What Is Stainless Steel?
Stainless steels are iron-based alloys containing a minimum of about 10.5% chromium; this forms a protective self-healing oxide film, which is the reason why this group of steels has their characteristic "stainlessness" or corrosion resistance. The ability of the oxide layer to heal itself means that the steel is corrosion resistant, no matter how much of the surface is removed. This is not the case when carbon or low alloy steels are protected from corrosion by metallic coatings such as zinc or cadmium or by organic coatings such as paint.
Although all stainless steels depend on the presence of chromium, other alloying elements are often added to enhance their properties. The categorization of stainless steels is unusual amongst metals in that it is based upon the nature of their metallurgical structure - the terms used denote the arrangement of the atoms which make up the grains of the steel, and which can be observed when a polished section through a piece of the material is viewed at high magnification through a microscope. Depending upon the exact chemical composition of the steel the microstructure may be made up of the stable phases austenite or ferrite, a "duplex" mix of these two, the phase martensite created when some steels are rapidly quenched from a high temperature, or a structure hardened by precipitated micro-constituents.
Types of Stainless Steel
Austenitic Stainless Steels
This group contains at least 16% chromium and 6% nickel (the basic grade 304 is referred to as 18/8) and range through to the high alloy or "super austenitics" such as 904L and 6% molybdenum grades. Additional elements can be added such as molybdenum, titanium or copper, to modify or improve their properties, making them suitable for many critical applications involving high temperature as well as corrosion resistance. This group of steels is also suitable for cryogenic applications because the effect of the nickel content in making the steel austenitic avoids the problems of brittleness at low temperatures, which is a characteristic of other types of steel.
Ferritic Stainless Steels
These are plain chromium (10.5 to 18%) grades such as Grade 430 and 409. Their moderate corrosion resistance and poor fabrication properties are improved in the higher alloyed grades such as Grades 434 and 444 and in the proprietary grade 3CR12
Martensitic Stainless Steels
Martensitic stainless steels are also based on the addition of chromium as the major alloying element but with a higher carbon and generally lower chromium content (e.g. 12% in Grade 410 and 416) than the ferritic types; Grade 431 has a chromium content of about 16%, but the microstructure is still martensite despite this high chromium level because this grade also contains 2% nickel.
Duplex Stainless Steels
Duplex stainless steels such as 2304 and 2205 (these designations indicate compositions of 23% chromium, 4% nickel and 22% chromium, 5% nickel but both grades contain further minor alloying additions) have microstructures comprising a mixture of austenite and ferrite. Duplex ferritic - austenitic steels combine some of the features of each class: they are resistant to stress corrosion cracking, albeit not quite as resistant as the ferritic steels; their toughness is superior to that of the ferritic steels but inferior to that of the austenitic steels, and their strength is greater than that of the (annealed) austenitic steels, by a factor of about two. In addition the duplex steels have general corrosion resistances equal to or better than 304 and 316, and in general their pitting corrosion resistances are superior to 316. They suffer reduced toughness below about –50°C and after exposure above 300°C, so are only used between these temperatures.
Characteristics of Stainless Steels
The characteristics of the broad group of stainless steels can be viewed as compared to the more familiar plain carbon "mild" steels. As a generalization the stainless steels have:
- Higher work hardening rate
- Higher ductility
- Higher strength and hardness
- Higher hot strength
- Higher corrosion resistance
- Higher cryogenic toughness
- Lower magnetic response (austenitic only)
- Must retain corrosion resistant surface in the finished product.
The surface of Stainless Steel is actually and extremely thin but stable and passive Chromium rich oxide film, on which Stainless Steel relies for its excellent corrosion resistance. The surface finish on Stainless Steel should therefore be developed and maintained to ensure this vital property, and also for the secondary reason of the pleasing aesthetic appearance of Stainless Steel.
STANDARD MILL FINISHED - FLAT ROLLED PRODUCTS
The Standard Mill Surface Finished are laid down in Specifications (BS 1449, Part 4, and the Committee of Stainless Steel Producers, American Iron & Steel Institute).
