Before or after heat up.......

[Gunner]

I have looked and not found an answer or suggestion on this subject.......

I normally lay the blank on top of my gas forge while it heats up, blank may get to 80-90 degrees depending on weather. Once the forge has been running 5-6 minutes I put the blank in and do the heat treat.

On tempering I just stick the HT'd blank into the oven and let it warm up with the oven.

Question- Is it better to let the blank heat up with the forge or stick it in at high temp. I have always felt jamming a cold blank into a hot forge could cause stress cracks but letting it warm up with the forge could cause a over heat problem.

Same as for the tempering oven, though it is a way lower temperature overall. What are your thoughts on this and how do you handle this issue?


Gunner

 

[J. Doyle]

Generally the standard accepted practice is to let the forge or oven come up to the target temp and equalize THEN put the blade(s) in.

I feel this is pretty important for the high heat operations such as thermal cycling and austenitizing.

It may not be as much of an issue with tempering but that could vary with ovens and tempering setups. So it's probably a good idea to build on the same good habits with tempering.

 

[Doug Lester]

If you are concerned with the steel heating up too fast and developing stress cracks you could bring the forge up to a lower operating temperature by turning your flame back and, if necessary, stick the blade into the forge for a minute or so and then take it out so the the heat can equalize. Just keep going back and forth until you are happy with the temperature of your bade. (Believe it or not steel is not all that good of a heat conductor)

As far as tempering in an oven I recommend that you allow the oven to come up to the temperature that you want and then stick the blades in. This will reduced exposing the steel to the heating elements cycling to heat up the oven.

Doug

 

[EdCaffreyMS]

Probably the most important thing that many miss when it comes to heating cycles, whether high or low temp types, is that for specific changes to occur within the steel, it requires a specific temp, for a specific duration. That being said, it's often more difficult to "nail" the exact temp desired in a "high temp" situation such as thermal cycling. Low temp situations such as tempering temps are easier to "nail", although they often require more patience. Over the years, I've found that tempering is very critical, therefore, it is where I exercise the most patience when prepping/actually conducting that step. I will turn on/set the desired temp in the oven I use for tempering, and allow that oven to cycle for at least 1/2 hour AFTER the beeper goes off (indicating that the set point has been reached).
I have a poster hanging on the shop wall that states: "Steel is the heart of the knife.....Heat treating is the Spirit" This reminds me to take as much care and as much time as necessary with heating/cooling operations, as I do with everything else.

There is a lot of conjecture concerning tempering, and the number of cycles used. Personally, based on testing/spectrographing I am convinced that at a minimum 2, 2 hour cycles are a necessity if you wish to fully achieve the desired change in the steel's matrix. I do 3 cycles, simply because testing has shown that in some instances a 3rd cycle is beneficial, but not always an improvement. Therefore, I choose to do the 3rd cycle simply as "insurance".

When it comes to heating/cooling cycles with blade steels, the one that usually represents the most potential for damage is "sub-zero quenching" or the use of liquid nitrogen. The "damages" often occur when an individual removes a blade from liquid nitrogen, and does not force it to warm up SLOWLY...... in other words the blade is removed, and placed on a bare bench, or other surface that causes it to warm too quickly, and the result is often micro cracks that only become evident during final finishing..... then you will see what appears to be a tiny/fine "hair" on your blade, and as you sand/finish, it becomes larger and more visible. The answer? Use a piece of kawool. Place the blade on the kawool, then either fold the kawool over the blade, or lay another piece of kawool on top and LEAVE IT. It will generally take 3-5 hours for the blade to warm up to room temp. Once that's completed, then run it through a "snap" temper to prevent any micro cracking, and continue your process. Generally, if using liquid nitrogen/sub-zero quenching, you'll want to temper a blade at 25-50F higher then you normally would for the same steel without a sub-zero quench.