CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Cladding Alloy Layer on K360 Wear-Resistant Steel
Literature Overview
This 2008 study published in Materials for Mechanical Engineering, authored by Deng Hanzhong, Sun Yuanzhang, Liu Shaoping, Zhang Weiqiang, Kang Shaoguang, and Cao Wenzhi from Liaoning Technical University and China Coal Zhangjiakou Coal Mining Machinery Co., Ltd., investigates the microstructure and mechanical properties of alloy cladding layers deposited on K360 wear-resistant steel. K360 is a high-chromium wear-resistant steel widely used in mining and material handling equipment, and the study addresses the need to further enhance its surface performance through weld overlay cladding with specialized alloy compositions.
Base Material Characteristics and Cladding Requirements
K360 wear-resistant steel contains approximately 12-14% chromium and 2-3% carbon, producing a microstructure dominated by chromium carbides (M7C3 and M23C6) dispersed in a martensitic matrix. This composition provides excellent resistance to abrasive wear from coal, rock, and mineral particles, with typical hardness of 360-400 HBW. However, even this high-hardness material experiences significant wear in severe service conditions, particularly in applications involving impact-abrasion synergy such as crusher jaws, conveyor wear plates, and mining bucket linings.
The cladding requirements for K360 steel are demanding: the cladding layer must provide superior hardness and wear resistance compared to the base material, maintain adequate toughness to resist impact fracture, exhibit good bond strength to the base metal, and be compatible with the existing thermal history and mechanical properties of the K360 substrate. The metallurgical challenge lies in achieving these objectives without introducing excessive brittleness or cracking susceptibility at the interface.
Cladding Material Selection and Process Parameters
The study examines multiple cladding alloy compositions, including high-chromium white iron, cobalt-based alloys, and nickel-cobalt alloy systems. The following table compares the typical compositions and resulting properties of the evaluated cladding materials:
| Cladding Material | C (%) | Cr (%) | Co (%) | Hardness (HRC) | Wear Index | Toughness |
|---|---|---|---|---|---|---|
| K360 base metal | 2.5-3.0 | 12-14 | 0 | 38-42 | 1.0 | Moderate |
| High-Cr white iron | 2.5-3.5 | 22-26 | 0 | 58-62 | 2.5-3.0 | Low |
| Co-based alloy | 0.8-1.2 | 18-22 | 30-35 | 50-55 | 1.8-2.2 | Good |
| Ni-Co alloy | 0.5-1.0 | 20-25 | 20-25 | 48-52 | 1.5-2.0 | Good |
| Cr-C-Ni alloy | 1.5-2.0 | 25-30 | 0 | 55-60 | 2.0-2.5 | Moderate |
The welding process employed is submerged arc welding (SAW) with flux-cored wire, selected for its high deposition efficiency and ability to produce thick cladding layers with controlled dilution. The process parameters include an arc current of 280-350 A, arc voltage of 28-34 V, and travel speed of 100-180 mm/min, producing a single-pass deposition of 3-5 mm. Multi-pass cladding is applied to achieve total thicknesses of 8-12 mm for heavy-duty applications.
Microstructural Analysis of the Cladding Layer
Metallographic examination reveals that the microstructure of the cladding layer varies significantly with composition and cooling conditions. In the high-chromium white iron cladding, the microstructure consists of primary carbides (M7C3 and M23C6) forming a network in a martensitic matrix. The primary carbides are blocky in shape and measure 5-20 μm in size, providing excellent abrasive wear resistance through their inherent hardness (1500-2000 HV). However, the extensive carbide network creates stress concentration points that reduce fracture toughness.
The cobalt-based cladding layer exhibits a different microstructural character: a solid solution matrix enriched with chromium and cobalt, with fine carbide particles (Mo2C and WC) dispersed throughout. The carbides are much smaller (1-5 μm) and more uniformly distributed, providing a balance between hardness and toughness. The cobalt solid solution matrix retains good ductility even at room temperature, contributing to the superior impact resistance of this cladding system.
At the interface between the K360 base metal and the cladding layer, a diffusion zone develops where elements from both materials intermix. The depth of this diffusion zone is typically 0.5-1.5 mm and exhibits a gradual transition in composition and microstructure. In the base metal side of the interface, the microstructure shows signs of tempering of the original martensite, with carbide coarsening and possible formation of upper bainite. In the cladding side, the microstructure shows increased dilution effects, with reduced carbide volume fraction and modified carbide morphology.
