Bill, maybe this will help. It's long but I've always felt every word was worth reading for the beginner.
HARDENING SIMPLE STEELS
All of the really low alloy steels have one feature which make them virtually foolproof when it comes to cooking them for hardening. When one of the low alloy steels reaches the critical temperature where it can be hardened by quenching, it turns non - magnetic. As the steel heats, check it with a magnet. At a certain point, the magnet won't stick. That's usually at a temperature ( color ) far lower than you would think. Once the magnet won't stick to the blade, give it a moment more in the fire and then into the quench it goes. ( An extra 50 degrees over the critical temperature insures better hardening and won't hurt the steel.)
The simple aloys can also be selectively hardened, not with a fancy temper line, but with a softer back that will make the spine and tang less susceptible to breakage. All you have to do is take the blade up to heat very quickly, getting the thin parts along the edge hot before the thicker spine. You could also just dip the cutting edge into the oil, allowing the spine to cool more slowly, not hardening it.
Should you err and get the spine too hot and inadvertently harden it, you can use a torch to partially anneal the critical areas. I recommend that all stick tang blades have a softer section where the tang joins the blade.
When the hot blade hits the oil, you will almost always get some fire. Don't leave the tang half out of the oil. It is near red hot, sticking out of the oil and acting like the wick of a candle to start a fire in the tank. NEVER use a small tank of oil to quench a lot of blades. Sooner or later, you'll find the flash point of the oil. That's where it starts burning all by itself, and you won't like that one little bit.
Forged blades will always have stress than needs to be worked out before hardening. They should be annealed, preferably several times before hardening. Some knife makers have had wonderful results by annealing their blade steel three times before hardening. It reduces grain size significantly, making a much tougher blade.
SERIOUS HEAT TREATING
1040 to 1050 steel Water quench from 1525 to 1550 F. Hardens to approx. RC 58. Very easy to get cracks with water quenching. Draw at 350 F. for spring temper, best for daggers, etc. Shallow hardening and can be done with beautiful, Japanese style temper lines.
1050 to 1095 steel Brine quench from 1475 to 1500 F. hardens to RC 60 to 65. Draw immediately. Oil quench at the same temperatures for slightly lower hardness. Shallow hardening and can be done in the Japanese style with a decorative temper line.
O-1, O-2, O-6, O-7 steel Preheat slowly to 1200 F. Oil quench from 1450 to 1500 F. Draw at about 350 F. O-6 reaches RC 65.
W-1, W-2, W-3 steel Preheat slowly to 1050 F. Water (brine) quench from 1400 to 1500 F. Draw immediately. May also be oil hardened if cross sections are radical, or simply for less chance of cracking. Shallow hardening and will work with Japanese temper lines.
L-6, L-7 steel Quench from 1450 to 1550 F in water or brine. Doesn't really need to be drawn. L-7 will give slightly more hardness.
D-2, D-7 steel Preheat at 1500 F. Harden from 1850 to 1875 F. Draw immediately.
A-2 steel Preheat at 1450. Air harden from 1700 to 1800 F. Draw at 350 F.
Water hardening is a mis-used term. In virtually all cases where a steel is referred to as water hardening, they're actually talking about quenching in brine, heated to a temperature of 170 degrees, (F) or above. Brine is made by dissolving non-iodized salt in water until a egg will float in it. Jim Hrisoulas uses bluing salts to make his brine. The whole idea is to raise the boiling temperature of the liquid and make it transfer heat better. Brine will eat right through an ordinary steel barrel in a very short time. Jim Ferguson just mixes in borax and detergent to reduce the surface tension.
Drawing is normally done for 30 minutes when one has the equipment to properly maintain the temperature. Flash drawing, that is, heating to show a certain oxide color on the metal surface and then letting it cool, is fine for softening a spine, but not best for the cutting edge.
Any steel which has a low draw temperature may be drawn to a softer temper along the spine to give it better shock resistance. If this is done, the blade will almost always curve a bit towards the softer part of the blade. This works better on shallow hardening steel than on others but can be done on 5160, 52100, A-2, 440-C and others.
