How to anneal a knife blank
This post will answer the question of why would you anneal a knife blank, what is annealing and how to anneal a knife blank in a home shop.
Why anneal steel?
When you anneal a knife blank, annealing it makes the steel much softer to work with and makes it faster to grind and easier to bend. Fully annealed steel will also bend with a smaller radius before cracking and failing.
If you buy fresh steel you can buy it in the annealed or hardened state. If you are buying fresh steel I highly recommend you buy in the annealed state and do most of your grinding before hardening. Reused steel is generally in a hardened and tempered state.
Trying to grind hardened steel will quickly dull your sanding belts and take much more work. It also can't be bent without risking cracking the blade and if you overheat it during the grinding process you will have to harden and temper the knife blade again once you are done as overheating it causes the temper to change and makes it soft.
Generally if you see the knife turn blue it has ruined its temper. If you plan to harden and temper the knife again anyway, there is no real risk of overheating the knife while grinding.
What is annealing?
Annealing is the process of taking steel up to a red hot temperature and then allowing it to very slowly cool. Doing so allows the steel to form very durable but soft carbon-iron structures inside the steel. The same steel can become soft or glass hard depending on how quickly it is cooled from above the recrystallize temperature.
How to anneal steel?
As for how to anneal a knife blank, that can be done many ways but the way I have found best for a home shop is a good airtight style wood stove.
First you must prep the blanks. I fit about 50 blanks tightly into a 2" x 1" x 14" chunk of C channel and bind it with iron rebar tie wire. This helps provide some strength to the blanks as they will be extremely soft once they get hot enough, as well as provides a large thermal mass to make sure it cools slow enough.
Preparing the stove
Next the stove must be filled with red hot coals. I recommend feeding a dried hardwood split into 1" thick sticks be fed to the stove until you see a coal bed at least 4" deep.
It may take several loads of wood before a deep enough bed of coals is produced. add more wood once all the existing wood has started glowing, do not let it turn to ash. Rearrange any wood that is not burning. Soft woods can be used but will produce very little coals for the amount of wood used.
Annealing knife blanks
Once a bed of coals is established, dig a small pit in the center for your blanks but leaving plenty of coals between the bottom of pit and stove floor. used a long metal rod to insert the blanks into the very center of the stove, making sure not to be too close to the back, back or sides of the stove. Use the rod or a fire poker to cover the blanks in glowing coals from the sides of the stove to provide more heat and insulation.
Close the air intake if available and allow the coals to burn to ashes slowly. You can check during the first few minutes to make sure the blanks are reaching red hot along the entire length but try to keep the door closed once they do. do not add more wood.
Once the fire has burned down, note the blanks may be hot for a few hours after that. leave them to cool in the oven buried in ashes. do not spray with water as we don't need any more rust then they have already acquired (known as fire scale) and we don't want to cool them quickly.
After annealing your knife blank
If successful you'll find the blanks can be very easily bent now and will spring back much less when bent. If done right you can also likely put a blank into a vise and bend it over sharply 90 degrees with a hammer and it won't break or crack. without annealing the blade you will find it generally snaps around a 45 degree sharp bend if it was hardened and tempered as a wood cutting blade.