ASM Metals HandBook Vol. 14 - Forming and Forging

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Fig. 5 Adjustable press-brake die f

or forming 180° bends in stainless steel sheet. Setup can be used for

forming bends to 3.2 mm (

in.) inside radius in sheet 0.30 to 0.46 mm (0.012 to 0.018 in.) thick, and it will

produce 4.0 mm (

in.) radius bends in half-

hard stainless steel. Detachable side of die can be shimmed for

bending thicker sheet or for bending with larger-radius punches. Dimensions in figure given in inches.

Springback is a function of the strength of the material, the radius and angle of bend, and the thickness of the stock; the

thicker the stock, the less severe the problem. Table 4 shows the relationship between radius of bend and springback for

three austenitic stainless steels. Ferritic steels usually exhibit less springback than austenitic steels, because the rate of

work hardening of ferritic steels is lower. As a practical guide, the amount of springback is normally proportional to

(0.2YS + UTS)/2.

Table 4 Springback of three austenitic stainless steels bent 90° to various radii

Springback

for bend radius of:

Steel and temper

1t 6t

20t

302 and 304, annealed

2° 4°

15°

301, half-hard 4° 13° 43°

Springback can be controlled by reducing the punch radius, by coining the line of bend (if the shape of the die is such that

bottoming is feasible), and by overbending. For overbending, it is sometimes necessary only to make the punch angle

smaller than the desired final angle of the workpiece, as in the following example.

Example 2: Setting a Flange Angle in a Press Brake.

The bracket shown in Fig. 6 was preformed in a U-die from a developed blank of type 302 stainless steel, half-hard, 1.0

mm (0.040 in.) thick. Only the punch angle needed to be reduced to set the angle on the flange.

Fig. 6 Production of a U-

shaped bracket from a developed blank by preforming, and restriking to set flange

angles, in a press brake. Dimensions given in inches.

As the bracket came from the U-die, the springback in each flange was 15°. To correct this spread, the piece was put in a

restrike die in a press brake, which set each angle separately. The restrike die angle was 90° with a 3.2 mm (

in.) radius.

The restrike punch was made to an angle of 86° with a 2.4 mm (

in.) radius to coin the bend, so that the flanges would

form to 90° ± 1°.

The lubricant was a water-soluble pigmented drawing compound. The workpiece was degreased after forming.

Lubricants. For ordinary press-brake operations (chiefly, bending and simple forming), lubricants are not used as

frequently as with higher-speed press operations. Convenience of use is a major factor in selecting lubricants for this type

of press-brake forming. Pigmented lubricants are not favored, and cooling effectiveness is of little significance at low

production rates. For severe forming and for operations that would ordinarily be done in a press, if available, the

recommendations in the "Press forming" column in Table 2 apply.

Applications of press-break forming are described in the following examples. Repetitive bends, as in corrugated stock,

are frequently made one at a time in a press brake if the quantity of production is not sufficient to warrant a special die, as

in the example below.

Example 3: Press-Brake Forming of Corrugations.

The corrugated sheet shown in Fig. 7 was formed from 0.41 mm (0.016 in.) thick full-hard type 302 stainless steel. The

finished sheets, after bending, were 419 mm (16

in.) long, as shown, but the width, w, varied according to the use of

the piece.

Fig. 7

Corrugated sheet in which corrugations were formed one at a time in a press brake, using tools shown.

Dimensions given in inches.

The corrugations were made one at a time by air bending in the tooling shown at lower right in Fig. 7. Pilot holes in the

workpiece and locating pins in the punch helped to keep the workpiece aligned. Deviation from flatness in the pieces was

corrected by restriking some of the bends.

Irregular contours on long, narrow parts are conveniently produced by bending in a press brake. Because of the strength

of stainless steels, the forming often must be divided among several successive operations, as in the example below.

Example 4: Forming of Stainless Steel Handrails in a Press Brake.

Figure 8 illustrates the shapes produced in five successive operations that were required for forming a handrail from 1.57

mm (0.062 in.) thick type 304 stainless steel. Because of flatness requirements and the resistance of the metal to bending,

a 3600 kN (400 tonf) press brake was used.

