Introduction: How to Temper Steel
Tempering is a type of heat treatment for iron-carbon alloys. These alloys are more formally called steel. In general, the process for heat treating steel is accomplished by heating, rapid cooling, and reheating of the chosen material. When steel is cooled quickly, the atoms are “frozen” in an unstable position. The steel is very hard, but very brittle. By tempering steel, the atoms are allowed to rearrange into a more stable position. The hardness of steel is sacrificed for ductility (the materials ability to stretch or deform). The end result is a material with increased strength and toughness. Given these properties, it allows for the material to bend before fracturing and not break in a brittle manner, which can be beneficial in many structural applications. The steps performed are a very general outline to show the how some mechanical properties of steel increase with tempering. Different compositions and other steel alloys may require different heating times and temperatures as well as different quenching mediums depending on the use of the material. For this general procedure, allow 10-15 minutes for completion.
More steps to ensure proper safety may need to be taken if steps deviate from what is given in this general procedure. Fire will be coming from the blowtorch, so use extreme caution when heating the the steel. Finally, it is recommended that safety gloves be worn at all times, especially for more complex treatments.
Things you'll need:
Two, six inch 1040 steel rods
Safety glasses and gloves
Cold tap water in a large bucket
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Step 1: Bend Rods
Make sure that the environment safe and clear from any potential hazards. Take the two steel rods and bend into a “U” shape. Be careful not to bend the rods too much and fracture the steel.
Step 2: Light the Blow Torch
Turn on the gas and light the blow torch. Be sure to have the blow torch pointed away from your body as shown. Make sure no one else is within three feet of the torch while it is lit. Have the fire extinguisher nearby in case of an emergency.
Step 3: Heating
Place the curved part of the rod (the “U”) in the hottest part of the flame by holding the two ends of the steel rod. The hottest part of the flame is the small cone about halfway into the flame. Make sure you rotate the rod while it is being heated by the flame to make sure that it heats uniformly. The heat from the blowtorch “excites” the atoms into a more mobile state allowing them to rearrange.
Step 4: Quench
When the rod becomes a deep orange in color, take the rod out of the flame and quickly dip it into the water inside the bucket, as shown in the photo (called quenching). Quenching drastically decreases the atoms’ motion, and can be illustrated as “freezing” the atoms in place. At this point, the atoms are in an unstable position.
Step 5: Repeat
Repeat steps 3 and 4 with the second steel rod.
Step 6: Temper
Take the second steel rod and temper it by reheating it with the torch for about 10 seconds. Do not temper the first steel rod! Quench again by placing it in bucket of water. This allows the atoms to “relax” and arrange themselves into a more stable position.
Step 7: Straighten the Rods
Attempt to straighten both rods to illustrate the difference in mechanical properties. The first rod (the rod that was not tempered, top photo), will be very brittle and will break since the atoms are not in a stable position. The second rod (the rod that was reheated or tempered, bottom photo), will bend but the “U” shape will remain. Since this rod was tempered and the atoms were allowed to “relax” to a stable position, the material becomes stronger and tougher.