Question about crack in forged blade...

D

DaQo'tah Forge

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Hi this is Daqo'tah

I finished the main forging of a John Deere load shaft into a knife last night,,,I took my time so as to not over-cook the blade this time....I watched everything as closely as i could,,,,there is just NO WAY I over heated this blade!

But as i started to grind off the scale today I noticed a hairline crack near the tip of the blade. (about 1 inch from tip)

I was bummed out at this, but was able to grind down into the steel deep enough to get past the crack...Then as I was grinding on the other side, I notised 2 more hairline cracks!(both cracks near tip, )

Clearly I did something majorly wrong when I was forging.

and your guess that would be?_______
 
What did you quench it in, and how warm was the quenchant?
 
If you found the cracks at the cleanup stage from the forge, I would suspect the steel. John Deere load shafts have a great reputation as being a clean steel, but there is always the chance that you got a shaft made of inferior stuff. New 5160 has developed a pretty bad reputation in the past few years for inclusions, and John Deere might have gotten some bad steel to make the shafts out of.

If the cracks developed in the quench, it could be that the quench medium was to fast, or cold. It could also be that the blade had some large scratches causing a stress riser and a crack. I finish all of my blades to 400 grit by hand before quenching, sometimes 600 grit. I also make sure that all of the scratches from the sandpaper run parallel to the cutting edge, although I don't know if that really matters.
 
I'm still learning here, but I thought I read somewhere that cracks like these can occur if you "overwork" the steel or let it cool instead of continual reheating/working?:confused:
 
OH,,,DARN!..
okay,,,,,I have to add one thing...

due to some advice I got a while back about quenching a blade as I forge it...Last night about 5 times I would work down the tang a bit, then just as the steel was not red in color anymore, I would take the hot steel outside in the snow and there I have a bucket filled with quenchingh oil...

I did this type of random cold-oil quenching while forgeing the blade...

The idea was told to me by a very well known knife maker, however I may stop this type of quenching...

actually I was told to quench in "Room temp" oil,,,well, thats different in the summer than in the winter,,,,and due to the chance of fire the can of quenching oil has to sit outside,,,,in the snow,,,temp outside is about 11 above.


so,,,Thanks for the advice, and I see now that my error was likely caused by the randon cold-oil quenches that I will now stop doing in the winter....
 
DaQo'tah, you can also get the cracks from working the steel at too low of a forging temperature. I had this happen a couple of weeks ago. I was forging a new prybar from a small load shaft, got careless, let the steel get too cool and cracked the tip of the bar and had to start over. The cracks may be from the cold quench or too low of a forging heat or a combination of both.

Todd - I have never seen an inclusion in a J.D. Deere load shaft. I have seen many in steel from Admiral.
 
Fox.....ummmmm...I dont know.

I know for sure that I did not over-heat the steel...but as far as working the steel too cold?.....

I didnt think so,,,however ...well..Im not good enough at this to actually know for sure.....

but,,,I did do about 5 "Cold-oil" quenches

and that will not be something I try again for a while...

And, ANYWAY,,,,I fixed the cracked blade just now by drawing a new outline on it that cut off the cracks, then I just used the belts grinder to grind down to my new outline...the knife is a bit shorter now than I had planned....

I have just this moment finished the Heat-treating with my O/A torch...and at this moment I have the blade in the kitchen oven tempering at 330-350.


I have always used up to 3 heat-treatments on my blades,,but as things were going so wrong with this blade anyway, I thought, "What the heck?". and stopped at one treatment...
 
