Improvement of Heat Treatment Process for Overlay Rolls

Literature Overview

This 2004 publication by Zhao Hui from the School of Materials Science and Engineering at Shenyang Institute of Technology addresses the improvement of heat treatment processes for overlay rolls (clad rolls). Overlay rolls are critical components in steel mills, paper mills, and other rolling operations where the surface layer must provide specific properties (wear resistance, corrosion resistance, or controlled friction) while the substrate provides structural support. The heat treatment of overlay rolls is a specialized process that must address both the overlay layer and the substrate without compromising the metallurgical bond or introducing detrimental phase transformations.

Core Technical Content

Types of Overlay Rolls and Their Heat Treatment Requirements

Roll Type	Overlay Material	Substrate	Heat Treatment Objective
Hardfacing roll	High-Cr cast iron, high-Mn steel	Low-carbon steel	Maximize surface hardness
Clad roll	Stainless steel, nickel alloy	Alloy steel	Maintain corrosion resistance
Composite roll	Ceramic overlay	Steel	Stress relief without cracking
Wear-resistant roll	Carbide overlay	Medium-carbon steel	Optimize hardness-toughness balance

Traditional Heat Treatment Approach and Its Limitations

The conventional approach typically involves:

Solution treatment at 950–1100°C followed by air cooling
Tempering at 500–650°C for hardness optimization
Surface grinding after heat treatment

Limitations of the conventional approach include:

Excessive grain growth in the overlay layer
Carbide coarsening leading to reduced hardness
Thermal distortion exceeding dimensional tolerances
Residual stress accumulation causing early failure
Inconsistent hardness distribution through the overlay thickness

Improved Heat Treatment Process

The improved process proposed by Zhao introduces several key modifications:

Process Step	Conventional	Improved	Benefit
Preheating	None or single stage	Multi-stage (300°C → 500°C → 700°C)	Reduce thermal shock
Heating rate	200°C/h	50–100°C/h	Minimize differential expansion
Soak temperature	950–1100°C	850–950°C (lower)	Reduce grain growth
Soak time	2–4 h	1–2 h (shorter)	Limit carbide coarsening
Cooling method	Air cool	Controlled rate cool (50°C/h)	Reduce residual stress
Tempering	Single stage	Multi-stage (two-step)	Optimize toughness
Post-grinding stress relief	200–300°C	400–500°C	Complete stress relief

Microstructural Control

The improved heat treatment achieves the following microstructural objectives:

Grain refinement: Maintaining overlay grain size in the ASTM 8–10 range (10–20 μm) by reducing soak temperature and time.
Carbide stability: Preserving fine M₇C₃ and M₆C carbide distribution through controlled cooling rates that prevent coarsening.
Phase equilibrium: Achieving the desired austenite/ferrite balance through precise temperature control during soaking.
Bond integrity: Maintaining the metallurgical bond between overlay and substrate by avoiding temperatures that could cause interfacial diffusion or phase instability.

Process Optimization Using PDCA Cycle

Plan (P)

Define target hardness profile: 55–60 HRC surface, 45–50 HRC at 2 mm depth, 35–40 HRC at substrate interface
Establish maximum allowable distortion: 0.05 mm/m
Set residual stress limit: <150 MPa tensile at surface

Do (D)

Execute improved heat treatment cycle with instrumented thermocouple monitoring
Record temperature profiles at multiple points (surface, mid-thickness, substrate interface)
Document cooling rates and hold times

Check (C)

Measure hardness profile using portable Rockwell and Brinell testers
Perform dimensional inspection using coordinate measuring machine (CMM)
Measure residual stress using X-ray diffraction
Conduct metallographic examination for grain size and phase composition

Act (A)

Adjust soaking temperature by ±20°C based on hardness results
Modify cooling rate if distortion exceeds tolerance
Extend or reduce soak time based on carbide distribution observations
Update process parameters for subsequent heat treatment cycles

Quality Control Parameters

Inspection Item	Method	Acceptance Criteria
Surface hardness	Rockwell C	55–60 HRC (typical)
Core hardness	Rockwell B/C	Per substrate specification
Hardness uniformity	5-point measurement	±3 HRC variation
Distortion	CMM measurement	≤0.05 mm/m
Residual stress	X-ray diffraction	<150 MPa tensile
Grain size	Metallographic examination	ASTM 8–10
Bond strength	Shear test	>200 MPa
Surface quality	Visual + profilometry	No cracks, Ra < 1.6 μm

Engineering Practice Integration

The improved heat treatment process for overlay rolls has been successfully applied in:

Hot strip mill work rolls: Where surface hardness must withstand temperatures up to 1000°C during rolling operations
Cold roll mill backup rolls: Where dimensional stability and surface finish are critical for product quality
Paper mill press rolls: Where smooth surface finish and controlled friction are essential
Aluminum rolling mill rolls: Where contamination sensitivity requires precise control of surface chemistry

Case Study: Hot Strip Mill Work Roll

A typical hot strip mill work roll with 3 mm high-chromium cast iron overlay on a 42CrMo substrate was treated using the improved process:

Parameter	Before Improvement	After Improvement
Surface hardness	58–62 HRC (variable)	57–59 HRC (uniform)
Distortion	0.12 mm/m	0.03 mm/m
Service life	80–120 heats	150–200 heats
Failure mode	Spalling	Gradual wear
Residual stress	+280 MPa (surface)	+80 MPa (surface)

Study Insights and Reflections

Zhao's work on improving the heat treatment process for overlay rolls addresses a fundamental challenge in the manufacturing of clad components: achieving the desired surface properties without compromising dimensional accuracy or introducing detrimental residual stresses. The key insight is that heat treatment for overlay rolls is not simply a surface treatment — it must be considered as a system-level process that accounts for the differential thermal expansion, phase transformation behavior, and mechanical properties of both the overlay and substrate materials. The multi-stage preheating approach is particularly effective in reducing thermal shock at the overlay-substrate interface, which is the most vulnerable location for cracking. For practitioners, the most important takeaway is that heat treatment parameters must be tailored to the specific overlay-substrate combination rather than applying generic procedures. The PDCA cycle approach to process optimization ensures continuous improvement and adaptation to new material combinations and service requirements.