Three Key Factors to Understand Machinability of Carbon and Alloy Steel

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

DSCF6546 — 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.

Silicon- The Great Influencer In Your Steel And In Your Machine Shop

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Silicon plays many roles in steel but its most important is deoxidation; it is detrimental to tool life, machinability and surface quality in low carbon and free machining steels.

Silicon makes up about a quarter of the earths crust. It is mined as sand, quartz, mica, talc, feldspars, vermiculite, and others; silicon is a key ingredient in glass, computer chips, and certain gemstones- rock crystal, agate, rhinestone, amethyst. Opal.

Opal5 — Opals are primarily silicon, but too precious to use for steel deoxidizing.

The human body contains approximately one gram of silicon, ranging from 4 ppm in blood, 17 ppm in bone, and up to 200 ppm in various tissues. Cereal grains are our primary source of dietary silicon.
Silicon is seldom found as a pure element, because it has a high affinity for oxygen. It is this ability to scavenge oxygen that makes silicon important in steelmaking.
Silicon’s primary role in steel making is as a deoxidizer. It makes steel sound, by removing oxygen bubbles from the molten steel. The percentage of silicon in the analysis was related to the type of steel, rimmed and capped steels (made by the ingot method) had no silicon intentionally added. Semi-killed steels typically contained up to 0.10% max silicon, and fully killed steels could have up to 0.60% maximum. Commercial practice in the US and Canada throughout my career was 0.15-.35 % silicon in SAE carbon and alloy steels.
In addition to deoxidiation silicon also influences the steel five different ways:

Silicon helps increase the steel’s strength and hardness, but is less effective than manganese in these functions.
In electrical and magnetic steels, silicon helps to promote desired crystal orientations and electrical resistivity.
In some high temperature service steels, silicon contributes to their oxidation resistance.
In alloy grades, silicon also increases strength (but not plasticity!) when quenched and tempered.
Silicon also has a moderate effect on hardenability of steel.

But there are always less desireable aspects of any element in an alloy

Silicon is detrimental to surface quality in low carbon steels, a condition that is especially magnified in low carbon resulfurized steels.

Silicon is detrimental to tool life in machining as it forms hard abrasive particles which increase tool wear and thus lower the steel’s machinability.

Bottom line, on plain carbon and alloy bar steels, silicon contents of 0.10, 0.15-.35 weight percent are typical; On resulfurized , and resufurized and rephosphorized free machining steels, silicon analysis above 0.02 wt % is cause for concern, due to potential surface quality and certain tool life issues.
Silicon metal photo
Opal

Carbon Equivalent And Weldability

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The weldability of steels is influenced primarily by the carbon content. At higher carbon levels, steels may need either pre- or post- weld heat treatment in order to prevent stress build up and weld cracking.
Generally speaking, if the Carbon Equivalent (CE) is 0.35 or below, no pre- or post- weld thermal treatment is needed. In our experience with maintenance welding, we have found that preheating was beneficial between 0.35 and 0.55 CE. Above 0.55 CE we usually both pre- and post- weld heated to relieve stress and prevent cracking.
So CE= .35 max.
However the other elements that are contained in the steel also have an effect on the steel’s “carbon equivalence.” These additional elements can really add up in scrap fed electric arc furnace steels that now predominate in our market.

EAF_20Emotion_20CON__430,property=Data — Electric Arc Furnaces are predominately scrap fed.

Photo credit.
Here are two formulas for calculating Carbon Equivalents.
CE=%C+(%Mn/6)+(%Cr+%Mo+%Va)/5 + (%Si+%Ni+%Cu)/15
This is the first formula I learned when I took over metallurgical support for maintenance ‘back in the day.’
In this formula you can see that 6 points of Manganese are approximately equal to one point of Carbon. 5 points of Chrome, Moly or Vanadium are roughly equal to a point of Carbon, while it takes about 15 points of Silicon, Nickel or Copper to get about the same effect as one point of Carbon.
The GE formula for Carbon Equivalency is CE= C+(Mn/6)+(Ni/20)+(Cr/10)+(Cu/40)+(Mo/50)+(Va/10). If this is less than .35 max, you should have no need to pre or post weld thermal treat in most cases.
As long as CE is no more than .35, you probably won’t need to preheat or post weld stress relieve your welded parts. above .35 CE, you may need either or both depending on section thickness and CE.
* (I) added (extra parentheses) to keep (the terms) clear in (this post); no (scathing rebukes) from (math teachers) please!