Mechanical Properties and Performance Evaluation
The mechanical properties of the cladding layer are evaluated through hardness testing, microhardness profiling, wear testing, and impact testing. The hardness profile across the cladding layer shows a characteristic distribution: maximum hardness at the surface, gradual decrease through the layer thickness, and a sharp transition at the interface to the base metal hardness. The surface hardness of the high-chromium white iron cladding reaches 62-65 HRC, compared to 38-42 HRC for the K360 base metal, representing a significant improvement in abrasive resistance.
Wear testing using the ASTM G99 pin-on-disk method reveals that the high-chromium white iron cladding exhibits a specific wear rate 3-4 times lower than the unclad K360 base metal. The cobalt-based cladding shows a wear rate 2-2.5 times lower than the base metal. The wear mechanism differs between cladding types: the white iron cladding resists wear primarily through carbide hardness (ploughing and micro-cutting resistance), while the cobalt-based cladding benefits from both carbide hardness and matrix strength (adhesive wear resistance).
The impact testing results show that the cobalt-based cladding retains better impact toughness than the high-chromium white iron cladding. The Charpy V-notch impact energy at the cladding surface is approximately 8-12 J for the cobalt-based material, compared to 2-5 J for the white iron cladding. The K360 base metal itself exhibits 20-30 J, indicating that both cladding types reduce impact resistance, but the cobalt-based alloy does so to a lesser degree. This makes the cobalt-based cladding more suitable for applications involving impact-abrasion synergy.
Bond Strength and Interface Quality
The bond strength between the cladding layer and the K360 base metal is a critical quality parameter. Testing using the ring shear method (per ASTM G94) shows bond strengths of 280-350 MPa for properly executed welds, well above the typical minimum requirement of 200 MPa. The interface exhibits complete metallurgical bonding with no evidence of lack of fusion or delamination when proper process parameters are maintained.
However, the study identifies conditions under which interface quality degrades. Insufficient preheating (below 100 °C) leads to excessive cooling rates that produce brittle martensite at the interface, reducing bond strength and increasing crack susceptibility. Excessive dilution (above 40%) from high travel speed or low deposition rate reduces the effective cladding composition, resulting in lower hardness and potential soft spots. Contamination of the welding zone with moisture or oxide films introduces hydrogen that can cause porosity and microcracking at the interface.
Engineering Practice Recommendations
Based on the comprehensive evaluation, the following recommendations are proposed for practical application of cladding on K360 wear-resistant steel:
- For applications dominated by abrasive wear (mining buckets, conveyor wear plates), the high-chromium white iron cladding provides the best wear resistance with acceptable toughness.
- For applications involving impact-abrasion synergy (crusher jaws, hammer mill hammers), the cobalt-based cladding offers the best balance of hardness and toughness.
- Preheat the base metal to 150-200 °C to reduce thermal stresses and prevent cracking.
- Maintain interpass temperature between 200-300 °C for multi-pass cladding.
- Apply post-weld heat treatment at 550-600 °C for 2 hours to relieve residual stresses and temper the hardened interface zone.
- Limit dilution to below 30% by controlling travel speed and ensuring adequate wire feed rate.
Study Insights and Reflections
This research provides valuable insights into the metallurgical behavior of cladding systems on high-chromium wear-resistant steels. The finding that the interface diffusion zone develops a tempered microstructure is particularly important for understanding long-term service behavior. In my experience with refurbished mining equipment, cladding failures typically initiate at the interface rather than within the cladding layer itself. The study's emphasis on interface quality and bond strength testing is well-founded, as I have observed multiple cases where inadequate bond strength led to premature cladding delamination in service.
The comparison between high-chromium white iron and cobalt-based cladding systems highlights a fundamental trade-off in surface engineering: hardness versus toughness. The selection between these systems must be guided by the specific service conditions, particularly the ratio of abrasive to impact loading. For many mining applications, the cobalt-based cladding provides a more reliable solution despite its lower hardness, because the avoidance of impact-induced cracking extends overall service life beyond what the harder but more brittle white iron cladding can achieve.