Normalizing is a thermal process where  steel is heated about 100-150 degrees F  above the critical range followed by cooling in still air to below that range.

Not a fan of expensive thermal treatments without a good reason...
Not a fan of expensive thermal treatments without a good reason…

On some steels, this normalize process is followed by a temper or stress relief anneal below the Ac1 to remove residual stresses resulting from the air cooling and to reduce hardness.
 Normalizing Steel gives you

  • Reduced hardness and removes residual stress
  • Improves machinability
  • Develops desired mechanical properties (especially in larger sections)
  • Improved austenitizing  for subsequent quench and tempering

Adding costly thermal treatments to a production process is seldom a good idea. But

  • if you need high side mechanical properties as a result of the quench and tempering operation you have planned,
  • if the heat is lean on chemistry,
  • if the part to be quenched has a large cross section or wall thickness,
  • if you know from experience that you have difficulty getting to high side with your quench,

Normalizing can help.
For the end quench position of the bar that corresponds to 90% martensite,  a non-normalized  4140,  austenitized at 1550 degrees can have anywhere from a 7 to 14 point Rockwell C hardness deficiency compared to the same steel that had been normalized.  Using an austenitizing temperature of 1650 (200 degrees F above the Ac3 temperature) the non-normalized 4140 could still exhibit as much as a 10 Rc hardness deficiency compared to normalized stock for the same time at temperature. For 4340 steel, the hardness deficit can  range from 10 to 18 points of Rockwell C hardness deficiency for the same austenitizing time.
Normalizing was a necessary step in the days of highly variable microstructures resulting from Ingot steels and analog controlled processes. Today’s modern computer controlled steel making processes provide more consistent products and structures, making normalizing a less utilized process. But normalizing remains a way to coax better properties or performance out of some steels.
We’re not a big fan of adding “fire for fire’s sake,” but if you suspect you may have difficulty in developing the full hardenability out of your steel, Normalizing may just give you the edge you need to assure you develop the as quenched hardness you need.
Photo credit: Above The Law Blog

If steel did not have the property of plastic deformation, the only ways to make parts from it would be casting  or cutting into shape.

No deformation processes like cold heading, cold rolling, swaging etc. would be possible.

Slip planes in the metallic crystal explain Plastic Deformation and Plasticity in Steel. This makes cold working processes like cold drawing possible.

If one subjects a piece of steel to a heavy load, the material will measurably stretch. When the load is removed, if the steel goes back to its original dimension, the deformation that it underwent when the weight was applied is called “elastic deformation.” In this case, the steel did not take a permanent “set.”

If one subjects a piece of steel to a much greater load, if, when the load is removed, the steel does not ‘spring back’ or recover to its original dimension, the new shape or dimension is a permanent deformation. (It is often said to have ‘taken a set,’)  This is called “Plastic Deformation.”

Plastic Deformation is explained by the movement of planes of atoms from their normal positions.

Steel and most industrially useful metals are able to withstand a great deal of this Plastic Deformation before they break.

Brittle metals will just fracture under such loads;

Cold drawing of steel is a process that applies a load to the metal to make the atoms in the steel take new positions with respect to each other, resulting in lowered ductility, increased  tensile and yield strength and new dimensions or shape. These in turn, are often helpful in improving the machinability of the steel, allowing you to more economically produce the parts and components that are essential for our current technologies.

Most people think of steel’s hardness as its main advantage. The facts of the matter are that it is steel’s plasticity or ability to plastically deform that makes it such a useful and versatile material for humankind.