Mill rolls are critical components in metal forming processes, especially in hot and cold rolling mills where they directly influence product quality, dimensional accuracy, and production efficiency. Due to the extreme mechanical and thermal stresses they endure during operation, mill roll materials must possess high hardness, wear resistance, thermal fatigue resistance, and structural stability. To achieve these performance requirements, various heat treatment techniques are applied during manufacturing. These treatments modify the microstructure of the roll material, enhancing its mechanical properties and service life.
Common Heat Treatment Types for Mill Rolls
The selection of heat treatment process depends on the roll material (such as alloy cast steel, forged steel, or infinite chilled cast iron), intended application (roughing, intermediate, or finishing stands), and required surface hardness. The most widely used heat treatment methods include:
- Stress Relief Annealing – Reduces residual stresses from casting or machining
- Normalizing – Refines grain structure and improves uniformity
- Austenitizing Diffusion Treatment – Homogenizes alloy element distribution
- Spheroidizing Annealing – Enhances machinability and prepares microstructure for hardening
- Quenching – Increases surface hardness through phase transformation
- Tempering – Balances hardness with toughness after quenching
- Cryogenic Treatment – Stabilizes microstructure and improves wear resistance
1. Stress Relief Annealing
Residual stresses develop during solidification, cooling, and machining of mill rolls. If not properly relieved, these internal stresses can lead to distortion or cracking during subsequent processing or service. Stress relief annealing is typically performed at temperatures between 550°C and 650°C, held for several hours depending on roll size, then slowly cooled in air or furnace.
This treatment does not alter the base microstructure but significantly reduces stress concentration risks. For large-diameter backup rolls made from forged alloy steel, stress relief is often conducted after rough machining and before final heat treatment.
2. Normalizing
Normalizing is commonly applied to forged or cast alloy steel rolls to refine coarse grains formed during solidification. The roll is heated to approximately 880–950°C, above the Ac3 temperature, held for sufficient time to ensure complete austenitization, and then cooled in still air.
The resulting fine pearlitic or bainitic structure provides better strength and toughness compared to as-cast structures. It also prepares the material for further heat treatments like spheroidizing or quenching. In some cases, double normalizing is used to further improve microstructural uniformity, especially for high-alloy rolls used in hot strip mill finishing stands.
3. Austenitizing Diffusion Treatment
In high-alloy cast steel rolls, dendritic segregation occurs during solidification due to non-uniform cooling rates across the roll cross-section. This leads to localized enrichment or depletion of key alloying elements such as chromium, molybdenum, and vanadium—phenomenon known as microsegregation.
To mitigate this, diffusion annealing is carried out by heating the roll to a temperature range of Ac3 + 150–250°C (typically 1100–1200°C), followed by prolonged soaking—often **40 to 100 hours**, depending on roll diameter. During this period, alloy atoms diffuse toward equilibrium concentrations, resulting in a more homogeneous austenitic phase.
For example, a 1200 mm diameter work roll made from Cr-Mo-V alloy cast steel may require up to 90 hours at 1150°C to achieve acceptable compositional uniformity. Without this step, uneven hardenability and unpredictable microstructures would occur after quenching, increasing the risk of spalling or premature failure.
4. Spheroidizing Annealing
Spheroidizing is essential for improving the machinability of high-carbon, high-alloy steel rolls before final hardening. The goal is to transform lamellar cementite in pearlite into spherical carbides dispersed in a ferrite matrix—a microstructure referred to as spheroidized pearlite or globular carbide structure.
The process involves heating the roll to just above the Ac1 point (~720–760°C), holding for extended periods (up to **72 hours**), followed by slow cooling through the pearlite transformation zone. Alternatively, cyclic spheroidizing (heating-cooling cycles) may be employed to accelerate carbide spheroidization.
A typical spheroidized structure should exhibit carbide particles with an average size of **0.5–2.0 μm**, uniformly distributed. This greatly reduces tool wear during turning and grinding operations. Moreover, it ensures consistent austenite formation during later quenching, minimizing soft spots and distortion.
5. Quenching Process
Quenching is the core hardening treatment that determines the final performance of the mill roll working layer. The roll is reheated to the austenitizing temperature—usually between 850°C and 980°C, depending on composition—and soaked to ensure full carbon and alloy dissolution.
Cooling rate is carefully controlled to avoid cracking while achieving desired phase transformation. Common quenching media include polymer solutions, oil sprays, or forced air jets. For alloy cast steel rolls, the objective is to form a **bainitic microstructure** in the outer layer (typically 30–80 mm depth), which offers an optimal balance of hardness, toughness, and thermal fatigue resistance.
For instance, a semi-steel backup roll used in a hot strip mill finishing stand might be quenched using water spray from radial nozzles, achieving a surface cooling rate of ~15°C/s. The resulting bainite content exceeds 85%, with secondary phases including retained austenite and fine carbides.
6. Tempering
After quenching, the roll contains high levels of internal stress and brittle martensite or lower bainite. Tempering relieves these stresses and converts unstable phases into tempered bainite or troostite, improving ductility without sacrificing excessive hardness.
