Carpenter Technology Corporation, whose Specialty Alloys Operations produces hundreds of stainless steels and specialty alloys, published the booklet "Heading Hints – A Guide to Cold Forming Specialty Alloys" to suggest proper cold-forming fabrication techniques. Request a free copy of "Heading Hints."

Multiple station machines often combine upsetting and extruding operations to form large-head, small-shank parts. The reason is that upsets can involve 6 to 10 diameters, rather than the 4-½ diameters maximum. Upsetting and extruding, however, are separate operations, so maximum deformation of a blank must be figured separately for upsetting and extruding limits even with multi-station headers.

By starting with wire stock larger in diameter than the required shank, then extruding the shank and finally upsetting the head, maximum deformations can be reached for both extruding and upsetting based on initial stock size. For instance, a ½’’ blank trap extruded to a reduction in area of 80 percent, and then upset 6 diameters, results in an actual upset having 70 diameters of the extruded shank size.

Since single-die, double-stroke headers (one die, two punches, two blows) are the norm in heading machines, combining upsetting and extrusion is common practice. The first blow extrudes the shank and partially forms the head; the second blow finishes the head.

An important rule for combining these two operations is that parts being formed in solid dies cannot have a shank length that exceeds 8 diameters (Figure 1). Since solid dies include a knockout pin, the knockout pin must overcome the great friction between the shank and the die as it kicks out the finished part. If more than 8 diameters of the knockout pin are unsupported outside the die, the knockout pin will usually bend as it pushes against the blank. Shank lengths over 8 diameters are produced using an open die, a two-part die that is spread apart by a cam mechanism as the part is finished. The next blank pushes out the finished part; no knockout pin is used with open dies.

Let’s assume a bolt has to be made from 1/4" wire on a two-blow, solid-die header. If we factor in the ground rules for upsetting and extrusion, the following limits apply to making this part:

1. The maximum length in inches of metal that can be used to upset the head is 1-1/8" (4-1/2 diameters of the 1/4" wire).

2. The maximum shank length is 2" (8 diameters of 1/4" wire).

3. The maximum blank length is 3-1/8", which includes 1-1/8" of material to upset the head, and 2" of material to form the shank.

Here’s another example. Assume you want to produce a part like the one shown in Figure 2. First, it would be uneconomical to machine this fastener from 1-1/4" stock in large numbers. Second, the part cannot be produced with conventional two-blow, solid-die heading. Here’s why. A 1/2" wire stock is required to form the head. Using 1/4" or 3/8" wire exceeds the 4-1/2 diameters rule for two-blow upsets. However, by using 1/2" wire, the 1/4" shank cannot be extruded in standard fashion. The 1/2" wire has an approximate area of 0.20 sq. in., while the 1/4" shank has an area of 0.05 sq. in. This is an area reduction of 75 percent, which exceeds the 30 percent ground rule for open extrusion. This part can be formed from the 1/2" wire, however, on a two-die, three-blow header:

1. Trap extrude the 1/2" shank portion, since this method allows for area reductions of up to 75 percent.

2. Transfer the part to the second die station and finish it in the normal one-die, two-punch sequence. The first blow extrudes the 3/4" shank portion and partially forms (or cones) the head. The second blow forms the 1-1/4" diameter head.

Warm and hot heading techniques involve the heating of wire or blanks during certain stages of the heading process and allow forming of more heavily alloyed metals, including precipitation hardening stainless steels and high temperature alloys. To assist in forming parts such as recessed-head screws from tougher metals, wire can be heated before it enters the header. This reduces yield and tensile strengths to improve forming characteristics.

Warm heading in the metallurgical sense is really cold heading since the metallurgical structure is not affected; the material is simply made more ductile. With hot heading, however, the metallurgical structure of the material is often altered. In both instances, less pressure is required to make the metal flow plastically since warm and hot heading techniques lower material strength and increase ductility.

Warm heading has been applied successfully in forming stainless steels and certain high temperature alloys. Heating is most effective in the 350° to 450°F (177° to 232°C) temperature range. While warm or hot heading is not normally required for highly headable stainless grades, it may be used with these metals to improve metal flow and avoid stress cracking in severe upsets.

Warm heading is accomplished by heating the wire before it enters the feed rolls or, when possible, between the feed rolls and the heading machine. Three types of heating methods are usually used - resistance, gas or induction.

Hot heading, on the other hand, means heating the wire to the 1100° or 1200°F (593° or 649°C) temperature range. It’s almost equivalent to forging.

Proper choice of lubricants is essential for effective warm and hot heading.

The fifth installment of this six-part series on heading basics outlines the types of heading machines.

Disclaimer

The information and data presented herein are typical or average values and are not a guarantee of maximum or minimum values. Applications specifically suggested for material described herein are made solely for the purpose of illustration to enable the reader to make his/her own evaluation and are not intended as warranties, either express or implied, of fitness for these or other purposes. There is no representation that the recipient of this literature will receive updated editions as they become available.