Fig. 8 Shapes progressively produced in the five-

operation forming of a handrail in a 3600 kN (400 tonf) press

brake. Dimensions given in inches.

Forming the 1.6 mm (

in.) radius beads (operation 1, Fig. 8) was particularly troublesome because of the difficulty in

retaining flatness. A force of 5300 kN (600 tonf), which exceeded the rating of the press brake, was used to form the

beads.

The second and third operations presented no problems, but the fourth operation was difficult because the workpiece had

to be held without marring the polished surface. Similar parts were produced from low-carbon steel without difficulty.

Forming of Stainless Steel

Revised by Joseph A. Douthett, Armco Inc.

Press Forming

Stainless steels are press formed with the same kind of equipment as that used in the forming of low-carbon steel.

However, although all stainless steels are not the same in strength or ductility, they all need more power to form than

carbon steels do. In general, presses should have the capacity for 100% more ram force than that needed for equivalent

work in low-carbon steel, and frames should have the rigidity and bulk necessary to withstand this greater force.

Dies. In addition to wearing out faster, dies may fracture more readily when used with stainless steel than when used

with low-carbon or medium-carbon steel. This is because of the greater forces needed for the working of stainless steel.

For the longest service in mass production, the wearing parts of the dies should be made of carbide, D2 tool steel, or high-

strength aluminum bronze. Carbide can last ten times as long as most tool steels, but it is more expensive and does not

have the shock resistance of tool steels and aluminum bronze. Tool steels such as D2 are preferred when resistance to

both shock and wear is required.

Aluminum bronze offers the most protection against galling and scuffing of the workpiece. An oil-hardening tool steel

such as O2 can be used for short production runs.

Austenitic Alloys. Workpieces can be stretched by applying high blankholder pressures to the flange areas to prevent

metal from flowing into the die. This causes severe thinning, but work hardening may cause the thinned metal to be as

strong as or stronger than the thicker unworked sections. Figure 9 shows a section of an automobile wheel cover made of

type 301 stainless steel; the central portion was purposely thinned and work hardened by stretching. In the following

example, one of the principal reasons for stressing the workpiece to the limits of formability was to work harden it for

increased strength.

Fig. 9 Profile of a press-formed automobile wheel cover showing thinning purposely

produced by severe

stretching. Dimensions given in inches.

Example 5: Severe Forming for Intentional Work Hardening.

The material for a muffler header (Fig. 10) for a small aircraft engine was intentionally stressed nearly to the limits of

formability to increase rigidity and to impart the necessary fatigue strength. The headers were made in two operations in a

530 kN (60 tonf) open-back-inclinable mechanical press having a 127 mm (5 in.) stroke. Each operation used a tool steel

die hardened to 59 to 62 HRC. Production was 400 pieces per month.

Fig. 10

Severe forming of an austenitic stainless steel aircraft muffler header to produce work hardening that

would increase the rigidity and fatigue strength of the part. Dimensions given in inches.

The first die (Fig. 10a) was a compound die that formed the dish of the part, formed the bead in the dish, and blanked the

inside and outside diameters. The blankholder at the outer edge of the workpiece was spring loaded, and a rubber pad

supported the inner surface of the workpiece against the center blanking punch. The sequence was programmed so that

the forming was completed before the outside and inside diameters were blanked, thus making the flange dimensions

more accurate and concentric than would have otherwise been possible. Die life was approximately 20,000 pieces.

The second die (Fig. 10b) formed both the inner (stretch) flange and the outer (compression) flange. A spring-loaded

pressure pad maintained the correct gripping pressure against the muffler-header body during this operation.

The blank was annealed type 321 stainless steel 0.81 mm (0.032 in.) thick, sheared to 216 mm (8

in.) square. The bead

formed in the first die was used as a locating surface in the second die. The dies were brushed with oil between pieces.

The production rate for both operations was seven pieces per minute. Setup time for the first operation was 0.17 h; for the

second operation, 0.31 h.

