PDA

View Full Version : R&P freezing pros and cons


Red_Chili
05-05-2009, 10:18 AM
Bobby Long offers cryo for ring and pinions. Kinda interesting given my recent experience. So what are the pros and cons? Increase strength at the expense of wear? Or is there no downside at all?

Looking at Trail Gear R&Ps again. Price is price, the quality seems to be there, and the wallet is screaming.

treerootCO
05-05-2009, 10:26 AM
Mountain High Performance

(303) 395-5289 Address: 5293 Ward Rd, Arvada, CO 80002
Cryogenic Processing Available

http://mountainhighperformance.com/index2.html

DaveInDenver
05-05-2009, 10:34 AM
Cryogenic treatment does increase both strength and wear, so there isn't a down side per say. Technically the part is more brittle, but it's hardness will be higher and so the point where it potentially will shatter is higher. But it will only work to make a properly treated object better, it does not make crap suddenly into good. So if the R&P are not done right to start, running a cryo cycle on it will not make a, poorly annealed, high ferrite or over tempered gear into something else. It's not a replacement for solid metallurgical practices (it's always a post hardening process, like peening, etc.). You still need to start with gears from the same good manufacturers, so don't approach it as a cost savings.

I should also add that if cyro'd gears are what make the difference between breaking and not, then you have zero design margin and are badly overloading the part. You need to solve the fundamental problem, you know all of this, though.

rover67
05-05-2009, 12:25 PM
Post pics of the peices...

Red_Chili
05-05-2009, 12:35 PM
It may be a while before I can move around the shop to pull the diff apart, but I will post pix.

Interesting... the tech at Marlin thought that cryo was a bad idea and made no sense to him. Said the dragster guys want their R&P to flex rather than break, and cryo makes breakage more likely in that situation due to increased brittleness. Harder though.

Continue...

Uncle Ben
05-05-2009, 02:03 PM
Cryoing does not make parts more brittle! Cryoing aligns the molecular structure of the metal so the strength is deep rater than just the case hardening. I already have more time and abuse on my mini truck crap in the front of the 80 since cryoing than the exact same R&P that I broke three years ago! I'm now on my second set of pads on the front cryoed rotors that basically are still like new...... believe what you want! Fortunately, the myth has enough believers in Longfields now that the Heep front axles are the laughing stock of breakage that Birfs once had! Cryoing hard parts is often two step process that involves heat treating afterwards (also at a very slow and controlled rate).

DaveInDenver
05-05-2009, 04:54 PM
It may be a while before I can move around the shop to pull the diff apart, but I will post pix.

Interesting... the tech at Marlin thought that cryo was a bad idea and made no sense to him. Said the dragster guys want their R&P to flex rather than break, and cryo makes breakage more likely in that situation due to increased brittleness. Harder though.

Continue...
I'm not sure why a drag guy wouldn't want to cryo his gears, it seems like they would want the sort of benefits it would give you. But relying on it is not good practice, it's not nearly as good as doing the heat treating (i.e. pre-heat, austenitizing, quench and first temper) properly at first to achieve your desired hardening.

Post hardening processes optimize what's done initially, so good gears only can gain certain properties with a cryogenic treatment and it's not absolutely necessary to the quality of the gear. Marlin's guy is not feeding you B.S. Good quality gears, set up and proper break-in are the 95%, the other processes are the 5%. It's not a bad idea to do another cryogenic temper step, I think it's just overstated. It can't fully fix what's not there to start with and the crystal lattice is formed when the iron is taken very hot (depending on alloy, the austenitize point will be a bit more than 700C and depending on the hardening process and carbon content might happen well above 1150C) and by the time/temp rate it's brought back down through its phases.

You don't do any more annealing or normalizing after cryogenic cycles (i.e., you only cross the eutectoid point during initial heat treating and don't cross back during post hardening cycles). You might (and probably will for stress relief) do more heat temper steps after cryogenic treatment, but the conversion from iron to steel that the part undergoes during manufacturing leaves you with the same structure to work with during post quenching operations. Temper, cryogenics and peening are ways to stabilize the existing martensitic structure and convert your retained austenitic structure. But freezing the part doesn't convert more iron or pearlite into anything useful within the structure, the crystal formation of heat treating happens at high temperatures and anything you do after quenching is just optimization to better align the martensitic crystals and reduce retained austenite.

FWIW, I attached a paper (admittedly a few years old now) talking about shallow and deep cryogenic treatments. It's still a developing field and although it's somewhat better understood now a decade later, the basics are still true as they've been for 50 years. It only optimizes what's there to start, it's not a magic do everything process.

Also the effectiveness of the cryogenic treatment is not uniform for all alloys. Nickel content seems to have great affect on the usefulness of the process. Very low nickel alloy takes a heat treat cycle with cryogenics better. For example LQT and QLT heat treatment cycles on mil spec alloys that requires lower nickel to produce similar results without cryogenics (i.e. a 5.5Ni alloy with cryo produces similar properties as a 9Ni alloy done without). Also the timing of the cryogenic temper produces different results, usually immediately following the initial temper after quenching produces the best results, with another heat temper following. Throwing room temp finished parts into a cryo bath might not even do anything to the structure and might result in higher internal stress if not relieved with another temper.

Surely the mechanical engineers here have some opinion, right? I just picked up some of this from the metallurgists I worked with years ago, so it's definitely not from the horses mouth by any stretch.