Heat Treating 101
Heat treating procedures are usually quite simple in terms of procedure and application. However, sometimes descriptions might seem inadequate as in many situations one may use the same technique to obtain different objectives. The best example for this would be stress relieving and tempering. Both are accomplished with the same equipment and use the identical time/temperature cycles however, the objectives for the two processes are inherently different. To help you to gain a better understanding we have outlined the principals of the heat-treating processes below.
Types OF HEAT-TREATING PROCESSES
Normalizing, by definition, is a heat treatment process where a material is heated to a pre-decided elevated temperature, held at that temperature for a certain period of time (usually 10-20 minutes), and then allowed to cool freely in the air to reach room temperature. The normalizing process is usually applied to metals that have been subjected to thermal or mechanical hardening processes such as forging and require their microstructure to be normalized.
Normalizing steel essentially resets the carbides back to uniformity. If the carbides have gotten all bunched up and oversized from forging, the steel won’t hold an edge as it potentially could. This process is primarily intended and beneficial for steel that has been forged.
Annealing is a heat treatment process that’s primary goal is to be able to manipulate and form metals by reducing their hardness. Annealing occurs when metal is heated above its re-crystallization temperature, held at an appropriate temperature for a prescribed set of time and then allowed to cool naturally. The primary method utilizes an oven because it has accurate control over the temperature throughout the process. The three steps to annealing are the recovery stage, re-crystallization stage and grain-growth stage. Each of these stages will allow you to reach the end game which is ultimately to make something bad-ass. It’s physics yo!
The heat treatment process known as hardening is used to enhance the hardness of the metal’s surface by heating and rapid cooling. The material is heated in a oven or furnace to a temperature that transforms its internal structure without melting it. The metal is then held at this temperature for one hour per every inch of thickness and then cooled rapidly. By cooling it quickly the metal establishes a harder, more stable crystalline structure. A common list of heat-treating recipes can be found here: https://www.americanrotary.com/wp-content/uploads/2021/10/Common-Heat-Treating-Recipes.pdf
There are many different types of tempering. The most common tempering process is known as differential tempering. Differential tempering is where the process produces a hardness which is non-uniform. This process is used when making swords, knives or other tools. The type of steel determines what the critical temperature is and how long they need to keep the material at that temperature. Other variables that impact tempering include the length of time the metal stays at its critical temperature as well as the rate of cooling. Both of these factors can significantly impact the mechanical properties of the material.
You want to temper your steel as soon as it gets to room temperature. Temper twice at 2 hours each allowing the steel to cool back to room temperature between cycles.
- 450F yields approximately 62RC
- 500F yields approximately 59-60RC which is where most will want this steel.
- 600F yields approximately 58RC
Cryogenic processing is performed by slowly cooling parts in a controlled bath of liquid nitrogen or a freezer, holding the parts until equilibrium is reached with the liquid's temperature. Nitrogen is a liquid at -320℉. Cryogenically processed parts are generally held for one hour per inch of thickness. This is a common process used in the production of blades to make them more durable and help prevent fracturing and/or chipping.