Carbon and Alloy Steel for Mechanical Fasteners

Extracto:

Material is furnished in many application variations. The purchaser should advise the supplier regarding the manufacturing process and finished product application as appropriate. Five application variations are:

  • Cold heading
  • Recessed head
  • Socket head
  • Scrapples nut
  • Tubular rivet
Forming is the primary manufacturing operation in the fastener industry and the term includes heading, upsetting, extruding, and forging. These formed parts are produced at very high speeds by metal flow due to machine-applied pressure.

Specification according to the Standard F 2282, establishes quality assurance requirements for the physical, mechanical, and metallurgical requirements for carbon and alloy steel wire, rods, and bars in coils intended for the manufacture of mechanical fasteners which includes: bolts, nuts, rivets, screws, washers, and special parts manufactured cold.

The term "quality" is being used to designate characteristics of a material which make it particularly well suited to a specific fabrication and/or application and does not imply "quality" in the usual sense.

Material is furnished in many application variations. The purchaser should advise the supplier regarding the manufacturing process and finished product application as appropriate. Five application variations are:

  • Cold heading
  • Recessed head
  • Socket head
  • Scrapples nut
  • Tubular rivet
Forming is the primary manufacturing operation in the fastener industry and the term includes heading, upsetting, extruding, and forging. These formed parts are produced at very high speeds by metal flow due to machine-applied pressure.

The primary forming operation self-inspects the quality of the raw material and imperfections such as seams, laps, and internal pipe which may not be visible are revealed when the material is upset. The absence of bursts, forging cracks, and open seams is strong evidence that the quality of material selected was that intended for the severe upsets of today’s fastener manufacturing.

Manufacture

Melting Practice: The steel shall be melted in a basic oxygen or electric furnace process.
Casting Practice: Steel shall be ingot cast, or continuous cast with controlled procedures to meet the requirements of this specification.
Deoxidation Practice
and Grain Size:
The material shall be furnished in one of the deoxidation and grain size. When not specified, the practice shall be at the option of the manufacturer.
Silicon killed fine grain shall be produced with aluminum for grain refinement. The material purchaser’s approval shall be obtained for the use of vanadium or columbium for grain refinement.
Silicon killed coarse grain practice.
Silicon killed fine grain practice.
Aluminum killed fine grain practice.
Hardenability: Hardenability for steels with a specified minimum carbon content of 0.20% or greater shall be determined for each heat and the results furnished to the purchaser when requested on the purchase order.
Thermal Treatments: The purchaser shall specify one of the following options for thermal treatment on the purchase order:
  • No thermal treatment.
  • Annealed.
  • Spheroidized.
  • Drawn from annealed rod or bar.
  • Drawn from spheroidize annealed rod or bar.
  • Spheroidized at finished size wire.
  • Annealed-in-process wire.
  • Spheroidized annealed-in-process wire.

Rimmed or capped steels are characterized by a lack of uniformity in their chemical composition, especially for the elements carbon, phosphorus, and sulfur, and for this reason product analysis is not technologically appropriate unless misapplication is clearly indicated.

Coarse Austenitic Grain Size: When a coarse grain size is specified, the steel shall have a grain size number of 1 to 5 inclusive. Conformance to this grain size of 70 % of the grains in the area examined shall constitute the basis of acceptance.

Fine Austenitic Grain Size: When a fine grain size is specified, the steel shall have a grain size number greater than five. Conformance to this grain size of 70 % of the grains in the area examined shall constitute the basis of acceptance. When aluminum is used as the grain refining element, the fine austenitic grain size requirement shall be deemed to be fulfilled if, on heat analysis, the total aluminum content is not less than 0.020 % total aluminum or, alternately, 0.015 % acid soluble aluminum. The aluminum content shall be reported.

Materials and Processing

While standard steel grades for rods and bars have been in existence for many years, and have, with modifications or restrictions of one or more elements, long been used for cold forming, ASTM standard presents a distinct selected series of twenty steel grades for cold forming. These have been jointly developed by steel producers and cold heading and forging users under the aegis of the Industrial Fasteners Institute. These twenty grades are designated steel grades and the ranges and limits for the thirteen carbon steel grades for carbon, manganese, phosphorus, and sulfur and alloy steels with copper, nickel, chromium, molybdenum, tin, and silicon.

A significant area of improvement is in the decarburization control and measurement for cold heading rods and bars.

To prepare a material for cold forming it is often spheroidized, which is an annealing treatment that transforms the microstructure of steel to its softest condition with maximum formability. In the hot rolled or normalized condition, steels containing less than 0.80 % carbon consist of the microconstituents pearlite and ferrite. Pearlite, the harder of the two constituents, causes the steels to resist deformation. The harder pearlite is comprised of alternating thin layers or shells of ferrite and cementite, a very hard substance.

