What's the difference between "quenching" and "tempering"?

I need a quick course in metallurgy. I came across the term “tempering” in regard to heat treating a gear cutter machined from tool steel. I’ve never machined anything or hardened any steel myself, but I’ve always thought what you did to harden steel (after machining it to the correct shape) was to heat it until it glowed a certain color, then cool it by dunking in either water or oil. So the only variables are what color to heat it to, and whether to dunk it in oil or water. Presumably you decide this by what you want the tool to do, and what type of steel it is made from. Anyway, after dunking, at that point it is done and ready to go. That’s what I thought anyway.

But now I’m reading about “tempering”. Is this the same thing as quenching? Just a different way to quench? Or is the way it is done, first you quench, then you temper?

Quenching is when you cool a solution treated steel quickly enough that carbides do not precipitate out of solution in a stable way. Tempering is when you take that quenched steel and heat it enough to begin precipitating the carbides but not enough to put everything back into solution. Quenched steels are brittle and tempering toughens them.

Ah, ok, thanks, I’m beginning to catch on I think. So if you didn’t temper after quenching, when you used the tool the teeth (brittle from the quenching) might chip while you were using it to cut the gear. So you add the temper process, and that removes some of the brittleness, but it will remain strong and won’t deform and will hold a sharp edge. Am I on the right track?

I’ve always thought “quenching” was a rather crude process where hot iron or steel was plunged into water primarily to cool it…Tempering is a more controlled process where the temperature of the material to be tempered is more closely regulated, determined by it’s color, it’s degree of incandescence, and then it is slipped into a container of oil (or water) in a careful and controlled manner to achieve the degree of temper desired…This becomes a fine art in knife and sword making…

The “tempering” process I was reading about was done very simply. By putting the part into an oven (in this case just a home oven, the kind used to cook dinner) and heating it for 20 minutes to something like 650 deg F. Then they just allowed it to cool off overnight, by turning off the oven. I don’t think my home oven will go to that temperature (except maybe for the cleaning cycle), but maybe some home ovens have that capability.

The article is in the current edition of the magazine “Home Machinist”.

Quenching, Tempering, Heat Treating are all techniques used to produce the desired characteristics in various metal alloys…Half magic and half science…

You might check out ‘metallurgy’ on Wikipedia. The follow links to the various terms. You’ve asked a good question that doesn’t have a short simple answer.

@GeorgeSanJose I think that would be more akin to annealing. Sometimes steel is annealed to alter the structural properties of the metal prior to being tempered in a separate process.


Quenching is a good way to case harden high carbon steel. There are a lot of names to learn if you want to know “everything” about metal. I don’t know many of the techniques.

Annealing is to bring the stock…or piece to be worked to a heat capable of allowing the molicules in the metal to relax and line up in a row. Depending on the makeup of the metal this could be a dull red heat…to… as hot as a bright orange heat (not quite to the white heat stage of melting. remove from the heat source and bury in lime or hardwood ash 6 inches surrounding the entire piece. This will take hours to cool depending on the size of the piece.

Now you work the piece to the desired finished product.

Reheat the item to harden and cool rapidy (opposed to the annealing over hours of cooling)
This reheating allows the molicules to move freely within the metal and by cooling (quenching) quickly, you trap the molicules in a random paterm (mixed directions) within the piece.
Remember, you cooled it slowly so those molicules could stay lined up as it cooled. Now you’ve jumbled them and this makes the item harder when quickly cooled…

To harden…this can be tricky because it all depends on the makeup of the base metal, some are air hardened, some oil quenched, some water quenched. And just becuase it is called tool steel the process can still be different. As an example you may use S7 tool steel and make a cold punch and harden it by oil bath

Or if the item is to be used and you still want a little flexability (not brittle at the tip) to it, you may air harden or forced air hardened with a fan, or even oven hardening.
Water hardening would be the hardest, oil hardening will cool it a bit slower making it less hard
Forced air hardening is a little less hard, air hardening will be softer yet, and oven hardening will be the softest that you can achieve. This would be like putting that red hot item in a oven and letting it cool slowly over maybe an hours time.
(when using oil bath do NOT remove the piece until it is cool.
By raising the item too soon smoke and gassed will raise from the bucket…and the hot item can easily ignite the gasses. You didn’t need those eyebrows anyway!!!

So it all depends on the type of metal and it’s alloy that determine the type of hardening.
I use H13 tool steel for some tools, but I prefer S7 tool steel for most. Though the S7 is trickier to forge it is a harder product. I just can’t let it get above a cherry red heat in the forge, so I get a real workout whailing on that to make any tools. The S7 makes better tools like punches that will have to absorb a lot of heat from the red hot piece being worked and yet hold their shape and they are best air hardened. H13 is a little more forgiving allowing me to punch cold steel and keep it’s shape yet for my work it’s best oil hardened. S7 would be too brittle and the tips would snap off in a cold hole.

