I am so often asked so many questions about steel I thought I would present a run-down on steel in laymans language, here it is. Your questions and comments are very welcome.
A well know statement was "Oils aint oils Sol" and neither is steel, steel.
Steel has been around for a very long time and heaps of info has been written about it. One of the very best reference books is "Machinerys Handbook, edition 24. All previous editions also have a tremendous amount of info on the subject. Steel comes in hundreds of different types. Each one is specificly designed for a particular application. I will only discuss the most common and most used steel types.
The steels discussed are:- Mild Steel, Carbon Steels, High Tensile Steel, Stainless Steel, Alloy and Spring Steel. Steel basiclly consists of two elements, Iron and Carbon. We will ignore most of the other elements that make up Alloy Steel. All steels contain carbon to some some degree. The carbon content ranges from < 0.01% to over 1.5%. A very small amount added makes a huge diference to the steels' properties. Unless you are seeking some specific property of steel, the carbon conten will be ignored for the moment. Mild Steel has very little carbon. It comes in "Black" which is rollled out red hot and "Bright", which is rolled out cold. Bright Mild Steel will warp and twist when you cut it from a sheet. Black MS is very stable and will retain its shape and is easy to weld and braze with brass or bronze rods.
Carbon Steels have more carbon added and they will become very hard (and brittle) when heated and dropped in water or oil. An example is Spring Steel (which is an alloy steel). When the carbon content reaches 0.4%, it can be heat-treated and made hard. As the carbon content increases, so does the difficulty of welding.
High Tensile Steel (eg. 4340) is a special Alloy Steel. It is very strong and can be heat treated to obtain and enhance its properties. There are many alloy steels and each one has a special property.
Stainless Steels are a peculiar combination of different elements. They are designed and created for just one job. To withstand corrosion in different conditions. None of these S/Steels are any good as structural members. Some can be hardened and made into a decent knife but these are the exception. The biggest problem with S/Steel is that it is soft and will stretch.

The most common question I am asked is "How can I heat-treat steel?"
Here then is the biggest can of worms ever I spilt over. To heat-treat properly, you need to be able to measure the (hot) temperatures very accurately. Just as a rough guide, the words:- Pearlite, Austenite, Matensite, Cementite, Critical Temperature, Annealing, Normalizing and many more strange words are precisely just that, strange words! But all these terms are very important and need to be understood. If you cant measure the temperature of the hot steel, then just forget the big strange words for they are all temperature dependent.

