May/June_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 | M A Y / J U N E 2 0 2 0 6 0 T hru-process” temperature profiling in the heat treat- ment industry has been around for over twenty years and the principle of operation of these systems is gen- erally well known. A multi-channel data logger, protected by an insulated thermal barrier, travels through a furnace mon- itoring the products being heat treated (Fig. 1). At the end of the process the complete temperature profile can be exam- ined, and critical calculations made using analysis software. This thermal fingerprint is unique and critical to the under- standing, control, improvement, and validation of the heat treat process being undertaken. BENEFITS OF THRU-PROCESS PROFILING Prior to the development of these systems, long “trail- ing” thermocouples were often used to determine the actual product temperature profile through continuous furnaces. Feeding thermocouples through a continuous furnace had obvious disadvantages, mainly the difficulty of the operation itself, the limited number of thermocouples that could be used, disruption to production, and the accuracy of the data, given that products could not follow the test basket into the furnace (due to the trailing thermocouples), so the furnace loading decreased as the trial progressed. As the thru-process method was adopted, the monitor- ing operation simplified, the disruption to production was minimized, and the measurement could always be carried out in a fully loaded furnace reproducing actual product con- ditions. Data obtained from thru-process profiling trials gives an accurate assessment of how long a product soaked at a specified temperature, the differences in product tempera- ture around the product basket, quench rates, and more. This data is used to calculate performance against specifica- tion, investigateprocess problems, andoptimize theprocess. MONITORING CHALLENGES Although the operating principle of these systems seems relatively straight forward, with the evolution of fur- nace technology and drive for automated systems, the de- sign is often complex, as the thru-process system needs to meet the unique challenges that come with different heat treatment processes. THRU-PROCESS TEMPERATURE PROFILING AND SURVEYING Data obtained from thru-process profiling is used to calculate performance against specification, investigate process problems, and optimize the heat-treating process. Steve Offley, PhoenixTM Ltd., U.K. 4 Thru-process temperature monitoring system loaded in a product basket traveling into and through a continuous sealed gas carburizing furnace. When designing or selecting the most appropriate thru-process system the following criteria need careful consideration: • Space or clearance in the furnace – Small enough to fit but with enough safe protection. Designed to either allow direct transfer by robot with or internally within the test product. • Furnace temperature – Materials (metals and insulation) that will survive temperature maximums and rapid temperature changes and routine temperature cycling. • Process duration – Protection to cover process delays whether anticipated or not. • Atmosphere in the furnace – Systems capable of handling difficult atmospheres (such as hydrogen) which may make thermal protection more challenging due to increased heat transfer. Design of systems to eliminate contamination/poisoning of critical controlled atmospheres (such as aluminum brazing (CAB)). • Quench within the process – Technology capable of handling not only heat but designed to allow safe “