Stretching. Stainless steel has high ductility but wrinkles easily in compression. Therefore, if there is a choice in the

direction of metal flow during forming, a better part is likely to be produced by stretching than by compression, as in the

following example.

Example 6: Use of Clamping Plates and Bead to Control Metal Flow.

The dome section shown in Fig. 11 was formed from a tapered blank of annealed type 302 stainless steel in a 2200 kN

(250 tonf) double-action hydraulic press. It was desirable to maximize metal flow from the narrow end of the blank in

order to cause stretching rather than contraction in the metal and thus avoid wrinkles.

Fig. 11 Tools and clamping plates for controlling metal flow in press forming the part shown.

Dimensions given

in inches.

The dies could not be oriented to let the blank lie flat, because of the necessity for forming the reentrant angle next to the

lower clamping plates. Both the upper and the lower edges of the blank were held between steel plates during forming.

The clamping force on each pair of plates was 320 kN (36 tonf). Because the upper plates were twice as long as the lower

and two-thirds as wide, the clamping force was distributed over a larger area and could have permitted most of the metal

flow from the larger end, with attendant wrinkling of the work metal. The addition of a bead to the upper clamping plates

improved holding at that end and caused most of the metal flow to occur at the small end of the blank. The application of

a fatty acid type, nonpigmented drawing compound to the lower plates further encouraged metal flow from the small end

of the blank. Scrap loss because of tearing over the relatively sharp lower die radius was 3%.

Ferritic Alloys. The formability of ferritic stainless steels, particularly the higher-chromium types, can be improved by

warm forming at 120 to 200 °C (250 to 400 °F), rather than cold forming. The metal is more ductile at the higher

temperatures, and less power is needed in forming. Some pieces that cannot be made by cold forming can be successfully

made by warm forming.

Lubrication. The lubricant used most often in the press forming of stainless steel is the chlorinated type. It has

unexcelled chemical EP activity, and the ability to adjust this activity and viscosity independently over an extremely wide

range makes it the most versatile lubricant for this purpose. All chlorinated lubricants are readily removable in degreasers

or solvents, and emulsifiers can be added to them for easy removal in water-base cleaners.

As shown in Table 2, pigmented pastes, sulfurized or sulfochlorinated oils, and dry wax or soap-borax films are also

highly effective lubricants for press forming, but are less convenient to use. Heavy-duty emulsions, because of their

superior characteristics as coolants, are preferred for high-speed operations. In the following example, high chlorine

content and high viscosity were needed to produce acceptable parts (see also Example 15, in which a low-viscosity

chlorine-base lubricant replaced a viscous mineral oil).

Example 7: Increase in Chlorine Content and Viscosity of Lubricant That

Improved Results in Forming.

A wheel cover was made from a type 302 stainless steel blank, 457 mm (18 in.) in diameter by 0.71 mm (0.028 in.) thick,

in two operations: draw, then trim and pierce. At first, a lightly chlorinated oil (10% Cl) of medium viscosity (1500 SUS,

Saybolt Universal seconds, at 40 °C, or 100 °F) was used in drawing. Even though the draw was shallow, 12% of the

wheel covers were rejected for splits and scratches.

A change was made to a highly chlorinated oil (36% Cl) of much higher viscosity (4000 SUS at 40 °C, or 100 °F). As a

result, the rejection rate decreased to less than 1%. After forming, the wheel covers were vapor degreased.

Forming of Stainless Steel

Revised by Joseph A. Douthett, Armco Inc.

Combined Operations in Compound and Progressive Dies

The use of compound and progressive dies for the mass production of parts that require many operations or for an

operation that is too severe to be done economically in a single-operation die is discussed in the article "Press Forming of

Low-Carbon Steel" in this Volume. The same principles apply to their use on stainless steel for blanking, piercing,

bending, forming, drawing, coining, embossing, or combinations of these operations.

