Bar Finishes

Hot-finished bars are commonly produced by hot rolling, forging, or pressing ingots to intermediate-size blooms or billets, which are subsequently hot rolled, forged, or extruded to final dimensions. (In some mills, the process starts with continuous cast sections.) The selection of rolling, forging, or extruding as the finishing method depends on several factors, including the composition of the steel and the final size. It is common practice to process bars in straight lengths, although smaller bars produced by rolling may be coiled.

Following hot rolling or forging, hot-finished bars may be subjected to various operations, including annealing or other heat treating; cleaning by pickling, blast cleaning, or other methods of descaling; rough turning; and machine straightening. When only improved surface is required, as for bars intended for forging, bars can be turned or ground.

Cold-finished bars are produced from hot-finished bars by additional operations to give close tolerance, improved surface finish, or specific mechanical properties.

Stainless steel bar is produced in the conditions and surface finishes given in Table 6. It is important that both condition and finish be specified, because each finish is applicable only to certain conditions.

Table 6 Conditions and surface finishes for stainless steel bar

Condition

Surface finish

Hot worked only

Scale not removed (except for spot conditioning)

Rough turned(a)(b)

Blast cleaned

Annealed or otherwise heat treated

Scale not removed (except for spot conditioning)

Rough turned(a)

Pickled or blast cleaned and pickled

Cold drawn or cold rolled

Centerless ground(a)

Polished(a)

Annealed and cold worked to high tensile strength1-0"1

Cold drawn or cold rolled

Centerless ground(a)

Polished(a)

Source: Ref 2

Source: Ref 2

(a) Applicable to round bar only.

(b) Bar of 400-series stainless steels that are highly hardenable, such as types 414, 420, 420F, 431, 440A, 440B, and 440C, are annealed before rough turning. Other hardenable types, such as types 403, 410, 416, and 416Se, also may require annealing, depending on composition and size.

(c) Produced only in mill orders; made predominantly in types 301, 302, 303Se, 304, 304N, 316, and 316N.

The finish on stainless steel bar is generally the result of processes used to size the bar or to improve the surface and usually not for purposes of achieving a certain surface appearance. Turning, for instance, improves the surface by removing undesirable defects. It is difficult, if not impossible, to obtain a hot-rolled surface that the customer can use without removal of a portion of the "skin." Turning is accomplished by passing the bar through a turning machine or lathe, using one or more passes depending on the amount of material to be removed.

Sometimes the final surface or dimension of round bar can be achieved only by centerless grinding. Centerless grinding differs from turning in that a grinding wheel is used for metal removal instead of a cutting tool, and more accurate dimensions and better surface finish are obtained. The centerless grinding machine is constructed so that the bar is supported under the greater portion of its length as well as under the grinding wheel, other than at the ends.

In some cases, a highly polished bar is desired rather than the standard centerless ground finish. In this case, centerless ground bars are passed through a polishing or lapping machine that imparts a higher degree of polish. Flat or shaped bars cannot be centerless ground, so any finish requirements are achieved by methods similar to those used for strip.

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