October 2, 2012
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.
August 10, 2010
Because the steel grain structure is cold forged, rather than cut, rolled threads are up to 30% stronger…
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.
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:
Blank diameter will be between major and minor diameter = $ Saved.