Cold Work And Steel Parts

March 14, 2013

Cold work is defined as the plastic deformation of a metal below its recrystallization temperature.

In the precision machining industry, cold working processes can include thread rolling, thread forming, swaging, crimping, staking, planishing, and metal spinning.

And the steel bars that we machine are typically cold drawn (cold worked.)

Our suppliers use cold work when cold drawing a bar from hot roll to make it more machinable.

How to recognize a cold work process: No heat is added and no chip is removed in the process of moving the metal into shape.

Cold working of steel

  • changes its mechanical properties
  • and improves its surface finish.

Tensile strength and yield strength are increased by the cold work while ductility (as measured by % elongation and % reduction in area decrease.

See our post here.

Steels with low carbon contents, low residuals, low Nitrogen levels, and made by the Basic Oxygen Process readily cold work- think 1008, 1010, etc..

Cracks can develop after cold work is performed on machined parts.

Cracks can develop after cold work is performed on machined parts.

Intentionally adding nitrogen  can make predispose a part to cracking during cold work. If a part needs to be crimped, swaged, staked or otherwise cold worked after machining, You should make certain that the steel is not renitrogenized. (Nitrogen intentionally added during the melt process).

Also, make sure that the cold work in cold drawing was standard draft rather than heavy draft. Heavy draft reduces the ductility remaining in the bar- but makes the chips easier to separate.

We posted about these issues here.

More information on Nitrogen in free machining steels.


8 Reasons to Stress Relieve Cold Drawn Steel Bars

October 2, 2012

Here are  8 reasons why you might want to consider stress relieving the steel before machining your parts.

  1. High carbon grade of steel. Alloy grades over 0.40 carbon and carbon grades above 0.50 carbon can often benefit from stress relief.
  2. Heavy draft to make size. Heavy draft can add cold working strain which can set up stresses in the part.
  3. Small diameter parts. The percentage of cold work (strain) is higher for the same draft reduction as diameter decreases.
  4. Long parts. Stresses tend to display  and their effects increase longitudinally.
  5. Assymetric parts– and parts with large differences in section or mass.
  6. To increase mechanical properties. At lower stress relieving temperatures, the hardness, tensile strength, and elastic properties of most cold drawn steels increase.
  7. To decrease mechanical properties. At higher stress relieving temperatures, hardness, tensile strength and yield strength are reduced while % elongation and 5 reduction of area are increased.
  8. To reduce distortion off the machine. Usually stress relieving is used as a last ditch effort to reduce the distortion  that presents after machining a part with some or many  of the characteristics given above.

There are certain applications where stress relief (of steel) is indicated

Stress relieving is a lower than  the material’s critical point thermal treatment also known as strain drawing, strain tempering, strain annealling, strain relieving, or pre-aging. It is performed to modify the the magnitude and distribution of of residual forces within a cold drawn steel bar, as well as to modify the mechanical properties.

Thanks Seth at Sixthman Blog for the photo.


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.


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

April 26, 2011

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…


Three Criteria for Selecting Bar Steels

March 23, 2011

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.


5 Benefits of Cold Work in Steels

June 22, 2010

Here are 5 benefits of cold working of steels that make a difference to your machining operations.

  1. Increased strength
  2. Improved surface finish
  3. Controlled dimensional tolerance and concentricity
  4. Improved straightness
  5. Improved machinability.

Increased Strength

It is widely known that cold working strain changes the properties of most metals. When as rolled steel bars are cold worked by cold drawing through a die, a significant increase in yield and tensile strength is obtained. At the same time, The reduction in area and percent elongation are reduced.

The graph below shows the effect of cold drawing on the tensile properties of 1 inch round diameter steel bars.

Mechanical Properties % Change resulting from % Cold Work

There are two important lessons in this graph: 1) As strength properties increase, ductility measures decrease; 2) Up to about 15% cold reduction, yield strength increases at a much greater rate than tensile strength. The first 5% of cold work results in the greatest increase in strength.

Improved Surface Finish

Hot rolled steel bars are finished at high temperatures, and so the surface has a hard abrasive scale made up of various oxides of Iron. This scale is hard and abrasive ranging from 270 – 1030 DPH (Vickers) microhardness depending on the type of oxide (s) formed. In order to cold draw the bars, cold finshers typically remove the sacle by shot blasting  or acid pickling. This results in the removal of the hard abrasive scale.

By pulling the bars though the die, the surface finish is also improved,  with Cold Drawn bars typically running 50 microinches maximum and modern equipment typically working at 25-30 micro inches. Compare this to a roughness height of  250 or more for hot rolled bars.

