Introduction: How to Temper Steel

Introduction:

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.

Safety:

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:

  • Blowtorch

  • Two, six inch 1040 steel rods

  • Fire extinguisher

  • Safety glasses and gloves

  • Cold tap water in a large bucket

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.

Comments

author
PardoByNight (author)2017-04-16

Very informative :)

author
caruncles (author)2017-03-06

Good presentation! However, if you are forging something more valuable you need to quench in oil. Cooling the metal too fast can cause it to crack or de-laminate, especially if you forge different pieces together.

I like the mini-forge!

author
Mistwalker (author)2017-03-05

This isn't tempering. This is heat treating to harden steel. Tempering is done after heat treat to reduce hardness of steel so that it isn't brittle.


This method only applies to certain steels. Heat treating and tempering vary greatly depending on grade and type of steel.

author
raverys12 (author)2017-03-05

thanks for the demo, but I'm afraid there is much more to this than mentioned here in your demo. Just fore the viewers .... the are many and I mean many different grades of steel. Water , Oil & Air hardening types. For the most part many from day to shop work will be exposed to non harden able cold roll steel. This can be surface hardened as you have explained with help of adding carbon to the material. For those that want to achieve this proscess without a lot of work. There is a product on the market called Kasenit. It's been around for a while. Makes easy work to case harden cold roll steel. By the way when your bending material back and forth and it till it breaks ... this is called work Harding. This happened in just about any type of material. Even brass. Hope this adds to your project. Take care.

author
DaveE63 (author)2017-03-05

Nice simple demonstration!

author
submariner_ss (author)2017-03-05

how long can you hold the rod with bare fingers?

author
Bradscopegems (author)2017-03-05

This is a great demonstration. Presumably you have to start with a low-carbon mild steel, or the rods will not bend easily. You could extend the demo by smearing the central part of the rod with case-hardening compound to introduce extra carbon before heating, then clean up and check the strength and hardening before and after annealing. As jtobako says, the interference colours are also instructive: yellow is a good tempering colour, blue is a nice decorative one which apparently gives a little corrosion protection. With such a thin rod the interference colours may run along very quickly, so heat carefully when tempering!

author
GavinS38 (author)2017-03-05

Another method would be to put it in the oven (after bending) at 300 degrees Celcius for 20 minutes. Then take it out the oven and let the air cool it down. That way it will not be so brittle. This is obviously more suited to spring steel than mild steel. At least most people have an oven handy :) In general the bend will close more after the heat treatment, so if 90 degrees is required an initial bend of 91 or 92 degrees might be more suitable.

author
jtobako (author)2017-03-02

Sounds like a science lab : )

The steel becomes brittle, but harder - you may want to try a scratch test with a file (glass is usually to hard).

Show that annealed metal is softer (heat to red and let cool) and that it can be hardened again.

If you take and harden the steel, remove the black iron oxide with sandpaper, then heat until you get a rainbow of colors on the steel (test the particular steel, not all show the colors well) with different hardness as the color changes.

author
cerberustugowar (author)2017-03-02

you could always temper back the hardness so it is a nice balance of strength and pliability. It's usually done in a regular oven

author
seamster (author)2017-03-02

Good info, thank you for sharing :)

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