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.
Our earlier post about Ductility showed how ductility can impact our shops. In this post, we will describe how we can measure ductility and use it to predict behavior  based on values reported on certs and test reports.
The percent elongation and percent reduction of area values shown on our test reports and material certifications from our material suppliers indicate the ductility of the material tested.
In the tensile test, a cylindrical specimen is gripped securely and subjected to  a uniaxial load and elongated until it breaks. At the end of the test, the pieces of the fractured specimen are fitted back together again and the change of length between the two gage marks put on the specimen before testing is determined. The change is then expressed as a percentage of the original gage length.

Fractured specimen fitted back together then measured
Fractured specimen fitted back together then measured

The percent reduction of area is determined by measuring the minimum diameter of the broken test specimen after the two pieces are fitted together and the difference is  expressed as a percentage of the original cross sectional area prior to the test.
The  differences in measurements after tensile test are used to calculate the % elongation and % reduction of area
The differences in measurements after tensile test are used to calculate the % elongation and % reduction of area

A minimum of 12% elongation  is recommended for  consistent, trouble free thread rolling applications.
Rolled threads are stronger, so having the ductility to thread roll is important. However, too much ductility makes it difficult to get the chip to separate by cutting.
Low ductility can be problematic for cold deformation manufacturing processes such as thread rolling, cold forming, swaging, staking and crimping.
This is the designer’s compromise: if it is good for cutting, it is probably not very good for rolling.
And Vise-Versa
And Vise-Versa

HSC online Graphic of test specimens
Yost made in USA vise photo credit

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.