Need help with broken blade

[Solmyr999]

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

 

 

[JHCC]

Bienvenue à IFI! Si vous n'avez pas déjà, LISEZ CECI EN PREMIER s'il vous plait.

[Steve Sells]

Used steels are always a risk for breaking

Water quench is too fast for most knifes

tempering at 500f is too hot for a knife, ok for a sword or an axe

bending it in a vise after hardening is asking for breaks

[Solmyr999]

Hello !

Thank you for your answer

I actually have round stocks of C70E2U (0.7% carbon) and 45Si7 (0.45% carbon) I should have used right away but didn't because I didn't want to have to flatten them down, as they are pretty thick.

Which one would you recommend I use for my next knife ? Would canola oil be better for quench ?

Is tempering between two steel plates a good idea for straightening a warped blade ?

[Steve Sells]

C70 is a good steel.  canola is fine.  if its warped bad re heat and harden it again

[Goods]

The 0.45C will work better for tough tools than knives. 0.7C is probably around the bottom end for straight carbon steel blades. Stick with the canola oil for quenching thin sections of medium carbon and most thicknesses of high carbon, but warm it up think coffee temperature. Doing so will decrease the viscosity and increase the cooling rate. To help prevent warping do a few normalizing cycles before quenching . That should help refine the grain structure and also relieve internal stresses left from forging. 
That’s my amateur input and may not be perfect, so I’ll defer to Steve’s much greater expertise.

David

(Steve, you posted just as I was writing...)

[ThomasPowers]

Don't use an angle grinder to remove scale---removes steel as well.  Use a wire brush like welders use.

Canola oil is great; use it around 120 deg F to slightly higher.  Be prepare for a flare up when the hot steel is quenched and move quickly from the forge to the quench tank!   Some steels have about a second to get under the nose on the chart!

For starting bladesmiths I suggest 5160 steel as it's quite forgiving and easily sourced in easily managed sizes.  (Coil and leaf springs for autos are often 5160---NOTE if you use used materials you want as few fatigue cycles on them as possible!  I used to buy 5160 from a spring repair shop that were cut off pieces from new stock they were making replacement springs from. )

[Solmyr999]

Thanks everyone for your insights and your time !

Now that I think about it, if this crop lifter piece has been used until it broke, it must be filled with fatigue cracks, which could partially explain my issues.

Where I live, finding anything other than mild steel is pretty hard, I had to drive quite a bit to find proper stocks. I'll try asking mechanics if they know where I could buy spring leftovers.

For now, I will try forging from C70 while taking everything you told me into account.

Thank you all again for your quick responses !

[ThomasPowers]

I'm lucky as I live in an area where lifting vehicles for 4 wheel drive purposes and lowering vehicles for lowriders is common and suspension shops generally scrap the springs the vehicles come in with--p-even if the car was brand new and just driven from the car dealer to the suspension shop!

I've even found a coil spring at the scrapyard that still had the paper label on it and they don't last long in our climate!

[Solmyr999]

I'd love to search in scrapyards not only for steel but other materials like wood, containers or electronics but picking up items from scrapyards is considered theft and is therefore forbidden in France, for stupid reasons like "people might hurt themselves with what they picked up and sue the scrapyard". Better to incinerate everything than let people recycle it I guess ...

[ThomasPowers]

Here some of the scrapyards sell to people.  The scrapyard owns it, they can sell it. Unfortunately many if not most scrapyards do not do that any more either because they have a contract to sell all they get to a specific company or they are worried about liability if someone gets hurt hunting for or using stuff the scrapyard sold.

I live in a rural area, a bit more than 1 person per sq km, and so things are a bit more relaxed and friendly as you see the local people at the cafe, church, schools, etc.  We are also a fairly poor area so lots of reuse of things is done by people in their homes.

[JHCC]

2 hours ago, Solmyr999 said:

Now that I think about it, if this crop lifter piece has been used until it broke, it must be filled with fatigue cracks, which could partially explain my issues.

Very good insight; this is also a problem with reusing old or broken springs.