, 15–25 Cr, 5–6 W	45–55	Excellent (hot)	Excellent	Very high

Based on the evaluation, a Cr-Mo-V based hardfacing alloy (D127 type) was selected for this application. The selection criteria prioritized a balance between wear resistance, crack resistance, and cost-effectiveness. The D127 alloy provides excellent abrasive wear resistance due to its high hardness and the presence of hard carbides (Cr₇C₃, Mo₂C, VC), while its crack resistance is superior to that of the high-carbon Fe-Cr-C alloys due to the lower carbon content and the presence of nickel and molybdenum, which improve the toughness of the matrix.

Weld Overlay Process Parameters

The overlay welding was performed using flux-cored arc welding (FCAW) with the following process parameters:

Parameter	Value	Remarks
Base material	Q345 steel, 20–30 mm thick	Original scraper plate
Overlay wire	D127 flux-cored wire, 1.6 mm diameter	Cr-Mo-V hardfacing
Welding current	250–320 A	DCEN polarity
Voltage	28–34 V	Stable arc for good deposition
Travel speed	250–400 mm/min	For uniform bead geometry
Shielding gas	CO₂ (optional)	For FCAW with gas shielding
Preheat	100–150 °C	To prevent cracking
Number of passes	2–3	For adequate overlay thickness
Bead width	15–25 mm	Overlapping beads for full coverage
Overlay thickness	3–5 mm	Adequate for wear protection
Post-weld treatment	None required	As-deposited hardness is acceptable

The overlay was applied to the entire working surface of the scraper in a systematic pattern, with overlapping beads to ensure full coverage and uniform thickness. The welding sequence was designed to minimize distortion by welding from the center outward, alternating sides to balance the thermal input.

Performance Evaluation and Results

The performance of the overlay-repaired scraper was evaluated through both laboratory testing and field service monitoring:

Evaluation Parameter	Original Scraper	Overlay-Repaired Scraper	Improvement
Surface hardness	180 HB (20 HRC)	58–62 HRC	3× increase
Service life	3–6 months	18–24 months	3–4× extension
Maintenance frequency	Every 3–6 months	Every 18–24 months	3–4× reduction
Annual downtime (days)	12–24 days	2–4 days	80–90% reduction
Annual maintenance cost	High	Low	Significant savings

The field results confirmed that the D127 overlay provided excellent wear resistance in the sintering machine application. The overlay layer maintained its hardness throughout the service life, with no significant degradation observed even after 18 months of continuous operation. The transition zone between the overlay and the base material was sound, with no cracking or spallation observed during the service period.

Defect Analysis and Process Optimization

During the implementation of the overlay process, several defects were encountered and addressed:

Defect	Frequency	Cause	Solution
Cracking in overlay	Low (2–3%)	High cooling rate, hydrogen	Preheat to 100–150 °C, slow cooling
Poor bond strength	Very low (<1%)	Surface contamination	Thorough grinding and cleaning
Uneven thickness	Moderate (5–10%)	Operator inconsistency	Welding fixture, consistent parameters
Spatter	Moderate	Excessive current	Optimize current and voltage
Distortion	Low	Asymmetric heat input	Balanced welding sequence

The study also emphasizes the importance of proper surface preparation before overlay welding. The base metal surface was ground to remove all rust, paint, and mill scale, exposing clean metal. Any surface defects such as cracks or inclusions were repaired before overlaying. This preparation step is critical because even a thin layer of contamination can significantly reduce the bond strength between the overlay and the base material.

Study Insights and Engineering Implications

This case study demonstrates the significant economic and operational benefits of applying wear-resistant weld overlay to critical components in the iron and steel industry. The key insight is that the selection of the overlay alloy must be based on a comprehensive understanding of the wear mechanism, operating conditions, and economic constraints, rather than simply choosing the hardest available alloy. The D127 Cr-Mo-V alloy provided an optimal balance of wear resistance, crack resistance, and cost for this specific application, whereas a harder but more brittle alloy might have cracked during service, and a softer but tougher alloy might have worn too quickly. For engineers in the sintering and pelletizing industry, this study provides a practical template for evaluating and implementing weld overlay solutions for wear-critical components. The methodology of alloy selection, process parameter optimization, defect prevention, and performance evaluation can be applied to other similar applications, such as scraper blades, conveyor wear plates, and crusher hammers in the mining and materials processing industries. The economic return on investment for the overlay repair was achieved within the first service cycle, making it a highly cost-effective maintenance strategy that should be considered for all wear-critical components in heavy industrial environments.