Carpenter

Fabricating Carpenter Stainless Steels

June 2006

Cold Heading, Warm Heading and Hot Heading Carpenter Stainless Steels

Cold Heading

Stainless steels continue to be used in the manufacture of cold-headed parts. This group of alloys provides several benefits, including corrosion resistance and high strength at room and elevated temperatures. Most stainless steels can be cold headed. Carpenter Technology has played an important role in the development and production of stainless steel cold-heading wire.

Comparison of Cold Headability: The diagram below presents a simple comparison of cold headability and corrosion resistance of the popular cold-headed grades of stainless steel.

The relative headability of martensitic stainless steels such as Type 410 and ferritic stainless steels such as Type 430 is affected primarily by carbon content and yield strength. Type 410 and Type 430 are relatively easy to cold head and are comparable to low carbon alloy steels. Martensitic stainless steels can be hardened by heat treatment but ferritic stainless steels cannot. Both types will harden slightly by cold working. Both are widely used for fasteners. 

The relative headability of the austenitic stainless steels is affected primarily by composition. That is, those higher in nickel, and in some cases copper, generally exhibit lower work-hardening rates because of the more stable austenitic structure. Stainless Type 305 was the original stainless grade developed for improved cold headability. The 12% nickel content accounts for this. Carpenter also produces Carpenter No. 10 (Type 384) and Carpenter 302HQ stainless, both of which exhibit low work-hardening rates and excellent cold headability for austenitic stainless steels. Typical austenitic stainless steels cannot be hardened by heat treatment; however, cold working will increase hardness. Carpenter 302HQ stainless has been used extensively to produce Phillips and other recessed-head fasteners. 

Product Forms

Carpenter manufactures three basic wire product forms in addition to specially finished wire or rod for special applications. The three basic forms include: 

1.       Annealed and Cold Drawn to Finish Heading Wire

2.       Cold Drawn and Annealed at Finish Heading Wire

3.       Hot Rolled and Annealed at Finish Rod 

Annealed and Cold Drawn to Finish Heading Wire is raw material in the finished condition. It is available in all sizes up to about 1.00" (25.4 mm) in diameter. Refer to Table I for specific size tolerances. Wire in this condition offers substantial surface integrity and the widest range of mill coatings. This product typically does not require additional sizing prior to entering the header. 

Table I - Standard Size Tolerances for Cold Drawn Heading Wire

Table II lists typical ultimate tensile strength maximums for annealed and cold-drawn wire for sizes greater than 0.100" diameter.

Table II - Typical Tensile Strength Maximums for Cold Drawn Heading Wire in Diameters >0.100”

Cold Drawn and Annealed at Finish Heading Wire is supplied cold reduced, annealed and coated. It offers the lowest mechanical properties and is suitable for redraw or heading. This product should be sized prior to entering the header. Cold drawn, annealed product is available in all sizes up to about 1.00" (25.4 mm) in diameter. Tolerances are double those available on annealed and cold drawn wire.

STARR®* wire (Stainless, Annealed, Ready for Redraw) is a modification of the basic cold drawn and annealed at finish form. Its manufacturing sequence typically includes additional operations to enhance surface quality. It is available in only a few stainless grades and is typically supplied with a cross sectional area about 5 percent over the cross sectional area of the finish drawn product. Typical maximum ultimate tensile strengths for annealed at finish wire are shown in Table III.

Table III - Typical Ultimate Tensile Strength Maximums for Annealed at Finish Wire in Diameters over 0.100" (2.54 mm) 

Hot Rolled and Annealed at Finish Rod is the least finished condition and must be sized prior to entering the header. It is supplied annealed, descaled and coated. Of the three wire forms, rod has the roughest surface and the widest tolerances. Rod is available in a size range from about 7/32" to 1-1/4" diameter. Tolerances may be as great as ±0.010" with a maximum of 0.015" out-of-round in the larger diameters. See Table IV.

Table IV - Standard Rod Tolerances

To reduce wire inventories and the number of wire sizes purchased, some headers draw wire or rod in front of the header with obvious savings. 

While rod is the lowest cost wire stock, this may not be an advantage. Many fabricators report that rod necessitates the addition of more in-house capabilities. When redrawing rod, scrap losses may increase and present overall quality control problems that often negate initial raw material savings. 

The best alternative for most headers is the use of wire that has been annealed and cold drawn to finish.  

Choice of the proper coating is influenced by the specific application; however, there are general considerations. The type of coating required depends on the alloy being formed, the degree of cold work needed, the temperature generated by the heading process, and the complexity of the part being formed. Additional factors influencing coating selection include availability and cost, compatibility with other mill coatings or fabricator lubricants, and the ease of coating removal from the finished parts.

For many years the most effective coating for stainless steel heading wire has been an electrolytically plated copper layer plus lime and soap drawn on during the final light draft made in finishing the wire. Today, however, coatings such as Carpenter’s Ecolube®* II coating may be used to eliminate the problems associated with disposal of cleaning acids containing metal ions. A key point to remember is that Carpenter, as a producer of stainless heading wire and rod and a variety of coatings, is totally equipped to help customers with coatings selection, as well as all other aspects of cold heading operations.

Coating classes are determined by selecting a coating option designated by a letter and a drawing option designated by a number. This is typically referred to as Carpenter's Alpha-Numeric Coating Classification System. The coating and drawing options are:

Warm Heading

Warm heading is a modified form of cold heading performed at 200° to 800°F (93° to 427°C), which is below the recrystallization or transformation temperature of the metal being formed. Ductility is improved without changing the microstructure. Warm heading allows working difficult-to-form materials, requires less deformation pressure, reduces tooling loads by as much as 50 percent compared to cold forming, and generally prolongs tool life.

Warm heading is especially applicable to parts of unusual shape and forming high strength alloys that are resistant to heat and corrosion. Usually alloys that work harden rapidly can be upset without cracking. The method generally works well for making high strength bolts.

With warm heading, the wire is usually heated before it enters the feed rolls, or, when possible, between the feed rolls and the header machine frame. The most commonly used methods of heating are:

1. Resistance heating - A contact stand is installed between the wire reel and the feed rolls. A low-voltage, high-amperage circuit is then connected to the contact stand and the feed rolls. The electrical resistance of the metal itself serves to produce the heat.
2. Gas heating - A series of burners is mounted on an adjustable stand and the wire passes over them. Variations include use of a tube, surrounded by a series of ring burners, which is mounted on an adjustable stand, heating the wire as it passes through the tube.
3. Induction heating - An induction coil is installed in front of the feed rolls, and the wire is passed through the coil.

Close control of wire temperature is important since erratic heating may cause uneven flow, and may result in uncontrolled head dimensions. If the wire is overheated, for instance, the material will tend to blob instead of flow. Also, the lubricity of the wire coating may be destroyed, and smearing may occur at the cutoff station. Close temperature control, on the other hand, improves plasticity and headability by reducing both the strength and work hardening of the material being formed. Consequently, less forming pressure is required to fill the cavity of the die or hammer, with a resulting improvement in sharper corners and shoulders and, in some cases, elimination of stress cracking. 

Surprisingly good warm heading results for the stainless steels are achieved in the temperature range between 350° and 450°F (177° to 232°C). Temperatures over 600°F (316°C) should generally be avoided.

Hot Heading

Hot heading or upset forging can be done on conventional heading equipment similar to cold heading machinery or on forging machines. Hot heading is generally performed on the larger diameters which cannot be obtained in coils. The same general principles applying to conventional forging should be utilized.

* STARR and Ecolube are registered trademarks of CRS Holdings, Inc., a subsidiary of Carpenter Technology Corporation.