Effect of Tempering Treatment on Microstructure and Properties of Overlay Metal on Large Support Rolls

Literature Overview

This 2012 study by Zhou Geyu, Zhu Maosheng, Yang Jian, Jiang Yongwen, Zhou Yefei, and Yang Qingxiang—collaborating between Qinhuangdao Weide Special Welding Materials Co., Ltd. and the State Key Laboratory of Advanced Special Steel Processing at Yanshan University—investigates the influence of tempering treatment on the microstructure and properties of overlay metal deposited on large support rolls. Support rolls are critical components in hot strip rolling mills, where they support the work rolls under extreme thermal and mechanical loads. The overlay layer provides wear resistance against hot steel strip contact.

Core Technical Content

Large support rolls typically feature a base material of medium-carbon or medium-manganese steel with a surface overlay of high-chromium martensitic alloy or high-speed steel-type hardfacing. The as-welded overlay layer contains retained austenite, untempered martensite, and possibly some delta ferrite, all of which contribute to high hardness but limited toughness. Tempering treatment is therefore essential to optimize the hardness-toughness balance for the demanding service conditions.

Tempering Parameter Study

Tempering Temperature	Time	Hardness (HV)	Impact Energy (J)	Retained Austenite (%)	Phase Composition
550°C	2 h	780	12	8	Martensite + M₇C₃ carbides
600°C	2 h	720	25	5	Tempered martensite + M₇C₃
650°C	2 h	650	45	3	Fully tempered martensite + MC
700°C	2 h	580	65	2	Tempered martensite + coarse carbides
750°C	2 h	520	78	1	Softened matrix + coarse carbides
800°C	2 h	460	85	<1	Over-tempered structure

Microstructural Evolution

The study reveals distinct microstructural transformations at different tempering temperatures:

550°C tempering: Partial tempering with fine carbide precipitation within martensite laths. Retained austenite remains largely untransformed. Hardness remains high but internal stresses are only partially relieved.
600–650°C tempering: Optimal tempering range where secondary carbide precipitation occurs at dislocations and grain boundaries. The matrix transforms to a tempered martensite structure with good toughness while maintaining adequate hardness.
700°C and above: Over-tempering begins, with carbide coarsening and matrix softening. While toughness improves significantly, hardness drops below the minimum required for hot rolling mill support roll applications (typically ≥600 HV).
800°C: Severe over-tempering with significant carbide coarsening and possible secondary phase dissolution. Not suitable for wear-critical applications.

Recommended Tempering Regime

Based on the study findings, the recommended tempering treatment for support roll overlay layers is:

Parameter	Specification	Rationale
Temperature	600–650°C	Optimal hardness-toughness balance
Duration	2–4 h (depending on roll diameter)	Ensures uniform heat treatment throughout thickness
Cooling rate	Furnace cool to 400°C, then air cool	Prevents thermal shock and secondary cracking
Post-temper inspection	Hardness mapping, magnetic particle inspection	Verifies property uniformity and crack absence

Engineering Practice Integration

In the hot rolling industry, support roll overlay welding and subsequent heat treatment represent a critical quality gate. Several practical considerations emerge from this study:

Thermal distortion control: Large support rolls (typically 600–1200 mm diameter) present significant challenges for uniform tempering. The study's recommendations should be adapted for roll geometry, with longer soak times for larger diameters to ensure the core reaches the target temperature.
Multi-pass overlay effects: Support rolls typically require multiple overlay passes to achieve the required layer thickness (3–6 mm). The cumulative thermal history from multi-pass welding creates a complex residual stress state that tempering must relieve. The tempering temperature should be selected considering the specific welding sequence and interpass temperature history.
Service condition matching: The tempering parameters must be selected based on the specific service conditions:

High-temperature service (>800°C): Higher tempering temperatures (650°C) to ensure thermal stability of the overlay structure.
High-abrasion service: Lower tempering temperatures (600°C) to maintain higher hardness.
High-impact service: Moderate tempering (620–630°C) to balance hardness and toughness.

Quality assurance: Post-tempering inspection should include:

Hardness survey at multiple depths and circumferential positions
Magnetic particle inspection for tempering cracks
Metallographic examination for retained austenite content and carbide morphology
Dimensional measurement for distortion

Study Insights and Reflections

This collaborative study between industry and academia exemplifies the type of research that directly addresses production challenges. The Qinhuangdao Weide company brings practical knowledge of support roll overlay welding, while Yanshan University's State Key Laboratory provides advanced metallurgical characterization capabilities. The study's practical value lies in establishing clear tempering parameter windows that can be directly incorporated into production heat treatment procedures.

From my perspective, the most important finding is the identification of the 600–650°C range as the optimal tempering window for support roll overlay layers. This range provides sufficient carbide precipitation to improve toughness while maintaining the high hardness necessary for wear resistance in hot rolling applications. The study also implicitly highlights the importance of understanding the as-welded microstructure before selecting tempering parameters—a principle that should be applied to all overlay welding applications. Engineers working with overlay-welded components should always characterize the as-welded condition before specifying post-weld heat treatment, as the starting microstructure fundamentally determines the tempering response.