Effects of accelerated microstructural processes on toughness and fatigue behaviour in heat-treated 51CrV4 spring steel

Kotous Jakub - COMTES FHT a.s. (Czech Republic)

Generally, the underlying motivation for shortening the manufacturing process is cost saving. Sometimes, this proves unfeasible. Final quality is often directly proportionate to the time expended. This is the case with soft annealing. The effects of spheroidisation annealing are diffusion-controlled, which means the process is time-consuming. When the annealing times are reduced, the proportion of spheroidised carbides decreases. Development and optimisation of soft annealing processes in furnaces led to a progress which cut the process times by several hours thanks to steps involving partial or full austenitizing. Still, the treatment requires several hours to complete. Recently, research into and development of the ASR (Accelerated Spheroidisation and Refinement) at COMTES FHT led to a spheroidising process with durations on the order of minutes. The ASR relies on rapid induction heating and cycling around Ac1. Alternatively, the process can be induced by thermomechanical treatment. In the absence of long holding times, grains and carbides cannot coarsen, leading to much finer microstructures than soft annealing. This is manifested in the subsequent quenching process and the final mechanical properties.
The present experiment focused on comparing mechanical properties of heat-treated ASR-spheroidised material and conventionally soft annealed material. The effects on fracture toughness and fatigue behaviour were closely monitored. The experimental material was 51CrV4 spring steel.

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