Cold Work And Steel Parts

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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.

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

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

High carbon grade of steel. Alloy grades over 0.40 carbon and carbon grades above 0.50 carbon can often benefit from stress relief.
Heavy draft to make size. Heavy draft can add cold working strain which can set up stresses in the part.
Small diameter parts. The percentage of cold work (strain) is higher for the same draft reduction as diameter decreases.
Long parts. Stresses tend to display and their effects increase longitudinally.
Assymetric parts– and parts with large differences in section or mass.
To increase mechanical properties. At lower stress relieving temperatures, the hardness, tensile strength, and elastic properties of most cold drawn steels increase.
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.
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.

Cold Drawing’s Effect on Mechanical Properties

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This is why yield strength doesn’t correlate well to hardness…

Cold drawing of steel bars changes the mechanical properties of the hot roll bar feedstock. With standard draft:

Tensile strength increases by 25-30%;
Yield strength increases 60 to 80%;
Brinell Hardness increases about 15%;
Ductility DECREASES 25-30% typically with standard draft.

As can be seen by the yield strength increase compared to hardness increase by cold drawing, the yield strength picks up about a 4 times multiple compared to hardness. Poor correlation.

Hardness correlates well with tensile strength in most steels.

In fact, my rule of thumb for estimating either tensile strength or hardness is that

Tensile Strength divided by 500= Brinell Hardness

for most low and medium carbon steels. Or

Brinell Hardness multiplied by 500= Tensile Strength.

Try it with your data.

Photocredit

Three Criteria for Selecting Bar Steels

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Three primary criteria for selecting bar steels are 1) suitability for end use, 2) suitability for manufacturing process, 3) economical delivery of the requirements.

Product_0022A corey — 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.

6 Reasons To Consider Rolled Threads

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Because the steel grain structure is cold forged, rather than cut, rolled threads are up to 30% stronger…

physical_5 — Sketch iluustrating grain flow lines due to cold work of rolling.

While the strength of a thread is a function of section thickness, a thread rolled rather than cut thread is usually superior in mechanical properties, all other things being equal- here are 6 reasons why:

The flow of the material by cold work during rolling reinforces the shape.
The cold working strain increases mechanical properties, Surface Hardness,Tensile Strength, Yield Strength, and the Yield Strength / Tensile Strength ratio.
The surface finish of the thread flanks is usually smoother due to the burnishing action of the rolls.Smoother finish means better fatigue life and fewer opportunities for stress risers.
There is more material (section thickness). This results in material savings*, since the diameter of the blank will be between the major and minor diameter of the thread, rather than greater than the major diameter for a cut thread.
The compressive stresses on the threads resulting from rolling improves the fatigue life.
The root of the thread has a smoother radius, improving fatigue life.

physical_2 — Schematic view of the thread rolling process.

Pictures courtesy PMPA member Ray Industries
Click here for a video of the thread rolling process in action from PMPA member Rolled Threads Unlimited LLC.
*I got my first “learnin” on rolled threads at my customer Keystone Threaded Products back when my hair was not silver and B.K. (Before Kids): “Why do you always buy funny sizes Jim?” I asked. That was a great first lesson on how engineering can add savings:

material saved — Blank diameter will be between major and minor diameter = $ Saved.

3 Reasons Why Mechanical Properties of Cold Drawn Steel Can Vary

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“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.

alab tensile — 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.

The original melt and cast process can affect chemical makeup;
The mechanical properties of cold drawn steel are affected by the amount of cold work;
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