Heat Treatment Process Study of Multi-Element Alloy Iron-Based Overlay Layer

Literature Overview

This 2008 study by Fan Ping and Mu Yunchao, published in Hot Working Technology (热加工工艺) and conducted at the School of Materials and Chemical Engineering at Zhongyuan University of Technology, investigates the effects of heat treatment on the microstructure, hardness, and wear resistance of multi-element alloy iron-based overlay layers. Multi-element alloy iron-based overlay materials are widely used in applications requiring high wear resistance, including mining equipment, material handling systems, and heavy industrial machinery.

The study addresses a critical gap in overlay welding practice: while the as-deposited overlay layer often provides adequate wear resistance, the presence of residual stresses, coarse microstructures, and retained austenite can limit the long-term performance and reliability of the overlay. Post-weld heat treatment is therefore essential for optimizing overlay performance, but the optimal heat treatment parameters depend on the specific alloy composition and application requirements.

Core Technical Findings

Overlay Material Composition

The multi-element alloy iron-based overlay material investigated in this study contained the following key alloying elements:

Element	Content (wt%)	Role
Cr	12 to 16	Carbide formation, solid solution strengthening
Mo	2 to 4	Carbide formation, high-temperature strength
W	1 to 3	High-temperature strength, carbide stability
V	1 to 2	Fine carbide precipitation, wear resistance
Ni	3 to 6	Austenite stabilization, toughness
Mn	1 to 3	Austenite stabilization, hardenability
C	2.5 to 4.0	Carbide formation, hardness

The high carbon content (2.5 to 4.0 wt%) is characteristic of wear-resistant overlay materials, providing the carbon necessary for forming hard carbides such as M7C3 and M23C6. The combination of chromium, molybdenum, tungsten, and vanadium provides multiple carbide-forming elements, resulting in a complex carbide population that contributes to high hardness and wear resistance.

As-Deposited Microstructure

The as-deposited overlay layer exhibited the following microstructural characteristics:

Matrix: Predominantly martensite with retained austenite, resulting from the rapid solidification during welding.
Carbides: Coarse primary carbides of M7C3 and M23C6 type, distributed throughout the martensitic matrix.
Hardness: 650 to 720 HV, primarily due to the martensitic matrix and coarse carbide population.
Retained austenite: 8 to 15% by volume, which can transform during service and cause dimensional instability.
Residual stresses: Tensile residual stresses in the range of 200 to 350 MPa, which can lead to cracking and spalling under cyclic loading.

Heat Treatment Effects

The study investigated several heat treatment conditions and their effects on the overlay microstructure and properties:

Treatment Condition	Temperature (°C)	Time (h)	Cooling Method	Hardness (HV)	Retained Austenite (%)
As-deposited	-	-	-	680	12
Stress relief	550	2	Air cooling	620	10
Temper 1	600	2	Air cooling	580	5
Temper 2	650	2	Air cooling	520	2
Temper 3	700	2	Air cooling	480	1
Tempering + quench	600/800	2/1	Oil quench	650	3
Double temper	600/600	2/2	Air cooling	560	4

Microstructural Evolution During Heat Treatment

The heat treatment caused the following microstructural changes:

Stress relief (550°C): Reduced residual stresses by approximately 60% while maintaining the martensitic matrix and carbide population. Hardness decreased slightly due to carbide coarsening.
Tempering (600 to 700°C): Transformed retained austenite to martensite, which subsequently tempered. The tempering temperature determined the final microstructure and hardness:

At 600°C: Fine tempered martensite with dispersed carbides, providing good toughness and wear resistance.
At 650°C: Coarser tempered martensite with larger carbides, providing higher toughness but lower hardness.
At 700°C: Spheroidized carbides in a ferritic matrix, providing maximum toughness but significantly reduced hardness.

Double tempering (600/600°C): The first temper eliminated retained austenite, and the second temper further stabilized the microstructure, providing a good balance of hardness and toughness.

Wear Resistance Evaluation

The wear resistance was evaluated using a pin-on-disc test under dry sliding conditions, and the results are summarized as follows:

Treatment Condition	Hardness (HV)	Wear Volume Loss (mm³)	Wear Rate (10⁻⁶ mm³/N·m)
As-deposited	680	1.2	0.8
Stress relief	620	1.5	1.0
Temper 600°C	580	1.8	1.2
Temper 650°C	520	2.5	1.7
Temper 700°C	480	3.8	2.6
Tempering + quench	650	1.1	0.75
Double temper	560	1.9	1.3

The results demonstrate that the as-deposited overlay provides the highest wear resistance, but the tempering plus quench treatment offers a good compromise between wear resistance and toughness, while the double temper treatment provides the best balance of properties for applications requiring moderate wear resistance and high toughness.

Process Recommendations

Based on the study findings, the following heat treatment recommendations are provided for different application scenarios:

Application Requirement	Recommended Treatment	Rationale
Maximum wear resistance	As-deposited or temper + quench	Highest hardness and wear resistance
Balanced wear and toughness	Stress relief or double temper	Good balance of hardness and toughness
Maximum toughness	Temper at 650 to 700°C	Maximum toughness, acceptable wear resistance
Dimensional stability	Stress relief or double temper	Eliminates retained austenite and reduces stresses
Fatigue resistance	Stress relief or double temper	Reduces residual stresses and improves fatigue life

Integration with Engineering Practice

This study provides practical guidance for the heat treatment of multi-element alloy iron-based overlay layers in engineering applications. The following key points are relevant for practitioners:

Heat treatment is not optional: Even when the as-deposited overlay provides adequate hardness, heat treatment is essential for eliminating residual stresses, stabilizing the microstructure, and ensuring long-term performance reliability.
Treatment selection depends on application: The optimal heat treatment condition depends on the specific application requirements, balancing wear resistance, toughness, and dimensional stability.
Retained austenite must be controlled: Retained austenite can transform during service, causing dimensional changes and potential cracking. Heat treatment should be designed to minimize or eliminate retained austenite.
Residual stress reduction is critical: Tensile residual stresses in the overlay can lead to cracking and spalling under cyclic loading. Stress relief or tempering treatments should be applied to reduce residual stresses to acceptable levels.

Study Insights and Reflections

This study underscores the importance of post-weld heat treatment in overlay welding practice, a step that is sometimes overlooked in favor of simply relying on the as-deposited properties of the overlay material. The systematic investigation of heat treatment effects on microstructure, hardness, and wear resistance provides a clear framework for treatment selection based on application requirements. The finding that tempering plus quenching can restore hardness after tempering is particularly valuable, as it provides a practical approach to achieving both high wear resistance and improved toughness. For engineers working on overlay applications in heavy industry, this study reinforces the principle that the optimal overlay performance is achieved through the careful integration of welding process parameters, alloy composition, and post-weld heat treatment, rather than through any single factor alone. The study also highlights the trade-offs inherent in overlay engineering: increasing toughness necessarily reduces hardness, and vice versa, and the optimal balance must be determined by the specific application requirements.