Investigation of heat transfer coefficient during quenching in various cooling agents

• The highest HTC during quenching in mineral oils occur in temperature 520–550 °C.
• The peaks of HTC for polymers exist at lower temperature compared to mineral oils.
• Temperature and utilization time of coolant affect the heat transfer coefficient.
• Ageing of mineral oils affects their severities; a direction of change is equivocal.

Heat transfer coefficients, HTCs, at the surface of a metal sample during immersion quenching were measured and evaluated using numerical procedures. The boundary inverse heat conduction problem has been defined and solved. A FEM self-developed computer code has been used to obtain a solution to the direct problem. The sensitivity of the method enabled us to examine the effect of various quenching parameters on the heat transfer for two mineral oils and a polymer quenchant. At 800 °C the HTC values were equal to ∼0.5 kW/(m2 K) and ∼3.2 kW/(m2 K), for mineral oils and a polymer coolant, respectively. They increased to ∼4.7 kW/(m2 K) – oil A, ∼6.0 kW/(m2 K) – oil B and ∼7.4 kW/(m2 K) – polymer, respectively. The peak of HTC was sharp and occurred at a narrow temperature interval between 520 and 550 °C for the oils, whereas for the polymer, the peak was lower by approx. 100 K and flat over 100–120 K interval. Subsequently HTC decreased, and at ∼150 °C the values were ∼0.5 kW/(m2 K) and ∼2.0 kW/(m2 K), for mineral oils and a water polymer coolant, respectively.