Eternite - Case Hardening

Following on from a previous thread, I have a couple of small components, a couple of "tappets" (EN1 bar turned to 10mm diameter then bored 9mm diameter, 9mm deep leaving a 1mm thick top and 0.5mm sides) I need case hardened and the corresponding cams which are machined from a piece of EN8 (we spoke about that earlier).
First test piece was a spare tappet, I put it in a small tin full of the Eternite, put the lid on and brought the whole lot upto big propane torch orange for 10 minutes, left it for 30 minutes as per instructions, at which point you are supposed to quench, but by that time the tappet was barely warm. I couldn't detect any hardening.
Tried again tonight, but this time got the tappet hot and dipped it into the Eternite, left for thirty minutes. This time, still barely warm so not a lot of point in the quench. However, the tappet was covered pretty evenly in a black scale, but not hard.
Thinking about it, heating to 900C, holding it for a while and cooling slowly is called annealing isn't it? How long does the component need to be held at temperature to absorb the carbon?
Tried the EN8 cam, covered in black scale, but no evidence of hardening and after 30 minutes, little point in quenching something that is already cooled off.
a) What I am missing here?
b) Whats the best treatment for getting the scale of and the component polished up?
c) Did I really pay £20 for a bag of charcoal and is someone having a laugh? (10Kg bags were 20pence at Sainsburys last Christmas)?
Steve

On Tue, 14 Aug 2007 22:42:17 +0100, "Steve W"
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Steve, I use Kasenite and still have a big tin. To use this I heat the part up to cherry red and stuff into the powder and leave it to cool. When it comes out the powder loads of crud is stuck to the part. I then heat to cherry red again, a bit hard to see as the part glows incandescent with the crud on the part.
Then I dunk it vertical into cold water. This usually makes a big bang, scares shit out next doors cat and knocks all the crud off to leave a gray mottled finish that then makes a noise like Yodie Margarine on his violin when you try to file it.

wrote:
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Thanks John. I've lost track of what I owe you for laughs and advice, not sure which is the most precious!

wrote:
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You've got the carbon into the part (possibly all the way through it with the tappet). You need to quench in water from about 840deg C to harden it. Immediately after you've done that put it in the kitchen oven at about 175-200 deg C for half an hour to temper it, otherwise the tappet might crack if the carbon has gone most of the way through the steel (die. 10 thou per side).
These thoughts are worth what you paid for them :-)
Mark Rand RTFM

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Thanks Mark - I'll give it a go tomorrow night, making sure the cat's locked up!

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You created a case, but it's not hardened. Heat it up again, and quench immediately!
Steve R.

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What you are missing is the basic mechanism by which steel is hardened. The process is to turn carbon in the steel into carbides of iron. To do this you need carbon in the steel in the first place (no shit Sherlock!). This can be achieved by either dissolving carbon in the alloy to start with (medium/high carbon steels) or adding it later by leaching carbon into the steel from carbon rich substances that the hot steel is in contact with. When you heat steel above what is called the 'critical temperature' its carbon and iron atoms move around to form a new structure with the carbon atoms inside the centre of a lattice of iron atoms. If this cools slowly the carbon atoms have time to move back out again to the outside of the lattice and the steel remains soft. If it cools quickly the carbon atoms become trapped inside the lattice and the steel becomes hard. It turns from austenite into martensite.
Medium/high carbon steels can be hardened by simply heating them above the critical point and quenching them quickly to trap the atoms into the desired structure. Low carbon steels can only be hardened by adding carbon atoms to their outer skin first and then quenching quickly. What you have achieved so far is to leach carbon atoms into the skin of the low carbon steel by heating the steel and dunking it into a carbon rich substance in which it cools slowly. What you have not done yet is to heat it above the critical temperature again and quench it quickly to turn that carbon rich skin into martensite.
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Elbow grease.
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You tell us.

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All the other replies seem to have covered most of the issues. As regards depth of the case, its depends on the temperature you reach and the carburising medium, and time. I used to be the metallurgist overseeing a large heat treatment plant (including gas carburising). For each batch of items case hardened I had to slice a specimen in half and check the case depth - after slicing using an abrasive disk the specimen was roughly polished and then etched heavily in an acid mix - the high carbon showed as a dark band. Why I am telling you this, is that even in industry with accurate firnace conditions there is quite a skill in getting the case depth required. This is partly because the rate at which items heat up in the furnace varies according to size and shape.
The depth of case was specified so as to allow a grinding tolerance for finishing (and a bit for wear too). The quenching tends to cause changes of shape because the shrinkage in the first bit to cool bends the part thats still hot - and if you quench a straight bit of steel on its side it will curve a lot - so always quench a long item on end. If you don't want case hardening everywhere, then copper plating will keep it out, so plate the items and then clean off where you want the case.
You can find some guidance on time for case depth on the interweb (have found it before, but can't find it now), but you only have rough temperature control, so its not easy to be precise. The diffusion slows as time goes on, so real thick cases - say 50 thou or more can take hours. If you jack the temperature higher it speeds up a lot, but that is not done in practice as you get coarsening of the crystal structure. Max is about 920C.
Hope this helps Cheshire Steve (who would signoff Steve R if it wasn't already taken - there's a lot of Steves about and it seems we all dabble in heat treatment !!)

Cheshire Steve wrote:
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Shielding with copper seems to be a bit problematic. There is a risk of hard spots. One solution is to use cases that only cover the part that has to be carbonized, with the rest sticking out. Another way that works sometimes (depending on the shape etc.) is to carbonize, then mill/turn off where the part has not to be hard and then harden.
Nick

Cheshire Steve wrote:
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Got any details of the acid mix?
thx.
Peter Fairbrother

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reach
metallurgist
carburising).
disk
acid
2N nitric works very well showing up the heat affected zone, and rod runs on welds, so I imagine the same would work for case depth.
AWEM

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It was a far stronger version of the normal mix used for etching steels for metallography, which we referred to as Nital. The normal mix is 2 to 5% concentrated nitric acid in alcohol (presumably ethanol). But for showing the case depth I think it was about 30% nitric, and you have to be dead careful mixing it because a lot of heat is generated and it can erupt violently from the container (woops). Its a bit like the adage that you should always add acid to water and not vice versa - but in this case you have alcohol in place of water.
Steve

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Here's quite a neat tool for nitriding
SteveR (another one)
http://www.matter.org.uk/steelmatter/manufacturing/surface_hardness/7_2_4.html

< Lots of good stuff snipped>
Cheshire Steve (who would signoff Steve R if it wasn't already taken - there's a lot of Steves about and it seems we all dabble in heat treatment !!)
I used to sign off with my last name, and was very active on RCM, and several other Usenet groups. All was well for several years, then too many people started to find me. Most were very nice people, who only wanted to discuss metalworking, and tecnical stuff with me. There were also a bunch of creeps, including some who were here on holiday trying to bum free accomodation in my home. Things got worse when I was in a short TV documentary fill in programme. People I had never met, though they knew me, and some turned up everywhere I went. No wonder the real "hollywood" people who lived in this area kept a low profile! By the way, they included star Brit Ecklund, and the late director(?) of Bridge on the River Kwai.
Sorry for the rant!
Steve R.

en8 will harden if you quench in water or oil when heated to a suitabl
temprature without using a case hardening compound MB
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