To elaborate on metes post,
Immediate means as soon as is practical ( not necessarily withing seconds or minutes). When a blade comes out of the quench, it is under extreme internal stress. It is also composed of very brittle untempered martensite. It can shatter, or crack easily, sometimes just sitting there, but surely if it is struck or dropped. The sooner it gets into the temper oven, the better. If doing several blades,set each aside laying flat on the bench until all are done. Carefully and gentle wash off all oil and put the batch into the oven to temper. Minutes or even an hour or two isn't necessarily a death sentence for the blade, but leaving around for days or weeks is asking for trouble.
( For plain carbon steels) When steel is quenched, the austenite that formed at 1400-1500F (called the A1 point) is converted to martensite. This conversion starts at about 400-450F ( called the Ms). It finishes somewhere between 200F and room temperature ( called the Mf). Thus, the steel must drop to room temperature before temper, so the conversion can be completed .The small amount of austenite that remains ( retained austenite) is converted when the steel is heated back up to near the start point again and then cooled back to room temperature. This new small amount of martensite is untempered, which is why you do two temper cycles. The second cycle tempers out the new martensite, and makes sure that the temper is evenly distributed.
(Note: On complex alloy steels, like stainless, the Mf can be 100F below zero. It takes a cryo treatment and two or three tempers to convert all the austenite to martensite. This is why cryo is recommended for such steels.)
As to temper times. Yes, you can temper with a torch in one or two minutes by drawing the blade spine and watching the colors, but it will be a very uneven and incomplete temper.A proper temper takes time.
The movement of atoms in steel is rapid at 1500F, but very slow at 400F. It takes at least an hour or more to allow the changes in structure to occur in the steel to get a good temper. The second temper finishes the job. While one ,one hour temper will do most of the job - two , two hour temper cycles is a much better standard.
Tempering is the movement of atoms, and as said, that takes time. The temperature allows this to happen, so the temper cycle is a function of time and temperature. Raise the temperature and it happens faster, do it longer and it happens more. Of the two, temperature is the active factor. The ratio of temperature to time in activity is about 10:1. That means a small increase in temperature will have ten times the impact on the hardness than a moderate increase in time. For practical purposes, assuming the temperature to be constant, a four hour temper would not appreciably lower the hardness ( maybe one point, max), but a ten degree increase in the temperature can affect it 1-2 points. This is why knowing the true temperature swings of your oven is important. If your home oven is set on 400F, that is the average that it is at. It may heat up to 425F and drop to 390F before the heat cuts back on during each heat/cool cycle.
Stacy
