Another cracked blade

Kevin McGovern said:
Thanks for all the feedback. I can most likely try most of this without any real negative impact.

Regarding

*1- I don't put blades in when it first shows 1975, but only once it is sitting at that temp without any real swings in temperature. This occurs about 20-30 minutes after the oven hits 1975 for the first time. I assume that the temperature holds consistently once the bricks are fully soaked.
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OK then, it would works however 2 segments ramp probably has lower temperature gradient from front to back.

*6- The heat treated blank sat on my bench fore 2-3 weeks before I finished it.
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Please hardness test this cracked blade to see whether finishing and aging increase its hrc. When I post-ht grind 0.05 to full thickness blades, often I do 2x 30 minutes temper again - as insurance against left over RA in middle of cross section after initial 2x temper cycles.
When a blade crack while sitting around - a simplest reasonable cause is ra to mart aging.

Btw - NJSB got NitroV from Lohmann (Germany), also same company produces Niolox.
 
bluntcut said:
OK then, it would works however 2 segments ramp probably has lower temperature gradient from front to back.


Please hardness test this cracked blade to see whether finishing and aging increase its hrc. When I post-ht grind 0.05 to full thickness blades, often I do 2x 30 minutes temper again - as insurance against left over RA in middle of cross section after initial 2x temper cycles.
When a blade crack while sitting around - a simplest reasonable cause is ra to mart aging.

Btw - NJSB got NitroV from Lohmann (Germany), also same company produces Niolox.
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I can test the blade, but not in the bevel cross section, as it's sides aren't parallel. I'm not sure that testing the tang will tell you anything.
 
Kevin McGovern said:
It was unground .090" thick prior to HT.
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This pic - i.imgur.com/e4DpsBx.jpg - to backup my yak about nitrov RA & HRC... I took it prior disassemble my knife shop and pack into moving boxes.
There are two 3/32" thick nitrov (near choppers) with 65rc tempered at 325F. Also a nitrov chopper at 63rc 400F tempered (92 seconds video: youtu.be/2DsR6DK_GFE)

iirc these nitrov aust at 1975F long soak (way longer than your), chopper with tang-in first. LN2.

I inferred your 61.5rc @325F (even though DI) is still 1-2rc below standard ht average, hence RA is my top suspect. 1975F is near upper aust temperature for aebl & nitrov, so back of oven at 2K+ would resulted with extra RA, where Mf temperature probably be lower than DI. Obviously this chef blade is long, so the tip is far toward the back of oven.
 
bluntcut said:
This pic - i.imgur.com/e4DpsBx.jpg - to backup my yak about nitrov RA & HRC... I took it prior disassemble my knife shop and pack into moving boxes.
There are two 3/32" thick nitrov (near choppers) with 65rc tempered at 325F. Also a nitrov chopper at 63rc 400F tempered (92 seconds video: youtu.be/2DsR6DK_GFE)

iirc these nitrov aust at 1975F long soak (way longer than your), chopper with tang-in first. LN2.

I inferred your 61.5rc @325F (even though DI) is still 1-2rc below standard ht average, hence RA is my top suspect. 1975F is near upper aust temperature for aebl & nitrov, so back of oven at 2K+ would resulted with extra RA, where Mf temperature probably be lower than DI. Obviously this chef blade is long, so the tip is far toward the back of oven.
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compounded by the fact that I routinely load my oven up with 2 rows of blades. Well, until recently I did.
 
Beside lacked of oven temperature uniformity issue. HT multiple blades with oven akin to bake multiple cup cakes at oven floor using ceramic (instead of aluminum) pan. Quite easy to get burnt on the edges and raw in the middle. 1 row of 6 is not as bad as 2 rows of 6 blades. Tried 2 rows of overlapped blades once - extreme warpage lession. I often ht 4-6 blades per heat and dealing with a tiny warpage due to lopsided heating. Get tricky when doing mixed steels with similar aust temp, need to arrange to make sure to put proper carbon in solution.

