Lets go back to basic metallurgy 101:
1)CONTENT - A knife made from an unknown steel may be problematic in HT. Your steel choice probably isn't the cause of the trouble, but this is always a concern with second use steel. Beyond the carbon and alloy content, which one can guess at somewhat reasonably based on its past life, there is the internal condition from the former use and HT...which you can't know. The grain size is unknown, and there may be many microscopic disruptions in the grains from repeated use and flexing. Consider a leaf spring or harrow tine. The steel may be assumed to be 5160 for a leaf spring and 1080-1095 for a harrow tine. That will be good enough for a guestimate HT regime. A grain reduction cycle will reduce any possible large grain size. The problem is that the flexing over many years of use may have done things to the arrangement of the grains that will not be repairable easily, if at all. This is why using a new and known alloy content steel is recommended.
2) CONDITION - If a blade is forged, it goes through many long and high temperature cycles, followed by cooling as it is forged. The normal forging range for a high carbon steel is between 2100°F and 1500°F. At that temperature and repeated cycles, the grain will grow...sometimes to huge size. This is of no concern until it is time to harden the finished blade. If you just austenitize ( heat up a little past non-magnetic) and quench in oil, the resultant grain will be roughly what it was before the quench - usually large. Large grains disrupt (breaks) much more easily than fine grain. A grain refinement cycle is how to reduce the grain size ( and subsequently the brittleness). Start with a soak at a temperature that is well above the HT target. For simple high carbon steels, the HT target is between 1450F and 1500F, so start about 1600F. Heat evenly and hold at that temperature for about 5 minutes ( if using a forge, heat as close as your eye can guestimate and hold for a minute or so). Air cool to black and cool off in water. This will get everything in solution and set the grain size at a starting point. Re-heat to 1500F and when the hold is done, quench in the appropriate oil. Reheat to 1400F and quench in the appropriate oil. Now the grain size has been reduced to a much smaller size, and the steel is in a condition that will allow a good final hardening. Heat to the target temp, usually about 1475F, and hold for up to five minutes, then quench. Temper twice for one hour each at around 400F. Cool between the tempers and after the second by dunking in water or holding under a running faucet. Judging the HT temperature by eye is very inaccurate...often by several hundred degrees too high. A magnet helps get you in the ball park, but you are still guessing in a critical place where 25° can make a lot of difference.
3) TEMPERING - Tempering converts the very brittle fresh martensite into a tougher tempered martensite. This is very important, or the blade will fail in use. Most people temper far too low. A properly hardened steel like 1095 will be at Rc663-64 with a temper of 400F, and at Rc60-61 with a temper of 500F. The words to notice are "properly hardened". Most people don't get the full potential from their steel. Unless using a non-standard knife steel, all tempering should be at or above 400F. Tempering will make the grains a bit less likely to come apart ( break/chip), but it won't cure large grain size. Blade steel is in thin sections, and the HT and temper are fairly basic, so two one hour tempers are all most blades need. A few very high alloy steels need other treatments ( sub-zero/cryo) and longer or additional tempers.
4) CARBON DISTRIBUTION - Lets discuss what we really need/want to happen in the steel. The carbon combines with the iron to make iron carbide - AKA cementite. The rest of the iron either stays as ferrite ( plain iron) or combines with excess carbon as other structures. Often things are added to the steel to make it harder or tougher......but it is the carbon-iron bonding that makes the steel work as a knife. There is a magical sweet spot in the balance of carbon and iron...called the eutectic point. It is when there is .77-.83% carbon ( .80% for basic knife steel discussion). Our beloved 1084 steel is at that ratio nearly exactly, and is called a eutectoid steel. This eutectoid property means that there is no extra carbon to be dealt with. That means no soak time to allow things to get into solution, a simple and complete hardening with minimal effort, and reliable/repeatable outcome. This is why 1084 is recommended to all smiths who use a forge for HT.
Steel below this point is called hypo-eutectoid, and steel above it is called hyper-eutectoid. Hypo-eutectoid steel is pretty fool proof. Once all the carbon is in solution, it combines with the iron and forms whatever percentage cementite it can, leaving the rest as plain ferrite. Hyper-eutectoid gets more complicated, as we have to direct the carbon to go where we want it and not where we don't.
5) PUTTING IT TOGERTHER - The above info tells us that the best mix of carbon and iron in out blades is about .84% carbon....so why do many steels have more carbon? The carbon is there to make the steel do different things and to alloy with other elements. In truth, we need a tad extra carbon to account for carbon loss, and to tie up the manganese and a few other minor elements that are in almost all steel. A tenth of a percent or so takes care of that. Any excess is going into carbides with other alloys, or remains as austenite. The subject of exactly what and how it combines is way beyond this simple explanation, so suffice to say that only about .85% of the carbon gets used to make a steel blade. The rest goes into making it have other attributes. That little tidbit tells up that we really don't want...or need....to dissolve all the carbon from where it is happily sitting when we harden the blade....if...IF...IF...we have properly conditioned the steel before the final HT. In the final hardening, lower austenitization temps will allow enough carbon to be available to form a perfect steel, without breaking up nice hard carbides that have already formed and well distributed. In some specialized hardening processes, like forming a hamon, this is a critical part of getting the maximum results. This is part of why different steels have different carbon and alloy content makes the target higher of lower. For basic simple carbon steels - 1050 hardens properly at 1525F, 1084 at 1500F, and 1095 at 1475F....see a pattern. The more excess carbon, the lower the austenitization temperature.
SUMMING IT UP - This is all a big part of why using found steel and judging temperature in HT by eye is not recommended for newer smith with simple equipment ( or at all). The steel will probably be hardenable, and the eyeball HT and quench will probably harden it....but how much of what makes a knife blade good is being accomplished ??? 30%, 50%, 75%.....you don't know......and it is a guarantee that it isn't 100%.
This isn't going to stop people from using old farm tools and car parts for making knives, or doing HT in a simple home built forge....and it should not be read as saying such.
By using as much of the above information as possible a smith can control the outcome to a higher degree.
