Stainless steels have some characteristics which should be considered when planning finishing operations:
- 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 behaviour.
- Low heat conductivity - This requires attention to precautionary measures that will prevent rapid or excessive rise in temperature which can cause heat tint discolouration, buckling 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 them clean and free of iron particles, which can be picked up if also used on carbon or alloy steel products.
In finishing, the dividing line between fine grinding and polishing is not always clear because both involve metal removal. Coarse or rough grinding, like a fine milling operation, removes excess metal from weld beads, flash on forgings, or run-outs on castings. Coarse or rough grinding is also used in centreless grinding of bars.
It is frequently necessary to employ a series of wheels of decreasing grit size in order to remedy an existing surface condition. The initial grit size is selected on the basis of what coarseness of abrasive is needed to remove the major portion of the unwanted condition. The operation is completed by using a graduated series of successively finer wheels until the desired final finish is attained. The bulk of the job is accomplished with the coarser grits; and the finishing is done with the finer grits. The direction of wheel traverse across the work is changed by 90 degrees with each grit in order to remove residual grinding lines.
Grinding weld beads
Cleaning welds of slag, spatter and discolouration can be achieved with stainless steel wire brushes followed by appropriate flap wheels. Excess metal in weld beads is normally removed by grinding, though an initial cut may be taken with a cold chisel when the size of the bead warrants. Procedures and precautions for grinding weld beads conform to those previously described except that the width of weld beads precludes right angle cutting with successive grit sizes.
Metal adjacent to beads being ground should be protected from flying bits of metal cuttings by covering with shields of paper or other materials. Wet rags may be laid on the work to absorb heat and thus reduce thermal distortion, particularly on light-gauge work.
Solid wheels used for coarse grinding include the vitreous and rubber-bonded or bakelite-bonded types. Grit sizes range from No. 20 to No. 36 for initial or coarse cutting and up to No. 60 for subsequent finishing work. Abrasives commonly employed are the aluminium oxide, zirconia and silicon carbide abrasives. It should be noted that each abrasive media has different characteristics. For a given grit size, each medium can produce a result which is very different from another. Surface speeds for solid wheels usually range around 1500 to 2000 metres per minute.
Care of Wheels
Grinding wheels should be maintained under clean, dry storage conditions. Those intended for use on stainless steel should never be used on other materials, as particles of such materials may contaminate stainless steel surfaces and seriously affect their corrosion resistance.¾ They should be kept dressed and true to shape.
Operation of Wheels
Grinding wheels should never be forced. Allowing them to ride on the work at proper speed (constantly maintained) makes for efficient cutting and checks the generation of unwanted heat. The elimination of heat build-up in localised zones is of major importance with stainless steels. Their relatively low thermal conductivity means slow diffusion of heat and consequently, increased thermal distortion. This is particularly the case with the austenitic grades whose coefficients of thermal expansion are relatively high.
Low metal temperatures avoid heat tinting which becomes evident at about 250 degrees Celsius, and other problems. It is good practice to keep a grinding wheel moving in as many sweeps or strokes as is possible when grinding flat or open work with a portable tool.
Belt Grinding (Linishing)
Belts carrying abrasives of various grit sizes are widely used for grinding and polishing stainless steel surfaces. They are commercially available in many widths ranging from those used for the grinding of wide sheet down to narrow sizes for work on relatively small parts. Examples of the latter are belt grinders and polishers used for cookware and furniture.
With refinement, grinding becomes polishing, either in preparing metal surfaces for subsequent buffing or in the actual preparation of a surface finish, such as a No. 4 polish in which the grit lines are clearly visible. Generally speaking, those operations which serve mainly to remove metal rapidly are considered as grinding, while those in which the emphasis is centred on attaining smoothness are classified as polishing. Grinding employs the coarser grits as a rule while most polishing operations are conducted with grits of 80 and finer.
If polishing is required, start with as fine a grit as possible to reduce finishing steps. There is a wide range of grinding and polishing tools on the market and advice is available from ASSDA members to assist in particular applications.
Polishing operations are conducted with the abrasive mounted either on made-up shaped wheels or belts which provide a resilient backing. The base material may be in either a smooth rolled or a previously ground condition. If the former, the starting grit size may be selected in a range of 80 to 100. If the latter, the initial grit should be one of sufficient coarseness to remove or smooth out any residual cutting lines or other surface imperfections left over from grinding. In either case, the treatment with the initial grit should be continued until a good, clean, uniform, blemish-free surface texture is obtained. The initial grit size to use on a pre-ground surface may be set at about 20 numbers finer than the last grit used in grinding, and changed, if necessary, after inspection. Upon completion of the initial stage of polishing, wheels or belts are changed to provide finer grits. Polishing speeds are generally somewhat higher than those used in grinding. A typical speed for wheel operation is 2500 metres per minute.
The precautions previously referred to under grinding apply also to polishing.
Buffed finishes are produced on stainless steel surfaces by equipment and buffing materials and handling procedures that are generally similar to those employed on other materials.
Buffing operations are generally composed of two stages. The first is known as cutting down buffing while the second is known as colour buffing.
The fine scratches left by previous polishing are cut down with a buff which carries no previously glued-on abrasive. Instead, abrasive is applied intermittently to the buffing wheel by rubbing a cutting compound in bar or stick form against it as it rotates. These cutting compounds are made to formulas containing very fine artificial abrasives (such as aluminium oxide) in the neighbourhood of 300 grit size with a stiff grease as a binder. They adhere to the wheel by impregnating the cloth discs.
