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

The machinability of steel bars is determined by three primary factors. Those factors are 1) Cold Work; 2) Thermal Treatment; 3) Chemical Composition.

Machinability is the result of Cold Work, Thermal Processing and Chemical composition- as well as the ability of the machine tool and the machinist.

Cold Work improves the machinability of low carbon steels by reducing the high ductility of the hot rolled product. Cold working the steel by die drawing or cold rolling results in chips that are harder, more brittle, and curled, prodcuing less built up edge on the tools cutting edge.. The improved Yield to Tensile Strength ratio means that your tools and machines have less work to do to get the chip to separate. Steels between 0.15- 0.30 wt% carbon are best machining; above 0.30 wt% the machinability decreases as carbon content (and hardness) increase.

Thermal Treatment improves the machinability of steel by reducing stresses, controlling microstructure, and lowering hardness and strength. While this is usually employed in higher carbon steels, sometimes a Spheroidize Anneal is employed in very low carbon steels to improve their formability. Stress Relief Anneal, Lamellar Pearlitic Anneal, and Spheroidize Anneals are the treatments applied to improve machinability in bar steels for machining.

Chemical composition is a major factor that contributes to the steel’s machinability or lack thereof. There are a number of chemical factors that promote machinability including

Carbon- low carbon steels are too ductile, resulting in gummy chips and the build up of workpiece material on the tool edge (BUE). Between 0.15 and 0.30 wt% carbon machinability is at its best; machinability decreases as carbon content increases beyond 0.30.

Additives that promote machining include

  • Sulfur combines with Manganese to form Manganese Sulfides which help the chip to break and improve surface finish.
  • Lead is added to steel to reduce friction during cutting by providing an internal lubricant. Lead does not alter the mechanical properties of the steel.
  • Phosphorus increases the strength of the softer ferrite phase in the steel, resulting in a harder and stronger chip (less ductile) promoting breakage and improved finishes.
  • Nitrogen can promote a brittle chip as well, making it especially beneificial to internal machining operations like drilling and tapping which constrain the chip’s movement.
  • (Nitrogen also can make the steel unsuitable for subnsequent cold working operations like thread rolling, crimping, swaging or staking.)

Additives that can have a detrimental effect on machining include deoxidizers and grain refiners.

Deoxidizing and grain refining elements include

  • Silicon,
  • Aluminum,
  • Vanadium
  • Niobium

These elements reduce machinability by promoting a finer grain structure and increasing the edge breakdown on the tool by abrasion.

Alloying elements can be said to inhibit machinability by their contribution to microstructure and properties, but this is of small impact compared to the factors listed above.