Page 41 - MetalForming June 2017

Fig. 1—Courtesy ArcelorMittal (Constructalia)
temperature high enough to create a solid solution of iron and carbon in a process called austenizing. Austenizing is followed by quenching to produce a hard, martensitic microstructure. Quenched martensite is strong but very brittle; only after it is tempered does it become tough and slightly more ductile.
Unfortunately, for most die shops and manufacturing companies, heat- treatment is a “black-box” process: Soft die details are sent out to the local heattreat facility and hardened details are returned. A cursory Rockwell hard- ness test may be conducted at the die shop when the parts return. If they meet hardness requirements they are usually accepted, regardless of how they may have been processed. This is a problem.
Case in point: Die failures can occur due to overheating damage during their heattreatment. Because most die steels are relatively high in carbon and/or alloy content, they are sensitive to dam-
age by overheating during austenizing. Overheating causes grain growth, coarse martensite formation and exces- sive amounts of retained austenite, all of which results in brittle dies that are susceptible to failure in service—even if the surface hardness is within spec- ification.
For stainless steel applications, com- mercial products such as kitchen uten- sils and appliances are common. In the automotive sector, it is exhaust sys- tems that usually come to mind, but there are other applications, such as stainless-steel body trim, windshield- wiper arms, cylinder-head gaskets and fuel-filler necks. Stainless steel also has appeared in structural components in some automotive structures as a high- strength solution to lightweighting.
Stamping companies and tool shops experienced with plain carbon steels often have problems making parts from stainless steel. Negative past experi- ences and poorly designed processes often produce poor results that can
fuel misconceptions and myths regard- ing these corrosion-resistant materials. For example, most stainless steels have work-hardening rates that are greater than those for plain carbon steels. A common misconception is that stain- less steels are less formable than low- carbon steels because they work-hard- en too much. In reality, stainless steels are very formable—many grades have substantially higher ductility than plain carbon steels. Stainless steels frequent- ly are deep-drawn into very complex shapes without the need for interme- diate annealing. Even the less formable ferritic grades have outstanding duc- tility (Fig. 1).
However, due to their high work- hardening rate, more press power and press energy is required to form stain- less steel. Frequently, a 100-percent increase compared to low-carbon steel of the same thickness is required.
A unique feature when cold-forming stainless steels is that more severe deformation is possible when slower forming speeds are used. This is dif- ferent from plain carbon steels that form virtually the same regardless of the forming speed. In general, when forming stainless steel into difficult shapes, you will want to slow down.
The required clearances between the draw post (punch) and die-cavity wall will be greater for stainless steels compared to plain carbon steels. Austenitic grades require sheet thick- ness plus an additional 35 to 40 per- cent; most ferritic alloys require sheet thickness plus 10 to 15 percent addi- tional clearance.
For more information regarding forming, punching and cutting high- er-strength steels, consider attending PMA’s Higher Strength Steel seminar in Detroit, MI, July 25-26, 2017. Visit the PMA website or contact Marianne Sichi at msichi@pma.org. MF
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MetalForming/June 2017 39
Tooling by Design
Martensitic stainless steels
HSLA steels
Austenitic stainless steels Type 304/316
600
500
400
300
200
100
0
Ferritic stainless steels Type 430
Carbon steels
A36
4
8 12 16 20 24 28 32 36 40 44 48 52 Strain %
Stress (MPa)