A Review in the Fundamentals of Heat Treating

30 May 2018

Much has been covered in past posts. The basics of heat treatment technology formed a foundation, then that groundwork received new and interesting additional levels. They dealt with surface hardening issues, quenching anomalies, and all manner of tricky alloy manipulating techniques. Now we’re reinforcing the groundwork and reviewing the fundamentals of heat treating science, plus many of the physical traits that characterize this essential post-processing phase.

Property Manipulating Essentials

Heat treatment engineers master many alloy altering disciplines, yet they never lose track of one basic truth. Qualified to wield all sorts of advanced engineering equipment, the goal is still to develop a raw metal workpiece until it exhibits the desired material profile. On balance, pure metals don’t slide into a furnace. No, it’s a specially amalgamated alloy that undergoes the process. Put it this way, thanks to added carbon, to property-altering trace metals, that selected material comes to the heat treatment area with a unique fingerprint. Faced with this specially tailored alloy, it’s the job of the engineering team to process that workpiece until it’s imbued with a nominated set of mechanical and physical properties.

In-Process Stress and Desirable End Properties

After the attached ASME or SAE-AISA labelling codifiers are interpreted, the project gets underway. The goal is to convert the microcrystalline structure of the alloy, to change it so that it becomes corrosion-resistant and harder. Other essential material properties include ductility and tensile strength. Take heed, processing environments and machine shops can also inadvertently alter the structure of a worked metal part. They cut and bend, drill and mill components until cold-worked stress is trapped inside the part. Therefore, not only must the heat treatment equipment add the desired end-process material characteristics, it must also dissolve in-process work stress.

The Different Heat Treatment Options

At the most fundamental level, there are furnaces and quenching pools. The furnace hardens or softens the part. Alternatively, the outer surface is subjected to a case hardening procedure. Next, the alloy part is rapidly cooled. The quench operation locks in the desired material grain, and thus the sought after metallurgical traits are achieved. Of course, a hard component isn’t much use if it’s going to fracture. To avoid this brittle condition, the alloy needs to be tempered. As the furnace imparts an element of toughness to the tempered material, it ends up malleable and workable.

Reviews of fundamental processes are always helpful. Still, that groundwork can’t hide the complex issues that come into play when the furnaces are lit. There’s numerous alloy grain types, heat treatment systems, localized flames, all-covering induction ovens, and vacuum or atmospheric gases employed during a working heat treatment operation. Driving away below these complex elements, however, the fundamentals keep the process in check.

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