Research on Surfacing Repair of Sintering Machine Toothed Rollers and Grates

Literature Overview

This study by Yu Guangming, Zhong Yi, and Duan Liren, published in 2006 in "China Surface Engineering" (Volume 19, Issue 3, pages 51-54), presents a comprehensive investigation into the surfacing repair of toothed rollers and grates (also known as grate bars or grate plates) used in large-scale sintering machines within the iron and steel industry. The authors conducted failure analysis, developed a proprietary high-chromium cast iron wear-resistant surfacing alloy, optimized welding processes, and established a mature manufacturing technology system.

Sintering machines are critical equipment in iron ore processing, where raw ore is agglomerated into sintered material for blast furnace feeding. The toothed rollers and grates operate under extremely severe conditions involving abrasive contact with iron ore, thermal cycling, and mechanical impact. Their service life directly impacts production continuity and operational costs.

Core Technical Content

Failure Analysis of Original Components

The failure modes identified for toothed rollers and grates include:

Abrasive wear on tooth tips and working surfaces: The primary failure mechanism, caused by continuous sliding contact with iron ore particles (typically 0.5-5 mm in size) containing hard minerals such as quartz (Mohs hardness 7) and magnetite.
Fatigue cracking at the root of teeth: Cyclic loading during the sintering process generates bending stresses at the tooth root, leading to fatigue crack initiation and propagation.
Thermal cracking: Temperature fluctuations between ambient conditions and the hot sintering bed (1100-1300 °C) cause thermal fatigue.
Adhesive wear: Under high contact pressure, material transfer occurs between the roller/grate surface and the ore bed.

Development of High-Chromium Cast Iron Surfacing Alloy

The authors developed a proprietary high-chromium cast iron alloy specifically designed for surfacing repair applications. The key compositional design principles include:

Element	Composition (wt%)	Function
C	2.5-3.5	Carbon source for carbide formation
Cr	25-30	Primary alloying element for Cr7C3 carbides
Mo	2-4	Enhances secondary hardening, improves red hardness
Mn	1.0-2.0	Austenite stabilizer, improves toughness
Si	1.0-2.0	Deoxidizer, promotes graphitization control
Ni	1.0-3.0	Improves toughness, reduces brittleness
B	0.05-0.15	Promotes fine carbide distribution

The microstructure of the optimized surfacing alloy consists of:

A matrix of martensite and retained austenite (providing toughness and wear resistance).
Primary Cr7C3 carbides (15-30 μm in size) providing primary wear resistance.
Secondary M23C6 and M7C3 carbides in the matrix (5-15 μm) providing secondary wear resistance.
A controlled amount of retained austenite (10-20%) for transformation toughening.

Welding Process Optimization

The surfacing process was optimized using multi-pass welding with the following parameters:

Process Parameter	Optimized Value	Rationale
Welding method	Submerged arc welding (SAW) or flux-cored arc welding (FCAW)	High deposition rate, good penetration
Current type	DCEN (direct current electrode negative)	Deep penetration, reduced dilution
Current	400-600 A	Sufficient for multi-pass buildup
Voltage	30-38 V	Stable arc, good bead profile
Travel speed	300-500 mm/min	Controlled heat input
Heat input	12-20 kJ/mm	Balances dilution and solidification rate
Preheat	150-250 °C	Reduces cracking susceptibility
Post-weld treatment	540-580 °C × 2-4 h	Temper martensite, reduce residual stress

Engineering Practice Integration

Manufacturing Technology System

The authors established a complete manufacturing technology system that can be summarized as follows:

Surface preparation: Complete removal of base metal material from the worn surface using CNC machining or grinding. The preparation depth should be 2-3 mm below the original surface to ensure adequate bonding. Surface roughness after preparation should be Ra 6.3-12.5 μm to promote mechanical anchoring.
Welding sequence: For large rollers and grates, a systematic welding sequence must be planned to minimize distortion. The recommended approach is symmetric welding from the center outward, with alternating passes on opposite sides.
Interpass temperature control: Maintained below 250 °C to prevent over-tempering of the previous pass. For high-chromium alloys, exceeding 300 °C interpass temperature can cause excessive grain growth and reduced hardness.
Post-weld heat treatment: A tempering treatment at 540-580 °C for 2-4 hours is critical to:

Temper the hard, brittle martensite to a tempered martensite structure.
Reduce welding residual stresses by 50-70%.
Improve toughness without significantly reducing hardness.
Stabilize retained austenite.

Final machining: The surfaced surface is machined to the required geometry and surface finish (typically Ra 3.2-6.3 μm for rollers, Ra 6.3-12.5 μm for grates).

Performance Evaluation

The following performance criteria were established for acceptance:

Test Parameter	Requirement	Test Method
Surface hardness	≥ HV550	Vickers microhardness, HV5
Core hardness (base metal)	No significant reduction	Vickers hardness, HV5
Wear rate (pin-on-disk)	≤ 1.5 × 10⁻⁶ mm³/N·m	ASTM G99 equivalent
Bend test	No cracking at 3 mm radius	Internal bend test
Impact toughness	≥ 27 J at 20 °C	Charpy V-notch
Service life improvement	≥ 3× original life	Field verification

Key Questions and Reflections

The study raises an important question about the balance between wear resistance and toughness in surfacing alloys for rotating components. High-chromium cast irons are inherently brittle, and their application on toothed rollers—which experience cyclic bending loads—requires careful consideration of fatigue resistance. The authors' approach of including Ni and Mn to improve toughness while maintaining high Cr for wear resistance represents a pragmatic engineering compromise.

Another consideration is the dilution effect. When surfacing onto a low-alloy steel base (typically 45 steel or 40Cr for rollers), the dilution of base metal into the weld deposit reduces the effective Cr and C content of the surfacing alloy. Multi-pass surfacing is essential to minimize dilution in subsequent passes. The first pass typically has 30-50% dilution, while subsequent passes may have only 5-15% dilution. This means the first pass may not achieve the full hardness of the designed alloy composition, and adequate buildup thickness is critical.

The post-weld tempering treatment deserves particular emphasis. Without proper tempering, the surfacing deposit would be extremely hard but brittle, susceptible to spalling under impact loading. The tempering process transforms the as-welded martensite (which can reach HV900-1100) to tempered martensite (HV550-650) with significantly improved toughness. This is a classic example of the hardness-toughness trade-off in engineering materials.

Study Insights and Implications

The most significant contribution of this paper is the demonstration that a systematic approach—combining failure analysis, alloy design, process optimization, and manufacturing standardization—can substantially extend the service life of critical mining and metallurgical equipment. The development of a proprietary high-chromium cast iron surfacing alloy tailored specifically for sintering machine applications demonstrates the value of application-specific material development rather than relying on generic commercial surfacing alloys. For engineers in the surface engineering field, this paper illustrates the PDCA methodology applied to a real industrial problem: the initial failure analysis (Plan), the alloy and process development (Do), the performance testing (Check), and the manufacturing standardization (Act). The practical results—achieved service life improvements of three times or more—validate the technical approach and demonstrate significant economic benefits for industrial users.