 |
| Main Menu | News By Date | News By Supplier | News By Category | About Us |
|
HEADING HINTS: ABOUT COLD HEADING (PART 4)
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. 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. Warm and Hot Heading 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.
|
| ©2008 New Materials International |