Selecting the right materials for roll forming—Part 2
How mechanical properties affect production
This is the second part of a two-part article. Read Part I. If you examine the mechanical properties of several materials, including carbon steel, alloyed steel, stainless steel, as they relate to roll forming, you'll gain an understanding of the influence of some primary metal processes on roll forming.
In addition to carbon, large varieties of metallic and nonmetallic elements are used to achieve the desired mechanical properties and corrosion resistance. The most frequently used alloying elements are manganese, cobalt, chromium, copper, molybdenum, vanadium, nickel, zirconium, and titanium.
The yield strength of the different high-strength, low-alloy (HSLA) steels is in the range of 60,000 to 120,000 PSI. The yield strength of the dual-phase martensitic steels can be as high as 180,000 to 220,000 PSI.
High-strength alloyed steels have unusual properties, strength, and springback that generate challenges for roll formers. In some cases, 25 degrees or more of overbend is required to achieve a 90-degree bend.
Furthermore, specifications cover only the minimum mechanical properties. Maximum yield can be considerably higher than the specified minimum, and it can fluctuate from coil to coil or even within a coil.
By increasing the chromium and other elements in steel, it is possible to make it resistant to rust and/or heat. Most stainless steels fall into one of three major classes, depending on their crystal structure and alloying elements:
1. Austenitic.200 and 300 series stainless steels usually have 16 to 24 percent chromium, 3.5 to 37 percent nickel, 0.8 to 0.25 percent carbon, and other elements. Austenitic stainless steels are ductile, but work hardening limits forming. The most commonly used alloys of this group are the 304 and 316 types. Austenitic stainless steels are nonmagnetic under most conditions.
2. Ferritic. Some 400 series stainless steels have limited ductility and, thus, restricted formability. Some alloys work-harden quickly, and it is difficult to weld them. However, they are usually less expensive than the 300 series steels. Most ferritic stainless steels are magnetic.3. Martensitic. This type of steel, with the exception of the 403, 410, and 414 types, cannot be cold formed. Most martensitic stainless steels are magnetic.
These properties are only typical. Stainless steels come in a large variety, and their properties fluctuate greatly, depending on alloying elements, cold work, and heat treatment.
Most stainless steel can be formed similarly to the carbon steels, but some of its characteristics must be considered: work hardening properties, high springback, more power required to form, better and different lubrication required (better wetting properties and higher pressure resistance), high-luster surface, and orange peel ridging and rapping.
An understanding of the effects of cold forming and the metallurgical and mechanical properties of stainless steel is essential for successful roll forming. Figure 1provides a guideline to the cold-forming properties of stainless steels.
Roll forming of aluminum is not as difficult as forming steel, but more attention has to be paid to setting and adjusting roll gaps, applying the right lubricant to prevent pickup of aluminum on the rolls, and using well-designed and finished rolls and good lubricant to prevent surface marks.
To specify the material simply as aluminum is one of the most common mistakes made by designers who are unfamiliar with metals. There are as many different aluminum alloys as steels, with vastly different properties. Therefore, it is essential to use the proper designation for aluminum used in roll forming.
Wrought Aluminum Alloy Designations. The mechanical and other properties of the wrought aluminum used for roll forming are influenced by the alloying elements, heat treatment, and cold work.
The numbering system used by the Aluminum Association standards classifies the commonly used aluminum alloys by their major alloying elements (see Figure 6). The 3000 series alloys, such as 3003, 3004, and 3105, are used most frequently by roll formers. Aluminum alloys called "building sheets" and other trade names usually belong to this group of alloy.
Basic Temper Designation. In aluminum alloys, temper designations are as follows:
F As fabricated, no mechanical limits specified
O Annealed to achieve the lowest strength temper
H Work hardened by rolling, with or without additional heat treatment
W Can be formed right after heat treatment, age hardened at room temperature
T Heat treated, aged at high temperature, with or without strain hardening
H1 Strain hardened only
H2 Strain hardened and partially annealed
H3 Strain hardened and stabilized by low-temperature heat treatment
A second digit after the H designations indicates a variation of the temper:
H12 1/4 hard H32 1/4 hard (and stabilized)
H14 1/2 hard H34 1/2 hard (and stabilized)
H16 3/4 hard H36 3/4 hard (and stabilized)
H18 Full hard H38 Full hard (and stabilized)
For three-digit designations such as H112, H364, and others, fabricators should consult the appropriate standards.
Numbers T1 to T10 refer to the combinations of hot and cold working, heat treatment, cooling, and aging. The basic treatment depends on the type of alloy and on the properties to be achieved.
Two-digit tempers such as T51 and others may refer to stress relieving by a stretching (or compressing) process.
Note that 1000, 2000, 4000, and 5000 series alloys cannot be heat-treated.
Cladding. The good corrosion resistance of the 1000 or other series of alloys can be combined with the high strength of other alloys. A thin layer of the low-strength, corrosion-resistant alloy is metallurgically bonded (hot-rolled) on both sides of the higher-strength, less-corrosion-resistant alloy. During roll forming, additional care has to be taken not to scratch or damage the protective surface of the clad alloys.
