The ability of a material to deform plastically without fracturing, is called ductility. In the materials usually machined in our shops, ductility is measured by determining the percent of elongation and the percent reduction of area on a specimen during a tensile test.
Ductility is often indicated by chip control issues in certain steels, as the chip readily deforms but does not separate from the work piece. This can result in persistent burrs attached to the work .
Ductility can also mean long stringy chips that can form a dreaded “birds nest” engulfing the tool and work piece.
Long necklace chips are another sign of ductile materials in machining.
Short chips curled into “sixes and nines” showing a bit of heat discoloration are typical of less ductile materials and dutile materials machined at proper parameters using chip breakers and high pressure coolant delivery.
In our machining practice we would prefer materials that are “crisp” rather than ductile.
In order to successfully deal with ductile materials, strategies such as chip control features on inserts, wiper style inserts, through tool coolant, interrupted cuts, chip breakers, and high pressure coolant can be considered.
Dialing in the appropriate feeds, speeds and depth of cut are crucial too.
Birdsnest photo courtesy Garage Journal
All other photos by author.
Tag: Plastic Deformation
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..
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.
In most of this footage you will see that there is a small mass of workpiece material (Built Up Edge or BUE) that is doing the “penetration.”
The fracturing ahead of the tool, and the occassional jamming of material under the tool, and the waviness of the generated surface are concepts to keep in mind when you try to understand why you are getting the finish on the surface that you get.
Also, a good way to visualize how the material is being workhardened by “rubbing” where the material is not separating easily ahead of the tool.
If steel did not have the property of plastic deformation, the only ways to make parts from it would be casting or cutting into shape.
No deformation processes like cold heading, cold rolling, swaging etc. would be possible.
If one subjects a piece of steel to a heavy load, the material will measurably stretch. When the load is removed, if the steel goes back to its original dimension, the deformation that it underwent when the weight was applied is called “elastic deformation.” In this case, the steel did not take a permanent “set.”
If one subjects a piece of steel to a much greater load, if, when the load is removed, the steel does not ‘spring back’ or recover to its original dimension, the new shape or dimension is a permanent deformation. (It is often said to have ‘taken a set,’) This is called “Plastic Deformation.”
Plastic Deformation is explained by the movement of planes of atoms from their normal positions.
Steel and most industrially useful metals are able to withstand a great deal of this Plastic Deformation before they break.
Brittle metals will just fracture under such loads;
Cold drawing of steel is a process that applies a load to the metal to make the atoms in the steel take new positions with respect to each other, resulting in lowered ductility, increased tensile and yield strength and new dimensions or shape. These in turn, are often helpful in improving the machinability of the steel, allowing you to more economically produce the parts and components that are essential for our current technologies.
Most people think of steel’s hardness as its main advantage. The facts of the matter are that it is steel’s plasticity or ability to plastically deform that makes it such a useful and versatile material for humankind.