Effect of Chromium and Molybdenum on Abrasive Wear Resistance of Clad Metal

Literature Overview and Research Background

Abrasive wear remains one of the dominant failure mechanisms in industrial components such as mining equipment, cement mill liners, hydraulic conveyors, and earth-moving machinery. The cladding of hardfacing alloys onto carbon or low-alloy steel substrates is a widely adopted strategy to extend service life. This literature study focuses on the systematic investigation of how chromium (Cr) and molybdenum (Mo) alloying additions influence the microstructure, hardness, and abrasive wear performance of weld overlay deposits produced by submerged arc welding (SAW). The research employs a range of Cr contents (0, 4, 8, 12, 16, 20, 25 wt%) and Mo contents (0, 1, 2, 3, 4 wt%) to map out the compositional response window, which is directly relevant to engineers selecting overlay consumables for specific abrasive environments.

Core Technical Findings

Influence of Chromium Content

Chromium is the primary alloying element governing the formation of chromium carbides (Cr7C3, Cr23C6, Cr3C2) in hardfacing alloys. The literature demonstrates that as Cr content increases from 0 to 25 wt%, the volume fraction of chromium carbides rises substantially, and the matrix transforms from a ferrite-pearlite structure to a martensitic or austenitic matrix depending on the carbon content and cooling rate.

The key insight is that Cr23C6 carbides (formed in the 8-16 wt% Cr range) provide the optimal balance between hardness and toughness for two-body abrasion, whereas Cr3C2 (dominant above 20 wt% Cr) offers superior hardness but reduced fracture toughness, making it more susceptible to chipping under impact-abrasion conditions. This distinction is critical when selecting overlay consumables for applications involving both sliding abrasion and impact loading.

Influence of Molybdenum Content

Molybdenum acts as a secondary but highly effective alloying element in hardfacing alloys. Its primary mechanisms include:

1. Solid solution strengthening — Mo atoms dissolved in the austenitic or martensitic matrix increase lattice distortion and dislocation mobility resistance.
2. Secondary carbide formation — Mo2C and MoC carbides (hardness ~1800-2000 HV) form as discrete particles, contributing to abrasive resistance through a particle-reinforcement mechanism.
3. Hardenability enhancement — Mo increases the critical cooling rate for martensite formation, allowing harder microstructures to develop even under slower cooling conditions typical of thick-section cladding.

The literature clearly demonstrates that the most cost-effective Mo addition lies in the 1-2 wt% range, where the wear resistance improvement per unit cost is maximized. Beyond 3 wt%, the marginal benefit diminishes while the consumable cost escalates significantly.

Microstructure and Wear Mechanism Analysis

The wear mechanism transitions systematically with increasing Cr and Mo content. At low alloy levels (Cr < 8 wt%, Mo < 1 wt%), the dominant wear mechanism is ploughing and micro-cutting, where abrasive particles deform and cut the relatively soft matrix. As Cr increases to 8-16 wt%, the mechanism shifts to micro-ploughing and micro-plowing around hard carbide particles, with the matrix serving as the sacrificial phase that protects the carbides from dislodgement. At high Cr levels (>20 wt%), the wear mechanism becomes carbide fracture and delamination, where the hard but brittle Cr3C2 carbides fracture under load, exposing fresh surfaces to further abrasion.

Molybdenum modifies this progression by increasing the matrix hardness and reducing the volume fraction of soft ferrite. When Mo is present at 2-3 wt% in conjunction with 12-16 wt% Cr, the resulting microstructure exhibits a martensitic matrix with a fine dispersion of Cr23C6 and Mo2C carbides. This composite microstructure achieves a hardness of 600-680 HV with a wear index 30-40% higher than a Cr-only alloy of equivalent chromium content, while maintaining acceptable fracture toughness.

Engineering Practice Integration

From a practical standpoint, the findings have direct implications for consumable selection in several industrial scenarios:

• Mining equipment (shovel teeth, draglines): High impact-abrasion conditions favor 12-16 wt% Cr with 1-2 wt% Mo, producing Cr23C6-reinforced martensitic deposits with hardness 550-620 HV.
• Cement mill liners: Predominantly two-body abrasion with moderate impact favors 8-12 wt% Cr with 1-2 wt% Mo, balancing wear resistance and cost.
• Slurry pump impellers: Three-body abrasion with corrosion demands higher Cr (16-20 wt%) for corrosion resistance, with Mo (2-3 wt%) to compensate for the toughness reduction.
• Coal handling chutes: Moderate abrasion with low impact favors 4-8 wt% Cr with 0-1 wt% Mo, optimizing cost-effectiveness.

Common Defects and Countermeasures

Study Insights and Implications

The most significant takeaway from this literature is that the Cr-Mo interaction is not merely additive but synergistic in the 8-16 wt% Cr and 1-3 wt% Mo compositional window. This synergy arises because Mo enhances the hardenability of the Cr-bearing matrix while simultaneously forming its own hard carbides, creating a dual-reinforcement microstructure. Engineers should recognize that simply increasing Cr content beyond 16 wt% without corresponding Mo addition yields diminishing returns and may introduce brittleness-related failure modes.

A practical recommendation is to adopt a "Cr-Mo matrix design" philosophy: first establish the Cr content based on the required carbide type and corrosion resistance, then optimize Mo content (typically 1-3 wt%) to maximize matrix hardness and hardenability without excessive cost escalation. This approach aligns with the PDCA (Plan-Do-Check-Act) cycle commonly applied in overlay process development, where the initial design plan is iteratively refined through trial welds, microstructural analysis, and wear testing.

The literature also highlights an often-overlooked aspect: the cooling rate sensitivity of Cr-Mo alloys. Because Mo increases hardenability, the same consumable may produce significantly different microstructures and hardness values depending on whether the cladding is applied to a thin section (fast cooling, full martensite) or a thick section (slow cooling, retained austenite or bainite). Process parameters must therefore be validated for each specific application geometry, and post-weld heat treatment (PWHT) at 550-650°C for stress relief and microstructure stabilization should be considered for critical applications.

In summary, the systematic understanding of Cr and Mo effects on clad metal abrasive wear performance provides a robust compositional design framework that bridges fundamental metallurgy with practical consumable selection. Engineers who internalize these relationships can make informed decisions that balance wear resistance, toughness, cost, and processability for any given industrial application.