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Alstone · Moderator & Site Supporter

Steel can be divided into two classes, there are those which respond to treatment and those which require further carbon before quenching can produce the desired result.
The effect of heating a piece of steel to its hardening or critical temperature, causes a change in its micro-structure, and if allowed to cool slowly the material returns to its normal condition, but if arrested at that point by rapid cooling it becomes hardened. It is the carbon content of the steel that is the hardening agent, the more carbon in the steel the lower the hardening temperature.

The methods used to bring the material to the desired temperature before quenching can be an open fire, torch, furnace or any other device you find suitable, the main criteria for good hardening is an even temperature and this is difficult to obtain by using a torch, a furnace would be more suitable or a lead bath, but as these would possibly not be available so a torch would do.

The temperature of steel can be roughly judged by its colour pale straw (220 C.) to dark blue (316 C.), dull red (515 C.) to white (1,320 C.). High carbon steels hardens in water with a film of oil on the top to stop hardening cracks in the surface, if you still get cracks then use just a high flash point oil as a coolant, most of the tool steels (High Speed Steel) in use now are oil hardening, for lower carbon steel clean warm water at is preferable. To test for different types of steel use a grinding wheel, the type of sparks produced will tell you what type of steel you have, wrought iron will have a longish spark of light straw colour, mild steel will produce longish light straw sparks with tails like arrow heads, tool steel sparks are white and shorter but have a greater number of arrow heads, while high carbon steel yields a cluster of fine white sparks, high speed steel very minute yellow sparks and tungsten steel orange sparks.

You really need to know the hardening temperature of the steel you are treating but on air guns this is not possible without reference to the manufactures data, so a bit of trial and error is needed, also knowing the temperature of the item you are hardening is difficult without a pyrometer, so below is a rough guide to metal temperature by the colour changes when heating up.

Lowest read heat visible in the dark 635F / 335C
Faint red 960F / 516C
Dull red 1,290F / 700C
Brilliant red 1,470F / 800C
Cherry red 1,650F / 900C
Bright cherry red 1,830F / 1,000C
Orange 2,010F / 1,100C
Bright Orange 2,100F / 1,200C
White heat 2,370F / 1,300C
Bright white heat 2,550F / 1,400C
Dazzling white heat 2,730F / 1,500C
Welding heat 2,800F / 1,550

Carbon steels having a low carbon content which do not respond to direct heating and quenching, may need to be surface hardened afterwards to induce additional carbon, this is known as case hardening in which a hard skin is formed around the item treated, the depth of which depends upon the duration of the process, for ordinary use a depth of between 1/32 and 1/16 will be enough.

The easiest method of case hardening is to heat the steel to a bright red and apply the powder by dusting it on to the surface or rolling the part in the powder. After allowing time for the compound to melt on the surface it is reheated and the process repeated two or three times, finally heating and quenching in water.

Carbonising material can be made from any animal carbon such as ground bone charred leather or the cutting from horses hoofs make excellent case hardening material, wood charcoal soaked in a aqueous solution of sodium carbonate dried and powdered makes a good case hardening compound. Or there are some good commercial compounds on the market.

Have Fun

Al

Amigo · Veteran Member

Al:
Will the next installment be about drawing/tempering and the use of color there?

Alstone · Moderator & Site Supporter

Amigo · Veteran Member

Al:
You might cover high carbon oil hardening tool steels first as they are pretty versatile. Ketos/O-1 seems to be more readily available in preground flat and round stock and in a wider variation of sizes than some of the other steels. The preground round sometimes called drill rod is handy for making that special hollow ground screw driver blade for that special screw that needs repeated snugging. O-1 is also ok for that light duty punch or chisel.
The air hardening higher chrome steels like A-2 & D-2 and the high shock silicon steels like S-5 & S7 could be covered at your later convenience.

Thank you for taking the time and explaining so concisely.

Best Regards.

Amigo

Alstone · Moderator & Site Supporter

Hi Amigo
Tempering as been touched on in the post on Hardening Steel so I will cover it fully in my next Guide, but seeing as you know something about steels and metal treatment you will understand that if I quote steel grades such as A1,O2,S7 or high carbon chrome steel etc, people will get lost in forest of figures. Also as this is an International site different countries have there own standards. I live in the UK and we use the Vickers scale of hardness or our BS EN standards which won’t mean a thing in other countries
Ketos steel is known as Silver Steel or High Speed Steel in the UK and as this is the most popular and easy to obtain, I will concentrate on it plus what can be found in the workshop

The whole idea of the guides is to help people to work metal using just what they have in there workshop or shed, or what they can obtain locally also to refurbish the tools they already have.

Anyway I’ll see if I can get around to it over the next week or so, and as it is a colour dependent process I’ll take some photos to go along with it.

Al

Amigo · Veteran Member

Al:
Here in the U.S. the scales most used for steels are Brinnel and for tool steels hardened, Rockwell-C.
Please pardon my ignorance in not knowing the U.K. used the Vickers scale, and not knowing how the scales compare. I'll have to either get busy or remain ignorant. I think I'll get busy doing some research.

What you are doing and planning on doing requires a lot of time and effort. I'm confident your efforts will be greatly appreciated by all the forum members.
The glaring difference I've noticed between AirGunHome and other forums is that the more frequent AGH posters primarily contribute, unlike some of the frequent posters on other forums who simply take and return nothing of any real value.
Thank you for you efforts.

Amigo

Alstone · Moderator & Site Supporter

Up until the ISO standards took off in the UK we also used Brinell and Vickers and still do depending upon the industry concerned, we are kind of stuck between Europe and America and years ago the States was one of our main areas of trade, and in military development we used the standards for the country concerned plus our own, but as I said ISO as taken over and one of the founder members is the American ANSI organisation, but I don’t see much use of it over there.

One company I worked for used the Brinell standard (which I think is Swedish) on all military equipment it made, as long as it came up to MIL specs of the product requirement, if not then we would change over to some other standard.
The thing is large companies that have been established for many years still use to a certain extent the standard they were brought up with, and only change when they start working in collaboration with another company, that requires then both to use the same standard.

Hardness tables are one of the most complicated things to cross reference and still make sense of, and to most people don’t mean a thing, thats why I prefer working with what you can see feel and hear, like the blacksmiths of old.

Al

Amigo · Veteran Member

Al:

"I prefer working with what you can see feel and hear, like the blacksmiths of old."
We are of like mind as I am in 100% agreement - it's real world usable results that count, especially when applied to a home shop.

Best Regards,

Amigo