When you add carbon to iron you get steel.
With the proper amounts of carbon along with other alloying elements you can harden the steel by heat treating.
That is the basis for using steel for blades.

Forging and heat treating of blades involves many changes in the steel and a better understanding of the process will help you make a better blade.

Metals by there nature are crystalline (there are no molecules in metals ).
Crystals stack up to form grains.

There are three different crystals that we deal with ;Ferrite, Austenite, Martensite .
Ferrite is found at room temperature,
Austenite at high temperature and
Marten site is what we get when the steel is hardened.

Other basic structures are ;
Cementite ( the iron carbon compound Fe3C ),
Pearlite ( a structure of fine alternating layers of ferrite and cementite ),
Bainite ( another structure we can get when hardening ).

Steels with small amounts of carbon cannot be hardened but when there is about .40% or more they can be.
When we get to about .85% (called the eutectoid point ) the marten site will be saturated with carbon.
Above .85% the structure will be martensite + carbides.
Those extra carbides will play an important part in edge retention ( wear resistance ) so if that is an important consideration pick a steel above ..85% carbon.
Steels with higher than 1.2% are not often used because of brittleness.
Generally the lower the carbon the higher the toughness.

The basic hardening process is to heat the ferrite /carbide structure to the austenitizing temperature where the ferrite changes to austenite ( ~ 1500 F ).
The steel is held at that temperature ( soak ) until the carbides dissolve ( 5-10 Min for blades ).Then , when cooled quickly enough the structure will transform to martensite.
There must be enough soak time to obtain full strength martensite and a homogeneous structure.
Excessive austenitizing temperature or excessive time will cause grain growth which will make the steel brittle.
How quickly it has to be cooled depends on composition.

Steels are classified by how fast they must be cooled to obtain martensite.
Either water, oil or air hardening.
Insufficient quench rate for a steel results in a structure of martensite with softer pearlite.
Too fast a quench might risk cracking.

The quench tank should be next to the furnace to avoid delay.
The quenching forms martensite but this is very brittle so it must be tempered.
Dropping an untempered blade will crack it , and even letting it sit may break it or cause micro cracks which will cause breakage later.

Temper immediately and for a minimum of 1 hour. I prefer a minimum tempering temperature of 400F to give a very stable structure.

Tempering to color - this in not satisfactory because the color is just a surface oxide .
It’s better and easier to use an oven. The higher the temperature the softer the steel will become.

Some alloys especially the more complex ones may not completely transform to martensite.
They may contain some retained austenite .
If we can reduce the retained austenite we may be able to increase hardness ..
We also minimize the chance that the austenite would transform over time . That would create untempered martensite which is brittle.

Double tempering ( temper ,cool to room temperature ,temper again ) will reduce retained austenite and stabilize any remaining.

We can also use cryogenics ( cooling to subzero in liquid nitrogen for example ) . Unfortunately much of cryogenics is hype. At this point in time the only sure benefit of cryogenics is to reduce retained austenite.
Cryogenics does NOT refine grain.
The retained austenite transforms to Untempered martensite so after cryogenics you must always temper.

We can manipulate things in heat treating also.
We can heat treat the spine and edge differently. Instead of a quench and temper of the whole blade we can ;

1- quench and differentially temper ,that is quench and temper then further temper the spine yielding all martensite but softer spine,
2-- differentially quench and temper ,the blade edge is quenched in the oil leaving the spine to cool slower . This produces a martensite edge and softer pearlite spine,
3 -- Edge harden, here only the edge is heated then the whole blade is quenched and tempered.
4-- Bainite , a special process where the steel is quenched in molten salt and held at about 500F for a few hours until a structure of bainite is produced. Not quite as hard as martensite but much tougher. We look for many properties in a blade but often it is defined by “hardness “ .

The hardness tests were developed to give a measure of tensile strength nothing else . When we apply it to things like edge retention it’s not very good, or toughness .

Carbon content ,composition and hardness all play a part.
All this takes us back to step #1.
We must carefully define our needs .
Once we do that we can pick an appropriate steel and suitable heat treatment.