Effect of Post-Weld Heat Treatment on Overlay Layer Microstructure and Properties of K360 Steel

Literature Overview

The study by Deng Hanzhong and Wang Dayong, published in the Journal of the China Coal Society (2011, Vol. 36, Issue 6, pp. 1036-1040), investigates the influence of post-weld heat treatment on the microstructure and mechanical properties of overlay layers deposited on K360 wear-resistant steel plates. The research was conducted at Liaoning Technical University, with contributions from the School of Materials Science and Engineering and the School of Mechanical Engineering. The work addresses a practical engineering challenge in coal mining equipment maintenance: the repair of worn K360 steel components while optimizing the balance between hardness, toughness, and wear resistance.

Technical Background and Material Characteristics

K360 is a high-carbon, high-chromium wear-resistant steel commonly used in coal mining equipment such as scraper conveyors, bucket wheels, and material handling components. The base material typically exhibits:

Hardness: HRC 36-40 (as-supplied)
Carbon content: 0.8-1.2%
Chromium content: 8-12%
Microstructure: Tempered martensite with carbide precipitation
Wear resistance: Superior to conventional carbon steels

When K360 steel components experience severe wear, overlay welding repair is a practical solution for restoring surface dimensions and extending service life. The CO2 gas shielded flux-cored wire overlay process was selected for this application due to its productivity, cost-effectiveness, and ability to deposit thick layers efficiently.

Experimental Methodology

The researchers performed CO2 gas shielded flux-cored wire overlay welding on worn K360 steel plates, followed by tempering treatment at approximately 400°C. The study compared the as-welded and post-heat-treated conditions through:

Microstructural analysis (optical microscopy, SEM)
Hardness testing (Vickers or Rockwell)
Impact toughness testing (Charpy V-notch)
Abrasive wear testing (dry sliding or pin-on-disk)

Microstructural Evolution and Performance Comparison

Property	As-Welded Overlay	After 400°C Tempering	Change
Primary phase	Martensite	Tempered troostite	Phase transformation
Carbide morphology	Network + blocky	Tempered carbides	Coarsening and spheroidization
Residual austenite	Minor amount	Reduced or eliminated	Decomposition during tempering
Hardness	High (HRC 55-60 estimated)	Moderate-high (HRC 48-52 estimated)	~10-15% reduction
Impact toughness	Low	Significantly improved	Major improvement
Wear resistance	1.46x base material	1.32x base material	~10% reduction

The microstructural transformation during tempering follows the well-established sequence:

As-welded condition - Hard martensite with network and blocky carbides, minimal residual austenite
Early tempering - Carbon precipitation from martensite, formation of transition carbides
Advanced tempering - Coalescence of carbides, ferrite matrix softening, troostite formation
Final state at 400°C - Tempered troostite (ferrite + fine dispersed carbides), reduced residual stresses

Heat Treatment Parameter Optimization

The selection of 400°C as the tempering temperature represents a deliberate engineering compromise. The tempering temperature selection for martensitic overlay materials follows these principles:

Temperature Range	Microstructural State	Hardness	Toughness	Application Suitability
200-250°C	Tempered martensite	Very high	Low	Maximum wear resistance required
300-350°C	Tempered martensite	High	Moderate	Balanced wear and toughness
400-450°C	Troostite	Moderate-high	Good	Toughness-critical applications
500-550°C	Sorbite	Moderate	High	Impact loading applications
600°C+	Coarse pearlite	Low	Very high	Not suitable for wear applications

The 400°C tempering temperature was selected because:

It provides significant improvement in impact toughness compared to the as-welded condition
It maintains adequate hardness for wear resistance
It reduces residual stresses that could cause cracking during service
It eliminates or reduces residual austenite, which can be detrimental in cryogenic or high-pressure applications

Engineering Practice Implications

The findings have direct implications for the repair of K360 steel components in coal mining operations:

As-welded overlay - Suitable for applications where maximum wear resistance is the primary requirement and impact loading is minimal (e.g., conveyor trough surfaces subject to material abrasion)
Post-tempered overlay - Preferred for components subject to impact loading, vibration, or thermal cycling (e.g., bucket wheel teeth, scraper chain components)
Decision framework - The choice between as-welded and tempered conditions should be based on:

Predominant wear mechanism (abrasive vs. impact)
Operating temperature range
Service life requirements
Safety considerations (brittle fracture risk)

Residual Stress and Distortion Control

The overlay welding process on K360 steel introduces significant residual stresses due to:

Thermal contraction of the weld metal upon cooling
Phase transformation stresses during martensite formation
Differential cooling rates between overlay and base material

The 400°C tempering treatment addresses these residual stresses through:

Stress relief through plastic deformation at elevated temperature
Accommodation of phase transformation strains
Reduction of thermal gradient-induced stresses

The residual stress reduction is particularly important for preventing:

Delayed cracking in the HAZ or overlay layer
Distortion of the repaired component
Premature failure under cyclic loading

FMEA Analysis for Overlay Repair of K360 Steel

Failure Mode	Cause	Consequence	Detection Method	Countermeasure
Overlay cracking	High residual stress	Component failure	MT/PT inspection	PWHT at 400°C
Delamination	Poor fusion	Loss of overlay	UT inspection	Preheat and control heat input
Excessive dilution	High heat input	Reduced hardness	Hardness testing	Optimize welding parameters
Brittle fracture	Low toughness	Catastrophic failure	Impact testing	Tempering treatment
Wear-through	Insufficient thickness	Base material exposure	Dimensional inspection	Adequate overlay thickness

Key Questions and Reflections

A fundamental question arises regarding the optimal tempering temperature for specific service conditions. While 400°C provides a good balance of properties, the actual optimal temperature depends on the specific wear mechanism and loading conditions. For purely abrasive wear applications, a lower tempering temperature (250-300°C) might be more appropriate to maintain higher hardness. For impact-dominated applications, a higher temperature (450-500°C) might provide better toughness without excessive hardness loss.

Another reflection concerns the interaction between the overlay layer and the base material during service. The coefficient of thermal expansion mismatch between the overlay and K360 base material creates thermal stresses during temperature cycling. The tempering treatment partially addresses this by reducing residual stresses, but the long-term thermal fatigue behavior of the overlay-base interface warrants further investigation.

The wear resistance reduction from 1.46x to 1.32x base material after tempering represents a 10% decrease, which is an acceptable trade-off for the significant improvement in toughness. This finding validates the engineering practice of applying PWHT to overlay weldments when toughness is a critical requirement.

Study Insights and Implications

This research provides valuable guidance for the repair of K360 steel components in coal mining applications. The systematic comparison of as-welded and post-tempered conditions demonstrates that the 400°C tempering treatment offers a favorable property balance for most practical applications. The microstructural evolution from martensite with network carbides to tempered troostite explains the observed property changes and provides a metallurgical basis for process optimization. For maintenance engineers, this work establishes a clear decision framework for selecting between as-welded and heat-treated overlay conditions based on service requirements. The research reinforces the principle that post-weld heat treatment is not merely a stress relief operation but a critical process step that fundamentally alters the microstructure and performance of overlay weldments.