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

[anthonyget]

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.

[suther51]

I admit, total shot in the dark here.

Wet coal, hydrogen embrittalment??

Hopefully those with much more knowledge than I on this will weigh in.

[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.

[anthonyget]

Thank you so much CanBeDone. Unfortunately I wasn't able to open your attachment however your comments are very much appreciated. I will bear all that in mind. I was worried I hadn't annealed it correctly. Thanks again.

[TheElderBrother]

Yeah! What he said!

(I hate it when I read a post about something I'm involved in, like knifemaking, and end up feeling like a total amateur...)

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.

[anthonyget]

Yeah! What he said!

(I hate it when I read a post about something I'm involved in, like knifemaking, and end up feeling like a total amateur...)

How do you think I feel.

[old kodger]

I'm not a forging guru, but I had the same thing happen to a piece of, what was known then (1961) as S90 hexagonal bar 1" a.f. In the process of drawing it out to make a cold chisel, the end cracked resulting in a much shorter chisel. When I enquired of the heat treating specialist in the factory complex "why?", his comment was that it wasn't hot enough when I was hammering it.

[anthonyget]

I'm not a forging guru, but I had the same thing happen to a piece of, what was known then (1961) as S90 hexagonal bar 1" a.f. In the process of drawing it out to make a cold chisel, the end cracked resulting in a much shorter chisel. When I enquired of the heat treating specialist in the factory complex "why?", his comment was that it wasn't hot enough when I was hammering it.

Thanks so much for taking the time.
I don't think it is that. I checked the video and it was hot enough. I think I made a mistake in the annealing process. You might be right, but not on that heat. I might have hammered it too hard on a previous heat that wasn't hot enough and caused the crack later. Very frustrating but I really appreciate your input.

[sossol]

Above all else, what I like most about your videos is how you present your successes and failures with equal objectivity. I feel like I'm learning alongside you, albeit without all of the actual work.


Neil

[anthonyget]

Above all else, what I like most about your videos is how you present your successes and failures with equal objectivity. I feel like I'm learning alongside you, albeit without all of the actual work.


Neil

Thanks so much Neil. Really appreciate your kind words.