Colour buffing is performed in the same manner as cutting down except that a colouring compound is substituted. A varied assortment of compounds (rouges) for use on stainless steel is available commercially; selection is best made by consulting with suppliers. When the desired final finish has been attained either by polishing or buffing, the work should be cleaned with a soft flannel cloth using a product like whiting (pulverised calcium carbonate) or powdered chalk.
Tumbling provides a combined rubbing and abrasive action which can be advantageously employed for the surface treatment of small parts. It can be adjusted to remove burrs, oxide scale and residual flux, and can also be used for light surface treatment such as cleaning, burnishing or colouring.
The action that takes place during the working of a charge is the result of forced movement of a tumbling material against surfaces of parts at a certain speed and for a definite length of time. All three of these factors - tumbling material, speed and duration of the working period - must be considered in combination when setting up to obtain any desired result.
In the painting of stainless steel surfaces, the main requirement to be satisfied is that of providing a good permanent bond. Stainless steel surfaces are dense, hard and smooth, particularly when in cold rolled or polished condition. For that reason, it is usually advisable to roughen them by means of strong pickling or glass-bead blasting.
Pickling etchants are solutions of either hydrochloric or hydrofluoric acid, both of which are commercially available proprietary products. The stainless steel has either a 2D or 2B finish. If 2B, a buff pass with Scotch-BriteTM provides a slightly better surface. Paint companies are best qualified to suggest paint types and procedures for prime and finish coats.
Etching/Glass Bead Blasting
Etching operations are conducted on stainless steel by dry or wet methods. The former include glass-bead and sand blasting and the latter employ chemical solutions. Desired changes in surface texture can be produced by either type of process; and the areas covered can be delineated by the use of masking or stop-off materials. Changes in surface texture produce marked differences in light reflectivity and as a result, contrasting effects are readily obtainable. Such contrasts are most pronounced when the etching is performed on cold rolled, polished or buffed finishes. Dulling to reduce reflectivity of stainless steel components is sometimes called for.
When glass bead or sand blasting, the medium must be clean and free from carbon steel contamination, which can cause stains.
Stainless steel surfaces can be readily blackened, the most common process is by immersion in a molten salt bath of sodium dichromate. This practice is widely used by the automotive industry to blacken stainless steel parts, such as windscreen wipers, and it is used by manufacturers of stainless steel solar collection panels and trivets for domestic gas stoves. The process applies a very thin smooth black oxide film to the surface of all stainless steel types. The film is normally dull black in colour, but it can be brightened by the application of oils or waxes. The film shows no tendency to age or lose colour in service; it is ductile, will not chip or peel, and it is resistant to heat-up to the normal scaling temperature of the stainless steel. A blackened stainless steel can be deformed moderately without harm and the film exhibits good resistance to abrasion.
A black surface can also be produced by black chromium plating.
A proprietary process used for colouring stainless steels entails immersing it in a hot chromic/sulphuric acid solution, followed by a cathodic hardening treatment in another acidic solution. The reaction of the base material with the hot acid produces a transparent film which in itself is basically colourless, but which shows colours through light interference. Colours produced in normal time sequence are bronze, blue, gold, red, purple and green, and within this range a wide variety of shades can be obtained. Black finish is also available. Appearance is also dependent on the nature of the starting surface; matt and satin surfaces produce matt colours, polished surfaces exhibit a high degree of metallic lustre.
Electropolishing of stainless steel is a method of imparting brilliance to its surface by removal of a thin surface layer. Studies have also shown that electropolishing has a beneficial effect on corrosion resistance. The work to be polished is the anode in a cell containing a suitable electrolyte. The process may be considered to be the opposite of electroplating. It has taken its place as an important production tool in the fabrication of the stainless steels along with mechanical polishing processes.
Generally speaking, the method supplements the mechanical polishing methods in that it provides an economical means of brightening many shapes or forms that cannot readily be finished by those means.
Electrolytic methods should not be employed to remove surface blemishes such as scratches, burrs, pits, scale patterns, forging marks and the like; although they will do so if such defects are very shallow. It is important to realise that defects initially present on surfaces may be greatly accentuated. Surface condition before electropolishing governs appearance as finished. These processes are also applicable to cast stainless steels. However, the resulting surfaces will not be as smooth as those on wrought materials unless they are mechanically prepared beforehand.
Matching Mill Finishes
In fabrication using No. 4 polished sheet, it is frequently necessary to refinish weld zones in order to blend them in with the original finish. While it is practically impossible to match a machine polished surface except by duplicating the original polishing, a fairly close blending-in may be obtained by skillful use of manual methods such as the following:
- If the original machine polished lines are parallel to the line of the weld, the bead can be dressed down by grinding with a hard or soft wheel and then finished off by polishing with No. 80 and No. 120 (and maybe No. 150) grit on a made-up wheel driven by a portable machine. The traversing of this wheel should be kept in line with the run of the bead so that its cut lines will be kept parallel to those of the original machine polished surface. Care should be taken to bring the metal of the joint flush with that of the base metal in order to avoid residual ridges or grooves. Because a grit of any original size will leave lines that vary with the amount of use it has received, it is good practice to run separate forerunner samples before starting on finish work.
- If the machine polished lines are not parallel to the lines of the weld, the final manual polishing should be conducted in the direction of the machine polishing. If the original polish lines on the two sides of a joint are not parallel to each other (say parallel to the bead on one side and perpendicular to the bead on the other), the best procedure would be to run the polishing cut lines along (not across) the bead. On work of this kind, abrasive paper (or pieces cut from a belt) may be backed up with a wood (or softer) block and guided by hand along the line of the weld joint.
It is also possible to repolish No. 8 finish to match the mill finish by polishing with suitably fine abrasives; a BA finish can also be repolished although there may be some colour differences.