Controlled Dimensions

Because the bars are cold reduced at room temperature by pulling through an oil lubricated die, the dimensional conformance of the steel is much more easily controlled. Typical tolerances for cold drawn 1 ” low carbon steel bars are +0.000″/ – 0.002″. this compares favorably to +/- 0.010 for hot rolled steel of the same chemistry and diameter. Concentricity is improved by the cold drawing operation.

Improved Straightness

The straightness of hot roll bars is generally 1/4″ max deviation in any 5 foot length. In cold drawn bars,  depending on size and grade this  deviation can be held to as little as 1/16″ in 10 feet.

Please see our post here for a more complete discussion of bar straightness.

Improved Machinability

Improved machinability is really the synergistic result of all of the above improvements made by cold work (cold drawing).

Higher yield to tensile ratio means the tool has less work to do to move the metal in the workpiece to its ultimate strength when it will separate as a chip. This translates into less force on the tool and greater tool life and productivity. Not putting hard abrasive scale and oxides into your cutting fluids nor on to your tool because the bar has been cleaned results in longer uptime and  less maintenance for tools, workholding, and machines. More tightly controlled dimensions and concentricity means that the bars can be run at higher speeds without creating harmful vibrations and chatter. Finer tolerances can be held by your equipment when bars are sized properly going into the machine. Similarly, improved straightness results in less runout and permits higher speeds in production.

Bottom line: Hot roll bars may be cheaper by the pound, but machining them will cost your company a lot more because they lack the benefits of cold drawing:

  • Increased strength
  • Improved surface finish
  • Controlled dimensional tolerance
  • Improved straightness
  • Improved machinability.
  • Graph and data: AISI Cold Finished Steel Bar Handbook, 1968. (Out of print)

    Share


    3 Reasons Why Mechanical Properties of Cold Drawn Steel Can Vary

    March 4, 2010

    Why do the mechanical properties on different shipments of the same size and grade of steel vary so much?

    To answer this, lets look at grade 1018, a non-free machining grade that we may encounter in our shops.

    We'll pull it until its two pieces!

    A cold drawn  1018 steel bar  1″ diameter typically has a Tensile Strength (TS) of 64000 psi.  Yield Strength (YS) of 54,000 psi; %Elongation in 2″  (%EL) of 15%; % Reduction of Area  (%RA) of 40%. (According to  Information Report SAE J 1397,  Estimated Mechanical properties and Machinability of Steel Bars,) Note, these are estimated values, not minimums!

    Your mileage (properties)  may vary– here are three reasons why.

    1. The original melt and cast process can affect chemical makeup;
    2. The mechanical properties of cold drawn steel are affected by the amount of cold work;
    3. The final steps of straightening and polishing can relax the steel.

    The original melt and cast process can affect chemical makeup. Basic Oxygen Furnace (BOF) steels are made from a high percentage of new metal, and so have lower levels of residual elements from scrap that could strengthen the material. Also BOF steels tend to run lower levels of nitrogen, which is a ferrite strengthener.  So BOF Melt steels tend to be on the low side of mechanical properties like tensile and Yield, and a bit higher ductility (%RA and %Elongation in 2″).

    The mechanical properties of cold drawn steel are affected by the amount of cold work. This can come about in two different ways: the first way is as the bar size ordered gets smaller, given a standard draft, the percentage of cold work increases. This increase in the percentage of cold work increases the mechanical properties of Tensile and Yield Strength and can decrease the ductility somewhat.

    The second way can be when different vendors use a different “drafting practice” resulting in a different amount of cold work to make the same size. Typical draft may be to use hot roll sized 1/16th” over the final size for drawing. Another vendor may choose 3/32″  oversize, and in rare cases a company my use 1/8″ to assure exceeding, not just  meeting,  minimum Yield Strength.

    The final steps of straightening and polishing can relax the steel. The amount of cold work done in straightening the bars can relax the steel because the force is applied transverse to the original drawing. So a supplier using a two roll straightener, all other things being equal, may produce bars with a different final set of properties than one using a train of planishing discs to get the bar commercially straight.

    So what values could you expect to encounter in grade 1018 steel when looking at all of these effects?

    We’ve seen 3/8″ 1018 with Tensile Strength (TS) in the high 80,000’s; Yield Strength (YS) in the high 70,000’s.%EL in 2″ as high as 26;%RA as high as 65.

    And in 4″ rd 1018, TS  as low as 58,000psi; YS of  about 42,000 psi; %EL in 2″ of 12%; % RA of 35%.

    The process path generally can explain the properties received.  And why those mechanical properties that you receive are sometimes so far from what you expect.

    Photo credit: A-Lab Dayton Ohio

    Share