Quenching illustrated and explained,

[Charlotte]

The following link to Houghton Intl. takes you to a page that has .pdf file discussion of what happens when you quench in various mediums.

Houghton

Click on heat treating in the left hand column. The discussion includes cooling curves for different kinds of oil, water, brine, etc.

They are of course selling their proprietary and safer, (radically more expensive at our level of use) quench liquids but the info is a good illustration of what some of the guru's have been telling us about what happens when we quench.

[Ratel10mm]

That looks like a very good link Charlotte, thanks very much. :)

[Rob Browne]

Thanks for that link

[pkrankow]

Very nice information. I'm saving this one for later use.
Phil

[Nakedanvil - Grant Sarver]

Charlotte: Thank you for posting that link! I had seen the research done on quenchants and could not get people to believe what was happening with brine. They would rather believe it works because salt raises the boiling point of the density. I'll put my money on experimental research any day. I've never seen any good research that points to anything being better than a concentrated salt solution other than lye solution. Despite all the "superquench" BS.

[John Larson]

This is good information. Thanks for sharing it. I just finished reading a somewhat simplified account of the time-temperature transformation diagram and appreciated this booklet's first few pages.

Let me share an idea that sprang from some www discussion with Grant Sarver on differential hardening. I harden power hammer dies of 4340 or 4140 by using stratified heating of the dies. I formerly used through-and-through heating and sometimes had cracking. I switched to stratified heating (hottest at the die working face, coolest at the opposite dovetail face) and full immersion quenching. I believe this has stopped the cracking problem, possibly from the exterior of the cooling die not totally encapsulating a hotter die core. All other aspects of my heat treating process were kept the same.

[Nakedanvil - Grant Sarver]

Glad that worked out for you John. As I recall we were talking about different ways to get a differential result and it boiled down to: differential heating, differential quenching and differential tempering.

Edited October 11, 2009 by nakedanvil

[John Larson]

Houghton and another of my sources speak of a very interesting quenching variation. Namely, quench the steel down to the upper temperature where martensite starts to form from austentite, and then hold that temperature until the whole piece is uniformly at that temp, and then let it air cool to almost ambient before tempering as usual. This prevents the martensite shell on the surface from constricting around the hotter core and thus quench cracking. Takes a pretty hot quench, or second soaking tank of molten metal or salts or whatever to implement, but they claim it is immently practical for manufacturing shops.

My stratified approach is simpler for me, but if a die had to be uniformly of a given hardness this scheme makes infinite sense.

[Nakedanvil - Grant Sarver]

The process you're referring to is called "Austempering" and is usually restricted to parts less than 1/8 inch thick or so. Even then it usually only gives results in the 30's Rockwell. Thin parts will quickly assume the bath temperature, thick parts take too long.

From http://www.austemperinc.com/
You should use the Austempering process if:

Material used: SAE 1045 to 1095, 4130, 4140, 5060, 5160, 6150
Material thickness between 0.008 and 0.150 inches
Hardness requirements needed in between HRC 38-52
Distortion prone

[Kenzie]

Austempering is used at around 600F. Typically in salt baths commercially. At a lower temperature it is called martempering or marquenching. You can also use interrupted quenching.

BTW, I am the metallurgist for Houghton.

Scott

[Kenzie]

Charlotte: Thank you for posting that link! I had seen the research done on quenchants and could not get people to believe what was happening with brine. They would rather believe it works because salt raises the boiling point of the density. I'll put my money on experimental research any day. I've never seen any good research that points to anything being better than a concentrated salt solution other than lye solution. Despite all the "superquench" BS.

"Superquench" is a nice quench and very fast. It is partially because of a wetting agent. But salt is also very fast because it tends to defeat the vapor phase. Agitation is also critical in any quenching operation.

[patrick]

Kenzie,

Welcome-It is good to have another metallurgist on the board. I am a plant metallurgist for Scot Forge, so tend to work with very large sections and a 50,000 gallon quench tank, but the principles are the same at the small scale.
There are a couple of other metallurgists floating around the blacksmiithing community, all with slight variations in experience and background. It provides a very nice blend of technical resources for the folks who are trying to work out forge and heat treat problems on a relatively small scale without a formal metallurgy background.

Patrick

[Nakedanvil - Grant Sarver]

Kenzie: What I said was "I've never seen any good research that points to anything being better than a concentrated salt solution other than lye solution. Despite all the "superquench" BS". I've gotten similar results with brine and never seen anything but anecdotal information on the use of sufficants. Is there any real research that shows wetting agents to be effective at the temperatures we're dealing with in quenching. It seem like if it was useful, that someone, somewhere would have researched it. It would seem that the effect salt has is similar to “wetting” in that it enhances contact of the quenchant with the steel. Great to have you on board.