CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Two Medium-Chromium Overlay Alloys
Literature Overview
This study, published in Heat Processing Technology in 2019 by Lei Sheng, Li Shuai, Ma Shihao, Sun Jun, and Zhu Jixiang from Anhui Jianzhu University, compares the microstructure and properties of two medium-chromium overlay alloys deposited by welding. Medium-chromium overlay alloys, typically containing 5–12 wt% Cr, represent an important class of wear-resistant cladding materials that offer a balance between hardness, toughness, and corrosion resistance. The study addresses the practical challenge of selecting between different medium-chromium compositions for specific service conditions, providing metallurgical justification for alloy selection.
Core Technical Points
Compositional Design and Alloy Classification
Medium-chromium overlay alloys are classified within the broader family of wear-resistant overlay alloys based on their chromium content and resulting microstructural characteristics. The two alloys studied differ in their chromium content, carbon content, and additional alloying elements, which lead to different microstructural evolution and property profiles.
| Property | Alloy A (Lower Cr) | Alloy B (Higher Cr) |
|---|---|---|
| Cr content (wt%) | 6–8 | 10–12 |
| C content (wt%) | 2.0–2.5 | 1.5–2.0 |
| Mn content (wt%) | 1.0–1.5 | 0.5–1.0 |
| Mo content (wt%) | 0.5–1.0 | 1.5–2.5 |
| Ni content (wt%) | 0–0.5 | 1.0–2.0 |
| Base metal | Carbon steel or low-alloy steel | Carbon steel or low-alloy steel |
| Intended service | Abrasive wear, moderate corrosion | Abrasive wear, higher corrosion resistance |
The lower chromium alloy (Alloy A) is designed for applications where abrasive wear resistance is the primary requirement and corrosion resistance is secondary. The higher chromium alloy (Alloy B) incorporates additional molybdenum and nickel to enhance both wear resistance and corrosion resistance, making it suitable for more demanding environments.
Microstructural Characteristics
The microstructure of medium-chromium overlay alloys is dominated by carbide phases formed during solidification. The type, morphology, and distribution of carbides are primarily determined by the chromium, carbon, and molybdenum contents, as well as the cooling rate during deposition.
For Alloy A (lower Cr, higher C), the microstructure typically consists of:
- A dendritic martensitic matrix with retained austenite
- Primary M7C3 carbides formed at the dendrite tips and along the dendrite boundaries
- Secondary M23C6 carbides precipitating within the matrix during cooling
- The carbide content is high (typically 20–35 vol%), providing excellent abrasive wear resistance
For Alloy B (higher Cr, lower C, with Mo and Ni), the microstructure is more complex:
- A tempered martensitic matrix with lower retained austenite
- Primary M6C and MC carbides (Mo-rich) formed at dendrite tips
- Secondary M23C6 carbides with higher chromium content
- The carbide content is moderate (typically 15–25 vol%), but the carbides are harder and more thermally stable due to Mo and Ni
The cooling rate during welding deposition, typically 5–30 °C/s for single-pass deposits, has a significant effect on the microstructure. Higher cooling rates promote finer carbide distributions and increase the volume fraction of retained austenite, while lower cooling rates allow more complete carbide precipitation and matrix transformation.
Mechanical Properties and Wear Performance
The mechanical properties of the two medium-chromium overlay alloys reflect their different microstructural characteristics:
| Property | Alloy A | Alloy B |
|---|---|---|
| Hardness (HV) | 800–1000 | 900–1100 |
| Compressive strength (MPa) | 2000–2500 | 2500–3000 |
| Impact toughness (J) | 5–10 | 8–15 |
| Dilution resistance | Moderate | Good |
| Corrosion resistance | Moderate | Good |
| Wear resistance (pin-on-disk) | Good | Excellent |
The higher hardness of Alloy B is attributed to the harder Mo-rich carbides and the higher chromium content in the M23C6 carbides. The improved toughness of Alloy B is due to the tempered martensitic matrix and the presence of nickel, which promotes solid solution strengthening without excessive embrittlement.
The wear resistance of both alloys is evaluated through pin-on-disk tests against alumina (Al2O3) and silicon carbide (SiC) counterparts. Alloy B exhibits superior wear resistance due to the combination of higher hardness, more thermally stable carbides, and better matrix support. The wear mechanism transitions from abrasive micro-ploughing at low loads to micro-cutting at higher loads, with the harder carbides in Alloy B providing better resistance to micro-cutting.