In spheroidize annealing, the cementite layers are caused by time and temperature to collapse into spheroids or globules of cementite. This globular form of cementite tends to facilitate cold deformation in such processes as cold heading, cold rolling, forming, and bending.

Boron is extremely effective as a hardening agent in carbon steels, contributing hardenability which generally exceeds the result of many commercial alloying elements. It does not adversely affect the formability or machinability of plain carbon steels. Actually, the reverse is true since boron permits the use of lower carbon content which contributes to improved formability and machinability.

In its early development, some unsatisfactory results produced product which did not have uniform hardness or toughness along with reduced ability to resist delayed fracture. However, many of these problems were overcome by exhaustive research which demonstrated that for boron to be effective as an alloying agent, it must be in solid solution in a composition range of 0.0005% to 0.003%. During deoxidation, failure to tie up the free nitrogen results in the formation of boron nitrides which will prevent the boron from being available for hardening. Research also revealed boron content in excess of 0.003% has a detrimental effect on impact strength because of the precipitation of excess boron as iron borocarbide in the grain boundaries. Many European steels contain higher boron levels than in North America.

When producing a boron steel, titanium and/or aluminum is added and the resulting product is subjected to thermal processing. These two additions are designed to tie up nitrogen to stop it from reacting with boron. The resulting free boron is available to provide excellent hardenability in steel. Both titanium and aluminum nitrides reduce the machinability of the steel, however, when the nitrogen becomes tied up, the formability of the steel is improved.

Silicon and aluminum act as somewhat similar elements with respect to their behavior when added during the steel making process. They both have a high affinity for oxygen and are, therefore, used to deoxidize or "kill" the steel. Deoxidation or "killing" is a process by which a strong deoxidizing element is added to the steel to react with the remaining oxygen in the bath to prevent any further reaction between carbon and oxygen.

When carbon and oxygen react in the bath a violent boiling action occurs which removes carbon from the steel. When the bath or heat reaches the desired carbon content for the grade being produced, the carbon-oxygen reaction must be stopped quickly to prevent further elimination of carbon. This addition is accomplished by the addition of deoxidizers such as silicon and aluminum which have a greater affinity for oxygen than does carbon. This effectively removes oxygen, eliminating the "carbon boil" and killing the heat. Elements other than silicon and aluminum can be used, but these are the most common.

Silicon and aluminum can be added together or individually. This is determined by the type of steel desired. If silicon only is added, that particular batch of steel is referred to as a silicon killed coarse grain practice grade because silicon acts as a deoxidizer without the formation of fine precipitates allowing the formation of large or coarse austenitic grains.

Austenitic grain size is not usually a factor for consideration in cold forming, but has a significant effect in subsequent fastener heat treatment. Aluminum, on the other hand, not only deoxidizes the steel, but also refines the grain size. Like silicon, aluminum removes oxygen from the bath, effectively killing the heat. Aluminum also reacts with nitrogen in the steel to form aluminum nitride particles which precipitate both at the grain boundaries and within the austenitic grains thus restricting the size of the grains; even when the steel is reheated for carburizing or neutral hardening, hence the term fine grain.

When aluminum only is added, the steel is referred to as aluminum killed, fine grain. A third group of steels are referred to as silicon killed, fine grain. In steels of this type, silicon is added as the deoxidizer followed by the addition of aluminum for grain size control.

In the two types where silicon is added, the silicon content can have several ranges with the most common being 0.15 % to 0.30 %. When aluminum is added to these steels for grain size control, the aluminum content is generally in the 0.015 % to 0.030 % range. The aluminum content in fully aluminum killed steels is generally 0.015 % to 0.055 %, somewhat higher on average since the aluminum must both deoxidize and control grain size at the same time.

In selecting the type of deoxidation practice for a particular carbon grade of steel to be used in fastener manufacturing, a number of factors should be considered, such as, heat treated property requirements, heat treat conditions, fastener size, and steel availability, to name a few. Silicon acts as a ferrite strengthener and, therefore, in the absence of aluminum, has somewhat greater hardenability. For the same carbon grade and heat treat conditions with and without aluminum, complete transformation of the fastener core during heat treatment can take place in a larger section using a coarse grain steel.

The disadvantage of silicon killed steels can be reflected in reduced ductility and tool life during cold heading because of its ferrite strengthening characteristic. Aluminum killed steels are usually more formable and hence provide somewhat improved tool life but reduced heat treatment response during heading, particularly in larger size fasteners. For this reason, the recommended maximum diameter for oil quenched aluminum killed carbon grades is typically 0.190 in.

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