Effect of Heat Treatment on D618 Cladding Layer Microstructure and Properties

Literature Overview

D618 is a cobalt-chromium-tungsten hardfacing alloy widely used for severe wear and corrosion applications. This paper investigates the effects of post-weld heat treatment on the microstructure, hardness, and mechanical properties of D618 cladding layers. The study addresses a critical practical issue: the as-welded microstructure of D618 often contains undesirable phases and residual stresses that limit performance. Appropriate heat treatment can transform the microstructure to achieve optimal properties.

D618 Composition and As-Welded Microstructure

Chemical Composition

Element	Range (wt%)	Primary Function
Co	55-65	Base matrix, high-temperature strength
Cr	28-35	Carbide formation, corrosion resistance
W	6-9	Carbide hardening, high-temperature strength
C	3.0-4.0	Carbide formation, hardness
Mo	2-4	Secondary hardening, carbide stability
Ni	0-2	Matrix solidification, grain refinement

As-Welded Microstructure

The as-welded D618 cladding layer typically contains:

Phase	Morphology	Hardness (HV)	Volume Fraction
Austenitic matrix	Dendritic	400-500	40-60%
M7C3 carbides	Dendritic, network	1500-2000	25-35%
M23C6 carbides	Blocky, network	1200-1600	10-20%
Sigma phase	Blocky, grain boundary	800-1000	0-10%
Eutectic	Network, interdendritic	1000-1500	5-15%

Residual Stress Distribution

The as-welded cladding layer contains significant residual stresses:

Location	Stress Type	Magnitude (MPa)
Cladding surface	Compressive	-100 to -300
Cladding center	Tensile	200 to 500
Cladding/base interface	Tensile	300 to 600
Base metal HAZ	Tensile	100 to 300

Heat Treatment Processes

Solution Treatment

Parameter	Typical Value	Purpose
Temperature	1100-1200°C	Dissolve carbides, homogenize
Holding time	1-4 hours	Complete dissolution
Cooling rate	Air cool or furnace cool	Control precipitation
Atmosphere	Protective (Ar or vacuum)	Prevent oxidation

Aging Treatment

Parameter	Typical Value	Purpose
Temperature	800-900°C	Precipitate fine carbides
Holding time	2-8 hours	Optimize carbide size
Cooling rate	Air cool	Retain precipitates
Quenching	Optional water quench	Retain austenite

Multi-Stage Treatment

For optimal properties, a multi-stage treatment may be used:

Solution treatment: 1150°C for 2h, air cool
First aging: 850°C for 4h, air cool
Second aging: 750°C for 4h, air cool
Final temper: 550°C for 2h, furnace cool

Microstructural Changes with Heat Treatment

Carbide Dissolution and Re-precipitation

Treatment Stage	Carbide State	Size Distribution	Hardness (HV)
As-welded	Coarse, dendritic	10-50 μm	1400-1800
After solution	Partially dissolved	5-20 μm	800-1000
After aging	Fine, dispersed	0.5-5 μm	1200-1600
Over-aged	Coarsened	10-30 μm	900-1200

Phase Transformation

The heat treatment sequence induces the following phase transformations:

Austenite decomposition: Austenite → martensite + carbides (during cooling)
Martensite tempering: High-carbon martensite → tempered martensite + carbides
Carbide coarsening: Fine carbides → coarse carbides (Ostwald ripening)
Sigma phase formation: At high temperatures, sigma phase may form at grain boundaries

Grain Structure Evolution

Treatment	Grain Size	Grain Boundary Character
As-welded	Dendritic, 50-200 μm	Dendrite arms, carbide networks
Solution treated	Recrystallized, 20-80 μm	Clean boundaries, few carbides
Aged	Recrystallized, 20-80 μm	Carbide precipitation at boundaries

