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.

Three primary criteria for selecting bar steels are  1) suitability for end use, 2) suitability for manufacturing process, 3) economical delivery of the requirements.

Shape can be an important selection factor.

Suitability for end use includes appropriate mechanical properties, physical properties and chemical compatibility. Mechanical properties can include hardness, tensile and yield strength, ductility as measured by % elongation or % reduction in area, and / or impact properties. Mechanical properties can be achieved by chemical composition, cold work, or heat treatment. Note: properties need to match the environmental conditions of the intended end use…  Physical properties that are often considered include magnetic properties for solenoid, actuator, or electronic applications. Process path of steelmaking can play an important role in determining these properties.
Suitability for manufacturing requires at least a cursory understanding of the intended process path. Will there be extensive stock removal by machining? Welding, brazing or other means of bonding? Heat treatment? Will the equipment used to machine require tight dimensional tolerances or straightness? Will the material be upset or cold worked? Will the material be cold worked (crimped, swaged, planished or staked) after machining? Bismuth additives can prevent achievement of bond strength in brazed joints unless special techniques and materials are employed. Various chemical constituents can have an effect on the cold work response of steel. These too can be determined by the melting and thermomechinical history of the steel before it arrives at your shop.
Economical delivery of requirements means choosing a materal that permits the creation of conforming parts that fully meet the requirements for end use and manufacturability at a total lowest cost. There are many ways to meet any particular set of requirements for steel in most uses. Chemistry, cold work, heat treatment, as well as design details can all be criteria used to select one material over another. Minimizing costs is clearly important, but most important is assuring that all of the “must have” properties (strength, hardness, surface finish, typically) needed in the finished product are delivered.
Costs of manufacturing can make up a large fraction of the final products cost. For some parts, the cost of manufacturing and processing can exceed the cost of the material. Choosing the lowest cost process path that will assure required properties often requires steel materials that are priced above the cheapest available. This is because free machining additives, or cold finishing processes  can reduce cost to obtain desired properties or product attributes when compared to those needed to get hot rolled product up to the desired levels of performance.
Bottom line: Buyers may want to get the cheapest price per pound of steel purchased; Savvy buyers want to buy the steel that results in the lowest cost per finished part- assuring that costs are minimized for the total cost of production of their product. Understanding the role of steel making and finishing processes can help the buyer optimize their material selection process.
Photo courtesy of PMPA Member Corey Steel.

Straight from the baker to you…

Making steel is just like this sort of ...

1) Inclusions are on the inside, not on the outside surface…
2) Inclusions are non metallic materials entrapped within a solid metal matrix.
3) Inclusions that are typically expected include Sulfides (Type A), Aluminates (Type B), Silicates (Type C) and Globular Oxides (Type D)
4) Other types of inclusions are called exogenous  inclusions as they come from materials not expected to be entrained or entrapped within the steel-  typically slag or refractory that might have broken off during steelmaking.
5) Inclusions are measured and rated in North America according to ASTM method E45
6) Bearing Quality Steels use a number of different practices in order to minimize the inclusion content (because inclusions would wear differently than the host metal, thus nucleating premature wear and failure.)
7) Steel Cleanliness, Steel Microcleanliness, and Inclusion content are all  different ways of talking about the presence of these non metallic particles within the steel itself.
Three reasons inclusions are normally expected in  plain carbon and alloy steel bar products  in our shops:
‘8) Manganese sulfides are expected to be present as they aid machining.
9) Silicates are expected to be in non- free machining steels as silicon is added as a deoxidizer to assure the soundness (freedom from gas bubbles and voids) of the steel
10) Aluminates are also expected if the steel is ordered as Aluminum Fine Grain. the Aluminum scavenges Oxygen and  nucleates the formation of fine grains of austenite.
11) The Manganese Sulfides promote free machining as they provide a place for the chip to break and help control welding of material (built up edge) on the tool edge. In leaded steels, the lead is closely associated with these manganese sulfide inclusions.
12) The Silicates and Aluminates in our common steel grades are of high hardness, abrasive, and are a primary reason for tool wear and edge chipping in ordinary steels.
13) A quick look at the certification tells us whether or not we will find these kinds of inclusions- just look at levels of Manganese, Sulfur, Silicon, and Aluminum.
For machining, in keeping with the baking theme, I like to think of Manganese Sulfide inclusions as “kinda like the raisins in raisin bread.”
Bakers dozen photo credit.

