Metal Forming Knowledge Base
Thinning and Circle Grid (Surface) Strain Analyses are Engineering Quality tools that can help determine where trouble spots could develop during the production processing of your parts. The forming limit of a given steel is based on the thickness and the strain hardening exponent (n-value). The forming limit for other alloys must be determined experimentally. Knowing these parameters, and their minimum values likely to be encountered in production, it is possible to assess if the stamping process is inherently robust enough to withstand the normal variation in material properties.
Engineering Quality Solutions team members have been conducting circle grid analysis for over twenty years. Our experience enables us to not only perform at a high level, but also teach circle grid analysis to companies and individuals who are interested in understanding the strains produced during the forming process.
Advanced High Strength Steels that exhibit high strength and enhanced formability are being offered around the world. These steels have the potential to effect cost and weight savings while improving performance.
The increased formability allows for greater part complexity, which leads to fewer individual parts (cost savings) and more manufacturing flexibility. Fewer parts mean less welding (cost and cycle-time savings) and weld flanges (mass and weight savings). Depending on design, the higher strength can translate into better fatigue and crash performance, while maintaining or even reducing thickness.
Understanding and compensating for the challenges associated with processing advanced high strength steels (AHSS) can help you minimize springback, edge cracking, trimming, wrinkling, and die wear.
Part I of this two-part series presented an overview of advanced high-strength steels (AHSS). This article addresses issues encountered when processing these grades.
Using AHSS in appropriate applications offers opportunities for reduced product weight, enhanced crash performance, manufacturing process consolidation, and cost reduction. However, because these grades have different microstructures, chemistries, and properties, die processing must be optimized to take advantage of the differences. Proactively addressing the associated challenges can help minimize costly tryout.
Challenges faced by stampers in the quest to produce a robust part are magnified when they form high strength steels. As materials increase in strength, the inherent tensile property variability increases. Using products from the family of grades known as the advanced high-strength steels (AHSS) complicate matters even more, since what is supplied from one mill may not be produced in the same way as that from another mill. Understanding how steel is made sets the stage for a more profitable relationship between the steel supplier and the steel consumer. The certified steel properties that come with the coil are useful, but more information about the sheet metal increases the likelihood of success.
Learn more about how the steelmaking process affects formability in this FREE Download.
Steel is, by minimal definition, an alloy of iron and up to 2 percent carbon (if it is more than 2 percent, the alloy is cast iron). Carbon is small enough to fit into the interstices (gaps) of a primarily iron matrix, making it an “interstitial element” in steel. If the steel alloy has an ultralow carbon level (typically less than 50 parts per million), most of these gaps will not be occupied and, as such, can be called interstitial-free (IF) steel. These very formable IF steels are extra-deep-drawing steels (EDDS) with a microstructure that is 100% ferrite (nearly pure iron). Achieving this low carbon level does not occur using conventional steel processing. Instead, the molten steel must be put under a vacuum that decarburizes it by removing carbon monoxide, as well as other gases like hydrogen and nitrogen. This process is called vacuum degassing, and it is done in the production of vacuum degassed interstitial-free steels (VD-IF).
Think about pulling a bar in tension. Load divided by cross-sectional area is force, or stress. But what cross section are you considering? Before starting that pull, the bar had a known cross-section of, let’s say, 0.5″ wide x metal thickness. It’s easy to measure these, since it is your starting material. At any load, the engineering stress is the load divided by this initial cross-sectional area. While you are pulling, the length increases, but the width and thickness shrink. At any load, the true stress is the load divided by the cross-sectional area at that instant. Unless thickness and width are being monitored continuously during the test, you cannot calculate true stress. It is, however, a much better representation of how the material behaves as it is being deformed, which explains its use in forming simulations. In circle grid analysis, engineering strain is the percent expansion of the circle compared to the initial diameter of the circle. The relationships between engineering values and true values are:
σ = s (1+e) ε = ln (1+e)
Where “s” and “e” are the engineering stress and strain, respectively, and “ σ ” and “ ε ” are the true stress and strain, respectively.
To determine if a grade of steel is prone to secondary cold work embrittlement, the ductile-to-brittle transition temperature (DBTT) is determined. The DBTT is the highest temperature at which a brittle crack is formed.
More information about the cause for secondary cold work embrittlement and how to test for it can be found in THIS FREE DOWNLOAD.
Let Engineering Quality Solutions determine if your grade is at risk for secondary cold work embrittlement!
Sheet metal production isn’t much different than baking a cake. It requires the right ingredients, added at the right time and processed at the right temperature for the right amount of time. Literally hundreds of different types of metals are available, each with it’s own blend of physical, chemical, and surface properties and characteristics.
Pure metals are relatively soft and malleable. When you move a carpet, it takes alot of force to pull the carpet from one end. However, if you create a little wave or ripple and propagate that through the carpet, it becomes much easier to move. Metal forming on the atomic scale is similar.
