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

Literature Overview

This study by Li Tao and Wang Xiangjie (2018, Foundry Technology, Vol. 39, No. 1, pp. 124–127) investigates the influence of post-weld tempering temperature on the microstructure, hardness, and toughness of an overlay weld deposited on 9Cr13 roller steel using a Cr-Ni-Mn system flux-cored wire. The research was funded by the Hubei Polytechnic University Talent Introduction Program (Project No. 14xjz02R). The work addresses a practical problem in the maintenance and refurbishment of steel mill rollers, where overlay welding is used to restore worn surfaces and improve wear resistance.

Core Technical Content and Microstructural Evolution

The authors systematically varied the tempering temperature from 50°C to 600°C and characterized the resulting microstructure using metallographic examination, hardness testing, and impact toughness evaluation. The key findings reveal a complex relationship between tempering temperature and mechanical properties, with a distinct temper brittleness temperature range identified.

Tempering Temperature Range	Microstructure	Hardness Trend	Toughness Trend	Fracture Morphology
50–200°C	Coarse lamellar tempered martensite	High	Low	Brittle fracture
300–550°C	Progressive reduction of lamellar martensite	Decreasing	Increasing	Mixed mode
550°C	Fine lath martensite + dispersed fine particles	Moderate-high	High	Ductile dimples
600°C	Large granular structure	Lower	Decreased	Reduced toughness

The temper brittleness temperature range was determined to be 150–200°C based on impact toughness values, which is consistent with the known Type I temper embrittlement phenomenon in low-alloy steels containing Cr, Ni, and Mn.

Metallurgical Analysis and Mechanism Discussion

The microstructural evolution observed in this study follows the classical tempering sequence for martensitic steels, but with specific characteristics related to the overlay welding process:

At low tempering temperatures (50–200°C), the martensite formed during rapid solidification and cooling of the overlay weld undergoes only partial decomposition. The retained carbon in the martensite provides high hardness but extreme brittleness. The coarse lamellar morphology indicates that the overlay weld experienced a relatively slow cooling rate compared to the base metal, possibly due to the thermal mass of the roller.
In the intermediate range (300–550°C), progressive tempering decomposes the martensite into tempered martensite with increasingly refined carbide precipitates. The transition from lamellar to lath morphology at 550°C is particularly significant — it suggests a transformation from a non-equilibrium to a more equilibrium microstructure, with improved dislocation mobility and crack deflection capability.
At 600°C, over-tempering leads to coarsening of the carbide precipitates and softening of the matrix. The large granular structure indicates excessive carbide growth, which reduces both hardness and toughness through different mechanisms — the former by reducing solid solution strengthening, the latter by providing crack propagation paths.

The identified temper brittleness range of 150–200°C is attributed to Type I temper embrittlement, which is associated with the segregation of phosphorus and sulfur to grain boundaries during tempering. The Cr-Ni-Mn composition of the flux-cored wire promotes this phenomenon, as chromium increases the solubility of phosphorus in austenite, facilitating its segregation during cooling.

Engineering Practice and Process Recommendations

For roller overlay welding applications in steel mills, the following process recommendations emerge from this study:

Avoid tempering in the 150–200°C range during any post-weld heat treatment, as this temperature window maximizes temper brittleness.
Optimal tempering temperature of 550°C provides the best balance of hardness and toughness, with a microstructure characterized by fine lath martensite and dispersed fine carbides.
Two-pass tempering may be beneficial: an initial temper at 550°C followed by a low-temperature temper at 250°C to relieve residual stresses without entering the embrittlement range.
Cooling rate control after tempering is critical — slow cooling through the 300–500°C range in air-cooled furnaces is preferable to avoid secondary precipitation effects.

For quality assurance purposes, the following inspection requirements should be specified:

Hardness measurement at multiple locations across the overlay surface, with acceptance criteria of 35–45 HRC for the 550°C tempered condition.
Impact toughness testing at the tempering temperature, with a minimum Charpy V-notch value of 27 J at 20°C.
Metallographic examination to verify the absence of coarse lamellar martensite and the presence of the expected tempered structure.

Study Insights and Implications

This paper provides a clear, practical framework for post-weld heat treatment of roller overlay welds. The identification of the temper brittleness temperature range is particularly valuable for engineering practice, as it prevents a common and costly failure mode — brittle fracture of the overlay layer during service. The systematic characterization of microstructure evolution across a wide tempering temperature range offers a comprehensive understanding of the metallurgical behavior that can guide process development for similar overlay welding applications.

One area for further investigation would be the effect of tempering time at the optimal 550°C temperature, as the study focuses primarily on temperature effects. Additionally, the long-term stability of the microstructure under thermal cycling conditions representative of roller service would strengthen the practical applicability of the findings. For engineers involved in roller refurbishment, this paper serves as an essential reference for establishing post-weld heat treatment specifications that ensure both wear resistance and fracture toughness of the overlay layer.