Questions About Soak Time When Hardening

M

marlin121

New Member
I recently bought my first HT kiln. After doing an initial firing to test the operation of the unit, I treated my first blade. During the process I made some notes and after considering it I came up with a few questions. Hopefully someone here will be able to shed some light on the subject for me. A little background before moving on to the questions: The steel I was treating is 3/16" ATS-34, the program I used for hardening is ramp up at full speed to 1925F, hold for 15 mins, plate quench.

1) Soak times are based on the thickness of the steel. If you finish a blade before heat treating and the thickness at the cutting edge is about .020", wouldn't that be the thickness you would use to determine your soak time at full temperature? Soak times seem to be based on the greatest thickness of the steel i.e. 1/8" or 3/16", but if the area of greatest concern is the cutting edge why wouldn't that be the thickness used to determine how long the blade should soak at full temperature?

2) Using a full speed ramp up to 1925F it took 96 minutes to reach full temperature. It took approximately 15 minutes from 1800 to 1925. Due to the length of time that the kiln takes to reach full temperature, is it really necessary to have an additional soak period?

Thanks,
Dan
 
A 15 minute hold is not enough for it to get to solution. Your blade is just getting to temp. After 30 minute hold time, quench the blade and when it gets to room temp, put it through a 375 F temper cycle for 2 hours. Then when it cools down, put it through another 2 hour temp. Then when cool to room temp, grind your blade to what ever finish you want.
 
Tom is right, soaking has very little to do with blade thickness. Its all about getting the elements/alloys into solution. I don't start timing my soak until 5 minutes after my furnace reaches the target temp.. Just because the furnace/kiln is at temp. that doesn't mean the steel is. If I err, I err on the side of more soak time than less. Those soak times are specified for a reason. If you wanna get the most from your steel you should use them fanatically.
Oversoaking for 10 minutes wont hurt anything but undersoaking 10 minutes almost definitely will.
 
As far as soak time goes, you are soaking the entire thickness of the blade to get the carbides to dissolve and release their carbon into solution so that can harden in the entire blade and not just on the edge only. The molybdenum in the alloy forms carbides that resist going into solution and requires higher temperatures for an extended times. Chromium dissolve at lower temperatures. I would place my blades in a hot oven and give the blades to chance to come up to temperature after the temperature recovers from having the door open and then start timing.

Doug
 
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