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.

Here are 5 reasons to anneal steel. 
To alter the grain structure;
To develop formability;
To improve machinability;
To modify mechanical properties;
To relieve residual stresses.
The annealing process is a combination of a heating cycle, a holding period or “soak” at temperature, and a controlled cooling cycle. Atmospheric controls are generally used to protect the steel from oxidation. 
The temperatures used and the cooling rates are carefully selected to correspond with each steel grade’s chemical composition in order to produce the results desired. 
For bar steels used in our precision machining shops, there are three kinds of annealing that may be encountered:
Subcritical Anneal
Solution Anneal
Spheroidize Anneal
Subcritical Anneal 
 A subcritical anneal is the metallurgical name for what is termed a process anneal or stress relief anneal in North American commercial practice. It consists of heating the steel to a temperature close, but below, the steel’s lower critical temperature or Ac1. This simple anneal reduces stress and hardness in the material and makes modest changes in its microstructure. Steel mills often employ this to improve cold shearing or cold forming. This is sometimes used between cold forming operations to reduce hardness. 
Solution Anneal 

Lamellar Pearlite.

Solution annealing is referred to in commercial practice as ‘LP Anneal’ or Lamellar Pearlite Anneal. Lamellar pearlite is the microstructure that predominates when doing this kind of anneal. The cycle for this anneal involves heating the material above the critical range (Ac3) and holding the steel (soaking) at that temperature for a length of time followed by slow cooling below the critical range (Ar1) temperature. This cycle reduces hardness and reprecipitates the carbide phase as lamellar pearlite. Controlling the time and temperature gives the metallurgist a means to alter the resulting lamellar pearlite structure, and refine the ferritic (as rolled) grain size. 
LP anneals are usually applied to medium carbon (0.40-0.65 weight %) plain carbon and alloy steels for precision machining in order to reduce hardness and improve machinability. 
Spheroidize Anneal   
Spheroidized Microstructure.

Spheroidize annealing is the term that describes a thermal process which results in a globular or spheroidal type of carbide after heating and cooling. There are several types of spheroidize cycles which we will write about in a future post. 
Spheroidized microstructures are desireable for machinability and improved surface finish when machining higher carbon steels. Spheroidized microstructures are also preferred when the steel is to be severely cold worked: cold extruded, cold upsetting, or bent. Most bearing steels are first spheroidize annealed prior to machining. 
Lamellar Pearlite photo 
Spheroidized Photo