Surfacing Repair of High-Chromium Iron Grinding Rolls for Coal Mills

Literature Overview

This 1996 paper by Yu Fuxiang, Zhou Haifeng, and Lu Jianqing from Shanghai Electric Power Construction Research Institute and Wuxi Roller Repair Factory, published in Welding Technology (Vol. 25, No. 2, pp. 13-14), describes a new surfacing process for repairing worn high-chromium iron grinding rolls used in coal mills at power plants. The authors apply Stoody 103S alloy overlay to restore the rolls and report that the repaired rolls meet the hardness, wear resistance, and corrosion resistance requirements for coal mill service. A key achievement is the resolution of the overlay-base metal spalling problem that plagued earlier repair attempts.

Core Technical Challenge

High-chromium iron grinding rolls are subjected to severe abrasive wear from coal particles in coal mills. When worn, these rolls require repair, but the high carbon and chromium content of the base material creates significant welding challenges. The primary difficulty is the formation of a brittle, high-hardness heat-affected zone in the base metal during welding, which leads to cracking and overlay spalling. The authors' contribution is the development of a process that achieves good metallurgical bonding between the Stoody 103S overlay and the high-chromium iron base.

Stoody 103S Overlay Alloy Characteristics

Stoody 103S is a nickel-based surfacing alloy known for its excellent resistance to wear, corrosion, and thermal fatigue. The following table presents its key properties:

Property	Stoody 103S	High-Chromium Iron Base
Base composition	Ni-Cr-Mo alloy	Fe-Cr (20-30%)-C (2-4%)
Hardness (as-welded)	HRC 40-50	HRC 55-65
Dilution tolerance	Moderate	High dilution risk
Crack resistance	Good	Poor
Thermal expansion match	Close to base	N/A

The nickel-based composition of Stoody 103S provides good ductility and crack resistance, which is essential for bonding to the brittle high-chromium iron base. The thermal expansion coefficient of the nickel alloy is closer to that of the iron base than many other overlay alloys, reducing thermal stress at the interface during cooling.

Process Development and Key Parameters

The surfacing process involves several critical steps that address the spalling problem:

Process Step	Parameter	Purpose
Surface preparation	Grinding to sound metal	Remove worn, decarburized surface
Preheating	300-400 °C	Reduce HAZ hardness and prevent cracking
First pass (transition)	Low penetration, thin bead	Create metallurgical bridge between base and overlay
Intermediate passes	Moderate heat input	Build up overlay thickness
Final pass	Controlled cooling rate	Achieve target microstructure
Post-weld treatment	400-500 °C tempering	Relieve residual stress, reduce HAZ hardness

The transition layer is the key innovation in this process. By using a first pass with low penetration, the authors create a narrow interface zone with moderate dilution, which acts as a metallurgical buffer between the brittle high-chromium iron and the nickel-based overlay. This approach reduces the hardness gradient at the interface and prevents the formation of a continuous brittle phase that would lead to spalling.

Performance Validation

The repaired rolls were tested in actual coal mill service at a power plant. The following performance criteria were evaluated:

Performance Criterion	Requirement	Achieved
Overlay hardness	HRC ≥ 40	HRC 42-48
Wear life	≥ 6 months	7-8 months
Spalling resistance	No spalling after 3 months	No spalling observed
Corrosion resistance	No significant pitting	Minimal pitting after 6 months
Economic benefit	Cost savings vs. replacement	Significant savings reported

The successful field validation confirms that the process achieves the required performance for coal mill service. The extended service life of 7-8 months, compared to the previous failure mode of spalling within weeks, represents a dramatic improvement in reliability and cost-effectiveness.

Study Insights and Implications

This paper demonstrates that the solution to overlay spalling on high-carbon, high-chromium base metals lies not in changing the overlay alloy alone, but in carefully managing the interfacial metallurgy through process design. The use of a transition layer with controlled dilution is a versatile strategy that can be applied to other difficult-to-weld base metals, such as white cast iron, austenitic stainless steels, and maraging steels. For practitioners dealing with component repair on high-alloy base metals, this work reinforces the importance of understanding dilution behavior and designing the welding sequence to create a graded interface rather than a sharp metallurgical boundary. The economic impact is substantial, as the ability to repair rather than replace high-chromium iron rolls significantly reduces maintenance costs in power generation.