I am in the process of making a surface grinding vice, all was going well until it came to hardening the main body.
The body of the vice I m/c out of mild steel, as I wanted to have this hard and wear resistant I wanted it case hardened. I packed the part in bone charcoal in a heavy walled steel box, sealed the top with fireclay and put it in a over ramped up to 900oC/1600f I let it do it's magic for seven/eight hours. I turned the oven off and let it cool overnight the next day I put the vice body back in the oven and heated it up to 840 oC and let it sit at temperature for a hour, the part was then quenched in a large bucket of water. Unfortunately on closer inspection of the part it is still quite soft and easily scratches with a scribe, I was hoping someone could point me in the direction of what I have done wrong or a solution on how to cure this problem. Thanks

Pray a lot more. Case hardening isn't as easy as many people want to think, try making it from another kind of steel, like 4140, and you will be happier, and have less problems.

I'd go with a commercial pack carburization material in that situation.

I have actually turned wrought iron into cast iron with 30 hours at temp in a powdered charcoal "case".

1600f is not hot enough to get any real carbon absorption in a reasonable time. You need 2000F+. In the manufacture of blister steel from wrought it was kept at this temp for a week!

So would you recommend casing it again at a higher temperature or at 1600 f for much longer than seven hours?

Both!

Simon,
I suggest using steel that has carbon in it already.
I have been hosting classes here making blister steel and bone charcoal is a very poor pack carburizer ..couple that with the short time at the low temp you cooked it at and you get three times failure.
The added cost of using a known steel with a known heat treatment is quickly offset by the chance of failure doing it any other way.

There area commercial case hardening compounds out there that should do what you need..casnit among others...you could hard face the sections with electric weld.

I have heard of folk running the oxy/acetylene torch with a sooty flame and holding it over the bright orange metal for a minute or two...this may work..may not..it is not something I have ever done.

Ric