196 Cold Working and Material Hardness
In this section the relationship between cold work and material hardness will be discussed in terms of bending a tube. Basically, cold working is deforming (or bending) a material like mild steel where the hardness value of the steel increases as a result.
First off - What is hardness in a metal? Hardness is a measurable property which describes how difficult it is to deform the metal. It is measured by pressing a sphere of known diameter with a known amount of force into a material sample. The distance the sphere travels into the sample is measured, and converted into a hardness value for a particular scale. Hardness scales include the Brinell for softer materials like aluminum, Rockwell f for copper, Rockwell b for mild steel, and Rockwell c for harder materials like tool steels.
In their softest or fully annealed condition, metals have a base hardness value. That base hardness can be increased by disrupting the microstructure of the metal either by a heat treating process or by deforming it during cold working.
 |
As a tube is being bent, the inside wall (closer to the center point) of the tube tries to grow thicker, and the outside wall (further from the center point) tries to grow
thinner. An area called the neutral axis which divides these
areas shows no wall thickness changes after bending.
Material tends to flow from the inside wall to the outside wall during the bending process, moving the neutral
axis slightly towards the outside of the bend.
Material is drawn towards the bend area on the outside (beyond the tangents) from the straights ahead and behind, so thinning shows
up there also. This is more prominent on tight radius bends. |
 |
| Radius |
Arc Length |
Change |
| 2.5 |
3.927 |
-.785 |
| 3.0 |
4.712 |
0 |
| 3.5 |
5.497 |
.785 |
|
To the left is an example of how much the outside wall must stretch and inside wall must shorten in a 1" OD tube bent on a 3" CLR through 90 degrees. After bending the outside wall stretched by .785" (17%)and the inside wall shortened by .785" (17%).
Without additional tooling, the outside wall will collapse towards the CLR instead of stretching and the inside wall will kink or form wrinkles instead of getting shorter. |
  |
| All this thinning and thickening is caused by the tension exerted on the outer wall as it gets stretched longer, and
by the compression forces exerted on the inside walls of the tube as it gets shorter. To the far left is what the grain cells look like at the neutral axis. Near left is what they look like after stretching or compression. |
 |
The diagram on the left shows the relationship between stress and strain, or how hard you push (stress) and how far it bends (strain). The red zone shows elastic deformation which is another term for spring back. If the amount of stress stays in the red zone, no bending will occur, when the stress is removed, the material will return to its' original condition. The yellow zone shows plastic deformation where the amount of stress results in bending. Apply enough force to go beyond the ultimate tensile strength and the material approaches its' breaking point. |
 |
As material is worked to and beyond its' ultimate tensile strength, dislocations appear. These are the result of chemical bonds in the microstructure being broken and result in tiny voids in the material. These voids actually move through the material to collect at what are called "stress risers". When enough collect, a crack forms. When the crack grows large enough, the material breaks. As material collects more and more dislocations, it gets harder and easier to snap.
This process is cumulative. With a tube, after roll forming into the tubular shape, the tube material will be harder than before. This is called cold working or work hardening. |
Hardening:
If a tube has been work hardened, like the extra passes through the rolls for pre-chrome or cold formed drawn over mandrel seamless tube, it can be more susceptible to cracking during further cold working like in the tube bending process. If enough scrap is generated, a cost added process like stress relieving or annealing may be called for. When material is annealed, it is heated in a controlled atmosphere and held at temperature for many hours. While at temperature, the microstructure of the material can reorganize and heal the broken bonds that cause dislocations. The result of the stress relieving or annealing process is a reduction in material hardness.
Percent Elongation:
Percent elongation of is another way (beyond a Stress Strain diagram) to describe how much cold work a material can take before breaking. Simply put, percent elongation is determined by stretching a sample of material to the breaking point - its' value is the difference in length before and after.
Back to basics
|