Tempering is less tricky to get it right. This is heating the item back up, but only to a blue or black heat to allow the item to soften just enough to allow some flexability as in a knife blade.
Hard… to keep a good edge, not too hard that you cannot sharpen it, yet you want the blade to flex enough so it doesn’t break at the first hard use.
Usually tempering is done with the lights off and the item is heated until just past the blue heat stage into a start of red heat. Quickly the item is pulled from the heat and the slag quickly filed away as it cools. When the item cools to the dark blue heat, it is quenched to hold that temper.
Because a knife blade edge will cool quicker than the back of the blade…the blade is rested on it’s sharpened edge on a piece of large iron heated to almost a melting heat. This keeps the edge hotter while the back cools.

You may not want to temper these machine tools to keep that hardness.

These guidelines are intended for the small shop. I don’t want to hear any "Well when I worked at the Purple Thumbnail hammer company we did XXX in a heat treating oven. I don’t think George has one!!!

Gosh I ramble on don’t I. Doesn’t that Yosemite guy ever shut up???


Wow, great summary! Ramble on all you want, that was informative and entertaining. I love the name of the hammer company too! Somebody may steal that one…

I know it doesn’t really apply here but when you said tempering was less tricky to get right, I flashed back to a time we tried to temper glass at work. We have some pretty sophisticated ovens here but couldn’t even come close to the profile needed to do that right…

THanks I’m glad someone got something out of it.
Or maybe I put everybody to sleep???


@Yosemite … great post, informative.

I’m still clueless though. … lol

Getting back to the OP, could you summarize the steps a home machinist would use to make a tool that would be used to cut the gears in, say, a differential pinion? I’m not intending to do this mind you, just curious. I know there’s a lot of details I’ll never understand, but I don’t quite comprehend the whole process even. Does it go roughly like this, are these the major steps?

  1. Start with a block of an appropriate tool steel bigger than the final product.
  2. Machine away everything that isn’t what the cutter is supposed to look like.
  3. Sharpen the cutting edges with files, etc.
  4. Heat it 'til it turns a certain color.
  5. Dip it in oil (or water) to quickly quench-cool it.
  6. Finally, heat treat it at a lower temperature for 30 minutes to several hours depending on its size.

very good Yosemite, you get a cookie

@GeorgeSanJose - " make a tool that would be used to cut the gears in, say, a differential pinion"

  1. Be a millionaire
  2. Buy a computer controlled milling machine

Seriously, you just asked about one of the most difficult things to do - even if you had the milling machine. To make a tool that could cut the helical gears is rocket science.

I guess I’m presuming the possibility of accomplishing that part, even though I understand it would be a very complicated task. The milling isn’t the part I’m interesting in though. It’s how the cutting edges are obtained using heat. Once you have the part all dimensionally shaped, the rest must be some kind of compromise between sharpness, hardness, brittleness, and maybe the ability to sharpen it after it becomes dull with use.

Sorry George, I’m not a machinist… my brother is but I got all the good looks!!!

From what I’ve seen of machining something like this…I’d say you would be using a dozen different chucked tools to make a pinion gear.

But to your question of which comes first.

  1. Start with a block of an appropriate tool steel bigger than the final product

  2. Anneal the steel

  3. Machine away everything that isn’t what the cutter is supposed to look like.

  4. Sharpen the cutting edges with files, etc.

  5. Heat it 'til it turns a certain color.

  6. Dip it in oil (or water) to quickly quench-cool it. Now it’s hardened

  7. Finally, heat treat it at a lower temperature for 30 minutes to several hours depending on its size. Not size but rather the makeup of the steel alloy, some combinations of alloy and final use… will not require this step.

Heat treating is hardening the whole piece to a temp and letting it cool. As in a hardened bolt
Tempering is done to achieve different hardness’s in different portions of the part. As in a knife back opposite the edge is softer to have flex, but the blade is harder to keep an edge better.


To do the job well, you need a time, temperature, transformation diagram for the specific alloy you will use. This diagram will tell you how quickly the alloy needs to be cooled to avoid forming an undesirable phase of the alloy. Then it will help you determine how hot the part can be before annealing begins.

"Tempering is done to achieve different hardness’s in different portions of the part. "

Bingo! In tempered glass, the surface of the hot glass is cooled by air jets. Cooling contracts the surface. Then when the center cools, it contracts into tension and puts the (now solid) surface into permanent compression. Brittle materials are strongest in compression and putting the surface into compression prevents crack propagation (to a point). The shattering of tempered glass into little pieces is from much of that tension/compression being relieved in milliseconds. The pattern of the air jets can be seen with polarizing sunglasses and the right light angle.