Tempering – To Quench or not

[Mike HurleyParticipant @mikehurley60381]

Can someone clarify my confusion on this matter. If I read a dozen articles on the subject I seem to get a dozen different answers. I'm talking here about tempering in the home workshop – just occasional on-off jobs with fairly bog standard carbon steels. Hardening OK, but I remain confused as many sources say 'Quench' when appropriate colour is reached (to halt the process) others indicate 'air cooling'.

I've a copy of Workshop Practice 1 (tubal Cain) on the subject which is very, very thorough, but I still can't seem to disseminate the correct answer which I feel comfortable with! I've hardened and tempered various stuff over the years, (with variable results) and have a little job on the go and so decided to raise the question to set my mind at rest once and for all!

I appreciate this can be a very complicated process when looked into in detail with specific steels / equipment / Industrial methods etc, but I'm looking for (if it's possible) a non-nonsense straightforward answer for Joe Public working in his shed-cum-workshop.

Regards, Mike

[Mike HurleyParticipant @mikehurley60381]

[John HaineParticipant @johnhaine32865]

If you are applying heat from behind the edge for example until the right colour runs to the edge then quench as soon as the colour is right otherwise it will continue to heat and spoil the temper. If you are able to raise the whole piece to the right temperature, then you can hold it there for a while then let it cool naturally.

[Kiwi BlokeParticipant @kiwibloke62605]

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.

[norm nortonParticipant @normnorton75434]

Very good question Mike because, as you say, writers often refer to quenching once the item reaches its desired tempering colour. Quenching a red hot steel to lock the crystals in their arranged shape is clearly important; that is how to lock in hardening. But I have always seen tempering as a stress-relieving, or crystal relaxation process, and once achieved there is nothing to lock as they will not continue rearranging as they cool to room temperature.

When tempering in big ovens, for longer times, the material is taken out or it cools in the switched off oven. But I agree that when doing a 'quickie' hand temper over a flame the use of an oil dip quench does the job of definitely stopping the heating.

[JasonBModerator @jasonb]

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.

[Graham MeekParticipant @grahammeek88282]

Hi Mike,

With 40 years of Toolmaking under my belt and also a Home Machinist for much of that time.

If a carbon Steel item is hardened then it will need Tempering. This will depend on the application the tool or item is being used for. Leave it dead hard and there is a pretty good chance the item will shatter.

Silver Steel is usually Hardened and Tempered in Water or better still Brine. Gauge Plate or Ground Flat Stock as it is called today is an Oil Hardening Steel. Use water with this material and you run the risk of surface cracks.

Oil can be used for Silver Steel but the hardness will be down when compared to water.

There are many steels that do require Air Hardening and Tempering, but I doubt you will come across these in the home workshop as many of these are done in controlled environment furnaces. Some steels even require to be heat treated in a vacuum.

Regards

Gray,

PS Jason wrote while I was writing,

Edited By Graham Meek on 19/01/2023 11:16:47

[SillyOldDufferModerator @sillyoldduffer]

Posted by Mike Hurley on 19/01/2023 10:00:18:…

I appreciate this can be a very complicated process when looked into in detail with specific steels / equipment / Industrial methods etc, but I'm looking for (if it's possible) a non-nonsense straightforward answer for Joe Public working in his shed-cum-workshop.

…

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

[jaCK HobsonParticipant @jackhobson50760]

I reckon the first three answers to be the best. I believe the OP was just about tempering, not hardening.

For most steels we come across that are easy to harden (silver steel, W1/2, O1/2, A1/2), the think the most important thing is the highest tempering temperature. Time held is less important for small tools or sections. Cooling speed is even less important. Water or oil or air doesn't really matter.

Small tools that I commonly rework I would use flame to get to right colour and then quench.

Things that need to be more critical for ultimate performance (fancy knife) then I would use an oven and follow the destructions on the packet – for W1/O1 that would probably be hold at temp for 1 hour.

[Mick B1Participant @mickb1]

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…

[David JuppParticipant @davidjupp51506]

When dealing with very large items which can be very slow to cool, there are circumstances where detrimental changes can take place due to diffusion whilst air cooling. Certain 'tramp elements' can migrate towards grain boundaries, leading to a reduction in toughness of the final product. In such cases quenching after tempering is important to maximise toughness. Such industrial practices may be at the root of some of what is written about heat treatment.

I doubt that many of us need to worry about such things.

[Martin KyteParticipant @martinkyte99762]

Posted by Mick B1 on 19/01/2023 15:04:35:

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?

[Mike HurleyParticipant @mikehurley60381]

Thanks to all who replied with plenty of detail and useful info. As I tried to make clear originally, my query was simply about tempering. I've never had an issue with the understanding of, or the practice of hardening in most cases. The general drift I get from the responses is, probably as I expected, there is no one-size-fits-all answer!
Discounting 'special' or 'exotic' metals, industrial practice or access to more advanced kit, and the fact that I am using a simple propane blowtorch directly or indirectly via dry sand or heated plate, and working typically on silver steel or very similar; trying to summarise –

• Once correctly hardened, the re-heating to a relatively low temperature makes the otherwise brittle material more malleable & less prone to fracture
• Quenching itself at these temperatures does not directly affect the structure of the material, it is simply a method of stopping the heating at a specific temperature (or as near as practical) which itself is what modifies the material's characteristics.
• If heating a thin / small piece by directly heating one end for example, and watching the colour change advancing, quenching when the required colour gets to a specifc point, as desired, is the only way to to stop the temperature rise moving on past the point desired.
• If heating indirectly, eg via sand or with a hot auxiliary plate, carefully watching the colour and removing the heat source / carefully modulating same to maintain an even heat for as long as appropriate for the size of metal, quenching is not necessary.

Within the simple practical constraints I outlined earlier, does this make sense or am I talking through my hat! (Not unusual) Further comments encouraged and most welcome

Regards Mike

[Kiwi BlokeParticipant @kiwibloke62605]

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

 

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

[Mike HurleyParticipant @mikehurley60381]

Thanks KB – It's nice to get something right occasionaly!

regards

[Graham MeekParticipant @grahammeek88282]

Posted by Mike Hurley on 19/01/2023 19:54:02:

• Quenching itself at these temperatures does not directly affect the structure of the material, it is simply a method of stopping the heating at a specific temperature (or as near as practical) which itself is what modifies the material's characteristics.

Regards Mike

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,

[John ReeseParticipant @johnreese12848]

As an example, suppose you had just forged a chisel and the body of the chisel was above critical temperature. You would quench the end of the chisel leaving the body hot. You would then polish the quenched end so you could see the temper colors. Then you watch the colors as heat from the tool body until you had the desired temper color. At that point you would quench the entire tool to prevent additional heat from migrating to the tempered end.

If the part were tempered in an oven and the entire part was at uniform temperature there would be no benefit quenching after tempering.

[Mike HurleyParticipant @mikehurley60381]

Again, thanks for the very useful contributions. I think that the crux of the matter, from what most people have said, is that in the majority of cases, working as I do, i do need to quench at the appropriate time!

I feel much happier with the tempering process now & confident my future results will be better. As usual this forum is quite amazing in the breadth of experience available and willingness of its members to share & help others.

Regards to all, Mike

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