Wear Resistance of Cemented Carbide Surfacing Layers After Heat Treatment

Literature Overview

This foundational study by Liu Yong and colleagues from Luoyang Institute of Technology (now Luoyang University of Technology) investigates the post-heat-treatment wear and corrosion-wear performance of cemented carbide surfacing layers. Published in "China Surface Engineering" (Vol. 15, Issue 3, 2002, pp. 17-19), this work is significant for establishing the effectiveness of heat treatment in optimizing the performance of hardfacing deposits, particularly cemented carbide type surfacing alloys.

Comparative Surface Treatment Approaches

The study evaluates three distinct surface treatment approaches and their post-heat-treatment performance:

1. Boron-Carbon-Nitrogen co-implantation (B,C,N co-diffusion layer)
2. Tungsten carbide type cemented carbide surfacing (D707 alloy)
3. High-carbon high-chromium cast iron type cemented carbide surfacing (D618 alloy)

This comparative approach is particularly valuable because it allows direct evaluation of surfacing technology against alternative surface hardening methods under identical post-treatment conditions.

Heat Treatment Effects on Microstructure

Air Cooling at 850°C

The 850°C air-cooling treatment serves two primary purposes:

1. Tempering of as-welded martensite: The rapid cooling from the welding process produces untempered martensite that is highly stressed and prone to cracking. Air cooling at 850°C provides controlled tempering that relieves residual stresses while maintaining adequate hardness.
2. Carbide modification: For WC-type deposits, this temperature is below the WC dissolution temperature but above the transformation temperature of the matrix, allowing selective tempering without damaging the WC particles.

For D618 (high-carbon high-chromium) deposits, the 850°C treatment promotes:

• Tempering of secondary carbides
• Possible transformation of metastable phases
• Stress relief without significant hardness reduction

Oil Quenching at 850±5°C followed by 400°C Tempering

This two-step heat treatment provides more comprehensive microstructural optimization:

1. Oil quenching at 850°C: Achieves uniform austenitization followed by controlled cooling that produces tempered martensite with fine carbide dispersion. The oil quench rate provides slower cooling than water, reducing cracking risk while maintaining hardenability.
2. 400°C tempering: Further refines the microstructure by:

• Precipitating fine secondary carbides
• Reducing internal stresses from quenching
• Optimizing the hardness-toughness balance
• Stabilizing the retained carbide structure

Performance Results Analysis

The key finding is that both cemented carbide surfacing types (D707 and D618) achieve high hardness and excellent corrosion-wear resistance after the optimized heat treatment (850±5°C oil quench + 400°C temper). This represents a significant practical advantage because:

• The heat treatment simplifies the overall surface engineering process
• Post-weld heat treatment can be applied to assembled components
• The combined corrosion-wear resistance exceeds that of the as-welded condition
• The process is compatible with standard heat treatment facilities

The comparison with B,C,N co-diffusion is particularly instructive, as it demonstrates that surfacing with subsequent heat treatment can achieve equivalent or superior performance to specialized surface diffusion processes, while offering advantages in coating thickness and load-bearing capacity.

Corrosion-Wear Synergy

The study's evaluation of corrosion-wear performance (rather than pure wear) is particularly relevant to industrial applications where the wear environment includes corrosive media. The corrosion-wear interaction is complex:

• Corrosion products may act as lubricants (reducing wear) or as abrasive third-body particles (increasing wear)
• Surface films may protect against corrosion but be removed by wear (exposing fresh material)
• The balance between protective film formation and mechanical removal determines overall performance

The cemented carbide surfacing layers with optimized heat treatment demonstrate that the hard carbide phases (WC in D707, Cr-carbides in D618) provide both mechanical resistance to wear and chemical stability against corrosion, creating a synergistic protective effect.

Process Integration and Simplification

A significant practical contribution of this work is the demonstration that heat treatment can serve as a process simplification tool:

Without heat treatment:

• Requires precise control of welding parameters to achieve optimal as-welded microstructure
• Limited ability to correct defects or optimize properties
• Higher process sensitivity

With heat treatment:

• Provides a "second chance" to optimize microstructure
• Can be applied after component assembly
• Offers flexibility in property optimization
• Reduces sensitivity to welding parameter variations
• Enables post-fabrication property adjustment

This process simplification has significant economic implications for industrial applications where component geometry may limit the practicality of specialized welding sequences.

Alloy-Specific Considerations

D707 (Tungsten Carbide Type)

• Contains WC particles as primary hard phase
• Matrix composition determines matrix hardness and corrosion resistance
• Heat treatment must avoid WC dissolution (limit temperature below 1000°C)
• WC particle distribution and size are set during welding
• Tempering primarily affects the matrix microstructure

D618 (High-Carbon High-Chromium Cast Iron Type)

• Contains Cr7C3 and Cr23C6 as primary hard phases
• Higher chromium content provides better corrosion resistance
• More responsive to heat treatment due to iron-based matrix
• Carbide type and distribution can be modified by heat treatment
• 400°C tempering optimizes secondary carbide precipitation

Engineering Application Scenarios

The findings have direct application in several industrial sectors:

• Chemical processing equipment: Where corrosion-wear is the dominant degradation mechanism
• Pulp and paper industry: Where abrasive slurries contain corrosive components
• Mining and mineral processing: Where slurry wear involves acidic or alkaline media
• Power generation: Where fly ash erosion occurs in corrosive flue gas environments
• Marine applications: Where seawater corrosion combines with biological and mechanical wear

Critical Technical Assessment

While the study is relatively brief, several aspects deserve critical examination:

1. Limited quantitative data: The study provides qualitative descriptions of wear performance but lacks quantitative wear rate data that would enable more precise comparison between treatments.
2. No coating thickness data: The thickness of the surfacing layers is not specified, which is important because wear life is directly proportional to coating thickness.
3. No adhesion strength data: The bond strength between coating and substrate is critical for practical applications but was not evaluated.
4. Limited corrosion testing: The corrosion-wear evaluation methodology is not described in detail, making it difficult to assess the test conditions and their relevance to actual service environments.
5. Single heat treatment cycle: The effect of multiple heat treatment cycles on cumulative microstructural evolution was not investigated.

Summary

This study establishes that appropriate post-weld heat treatment significantly enhances both the wear resistance and corrosion-wear performance of cemented carbide surfacing layers, while simultaneously simplifying the overall surface engineering process. The optimized treatment of 850±5°C oil quenching followed by 400°C tempering provides a practical and reproducible approach to achieving high-performance surface layers on industrial components. The demonstration that surfacing combined with heat treatment can compete with or exceed specialized surface diffusion technologies (such as B,C,N co-implantation) validates the surfacing approach for applications requiring substantial coating thickness and load-bearing capacity. For engineers selecting surface engineering solutions, this work provides clear evidence that the combination of appropriate alloy selection and post-weld heat treatment offers a cost-effective and technically superior approach to combating corrosion-wear degradation in industrial equipment.