Ultrafast cooling of medium carbon steel strip by air atomised water sprays with dissolved additives

Ultrafast cooling is a newly developing heat treatment technique that can be used for the production of advanced high strength steels. Conventional laminar jet cooling is generally fitted in the runout table; however, it cannot provide the higher cooling rates necessary, and air atomised spray cooling can be the best alternative for ultrafast cooling rates. As water is the commonly used coolant, it is beneficial to know the effect of surface active or volatile additives on the cooling characteristics for high heat flux applications. Thus, this study examines the two aspects of the use of enhancement of air atomised water spray cooling process with a mixture of ethanol and surfactant additives. First, additive based cooling of stainless steel strip was evaluated to optimise the cooling rate, then these optimum parameters were used to study the improvement in mechanical properties and microstructure of medium carbon steel compared to pure water ultrafast cooling. The cooling experiments were carried out at a temperature above the Leidenfrost point of the hot strip. A commercial inverse solver called INTEMP was used for the calculation of surface temperature and heat fluxes. It was observed that the addition of surfactants and alcohol enhanced the cooling rate and critical heat flux of the test strip and has higher impact on martensite phase evolution.