Heat treating steel

[Drains]

We have a lovely new (2nd hand) Ramsell Naber tool-room furnace at work, so we're now heat treatment central. What fun.

However, one consistent problem with heat treatment (we're talking annealing, stress relieving, normalising, hardening etc) is the formation of surface oxides. There are one or two straightforward remedies - eg wrapping the object to be treated in stainless foil, purging the oven furnace with nitrogen, argon etc ...

Also possible to use a box and put some form of reducing substance in to absorb oxygen...

Anyone here have experience of this, or advice on techniques and substances to use?

 

[graffian]

Another thing that works if your doing small stuff is coating with borax/water.
I coat small parts made from silver steel with Borax before hardening and afterwards they come out shiny/grey. Reminds me I have run out of Borax.
Bloke who told me to use Borax told me to use Borax and Meths, water seems to work.

 

[chunkolini]

I have read somewhere about using sawdust as a reducing agent. Ancient memory residues here, a reducing environment is one where available oxygen is used up reducing the amount available to affect the work piece? I think it was for carburizing knife blades, get them to the right temperature and stick em in a box of sawdust which burns and takes up all the oxygen. Giving the blade a higher carbon content in it's surface.

Or is that complete balloks.

Chunko'.

 

[TechnicAl]

I think your talking about carburising (google it). There are branded products but I think they are usually based on charcoal.

 

[Drains]

I think Chunko it's not complete ballocks... the carbon in the sawdust gets added to the knife blades - a tad of case hardening going on there? Doubtless there is some reducing going on as the spare carbon absorbs oxygen and becomes co2. Or something.

Another thing i've read about re reducing is using "neutral coal". I think in the old days my grandad used it on the fire, but he called it coke, the down to earth fella that he was. I reckon the last coking plant round here closed down about 35 years ago...

 

[hareng]

This is coke and it dont work, masses of scale after annealing.
http://pic20.picturetrail.com/VOL15/728921/3960275/49395040.jpg

 

[TechnicAl]

Stretching my memory here but you can use coke but you need to introduce a gas into the atmosphere........I think it was Ammonia. Something you wouldnt want to do unless you had all the correct equipment. I think the Ammonia splits into Nitrogen and Hydrogen, the Nitrogen forming nitrides and the Hydrogen reducing that atmosphere so no oxides form. Therefore the steel gets hardened but remains bright......well thats the theory.

Coke has had all the active ingredients removed so its really just a heat source.

 

[Drains]

Aye Al, but in these modern times BOC do a nitrogen/2% hydrogen mix, saving the bother of messing about with Ammonia (remember those printers? - I spent years churning out A1s stinking of NH3), [... edit... I notice ammonia is used specifically in nitriding ....] the Ni is the purge and the H2 the reducer. Not sure what heat will give nitrides [yes I am: 500-575 degC], and that's not necessarily what we want at this stage of treatment, although we do have a call for nitriding some components.

"Inert Carbon" is the current word on the street - we'll see if we can get some. I'm going to knock up some s/s annealing boxes c/w gas entry option, find carbon, plumb in our nitrogen bottle that's not got enough pressure to pressurise our hydraulic accumulators any more, and see what the results are with various configurations.

 

[Welderpaul]

Boys and their new toys

If the oven is anything like the one our profilers have, its used to store old bits of broken machines.

Be honest now...are you just playing?

 

[Drains]

Cheeky laddie....

Ok, I confess, if I spend time getting the furnace sorted, I can put off doing the other jobs I am paid a to do. So sack me.

No Paul, this is proper stuff. We have rakes of experimental components which get sent off for stress relieving, hardening, tempering, annealing, nitriding, pre-heating, etc. It takes ages and costs plenty. A working and reliable oven has been sorely missed here, but there's no point in doing it badly, as these bits and pieces are expensive one-offs or small batches and ruining them by getting them all scaled up is a waste of time and money.
A vacuum furnace would be a winner, but it was a struggle to get the one we have, so we need to make the most of that.

Graffian - the borax/boric acid thing may well be included in our heat treating tool kit, but we really want a remedy we don't have to apply and wash off, if we can get it.

 

[Welderpaul]

Off topic: Did you ever get that saw repaired/replaced?

 

[Drains]

Yes. The foreman slyly ordered a Startrite horizontal bandsaw without telling the boss, who was still going on about how he thought mech hacksaws were better and more accurate as it turned up on the wagon....

He likes it now of course. Almost admitted it at one point.

This Ramsell Naber oven is a replacement for an ancient and highly Hasawa illegal Wild Barfield which stopped working. In this case, it was me who put the order through with a quiet nod from one of the directors.

 

[graffian]

What happened to the old oven?

 

[Drains]

What happened to the old oven?

It's being fixed. After much shenanegens it turns out there was a fusable link in there which blew after a 1000 degree session. There was nothing else wrong with it, although the old controller was iffy - you had to open up to adjust the temp, and there were exposed wires inside. It was accurate though. Now replaced by a digital controller and and relay which are legal...

It'll be a standby. Sorry!