If you're 'running ' the colour up the shank of a tool, towards its cutting edge, you need to stop the process quickly, as soon as the desired colour reaches the edge. hence quench. But there's no need to quench if you can control the temperature accurately (as in an oven, etc). This answer is appropriate to the average Joe, working with silver steel, high-carbon steel, etc. No need to quench copper, when annealing, either.

Depends somewhat on what grade of steel you are hardening as quenching is more suited to oil and water(brine) hardening and air hardening steel is well best suited to the use of air.

Alas, no such thing. If it existed everyone would be doing it!

However, I manage fairly well by keeping it simple.

Making cutting tools is my most common reason for hardening and tempering. For that I invariably use Silver Steel. Silver steel, aka Drill-rod, is an alloy specifically formulated to harden and temper over a wide-range of conditions. The wide-range means that the operator need not be particularly skilled, or require special equipment. It does require a certain amount of care and practice, but silver steel can be heated 'cherry red', whatever that is, in a dimmish workshop for about the right length of time, and quenched in water with a high chance of success. Likewise, after cleaning up it's fairly easy to temper the result by gently heating towards the cutting edge whilst watching the oxide colours change: these indicate temperature well enough for the operator to stop heating the instant the edge is at the right temperature, and then allowed to cool slowly. Alternatively the whole thing can be baked in an oven.

Judging temperature by oxide colour and sliding up towards the edge has a hidden benefit. Although it seems simple, the operator has improved the cutter by altering the metallurgy to get a hard but brittle cutting edge supported by a softer, but tougher shank. This may not be what's wanted! In other cases, it's desirable to have the part hardened and tempered to the same degree throughout. An oven is the best way of doing this.

Gauge Plate is a close relative of Silver Steel. The main difference is it's Oil rather than Water hardened, again to make it easier for the operator to get a decent result without special equipment.

Other hardenable steels are progressively more difficult because they harden. quench and temper over a narrower range of temperatures, and are more time critical. Easier to misjudge.

Silver Steel and Gauge Plate are easy but expensive. Medium and High-Carbon steels are cheap, but the process is fussier; the operator has to know what the steel is, what it's parameters are, and stick more-or-less closely to them. Many steels are unforgiving – even small errors of temperature and timing spoil the result. So far I've not experimented much with any of them, but it can be made to work. The secret is judging time and temperature accurately enough at all stages: mustn't heat the metal too quickly or too slowly; mustn't be too cool or too hot in the quench; must be heated the same temperature throughout and for long enough for the structure to change; quench liquid must remove heat evenly and at the correct rate; and then tempering has similar fussy rules.

High Speed Steels are spectacularly difficult to harden and temper. Takes over 24 hours with a complicated sequence of timed temperature stages. Requires ovens that are accurately temperature controlled and close attention to detail throughout. It's an industrial process beyond anything man-in-shed could do without spending a fortune on equipment.

My advice is to practice on Silver Steel and Gauge Plate, and use them as much as possible. Once mastered, move on to cheaper steels and practice with them. Not rocket science, but having the equipment to heat and cool metal controllably helps develop the skill and judgement needed. I learned a lot by watching the US 'Forged in Fire' TV programme. It's like the 'Great British Bakeoff' except the competitors make knives and swords instead of cake. The competitors are usually professional or semi-professional, certainly not inexperienced, and they have good equipment. Even so, they often have serious trouble heat-treating their blades correctly. Warped, unable to take an edge, cracked, microcracked, and/or brittle. Part of the fun is watching beautifully finished knives fail the tests. Another is judging their general metalworking skills, which are often seriously lacking. Knide-makers who are brilliant at design, forging, heat treating, grinding, sharpening and handle-making often horribly abuse a pillar-drill and other tools. The workshop has a Bridgeport in the background – so far never been used, even though it would eat certain problems.

Dave

Small items like silver steel slot cutters and punches don't normally need quenching for temper because they lose heat faster than you can get 'em to the quench tank.

Wave the item gently through the flame and give it a second or three to take on oxide colour. Once you reach light straw, stop. Quench if you want to handle it immediately, but IME it doesn't make a deal of difference.

Items with greater heat capacity would need more measured treatment. Worst thing I ever did was oil-quench about a pound of gauge plate and cracked it irreparably. That was in 1976, and I've somehow recovered from the shame although I can't remember how I retrieved the tool I was making – I must've edited the memory…

That’s an odd story as presumably you oil quenched to harden it from red heat which I would have assumed would have caused a greater thermal shock. Maybe that’s when the damage started?

I think that's about it, and beautifully summarized!

Edited By Kiwi Bloke on 20/01/2023 08:48:47

Hi Mike,

You generally have the Gist of what is going on with your summary.

However the Tempering process is a Refining Process in that it modifies the grain structure of the material. As hardened the structure is coarse and therefore Brittle. Tempering modifies the grain structure by making it finer. More grain boundaries improves the internal bond making it less brittle.

Quenching stops the grain refinement at that particular stage and this is denoted by the colour change.

Allowing the part to cool in air will not stop the grain growth at that particular point as effectively as quenching.

Like you I have nothing more than a Propane torch.

Regards

Gray,