Page 14 - MetalForming-Jul-2018-issue

The Science of Forming
By Daniel J. Schaeffler, Ph.D.
Material Specs: You Get What You Order
With a part to manufacture, your first decisions revolve around how to generate that part and what material to use. Manufacturing options include casting, hydroforming and 3D printing, but you already have the experience and infrastructure for stamping. Factors such as cost, weight, corrosion resistance, strength and stiffness go into the discussion of what sheetmetal to select. With the material type selected—steel, aluminum, stainless, brass, etc.—you must order it from a mill or service center. At this point, “Give me what I need,” is not good enough. Here, specifications come in to play.
A materials specification simply is a listing of the require- ments and characteristics that must be met for the supplied product to be considered satisfactory. The spec can call out limits of alloying-element composition levels, thickness, flatness and tensile properties. Any organization can create this listing, including industry organizations such as Amer- ican Society for Testing and Materials, American Iron and Steel Institute or the Aluminum Association. End users such as General Motors or Boeing, or the materials producers themselves, often write specifications as well.
Even if you seek generic grades, chances are that you are ordering to these documents whether you know it or not. When specifying only AA6061 aluminum or AISI 1010 steel, your supplier will default back to these and related docu- ments to determine what to supply. When ordering these generic grades, several suppliers are available, with a greater chance that the materials will be in stock. And, these materials often cost less than those with tighter constraints.
Specifications May Be Too Generic
The risk with this approach: The material supplier must meet only what is stated in the specification, and not what is otherwise expected based on what you had received pre- viously. For example, suppose you have been ordering “1008 steel.” SAE J403 Grade 1008 represents the only specification
Danny Schaeffler, with 30 years of materials and appli- cations experience, is co-founder of 4M Partners, LLC and founder and president of Engineering Quality Solu- tions (EQS). EQS provides product-applications assis- tance to materials and manufacturing companies; 4M teaches fundamentals and practical details of material properties, forming technologies, processes and trou- bleshooting needed to form high-quality components. Schaeffler, who also spent 10 years at LTV Steel Co., received his Bachelor of Science degree in Materials Science and Engineering from the Johns Hopkins Uni-
versity in Baltimore, MD, and Master of Science and Doctor of Philosophy degrees in Materials Engineering from Drexel University in Philadelphia, PA. Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
document that defines what 1008 steel entails. This document defines chemistry limits, and little else.
The allowable range of carbon for this grade lets the service center provide anywhere from ultra-low carbon, extra-deep-drawing steel with carbon levels close to 0.002 percent, to a higher-strength steel with “10 points” of carbon, or 0.10-percent carbon content. This chemistry range alone may result in a yield-strength spread of 140 MPa, and the steel service center can provide either one or something in between that still meets the requirement of 1008 steel. Again, the specification does not call out any limits on tensile prop- erties such as strength or elongation. Specifications for high- er-strength grades likely list minimum required strength and elongation levels, and some may cap yield strength.
Services centers can profit by buying coils that may not be suitable for their initial application but are otherwise satis- factory, and, therefore, have a relatively low purchase cost. A common issue in as-produced coils is flatness outside of allow- able limits. Many material suppliers may have the equipment needed to correct flatness, so this is a good way for them to buy what was initially deemed a sub-prime coil and rehabilitate it to prime condition. The leveling and flattening processes involve passing the coil through many rolls, which results in increased strength and decreased ductility of the material.
Even though the mechanical properties have changed, the chemistry remains the same. This means that your service center may ship you a flattened coil with different strength and elongation than coils received in the past. This reprocessed coil will differ from what you are used to receiv- ing. It may not form the same way as prior coils—you could see increased scrap—but since the composition conforms to 1008 steel, you received what you ordered.
Be Specific if the Application Calls for It
If a process requires a tighter range of incoming properties, place this information on the order, by using a different spec- ification or by adding line items. Not surprisingly, the more constraints placed on an ordered product, the greater the possibility of a higher cost. When deciding what specifications to include on a material order, balance price savings of order- ing to a generic specification against the cost to any downtime associated with additional variability, manufacturing-pro- ductivity losses and increased scrap. MF
Learn about high-strength-steel grades and formability, tool steels and coatings, presses and die design, and effective lubricant strategies at PMA’s two-day Stamping Higher- Strength Steels Seminar in Nashville, TN, September 12-13. Visit www.pma.org for details and registration, or contact Marianne Sichi at msichi@pma.org for more information.
12 MetalForming/July 2018
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