KC wrote:
Spoke to Paul Ivey. He says that when he was at Morris Motors developing
the S stuf, they used to end up with
cranks heat treated to 0.060" deep, or furtehr since they initiall used to run them at -0.060" to reduce
drag/friction, and they were still hard at that. He said these cranks
were simply left in the oven for over a week to get the depth. Obviously
impossibly expensive to do that now. After testing engines back to back
on the dyno with the standard journel cranks and the -0.060" ones and
finding no gain whatsoever, the depth of hardening was stopped and
standard size journals used. KC
--
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Ok kiddies......
Another thing I should have spelt out in my Epic post on metallurgy... (I think GT hinted at it)...
HEAT TREATMENT means the use of heat to alter the make up of a metal, in order to inhance its suitibility for a particular application.
Before I start, yes it's obvious that Keith is refering to the use of heat treatment for hardening and I will get to hardening in a sec.... but still.... while I've been sitting quitely on the topic of metals for the last year or so while people make comments like
Quote:
Mild Steel doesn't harden with heat treatment,
..... I've noticed that heat treatment is another miss understood topic.
Ok, so heat treatment. What goes on? With the use of heat, you can change the mechanical properties of a metal.
Heat Treatment, does NOT always mean hardening. I'll explain how and why.....
So, what happens when we apply heat to a steel? It starts moving. The obvious changes we start to see are changes in colour and diamentional change...
STRESS RELIEVING is the first thing that starts to happen, this hppens in most steels at around the 600-650 mark. When you machine a metal, you place stresses in it (you can also relieve other stresses but thats got nothing to do with heat treatment)... so machining or trying to mechanically straighten (by bending) a metal will place stresses in it.... as will for forging... casting in somecases... basically a stress is a stored energy... if stress is rapidly released from a metal, it will cause cracking or excessive distortion/warpage.
...... So if you were to use
heat treatment for stress relieving, you're gently removing stress out of the steel at a controlled rate. The work piece is heated up to that 600-650 degree mark, held there for the desired length of time (this will change depending on the material we're dealing with... but generally 600-650 for 30 min to 90 min) and then cooled at a very slow controlled rate. Normally you would control the cooling in a furnace with an inert atmosphere... problem with air cooling (which I mentioned much earlier in either thread or another one of the crank threads) is that it draws any carbon below the surface to the surface forming that black scale you always see on black bar... that black stuff is pure carbon, EXTREMELY HARD, and the material directly under it usually has no carbon in it at all (because its all been drawn to the surface) making it EXTREMELY SOFT and pretty much useless.... so if a material is air cooled, allowances need to be made so that there is enough material left to machine off the scale and soft layer.
Transormation Stage or critical point is where stuff gets exciting

This is at about the 720-730 mark....
So, as I mentioned in my last post on metals, steel always has Iron, and Carbon in it.... when there is less than 0.2-0.3% carbon, what you see under the microscope is a crystal structure called "ferrite",, as the amount of carbon increases we start to see more of what is called "Cementite" which is an iron carbide... and if there is as much as 0.8% carbont it, it forms what is called "Pearlite" (gets its name cos the excessive abouts of carbon give it a "mother of pearl" appearance)..... Right, won't go into that anymore, people
will start falling asleep
NOW.... what happens when we heat up this cocktail?????? Well, first off.... when you look at steel under the microscope (when its cold), the carbon and iron are clearly visable... and is sort of looks like a poorly mixed cake. You have clusters of iron without much carbon around them here, and clumps or carbon there.... this is what "Annealed" steel looks like (annealed meaning softened steel)..... and when you have extra crappy chinese "
steel" you can measure the spacing between the iron and carbon with a rule! Meaning bits of it will machine really easy, and other bits will blunt your tools!

But good steels won't have this problem,,, you'll only see the spaces under a microscope.... (incidently, for anyone interested, what I am talking about is called the "lattace structure".. and it looks just like that... lattace)
Soooooooo

