Ran into an annoying problem

If these are knives you plan to sell and you are limited on budget, I'd recommend having the HT professionally done. It costs a bit, but if you are putting out bad HT knives, you won't get much repeat business or recommendations. To me, the HT is the most important part of a knife and if you mess that up, you are throwing away money and time. Many places will give you a bulk discount if you send multiple blades of the same material. Even if $10 a blade, you can incorporate that into your price, or know it was money well spent on a keeper. Just my .02.
 
If it helps, many of us have been in your boots. The good news is that many of us also now make MUCH better knives. Heat treating is critical to performance as you just experienced. May have missed it but did you say if you forge or made these by stock removal?

Coarse grain, probably resulting from substantial overheating at quench (or earlier without being normalized), can give you a “crumbly” edge in carbon steel. Sure you considered decarb, but you probably ground away (cool and carefully) far more than would be needed to remove that soft material. I’d bet these blades probably went through a “roller coaster” of temperatures over a fair amount of time at HT; so coarse grain and decarb are likely conspiring against you.

So for now you are limited to a 2BF, a known quality steel, no money and no thermocouple. Could be a lot worse (O/A torch and A36 from the bigbox store – there’s always that guy…). To my mind, you can still learn to do a decent job heat treating as long as you don’t fight your limitations. No swords and maybe no blades at all >3” or so. Stick with a single eutectoid alloy (1075, 1080, or 1084) that requires little to no soak at critical and use that fact to your advantage. Learn how your steel changes as related to temperature and forget the idea that a loss of ferromagnetism indicates critical temp. It is simply not true for these steels - regardless of who tells you so. You are shooting for ~1475F and coincidentally NaCl melts at that temp.

Suggestions:

Wait till after dark so you can see the important stuff happening in your fire. Close off your 2BF as much as possible and soak empty for several minutes with your MAPP torch wide open. Insulating (soft) firebrick doesn’t have the heat capacity of some other common forge liner materials so I don’t know how big a difference this will make, but the goal is to put heat into your forge walls so that when you introduce your steel, it will have an energetic “head start” and a more uniform interior. Then back off the heat and set the blade out of any hot spots (micro-muffle?). Now be patient and watch carefully what happens as the steel comes up in temperature. Take your time. You will notice the thinner edge and point will begin to glow deep red. Try to get this sub-critical temp as uniform as you can across your blade. Bump the heat slowly and the point/edge will eventually “flash” in a pretty obvious way as they reach critical. This color will spread through the blade quickly with a decent preheat. Avoid distractions and pay attention, or just wait for a better time.

I suggest thermal cycling one of your already-broken junker 108X blades several times first. Just hit critical, yank and let air cool. Learn what the transition to critical looks like as folks have been doing for a very long time. Get your confidence up this way and full quench into a quart+ of warm canola oil. If you go through this successfully with your course-grained knife you mentioned and dull/score/break again, you should notice much finer grain than you got in round one. If you avoid overheating the thin pointy parts on a new blade and you temper at 400 to 425F range for 2h, you will be quite happy with the result I expect. Study, ask questions, be patient and try again if it doesn’t work out.
 
Really appreciate that advise. The firebrick I'm using actually is lined with a furnace compound, but I havn't been presoaking the interior. I'll make that change, drill and sheer the handles off the broken blade, cycle it several times, then experiment a bit and repeat the thermal cycling as needed. Or is there a limit to how many times that can be done?
 
That...and use a gallon of canola. A quart isn't enough thermal mass to absorb the heat efficiently. Using too small a quenchant volume can lead to blades that won't stay sharp :)
 
Lone Fortress said:
...cycle it several times, then experiment a bit and repeat the thermal cycling as needed. Or is there a limit to how many times that can be done?
Click to expand...


Sounds like a plan, but I honestly don’t know the answer regarding a limit on the number of times you can hit critical and not trash carbon steel. For practical purposes, I expect that rarely/never happens for most makers. Thermal cycling should be done with a clear purpose though.

The short-term goal is to get control of your temperatures and become familiar with what the decalescent transformation (critical) looks like, using your gear. Then chase a basic normalization process that will take maybe 3 heats at descending temps and solve several practical issues. (That is the study part mentioned before.)

And Stacy is right on the quench volume issue. With agitation, I believe a quart has enough mass to beat the pearlite nose and hit Mf within a minute or two on a typical 3” blade, but you would likely be going with a volume that is about 4.5” tall and 4” in diameter. You would also likely have an inch or two of glowing material that you could not quench. Really bad idea. At best, this equals some flames and oxidized quenchant that needs replacement too soon. At worst, this means explaining the ash dump where your shop used to be to the insurance agent. Plastic quench containers also are a bad idea – use metal. I used a one gallon galvanized farm pail for a few years and would just pour my (precious) Parks50 back into its original 5 gal plastic bucket afterwards. If you can’t completely submerge your cold blade, you might consider waiting till you find a better alternative. Or at least a tight lid and extinguisher...

One other thought; ditch the propylene ASAP and get yourself the adapters to connect your torch to a standard 20# bottle of propane. IME there is not enough practical difference in this application to make the expense and hassle of those 1# MAPP bottles worth it. (Many here would say the exact same of the 20# bottles though.)

BTW Stacy; I noticed you hit 16,000 posts right here this morning. Wow! Thanks so much for your time and dedication to all the folks here on this forum. You have been a clear-headed and reliable resource to so many for a long time. It is sincerely appreciated!:thumbup:
 
Just done a second attempt today and the results are much more satisfactory. The steel is visibly much smoother at the break zones isntead of having that rough appearance. The problem is that the steel was a bit brittle this time around, although I suspect that's a simple case of the martensite not properly being tempered back as I only done one tempering cycle before doing the test.

The container I've been using for quenching is a old metal coffee can which is 33oz-but far now it seems to be sufficient since I'm only making small blades.

I think I'm going to repeat this process on all of them (except two, which are too large for my equipment), and vary the tempering temperatures then compare the results.
 
Sounds like you just dumped the clutch on second gear - good to read. Finer grain will lead you to finer and more durable edges.

Even though you're not doing an extended soak at HT, remember to dress the edge back a few thousandths to get to reliable steel before your testing.

Also wrap your blades loosely in foil or place in a pan of clean dry sand during tempering to smooth out the wildest temp swings. Using caveman gear/technique without protection - your thin blade parts will likely overheat multiple times, over-temper and behave in a way that doesn't match the apparent Rc of your ricasso or spine. That nonsense will drive you to drink until it clicks.

Keep testing some of these early efforts to scrap and you WILL learn some seriously useful stuff that many new makers put off too long. Much harder to trash your first decent blades when you could be using them to impress the girls...
 
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