Hello everyone, this is my first time posting on this forum,
I started forging as a hobby a couple years ago and I am a beginner in the craft. I forged three blades so far, and only the first didn't break in half and actually made it to being an actual knife, but it was very short in comparison to my third blade. The second blade broke when I tried, foolishly, to straighten it in the vise right after quenching.
I come to you for advice on why my third blade broke, so here is the detailed process it went through, from forging to tempering :
The steel is from a used crop lifter spring my uncle gave me after it broke : here is a description ; I attached a picture of the specific part I used;
I forged it into the rough shape trying to never forge it when it wasn't at least red/orange hot, ground the profile and then the bevels until the cutting edge of the blade was around 1.5 mm thick (thickness of a dime, as many people recommend across all the forums I searched); I use a homemade charcoal forge made of refractory cement bricks;
At this point the blade is 14 cm (5.51") long, 24.5 cm (9.65") long in total with the tang, 3 cm (1.18") wide and nearly 4 mm (0.16") thick in the middle with an apple seed grind;
I then proceeded to heat it to bright orange before quenching it in brine (10% weight salt solution), as someone recommended on some forum, because oil quenches supposedly too slow for spring steel; the "quench tank" is a piece of PVC pipe
It warped badly, so I tried straightening it by first putting it in the jaws of my vise and tightening gently (I didn't hear any cracking noise) right after the quench;
Seeing it didn't do anything, I tempered it for 2 hours at 270°C (518°F), tightened between two steel plates;
This straightened it a bit, but it was still badly warped, so my plan was to tighten it to a steel plate with a piece of round wire below it to give it some flex in the opposite direction of the warp, and temper it for another hour at 518°F;
At this point it started clicking and cracked in half, before I even put it in the oven;
Looking at the fracture surfaces, there is a purple coloration in some areas, which means a crack formed before the temper cycle and oxygen got in during the temper cycle (purple corresponding to ~270°C (518°F)). I attached pictures of the fracture surface.
My problem here is that I can't determine if this crack originated from the forging or the quench. So I wanted to know if, by looking at it and knowing the process I used, you could help me determine the origins of this crack.
By closely looking at the rest of the blade, I can even see another crack, on the same face of the blade as the one who lead to failure.
So my conclusion is that the blade would have failed eventually, because there were cracks before the temper.
My ideas of improvement are :
Quench in oil; I was thinking of canola oil because I have an expired bottle of it;
Regularly clean the scale during forging with an angle grinder;
Try to straighten only during temper and not in the vise right after quench;
I'm in my last year of a material science masters so I have a good understanding of theoretical metallurgy, so don't hesitate to go specific in your explanations.
I'm sorry if I sound weird, arrogant or contemptuous but I am French and I don't write or speak English daily.
This blade breaking defeated me because I was very satisfied with it. This broke my heart.
I come here seeking advice in all humility.
Thank you very much in advance for your time.
ATTACHMENTS : I put my three pictures in this imgur url : https://imgur.com/a/WU0MAOD
