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News

Heading hints: About cold heading

Carpenter Technology Corporation : 13 May, 2006  (New Product)
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.' Although at one time it was felt cold heading techniques were somewhat limited to ductile materials with low work hardening rates, that is no longer the case. With today's more advanced equipment, techniques and tooling, higher strength materials like stainless steels and high temperature alloys are routinely being cold headed. A number of stainless steels are produced with modified compositions to provide lower work hardening rates.
This is the final installment of a six-part series on heading basics. This section discusses materials that can be headed, as well as formability and wire considerations.

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.'

Although at one time it was felt cold heading techniques were somewhat limited to ductile materials with low work hardening rates, that is no longer the case. With today's more advanced equipment, techniques and tooling, higher strength materials like stainless steels and high temperature alloys are routinely being cold headed. A number of stainless steels are produced with modified compositions to provide lower work hardening rates.

Strength of materials is the determining factor in the ease of cold forming. The yield and tensile strength of an alloy governs formability. Yield strength is the point at which the metal begins to deform permanently. Tensile strength is the point at which the metal begins to tear apart. Plastic flow occurs when the force applied exceeds the material's yield strength. If the metal is stressed beyond its tensile strength during forming, the blank splits, cracks or breaks. The range in which a metal can be cold worked lies between its yield and tensile strength values.

Tensile strengths (ultimate strengths) found in most technical literature differ from the actual strengths of the materials being formed in the header. These strengths, therefore, must be considered in cold forming.

The strength of a material is affected by both the temperature at which it is being formed, and by the speed with which it is being formed. It is also affected by the geometry of the part being made.

What occurs physically with materials during the heading process is very complex. While it can be frustrating, for example, when trying to determine exactly why certain materials may crack, it is comforting to know that modern technology is addressing this problem. CAD/CAM techniques are being employed to more accurately predict and control what happens to materials in the headers. Software packages are available that can allow design engineers or operators to precisely determine if slowing the part production rate per minute would eliminate cracking problems.

Cold worked metal work (or strain) hardens due to a reorganization of its microstructure. A series of cold forming operations means both the yield strength and tensile strength increase. However, the yield strength increases faster than the tensile strength, which narrows the metal's formability range mentioned above. Depending on the type of metal, this range can become so narrow that further attempts to cold work the metal result in fracture.

Work hardening accounts for the increased strength of formed parts, and there is an associated increase in tooling pressure required to deform them. The standard specialty alloy grades designed for cold heading take into account the desirability of low cold working rates.

An important part of good cold headability is the soundness of the wire. Sound centers are promoted by close controls during melting and hot working operations. In addition, a thorough billet inspection for surface defects is important. Quality wire offers fabricators consistent, lot-to-lot uniformity; excellent formability; optimum cut-off results; controlled sizes in a wide range of diameters; close tolerances and uniform coils; and superior surfaces.

A broad range of wire sizes and alloy types are available to match expanding cold-forming opportunities. Because the majority of headed parts are made from wire in the 0.062'' to 0.750'' diameter range, producers of headable alloys should manufacture a number of different alloys in standard wire sizes up to 1.00'' in diameter.

Equally important is the capability of the producer to match specific fabricator needs. Producers should manufacture wire in three basic conditions: hot rolled annealed, cold drawn annealed, and annealed cold drawn. Cold finished wire is ready for the heading operation and the manufacturer controls the consistency and tolerances. Wire ready-for-redraw and hot rod receive less processing at the mill and so have more variation in size, mechanical properties and finish. With that in mind, the heading shop assumes a greater share of responsibility for product quality in return for reduced material cost. Many headers prefer annealed, cold finished wire since it can go right into the header for production. Others prefer partially finished wire because they have drawing capabilities in line with their heading operation and want more precise control of the wire diameter.

Design Versatility: Cold heading or forming opens unlimited possibilities to the part designer:

1. Cold forming allows use of high strength parts to be produced from non-heat-treatable alloys.

2. Cold forming is often the most cost effective way to produce complicated configurations compared with profile milling, electric discharge machining, hobbing or chemical etching.

