Page 42 - MetalForming March 2017

The Science of Forming
By Stuart Keeler
How Grain Structure Impacts Steel Strength
Springback SA
Springback is proportional to elastic modulus.
(s)Steel=30×106
(A) Aluminum = 10 × 106
For equal YS, aluminum has three times the springback.
Strain
Many in the metalforming industry assume that steel- makers insert strong items into steels to increase their strength. While true in some cases, the opposite occurs nearly all the time, for it is the microscopic atomic cell (Fig. 1) that can have the biggest impact on steel strength.
Steel Versus Aluminum
Steel
BCC = Body-Centered Cubic
Aluminum
FCC = Face-Centered Cubic
Fig. 2—Steel has the highest elastic modulus and the least amount of springback.
Making Steel Stronger
Substitutional
Exchange Atoms: Mn – Manganese P – Phosphorus Ti – Titanium
Si – Silicon
Interstitial
Extra internal atoms: C – Carbon
N – Nitrogen
Interatomic bond
Substitutional alloying element that is bigger than the matrix element
Substitutional alloying element that is smaller than the matrix element
Interstitial alloying element
Fig. 1—Different metal alloys have different atomic cell structures, such as BCC and FCC.
The atomic cell is the cubic structure that holds the atoms in a neutral condition. If the cube is elongated in one direc- tion, a positive elastic stress results. When the ends are released, the cell returns back to the original configuration and size. And, when compressing the atomic cell, it shrinks and creates a negative elastic stress.
The atomic cell for steel is body-centered cubic (BCC), with one atom at each corner and one in the center of the cube. The cell has nine atoms. For comparison, a nonferrous-alloy cell (such as aluminum) is face-centered cubic (FCC), with an atom at each corner and one in the center of each face (Fig. 1). This cell has 14 atoms and functions similarly to a steel cell.
With the BCC cell of only nine atoms, one might assume the lighter weight of BCC would be relatively weak. However, test results (Fig. 2) show that for equal yield stress, steel has three times the elastic modulus and three times less spring- back than alloys with FCC cells.
During the steelmaking process, as the liquid steel begins to solidify the atomic cells line up in all directions to form
Stuart Keeler (Keeler Technologies LLC) is known worldwide for his discovery of forming limit diagrams, development of circle-grid analysis and implementa- tion of other press-shop analysis tools. Keeler’s metal- forming experience includes 24 years at National Steel Corporation and 12 years at The Budd Compa- ny Technical Center, enabling him to bring a very diverse background to this column and to the semi- nars he teaches for PMA.
Keeler Technologies LLC
P.O. Box 283 | Grosse Ile, MI 48138 keeltech@comcast.net
Fig. 3—Atoms can be exchanged to make steel stronger at specific locations.
three-dimensional steel (Fig. 3). Segments of the steel can be made stronger by exchanging different atoms. Once all the steel has solidified, the microstructure has formed. Some grains are small, some are large. Small grains will strengthen the steel—the grain boundaries intersect with neighboring grains to add strength, while the inner portion of the grains remain soft. The more grains in a given amount of space, the stronger the steel. Steelmakers seek to create a specific number of grains in the microstructure.
The steelmaking industry is on the edge of complete change. Rather than insert a chunk of martensite into a grain of ferrite to create a dual-phase steel, they may add millions (maybe billions) of nano-sized particles to the steel, evenly distributed to add strength and develop other qualities.
Here, the smallest additions will make the strongest steels.
MF
40 MetalForming/March 2017
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Stress