Heat Treating Question

[walterwoj]

I have posted before about my heat treat oven rebuild and have been messing around with it today.

This has raised a few questions I thought I'd ask about heat treating In general:

Where do you find info on how to heat treat and temper various metal?

I have seen some documents that state: "Raise heat to 1550 at a rate of no more than 400 per hour".

How important is that to do?

What happens if I chuck a 70 deg part right into the heated oven?

The specs usually say "30 mins per inch", what happens if I leave it in longer?

They also usually say to temper before the part drops below 125 deg right after heat treating. Does it make a difference if I let the part fully cool first? (My oven goes up to 1550 in 45 min, but DOWN to 400 takes several hours. Even with the door open)

I have attached a pdf I found on the topic so you can see what I'm referring to.

Also, below is why I do not recommend quenching in a PLASTIC bucket! LOL!

 

[durableoreo]

Now you have a reason to buy/make another oven Sometimes you can get away with a toaster oven, if you're going for something really hard like a knife blade. For RC = 35 (hard but machineable) you need a lot more degrees that kitchen appliances can provide.

To hit the 400 degF / hr, I usually don't worry about it. I don't have a good reason. No problems have appeared so I've been lazy, I suppose. Heat transfer is exponential in the simplest model, known as Newton's law of heating and cooling. You could probably model temperature on a spreadsheet or with a computer program. There would be assumptions, of course, but it's a start.

 

[Asm109]

First off, heat treating is all about changing the crystalline microstructure of the metal. Nobody makes the instructions more complicated without a reason. My engineering degree is way too far in the past to remember the the reasons for all these limitations. But I remember a couple.

Anything that is a time/inch is to ensure that the center of the part gets up to temp.
Longer bad? well maybe because the crystal grains grow with time at temp. Smaller grains are stronger. So you make the tradeoff, 10-15 minutes no biggie, overnight not good.

You did not mention but the note in the pdf to increase rapidly from 1250 to 1500 is important because at certain temps you get microstructure changes that usually are very brittle, so you want to spend a little time as possible in those temp ranges.

 

[walterwoj]

@durableoreo : I have a toaster oven too! But my big oven is much more accurate!

@Asm109 : Thank you for those details! My main complaint is already that the oven takes a minimum of 4 hours to do anything, I didn't like that those instructions call for adding at least 4 hours to the time. (Plus it's faster to get from 0 to 1000 degrees than from 1000 to 1500 so the part they want slow is fast and the part they want fast is slow). I'll probably save the slower version for important things and do the quick way for less important things.

 

[pontiac428]

I would reconsider your used motor oil quench. It's a foul, toxic brew that leaves a chemical residue on everything around it when you perform a quench. If you don't want to buy quench oil, it's okay- any non-detergent 30 wt HDO will work. I use and re-use a quart of clean, cheap jack oil for quench. It doesn't have the juices of incomplete combustion, thermal breakdown, or microparticulates from leaded babbitt or chromium rings in it like used oil.

Sorry, but whenever I see anyone trying to make use of used oil it makes me cringe.

 

[durableoreo]

RE quench oils. I was quite skeptical that there was any difference. Used canola for years. There's a test where you heat a sphere, then hang it in oil next to a magnet. Using the timing results from that test fixture, it's easy to see that the various oils perform differently. There are other test, too, but I can't think of them right now.

If you've made your own heat treat oven, you've saved so much money that you may as well treat yourself to a gallon of AAA or the the 11-second quench oil from McMaster.

Here's an interesting video on the topic:

 

[walterwoj]

The used motor oil was a test since my wife hadn't gotten back from the store with the canola oil. that's also why I used the bucket, I have a new empty paint can ready for the canola and didn't want to taint it with the motor oil first.

 

[Video_man]

Regarding Canola oil, and slightly off-topic, I've found that steel heated to just where the oxide colors turn to grey, and quenched in Canola leaves a very attractive blue-black coating, looks much like the classic "nitre blue." Pretty durable, as well.