View RSS Feed

Beo

Tempering Steel

Rating: 2 votes, 4.00 average.
Tempering steel for your flintlock
By Beowulf65

Many of us will eventually make parts for our flinters which require hardening. This presentation is an attempt to help people understand what they are doing and how to accomplish it. A more complete explanation can be found in Machinery's Handbook in its many editions.
Tempering refers to heating steel above a critical temperature, then cooling it rapidly to freeze it in a very hard state followed by rewarming it to an intermediate temperature to give a hardness suitable for the job intended. There are several technical points to grasp;
1. Iron requires some carbon in it in order to be hardened. This is normally between 0.4% and 1.2% carbon. The carbon is present in the form of iron carbide scattered amongst the iron molecules.
2. Heating the metal above a critical temperature (usually red hot and non-magnetic) causes this iron carbide to mix uniformly with the iron in a non-crystalline form (called austenite). Slow cooling at this point (eg buried in wood ash or lime) results in a soft layered crystalline structure and is referred to as annealing. Rapid cooling in water or oil results in a very hard crystalline structure (called martensite). The creation of either extreme requires, however, heating to the critical temperature first to form austenite.
3. Workable properties are achieved by reheating the martensitic iron to an intermediate temperature which alters the crystalline structure and makes the metal softer with progressively higher temperatures.
So much for theory. In practice you can buy the raw material with a known carbon content or test scrap material around your shop for carbon content and use what appears to be high carbon. Shovel blades make excellent springs, for example. High carbon steel touched on a grind stone will give a shower of fizzy, bursting sparks. I suggest trying this on a file as an example and compare it with what you know to be low carbon such as angle iron.
After shaping the part to be hardened to its final form, polish it and heat it red hot. Plunge it into water until cool. For long round parts put them in a drill chuck and quench them spinning to prevent warping. A long piece of steel quenched on its side will shrink rapidly on the cool side (and bend towards the cool) then freeze so that the shrinking of the opposite side is unable to pull it straight again. After quenching, test the metal with a file ; it should be hard enough that the file slips over it without cutting. Repolish the surface with oiled abrasive paper or steel wool.
I use molten wheel weights (because they melt at a lower temperature than lead) to draw the temper of the work to a useful hardness. I use an electric lead pot and after plugging it in, place the thoroughly dry work on top of the lead and allow it to warm slowly as the lead melts. If you have a lead thermometer, use it to measure the temperature of the lead. If not, float a thin piece of polished steel on the lead and watch the colour changes (thin metal changes temperature more quickly). When the colour of this thin piece indicates the correct maximum temperature, pull the plug on the pot so that the temperature will not rise further and totally immerse the work in the lead for 2 -3 minutes. Then take it out and shake any loose lead off and allow it to cool. Finish by cleaning any remaining lead off with steel wool or abrasive paper.
All iron oxidizes to different colours, depending on temperature, and this can be used as a crude thermometer. The colour progression from cool to hot is; straw to brown to purple to darkening blue to dark blue to light blue to grey. This corresponds to about 450 degrees Fahrenheit for straw to about 650 degree Fahrenheit for light blue/grey. These colours should also be compared with your particular lead thermometer
I find I use 2 colours/temperatures primarily in practice. For cutting tools such as threading taps, milling cutters, and for gun parts of minimal wear such as sear noses and tumblers I use dark brown or about 500 degrees Fahrenheit. For springs and things flexible such as the necks of goose neck chisels, I use light blue/grey which on my thermometer is about 660 - 670 degrees F. I feel it is far easier to reharden a soft spring than to refile a broken one.
Hope this helps anyone who needs the help.
Beo,

Submit "Tempering Steel" to Digg Submit "Tempering Steel" to del.icio.us Submit "Tempering Steel" to StumbleUpon Submit "Tempering Steel" to Google

Categories
Uncategorized

Comments