21-Mat-A6 Materials Selection and Design for Materials Processing · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
The alloy chosen is a medium-carbon (1040) steel requiring stress-relieving, normalizing and tempering treatments across the stated 425–750 °C range, at a moderate industrial production volume (mixed part sizes, batch-to-batch flexibility valued over single-part maximum throughput).
An electrically heated, gas-tight batch box (or car-bottom, for larger loads) furnace is the most economical choice at this scale and flexibility requirement, over a continuous (roller-hearth or pusher) furnace. Continuous furnaces are more capital-intensive and are only economical when a single part geometry runs at high, steady volume; a batch furnace instead lets the same installation switch between stress-relief, normalizing and tempering loads at different temperatures and hold times without re-tooling, which matches a heat-treater serving mixed orders.
Electrical resistance heating (Ni–Cr alloy elements, adequate to 750 °C with wide margin) with a multi-zone, closed-loop PID controller referenced to type-K thermocouples distributed through the load and the furnace chamber gives the tightest, most repeatable control (typically ±3 °C) of the options available, and is simpler to instrument and automate than a gas-fired system, which needs additional air/fuel ratio and trim control to hold the same tolerance. The modest maximum temperature (750 °C) keeps element life and electricity cost reasonable, favouring electric heating's control advantage over any fuel-cost advantage gas firing might otherwise offer at higher temperatures.
A dry, low-dew-point, slightly reducing endothermic or nitrogen–hydrogen blended atmosphere (rather than air) protects the steel surface from oxidation and, importantly in this carbon range, from decarburization during the longer holds associated with normalizing and stress-relief; a pure inert (dry N2) blanket is an economical minimum for the lower-temperature stress-relief loads where scale formation is the main concern rather than active decarburization control.
Continuous monitoring of the atmosphere's dew point (or, for endothermic gas, an in-line oxygen probe/CO2 infrared analyser) against a target carbon potential, with automatic trim of the gas generator's air/fuel mix, keeps the surface carbon content of the steel matched to the bulk and avoids both decarburization and unwanted surface carburization. A slight positive furnace pressure, maintained by continuous gas purge flow, prevents ambient air ingress at door seals and loading openings, which is the dominant, cheaply avoidable leak path for atmosphere contamination in a batch furnace.
An attached vestibule (cooling chamber), maintained under the SAME protective atmosphere as the hot zone (or purged with dry N2) and fitted with a recirculating fan for controlled forced-air or fan-assisted cooling, allows the load to cool from the treatment temperature down through the range where oxidation/scaling is still active before it is ever exposed to ambient air. Since the whole economic point of a controlled atmosphere in the hot zone is defeated if the part is pulled out hot and scales in the doorway, the cooling chamber is what actually delivers the bright, scale-free as-processed surface finish the treatment is paying for, and avoids a separate downstream pickling/cleaning step.
