Bowie-Rockwell Hardness (at Tip)

P

PeterTheWolf

Joined
Jan 1, 2013
Messages
6
Hello,

I have one question.

What should the Rockwell hardness be at the tip (and let’s say 1/2” back) on a Bowie Knife to ensure it will not snap off under some normal prying. (I know prying is subjective … but under normal in the field abuse).

My process for this blade is as such:
This is a blade forged in gas forge at 1600-1700 degrees of 5160 steel
Once forged to size, I then did three thermal cycles to normalize the steel with:
Bringing the blade up to 1550-1600 degrees (in gas forge) and then pull it letting it cool in ambient still air conditions.
Then I annealed the blade to bring it to its softest state by:
1.) With blade (Wrapped in Stainless Steel Envelope with a piece of paper) in heat-treat oven, bring it up to 1525 degrees.
2.) Hold it at 1525 degrees for 10 minutes.
3.) Open the heat-treat oven door and cool as fast as possible to 1245 degrees.
4.) Close the heat-treat oven door and then hold at 1245 degrees for 6 hours.
5.) Then shut the heat-treat oven off and let it cool to ambient temperature … about 10-12 hours.
6.) This leaves the blade so very soft with no carbon scale.
Then grind/holes/sand to about 220 grit, leaving the cutting edge about .025”-.030” thick.
Then I heat-treat the blade at such:
1.) Bring the blade in a gas forge up to 1550-1570 degrees.
2.) Pull and cooled as fast as possible in Transmission fluid at 165 degrees with a plunge/lift/plunge/lift motion until it is cool to about 500 degrees.
3.) Pull from fluid and air cooled to ambient temperature.
Then I temped the blade at 300 degrees for 90 minutes, Which should have left the blade at about 60 HRc.
However, it ended up at about 61.5-63 HRc.
Then I applied my differential Heat-Treat to the spine by placing the blade in a trough of cold water so the cutting edge was cover by at least 3/8-1/2” and used an oxy-acetylene torch to heat the spine and handle up to just blue.
This gave me my Rockwell-C hardness shown in the picture.

BowieHardness.jpg
 
IMO 61 to 63 is way too high for a big bowie knife that will be used hard. Should shoot for 58-59 for the cutting edge and then temper back the spine.

To answer your question' the cutting edge including the tip should be the same hardness along the entire edge.
I think you are seeing such varied Rockwell numbers along your edge because you are heating in a gas forge. The tip is likely cooling down before the rest of the edge, causing differential hardening. It could also be due to your quenching medium. Try using canola oil warmed up to 130 degrees. Works well for me.

Try this tempering chart.
index.php
 
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Hardness was tested on a Rockwell tester with scale C calibrated to certified test block.

When I heated the blade up in the gas forge I run two thermocouples. One at each end of the forge. Both were running within 15 degrees of each other and both thermocouples were also calibrated before use.
I do believe I see the HRc deveation of the main edge and tip because of the error on the Rockwell tester penitrator on a bit of an angle at the tip.

I was shorting for 57-59 HRc on cutting edge; however, I do not think I tempered it long enough.
All this was before I purchase my Rockwell tester.

I have also purchased some 10-12 second quench oil and some 7-9 second quench to replace the transmission fuild from Heatbath Corporation. Have not tried either of these yet.
 
The temper time was not the problem, it is the temperature. If you look at the graph above it doesn't even register 300 faranheit. I would temper at 400, test it, then raise the temperature incrementally until you get your desired hardness.
 
Besides the temper being far too low, ...and the desired hardness too high - the actual testing of the blade is probably giving false readings.

With a standard tester in Rockwell C scale, you can only do a test on a blade at a flat, smooth, parallel surface. That is basically the tang and ricasso. On your blade, if the upper side bevel is truly parallel (side to side) then it could be tested between the grind line and the spine. The tip and main bevel would be neatly impossible to get an accurate reading on. Any angle skews the reading.

Just another comment, but your forging temps are too low, also. For 5160, start forging at 2200F and quit at 1600F.
 
Ok ... This is the information I was hoping to get from a forum of MasterCraftsMan/BladeSmiths.

I will indeed change my process according to the expert advice given.

And I thank you both for taking the time to share with a novice.

Shane_O_CA ....... I just have to ask .... Why the McDonald's Chicken Nugget Process Video on your website?
 
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