Simple case hardening recipe

The way I understand "case hardening" describes any process where a hard outer "case" is applied to a part. The classic process is as Bill Davies said "pack carburizing" where carbon is added to low carbon steel by heating it in a closed box with charcoal. Other processes adds nitrogen (nitriding) or a combination of carbon and nitrogen ( carbonitriding) using either a molten salt bath or a controlled gas atmosphere.

A simple yet effective concoction is regular BBQ charcoal with say 5% of calcium carbonate (limestone) and/or sodium carbonate (washing soda). The carbonates provides a source for CO2 which combined with the charcoal forms carbon monoxide. This is the actual carrier that transports carbon into the part.

The deposition rate is slow. You can expect a case depth of 0,1mm per hour at the beginning, but this quickly drops as case depth increases.

That's the first time I've seen the OED quoted as an authority on metallurgy. SOD's reference might well have been the only way in ancient times, (they also used bone dust, a byproduct of making buttons) but for a long time Fulmen's definition is correct

@SillyOldDuffer: Various carbonates are added as "activators", but I haven't found any good info on the reason for the different metals used. But I suspect it has to do with the temperature and rate of decomposition (into CO2 and the corresponding metal oxide). Barium carbonate breaks down more slowly than sodium or calcium carbonate so I suspect it's mostly needed for very thick casings on large parts. These can take days to form.

I actually have suitable barium salts, but never tried it due to it's toxicity. In my experience a 50/50 mix of sodium and calcium carbonate will work just fine.

IIRC Cherry Red was primarily ferrocyanides, forming a very thin nitrocarburizing case. While nitrides provide a very hard, wear resistant case they don't really provide the increased structural strength that carburizing does.

While working the plating shop as part of my training there was a process called Black Chrome.The parts were first Copper plated and immersed in what looked like boiling sawdust mixture. Upon removing the parts were then brushed to get the copper plating off, and up came a lustrious black chrome. I later learnt that Browning shotguns had black chrome barrels to prevent wear.

The application I saw was for the internal parts of large gun recuporators.

Well, that's very interesting. I always thought Iron at 2000°C would have the viscosity of running water. So they had a method to keep the original form after the case hardening?

Regards,
Hans

2000C is slightly on the hot side of things My notes say 900-950°C, and it has worked very well for me in the past.

One source suggested the following mix: 50% charcoal, 35% coke, 10% barium carbonate, 3% calcium carbonate and 2% sodium carbonate. I've only used straight BBQ charcoal with 10-20% carbonates. As noted earlier the barium is probably not needed for our kind of work, any carbonate will work fine or at least better than none.

Dirty engine oil I believe

Make sure you plunge "straight" along the axis

in the 30s and 40s velocette used to do black chrome handlebars which mine were lucky enough to have.

I plead guilty. No idea where 2000° came from. My book says 900 to 920°C and all I did was copy it. Should have realised it was stupid because I know steel melts at about 1250°C.

Time to take up knitting...