So lets use my favorite system of heat-treatment, its called the "TLAR" system and it does work. TLAR stand for:- That Looks About Right.
There are (very) basiclly, only three kinds of steel. One is hardened in water, one in oil and one in air. Forget the last one.
Machinerys Handbook is full of info for heat treatment and will give a recomendation for either water or oil for hardening the steel. Try and get the book from your favorite libary. Having made the part and decided on oil or water to quench it, lets begin the hot part of the process.
It's a great idea (and prefered) to heat the part/s within the confines of some fire bricks. This keeps the heat in and helps uniform heating. An LPG torch is the best way to heat the part. Only use oxy-acetylene if you are very skilled in its use. The job to heat-treat, is a piece of round spring steel about 9mm in diameter and 150mm long. (Hey, that sounds familiar). The first heating is to harden the rod.
Heat the bar untill it's a bright red /dull orange colour, do this in a subdued light. Bright light stops the colour identification. When "TLAR", pick it up at one end and quickly lower it vertically into a can, drum, container of oil. You need an oil with a high flash point. Take it out, wipe the oil off and try to mark it with a file. If the file "sings" over the rod, it is hard. If it bites into the steel, heat the bar again and make it hotter before quenching it. When it is hard, be very carfull as the steel is now very brittle. It must be Tempered. Tempering is simply reheating the part to a specific temerature. As the temperature is increased, so does the hardness decrease. Heat too much and you will be back at square 1 with a soft piece of steel. It will need to be hardened again.
To begin tempering, the steel must be oil free and polished from end to end. Polishing is very important as we will use the colour changes to gauge the temperature of the steel. It is essential that your sticky grotty oily fingers dont touch the clean and polished steel.
Lay the steel on the cool fire bricks and very gently, begin to heat the steel. Keep the flame moving uniformly over the full length of the steel bar. As the steel heats it will turn different colours. The first colour is a pale yellow straw. Then it will darken into a brown/bronze then into a dark blue. From dark blue it goes to purple, light blue, to grey and then to black and then red as in red hot! To give you an idea of the temperature, when its dark blue/purple, its as hot as molten lead. We want to stop heating when it's dark blue, almost purple. When you're satisfied (TLAR) with the temperature, drop it vertically into the oil. Do this tempering stuff three times. Be very carefull to keep the heat to the same degree if you can.
Take the cooled bar, wipe it clean and place it in the vice holding it by about 20 or 25mm. Now get a hammer and smack the bar gently on the end a few times. Yes! really do this. If you have done all the steps correctly, the hammer will bounce/spring off without damage to the bar. The hammer will really fly as you hit the bar. When you do this test, WEAR FACE and EYE protection and also wear stout leather gloves!! If the bar fails this simple (?) test and shatters/snaps, it will take off like a bullet. If it breaks, the bar was not tempered at a high enough temperature. If it bends, it was heated much too much.
The tempering three times is to obtain a uniform heating throughout the length of the bar.
If you are able to obtain some "Temperature Crayons", these will be a great aid to get the hardening and tempering correct. You will also need to know the properties of the steel to simplify the process.
Dont be afraid to try this technique. Practice on an unwanted piece first. When you have mastered the process, a whole new world will open up before you. You can make things like scribers, center punches, chisels, bowie knives and all things that cut and shape other metals.

This has been written for simplicity in understanding steels and heat-treatment. There is so much more to know and understand. I hope the above has filled in a few gaps in your knowledge of steel and has been a great help for you. If your appetite is fully whetted, check out your libary for more detailed info that suits your requirements.

Kody

Hi Kody,
That was very enlightening as to the hardening and tempering of a piece of spring steel. Your T.L.A.R. sounds like a bit of R.O.T (rule of thumb) to me.

My questions are in relation to the starting point.

1. Are there different grades of spring steel and if so how is the grading defined?

2. When a piece of spring steel is purchased as new stock from a metal merchant, what is its state? That is, does it come to you as a piece of steel that has x% carbon content and x% of x metal as an alloy in this steel but, as yet is still in a soft state that may be easily drilled, tapped, threaded, machined, filed, bent or otherwise cold worked to a desired shape prior to hardening and tempering? I guess what all that means is, is it in an annealed state?

3. If it isn't in an annealed state, what is required to make the metal workable?

4. Lastly, what are the limitations on working of spring steel as referred to above. I would imagine that sharp corners at high stress point are to be avoided for one. I would also imagine that after hardening and tempering any threads would need a recut which probably would not be possible with a standard tap or die. Is threading of spring steel something that should be avoided where possible, or, is it something that should be designed out of spring steel components, or, is this something that can be accommodated by selective/localised hardening and tempering/annealing???

Cheers from Cisco. P.S. Haven't seen recent posts on your project. How goes the progress pilgrim??

You're right on with the safety glasses Paul. Most of the industrial sites in Gladstone now require wearing of full goggles everywhere except indoors ie offices, crib rooms or dunnies.

I think even private health insurance would wipe you if you suffered an eye injury while mowing the yard without eye protection, let alone the home work shop.

Who would need to go to TAFE when you have a forum like this one.

Thanks Joe.

So Kody,

If I wanted some steel to be in the stiffest state possible, how would I go about heat treating it?

That's funny, at BHP we were taught never to use gloves when grinding/lathing as it's too easy to get one sucked into the machine, and then you lose your whole hand instead of just the tip of a finger