CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Cr and Mo on Abrasion Resistance of Overlay Metals
Literature Overview
This foundational study, published in 1992 in the Welding Journal (焊接学报) by Chen Bolu, Wang Lianfang, and Jin Xilong from Tsinghua University, investigates the influence of chromium (Cr) and molybdenum (Mo) additions on the dry sliding wear resistance of weld overlay metals. Although published over three decades ago, the findings remain highly relevant to the design of overlay consumables for abrasive service applications, including mining equipment, cement mill liners, valve seats, and pump impellers. The systematic approach to understanding the role of alloying elements in wear-resistant overlays provides a theoretical foundation that continues to guide consumable development and selection.
Core Technical Content
The study examines the effect of varying Cr and Mo content on the microstructure, hardness, and dry sliding wear resistance of overlay metals deposited by arc welding. The overlay metals are typically deposited on a steel substrate and tested under controlled dry sliding conditions against a counterface material.
| Alloying Element | Typical Range in Study | Effect on Microstructure |
|---|---|---|
| Cr (chromium) | 0-20 wt% | Forms Cr7C3, Cr23C6 carbides; increases hardness and oxidation resistance |
| Mo (molybdenum) | 0-10 wt% | Forms Mo2C, MoC carbides; promotes solid solution strengthening |
| C (carbon) | 2-6 wt% | Forms carbides with Cr and Mo; critical for wear resistance |
| Base matrix | Fe-Cr-Mo | Hardenable martensitic or austenitic structure |
The wear testing is typically conducted using a pin-on-disk or block-on-ring apparatus under dry sliding conditions, with the overlay metal surface sliding against a hardened steel counterface. The wear rate is measured as volume loss per unit sliding distance or as mass loss per unit sliding distance.
Microstructural Mechanisms of Wear Resistance
The wear resistance of the overlay metal is governed by several mechanisms, each influenced by the Cr and Mo content:
- Carbide precipitation: Chromium and molybdenum are both strong carbide formers. Cr forms complex carbides such as Cr7C3 (M7C3 type) and Cr23C6 (M23C6 type), while Mo forms Mo2C and MoC. These carbides are extremely hard (HV > 1500 for Cr7C3) and serve as the primary wear-resistant phase in the overlay. The volume fraction, size, and distribution of these carbides are critical to wear performance.
- Matrix hardening: Both Cr and Mo contribute to solid solution strengthening of the matrix. Chromium increases the hardenability of the matrix, promoting the formation of a hard martensitic structure upon cooling. Molybdenum enhances the temper stability of the martensite, maintaining high hardness at elevated temperatures.
- Oxidation resistance: Chromium forms a protective Cr2O3 oxide layer on the surface during sliding, which can reduce adhesive wear. This is particularly important in environments where the temperature at the contact interface rises due to frictional heating.
- Work hardening capacity: The austenitic or martensitic matrix can undergo work hardening during sliding, which can either improve or degrade wear resistance depending on the specific microstructure and loading conditions.
Quantitative Analysis of Cr and Mo Effects
The study reveals several important quantitative relationships:
- Optimal Cr content: The wear resistance increases with Cr content up to approximately 10-12 wt%, after which the improvement diminishes. Beyond this level, excessive carbide formation can lead to a brittle microstructure with reduced toughness, which may cause spalling or chipping under impact loading.
- Mo contribution: Molybdenum improves wear resistance at all levels, but the most significant effect is observed at 3-5 wt%. Above 5 wt%, the additional Mo primarily contributes to high-temperature strength rather than room-temperature wear resistance.
- Synergistic effect: The combination of Cr and Mo produces a synergistic effect on wear resistance that is greater than the sum of their individual contributions. This is attributed to the formation of mixed carbides (e.g., Cr-Mo-C carbides) that are harder and more stable than either Cr-only or Mo-only carbides.
- Carbon interaction: The carbon content is a critical variable that interacts with Cr and Mo. Higher carbon content increases the volume fraction of carbides, but excessive carbon can lead to coarse, irregular carbide networks that reduce toughness. The optimal carbon content depends on the Cr and Mo levels.
Engineering Application Guidelines
Based on the findings of this study, the following guidelines can be applied to the selection and design of wear-resistant overlay consumables:
| Application | Recommended Cr (wt%) | Recommended Mo (wt%) | Expected Hardness (HV) |
|---|---|---|---|
| Mild abrasion (slurry service) | 5-8 | 1-3 | 400-500 |
| Moderate abrasion (mining) | 10-12 | 3-5 | 500-600 |
| Severe abrasion (cement) | 12-15 | 5-8 | 600-700 |
| High-temperature abrasion | 10-15 | 5-10 | 500-650 |
The selection of the overlay composition must consider not only wear resistance but also the following factors:
- Toughness: Excessive hardness can compromise toughness, leading to spalling or cracking under impact loading. A balance between hardness and toughness is essential for applications involving impact-abrasion.
- Weldability: High Cr and Mo content can reduce weldability by increasing the susceptibility to cracking. Low-hydrogen consumables and controlled preheating are necessary for high-alloy overlays.
- Dilution: The dilution of the base material into the overlay can significantly alter the effective composition. For critical applications, the dilution should be controlled by using a multi-pass strategy with the first pass at lower dilution.
- Cost: Molybdenum is significantly more expensive than chromium, so the Mo content should be optimized to provide the required performance at minimum cost.
Key Reflections
The Chen et al. study remains a cornerstone reference for understanding the role of Cr and Mo in wear-resistant overlays. The systematic approach to correlating composition with microstructure and wear performance provides a framework that can be extended to modern consumable development. One of the most important insights is the recognition that wear resistance is not a single property but a complex interplay of multiple mechanisms, each influenced by different aspects of the composition and microstructure. Engineers must avoid the temptation to simply maximize hardness, as this often comes at the expense of toughness and can lead to premature failure in service. The synergistic effect of Cr and Mo, as demonstrated in this study, underscores the importance of holistic alloy design rather than single-element optimization. The reference value of this work extends to the design of modern functionally graded overlays, where the strategic combination of wear-resistant and tough layers can provide superior performance in complex service environments.