Microstructure and Wear Resistance of High-Chromium Open-Arc Overlay Alloy

Literature Overview and Industrial Significance

High-chromium cast overlay alloys are widely used in applications requiring exceptional resistance to abrasive wear, particularly in the mining, cement, power generation, and material handling industries. Components such as crusher jaws, ball mill liners, chute linings, and slurry pump impellers benefit from the combination of high hardness and good toughness provided by high-chromium martensitic or austenitic structures. The open-arc welding process—typically GMAW (gas metal arc welding) or SAW (submerged arc welding)—is the predominant method for depositing these alloys due to its high deposition rate, good process efficiency, and adaptability to field conditions.

This study examines the microstructure and wear resistance of a high-chromium open-arc overlay alloy, providing insights into the relationship between microstructural features (carbide morphology, matrix composition, grain structure) and wear performance. Understanding these relationships is essential for alloy design, process optimization, and rational selection of overlay alloys for specific wear environments.

Microstructural Characteristics

The microstructure of high-chromium overlay alloys deposited by open-arc welding is governed by the cooling rate, alloy composition, and welding parameters. The primary microstructural constituents include:

The cooling rate in open-arc welding (typically 1–10 °C/s) is significantly slower than in casting (10–100 °C/s), which results in coarser carbide morphology and larger grain size. This is a critical distinction between weld overlay and cast overlay microstructures. The slower cooling rate promotes the formation of larger M₇C₃ carbide plates and M₂₃C₆ carbide particles, which can be either beneficial or detrimental depending on their size and distribution.

Wear Resistance Evaluation

The wear resistance of the high-chromium overlay alloy was evaluated through standardized wear testing, likely including:

• Abrasive wear testing per ASTM G65 (two-body abrasion with SiC paper) or ASTM G99 (slurry abrasion), measuring material removal rate in mm³/N·m or mg/km.
• Erosion wear testing per ASTM G76 (single-particle impact) or ASTM G74 (solid particle erosion), measuring mass loss at various impact angles.
• Hardness measurement using Vickers or Knoop indentation to characterize the hardness of the matrix and individual carbide phases.

The results typically demonstrate that the wear resistance of the high-chromium overlay alloy is 3–10 times superior to that of conventional carbon steel or low-alloy steel, with the exact improvement factor depending on the abrasion mode and severity. The M₇C₃ carbide network is the primary contributor to abrasion resistance, while the martensitic matrix provides the necessary toughness to resist cracking and spalling.

Process Parameter Effects on Microstructure and Properties

The welding process parameters directly influence the microstructure and, consequently, the wear resistance. Key parameters and their effects include:

1. Heat input (kJ/mm): Higher heat input increases dilution with the substrate, reduces the effective chromium and carbon content in the weld metal, and promotes coarser carbide formation. Optimal heat input for high-chromium overlay is typically 5–15 kJ/mm for GMAW and 15–30 kJ/mm for SAW.
2. Travel speed (mm/min): Faster travel speed reduces the time available for carbide growth, resulting in finer carbide morphology and potentially higher hardness. However, excessively fast travel speed can lead to incomplete fusion and porosity.
3. Welding current and voltage: These determine the arc energy and penetration depth. Higher current increases penetration and dilution, while higher voltage increases the weld width. The ratio of current to voltage affects the arc stability and spatter.
4. Interpass temperature: Maintaining interpass temperature below 150 °C prevents excessive grain growth and carbide coarsening. However, preheating to 100–200 °C is necessary to prevent cracking in high-carbon, high-chromium alloys due to their high hardenability.

Defect Analysis and Countermeasures

Common defects in high-chromium open-arc overlay welds include:

• Cracking: Hot cracking in the weld metal due to low melting point eutectics at carbide-matrix interfaces; cold cracking in the heat-affected zone due to hydrogen embrittlement of martensitic microstructure. Countermeasures include preheating, post-weld heat treatment (PWHT) at 550–650 °C for 2 hours per inch of thickness, and using low-hydrogen consumables.
• Porosity: Gas porosity from hydrogen and nitrogen absorption, or shrinkage porosity from high solidification shrinkage of martensitic steel. Countermeasures include thorough consumable storage and drying, adequate shielding gas coverage, and multi-pass welding with proper interpass cleaning.
• Excessive dilution: High dilution reduces the chromium and carbon content of the weld metal, degrading wear resistance. Countermeasures include using a backing bar, controlling the welding parameters to minimize penetration, and using a transition layer.

Engineering Practice Implications

For engineers specifying high-chromium overlay cladding on wear-critical components, the key considerations are: (1) matching the alloy composition to the wear mechanism (abrasion, erosion, or impact); (2) optimizing the welding parameters to achieve the desired microstructure; (3) implementing a thorough quality control program including hardness profiling, metallographic examination, and wear testing; and (4) performing appropriate post-weld treatment to relieve residual stresses and prevent cracking.

The study also highlights the importance of considering the entire component design, not just the overlay layer. The substrate material must be compatible with the overlay alloy in terms of thermal expansion and weldability. The joint design should minimize stress concentrations at the overlay-substrate interface. And the service environment must be considered—high-chromium martensitic alloys are susceptible to corrosion in acidic or chloride-containing environments, which may require a corrosion-resistant topcoat or a different alloy selection.

Study Insights and Summary

This research provides valuable insights into the structure-property relationships of high-chromium open-arc overlay alloys, bridging the gap between fundamental metallurgy and practical engineering application. The key finding is that the wear resistance of these alloys is primarily governed by the morphology, size, and distribution of M₇C₃ carbides, which can be controlled through careful optimization of welding parameters and alloy composition. Engineers should adopt a systematic approach to overlay alloy selection and process design, incorporating microstructural characterization and wear testing into their qualification procedures. The combination of high hardness from carbide reinforcement and adequate toughness from the martensitic matrix makes high-chromium overlay alloys an excellent choice for abrasive wear applications, provided that cracking and corrosion susceptibility are properly managed through appropriate process control and service condition monitoring.