Eight reasons for the significance of grain size in metal sheet bending.

Question: First of all, I would like to thank you for all the worthy information and instructions you provide readily. Due to it we gain thorough competence and clear idea on major issues in the formation of metal sheets.

More than once you have mentioned the importance of metal texture, its influence on the formation process through press brakes. And what do the crystals and grains differ in? What is the role of grains in metal formation? What are the ways to withstand the changes brought about by manufacturing parts along with varying features?

Answer: We are on our go to keep up with the latest advances in sheet metal formation. We are willing to provide you with the best and most suitable solutions to production challenges. 

Formation of sheet metals through press brakes cannot always be predictable. Part formation through air-bend may lead to distortion without any apparent reasons. With a long-year practice, being well-aware of the machine and material features, a technician is able to restrict the extent of distortions.

To gain the most efficient outcomes, a skillful wood-worker considers the wood textures prior to starting their task. So does an experienced press brake operator while processing metal sheets. Awareness of grain directions and a range of features of materials have an effect on bending angles, bending radii, bending tolerance as well as bending deductions.

You`ll realize that the sizes of the crystals are largely dependent on the process of fabricating metals, this, in turn, changes grain sizes and, thus affecting the bend stability in a while. For instance, if bends crack outside the radii, they weaken, require lower pressures for formation, and cause over-bending of flanges.  

Just as wood grains, grains in metals affect the part behavior. Change of grain features produce some unpredictable difficulties from sheets to sheets and batches to batches. In case of cold-roll metals, crystals extend towards the roll directions. (Fig. 1) Due to this process crystals get thinner and produce grains seen by us. These new grain-formation determines the direction.

Lateral grain-bend for certain sheets might cause cracks, lower the desired tonnages, change the back spring and even bring about breakage. Grain-bend might also lead to the orange-skin effect on the outer side of bends. This depends on applications and mainly happens in the formation of inner bend radii that are no more than the least radii appropriate for the certain kinds of materials. The most suitable solution is keeping inner bending radii at or more than the least bending radii, particularly if bending is grain lateral. For vertical bending lines or diagonal lines you may have inner bending radii a bit less if needed.

Because of these changes, the part formation dimensions may alter creating post-processing troubles. It takes rather long a welder or an assembler to manually adjust to assemble parts together. Qualities of finished parts are declining and costs are rising.

Fig. 1

To cut down the troubles, it is advisable to identify the difference between materials and perform in accordance with those properties but not contradict them. This way you will avoid problems and gain excellent results. The essence of metal grains, their derivation will be the matter of our further discussions.

Eight Factors

In the formation process metal sheets expand upon the outer sides of bends and compress upon the inner sides. The expanding and compressing performance of sheets can determine the amounts of unpredictable deformity. (Fig. 2)

Each factor mentioned below affects the formation of metal sheets, and mostly they are connected with grains. Each factor should be studied, understood and observed to minimize alterations from parts to parts. Metal properties determine the course of deformation as well as the formation force. Surely, even a most skilled operator experiences problems. Yet, considering the following factors will reduce the error amounts and will better the final qualities.

1. Work Tempering. Plastic deformity brings on dislocation in the metal texture. This produces even more dislocation. Each dislocation interacts with another, and they get more and more solid. This fastening decreases the mobility of dislocations when the sheet is being strengthened. This is called as cold processing. Deformity, caused by cold processing, raises the metal force if work tempering accompanies it. It occurs if cold processing is the last treatment step.

Work tempering could be controlled somewhat if the bend speed is changed. Slow speed allows more work tempering. On the other hand, higher rate will provide less work tempering. While bending already work-tempered metal, greater pressures are required. 

To make it more clear take a paper-clip. Unwrap it, then curve a portion backward and forward several times at the same point. Note how hard it is to shape the wire at the same point after the first time. The further bending gets even more difficult.

Curving the clip more than once creates a strain-tempered space of dispositions, jumbled together after the formation, causing the material force to increase. The increase of material force through cold processing, causes reduction of metal viscosity. Continuing the clip bend will finally lead to the cracking and breakage.

