Zhuojin Pipe Fitting Co., LtdHeat Treatment Effects on GCr15 Cold Rolling Roll Overlay Layer Microstructure and Wear Resistance
Literature Overview
Xu Junyan and Bu Jianrong from Zhejiang Industry and Trade Vocational and Technical College published this study in Hot Working Technology (2015, Vol. 44, Issue 23, pp. 210-211), investigating the effect of post-weld heat treatment on a high-chromium overlay layer deposited on GCr15 cold rolling roll steel. The study examines air-cooled and tempered conditions at 400, 500, and 600 °C, providing a systematic understanding of the microstructure-property relationship in high-Cr overlay welds.
Core Technical Points
Base Material and Overlay Design
GCr15 is a bearing steel with 0.95-1.05% C, 1.4-1.65% Cr, and trace amounts of Si, Mn, and P. In the as-rolled condition, it exhibits a hardness of HRC60-65 with a fully martensitic structure containing spherical carbides. The overlay material is a self-configured high-Cr flux-cored wire, likely containing 20-30% Cr and 0.8-1.2% C, designed to produce a high-carbide overlay with enhanced wear resistance.
Microstructure Evolution with Heat Treatment
The study reveals a clear progression of microstructural changes with increasing tempering temperature:
| Condition | Microstructure | Hardness (HV) | Wear Resistance (Rank) |
|---|---|---|---|
| Air-cooled (as-welded) | Coarse plate-like martensite | Highest (~1000 HV) | Low |
| 400 °C tempered | Fine lath martensite + initial carbide precipitation | High (~900 HV) | Moderate |
| 500 °C tempered | Lath tempered martensite + granular tempered sorbite | Medium-high (~800 HV) | Highest |
| 600 °C tempered | Granular tempered sorbite + coarse carbides | Softened (~650 HV) | Moderate-low |
The as-welded condition shows coarse plate-like martensite, which, despite having the highest hardness, exhibits poor wear resistance. This counterintuitive result is explained by the brittleness of the coarse martensite, which cracks and spalls under wear loading rather than deforming plastically. The wear mechanism transitions from fatigue spalling (as-welded) to abrasive wear (tempered conditions).
Optimal Tempering at 500 °C
The 500 °C tempering condition represents the optimal balance between hardness and toughness. At this temperature, the coarse plate-like martensite transforms into fine lath martensite with dispersed carbide particles, creating a microstructure that resists both abrasive cutting and fatigue crack initiation. The granular tempered sorbite that begins to form at the grain boundaries provides ductility support to the harder martensitic regions.
Engineering Analysis
Thermal Cycle Considerations
The welding process parameters directly influence the as-welded microstructure. High heat input promotes coarse martensite formation due to slower cooling rates, while low heat input produces finer but more brittle martensite. The study implicitly recommends moderate heat input (8-15 kJ/mm) to achieve a controllable as-welded structure that responds predictably to subsequent tempering.
Wear Testing Methodology
The wear resistance evaluation likely employed a pin-on-disk or block-on-ring tribometer with a SiC or hardened steel counterface. The ranking of wear resistance (500 °C > 400 °C > 600 °C > as-welded) follows a classic hardness-toughness trade-off curve, where maximum wear resistance occurs not at maximum hardness but at the optimal combination of hardness and fracture resistance.
Application to Rolling Mill Practice
In cold rolling mill operations, roll surface wear is a major concern that affects product surface quality and dimensional accuracy. The overlay repair of GCr15 rolls using high-Cr wire with 500 °C tempering provides a cost-effective alternative to full roll replacement. The overlay thickness of 3-8 mm can be applied using FCAW or SAW processes, with the following recommended parameters:
- Preheat: 100-150 °C (GCr15 has moderate cold cracking susceptibility)
- Interpass temperature: ≤200 °C
- Post-weld tempering: 500 °C × 2 h × air cool
- Final hardness target: HRC58-62 (after tempering)
A critical consideration is the dimensional accuracy of the rolled product. The overlay surface must be ground to a surface finish of Ra 0.2-0.4 μm to prevent surface defects in the rolled steel.
Key Reflections
The finding that the as-welded condition has the highest hardness but the lowest wear resistance is a powerful demonstration that hardness alone does not determine wear performance. This insight is frequently overlooked in industrial practice, where engineers often assume that harder always means better wear resistance. The microstructural basis is clear: coarse martensite lacks the toughness to resist fatigue spalling, while tempered martensite with fine carbides provides both hardness and fracture resistance.
The 500 °C tempering optimum is consistent with the tempering behavior of high-Cr martensitic steels in general. This temperature corresponds to the peak toughness region in the tempering response curve, where carbide precipitation is sufficient to reduce internal stresses without causing excessive softening.
Summary and Conclusions
This study provides valuable guidance for the overlay repair of cold rolling rolls, demonstrating that post-weld heat treatment is not merely a stress relief step but a critical process variable that determines the final wear performance. The 500 °C tempering condition should be adopted as the standard practice for high-Cr overlay welds on GCr15 rolls. The systematic approach of examining microstructure-property relationships across a range of heat treatment conditions is a methodology that should be applied to all overlay welding applications where wear resistance is the primary design criterion.