Metallurgy of Stainless Steel and Cobalt – Chromium alloys

THE FOLLOWING BRANDS OF STEEL MAY BE CLASSED AS TYPICAL:

  

   MANUFACTURER                                             BRAND

 

Krupp _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _V2A

Thomas Firth and Son, Ltd. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _Staybrite

Brown Bayley’s Steel Works, Ltd. _ _ _ _ _ _ _ _ _ _ _ _ _ Anka

Rocky Mountain Metal Products Co. _ _ _ U.S.S. stabilized 18-8

Avesta _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _832M

Nippon Kinzoku Kogyo K.K._ _ _ _ _ _ _ _ _ _ _ _ _ _ N.T. K. 8

Yawata Seitetsujo_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _NC. 6

Nippon Tokushuko G.K. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ST. 5

 

MAKING ORTHODONTIC WIRE

A look at the processes and problems involved in wire manufacturer leaves one with about the same impression as a look at the developing dentition. It is remarkable that the final result can be as dependably good as it actually is.

Melting

The physical properties of metals are influenced at every step in production beginning with the selection and melting of the alloying metals. This composition which fixes the general properties of the metal is more variable than might be expected.     

The ingot

One of the critical operations in wire making is pouring the molten alloy into a mold to produce an ingot. This ingot will be having varying degrees of porosity and inclusions of slag in different parts. The inside of the ingot will show crystals of the component metals called grains and it is this granular structure that controls many of the ultimate mechanical properties. The size and distribution of the grains depend on the rate of cooling and the size of the ingot.

 

The porosity in ingot comes from two sources.

 

1)            Gases that are either dissolved in the metal or produced by chemical reactions within the mass from bubbles that are trapped in the metal.

 

2)            As the ingot cools the bulk cooling of the interior section compared to the hardened outside shell will produce vacuum voids.

 

Rolling

The first mechanical step is rolling the ingot into a long bar. This is done by a series of rollers that gradually reduce the ingot to a relatively small diameter. The different parts of the original ingot never completely lose their identity.

The individual grains of the ingot also keep their identity through the rolling process. Each grain is elongated in the same proportion as the ingot. The squeezing massaging action of rolling the ingot has a very important effect on the grain structure actually increasing the strength of the metal. Rolling causes increase in the hardness or brittleness of the metal as the grains are forced to interlock even more tightly with one another in a form of work hardening.

Each pass through the rollers increase the work hardening until finally the structure becomes so “locked up” that it can no longer adjust enough to adapt to the squeezing of the rollers.

 

If rolling is continued beyond this point the surface will start to show many s0mall cracks and begin to crumble. Before this happens the rolling process is interrupted and the metal is converted by heating to a suitably high temperature.

Drawing

The wire is reduced to its final size by drawing. This is a more precise process in which the wire is pulled through a small hole in a die. One hole is slightly smaller than the starting diameter of the wire, so that the walls of the die squeeze it uniformly from all sides as it passes through reducing the wire to the diameter of the die. Before it is reduced to orthodontic size a wire is drawn through many series of dies and annealed several times along the way to relieve work hardening. The actual no. of drafts through the dies as well as the frequency of annealing depends on the alloy being drawn. Stainless steel needs more frequency than carbon steel and gold.

 

RECTANGULAR WIRES

Rectangular wires can be made by drawing the material through a rectangular die or by rolling round wires to a rectangular shape using Turk’s head, which contains pairs of rolls. There appears to be very little significant difference in the wires formed by the two processes. During drawing, sharp corners are produced on rectangular wire and the engagement of these edges on a rectangular slot can be an advantage in the application of torque. But the rolled rectangular wires will have some degree of rounding at the corners.