Tag: Machinability

3 Facts About Scale (Iron Oxide) Rust On Steel

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On hot rolled bars to be cold drawn, the dark oxide surface is called scale.

What we see on the surface is "scale"- a combination of oxides of iron.

Scale is the name given to the oxides of iron that are formed on as wrought products as a result of mill operations (high temperature rolling or furnace treatment)

Rust is the commonly used term for iron oxide from weathering or corrosion.

Scale is

  • Hard
  • Brittle
  • High Coefficient of Friction

So we need to get it off the steel if you are to have any chance of keeping the tool edge sharp.

There are 3  oxides of iron:

Hematite   ( Fe2 O3)  has a microhardness of  ~ 1030 D.P.H., is red  in color, and is not soluble in acid.

Magnetite  (Fe3O4)  has a microhardness of  ~ 420-500 D.P.H., is black in color, and is not soluble in acid. 

Wustite  (FeO) has a microhardness of  ~ 270 -350 D.P.H., is blueish in color, and is soluble in acid. Wustite is the phase that makes up the innermost scale on the bars or rods.

Hematite and Magnetite make up the outer layers of the scale, and due to their composition, make up the larger mass of scale present. Due to their hardness and quantity they are the real dealbreakers for machining as they create tool edge wear.

One of the ways that Cold Finished Steel bars aid machinability is  by removing these hard abrasive oxides from the workpiece, so that they don’t destroy your tools and contaminate your cutting fluids.

Bar Coils

Role of Phosphorous, Nitrogen and Coldwork on Machinability of Carbon And Alloy Steels

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Machinability of carbon and alloy steels is a shear process. Working the metal (Shearing to create chip) provides heat. The subsequent sliding of the produced chip on the face of the cutting tool provides heat as well.

Three ways to improve machinability include

  1. Optimizing the chemistry to provide for a minimum shear strength
  2. Adding internally contained lubricants
  3. Adjusting cold work

The steels that we are talking about are in large part composed of the ferrite phase. This is advantageous to us as machinists, because it has a relatively low shear strength.

Because ferrite is also ductile, it does not cut cleanly and tends to tear. Grade 1008 or 1010 are prime examples of  how pure ferrite machines. Long stringy, unbroken chips, torn surface finishes and lots of machine down time to clear “birds nests” are typical results.

Adding carbon up to a point improves machinability by adding a second harder phase (pearlite) into the ferrite. The good news is that up to a point, the chip formation is greatly improved, and surface finish improves somewhat. The bad news is that the shear strength of the steel is also increased. This requires more work to be done by the machine tool.

Addition of Nitrogen and Phosphorous can not only increase the shear strength of the ferrite, but also reduce the ductility (embrittle it).This ferrite embrittlement promotes the formation of short chips, very smooth surface finishes, and the ability to hold high dimensional accuracy on the part being produced. The downside is that these additions can make the parts prone to cracking if subsequebnt cold work operations are performed.

The graph below shows how cold work (cold drawing reduction) works in combination to reduce chip toughness, resulting in controlled chip length, improved surface finish, and improved dimensional accuracy of the part. To read the graphs, the Nitrogen content is shown in one of two ranges, and Phosphorous content is varied as is  the amount (%) cold work. You can see how the synergistic effects of these two chemical elements  when appropriately augmented by cold work, can drop the materials toughness  by as much as 80-90%.

 
 

Phosphorous and Nitrogen affect ductility; Cold work further activates their effect.

Add to that internal lubrication by a separate manganese sulfide phase or a lead addition, and now you can see how these factors can make grade 1215 or 12L14 machinable at speeds far, far, faster than their carbon equivalent 1008-1010. With greater uptime and tool life.

Internal Lubricant- Manganese Sulfides

And you thought that cold drawing just made the bar surface prettier and held closer in size…

Why Lead Is NOT An Alloying Element In Steel

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Lead is added to steels to improve their machinability. But Lead is not considered an alloying element.

An Alloying element is “An element which is added to a metal (and which remains within the metal) to effect changes in properties,” according to my copy of the Metals Handbook Desk Edition.

  
Arrows point to lead.

While lead is an element that is added to a metal:

  • It does not remain in the metal, it remains separate from and mechanically dispersed in the steel  as ‘inclusions’ when it solidifies. It is the dark material on the ends of the manganese sulfides in the photo above.
  • It does not change mechanical properties of the steel.

“Lead can be added to both carbon and alloy steels to improve machinability…The lead is present as small inclusions that are usually associated with the manganese sulfide inclusions…Lead has no apparent effect on the yield strength, tensile strength, reduction of area, elongation, impact strength, or fatigue strength of steel. “- Cold Finished Steel Bar Handbook
For this reason, the addition to lead to steel is not considered an alloying addition. The addition of lead is a great way to improve the economics of machining and improving the surface finish of  complex parts from steel.
Photo from L.E. Samuels Optical Microscopy of Carbon Steels

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 is an important ingredient for quality steel.


