Three Key Factors to Understand Machinability of Carbon and Alloy Steel

March 13, 2012

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 Considerations- Gaging Inspection Lab

March 8, 2011

Thermal effects can affect your results.

Its consistency of temperature, not the actual temperature, that is important.

Thermal errors can stack up.

Consistent temperature is more important than the actual temperature

For measurement uncertainty purposes, you want to assure that linear expansion dimensional errors attributable to temperature variation are minimized- less than 10% of your intended accuracy.

Thermal Expansion Coefficient – The thermal expansion coefficient (CTE) of tool steel is added to the measurement uncertainty calculation where relevant. The Testing Laboratory considers consistency in temperature most important. This policy was derived from MIL-STD-120 which states: “Whenever precision measurements are to be made, the temperature should constantly be kept as near to 68 degrees as possible. Since most gages and measuring instruments are usually made of steel…..the requirement that the temperature remain constant is more important than the actual temperature.”

Based on the above statement in bold, our laboratory tracked the temperature with its computerized temperature control system over a period of a month in order to determine the amount of deviation from 68 degrees. The amount of this deviation is used to calculate the Linear Expansion per unit length per degree Fahrenheit. This amount is used in the calculation of relevant measurement uncertainties.

For steel, the coefficient we used was 0.000006″ per degree of temperature change. (That’s six millionths of an inch per degree F)

For copper and copper alloys we used 0.000009″ per degree of temperature change. (That’s nine millionths of an inch per degree F.)

For aluminum, the figure we used was 0.000013 ” per degree of temperature change. (That’s thirteen millionths of an inch per degree F.)

While room airconditioning is important don’t forget that handling gages can affect your measurement system too.

Chart From Kennedy and Andrews Inspection and Gaging

Note that gaging can pick up operators body heat and that temperature errors can thus stack up…

http://www.commodorecomputerclub.com/images/020411/IMG_1868.jpg


Charles Martin Hall- Enabled Our Modern World

February 22, 2011

Charles Martin Hall discovered the electrolytic process for extracting Aluminum from its oxide, 125 years ago from tomorrow. Hall later went on to co-found ALCOA, and gifted his Alma Mater, Oberlin College, with 1/3 of his estate.

Patent number 400664 was issued to him on 04. 02. 1889. See the patent here.

Better living through electrochemistry...

Paul T. Heroult made the same discovery around the same time, and history credits both men for this accomplishment by calling it the Hall-Heroult process.

Aluminum is a critical material of our modern technologies- airplanes, air conditioning and refrigeration parts, engine blocks, cookware, beverage cans. As copper prices continue to escalate, our customers are finding aluminum parts are becoming viable substitutions. And the price of aluminum seems less variable, too. Thats good news for shops that make parts out of aluminum.

According to the Metal Service Center Industry association:

U.S. aluminum shipments finished 2010 some 25.8 percent higher at 1.3 million tons and rose 7.7 percent in Canada, to 135,200 tons than 2009.

U.S. metal centers shipped 100,300 tons of aluminum products during December, or 26.7 percent more than during December 2009. Aluminum inventories at the end of the year totaled 347,900 tons, 33.5 percent above the stockpiles of a year ago and equal to a 3.5-month supply.

In Canada, service centers shipped 9,000 tons of aluminum during December, up 16.7 percent from the same month last year. Aluminum inventories at year end of 31,300 tons were 7.2 percent above stocks at the end of 2009 and equal to a 3.5-month supply.

Aluminum shipments indicate economic recovery is in process.

We believe that continued demand for copper  in global developing economies will increasingly make aluminum a cost effective substitute. Add demand for lighter weight vehicles and improved fuel mileage and we can see that aluminum will continue to increase in its use in our shops.

And to protect our critical thinking…

Hall and Hall Cell

MSCI Link

Hat


Aluminum’s Role In Steel

November 16, 2010

Aluminum is a critical ingredient of  steel in our shops, not just as a stand alone material for machining.

And it makes a darn nice container for pressurized carbonated beverages like Pepsi...

Aluminum metal is used to make many parts produced by precision machining, and is finding increasing application in automotive because of its light weight and high strength to weight ratio.

But aluminum plays a key role in some steel applications that you should know about.

  • Aluminum is used as a deoxidizer.  Aluminum scavenges Oxygen from the melt reducing porosity in the solidiied steel.
  • Aluminum is used to produce a fine austenitic grain size. (Aluminum is the most effective element to control grain growth in steel.)
  • Aluminum is also used as an alloying addition in the amounts of 0.95- 1.30 weight 5 to make Nitriding steel. Nitriding increases the hardness of the steel by the formation of a hard, stable aluminum nitiride compound.

This is what nitrided steel looks like under the microscope.

Aluminum’s ability to scavenge Oxygen results in tiny aluminum oxide particles dispersed throughout the steel. As aluminum oxide is hard and abrasive, Aluminum is not deliberately added to free machining steels where it would destroy tool life.

