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The amount of technical information available on the subject of raw material selection is somewhat limited. End users need a solid foundation for decision making and normally look for guidance from fastener manufacturers. Today, distributors are serving as a key link in fastener supply to end user clients. The need to provide distributors with special insight to raw material selection, therefore, deserves clear evaluation. This brochure serves as a resource in this direction and was developed by IFI's Raw Materials Study Committee whose mission statement follows -
"The Raw Materials Study Committee encourages a continuing education and improvement program for customers, users, producers, suppliers, and processors by -
- Identifying the physical, mechanical, and metallurgical properties or characteristics of carbon and alloy steels required to manufacture fasteners and other formed parts.
- Joint development of information by fastener producers and raw material suppliers based on present needs and anticipated future requirements.
- Providing guidelines for continued improvement in raw material quality which will serve to improve specifications and enhance manufacturing and finished product quality.
How clean is today's steel; how is cleanliness measured or rated? What is its relationship to cold forming?
Dirty steel causes problems in fastener manufacturing. Specifically, inclusions in raw material create difficulties for thread rolling. Also, pipe is a problem for heat treaters, but appears to be rare in occurrence.
Inclusion levels are highly dependent on the steelmaking method. The four common groups of inclusions are sulphide, alumina, silicate and globular oxide, with alumina and globular oxide being the most detrimental to cold heading applications. General limits for inclusions cannot be quoted since they may vary according to the fastener's end application. Typical cleanliness levels are A2, B2, C2 and D2 in accordance with ASTM E45 method A=2 thin. A recommended microscopic practice for evaluating the inclusion content in steel is found in SAE J422.
The nonmetallic cleanliness of steels for fasteners should be a non issue in that it is relatively easy to assign root cause of nonmetallic cleanliness to many material problems. Trained personnel can identify nonmetallic inclusions by microexamination and can usually postulate that a quench crack, surface defect or fatigue crack was caused by the unfortunate location of a nonconforming inclusion.
The agreement of the conforming level of nonmetallic cleanliness is both a commercial and a technical issue. One should not specify cleaner steel than one really needs because of economic issues. Questions need to be asked. Is it a safety critical application? Is there any evidence that nonmetallic inclusions have caused the consumer or supplier to be economically disadvantaged? Is the current level of steel technology supplying a product that my technical resources say is clean steel? Size and required strength of the intended cold formed part should also be considered.
What is the current situation related to control of raw material surface discontinuities?
Currently, raw materials are offered as SQ (Special Quality) or CHQ (Cold Heading Quality). Both of these are considered cold heading materials; however, CHQ will generally be chosen when selected characteristics are to be closely controlled. An example of such a characteristic is seam depth which, for CHQ, has a max allowable of 0.025 inches. However, no known technical method allows reasonable inspection of 100% of raw material surfaces. In a practical sense, cold forming offers 100% inspection of the raw material in terms of its suitability. It is suggested that ASTM F788/F788M be used for guidance.
What is the difference between casting production methods?
Ingot, bloom and billet cast steel all begin with steel melted either in a basic oxygen or electric arc furnace. Other intermediate processes such as ladle refining may take place before the steel is cast. The casting operation is where the differences occur. In ingot casting, the steel from the ladle is taken to a pouring deck where the molten steel is poured into large cast iron molds and allowed to slowly cool into large ingots. After solidification has progressed sufficiently, the molds are stripped from the ingots and the ingots removed to the next operation which is reheating for rolling to a billet. In bloom and billet casting, the steel from the ladle is poured into a large "pot" called a tundish from which it is fed into a water-cooled mold and extracted in a solidified section as either a bloom or billet. Blooms are an intermediate product usually having a much larger cross section than billets. These blooms are rolled into billets which are subsequently rolled into bars or rods.
As a final note, ingot casting is not readily available for fastener raw material and is rapidly disappearing.
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