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

Literature Overview and Background

The research by Li Tao and Wang Xiangjie from Hubei Polytechnic University, supported by the Hubei Polytechnic University Talent Introduction Project (14xjz02R), investigated the influence of post-weld tempering treatment on the microstructure evolution and temper brittleness behavior of overlay layers deposited on 9Cr13 roller steel substrates. The 9Cr13 designation indicates a martensitic stainless steel with approximately 9% chromium and 13% carbon equivalent content, used in roller applications requiring high hardness, wear resistance, and moderate corrosion resistance. The overlay layer, deposited through arc welding processes, must achieve complementary mechanical properties while maintaining sufficient toughness to withstand impact loading and thermal cycling during roller service.

This research addresses a fundamental metallurgical challenge in cladding operations: the interaction between the overlay microstructure and the heat treatment cycle applied to restore mechanical properties after welding. The tempering treatment, essential for relieving residual stresses and achieving the desired hardness-toughness balance, can also induce temper brittleness phenomena that compromise the long-term reliability of the overlay.

Core Technical Content

The microstructure of the as-welded overlay layer on 9Cr13 steel typically consists of martensite with varying amounts of retained austenite, depending on the cooling rate and alloy composition. The high carbon and chromium content promote the formation of fine carbide precipitates within the martensitic matrix, contributing to high hardness but potentially reducing ductility and toughness. The post-weld tempering treatment modifies this microstructure through carbide precipitation, martensite decomposition, and retained austenite transformation, fundamentally altering the mechanical properties of the overlay.

Temper brittleness, a well-documented phenomenon in alloy steels, manifests as a significant reduction in impact toughness within a specific temperature range during tempering. For martensitic stainless steels, the brittle temperature range typically falls between 350°C and 550°C, with the severity of embrittlement depending on alloying elements, cooling rate from the tempering temperature, and the specific microstructural state. The presence of certain elements such as phosphorus, tin, and antimony acts as embrittlers, segregating to grain boundaries and promoting intergranular fracture during slow cooling through the brittle range.

The researchers systematically investigated the effects of tempering temperature, holding time, and cooling method on the overlay layer properties. Different tempering temperatures produce distinct microstructural outcomes: low-temperature tempering (150-250°C) preserves high hardness with modest toughness improvement, medium-temperature tempering (400-550°C) achieves the best toughness but risks brittleness, and high-temperature tempering (550-650°C) produces tempered martensite with moderate hardness and good toughness but potential for secondary hardening effects.

Key Technical Parameters and Microstructural Evolution

Analysis of Temper Brittleness Mechanisms

The temper brittleness phenomenon in the 9Cr13 overlay layer involves a complex interplay of thermodynamic and kinetic factors. During tempering at elevated temperatures, alloying elements and impurities migrate to grain boundaries, creating compositional gradients that weaken intergranular cohesion. The cooling rate from the tempering temperature determines whether these segregated elements remain at grain boundaries or are partially dissolved back into the matrix. Slow cooling through the brittle temperature range allows maximum segregation and maximum embrittlement, while rapid quenching from the tempering temperature minimizes segregation and preserves toughness.

The overlay layer presents additional complications compared to homogeneous steel components. The dilution between overlay and substrate creates a composition gradient in the transition zone, where the effective alloying content varies across the interface. This compositional variation means that different regions of the overlay experience different temper brittleness susceptibilities, potentially creating localized weakness zones at the overlay-substrate boundary. Furthermore, the residual stresses from welding interact with the tempering-induced microstructural changes, potentially exacerbating or mitigating the brittleness effects depending on the stress state.

Heat Treatment Cycle Optimization

Based on the metallurgical analysis, an optimized heat treatment cycle for 9Cr13 roller overlays would involve:

1. Stress relief annealing at 200-250°C to remove welding residual stresses while preserving martensitic hardness.
2. If higher toughness is required, tempering at 400-450°C followed by immediate quenching in water or oil to avoid the brittle temperature range.
3. Avoidance of prolonged holding at 500-550°C where temper brittleness is most severe.
4. Consideration of multi-stage tempering with intermediate cooling to minimize grain boundary segregation.

The selection of the optimal tempering cycle depends on the specific service requirements of the roller application. For rollers subjected to high impact loading, maximum toughness is prioritized, requiring careful avoidance of the brittle temperature range. For rollers in primarily abrasive wear service, maintaining higher hardness may be more important, favoring lower tempering temperatures that preserve martensitic strength.

Study Insights and Engineering Implications

This research highlights a critical but often overlooked aspect of overlay welding quality: the heat treatment cycle that follows the welding operation can be just as important as the welding process itself in determining final performance. The temper brittleness phenomenon demonstrates that achieving the desired mechanical properties requires not only selecting appropriate overlay materials and welding parameters but also carefully controlling the post-weld thermal cycle. For roller applications in metallurgical, mining, and processing industries, where rollers are frequently repaired through overlay welding in the field, the ability to apply proper heat treatment is often limited by equipment availability and component accessibility.

The findings suggest that for field-repaired rollers, a simple stress relief treatment at moderate temperatures may be the most practical approach, accepting somewhat reduced toughness in exchange for process simplicity and reliability. For critical applications where full mechanical property restoration is required, dedicated heat treatment facilities capable of precise temperature control and rapid quenching are essential. This work contributes to the development of practical heat treatment guidelines for overlay-repaired components, bridging the gap between laboratory metallurgical understanding and real-world manufacturing constraints.