DaQo'tah, quenching the blade in cold oil goes against everything that I have learned. Are you sure that who ever gave you that advice wasn't just pulling your leg, hard?
After forgeing I normalize, ie. heat to non-magnetic (critical) plus a little (should be about 50 degrees) then cool to black in still air. That step probally isn't necessary for something like 5160 (your bar) but makes me feel better and doesn't hurt anything. Then anneal, meaning heat to critical and cool slowly. Lots of people use vermiclite but I find that just leaving in the forge (gas) while it cools does well.
After grinding and before heat treating you neen to normalize at least twice. Finally heat to critical, quinch in 140 degree oil, degrease and put it in the oven at 375 for a half hour, allow to cool and repete. 3 times is probally better.
Then the file test, sharpen then brass rod test. Reheat treat if it fails eather.
If I haven't been clear or if anyone disagrees please let me know.
Still learning, Lynn
 
I can sort of understand the idea of working a section of blade...quenching...reheating next section and working it etc.,etc.,but it seems to me that it's more time consuming than continual reheats with the quenching at the end. As far as a cold quench, everything I've read says to preheat the oil to 160 degrees or there abouts to prevent the steel from busting up:confused:
 
Lynn has a good point, the best practice is to normalize and anneal after forging. Cracking may be a decrburization problem also. In industrial practice typical oil temperature is 135 F, there isn't any need to go higher.
 
This is a private message I received. I am deleting the name because I don't have permission to quote.

<message>

This is the way I would do it:
Toward the finishing forging cycles I would take the blade just above critical and quench it in room temp Type A oil, down to a black heat, do this several times while working down the tang.

(me) [I don't understand!] This seems like heat treating during forging. While some steels will accept the stresses involved in the rapid cooling it seems unnecessary at this point. The heating of another porton of the blade would seem to make the rapid cooling redundant.

After the blade is forged, heat to above critical, hold in a shadow and watch the blade cycle down to magnetic, first it will be nonmagnetic andlight colored, it will darken in color, then lighten back up again then back in the forge to heat back up. This is what I call "the blade
smith normalize", do this twice, then heat back up to critical and allow to cool to room temp in still air (a complete normalize critical).
Then back into to forge to about 900 degrees and allow to cool down in
the forge or what ever you have to cool it down slow, should take about 7 hours to reach room temp.

(me) This is the best description of normalizing I have ever read. Great!

Quench three times in Texaco Type A heated to about 135 degrees f. (the
higher temp 165f that I use is fine tuning for my 52100.) Let it cool
to room temp every quench in the oil.

I would temper the first time at 350f and see if it chips (edge flex),
then go higher if necessary.

(me) While 3 quenchs is workable I agree with the previous post about this subject that holding for a few minutes gives the same results for complex steel like 52100. I think it may be unnecessary for 5160 and 10XX steels.
As always this is just my ideas and I appriciate diferent openions.
Lynn


If you follow
 
I agree with Fox that the cracks could be from working the steel to cold. I cracked a bearing race the same way a couple of years ago. I have never heard of anyone encountering inclusions from a John Deere load shaft, but it is possible. I'd guess that the cracks came from the cold oil quenching, but there's no way to really know. If you're worried about flame up, you could keep a metal lid handy and just let the blade fall and cover your quench bucket if it flames up. Then you could have your quench medium inside. I bet it gets really thick in the winter up there.:) :) I've had several blades crack in the quench, but it was obvious when it happened. I have always heard that dreaded "clink" when it cracked. I haven't had any 5160 crack, but I haven't worked with it much.
 
Cracking doesn't have to be obvious. You can get , by poor practices, microcracks which can only be seen under a microscope .These can later ,in use , propagate and cause blade breakage.
 
Anyway,,,thanks everyone for your advice,,,I do now believe that the cold-oil quench during the last bit of the forgeing was the reason I ended up with cracks...

This blade was the 3rd from the same shaft,,,the first blade was the best I have ever made, the 2nd had "dots" on the cutting edge that I was told might be due to over-cooking the blade in the Heat-treatments...

This now is the last blade I will get from the same shaft,,This didnt turn out to be my lucky shaft to use for blades,,,

I was able to scratch a line that was clear of the cracks and then grind down to that line on the blade and this was to have the effect of cutting off all the cracks,,but guess what...as I was sanding again on the other side, I yet again found more cracks,,,,

by now my knife blade that started at 7 inches, is now down to 4 &1/2 inches,,,,it's still a workable length, but I cant go any shorter....

to sum up.....I will not do a cold-oil forge quench during the winter in North dakota again.....LOL
 
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