Finishing Methods

This section deals with the finishing of stainless steels as practiced by the fabricator or manufacturer--specifically, grinding (used here to mean metal removal for the purpose of altering surface finish, not to achieve close dimensional tolerances), polishing, and buffing. Table 8 summarizes the sequence of operations for the grinding, polishing, and buffing of stainless steels.

Table 8 Sequence of grinding, polishing, and buffing operations for stainless steels

Operation

Stage

Wheels

Grit size

Abrasive

Method

Purpose

Grinding

rubber or bakelite bonded, soft body

20-30

Manufactured types, such as AI2O3 or SiC

Dry

For rough work on castings, forgings, and weld beads that may require it. Castings usually sandblasted initially

Grinding

Finish

Same as above; or setup disk wheels or abrasive belts

36-80

Manufactured types, such as Al2O3 or SiC

Dry

For subsequent operations on the above and also preliminary to polishing operations on hot-rolled plate. May also be employed for touch-up work (removing deep scratches and tool marks prior to polishing)

Polishing

Rough

Setup disk canvas

80-100

Manufactured types,

Grease stick

To produce a rough polished finish and to remove imperfections left

compress wheels or abrasive belts

such as Al2O3 or SiC

by hand wheel grinding. Also use as a preparatory operation for a final polish corresponding to standard No. 4 mill finish. This is the usual starting on cold-rolled sheet.

Polishing

No. 4 finish

Setup disk canvas compress wheels or abrasive belts

120150

Manufactured types, such as Al2O3 or SiC

Grease stick

To provide a finish similar to standard No. 4 mill finish. Variations in fineness of finish should be expected with variations in condition of grit as between a new or worn belt.

finish

(tampico)

Tampico brush type

Start with No. 4 finish

Pumice pastes made up on the job, or commercially available manufactured compositions

To provide a finish corresponding to standard No. 6 mill finish

Polishing

Pre-No. 7 finish

Made-up cloth wheels or abrasive belts

180

Manufactured types

Grease stick

To provide a finish preparatory to buffing for standard No. 7 finish. May be used as final finish on drawn parts and architectural trim

Polishing

Pre-No. 8 finish

Made-up cloth wheels or abrasive belts

240 to flour

Manufactured types or Turkish emery

Grease stick

To provide a final polished finish preliminary to a mirror finish

Polishing

Satin finishing

Full disk buffs, loose or concentric sewed

Greaseless compounds applied to the wheel as required. These are available in many grit sizes.

A method of producing satin finishes. Fineness of finish varies with composition employed.

Buffing

Cutting

Bias-type, sewed piece, or fill disk buffs

Use stainless steel cutting compound on buff.

Apply as needed to remove any residual polishing lines or other imperfections prior to color buffing for high-luster finishes such as No. 7. May be applied directly to bright cold-rolled strip

Buffing

Coloring

Full disk or bias-type buff

Use stainless steel coloring compound.

For final coloring to bright lusters such as No. 7 and mirror. High speeds for brightness (approximately 50 m/s, or 10,000 sfm). Lower speeds for satinlike finish (approximately 20 to 30 m/s, or 3500 to 6000 sfm)

Source: Ref 2

Before discussing each of these processes, it is useful to review the inherent characteristics of stainless steels that set these materials apart from other metals, and to emphasize the ever-present need to avoid any practices that may result in a loss in corrosion resistance or a reduction in mechanical property values. For example, stainless steels, as a group, are characterized by:

• High tensile strength. This fact determines the power necessary for efficient metal removal in grinding and polishing.

• High surface hardness. This governs the selection of abrasive materials and their cutting behavior.

• Low heat conductivity. This requires attention to precautionary measures that will prevent rapid or excessive rise in temperature, which can cause heat tint discoloration and, possibly, a reduction in corrosion resistance.

• Rapid work hardening. This necessitates proper grinding techniques, especially of austenitic grades in which residual stresses at the metal surface may affect in-service corrosion resistance.

• Need for cleanliness. This means attention must be given to grinding and polishing media to keep it clean and free of iron particles, which can be picked up if used with carbon or alloy steel products.

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