1) Martensite is the hardest and most brittle microstructure obtainable in a given steel.
2) Martensite hardness of the steel is a function of the carbon content in that steel.
3) Martensite results from cooling from austenitic temperatures rapidly by pulling the heat out using a liquid quenchant before pearlite can form.
4) As quenched Martensitic structures are too brittle for economic use-they must be tempered.
5) Reheating as quenched Martensite to a temperature just below the AC1 results in the best combinations of strength and toughness.

This is what you get when you cool faster than the critical cooling (pearlite transition) rate- Martensite

Hardness of martensite is a function of carbon content

Softening of martensite in 0.35%C, 0.8% C, and 1.2% C carbon steels by tempering at the indicated temperature for 1 hour.

Because Martensite transformation is almost instantaneous, the Martensite has the identical composition of the parent phase, unlike ferrite and pearlite which result  from a slower chemical diffusion process, so each have different chemical compositions than the parent austenite.
Formation of Martensite involves a transformation from a body-centered cubic structure to  body-centered tetragonal structure. The large increase in volume that results  creates a highly stressed structure. This is why Martensite has a higher hardness than Austenite for the exact same chemistry…
Photo  and Graphs Credit: Cold Finished Steel Bar Handbook

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 

The role of Manganese in steel in our precision machining shops.

Manganese ore like this comes from Turkey.

Carbon is a chemical element that is the primary hardening constituent in steel. Manganese is a chemical element that is present in all commercial steels, and contributes substantially to a steel’s strength and hardness, but to a lesser extent than does carbon.

  1. The effectiveness of Manganese in increasing mechanical properties depends on and is proportional to the carbon content of the steel.
  2. Manganese also plays an important role in decreasing the critical cooling rate during hardening. This means that manganese helps to increase the steel’s hardenability. It’s effect on hardenability is greater than that of any of the other commonly used alloying elements.
  3. Manganese is also an active deoxidizer, and is less likely to segregate than other elements.
  4. Manganese improves machinability, by combining with sulfur to form an soft inclusion in the steel that promotes a steady built up edge and a place for the chip to break.
  5. Manganese improves yield  at the steel mill by combining with the sulfur in the steel, minimizing the formation of iron pyrite (iron sulfide) which can cause the steel to crack and tear during high temperature rolling.

Manganese is an important constituent of today’s steels.
Now you know a few reasons why Mn (the abbreviation for Manganese) is the second element shown on the chemical analysis report (right after carbon).
It’s That Important!
Mn Ore Photocredit.

Don’t confuse hardness and hardenability. Hardness is a material property. Hardenability is a way to indicate a material’s potential to be hardened by thermal treatment.
Hardness is resistance to penetration. Hardenability describes how deep the steel may be hardened upon quenching from high temperature. The depth of hardening is an important factor in a steel part’s toughness.
The brinell test uses a 10mm hardened steel (sometimes carbide) ball and various levels of force applied over a specified time.

The softer the material, the deeper the penetration, the wider the impression.
The softer the material, the deeper the penetration, the wider the impression.

The width of the impressions is measured optically and averaged. (Wider impressions mean the ball penetrated deeper, thus, the material is less hard.) The Brinell hardness number is calculated by dividing the load applied by the surface area of the indentation. Prior to today’s direct reading instruments, the measured indentation diameters could be looked up on a reference chart and the corresponding Brinell hardness number given.
The Rockwell test is similar, but uses different forces and either a smaller ball indenter (Rockwell B scale ) or a diamond indenter (Rockwell C scale).
Hardenability- Jominy Test
In the Jominy test, a standard specimen is heated then water quenched from the end, and a series of rockwell hardness tests are taken in 1/16th inch increments along the length of the specimen.
Jominy test measures potential depth steel will harden.
Jominy test measures potential depth steel will harden.

It is the influence of the steel’s chemical makeup (Carbon and Alloying elements) that determine how a  deeply a grade of steel will transform to martensite for a particular quenching treatment. This means that for each grade being heat treated,  mechanical properties are a result of cooling rate (quench). An excellent web page on this can be found here.
So what of the difference between hardness and hardenability?
Hardness is resistance to penetration under specified conditions of load and indenter.
Hardenability is the ability of a steel to acheive a certain hardness at a given depth, upon suitable heat treatment and quench. Hardness can be measured in steels in any condition. Hardenability presumes that the steels will be heat treated to acheive a targeted hardness at a given depth.
One is an actual property, one is a measure of potential.
And now you know.
Web resources:
Gordon England Thermal Spray Coatings
Farmingdale State College School of Engineering Technologies.