Process and Standards Analysis
Medium-chromium overlay alloys are typically deposited by GMAW, FCAW, or submerged arc welding (SAW). The process parameters for welding deposition include:
| Parameter | GMAW | FCAW | SAW |
|---|---|---|---|
| Arc voltage (V) | 28–35 | 25–32 | 25–35 |
| Current (A) | 200–350 | 300–500 | 400–700 |
| Travel speed (mm/min) | 200–500 | 150–350 | 100–300 |
| Shielding gas | Ar or Ar+CO2 | Flux | Flux |
| Preheat (°C) | 150–250 | 150–250 | 150–250 |
| Interpass temp (°C) | < 250 | < 250 | < 250 |
| Dilution ratio | 20–40% | 30–50% | 40–60% |
The dilution ratio is a critical parameter for medium-chromium overlay alloys, as excessive dilution reduces the chromium and carbon content of the overlay layer, leading to lower hardness and reduced wear resistance. The dilution ratio is controlled by the heat input, travel speed, and the number of passes. Multi-pass deposition with controlled interpass temperature is essential for achieving the target overlay composition and thickness.
From a standards perspective, medium-chromium overlay alloys fall under several classification systems:
- ISO 2553: Classification of overlay welding consumables, where medium-chromium alloys are classified as type 2 (high hardness) or type 3 (high hardness with corrosion resistance)
- ASTM A263: Overlay welding electrodes for carbon and alloy steels, which includes medium-chromium compositions
- NB/T 47014: Qualification of welding procedures for pressure equipment, requiring demonstration of mechanical properties and bond strength
- ASME Section IX: Welding qualification procedures, applicable when the overlay is part of a pressure vessel
The weld procedure qualification requires testing of the deposited weld metal for hardness, impact toughness, and chemical composition, as well as bond strength testing between the overlay and the base metal. The bond strength is typically evaluated by macrograph examination of a cross-section through the weld and base metal, with acceptance criteria based on the absence of cracks, lack of fusion, or excessive dilution at the interface.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracking in overlay | Excessive carbon; high hardness; low toughness | Reduce carbon content; add Ni; control cooling rate |
| Excessive dilution | High heat input; low travel speed | Reduce heat input; increase travel speed |
| Hardness inhomogeneity | Uneven cooling rate; segregation | Multi-pass deposition; controlled parameters |
| Porosity | Gas absorption; poor shielding | Improve shielding; use low-hydrogen consumables |
| Poor bond strength | Contamination; insufficient fusion | Clean substrate; ensure proper fusion |
Engineering Practice Integration
Medium-chromium overlay alloys are widely used in applications where a balance of wear resistance and toughness is required:
- Coal handling equipment: chutes, slides, and hoppers where abrasive wear from coal and ash is severe
- Mining equipment: bucket teeth, crusher liners, and conveyor rollers where impact and abrasion combine
- Cement industry: mill liners, grinding elements, and chutes where wear from cement clinker and aggregates is dominant
- Power generation: boiler tubes and heat exchanger surfaces exposed to fly ash erosion
- Petrochemical industry: pump impellers and valve components exposed to abrasive slurry
The selection between Alloy A and Alloy B depends on the specific service condition. Alloy A is preferred for dry abrasive wear applications where corrosion resistance is not a concern and cost is a factor. Alloy B is preferred for wet or corrosive abrasive wear applications where the combination of erosion and corrosion requires enhanced corrosion resistance.
Key Questions and Reflections
The comparison of two medium-chromium overlay alloys highlights the importance of alloy design in tailoring properties to specific service conditions. The addition of molybdenum and nickel in Alloy B provides clear benefits in terms of hardness, toughness, and corrosion resistance, but at the cost of increased material expense. The economic justification for Alloy B depends on the service life improvement and the cost of downtime due to premature failure.
A key question for future investigation is the long-term performance of these alloys under cyclic loading and thermal cycling conditions. The retained austenite in Alloy A may undergo strain-induced transformation during service, leading to changes in hardness and residual stress. The stability of the microstructure under thermal cycling is particularly important for applications involving temperature variations, such as boiler tubes or heat exchanger surfaces.
Study Insights and Implications
The fundamental insight from this work is that medium-chromium overlay alloys offer a versatile solution for wear-resistant cladding applications, with the ability to tailor properties through compositional modification. The addition of molybdenum and nickel to a medium-chromium base provides significant improvements in hardness, toughness, and corrosion resistance, making Alloy B suitable for more demanding service conditions.
For engineers involved in cladding design, the key takeaway is that alloy selection should be based on a thorough understanding of the service environment, including the type of wear, the presence of corrosive media, the temperature range, and the loading conditions. The microstructural analysis provides the metallurgical basis for understanding how compositional changes affect properties, enabling rational alloy selection rather than empirical trial and error.
The practical implication is that medium-chromium overlay alloys can be optimized for specific applications through careful compositional design and process parameter control. The dilution ratio must be carefully managed to ensure that the deposited overlay achieves the target composition and properties, and the bond strength must be verified to ensure reliable performance in service.