The machinability of steel bars is determined by three primary factors. Those factors are 1) Cold Work; 2) Thermal Treatment; 3) Chemical Composition.
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
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.
Thermal effects can affect your results. Its consistency of temperature, not the actual temperature, that is important. Thermal errors can stack up.
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.
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.
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 is a critical ingredient of steel in our shops, not just as a stand alone material for machining.
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.
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.
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
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.
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.
Some things you want to have bubbles, some you don’t.
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.
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.)
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.
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.
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.
11 industries reported expansion in February. The Precision Machined Products Industry, a sub industry of Fabricated Metals, serves 7 of these industries showing the greatest recovery.
Here are the sectors that reported expansion that precision machining serves:
Computer & Electronic Products;
Appliances & Components;
Fabricated Metal Products;
Economic activity in the manufacturing sector expanded in February for the seventh consecutive month, and the overall economy grew for the 10th consecutive month, say the nation’s supply executives in the latest Manufacturing ISM Report On Business®.
The PMI index for February was 56.5 down 1.9 percentage points from January. Because the PMI is above 50, the manufacturing economy is expanding.
According to ISM steel, stainless steel, and aluminum are increasing in price.
Anecdotal data from our conversations with members confirms the ISM numbers, and points out that the metals named above are both more expensive and in short supply.
You know business is improving when they are out of plain vanilla. Photocredit.
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:
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.)
Poorer Plastic Forming than Coarse Grained Steels.
Less Distortion in Heat Treating than Coarse Grained Steels
Higher Ductility at the same hardness than Coarse Grained Steels
Shallower Hardenability than Coarse Grained Steels.
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.
The current service center destocking will mean shortages and delays when manufacturing recovers. This makes raw material price increases inevitable.MSCI Report Steel: US and Canadian first-half steel shipments totaled 14.8 million tons, down 43.9% year-over-year. US steel inventories at the end of the month were reported to be 5.98 million tons; down 44.4% from last year. Canadian steel inventories at the end of June totaled about 1.05 million tons, 33% below last year. Aluminum: First-half shipments of 524,600 tons of aluminum were down 43.2% year-over-year for US Service Centers. US inventories at the end of June totaled 269,800 tons, a reduction of 44.1% from a year ago.
According to The Metal Service Center Institute-In Canada, first-half aluminum shipments totaled 65,100 tons, a decline of 26.4%. Month-end inventories totaled 31,700 tons, a decline of 15.7% from a year ago. Sensemaking:Beware the “at current shipping rates, months of shipments fallacy” in the MSCI press release. This is simple arithmetic, not critical thinking. When demand recovers, “today’s current shipping rates ” are not going to be relevant at all.
The fact is these inventories are lowest since the early 1980’s, and when business resumes just a little bit, there will be nothing in the cupboard. And weeks and weeks of lead time to refill the pipeline.
The current service center destocking will mean shortages. Shortages will mean delays and raw material price increases. Delays and raw material price increases will mean higher prices for precision machined parts for finished products. You can bet dollars to donuts that the spotlight will be on you and your shop as you try to recover these increases- You’ll be called “Greedy business men fueling inflation! ”
So much for sensemaking.
Have a nice day!