Tempering temperatures vary based on roll type:
- Hot mill rolls: 500–600°C
- Cold mill rolls: 180–250°C
Dwell times range from **10 to 40 hours**, again dependent on mass. Multiple tempering cycles are sometimes applied to minimize residual stress and stabilize dimensions. A well-tempered roll will show Rockwell hardness values within tight tolerances—for example, HRC 60–63 for cold work rolls and HRC 45–55 for hot mill backup rolls.
7. Cryogenic Treatment
Deep cryogenic treatment involves cooling the mill roll to sub-zero temperatures (typically **-70°C to -196°C**) using liquid nitrogen, usually after quenching and before tempering. This promotes the transformation of retained austenite into martensite and enhances fine carbide precipitation.
Although not universally applied, cryogenic treatment has proven beneficial for high-speed steel (HSS) rolls and ultra-high-carbon cold rolls requiring maximum wear resistance. Studies have shown that deep freezing can reduce retained austenite content from over 15% to less than 5%, significantly extending roll life in precision cold rolling applications.
The process requires careful control of cooling and warming rates to prevent thermal shock. Typical soak duration ranges from **12 to 24 hours** at -190°C.
| Heat Treatment Type | Temperature Range | Hold Time | Cooling Method | Typical Application |
|---|---|---|---|---|
| Stress Relief Annealing | 550–650°C | 4–24 h | Furnace cooling | Post-machining, pre-hardening |
| Normalizing | 880–950°C | 8–20 h | Air cooling | Forged steel rolls, pre-spheroidizing |
| Diffusion Annealing | 1100–1200°C | 40–100 h | Furnace cooling | High-alloy cast steel rolls |
| Spheroidizing Annealing | 720–760°C | 48–72 h | Slow cooling | High-carbon alloy rolls |
| Quenching | 850–980°C | 10–30 h | Polymer/water spray | Final hardening stage |
| Tempering | 180–600°C | 10–40 h | Air/furnace cooling | Post-quench stabilization |
| Cryogenic Treatment | -70 to -196°C | 12–24 h | Controlled warming | HSS and cold mill rolls |
Microstructural Evolution in Alloy Cast Steel Mill Rolls
The effectiveness of heat treatment is ultimately judged by the resulting microstructure. A properly treated roll should exhibit:
- Uniform distribution of fine carbides
- Minimal retained austenite (<10%)
- No visible segregation under macroetch testing
- Consistent hardness profile from surface to core
Optical microscopy and scanning electron microscopy (SEM) analyses confirm that diffusion-annealed rolls display significantly reduced dendritic patterning. Similarly, spheroidized samples show round, discrete M7C3 and M23C6 carbides, whereas un-treated counterparts retain sharp, interconnected networks that act as crack initiation sites.
Performance Impact of Proper Heat Treatment
Field data from integrated steel plants indicate that fully processed mill rolls last **30–50% longer** than those subjected to incomplete heat treatment cycles. For example, a work roll used in a tandem cold mill experienced an average campaign life of 18 days when standard quenching and single tempering were applied. After introducing diffusion annealing and cryogenic treatment, campaign life increased to 27 days—a 50% improvement—with fewer surface checks and reduced grinding frequency.
In another case, a backup roll made from 2.25Cr-1Mo-0.25V cast steel failed prematurely due to spalling after only two months in a hot strip mill. Metallurgical analysis revealed severe centerline segregation and insufficient diffusion treatment. Upon implementing a revised cycle with 96-hour soaking at 1150°C, the same roll design achieved over 14 months of service without major defects.
Quality Control and Testing Procedures
Rigorous inspection protocols accompany each heat treatment stage. Key verification methods include:
- Hardness mapping: Surface and cross-sectional Rockwell C tests at multiple points
- Ultrasonic testing (UT): Detects internal cracks or delaminations post-quenching
- Metallographic examination: Verifies phase composition and carbide morphology
- Residual stress measurement: Using X-ray diffraction or hole-drilling method
- Chemical homogeneity check: EPMA (Electron Probe Microanalysis) for elemental mapping
Any deviation from specified parameters triggers corrective actions such as re-heat treatment or rejection. Modern facilities use automated furnace controls with real-time data logging to ensure traceability and compliance with international standards like ISO 9001 and ASTM A648.
Summary of Best Practices in Mill Roll Heat Treatment
Successful heat treatment of mill rolls requires precise coordination of time, temperature, and cooling dynamics. Each step serves a distinct metallurgical function:
- Diffusion treatment eliminates chemical segregation in large castings
- Spheroidizing enables efficient machining and consistent hardening response
- Quenching establishes the primary wear-resistant microstructure
- Tempering optimizes toughness and dimensional stability
- Cryogenic treatment maximizes phase stability in high-performance rolls
Manufacturers aiming to produce reliable, long-lasting mill rolls must invest in advanced furnace systems, skilled metallurgists, and comprehensive quality assurance programs. As rolling mill speeds and loads continue to increase globally, the demand for thermally optimized rolls will only grow.