tempering times?

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calamari26

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Apr 7, 2007
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I've been reading about tempering lately and have come across different recommendations for tempering times. Generally i've tempered my blades two times at two hours each but have read some recipes lately going from 20min cycles to 2hrs/inch thus making me wonder if two hours are really neccessary. I guess i'm wondering if anyone has done any testing with different temper lengths?

thanks
Ryan
 
Temper for two hours minimum each temper unless the thickness of the material exceeds 1 inch in total cross section. After that initial inch add 1 hour per inch of thickness. A 4 inch section would have a temper cycle of around 5 hours, etc. Knife blades are so thin that 2 hours will suffice. The part that most people don't do properly is making sure that the steel reaches room temperature or below between tempers. This promotes full transformations to hardened structure.
 
Tempering times have much less to do with the thickness in the piece of steel with the sizes we work with. Instead it takes time for the process to occur. Tempering is a very slow low level diffusion of the carbon trapped in solid solution during the quench. Above 1350F the carbon atoms move in seconds, below 500F the carbon atoms move very little and it takes hours. But this is no different than switching to a slower moving tool when doing any fine detail work. At this stept you just want to move enough carbon atoms to drop the HRC hardness one point at a time, that is much touchier than moving from taking 2 mintues on a 60 grit hogger belt to a needle file for 15 minutes in order to be very precise.

If ever you see me describing my process with cycles less than 2 hours it is because I use equipments and mediums that heat very differently than ovens. Ovens are full of air and air is not a great conductor. At least 15 to 20 minutes of a temper cycle in an oven is taken up by getting the heat into the steel, the rest is allowing the heat to do its job (it is different at higher temperatures but at 400F heating is slow).

If working in an oven do your first temper slightly lower for the two hours described, then cool it, I quench mine in water and get to work quicker. Then do whatever tests you have available to check for hardness levels and go for a second cycle.
 
Great 411 there. Thanks!
 
thanks for the replies, but...

then what about tempering on a forge?
i don't know much about forging, but i'm pretty sure it's done in less than two hours.

ryan
 
i think your getting autensizing and tempering confused. i dont think you will be tempering in a forge. tempering is 500 f and below range autensizing temps are 1450ish and above. tempering is hrs , autensizing ort how ever it is spleed is done in 10 ish minutes some steels with high alloys like cpm 154 take 45ish minutes
 
calamari26 said:
thanks for the replies, but...

then what about tempering on a forge?
i don't know much about forging, but i'm pretty sure it's done in less than two hours.

ryan
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If you are referring to actual tempering it would be accomplished by overshooting temperatures to compensate for time. It would be like doing brain surgery with a chainsaw, you can get inside the skull MUCH quicker than with proper surgical instruments, but once there you had better be damn good with a chainsaw:eek:!
 
Kevin, does the same apply to austenitising for hardening? can you jump the temperature up quickly instead of soaking at a lower temperature to quickly homegenise things ina short period of time? As long as you are accurate with temperatures, would chainsaw-austenitising and tempering be as effective?
 
Take a look at a TTT diagram .Note that the temperature is a linear scale , 600, 700, 800 F, etc. Now look at the time scale ,that's logarithmic, 1 sec, 1 min, 1 hour, 1 day. That tells you that temperature is far more influential than time .
In a practical sense while soaking to dissolve carbides during austenitizing - excessive time is far less damaging than excessive temperature....In another part of the forum I've been talking about tip breakage .I've had a few good quality blades break off a small bit of the tip.I have to assume it's from grinding damage .Grinding time is very short yet if not done properly you can temper back the tip or worse reharden the tip with large grains and untempered structure .....For those who harden with a torch - never heat the edge or tip , let the heat from the spine diffuse to the tip. Otherwise the tip can easily overheat and get very large grain and become brittle!
 
Newby here...
How can I temper knife blades in a regular kitchen oven? Do I need to get a thermometer I can fit inside to check the temperature or just crack it up to full power for a while? I asked at home but seems we don't have the instruction manual any more... so I can't translate the settings to ºC.

Two hours af full throttle and then let it cool off inside of the oven? (overnight for example). I am talking about small blades (25mm long, 30mm wide and 5mm thick).
Mikel
 
Use an oven thermometer and do the 2x2 temper. Kitchen ranges, whether gas, or electric are notorious for inaccurate temperature controls.
 
Thanks again for the replies, i think i may be asking the wrong questions though.

What i'm wondering is how it was determined that 2 hours was optimal? why not 3 or 4? Is there some research this is based on or is it just because "granddad said so".
And why two times, why not once at 4 hours? What does the cooling in the middle do?
any metallurgists on this site want to chime in?

thanks
ryan
 
It's not an exact thing but it's been found that 2 hours does the proper job.Don't argue just do it ! As for how many that depends on the complexity of the steel .The simplest steel such as a 1070 would be OK with one temper.An O-1 would be better with two and an S30V better with three. The simple steels involve just tempering martensite while the complex ones involve retained austenite also.
 
Calamari26 (by the way I really like calamari when done right:)) I like your way of thinking, in that you ask if there is a real reason instead of people just doing it because they have been told to do it. A vicious cycle of perpetual ignorance is fostered when people do not ask the questions like what you are asking.

However, the times and temps of tempering of steel has been verified and established by a fair bit of research in the metallurgical field. It would be based upon the effects from oven tempering on Rockwell hardness, the rate at which it drops and the resulting strength. Each steel has its own optimum tempering range for the given properties that are desired and has been meticulously mapped out for the same. For example- while increasing tempering temperatures will continue to lower Rockwell hardness, it will also increase impact toughness, but in many steels there will be a range that will either not see gain in toughness or even a loss. The fact that this has been charted for almost any given steel is an indication as to how thoroughly the process has been studied.

Many steels actually retain a small given amount of austenite regardless of what we may wish, tempering can have an effect on this as it destabilizes the austenite and allows it to convert to other structures, thus increasing hardness and necessitating yet another temper to compensate, however with the percentage of austenite shrinking on every cycle it can be found that there is little more to gain after 2 or 3 cycles. My own independent research and study entirely confirms this.

Bladesmiths can afford to fiddle and do things just because Granddad said so, in industry millions of dollars are at stake if they don’t know what they are doing and why. The bladesmiths who pay attention to that and decide to finally quit reinventing the wheel are well ahead in the game eventually.
 
Cooking calamari has to be done right too ! It has to be cooked quickly or a long time anything in between and it's tough !
 
mete said:
Cooking calamari has to be done right too ! It has to be cooked quickly or a long time anything in between and it's tough !
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I hear yah! I love this spicy clamari ring Italian salad we make, but if you don't cook it quick it is like chewing on little car tires:(. While another dish that uses a home made red sauce and angel hair pasta has to be cooked good and long or you get those same little tires on you plate.
 
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