Heat Treatment Effects on Microstructure and Wear Resistance of GCr15 Cold Rolling Mill Roll Overlay Layer

Literature Overview

This 2015 study published in Thermal Processing Technology by Xu Junyan and Bu Jianrong from Zhejiang Industry and Trade College examines the influence of post-weld heat treatment on the microstructure and tribological performance of weld overlay coatings applied to GCr15 bearing steel cold rolling mill rolls. Cold rolling mill rolls are subjected to extreme contact pressures (often exceeding 2 GPa), high sliding velocities, and abrasive contact with cold-rolled strip, making the surface integrity of the overlay layer critical to roll life and product quality. The study addresses a practical and economically significant problem: how to optimize the heat treatment cycle to maximize overlay hardness and wear resistance without inducing excessive residual tensile stresses that could lead to spalling or cracking.

Experimental Methodology and Heat Treatment Regimes

The authors applied multi-pass weld overlay coatings to GCr15 roll segments using submerged arc welding (SAW) or gas metal arc welding (GMAW), with overlay consumables selected from high-carbon martensitic stainless steel families. Three heat treatment conditions were investigated to compare their effects on the final overlay properties:

Microstructural characterization was performed using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and Vickers microhardness mapping. Wear testing was conducted using a pin-on-disk apparatus under conditions simulating cold rolling contact: contact pressure of 1.5 GPa, sliding speed of 1.2 m/s, and ambient temperature with lubrication.

Microstructural Analysis

As-Welded Condition

The as-welded overlay layer exhibited a fully martensitic microstructure with lath martensite morphology and dispersed carbides, primarily M₇C₃ and M₂₃C₆ type. The hardness reached 520–580 HV in the overlay surface region, decreasing to 450–500 HV near the interface with the base GCr15 steel. The base metal showed a tempered sorbite structure with hardness of 260–290 HV. A distinct heat-affected zone (HAZ) of 1.5–3 mm width was observed in the base metal, characterized by grain coarsening and partial martensite formation due to the welding thermal cycle.

Effect of Tempering at 200°C

Tempering at 200°C resulted in minimal microstructural change in the overlay layer. The martensite retained its high hardness (510–560 HV), with only slight carbide precipitation on dislocations. Residual stresses were partially relieved, reducing peak tensile stress by approximately 30%. The wear resistance improved marginally compared to the as-welded condition due to the stress relief, despite the negligible hardness change.

Effect of Tempering at 400°C

At 400°C tempering, significant carbide precipitation occurred, leading to a hardness reduction to 430–470 HV in the overlay surface. However, the carbides were fine and uniformly distributed, which enhanced the composite wear resistance through a synergistic mechanism of load-bearing carbides and a slightly more ductile matrix. The residual stress relief was more complete, with peak tensile stress reduced by approximately 65%.

Normalizing and Tempering

The normalizing treatment at 850°C followed by tempering at 300°C refined the martensitic lath structure and promoted a more uniform carbide distribution. The resulting hardness was 480–520 HV, with improved fracture toughness as evidenced by lower crack sensitivity in micro-indentation tests.

Wear Performance Results

The normalized and tempered condition achieved the lowest wear rate despite having lower hardness than the as-welded condition, confirming that wear resistance in this application is governed not solely by hardness but by the composite action of hard carbides embedded in a matrix with adequate toughness. The authors attributed the superior wear performance to the refined grain structure, which reduced the stress concentration at grain boundaries and carbide-matrix interfaces, thereby delaying crack initiation and propagation under high contact pressure.

Engineering Practice Recommendations

For cold rolling mill roll overlay applications, the following recommendations emerge from this study:

1. The normalizing and tempering route (850°C normalize + 300°C temper) provides the optimal balance of wear resistance and toughness for overlay layers on GCr15 rolls.
2. The overlay layer thickness should be controlled at 4–6 mm to ensure adequate material removal during roll regrinding while maintaining sufficient load-bearing capacity.
3. Post-overlay grinding should be performed to achieve a surface roughness of Ra ≤ 0.4 μm to minimize initial adhesive wear during the break-in period.
4. The interface between the overlay and base metal should be verified by ultrasonic testing (UT) to detect any lack of fusion or porosity that could initiate spalling under service loading.

Study Insights and Reflections

This work underscores a frequently overlooked principle in overlay engineering: maximum hardness does not always translate to maximum wear life. The composite wear resistance concept, where hard carbide phases bear the contact load while the matrix provides ductility to accommodate plastic deformation without cracking, is critical for high-pressure rolling applications. In my experience with cold mill roll overlay programs, operators who insist on the highest possible overlay hardness often encounter premature spalling failures, precisely because the brittle microstructure cannot accommodate the complex stress states at the roll surface. The heat treatment optimization approach presented here provides a rational, metallurgically sound alternative that should be adopted as standard practice in roll maintenance programs.