Effects of Chromium and Molybdenum on Abrasive Wear Resistance of Cladding Metals

Literature Overview

This paper examines the influence of chromium (Cr) and molybdenum (Mo) alloying elements on the abrasive wear resistance of weld overlay metals produced by submerged arc welding (SAW) and gas metal arc welding (GMAW) processes. The study is particularly relevant to industries such as mining, cement, and power generation where equipment components are subjected to severe sliding and impact-abrasive wear. The research systematically varies the Cr content from 8 wt% to 22 wt% and the Mo content from 0 wt% to 4 wt%, and evaluates the resulting microstructure, hardness, and wear volume loss under standardized ASTM G65 pin-on-disk and ASTM G99 sand rubber wheel test conditions.

Core Technical Findings

The wear resistance of the cladding metals is governed primarily by the volume fraction and morphology of hard phases such as Cr7C3, Mo2C, and Cr23C6 carbides. As the chromium content increases from 8 wt% to 22 wt%, the hardness rises from approximately 380 HV to 620 HV due to the formation of a continuous network of Cr-rich carbides. However, beyond 20 wt% Cr, the ductility of the matrix decreases significantly, leading to microcracking under impact-abrasive conditions and a marginal decrease in overall wear life. Molybdenum addition up to 2 wt% refines the carbide distribution and increases the hardness from 620 HV to 700 HV without causing excessive brittleness. Beyond 2 wt% Mo, the formation of Mo2C particles at grain boundaries can promote intergranular cracking during wear testing.

The optimal composition identified is approximately 20 wt% Cr with 2 wt% Mo, which provides the highest wear resistance index of 4.91 relative to the baseline. This composition forms a tempered martensite matrix with a fine dispersion of mixed Cr-Mo carbides, which offers an excellent combination of hardness and toughness.

Microstructure Analysis

The microstructure evolution with increasing Cr and Mo content was characterized by optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). At 8 wt% Cr, the microstructure consists of ferrite and pearlite with isolated carbide particles. As Cr increases to 16 wt%, the matrix transforms to martensite with Cr7C3 carbides at the grain boundaries. At 20 wt% Cr, the carbide network becomes continuous, and the martensite laths are refined due to the increased hardenability. The addition of Mo promotes the formation of Mo2C particles within the martensite laths, which act as additional strengthening phases. However, excessive Mo (above 3 wt%) leads to the formation of coarse Mo2C at the grain boundaries, which can serve as crack initiation sites during impact-abrasive wear.

The grain size of the cladding layer was measured according to ASTM E112 and found to decrease from approximately 35 μm at 8 wt% Cr to 18 μm at 20 wt% Cr, which contributes to the Hall-Petch strengthening effect. The Mo addition further refines the grain structure to approximately 15 μm at 20 wt% Cr with 2 wt% Mo, enhancing both hardness and fatigue resistance.

Process Considerations

The cladding metals were produced using both SAW and GMAW processes. The SAW process was conducted with a flux-cored wire and a single-pass overlay with a deposition rate of 4.5 kg/h, while the GMAW process used a solid wire with a deposition rate of 2.8 kg/h. The SAW process produced a coarser grain structure due to the higher thermal input, resulting in a slightly lower hardness (by approximately 20 HV) compared to the GMAW process. However, the SAW process offers a higher deposition efficiency and is more suitable for field repair applications.

The selection between SAW and GMAW should be based on the balance between deposition efficiency and microstructure refinement. For large-area cladding of mining equipment, SAW is preferred for productivity, while GMAW is suitable for precision repair of critical components where maximum hardness is required.

Engineering Practice Implications

For engineers selecting cladding materials for abrasive wear applications, the key insight is that the Cr-Mo combination provides a synergistic effect that cannot be achieved by Cr alone. The recommended composition of 20 wt% Cr with 2 wt% Mo should be specified in procurement documents and welding procedure specifications. The FMEA analysis of potential failure modes indicates that the primary risk is microcracking under impact-abrasive conditions if the Mo content exceeds 3 wt%, which can lead to spalling of the overlay. Countermeasures include limiting the Mo content to the 1.5–2.5 wt% range and ensuring that the post-weld heat treatment (tempering at 550–600 °C) relieves residual stresses without causing carbide coarsening.

Summary

This study provides a clear quantitative relationship between Cr and Mo content and the abrasive wear resistance of weld overlay metals. The optimal composition of 20 wt% Cr with 2 wt% Mo achieves a hardness of 700 HV and a wear rate index nearly five times that of the baseline 8 wt% Cr composition. Engineers should incorporate these findings into material selection and welding procedure design, paying particular attention to the balance between hardness and toughness to avoid impact-abrasive failure. The process choice between SAW and GMAW should be guided by the specific application requirements, with SAW preferred for productivity and GMAW for maximum hardness. Adhering to these recommendations will extend the service life of cladded components in severe abrasive environments and reduce unplanned maintenance costs.