Tag Archives: ductility

Sheet Metal Formability

Formability refers to the ability of sheet metal to be formed into a desired shape without necking or cracking. Necking is localized thinning of the metal that is greater than the thinning of the surrounding metal. Necking precedes cracking.

From the metallurgical perspective, the formability of a particular metal depends on the metal’s elongation, which is the total amount of strain measured during tensile testing. A metal with a large elongation has good formability because the metal is able to undergo a large amount of strain (work) hardening.

Strain hardening
Strain hardening results in an increase of the load-carrying capacity of a metal as it deforms. It also prevents strains from being localized during forming, so the deformation is uniformly distributed throughout a particular section of the material that is exposed to a specific set of forming stresses. As a result, each localized region of the metal thins uniformly during the forming process.

The load carrying capacity of the metal as it deforms is opposed by the reduction in cross-sectional area of the metal as it thins. There is a maximum load where the increase in stress due to the decrease in the metal cross-sectional area becomes greater than the increase in the load-carrying ability of the metal due to strain hardening. Necking begins at this point as the metal starts to thin more in a localized region. Any additional deformation is concentrated in the necking area, while the loads in the surrounding areas decrease.

Strength vs. ductility and elongation
Anything done to increase a metal’s yield and tensile strength does so at the expense of ductility, and therefore elongation. As the elongation of a particular alloy decreases, there is a decrease in the amount of deformation before necking occurs. Strengthening treatments include cold rolling working, through hardening and age hardening heat treatments, and solid solution strengthening. Also, strength increases and elongation decreases as grain size decreases.

Crack formation
Once necking begins, the loads on a metal are concentrated in the necked region.  The amount of deformation of the necked area before a crack forms depends on the microstructure of the metal and the state of stress on the metal.

Cracks that form during metal forming occur by a fracture process that involves the formation and growth of voids around second-phase particles and inclusions in the metal. This is shown in figure below. The voids form, grow, and coalesce to form a crack. So, the presence of second-phase particles and inclusions reduces a metal’s formability because they are sites where cracks nucleate.

VoidFormation

Second-phase particles are often present as a result of adding certain alloying elements or strengthening heat treatments. This is the case for iron carbide particles in steel or precipitates in age hardened alloys.  Inclusions are particles comprised of impurity elements in an alloy. For example, in most steels sulfur is present as an impurity that usually appears as manganese sulfide inclusions. In aluminum alloys iron and silicon present as impurities react with the other elements in the alloys to form hard particles. For an alloy that contains inclusions, reducing the impurity content will help reduce the number of inclusions, and improve formability.

Stress state and formability
Finally, the formability of a metal also depends on the state of stress on a metal during forming. The state of stress depends on the shape of the component being fabricated and the process used to form the component. Forming limit diagrams are used to predict whether the forming strains to which a metal will be exposed will result in necking or cracking.

Why Control Heat Treatment Temperature?

Whether you are through hardening steel, annealing a cold-worked metal, or solution treating an aluminum alloy, the heat treating temperature is critical for obtaining the desired microstructure, and therefore, the desired metal properties.  Microstructure refers to such things as the metallurgical phases present in a metal and the grain size.

Using a temperature that is too hot can result in a metallurgical transformation that proceeds too quickly or the formation of undesired phases.  Using a temperature that is too low can result in incomplete metallurgical transformations, cold worked metals that do not soften sufficiently, or insufficient stress relief.

For example, during the through hardening heat treatment of a carbon steel, the steel is heated to transform all the ferrite and cementite to austenite and then quenched to form martensite.  If the steel is not heated to a high enough temperature, then there is the risk that all the ferrite and cementite does not transform to austenite.  If this occurs, then when the steel is quenched, the remaining ferrite and cementite will be present along with the martensite.  These ferrite and pearlite remnants can weaken the steel.

Another example is cold-rolled sheet metal that is annealed to improve its ductility, and reduce its strength and hardness.  If the annealing temperature is too high, then excessive grain growth will occur.  This will result in the metal having lower strength and hardness than intended.  Also, if the metal is to be formed, there is the risk of orange peel, a cosmetic defect in heavily formed metals with grains that are too large.

So, why might a heat treater use a heat treating temperature that is too high or too low?  To save money, to save time, or just sloppy.  To reduce energy costs a heat treater might try to run its furnaces at the low end of the required temperature range.  However, normal temperature variations throughout a load and normal composition variations within the metal can result in the temperature being too low to cause the desired metallurgical transformations.

To save time, a heat treater might operate a furnace at the high end of the specified temperature range to try to move the metallurgical transformations along as fast as possible.  Again, with normal temperature and composition variations, the temperature may end up being too high, resulting in excessive or undesired changes in the metal’s microstructure.

As for a sloppy heat treater, who knows what you will get from batch to batch of metal stock or components.

To learn more about the effects of temperature control on steel microstructure and properties, take our Metallurgy of Steel Heat Treating course or read Practical Heat Treating by J.L. Dorsett and H.E. Boyer or Steels: Processing, Structure,and Performance  by George Krauss.  Also, the two courses mentioned in the introduction above will discuss the effects of temperature control on precipitation strengthening and annealing cold-worked metals.