I am going to be heat treating my first knife soon (1084) I have purchased a kiln (thanks USAKMS) and the heating part seems pretty strait forward but I have a few questions.
First, does 1084 need to be foil wrapped for heat treating.
Second, I have been reading that the tempering sequence needs to happen very soon after HT.How soon is very soon? I was planning on being able to use my kiln for both HT and tempering. Will the kiln cool down to the proper tempering temp soon enough to temper after HT or do I need a seperate tempering oven.
Third, If I need to get the steel from quench to tempering soon, what kind of temp should the steel be at that time. Should it be basically room temp or something else.
Thanks for the help. I have been reading as much as I can on this but havn't specifically seen this.
I was once asked what things I would change if I could do the knifemaker thing it all over again. I didn’t take very long to assess the two easiest things that I felt would have eliminated years from my learning curve and put me that far ahead of the game with little to no effort, or expenditure, on my part:
1. Using a good known steel, (both chemistry and heat treatment history).
2. Using a predictably consistent and reliable quenchant (note the lack of mention of speed alone).
These answers immediately elicited an overly defensive debate that continues to this day. The underlying psychology for such strong resistance for control over what are arguably the most critical aspects in the making of a quality cutting tool is beyond my area of expertise and the scope of bladesmithing forums, however, occasionally I see a self-professed new maker who I envy for naturally having the keen insight that I lacked.
Of course, with the most critical part of heat treating any steel being the ability to control temperature, a good oven undoubtably ranks up with the other two, but iIts omission from the list is due to its expense. Most beginners understandably rank cost equally with the effectiveness of the tool so the two items listed above top the list simply because of their “bang for the buck” factor cannot be beat.
But you, Cojab, are not the average beginner with how serious you appear to be about heat treating your first knife and what you have invested in obtaining the most control over the heat as you can; you have a good steel and a good heat source. Spending some time learning to do it by eye does have its benefits but so does the determination to have the most control possible. Now with your heat source in place you are looking for knowledge on improving the heating conditions and follow up treatments, a good systematic approach that I hope we can assist with.
Although some steel are more prone to problems from the atmosphere when at heat, all steels can benefit from some shielding in a kiln. While foil may be a bit much for 1084, I think a standard anti-scale compound would be nice. The iron-oxide scaling in a kiln can often offset the more dreaded effect of decarburization, but it will still create a headache for you in the subsequent cleanup, not to mention affect the conditions in the quench.
Your tempering should always be ASAP after the steel has assumed room temperature with any alloy exceeding .6% Carbon. It gets quite complicated, but high carbon steels will form a particular type of martensite (the hard stuff) that has an awful lot of stress points and internal “concerns” and the sooner these are addressed, the better.
However many folks get too jumpy over this and are too hasty to temper steel that is not done hardening. High carbon steel does not actually begin to harden in the quenching process until it reaches 500F or below at which point a very stressful and dramatic process requiring heavy deformation of the microscopic structure begins. This process is entirely temperature dependent and requires continuous cooling until it is complete, if you stop the cooling or reheat before it is complete the hardening will not be complete, it is that simple. Remember this- the higher the carbon content the lower the temperature that the hardening will be complete and this is even more so with added alloying. 1084 is an easy one, it will begin hardening around 430F and will be all done around 200F.*
I am a very big proponent of the kitchen oven Darrin suggested, but if that is out of the question, and you need to use the same kiln, you could use a modified method of quenching to give yourself a cushion. At any point from 450F down you can remove the blade from the quench and allow it to air cool; remember, so long as the cooling is continuous things will work, that doesn’t mean that cooling has to be as fast. When air cooling, the thermal mass of the steel will allow a jump start on the tempering and as much as 50% of the hard stuff can be exposed to some tempering effects. This will give an enormous amount of wiggle room over traditionally quenched steel to allow your kiln to cool to tempering temperatures.
*Those who want to get an idea of how noticeable this process is can quench a piece of steel and interrupt at around 450F to check it with a file, it will bite like it is dead soft. Allowing it to air cool, you will notice the file biting less with every minute that goes by until the steel is cool and fully hard. You can also play with the magnet and watch the non-magnetic unhardened steel slowly grab the magnet more and more as the temperature drops.