Kevin McGovern said:
compounded by the fact that I routinely load my oven up with 2 rows of blades. Well, until recently I did.
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bluntcut said:
Beside lacked of oven temperature uniformity issue. HT multiple blades with oven akin to bake multiple cup cakes at oven floor using ceramic (instead of aluminum) pan. Quite easy to get burnt on the edges and raw in the middle. 1 row of 6 is not as bad as 2 rows of 6 blades. Tried 2 rows of overlapped blades once - extreme warpage lession. I often ht 4-6 blades per heat and dealing with a tiny warpage due to lopsided heating. Get tricky when doing mixed steels with similar aust temp, need to arrange to make sure to put proper carbon in solution.
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except a cupcake doesn't get baked in an oven that is designed to become a thermal mass.
 
Kevin McGovern said:
compounded by the fact that I routinely load my oven up with 2 rows of blades. Well, until recently I did.
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I think the problem is with tempering, low temperature + a lot of blades takes an awfull lot of time to get the temperature uniform on all the blades, 2 hours is not enough for sure. I would bet that knife is at least 2RC higher than the desired hardness

Pablo
 
PEU said:
I think the problem is with tempering, low temperature + a lot of blades takes an awfull lot of time to get the temperature uniform on all the blades, 2 hours is not enough for sure. I would bet that knife is at least 2RC higher than the desired hardness

Pablo
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Nope. It was right on target at 61.5. I tempered them in batches in a kitchen oven with a separate thermometer as I have always done. The same way I have done many AEB-L blades, without incident.
 
Well even if it did hit desired RC, it doesn't mean it was long enough at temper to mitigate stress. If I had rows of blades hanging, I'd be tempering for 3-4 hours personally. With this steel in particular, I'd also do a final temper after all finish grinding, or anything else that may introduce stress was done.

I've said before, but doing a final "stress relieving temper" after any finish work on a part has been done, is very common in industry, and even after a number of use cycles of the part, especially cutters/blades of any type. Usually it's done 25 degrees below the final primary tempering temp, but there's no reason you can't just run it another 2 hours at the same temp.
 
I use a PID controlled toaster oven with a piece of 1/2 plate in it for thermal mass and my thermal couple tip goes in through the edge of it a couple inches into the middle of the plate. I reset the timer on it once it reaches back up to temp so I know everything inside the oven is at actual temperature. The cavity around the oven has a layer of kaowool to help retain the heat. once it levels out my temps will hold to withing +- .5 degrees. I can't do very large knives in it, but it will fit regular sized kitchen knives.

The only reason I really bring this up, is so much talk is said of how critical the heat treatment part of knife making is. When I built my forge I built it specific for heat treating since I couldn't afford a kiln so I built is with the mindset to keep the heat as accurate and even as I possibly could. I used that same mindset when I built my tempering oven which is a part of the heat treating process I think a lot of people overlook the accuracy of.
 
KnuckleDownKnives said:
I use a PID controlled toaster oven with a piece of 1/2 plate in it for thermal mass and my thermal couple tip goes in through the edge of it a couple inches into the middle of the plate. I reset the timer on it once it reaches back up to temp so I know everything inside the oven is at actual temperature. The cavity around the oven has a layer of kaowool to help retain the heat. once it levels out my temps will hold to withing +- .5 degrees. I can't do very large knives in it, but it will fit regular sized kitchen knives.

The only reason I really bring this up, is so much talk is said of how critical the heat treatment part of knife making is. When I built my forge I built it specific for heat treating since I couldn't afford a kiln so I built is with the mindset to keep the heat as accurate and even as I possibly could. I used that same mindset when I built my tempering oven which is a part of the heat treating process I think a lot of people overlook the accuracy of.
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I have a thermocouple and digital thermometer that I use. A large try of pearlite goes in the bottom. It holds temps pretty well.
 
Kevin McGovern said:
I have a thermocouple and digital thermometer that I use. A large try of pearlite goes in the bottom. It holds temps pretty well.
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I wasn't necessarily directing that comment towards you, I was just making a blanket statement as the thread has geared towards a tempering issue may solve the issue, which I hope you have success in finding the cause of. I was basically just trying to add to the thread as a statement to anyone who comes across this to pay attention to the tempering process as it is a very important step in the process of heat treatment and shouldn't be a step taken for granted.
 
KnuckleDownKnives said:
I wasn't necessarily directing that comment towards you, I was just making a blanket statement as the thread has geared towards a tempering issue may solve the issue, which I hope you have success in finding the cause of. I was basically just trying to add to the thread as a statement to anyone who comes across this to pay attention to the tempering process as it is a very important step in the process of heat treatment and shouldn't be a step taken for granted.
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No worries
 
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