3. Cold forming has inherent capabilities for greater strength and high production rates.

Designers can, and should, rely on the expertise of cold forming production people who are most familiar with machining capabilities. Extruding, for example, is an efficient and highly economical method for creating two or more different diameters on a part. With backward extrusion, the designer has an excellent way to form tubular shapes, including those produced with double reverse extrusions. Some part configurations achievable with cold forming are indicated in Figure 1. Multi-station headers also contribute to the designer's ability to produce a component that requires closely allied cold forming operations.

Variety of Sizes, Shapes, Part Complexity: Economically produced, cold formed parts today include bolts, studs, screws, rivets, special fasteners, cams, valves and many other components requiring the diameter of the head to be substantially larger than the shank. Total upsetting of the blank also is performed on cold headers to mass produce nuts and balls for ball bearings. Symmetrical parts are the easiest to cold form and cylindrical parts require only that a transfer mechanism move them from one die to another. Asymmetrical parts require positioning at each station, which calls for close cooperation between the product designer and tool engineer.

Tolerances: Tolerances vary with the style of the upset and, as with any other manufacturing process, closer tolerances require greater cost and precision. Diameter tolerances in cold forming operations are easily held within acceptable limits for standard fasteners. Tolerances are naturally affected by tool wear, so a check on die wear is mandatory when running parts with tighter tolerances.

Positive Metallurgical Effects: A primary advantage of cold forming is maximization of metallurgical properties in the finished part. The upset process actually causes the metal to flow along the axis of the blank; the grain structure is rearranged in the process to follow the contour of the part (Figure 2). This new grain structure supports the part and adds strength to it. Cutting, on the other hand, weakens grain structure. Metal cut away from underneath a bolt head means cutting the grain structure at the same time, so the bolt head is now weaker than the stock from which it was cut.

Cold heading involves working of the metal far below its recrystallization temperature. Existing grains are worked and no new grains are formed. This improves strength, hardness, toughness and fatigue resistance. Cold worked grains are usually finer than in hot-forged parts and the grain flow lines established by the various cold forming operations remain uninterrupted in the finished part. The result is enhanced strength at critically stressed corners. This metallurgical advantage often allows headed parts to be smaller without sacrificing properties.

High production Rates, Repeatability: Today's headers can turn out parts at a rate as high as 100 times greater than that achievable with machining. While production rates are controlled by part size and complexity, heading and cold forming machines are automated production lines that take raw material and convert it into to finished parts, ready for use. In a multi-station header where all the die units are working in unison, a finished part is ejected with every stroke. With good die design, low temperature and good lubrication, repeatability is excellent.

Material Savings: Cold forming is a type of 'chipless machining.' Parts are produced to net or near net shapes. The only waste comes from piercing and trimming. Heading scrap losses average from 1 to 3 percent, while turning or forging can produce scrap losses as high as 75 percent. An excellent example of material savings occurs in the cold forming of spark plug bodies. Prior to using cold forming, the pieces were cut with scrap losses averaging 74 percent. Now, bodies can be cold formed 10 times faster and with scrap losses of only 6 percent.

The weight of a finished part that is headed can generally be held within 1 percent when required, or to even 0.2 percent when more precise cut-off is used to produce blanks. Oftentimes further machining is not required, and on many jobs cold forming eliminates secondary grinding.

Finished Quality: Longer part life is a benefit of cold formed parts since the controlled flow lines offer added resistance to impact, fatigue and shear failure. Cold forming means improved surface finishes. Extruding may improve surface finish from 10 to 100 micro inches and upsetting results in high quality finishes when the part is confined in the tooling or comes in contact with tooling surfaces. High quality finishes result when a high quality wire feed stock is used.

Heading, like any other metalworking process, has its limitations. Relatively simple designs that can be produced on standard one- or two- blow headers usually require a minimum quantity of about 5,000 parts to cover tooling cots and set up. Complex parts that call for multiple dies, development work and other procedures usually require a quantity of at least 25,000 to 30,000 pieces. Larger or more complicated designs may not lend themselves to cold forming, but require machining instead. Some materials, because of their exceptionally high strength levels, may exceed the formability range limits for cold forming.
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