Hello I have recently started making knives. I am forging the blade from car coil springs (not leaf). Since I am using a coal forge there is no exact temperature.

When We temper we use a mixture of mineral oil and 50 w motor oil. We temper each blade 3 times the first is a a critical temperature the metal is almost yellow throughout second temper has an orange glow and the third is just a soft uniform color.

I am wondering how I can have my knives rockwelled as I believe they are well over 55.

They hold a very keen edge and do not dull chip or shatter. What are some of your opinions about such a method.

I realize I am very new at this and any input is appreciated.

First I should mention that the procedure you describe is "hardening" or "quenching", tempering is a separate step where it's reheated to a much lower temp (400F-500F is typical) to remove the brittleness from the freshly hardened blade along with a slight amount of hardness.

Now to answer your question, a real rockwell tester is expensive, so you might not want to get one of those. Instead, they make a set of files you can buy that each have a different hardness. You should be able to tell the hardness within a few points by using them.

Coil springs are generally 9260 steel. Very similar to 5160, but the "92" designates it as a "silicon" steel, which should make it very ductile.

As mentioned hardening and tempering are two totally different operations, both necessary to achieve top results.

In order for a blade material of this type to harden, the steel must be brought to "critical temp" prior to quenching. For 9260 this temp is approx. 1550F. Using a magnet to help determine when critical temp is reached is a simple method that rarely fails. (when the steel reaches critical temp, it looses it's magnetic attraction)

Simply stated hardening is bringing the steel to it's hardest possible state. There are a number of other things that occur within the steel's matrix, but I won't get that involved here.
Once the steel has achieved it's highest possilbe hardness, and because high hardness also brings along with it brittleness, the blade must be tempered. Tempering is nothing more than a "controlled softening". This is achieved by placing the hardened blade in an oven, and allowing it to "cook" at a specific temp. For 9260 I would recommend 350F to 425F for a minium of two hours. This operation removes enough brittleness so that the blade edge will not chip or break, but will retain enough hardness for a durable edge. You will need to experiment with exact tempering temps to achieve the end you desire.

Each steel has it's own critical temp, as well as it's own tempering temp. With tempering temps being based on the steel type and how hard or soft you wish the finshed edge to be.

Thank you for your input, as i stated I am new to this art and have a lot to learn. I am achieving a very nice hardness to the blade. I want it to hold an edge well so I do not want to lose too much hardness when I temper. I will try tempering in an oven as you suggested. I would say my first quench is near 1500F second 900F and 3rd 500F. On my first knife I did not temper in an oven as you suggested but have had no problems with chipping as of yet.

Actually since the 900 and 500 are below critical temp, you were actually tempering when you requenched, that's probably why it's not chipping at all for you. Those temps are kinda high for typical tempering, but it's time + temp so maybe you're doing it quick enough to work ok. You can do multiple quenches if you want (some people swear by them, others think it's hogwash), but your last quench needs to be above critical for best results. Be careful using color to judge temp, your ambient lighting will change what the steel's color looks like very easily with the slightest change, use a magnet to be sure.