Mechanical Property Changes

Hardness vs. Treatment Temperature

Treatment Temperature (°C)	Hardness (HRC)	Hardness (HV)
As-welded	58-62	1400-1800
800°C / 4h	55-58	1200-1500
850°C / 4h	52-55	1100-1400
900°C / 4h	48-52	1000-1300
950°C / 4h	45-48	900-1200
1000°C / 4h	42-45	800-1100

Wear Resistance

Treatment Condition	Wear Rate (mg/1000 cycles)	Relative Wear Resistance
As-welded	100-150	1.0 (baseline)
850°C / 4h	60-90	1.5-2.0
900°C / 4h	80-120	1.2-1.5
950°C / 4h	120-180	0.8-1.0

Impact Toughness

Treatment Condition	Impact Energy (J)	Toughness Rating
As-welded	5-10	Low
800°C / 4h	10-15	Low-Medium
850°C / 4h	15-25	Medium
900°C / 4h	20-30	Medium-High
950°C / 4h	25-40	High

Defect Analysis and Countermeasures

Defect	Cause	Countermeasure
Cracking during treatment	Thermal stress, phase transformation	Slow heating rate, preheat, stress relief
Decarburization	High-temperature oxidation	Protective atmosphere, vacuum treatment
Grain growth	Excessive temperature or time	Control temperature, limit holding time
Sigma phase formation	High-temperature exposure	Avoid temperatures >1000°C, limit time
Soft spots	Incomplete treatment, composition variation	Uniform heating, verify composition

Engineering Practice Considerations

Treatment Selection Guide

Application Requirement	Recommended Treatment	Expected Properties
Maximum hardness	As-welded or light temper	58-62 HRC, low toughness
Balanced hardness/toughness	850°C / 4h aging	52-55 HRC, medium toughness
Maximum toughness	950°C / 4h temper	45-48 HRC, high toughness
Corrosion resistance	1150°C solution + 850°C aging	Homogeneous, corrosion resistant
High-temperature service	900°C / 8h aging	Stable at elevated temperatures

Equipment Requirements

Equipment	Specification	Purpose
Furnace	1200°C max, ±5°C control	Solution and aging
Atmosphere	Argon or vacuum	Prevent oxidation
Thermocouples	Type K, calibrated	Temperature monitoring
Cooling rate control	Air cool or controlled	Property optimization
Quenching facility	Water or oil	Rapid cooling if needed

Quality Verification

Test	Method	Acceptance Criteria
Hardness	Vickers or Rockwell	Within specification
Microstructure	Metallographic examination	No harmful phases
Bond strength	Tensile or bend test	Minimum 200 MPa
Residual stress	X-ray diffraction	<300 MPa tensile
Corrosion resistance	Salt spray or immersion	No pitting or cracking

Study Insights and Reflections

The research demonstrates that heat treatment is a powerful tool for optimizing D618 cladding performance. The key insight is that there is no single optimal treatment; rather, the treatment must be tailored to the specific application requirements. For maximum wear resistance, the as-welded or lightly tempered condition provides the highest hardness. For applications requiring toughness, such as impact loading or thermal cycling, more aggressive tempering is necessary.

One important finding is the role of carbide size and distribution in determining wear resistance. Fine, uniformly dispersed carbides provide superior wear resistance compared to coarse, network carbides. This is because fine carbides provide more effective obstruction to dislocation motion and abrasion without creating stress concentration points that could initiate cracking.

The research also highlights the importance of avoiding sigma phase formation, which is a brittle intermetallic that can severely degrade toughness. Sigma phase forms preferentially at grain boundaries during prolonged exposure to temperatures above 950°C. Avoiding this phase requires careful control of treatment temperature and time.

In conclusion, the heat treatment of D618 cladding layers offers significant opportunities for property optimization. The systematic understanding of microstructure-property relationships enables engineers to select appropriate treatments for specific applications. The key to success lies in balancing hardness, toughness, and corrosion resistance through careful selection of treatment parameters. Future research should focus on developing accelerated treatment cycles that achieve optimal properties in reduced time, as well as exploring advanced treatments such as tempering with compressive stress or cryogenic treatment for further property enhancement.