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SURFACE HARDENİNG

Surface hardening, a process that includes a wide variety of techniques, is used to improve the wear resistance of parts without affecting the more soft, tough interior of the part. This combination of hard surface and resistance to breakage on impact is useful in parts such as a cam or ring gear that must have a very hard surface to resist wear, alonf with a tough interior to resist the impact that occurs during operation. Further, the surface hardening of steel has an advantage over through hardening, because less expensive low- and medium- carbon steels can be surface hardened without the problems of distortion and cracking associated with the through hardening of thich section.

(Joseph R.D., Surface hardening of steels: understanding the basics, page 1)

DOUBLE ACTION DRAWING

In drawing and redrawing of thin stock into cylindrical shells with no ironing, the diameter reduction should be 35 to 40% on the first operation and 15 to 25% on redraws. If the walls are held to size, the first and second operations may be the same as suggested above, but the amounth of reduction should be dimished by about 5% on each successive redraw.

Although reductions of up to 50% can be made in one operation, this is not advisable because of the possibility of excessive shell breakage. Also, large reductions may open the surface of the metal and cause difficulty in finishing.

(Joseph R.D.,ASM International,Nickel, cobalt, and their alloys,page 193)

SPINNING TOOLS

The bowls, stems and feet of cups,and hollow vessels of almost any profile may be produced from discs of sheet metal by burnishing them to shape on a lathe. This process is known as spinning. It is applicable to shapes which are circular in plan: though the form of the work produced may, of course, be afterwards altered by the use of snarling irons, or by shaping and planishing it on a suitable stake. Thus, the general form could have been produced on the spinning lathe, the flutes or lobes being afterwards snarled and hammered out. But, primarily, spinning is employed for shapes which are circular in plan, whatever their profile may be. The process is in very general use among manufacturing silversmiths, to whom the ease with which a number of exactly similar copies of a shape may be produced is of importance. It has, however, little interest for the artist, for to him variety and a less mechanical form than that so generally adopted commercially will naturally appeal.

(Maryon H.,Metalwork and enamelling:

a practical treatise on gold and silversmiths' work and their allied crafts, page 104)

THROUGH HARDENING

Through-hardening bearing steels which contain about 1 % carbon and 1 to 2 % chromium. These steels are used in the through-hardened condition for normal service with operating temperatures less than 150 C. Modification of the basic chemical composition, by an increase in manganese content or an addition of molybdenum, is used to increase the hardenability of heavier sections and ensure freedom from nonmartensitic transformations in hardening.

(Nayar,The Metals Databook, page113)