.......
When we heat it up
past that critical point (by 50-90 degrees C .... up to as much as 950 degrees C in some cases) you notice a few changes... While it is held at these tempretures, it is more malleable (hence blacksmiths keeping steel at this temp while they work them).... what is happening here is a change in the Lattace Structure... the carbon starts to move freely around the iron and begins to dispurse itself more evenly through the mix.... While its in this hot stage with the carbon evenly spread thru the mix, it is refered to as an "Austenitec State".... While its here, the carbon is moving around amoungst the iron at a rate of about 10 micron a minute (0.010
mm/min) and this changes depending on the temp.
NOW what happens from here is very important as to what mechaical virtues we want the steel to hold....
If we are
"Normalising" (as in
full stress relieveing and bring back to original state) we would slowly let it cool,, and as it does so, the Carbon would slowly go back to where ever it was to start with, and it would no longer have that fine distribution through the mix...
If we are
Hardening you need to quench it while its Austenitic.... what this does, is capture the steel in that state with the carbon finely spread around and amoungst the iron... once it is quenched, it forms "Martensite"... which is the name for the new crystal structure.
IT'S NOT THE GETTING HOT PART THAT MAKES STEEL HARD, IT'S HOW QUICKLY IT'S COOLED DOWN
To work how hard to what depth, you need to refer to a relevant TTT diagram (Time, Temperature, Transformation) for the steel you have.... they all behave differently. What a TTT diagram is, is basically a graph that you use for calculating what temp you need to get the metal to, how long you need to hold it there.. and how quickly you need to quench it/what temp to quench it too.
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The above applies to nearly all common forms of heat treatment methods (Gas furnace, Oil furnace, Induction, Electric)..... Using these methods, you can harden a steel from a couple of thou (provided you don't need to do any more machining) to through harden.
So when someone says "heat treated to 0.060".." then yeah... sure... thats possible....
depending on the method of treatment....
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Actually, this all reminds me....Anyone here ever bought after market conrods? And recieved one of those letters with them giving a full break down of the conrods you bought? A friend of mine just bought a set of conrods for a Subbie... They mentioned that they were "heat treated" ... he didn't read the whole letter, just skimmed it.. when I was talking to him about it later,,, he goes "Oh yeah, those rods, they've been hardened! They must be awesome!"....

..... What forces are conrods subject to????? Compressive, and Tensile.... What do we know about hard material???? Good compressive strength,,, poor tensile..... it copes well with being squashed, but doesn't cope with being pulled............ so.... "why would you harden them?" I thought..... I asked him the quesiton very specifically "Did it say
hardened or
heat treated"..... cos heat treated could mean stress relieved... he came back "Oh yeah.... it says heat treated
for stress relieveing"....
I had to laugh at this exact same thing again very recently.... I was reading through this article in a mini magazine,, and this "engineer" was talking about how he had his conrods "hardened to 30HRc" .... I read it,,, I read it again.... and I had a bit of a chuckle to myself... the material most of these after market conrods are made out of, comes supplied (as in an annealed state) at between 250 - 300 Brinell Hardness (HB) which is 24 - 32 HRc

..... always makes me laugh, watching someone tying to claim someone elses work as their own,,,
THEN stuff up when they try and provide technical data on it.... chances are, this misguided "engineer" was told by the people he paid to make these rods for him that they were heat treated, and he assumed that they meant hardened.... and knowing nothing about metallurgy or mech engineering, he went off and did a hardness test on them
Anyway.... back on topic...
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Nitriding works differently.
For a start, you don't need to get it nearly as hot. Only between 520-650 degrees... so as GT and David alluded to earlier, at this temp the material will stress relieve.
Second of all... remember how I was just saying that in other forms of hardening you need to get steel right up into the Austenitic State so that the carbon becomes soluble and starts moving amoungst the iron? And then you
quench it to form a hard martensite structure? Well, with nitriding, the work piece is heated up to 520-650, juuuust enough to start moving, in a nitrogen atmosphere.... the nitrogen is drawn into the steel and diffuses itself amoungst the ferrite to form hard "Nitrides".
Because this method relies on a gas diffusing itself into the parent material, the depth to which in can be absorbed is limited.... if there is no Nitrogen, there are no nitrides.... you need elements like Al, Cr and Mo to form nitrides,,, and they can pass nitrides between themsleves (which is how you get your depth).... but the depth that can be achieved is limited.... this reaction "runs out of puff" pretty quickly.... the temperature will have
some bearing on the depth of diffusion as well obviously....
With other methods of hardening, ones that rely on temperature changes alone to produce changes.. the reason you
ARE able to get deep penetration or even through hardening,, is because the work piece is Through Heated.... that is, the
entire piece is Austenitic.... so the change (Transformation) is occuring all the way through. Steel isn't a sponge... you won't "soak" a gas the whole way through it.
So, the limit, for even the best nitriding steels, is ...... (drum roll) actully, in fact, only ever about 0.7-0.8
mm.
The calcs I did on GR's sample, suggest it was immersed for 25-30 hours. Thats about it. And with nitriding (EN40b), it really does plateau out after a couple of days.... even at a couple of days, all that you are achieving is better consistancy (and stress relieving).
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So... why would you use nitriding at all, if the diffusion depths are so low????
Horses for courses.
Nitriding, even at the softest end of the scale, will still produce higher hardness levels than you will get with most normal heat treatment methods.
Another benefit, because its done at much lower temperatures, there is much much much less diamentional change.... when you quench harden (as in normal heat treament) you need to get it much hotter, and there is more change as a result.
And of course there is the bit of corrosions resistance.... but.. errr.... what was everyones remarks when I said "Resistance, not proofing... I've seen rusty nitrided tools"????? Well,,,,, what do you make of this??
That wouldn't be surface oxidisation... surely.....