Aluminum is more effective at grain growth control than elements like vanadium, titainium and zirconium. These three elements adversley affect hardenability because they form carbides that are both  quite stable and difficult to dissolve in austenite prior to quenching.

In the nitriding steel, this recipe is relatively distortion free at the temperatures up to the nitriding temperature. 

    Nitralloy “N” Nitralloy 135
C   0.22-0.27 0.38-0.43
Mn   0.50-0.70 0.50-0.80
P   0.035 0.025
S   0.040 0.025
Si   0.15-0.35 0.20-0.40
Ni   3.25-3.75 0.25
Cr   1.00-1.35 1.40-1.80
Mo   0.20-0.30 0.30-0.40
V  
Cu   0.35
Other   Al, 0.95-1.30 Al, 0.95-1.30
Source   ASTM A355-89 AMS 6470J

So yes Virginia, you may have more Aluminum in your shop than the number of aluminum bars, soda cans and foil wrappers might lead you to believe. Hiding in your steel!

 Nitride structure

Nitralloy Table

 

 

 

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Prices Rise As Manufacturing Recovers

September 2, 2010
The prices of all raw materials that we track rose as follows over the past year:
Aluminum: Up 23% from July 2009.
Brass: Up 28% from July 2009.

Copper: Up 17% from July 2009.

Nickel: Up 70% from July 2009.

Stainless: Up 37% from July 2009.

Steel, Busheling: Up 51% from July 2009.

China Coke, Up 3% from July 2009.

Up some serious double digits over a year ago.

You can download the August Material Impacts report free here

We track these items as they indicate the direction that we wll be paying for our raw materials in our precision machining shops, Steel, Aluminum, Brass and Stainless barstock. These items are critical to the manufacture of those materials. 

 

 

 

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Killed Steel

July 20, 2010

Some things you want to have bubbles, some you don’t.

Usually, Bubbles are good.

In beermaking, yeast consumes the sugars in the wort and convert them into CO2 gas bubbles- carbonation.

In steel making the main reaction is the combination of Carbon in the melt with Oxygen to form a gas. At the high temperatures involved, this gas is very soluble in the molten bath.

If the Oxygen that is available for this chemical reaction isn’t completely removed before the steel is cast the gases will continue to be forced out of the melt during solidification, resulting in porosity in the steel.

Bubbles and where the gas goes can be important in your steel part.

In order to control the evolution of gas, chemicals called deoxidizers are added to the steel. These chemicals, Silicon or Aluminum, Vanadium, Columbium, Niobium scavenge the available oxygen in the molten steel, react chemically to form solid oxide particles dispersed throughout the steel, rather than bubbles of Carbon Dioxide.

The amount and type of deoxidizer added determines the type of steel. If sufficent deoxidizers are added, no gas is evolved from the solidifying steel, and the steel is said to be “killed.” The ingot drawing labelled number 1 shows a fully killed (deoxidized) steel showing only a shrinkage cavity, and no bubbles or porosity. ( This shrinkage cavity would be cropped off in normal rolling practice.)

Because gas is still evolving, this beer is NOT KILLED.

Killed steel has more uniform chemical composition and properties than rimmed, semi-killed, or non-killed steels, and generally less segregation. The uniformity of killed steel and and its freedom from porosity makes these steels more suitable for critical components and for applications involving heat treatment.

Killed steels generally contain 0.15-.35 weight percent Silicon as a deoxidizer, and may contain  some of the other elements as mentioned above. These other elements may be used as deoxidizers or as grain refiners.

Steel grades with a Carbon maximum of 0.30 weight % and above, and all alloy steels are typically provided as “killed steels.”

Free machining steels such as 12L14, 1215, and some 11XX series steels are not “killed” with Silicon, Aluminum, etc., due to their deleterious effects on tool life and machinability. The high amounts of Manganese  in these steels form Manganese Sulfides to promote machinability, and also the Manganese scavenges excess Oxygen, preventing  evolution of CO2.

Killed steel is specified so your critical parts won't have bubbles in them.

Killed steel- for critical parts. Non-killed beer for critical  after work down time.

Cheers!

Beer Bubbles Photo Credit

Ingot scan from a handout in my files originally after Making Shaping and Treating of Steel.

 Beer Head Photo Credit

Bread with Holes

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Materials Prices Increase-Through The Roof

April 8, 2010
6 of 7 materials we track up 44-114%

6 of 7 materials we track up 44-114%

Prices of raw materials used to make precision machined products are up substantially,  ranging from 44% to 114%  from March 2009- March 2010 for 6 of the 7 materials we track.

 Low inventories, increasing demand, idled production facilities, are among the factors involved here in North America.

As are the historic iron ore agreement  and continued high demand in China. 

We think this trend will be around for a while...

Fuel price increases also impact freight, which is an important factor in our business.

We will not be shocked to see monies paid for steel in May to be $80 per ton higher than they were in April based on already announced price increases and the current price on  #1 busheling which determines surcharges.

Read more and download the .pdf report  here.

Photocredit.

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