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.
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:

  1. Silicon helps increase the steel’s strength and hardness, but  is less effective than manganese in these functions.
  2. In electrical and magnetic steels, silicon helps to promote desired crystal orientations and electrical resistivity.
  3. In some high temperature service steels, silicon contributes to their oxidation resistance.
  4. In  alloy grades, silicon also increases strength (but not plasticity!) when quenched and tempered.
  5. 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
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    5 Reasons To Anneal Steel

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    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 
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    What is Machinability?

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    Machinability is one of those words that everyone uses but everyone also seems to have a different meaning.

    Nudge, Nudge. Know what I mean?

    Here is a look at just a few of the aspects of what that person you are talking with might have in their mind when you say “machinability.”
    1) Surface feet per minute. High surface feet per minute equates to fast cycle times. Fast cycle times mean lots of finished parts per hour. Thus surface feet per minute equals machinability. (But too high surface feet per minute can mean premature tool failure and higher costs and downtime).
    2) Tool life. Rapid tool wear is a sign of poor machinability. Long tool life equals better machinability. (Too long tool life can mean overpaying for tools or too slow cycle times).
    3) Ability to hold surface finish and close tolerance. If you are constantly fighting the setup to keep the finish acceptable or to hold the specified tolerance, you are not experiencing “good machinability.”
    4) Uptime. If the doors are open and your operators head is in the machine and his backside is pointing out, you aren’t making parts. Downtime equals not so machinable.
    So what are the units of machinability? Is machinability measured in surface feet per minute? Tool Life? Surface finish or tolerance? Machine uptime?
    In order to measure anything, you first have to have units with which to measure.
    May I humbly suggest that the proper units of machinability are parts produced by the end of the shift, conforming to print, and requiring the least amount of operator intervention to produce at the quoted cost?
    Only when we agree on this definition can we get a meaningful discussion between Purchasing, “I want the cheapest material.” Operations, “If you gave me better tools or material I could get this job running.” Engineering, “Why can’t you guys hit the cycle time, we figured that job ourself?” And Management, “Why can’t you guys hit plan? I buy you everything you want…”
    The value that shop management adds is to facilitate the organization’s arrival, together, to the optimum state for the shop to produce given the resources available. To do that everybody needs to be on the same page.
    When we’re talking about machinability, that page ought to read “parts produced by the end of the shift, to print, and requiring the least amount of operator intervention to produce.”
    So how do you define machinability?
    Have you seen the tragic results of a department maximum that cost the rest of the organization dearly?
    (Not at your current employer, of course!
    ” Nudge. Nudge. Know what I mean?”


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    5 Ways Fine Austenitic Grain Size Affects Your Machine Shop

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    While Austenitic Grain Size is a result of chemistry (composition), the changes that it evokes in our process are a result of material structure and properties, not just the chemical ‘ingredients.’
    Steel that is fully deoxidized and grain refined is more sound, less susceptible to cracking and distorting, and more easily controlled in heat treat. Well worth it in final performance compared to the machinist’s increased tooling costs.
     Here are 5 Ways Austenitic Fine Grained steels can affect your shop:

    1. Poorer Machinability than Coarse Grained Steels. (The hard oxides and nitrides resulting from deoxidation and grain refinement abrade the edge of tools and coatings- this is one reason that you go through more tooling on Fine Grained Steels.)
    2. Poorer Plastic Forming than Coarse Grained Steels.
    3. Less Distortion in Heat Treating than Coarse Grained Steels
    4. Higher Ductility at the same hardness than Coarse Grained Steels
    5. Shallower Hardenability than Coarse Grained Steels.

    This is a look at Austenitic Fine Grain Steel.

    Fine Austenitic Grain Size is a result of  DELIBERATELY ADDDING grain refining elements to a heat of steel. Because these grain refining elements have been added, the steel has a “Fine Austenitic Grain Size.”
    In order to make steels with this Austenitic Fine Grained Structure, the steel is first deoxidized , (usually with  Silicon) and then Aluminum, or Vanadium or Niobium are added. Aluminum, Vanadium, and Niobium are called grain refiners.
     After  the Silicon has scavenged most of the Oxygen out of the  molten steel, the grain refiner is added. (In this post I’ll stick with Aluminum as the example.) The added Aluminum reacts with Nitrogen in the molten steel to form Aluminum Nitride particles. These tiny particles precipitate along the boundaries of the Austenite as well as with in the Austenite grains. This restricts the  growth of the grains.
    Because the deoxidation and grain refinement  create hard abrasive oxide and nitride particles, they machine and process differently than coarse grained steels.
    Fine Austenitic Grain Size appears on the material test report as an ASTM value of 5 or greater. Values of 5, 6, 7, 8, or “5 and finer”  indicate that  the material is Austenitic Fine Grained. Typically 7 or 8 was  reported for the Aluminum  Fine Grain steels that I certified.
    The methods for determining Austenitic Grain Size are detailed in ASTM Standard E112, Standard Test Methods for determining Average Grain Size.
    To get the Coarse Austenitic Grain Size Story, see our post here.
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