Tried to forge a blade out of a big file. Disaster struck

[CanBeDone]

I found a beautiful huge file in a salvage yard. Not good enough to use as a file so thought I would forge it into a Japanese Tanto.
It was all going so well until... If anyone can tell me what I may have done wrong I'd be very grateful. Thanks in advance.

There are several things you should have been done:
1. Run your angle grinder over it and check the color of the sparks produced. If brightly yellow, it is a plain carbon or low alloy steel. If dark red, it is rich in chromium / nickel, and then you better know exactly what its chemical composition is, or follow slavishly the heat treatment / forging procedures prescribed for it.
For a plain carbon or low alloy steel, guesswork is good enough - mostly. These you may treat as plain iron-carbon alloys, and the iron-carbon phase diagram tells you the whole story. If you cannot read a phase diagram, you are well advised to get a book and learn about it. Here I do only the very basics.
Attachment 36514
Since your raw material is a file, the sensible assumption is to consider steel with 0.8..1% carbon and thus you have ferritic steel. I am putting emphasis here on how the steel acts under observation by x-rays, ferritic meaning it's crystallographic structure is body-centered cubic. Ignore the phase descriptions given in the attached phase diagram, they refer to observations under a light microscope.
If you do heat treatment to just soften the steel, stay below the A₁ transition temperature, i.e. below 723^oC. 500..600^oC is a good choice. If you do want to go higher, all your steel must be hot enough to be in the γ (gamma) phase, also called austenite. If not, you will have problems with toughness / cracking later. Use of the magnetic transition temperature (A₂ can give you guidance, but is no guarantee that your steel is fully austenitic. Small remainders have huge consequences for toughness / cracking. And if you want to do forging just below A₁, i.e. what is commonly called thermomechanical treatment, you need huge deformations per blow. Hand forging in that temperature range is just calling for trouble.
The most likely reason for the steel cracking is your prior heat treatment, and here not the maximum temperature obtained, but the time it was held there leading to excessive grain coarsening. The fracture pattern you are showing is consistent with that - if the steel did contain no grain refiners (Al, Nb, V).
But if the steel was fine grained, and a metallurgical investigation would show that, it might also be due to copper, phosphorus or sulphur segregation to austenite grain boundaries. I doubt it, though, since it would require liquid on at least some grain boundaries - and for that the temperature is wrong.