Both compound and progressive dies must be made of die materials that are hard enough to withstand the most severe

demands of blanking and are tough enough for the most severe forming or coining operations. The lubricant must have

enough body for the most severe draw, yet must be light enough not to interfere with the production of coined or

embossed details or to gum up cutting edges. In a compound die in a double-action press, two draws can be made in

stainless steel if the press capacity is not exceeded. The following example demonstrates the near-maximum severity of

forming that can be achieved in a blank-and-draw compound die.

Example 8: Blanking and Severe Drawing in One Operation in a Compound Die.

The shell illustrated in Table 5 was blanked and drawn in a severe forming operation in a compound die at the rate of

16,000 pieces per year. The die was used in a 400 kN (45 tonf) mechanical press with an air cushion. The formed piece

was restruck in the same die to sharpen the draw radius and to flatten the flange within 0.15 mm (0.006 in.). The die was

made of A2 tool steel and had a life of 50,000 pieces per grind. An emulsified chlorinated concentrate was used as a

lubricant.

Table 5 Production rates and labor time for making a severely drawn shell

Operation Production,

pcs/h

Labor

per 100

pcs, h

Blank and draw

(a)

922

0.108

Restrike

(a)

1429

0.070

Pierce center hole; trim

(b)

845

0.118

Pierce side holes

(c)

786

0.127

(a)

In a compound die, in a 400 kN (45 tonf) mechanical press with an air cushion.

(b)

In a 200 kN (22 tonf) mechanical press.

(c)

In a horn die in a 200 kN (22 tonf) mechanical press

After forming, the piece was then moved to a 200 kN (22 tonf) mechanical press, in which the 2.4 mm (0.093 in.) diam

hole was pierced and the flange was trimmed to an oval shape. A second piercing operation, in a horn die in a 200 kN (22

tonf) mechanical press, pierced two 1.6 mm (0.062 in.) diam holes in the side of the shell. Air ejection was used in all

operations except the final piercing, where the piece was picked off.

The material was type 302 stainless steel, 0.94 mm (0.037 in.) thick and 57 mm (2

in.) wide, which had been annealed.

Table 5 lists the production rate and labor time for each of the four operations.

Small, complex parts that must be made in large quantities are well suited to production in progressive or transfer

dies. A transfer die uses a minimum of material and can accept coil stock, loose blanks, or partially formed parts. Scrap

removal problems are lessened. A progressive die is preferred when the piece can remain attached to the strip.

In the following example, piercing, blanking, and forming were combined in a seven-stage progressive die. Although

progressive, it was hand fed--a rather unusual combination.

Example 9: Producing a Small Bracket in a Progressive Die With Hand Feeding.

The small bracket shown in Fig. 12 was made in a seven-station progressive die from 9.5 mm (

in.) wide stock that was

hand fed into the 50 kN (6 tonf) press. Hand feeding was done because close operator attention was required to prevent

jamming, which would have damaged the frail dies. The sequence of operations was as follows:

• Feed strip to finger stop; pierce

• Feed to notch-die opening; pierce

• Notch and trim lugs

• Form lugs

• Form and cut off

• Unload by blast of air

Fig. 12 Bracket that was made by hand feeding stock into a pr

ogressive die to avoid the jamming that would

have been likely from automatic feeding. Dimensions given in inches.

The parts were barrel finished to remove burrs and to provide a smooth finish and high luster. The production rate was

2175 pieces per hour. Annual production was 2 million pieces. A chlorinated and inhibited oil was used as a lubricant.

Progressive Dies Versus Simple Dies. There is often a choice as to whether a stainless steel piece is to be made in

a progressive die or in a series of single-operation (simple) dies. Deep forming usually presents difficulties in designing

and constructing efficient and long-life progressive dies. The cost and delay involved in developing progressive tooling

was justified for producing the frame described in the following example in quantities of 100,000 or more per year.

Example 10: Use of Progressive Dies for High-Quantity Production of Frames.

The frame shown in Fig. 13 was made of 0.56 mm (0.022 in.) thick type 430 stainless steel coil stock, 95.3 mm (3

in.)

wide. The maximum hardness was 83 HRB.