2. Hardness Alteration. Sheet metals are produced in the boundaries of permissible hardness. Due to this, 2 same sheet metals can possess absolutely other back spring features, that a press brake operator should know.

3. Alterations in Thicknesses. The same applies to the thicknesses of materials indicated within the tolerances corresponding to a particular material gauge. Thus, in case this is not considered, 2 same sheets will create absolutely varied parts. A change in thickness results in variations in back spring as well.

4. Positioning of Bending Lines. Features at the bending points can determine the metal expansion, compression and deformation. The bending center line has to be located 2.5-3 times as far from the edges of holes or features as the thicknesses of materials.

Fig. 2

5. Formation Methods. The most widespread bend methods through press brakes include air formation (air bend), bottom formation (bottoming) and coin bending (coin formation). Bottom and coin bendings are quite different. In any case, if you deal with upgraded press brakes and tools in precision metal workshop, you are likely to air bend.

6. Bending Lengths. Bending lengths depend on the tonnages needed to produce bends. Bends applying excessive tonnages for each in.² might distract the machine plunger and sidewalls. Deflecting the plunger to a degree more than the center-line loading restriction of the machine will bring serious damage in the form of plunger disturbance. This, in its turn, will make impossible to produce direct bends.  

7. Direction of Grains. While making sheets from metal bars in the factory, a stream of rolls imposes force to them. Thus, each metal crystal gets lengthened. Due to this, grains are produced. They possess a preferred direction in cold rolling and are just partly directed in hot rolling.

8. Sizes of Grains. Grains determine the sheet strengths. Grain boundaries behave as barriers for dislocations. As a result, slipping decreases for the adjacent grains differ in their orientation. Small grains provide small area for particles to pass alongside certain slipping planes. This is the reason why small grains provide more strengths for materials. In factories rates of hardening or solidifying control grain sizes and numbers for materials.

Grain Definition 

When looked by microscopes or some glass that magnifies the image, grains can be more clearly visible in metal sheets. See Fig.3.

As for the directions of grains, they are not always seen. Certain sheet metals require no microscopic vision. Yet, sizes of grains remain major influence in formation process. They determine the material yielding strength as well as the crack extent occurring on the outer sides of bends. Steel making implies solidification of molten metals, when they get cooler.  During cooling re-crystallization happens. In conditions of high temperatures newly created grains, still without deformation, nucleate growing within previously deformed grains or most commonly at the borders of grains. Newly formed grains grow replacing the distorted ones. As re-crystallization occurs, the material industrial characteristics come to their initial state and gain higher ductility. During this process a host of bits combines forming crystals. These produce grains in materials after cold rolling. In the melting process, when lots of grains get formed meanwhile, the cooling metal produces more grain amount. If smaller crystal amount is formed, bigger space is provided for grain growing, which leads to large-sized grains and smaller number of grains.

Each crystal glides over the other in the formation process. The area where this happens is called a cleavage plane. The tight-bonded crystals ensure toughness for materials. The tonnages needed for formation through press brakes increase because of this, and metals become sensitive to cracks and more inclined to orange-skin effect.

In some spaces of crystal texture there exist certain particles not included in the symmetric crystalline formations. The spaces mentioned are known as grain borders. These are not involved in crystalline texture, they do not possess any splitting plane. Due to it borders strengthen. (Fig. 4)

Fig. 3

Movements occur alongside the crystal splitting plane, not inside the borders. The small sizes of grains cause the amount of borders and their thickness to rise. The amount of boundaries gets more prior to any move happens. 

Thus, a strong material has got a small-grain texture. Large-grain materials have more ductility, weakness and provide easiness in formation. On the hand, a small-grain material may offer easy formation as well. It depends on the grain features and arrangement. This is going to be one of our further matters of discussion. 

All this is the preface of a book. In the bend process through press brakes grain size appears a significant factor. It has to be considered by fabricators.

Fig. 4

In our upcoming columns we intend to go over details concerning the annealing, normalization of materials and application of grain sizes in the formation process.