Cracks can develop after cold work is performed on machined parts.
Cold work is defined as the plastic deformation of a metal below its re-crystallization temperature. In the precision machining industry, cold working processes can include thread rolling, thread forming, swaging, crimping, staking, planishing and metal spinning. The steel bars we machine are typically cold drawn (cold worked).
Suppliers use cold work when cold drawing a bar from hot roll to make it more machinable. No heat is added to a cold work process, 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 percent elongation and percent reduction in area, decreases. See “5 Benefits of Cold Work in Steels.”
Steels with low carbon contents, low residuals, low nitrogen levels and steels made by the basic oxygen process readily cold work: think 1008, 1010, and so on.
Intentionally adding nitrogen can predispose a part to cracking during cold work. If a part needs to be crimped, swaged, staked or otherwise cold worked after machining, make certain that the steel is not re-nitrogenized (when nitrogen is intentionally added during the melt process).
Also, make sure that the cold work in cold drawing is standard draft rather than heavy draft. Heavy draft reduces the ductility remaining in the bar, but makes the chips easier to separate. Read about these issues in “Role of Phosphorous, Nitrogen and Cold Work on Machinability of Carbon and Alloy Steels.”
Read more at “Role of Nitrogen in Free Machining Carbon Steels.”
Originally posted at PMPASpeakingofPrecision.org.comments powered by Disqus