Nov_Dec_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 0 6 0 E ach and every daymillions of gallons of synthetic aque- ous quenchants are cycled, filtered, cooled, or heated, as process cooling media for heat treating both ferrous and non-ferrous parts. A common method for quenching parts is to utilize a synthetic aqueous quenchant such as polyalkylene glycol (PAG) in water. Cooling rates, as defined by Newton’s law of cooling formula , show that the cooling coefficient has an exponential role in heat transfer. The per- formance of the quenchant, the delivery device between hot part and ambient conditions, depends on several variables with quenchant concentration being a prominent factor in the cooling rate. Methods of checking the quenchant’s concentration have advanced into the digi- tal world yielding a higher level of process control and cost savings available, whether it is for a new installation or retrofit to all aqueous quenchant based heat treatment processing systems. QUENCH CONCENTRATION Determining the role of quench con- centration as a key factor for heat treat- ment process control begins with the eval- uation of the formula for Newton’s law of cooling: T = T ambient + (T initial – T ambient ) * exp(-kt) (Eq 1) Where: T [K] is the temperature of the object at the time t T ambient [K] is the ambient temperature of the object T initial [K] is the initial temperature of the object k [1/s] is the cooling coefficient t [s] is the time of the cooling The cooling coefficient k[1/s] or the transfer coefficient is the proportionali- ty constant between the heat flux (W/m²) DIGITAL REFRACTOMETRY OFFERS IMPROVED PROCESS CONTROL FOR HEAT TREATMENT A digital refractometer gives more accurate quench concentration readings thereby reducing maintenance, increasing productivity, and ensuring desired metallurgical properties. Robert John Madeira* Inductoheat Inc., Madison Heights, Michigan and Inductotherm Heating & Welding Mexico *Member of ASM International and the thermodynamic driving force for the flow of heat (T initial – T ambient ). The overall heat transfer rate is usually ex- pressed in terms of an overall conductance or heat transfer coefficient. The heat transfer coefficient can be expressed as W/(m 2 (T initial – T ambient )). The cooling rate of the quench fluid is expressed in F ° /sec when plotted against tempera- ture and time for the heat transfer function. The cooling coefficient for quenchants is well de- scribed and Fig. 1 shows cooling curves for a common aqueous quenchant, Aqua-Quench 245. The cooling rate data covers the three phases of quenching, vapor, nucle- ate boiling, and convection phases. It is evident from the Fig. 1 — Cooling rate curves for various concentrations. Courtesy of Quaker Houghton. 6