Atomically, a pure metal can be pictured like 3-D network of racked billiard balls all the same size. The gaps in the atomic structure, called dislocations, are necessary for metal flow. As these dislocations propagate through the workpiece, any deviations from the homogenous pure matrix element will require more effort for the dislocations to move around it. All pure metals are relatively soft and malleable for this reason.
Danny Schaeffler may be the current Science of Forming columnist, but Dr. Stuart Keeler was the founding columnist where he wrote nearly 200 articles. Engineering Quality Solutions is a proud sponsor of the Science of Forming Vol. 2, a CD-ROM containing many of the material-formability articles and tutorials by metalforming expert Stuart Keeler. Dr. Keeler is best known worldwide for his discovery of forming limit diagrams, development of circle grid analysis and implementation of other press shop analysis tools.
Articles you will find on the CD include:
Virtual Sheetmetal Forming–An Overview
What is Science of Forming?
Help Your Sheetmetal Supplier Help You
Material Properties: Typical or Worst-Case
Learning About New Forming Technology
Statistical Deformation Control for Stamping Evaluation
Cups and Boxes are Different
Deformation Through a Draw Bead
Troubleshooting — Using an Extra Eye
Attacking Process Variation
How Important is Die Transition?
The Die Has a Fever?
A Tale of Three Press Shops
A Complaint Heard Too Often
Ultrasonic Thickness Gauges Show Forming Severity
Why Certain Defects Occur
Troubleshooting — Lost in the Jungle?
Stamping Tears Can Confuse Troubleshooting
Forming Characteristics of Higher-Strength Steels
What is Dual-Phase Steel?
What Really is Grain Direction?
Maximum Forming Speeds?
Myth or Truth in Metalforming
Deform in One Direction or Two
How Much Does Metal Thin?
Forming Problems with Higher-Strength Steels
A Forming Limit for Thickness Strains
The New Dual-Phase Steel
The New TRIP Steel
Does Steel Get Brittle as It Gets Stronger?
What is the r Value?
To order the Science of Forming Vol. 2, email us at KeelerCD@EQSgroup.com
The Tool & Die Authority was published by the Precision Metalforming Association as a combination of blog-style news and exclusive information about tool and die companies, markets, customers and much more. TDA provided solid technical tips not found anywhere else that helped tool and die operations solve a range of challenges, enabling them to offer top-notch service to their customers while improving their bottom line.
The correct way to use the Mylar strip is to measure from the center-width locations of the boundary line around the circumference of the now-deformed ellipse. Measuring from inside-to-inside or outside-to-outside is wrong! With a fuzzy ellipse boundary line (old stencil, poor gridding technique, etc.), it is not hard to make measurement errors more severe than just measuring the outside-to-outside dimensions of a crisp circle/ellipse. The width of the line forming the boundary of an etched circle is about 0.008″ which is also the thickness of the lines of the Mylar strip commonly used to measure the deformed ellipse manually (the diverging railroad tracks). If you are measuring inside-to-inside of a ellipse that was formed after starting with a 0.100″ diameter circle that was stretched 20% in one direction, you’ll measure the major axis as 0.120″-0.008″, or 0.112 inch, which is 12% on the major strain axis. You’ll also run the risk of measuring the minor strain wrong at 0.100”-0.008”, or 0.092 inch, which corresponds to a minor strain of -8%. Similarly, if you are measuring the outside-to-outside dimensions, you’ll wind up with 28% on the major strain axis and + 8% on the minor axis. A poor technique can turn a correct (20%, 0%) reading into anything between (28%,8%) and (12%,-8%)! Unless you are measuring from exactly the center- width position on the line making up the circumference of the circle/ellipse, you can get vastly different results, confusing the strain analysis interpretation.
Everyone likes to have a “Rule of Thumb” to use as a quick and easy guide. To make the best use of these maxims, it helps to understand where they came from, and what the limitations are in their use.
When it comes to taking the right steps to ensure a robust stamping process, a surface strain analysis using a forming limit diagram is recommended. The forming limit curve should be generated from the minimum allowable thickness and the lower mill production limit (or the -3σ value) for the strain hardening exponent, or n-value. To bypass some of the work involved in generating this information, some companies have chosen to use a rule of thumb that calls for a maximum 20% thickness reduction on a formed part compared with the initial flat blank thickness. In some cases, this is an acceptable substitution, but in many cases, using this 20% threshold only confuses the proper course of action.
More information about how the maximum thinning rule should be applied can be found in THIS FREE DOWNLOAD.
Let Engineering Quality Solutions teach you when and how to use this shortcut!
Advanced High Strength Steels have been gaining ground in an automotive industry hunting for all the weight savings it can while satisfying or surpassing safety standards. The material could be called the next step in steel’s evolution toward exhibiting the high-strength, lightweight characteristics of more expensive materials, like aluminum and magnesium.
Challenges arise, however, from a formability standpoint. The varieties of AHSS do exhibit high formability, but in entirely different ways from legacy materials. Springback isn’t the only concern – metal formers need to throw out the old rulebooks. AHSS has spurred stampers to think about metal forming in new, unconventional ways.