Bimetals. Aluminum also can be bonded to other metals such as stainless steel. The combination provides a relatively lightweight product with the stainless steel's (or other metal's) properties.
It is not too difficult to roll form these types of bimetals, but more attention must be paid to the forces applied by the rolls. The larger force required to form stainless steel reduces the thickness of aluminum. Uneven thickness reduction can create an unacceptable camber, twist, or other imperfections.
Influence of Primary Metal Processes on Roll Forming
Several processes are applied to metal during production, and these processes have an effect on roll forming results:
1. Hot and cold rolling
Hot and Cold Rolling. Thick cast metal slabs usually are hot-rolled to a minimum practical limit and then cold-rolled further to reduce thickness.
Both hot and cold rolling are accomplished by reducing the thickness between pairs of large-diameter rolls or between smaller-diameter rolls supported against the deflection by larger rolls. Regardless of their diameter, the rolls will be deflected by the large forces required to reduce material thickness. The forces can be in the millions of pounds.
If straight rolls deflect, then the metal in the longitudinal center of the coil will be thicker than at the edges. The thinner the material gets during rolling, the longer it will be. The thinner, longer edges of the coil cannot "run away" from the center, so they will take a wavy shape.
To counteract the deflection, the center of the rolls are ground to larger diameters, creating a crown on the barrel-shaped rolls. If the increase in diameter is twice the anticipated deflection, then the working side of the roll will become straight under the load. Coil rolled under this condition will have the same thickness at any point.
The actual deflection, however, can be controlled by adjusting the gap between the rolls. Rolling temperature, uniformity of the starting thickness, and distribution of alloying elements will change the force required for rolling and, hence, the deflection.
As forces change, deflection fluctuates. If the force is higher than the crown is prepared for, the center of the coil will be thicker. If the actual force is less than the anticipated one, the center of the sheet will be thinner and the edges will be thicker. If the edges are thin, a visible edge wave results; if the center is thin, visible center wave (oil canning) can develop.
If an oil-canning sheet is slit in half longitudinally, the long center part will create an outward camber in the sheet. If the edges are wavy, then the camber will be in the opposite direction. Slitting will not take out the camber from these coils.
Nonmetallic Coating. Coil-coated finishes are available in several varieties. Common problems with coil-coated materials can be the abrasion of the coated surface, its sensitivity to pressure, and cracking of certain types of paints at the bend lines.
Today, paints usually have excellent surface characteristics for roll forming. Typically, lubrication is not required because paints are relatively soft compared with the metals. Operators must ensure that the gap between the rolls is sufficient and that no undue pressure is applied on the coated surface. The surface finish of the rolls and other tooling should be good and free of chips and cracks. A rough roll surface may pick up paints and/or mar the surface.
Paint cracking may occur if less ductile paint or plastic is used. A larger bending radius can overcome this problem. If the bending radius must remain small, then preheating of the bend lines in front of the forming rolls can make the paint flexible and elastic enough to be formed without making it too soft. Depending on the type of paint or laminate used, the surface temperature may be raised to 130 to 230 degrees.
On the other hand, paint is less flexible at low temperatures. Painted coils that are transported and/or stored below the freezing point may develop paint cracks if they are roll formed shortly after bringing them inside the plant.
Exposed surfaces must be checked carefully for paint cracks. Almost invisible small cracks at the bend lines allow water and corrosive atmosphere to pass through, and corrosion may start during storage and/or after installation. Correcting problems caused by paint cracking is extremely expensive.
In the case of laminated material, the thickness of lamination should be added to the thickness of material when rolls are designed and installed. For example, rolls designed for 0.036-inch-thick material will not be able to process 0.034-inch-thick steel with 0.010-inch-thick laminate.
Slitting. High-speed slitting is the most economical way to cut wide mill coils into narrower strips. If the thickness varies significantly within the width of the coil, then during rewinding, the thicker, shorter coil segments will remain tight, and the thinner, longer ones will hang in a loop.
In some older slitter installations, 20- to 40-foot-deep pits were built to accommodate these loops. To keep a tension on all recoiled narrow strips and to eliminate the loop, cardboard pieces were placed between the wraps to increase the rewinded coil's diameter. Newer, better slitting equipment can rewind the strips under tension and eliminate the dangerous practice of inserting cardboard pieces.
As mentioned previously, slitting will not eliminate camber in the strip. Actually, dull slitting knives, applying more force, and/or creating burr may induce camber in the formerly straight material.
Choosing the right material at the right price is an important contribution to successful roll forming. To select the right material, an understanding of both the properties of the material and principles of roll forming is important.
Practically every material that can be formed with other processes can also be roll formed. However, the material's mechanical properties, thickness, straightness and flatness tolerances, surface quality, and finished-product requirements should influence the number of passes and shaft diameters selected and how the rolls are designed. To achieve the best results, cooperation among the customer, product designer, manufacturing plant, and suppliers is essential.