Rehardening drill rod

[lensman57]

Hi,
This may sound silly but could a piece of drill rod be rehardened if the first attemt fails?
Tonight I tried to harden the hob that I had cut for cutting gear and the temp in the garage was so cold I don't thik all the teeth reached the temp, they were red but ti varying degree.
I may have to give it a second try to get it bright red.

Regards and thanks,

A.G

 

[gbritnell]

Absolutely. Rotate your piece while heating to get a more even heat. It should be more orange than red, or when it's no longer magnetic.
gbritnell

 

[lensman57]

Hi,
Thanks for the advice.

Regards,

A.G

 

[chrsbrbnk]

also making sure to use the right quench medium

 

[Mo deller]

also making sure to use the right quench medium

I have some silver steel as we call drill rod here in Blighty. I was aware that there are different ways of quenching but dont know what the difference is in the rod or how they are marked.
I just wondered if the piece you are using is unknown how best to proceed and if you get the wrong medium could you try again with another?

Peter

 

[arnoldb]

Hi Peter

There are three main types of silver steel - air hardening, water hardening and oil hardening, but there's no easy way to differentiate between them if markings have been removed (or if your supplier does not even know which type they are selling you!)

I've gotten to a point where I'll saw a bit off the rod and then follow these steps to figure out which it is:
1. First, heat bright red, put somewhere to cool down on it's own. Then try and file a bit off it. If the file just skids over it, it's air hardening.
2. If it was not air hardening, then I re-heat to bright red again, and dunk it in oil. Same test with the file. If the file won't touch it at this point, it's oil hardening.
3. Repeat test with water if the oil didn't work. If this still did not harden it, it's either not silver steel, or it wasn't heated enough.

Like gbritnell said, it must be heated to a bright red/orange, or test with a magnet.

Regards, Arnold

 

[Mo deller]

Thanks Arnold, that's exactly what I needed to know.

Peter

 

[Ned Ludd]

If I can add one little bit, once hardened silver steel should be annealed before rehardening this is to prevent cracking from occurring.
Ned

 

[Swede]

May I also add that it is helpful to hold the component at the transformation temperature if you can, for a period of time. The normal oven process is (IIRC) 15 minutes per square inch of cross section. Got to give those molecules and atoms time to arrange themselves properly. It's hard to do with a hand-held torch setup, but a slow count to 20 or whatever is going to be better than an instant quench.

Watch out for scaling, though.

 

[mu38&Bg#]

I've gotten to a point where I'll saw a bit off the rod and then follow these steps to figure out which it is:
1. First, heat bright red, put somewhere to cool down on it's own. Then try and file a bit off it. If the file just skids over it, it's air hardening.
2. If it was not air hardening, then I re-heat to bright red again, and dunk it in oil. Same test with the file. If the file won't touch it at this point, it's oil hardening.
3. Repeat test with water if the oil didn't work. If this still did not harden it, it's either not silver steel, or it wasn't heated enough.

I thought oil was the fastest quench? You get the http://en.wikipedia.org/wiki/Leidenfrost_effect effect with water that you don't get with oil.

Greg

 

[rkepler]

I thought oil was the fastest quench? You get the http://en.wikipedia.org/wiki/Leidenfrost_effect effect with water that you don't get with oil.

Even with the Leidenfrost effect water is a faster quench than oil. Brine is even faster, the salt provides places for the water to grab the steel - at least it looks that way. (I understand that mercury is even faster still, but have never tried it. Anyone who did would have to arrange for the mercury vapor to stay out of their lungs.)

Remember that a fast quench speed is to maximize hardness and can be detrimental to toughness. You can regain some of the toughness in the tempering cycle but sometimes a marquench gives a better overall result.

 

[arnoldb]

Greg, I'm not too sure about that; you may be correct.

My order of choice between oil and water is purely based on the fact that a dip in water cools a workpiece down to a handle-able temperature much quicker than the oil does.

I think oil also produce the Leidenfrost effect - I get a lot of bubbling around parts when I dunk them in oil...

A quick search with "water oil quenching speed" on Google came up with sites with these comments:
"As such, the "quenching power" of oil is far less drastic than that of water."
"The quenching velocity of oil is much less than water. "
"The quenching velocity (cooling rate) of oil is much less than water. "
Note that I didn't really confirm the sites' credibility - more research is required.

Kind regards, Arnold

:big: rkeppler; you pipped me to the post.

 

[mu38&Bg#]

OK, it's just what I remembered from school. Not the first time I learned something that I thought was right. oh:

 

[Gedeon Spilett]

Hi
when heating or reheating, don't overdo it, carrot red is enough,
if you heat with a gas torch, it is very easy to burn all the carbon content of this nice piece of tool steel, and to obtain in the end a useless piece of iron !
cheers

 

[rkepler]

The very best indicator of reaching critical temperature (with any normal alloy) is that you've reached the Curie point where the steel has become paramagnetic - not responsive to a magnet. I use this when using a torch since I simply can't identify the proper color but a magnet works every time.

I failed to mention that if you're rehardening you will want to run a couple of normalization cycles (heat a little above critical and air cool), this will improve the grain that will have been coarsened in the hardening attempt. If you're doing this to an air cooled tool steel you'll have to use a furnace as the cooling rate is critical. With all the time at temperature decarb of the top layer of the steel will be a problem with that so you might want to look at coating with a scale reducing coating.

Finally there is a water quench that doesn't use salt to speed the quench - Rob Gunter's Superquench. You can google it for the exact mix but it uses a surfactant to improve contact between the steel and the quench. It's superfast, faster than saltwater alone.

If you do use a really fast quench be sure to run the temper cycle as soon as practical. I've had high carbon stuff break from the internal stresses when left full hard overnight. Never had that with an air hardening steel, that and minimal warping is why I use it for most anything anymore.

 

[lensman57]

Hi,
Thank you all for your replies.
I have reheated the hob but I don't seem to be able to get it to the cooked carrot colour, I also tempered it by putting it in the oven for an hour at about 250C. The quenching was in the salted water and still the file seems to mark it meaning that it has not really hardened, I will have another crack at it tomorrow.

regards,

A.G