Effect of Post-Weld Tempering on Microstructure and Temper Embrittlement of Overlay Layer on 9Cr13 Roller Steel

Literature Overview

Published in Foundry Technology (2018, Vol. 39, No. 1, pp. 124-127), this study by Li Tao and Wang Xiangjie from Hubei Polytechnic University investigates the influence of post-weld tempering temperature on the microstructure, hardness, and toughness of a Cr-Ni-Mn flux-cored wire overlay deposited on 9Cr13 roller steel. The research was funded by the Hubei Polytechnic University Talent Introduction Project (No. 14xjz02R). Roller steels are critical components in hot rolling mills, and their surface restoration through overlay welding is a common industrial practice. However, the post-weld heat treatment of the overlay layer is often overlooked, leading to premature failure in service.

Core Technical Analysis

Microstructure Evolution with Tempering Temperature

The study systematically examines the microstructural changes across a wide tempering temperature range, revealing distinct transition zones:

Temper Embrittlement Analysis

The most significant finding is the identification of the temper embrittlement temperature interval as 150-200°C based on impact toughness values. Temper embrittlement in this context is associated with the precipitation of carbides and impurity elements (such as phosphorus, sulfur, and antimony) at grain boundaries during slow cooling through the critical temperature range. This phenomenon is particularly relevant for 9Cr13 steel, which contains approximately 13% chromium, making it susceptible to intergranular embrittlement.

The mechanism of temper embrittlement in Cr-Ni-Mn overlay deposits on 9Cr13 steel involves several factors:

1. Segregation of impurity elements: During tempering, phosphorus and sulfur segregate to grain boundaries, reducing intergranular cohesion.
2. Carbide precipitation: Cr-rich carbides (M7C3, M23C6) precipitate preferentially at grain boundaries, creating embrittling phases.
3. Residual stress interaction: The welding residual stress state interacts with the tempering process, potentially promoting crack initiation at weakened grain boundaries.

Optimal Tempering Parameters

The study identifies 550°C as the optimal tempering temperature, where:

• The coarse plate-like tempered martensite has completely transformed into fine lath-like tempered martensite.
• A well-dispersed granular carbide structure provides good hardness-toughness balance.
• The fracture morphology shows abundant dimples, indicating ductile fracture behavior.
• Both hardness and toughness reach a favorable equilibrium.

This optimal condition is consistent with the general principle that medium-temperature tempering (450-550°C) for high-carbon martensitic steels produces the best combination of mechanical properties. The 550°C temperature is high enough to fully spheroidize carbides and relieve residual stresses but low enough to avoid excessive softening.

Engineering Practice Implications

For industrial overlay welding of roller steels, the following recommendations emerge:

• Avoid the 150-200°C temper embrittlement range: If post-weld heat treatment is required, tempering temperatures below 150°C or above 200°C should be selected to bypass the embrittlement zone.
• Prefer 550°C tempering: This temperature provides the best mechanical property balance and should be the default choice for overlay layers on 9Cr13 steel.
• Control cooling rate: Slow cooling through the 150-200°C range after tempering at higher temperatures should be avoided; rapid cooling through this range (furnace cooling with reduced temperature or air cooling) minimizes embrittlement risk.
• Consider multi-stage tempering: For critical applications, a two-stage tempering process (e.g., 550°C followed by 200°C) can further improve toughness by preventing carbide coarsening while eliminating the embrittlement zone.

Key Reflections

The identification of the temper embrittlement zone at 150-200°C is practically significant. In many industrial settings, overlay welds are left in the as-welded or stress-relieved condition, which often places the material in or near the embrittlement range. This study provides clear evidence that such practice is detrimental to the long-term performance of the overlay layer. The transformation from coarse brittle martensite to fine lath martensite with dispersed carbides at 550°C is a classic example of the beneficial effects of tempering on high-carbon martensitic microstructures.

The work also highlights the importance of fracture surface analysis in understanding failure mechanisms. The transition from cleavage fracture at low tempering temperatures to dimple fracture at 550°C provides visual confirmation of the toughness improvement and is a valuable diagnostic tool for quality assurance.

Concluding Remarks

This study provides a comprehensive understanding of how post-weld tempering temperature governs the microstructure and mechanical properties of Cr-Ni-Mn overlay deposits on 9Cr13 roller steel. The identification of the 150-200°C temper embrittlement zone and the recommendation of 550°C as the optimal tempering temperature are directly actionable for industrial practice. Engineers involved in roller repair and overlay welding should incorporate these findings into their process specifications, particularly by avoiding stress-relief temperatures that fall within the embrittlement range and by selecting tempering parameters that promote fine, well-dispersed microstructures. The work reinforces the broader principle that post-weld heat treatment is not merely a stress-relief step but a critical process variable that determines the service life and reliability of overlay weldments.