Effect of Tempering Temperature on Microstructure and Wear Resistance of Iron-Based Multi-Component Alloy Weld Overlay Layer

Literature Overview

This 2006 study, published in "Heat Processing Technology" (热加工工艺), investigates the influence of tempering temperature on the microstructure and wear resistance of an iron-based multi-component alloy weld overlay layer. The research was conducted by faculty at Hebei Building Materials Vocational and Technical College, representing academic research focused on fundamental understanding of post-weld heat treatment effects on overlay performance. The study addresses a critical practical issue: many iron-based overlay alloys are deposited in an as-welded condition with a hard but brittle microstructure, and tempering is often required to optimize the balance between wear resistance and toughness.

Core Technical Content

Alloy System and As-Welded Microstructure

The iron-based multi-component alloy overlay typically contains a combination of alloying elements designed to promote the formation of hard phases while maintaining adequate weldability. A typical composition might include:

• Carbon (C): 2.0–4.0 wt% to promote carbide formation
• Chromium (Cr): 8–20 wt% for carbide stability and corrosion resistance
• Molybdenum (Mo): 2–6 wt% for solid solution strengthening and high-temperature performance
• Vanadium (V): 1–3 wt% for fine carbide precipitation
• Tungsten (W): 0–3 wt% for high-temperature wear resistance
• Iron (Fe): Balance

The as-welded microstructure typically consists of a martensitic matrix with dispersed carbides (M₇C₃, M₂C, VC, WC). The hardness in the as-welded condition can reach 60–70 HRC, but the material is brittle and susceptible to cracking under impact loading.

Effect of Tempering Temperature on Microstructure

Tempering induces several microstructural transformations that affect the mechanical properties of the overlay:

1. Low-temperature tempering (200–350°C):

• Carbides remain largely unchanged
• Dislocation density decreases slightly
• Hardness decreases marginally (1–3 HRC)
• Toughness improves slightly

1. Medium-temperature tempering (350–550°C):

• Tempered martensite forms
• Some carbide coarsening occurs
• Significant hardness reduction (5–10 HRC)
• Substantial improvement in toughness

1. High-temperature tempering (550–700°C):

• Spheroidization of carbides begins
• Tempered sorbite or troostite forms
• Further hardness reduction (10–20 HRC)
• Maximum toughness achieved
• Risk of over-tempering and loss of wear resistance

Wear Mechanism Analysis

The wear resistance of iron-based overlay alloys is primarily governed by:

• Carbide hardness and volume fraction: Harder and more abundant carbides provide superior abrasion resistance.
• Matrix toughness: A tougher matrix prevents crack initiation and propagation around carbides.
• Carbide distribution: Uniformly distributed carbides provide consistent wear resistance; clustered carbides create weak points.
• Thermal stability: At elevated temperatures, carbides may soften or transform, reducing wear resistance.

The optimal tempering temperature depends on the specific service conditions:

• Room-temperature dry abrasion: Lower tempering temperatures (300–400°C) maintain high carbide hardness while improving toughness.
• Elevated-temperature wear: Higher tempering temperatures (500–600°C) improve thermal stability and reduce thermal cracking susceptibility.
• Impact loading: Moderate tempering (400–500°C) provides the best balance between hardness and toughness.

Process and Standards Analysis

Welding and Heat Treatment Parameters

For iron-based multi-component alloy overlay welding, the following parameters are typical:

• Welding process: SAW or FCAW for high deposition rates; GTAW for thin overlays or critical applications
• Preheat temperature: 100–200°C depending on substrate and overlay thickness
• Interpass temperature: 150–250°C
• Post-weld heat treatment: Tempering at 300–650°C for 1–4 hours depending on overlay thickness

The heat treatment cycle is critical for achieving the desired microstructure. A typical tempering cycle includes:

1. Preheating to 200°C to reduce thermal gradients
2. Heating to the target tempering temperature at a rate of 50–100°C/h
3. Holding at temperature for 1 hour per 25 mm thickness
4. Cooling in air or furnace depending on the required final properties

Applicable Standards

Integration with Engineering Practice

Iron-based multi-component alloy overlays are widely used in applications requiring high wear resistance, including:

• Mining equipment (crushers, conveyors, excavators)
• Cement industry (mill liners, kiln components)
• Power generation (turbine blades, hammers)
• Material handling (chutes, hoppers, wear plates)

The tempering treatment is often required to optimize the overlay performance for specific service conditions. For example:

• Mining crushers: Operate at room temperature with high impact loading; tempering at 400–500°C provides optimal balance.
• Cement mill liners: Subjected to high temperatures and severe abrasion; tempering at 550–650°C improves thermal stability.
• Power plant components: May operate at elevated temperatures; tempering at 500–600°C maintains hardness at operating temperature.

Key Questions and Reflections

A key question is: what is the minimum tempering temperature required to achieve acceptable toughness without significantly compromising wear resistance? The answer depends on the specific alloy composition and service conditions, but generally, tempering at 400–500°C provides a good compromise for most applications.

Another important consideration is the effect of tempering on the bond strength between the overlay and the substrate. Excessive tempering temperatures can lead to softening of the heat-affected zone in the substrate, potentially reducing the bond strength. This is particularly relevant for thick overlays on thin substrates where the heat treatment cycle can significantly affect the substrate properties.

Study Insights and Implications

This research provides valuable guidance for optimizing the tempering treatment of iron-based overlay alloys. The findings have direct implications for:

• Process optimization: Establishing recommended tempering parameters for different alloy compositions and service conditions.
• Quality control: Defining acceptance criteria for overlay deposits based on hardness, toughness, and wear resistance.
• Application guidance: Identifying the most suitable tempering temperature for specific applications.
• Life prediction: Understanding the relationship between microstructure and wear performance enables more accurate life predictions for overlay-clad components.

The study exemplifies the importance of post-weld heat treatment in optimizing the performance of weld overlay deposits, demonstrating that careful selection of tempering parameters can significantly improve the service life of overlay-clad components.