Many of the low alloy steels, like 0-1 and 5160, will show a temper line if the blade is selectively hardened, but the line is rather plain, merely a division between hard and soft. A proper, decorative line, needs either the 10 series, or W series of steel. Oil quenching will produce a temper line in those steels, but water quenching is necessary if one desires the more intricate details of the Japanese style line.
SPARKS GIVE A CLUE TO COMPOSITION
The spark from a steel with .15 to.40 of one percent carbon will simply fork.
Steel with at least .45 of one per cent carbon will show a small but distinct secondary burst. This is considered the minimum for hardening into a useful knife.
Carbon over one per cent will give an intense and multiple bursts.
Moderate silicon makes a spark that is short and ends with a sharp white flash.
Nickel sparks have a small, very intense and bright white color.
Molybdenum gives a spark with a distinct, separate head.
Sparks are best judged by looking at the last third of the trail. Alloys with several additives will confuse the heck out of anyone.
OVEN DRAWING COLORS
If you want to do your own heat treating on some of the easier steel types, you will have to first harden the steel all the way, then draw its' hardness back a bit to keep it from being too brittle to use as a knife. You can do this in an ordinary kitchen oven, but most ovens have a crude thermostat that is not much better that taking an educated guess at the temperature. The scientific term is SWAG, scientific wild assed guess.
Oxide colors will form on clean steel as it is heated. You've seen them many times, as the steel got hot where you're grinding.
Contrary to some belief, these do not indicate that the steel has had its' carbon burned out. All it means is that the steel got hot, and if hardened, lost some of the hardness. ( Burning carbon out of a blade takes bright yellow heat applied for more than just a minute.)
The colors are a very precise indicator of temperature with simple carbon steel.
400 degrees, pale straw
425 degrees, straw
490 degrees, golden yellow
500 degrees, brown
525 degrees, brown purple
570 degrees, purple
600 degrees, bright blue
650 degrees, pale blue
There is one little catch when using the color with any steel more complex than 0-1. They don't all react to a given temperature with the same color. D-2, for instance, looks a lot different from 0-1 at 450 degrees. It's best to put a scrap piece of 0-1 or 1095 in with the complex stuff when running the draw cycle, and use it as a precise indicator.
RED HEAT TEMPERATURES
Faintest dull red 900F
Dull red 1200F
Cherry red 1400F
Orange red 1500F
Bright salmon orange 1600F
Brilliant orange 1640F
Temperatures should be judged in a dimly lit area. If you are working outside, colors will appear at least one or possible two steps too low, and you will severely overheat the steel. I have become intimately familiar with this problem. One fellow has a open barrel set horizontal beside the forge, providing a shaded spot to stick the blade into for judging steel temperature.
SELECTIVE TEMPERING
If you want the spine of a carbon steel blade drawn softer than the cutting edge, the process isn't nearly as difficult as you might think. All you need is an inexpensive wallpaper tray, filled with water, and a torch. A propane torch will work, but the oxyacetylene type works a lot better.
You hold the blade above the tray, edge down, and begin heating the spine until you start to see some color. You always start at the heaviest part of the blade. The first color will be a pale yellow.
At this point, try to keep at least half an inch of the cutting edge in the water. This will keep the heat from running down to the parts that have to stay hard. Work the heat only on the very spine, and work slowly towards the tip. Stop well back from the tip. It will heat easily and you want it to stay hard.
If you are using a acetylene torch. the flame must be aimed directly down at the spine of the blade. If you heat from the side, the blade will warp. This is not a problem with the slower heating L P torch. After you melt a tray or two, you'll also want to use a metal tank.
When the length of the spine has a rich blue color, it will be drawn to a high 40 or low 50 Rockwell, which is right where you want it. On stick tang blades, you should cook the junction of tang and blade to at least a good rich brown to prevent any brittle fracture at that point. The end half of a tang doesn't need to be hardened at all.
Some of the more complex alloys with low drawing temperatures, like 440-C and A-2 may also be treated to soften the back with this technique.
Sorry Bill but the system will only let me post half of it. If someone wants the whole thing, email me and I'll send it to them.
