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Heat Treatment: For Knives or Knockout Drums

By Marc McConnell, P.E., Metallurgy and Fixed Equipment Engineering Coordinator at Pro-Surve Technical Services, and Andrew Bohm, Mechanical Engineer at Fulcrum Reliability. This article appears in the January/February 2019 issue of Inspectioneering Journal
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Editors Note: Here are a few definitions, in context, to assist the reader, courtesy of Wikipedia:

  • Ductility: a measure of a material's ability to undergo significant plastic deformation before rupture, which may be expressed as percent elongation or percent area reduction from a tensile test. 
  • Strength: the strength of a material is its ability to withstand an applied load without failure or plastic deformation. 
  • Brittle: A material is brittle if, when subjected to stress, it breaks without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength.

The Origins of Heat Treating

Civilization as we know it today would not have been made possible had man not learned how to utilize iron and steel to the high degree of which it is currently employed.  Steel can be heat treated so that it is hard, springy, or relatively soft. In ancient time, when all combat was face to face, a warrior depended on the quality of his sword.  He would neither want it to break nor bend in battle. As a result, the sword quality often resulted in a situation of life or death during combat.

Make no mistake; the consequences of improperly heat-treated pressure vessels can be more substantial than a broken blade in battle.  As is often the case, knowledge is power. Ancient swordsmiths understood the techniques to selectively harden and temper blades capable of holding an extremely tough edge that could pass a 90-degree flex test.  Modern heat treatment draws on these early pioneers and aims to resolve the same metallurgical issues. In the more than one thousand years since the early blades were formed today’s processes have improved greatly, but mistakes are still made, proving the age-old saying “just because something is common sense doesn't mean it is common practice.”

The Creation of Steel

Early swordsmiths probably didn't realize that, through the process of smelting, they were introducing trace amounts of carbon into the iron that they were purifying from iron ore.  Adding carbon to iron in the proper quantities gave rise to the vastly superior alloy commonly known as carbon steel. However, even when the correct amounts of carbon and iron were mixed together, the steel still required proper heat treating to become a usable sword.  In much the same way, a vessel built with the appropriate metallurgy is simply one step in the right direction. Appropriate heat treatment of the steel is essential to obtain the properties required for full-service applications. A good sword has to be hard enough to hold an edge along its length.  At the same time, it must be strong enough and flexible enough that it can absorb massive shocks at just about any point along its extent and not crack or break.

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