Hardfacing of Toothed Rolls in Sintered Hot Ore Crushers

Literature Overview

This paper by Wang Liang and colleagues from Handan Iron and Steel Company's Sintering Plant (published in Sintering and Pelletizing, Vol. 26, No. 6, 2001) addresses a critical industrial problem: the severe abrasive and thermal wear experienced by toothed rolls in hot ore crushers used in the sintering process. The authors report their research and improvement of hardfacing processes for both toothed rolls and heat-resistant grates, achieving a service life exceeding one year even without water cooling. This work is particularly significant because it represents a practical engineering solution developed through消化吸收引进技术 (absorption and digestion of imported technology), reflecting the transition from technology transfer to independent process optimization in Chinese heavy industry.

Core Technical Content

The fundamental challenge addressed here is the extreme operating environment of sintered hot ore crushers. The toothed rolls are subjected to:

Continuous impact and abrasion from hot sintered ore particles
High temperatures that can exceed 600–800°C in the crushing zone
Thermal cycling that induces thermal fatigue cracking
Severe adhesive and abrasive wear mechanisms

The authors selected high-chromium alloy hardfacing materials as the primary solution. High-chromium cast irons (typically 12–30% Cr) are well known for their excellent wear resistance due to the formation of hard, wear-resistant chromium carbides (primarily M7C3 and M23C6 type carbides). However, the application of such materials to toothed rolls presents several welding challenges:

Parameter	Typical Requirement	Challenge
Base material	Low-carbon steel or medium-carbon steel	High carbon content in hardfacing causes cracking
Preheat temperature	200–350°C	Must be controlled to prevent thermal cracking
Interpass temperature	Maintain ≤350°C	Thermal stress management
Cool-down rate	Slow, controlled	Prevent martensitic transformation cracking
Dilution rate	<20%	Excessive dilution reduces hardness and wear resistance
Hardness requirement	≥58 HRC	Must maintain at operating temperature
Service life target	>1 year without water cooling	Significant improvement over previous designs

Technical Analysis of the Hardfacing Process

Material Selection and Metallurgy

The selection of high-chromium alloy hardfacing materials involves careful consideration of the carbide type and matrix microstructure. In high-chromium hardfacing alloys:

M7C3 carbides (hexagonal structure) dominate when the carbon content is relatively low and the cooling rate is moderate. These carbides are harder and more wear-resistant but more brittle.
M23C6 carbides (orthorhombic structure) form at higher carbon contents or slower cooling rates. They are slightly softer but more crack-resistant.

The key metallurgical concern is the formation of brittle martensite in the transition zone between the base metal and the hardfacing layer. The authors' approach to achieving >1 year service life without water cooling suggests several process innovations:

Multi-pass hardfacing to manage dilution and thermal stress
Optimized preheating and interpass temperature control to reduce residual stresses
Selection of appropriate welding consumables (electrodes or flux-cored wires) with controlled carbon and chromium content
Possible use of intermediate layers to buffer the thermal expansion mismatch

Welding Cracking Prevention

The paper specifically mentions welding cracks (焊接裂纹) as a key concern. In high-chromium hardfacing applications, cracking can occur in several forms:

Crack Type	Location	Cause	Countermeasure
Hot cracking	Hardfacing surface/interpass	Low melting eutectics (S, P), high sulfur	Use low-sulfur consumables, proper preheat
Cold cracking	HAZ/transition zone	Hydrogen embrittlement, martensitic transformation	Preheating, low-hydrogen consumables, controlled cooling
Thermal fatigue cracks	Hardfacing surface	Cyclic thermal stress	Optimized microstructure, residual stress relief
Intergranular cracks	Base metal HAZ	Sensitization, grain boundary weakening	Limit interpass temperature, avoid sensitization range

The achievement of >1 year service life without water cooling is remarkable because it implies that the hardfacing layer must withstand both mechanical wear and thermal degradation simultaneously. This likely required:

A carefully designed microstructure with a balance between hardness and toughness
Controlled residual stress levels to prevent thermal fatigue initiation
Adequate thickness of the hardfacing layer to resist through-thickness wear
Proper geometric design of the tooth profile to distribute contact stresses

Engineering Practice Implications

Application to Pipe and Fitting Manufacturing

While this paper focuses on sintering plant equipment, the principles are directly applicable to several areas in steel pipe and fitting manufacturing:

Wear-resistant linings for material handling equipment: Pipe fabrication shops often use rollers, conveyors, and handling equipment that suffer from abrasive wear. The high-chromium hardfacing approach can extend the service life of these components.
Die and tool repair: In pipe fitting manufacturing (especially for forged fittings), dies and punches are subject to severe wear. Hardfacing with high-chromium alloys is a common repair strategy.
Cutting and forming tool maintenance: Tools used in pipe cutting, beveling, and forming operations can be restored through hardfacing, reducing replacement costs.

Process Optimization Considerations

For engineers implementing similar hardfacing solutions, the following process parameters should be carefully controlled:

Process Parameter	Recommended Range	Monitoring Method
Preheat temperature	250–350°C	Infrared pyrometer
Interpass temperature	≤350°C	Surface thermocouple
Welding current	150–250A (depending on electrode)	Ammeter
Travel speed	50–100 mm/min	Encoder or manual control
Arc length	3–5 mm	Visual + acoustic monitoring
Layer thickness	3–5 mm per pass	Ultrasonic thickness measurement
Total build-up thickness	15–25 mm	Ultrasonic thickness measurement
Post-weld cooling	Furnace cooling or insulated blanket	Thermocouple monitoring

Quality Control and Inspection

The quality of hardfacing deposits should be verified through:

Macroscopic examination: Visual inspection for porosity, cracks, undercut, and uniformity
Hardness testing: Vickers or Rockwell hardness across the cross-section to verify dilution profile
Metallographic examination: Cross-sectional analysis to assess microstructure and bonding quality
Penetrant testing (PT): Surface crack detection
Ultrasonic testing (UT): Internal defect detection and thickness measurement

Key Questions and Reflections

One of the most interesting aspects of this paper is the claim of >1 year service life without water cooling. In modern practice, water-cooled toothed rolls are still common in sintering plants because they provide additional thermal management. The fact that the authors achieved comparable or better performance without cooling suggests that the hardfacing process was significantly improved beyond the originally imported technology. This raises important questions about the specific metallurgical and process innovations that were implemented.

Another point worth reflecting upon is the evolution of hardfacing technology since 2001. Modern hardfacing processes now include:

Plasma transferred arc (PTA) hardfacing for thinner, more uniform deposits
Laser cladding for localized repair with minimal heat input
Automated multi-axis hardfacing systems for complex geometries
Advanced consumable development with rare earth additions for microstructure refinement

These modern techniques could potentially further extend the service life of toothed rolls beyond the 1-year benchmark established in this paper.

Study Insights and Implications

The Handan Iron and Steel Company's work represents an important example of indigenous technological improvement in Chinese heavy industry. The key lesson is that process optimization—rather than simply importing new equipment or materials—can yield substantial improvements in component service life and operational reliability. For engineers working in pipe and fitting manufacturing, this paper underscores the importance of understanding the fundamental metallurgy of hardfacing deposits and the critical role of process parameters in achieving desired performance. The principles of preheating, interpass temperature control, dilution management, and residual stress mitigation are universally applicable to any hardfacing application, whether on sintering plant equipment, pipe handling rollers, or cutting tools in fabrication shops.