Effect of High-Chromium Alloy Open-Arc Surfacing Process Parameters on Microstructure and Hardness

Literature Overview

The paper by Zhang Xiaohua, Tan Xiaobo, Zuo Zhijian, and Hong Min, published in Hot Working Technology (Vol. 50, No. 7, 2021), investigates the influence of open-arc surfacing process parameters on the microstructure and hardness of high-chromium alloy coatings applied to grates of single-roller crushers. Single-roller crushers are widely used in mining, cement, and aggregate processing industries, where the grates (or breaker plates) are subjected to severe impact loading, abrasion, and thermal cycling. The surfacing of high-chromium alloys on these components is a standard practice to extend their service life.

Core Technical Content

Application Background and Requirements

Single-roller crusher grates experience a combination of:

Impact loading from feed material (rock, ore, or aggregate)
Abrasive wear from material sliding across the grate surface
Thermal cycling from frictional heating and cooling
Corrosive attack from moisture and chemical agents in the feed material
Fatigue loading from cyclic impact and vibration

The high-chromium alloy surfacing layer must therefore possess high hardness, excellent abrasion resistance, good impact toughness, and thermal stability. The microstructure of the deposited layer, particularly the morphology and distribution of chromium carbides, is the primary determinant of these properties.

Process Parameters Investigated

The authors systematically varied three key process parameters:

Welding speed: affects heat input per unit length, bead geometry, and cooling rate
Bead overlap ratio (压道比率): affects bead profile, surface appearance, and layer uniformity
Welding curvature radius: affects bead geometry on curved surfaces and crack tendency

Key Findings

Parameter	Effect on Microstructure	Effect on Hardness	Effect on Surface Quality
Welding Speed	Higher speed → finer, more uniform microstructure (lower heat input)	Higher speed → higher hardness	Higher speed → narrower beads
Overlap Ratio	Higher ratio → more uniform layer	Moderate effect	Higher ratio → more aesthetic bead profile
Curvature Radius	Larger radius → fewer cracks, more uniform cooling	Moderate effect	Larger radius → flatter surface, fewer cracks

The paper reports that:

As welding speed increased, the microhardness of the surfacing layer increased. This is attributed to the lower heat input at higher speeds, which results in faster cooling and finer microstructure with more uniformly distributed carbides.
As the overlap ratio increased, the surfacing layer profile became more aesthetically pleasing, with smoother transitions between adjacent beads.
As the curvature radius increased, the surfacing layer surface became flatter and cracks were significantly reduced. This is because a larger curvature radius reduces the tensile stresses developed during solidification due to differential contraction between the bead center and edges.
At lower heat input, the microstructure was fine and uniformly distributed. At higher heat input, large plate-like carbides appeared. With further increase in heat input, the quantity of plate-like carbides decreased, likely due to over-melting and dissolution of carbides.

Optimal Process Parameters

The authors identified the following optimal parameters:

Welding speed: 475 mm/min
Curvature radius: 20 mm
Overlap ratio: 50%

Under these conditions, the surfacing layer exhibited the best macroscopic appearance, suggesting an optimal balance between bead geometry, surface quality, and crack resistance.

Metallurgical Analysis

The microstructure of high-chromium surfacing alloys is dominated by chromium carbides, primarily M7C3 and M23C6 types, embedded in an austenitic or martensitic matrix. The morphology and distribution of these carbides are strongly influenced by the solidification conditions:

Low heat input (high welding speed): rapid solidification produces fine, uniformly distributed carbides. The high cooling rate also promotes a martensitic matrix, which contributes to high hardness but may reduce toughness.
Moderate heat input: carbides begin to coarsen and may develop plate-like morphologies. The matrix may transition from martensitic to austenitic due to the lower cooling rate.
High heat input (low welding speed): excessive heat causes carbide coarsening and dissolution. The matrix becomes more austenitic, and the overall hardness may decrease due to the reduced carbide volume fraction and coarser carbide morphology.

The non-monotonic relationship between heat input and carbide morphology is a critical insight for process optimization. There exists an optimal heat input window that maximizes hardness while maintaining adequate toughness and crack resistance.

Key Questions and Reflections

The paper focuses primarily on microhardness as the performance indicator, but hardness alone is not sufficient to characterize the wear resistance of high-chromium surfacing layers. In service, the grates are subjected to impact loading, which requires good toughness in addition to hardness. A very hard but brittle surfacing layer may spall under impact, leading to premature failure. Engineers should consider incorporating impact toughness testing and abrasion testing in their process qualification programs.

The curvature radius parameter is particularly relevant for surfacing on curved components. The reduction in cracks with increasing curvature radius is attributed to the reduced tensile stresses during solidification. However, in practice, the curvature radius of the component is fixed and cannot be adjusted. Instead, engineers must select process parameters that accommodate the given curvature, or employ techniques such as multi-pass surfacing with strategic bead placement to manage residual stresses.

The overlap ratio of 50% identified as optimal is consistent with general surfacing practice, where 30-50% overlap is typically recommended to ensure complete coverage without excessive overlap that could cause undercut or excessive heat input at the overlap zone.

Study Insights and Reference Value

This paper provides valuable quantitative data on the influence of open-arc surfacing process parameters on high-chromium alloy coatings. The systematic investigation of welding speed, overlap ratio, and curvature radius offers practical guidance for engineers optimizing surfacing processes for crusher grates and similar components.

The finding that there is an optimal heat input window for carbide morphology is particularly important. It demonstrates that simply maximizing hardness by increasing welding speed is not always the best strategy, as it may compromise toughness and crack resistance. Engineers must seek a balance between hardness, toughness, and crack resistance when optimizing surfacing processes.

The paper has significant reference value for engineers in the mining, cement, and aggregate processing industries who rely on high-chromium surfacing to extend the service life of crusher components. The identified optimal parameters can serve as a starting point for process development, with further optimization based on specific component geometry, substrate material, and service conditions.

The work also highlights the importance of considering the component geometry (curvature radius) in surfacing process design. This is often overlooked in practice but can have a significant impact on surfacing quality, particularly with respect to crack formation. Engineers should incorporate geometric factors into their process qualification programs to ensure reliable surfacing on curved components.