The finished are designated by a system of numbers, and these are broadly described hereunder relative to the finishing operations employed. It should be remembered that different grades of Stainless Steel can result in a variation of visual appearance for the same finishing operation. The thickness can also have an effect, generally the thinner the material the smoother the surface finish.The thicker gauge sizes of Stainless Steel are hot rolled. This is done at high temperatures and will always result in a scaled surface.
Stainless Steel Flat Product is supplied in the annealed ie fully softened condition. This is also a high temperature operation and unless carried out in a very closely controlled inert atmosphere, will result in oxidation (scaling) of the surface.
The scale is usually removed by a pickling process, that is the removal of the scale by use of suitable acids, and the passivated by the use of Nitric Acid.
No. 0 Finish
Also referred to as Hot Rolled Annealed (HRA). The plate is hot rolled to required thickness, and then annealed. No pickling or passivation operations are effected, resulting in a scaled black finish.
This does not develop the fully corrosion resistant film on the Stainless Steel, and except for certain high temperature heat resisting applications, this finish is unsuitable for general end uses.
No 1 Finish
Plate is hot rolled, annealed, pickled and passivated. This results in a dull, slightly rough surface; quite suitable for industrial applications which generally involve the range of plate thicknesses. Grinding marks may be visible in isolated areas.
Some of the thinner thicknesses within the plate range are Cold Rolled; but Sheet, Coil and Strip gauges are produced by Cold Rolling, ie rolled without and heating of the material. Cold Rolling hardens the material, and the thinner sizes may have to be subjected to an intermediate anneal and pickle, or bright annealed, during the reduction of thickness to final gauge.
The starting material for Cold Rolling always has a No. 1 finish. Cold Rolled material is supplied with the following standard mill finishes.
No 2D Finish
A no. 1 Finish after being Cold Rolled, Annealed, Pickled and Passivated. This results in a uniform dull matt finish, superior to a No. 1 Finish.
Suitable for industrial application, and eminently suitable for severe deep drawing as the dull surface, (which may be polished after fabrication) retains the lubricant during the drawing operation.
No 2B Finish
A 2D Finish is given a subsequent light skin pass cold rolling operation between polished rolls. This is the most common finish produced and called for on sheet material. It is brighter than 2D and is semi-reflective. It is commonly used for most deep drawing operations, and is more easily polished to the final finished required than is a 2D finish.
No 2BA Finish
This is more commonly referred to as a BRIGHT ANNEALED (BA) FINISH. Material with a No. 1 finish is Cold Rolled using highly polished rolls in contact with the steel surface. This smooths and brightens the surface.
The smoothness and reflectiveness of the surface improves as the material is rolled to thinner and thinner sizes. Any annealing which needs to be done in order to effect the required reduction in gauge, and the final anneal, is effected in a very closely controlled inert atmosphere. No oxidation or scaling of the surface therefore occurs and there is no need for additional pickling and passivating.
The final surface developed can have "MIRROR" type finish similar in appearance to the highly polished No. 7 and No. 8 Finishes.
Much of the 2B Finish sheet imported is not a true 2B Finish. Mills which operate bright annealing facilities will often carry out all the annealing operations of Cold Rolled material in such facilities. This leads to a superior "2B" finish as no oxidation or scaling takes place during the annealing operation, even though the actual rolling may be effected on polished rolls as for normal 2B Finish, but not highly polished as would be needed to produce a BA finish.
The following finishes are all mechanically produced polished finishes. As well as being standard mill finishes, they are also applied to stainless steel articles and components to meet the required aesthetic criteria. It should be appreciated that factors such as hand polishing vs. mechanical polishing; polishing a flat product as against a component of complex shape; thickness and composition of material can affect the visual appearance of the final surface.
No. 3 Finish
This is a ground unidirectional uniform finish obtained with 80 - 100 grit abrasive. It is a good intermediate or starting surface finish for use in such instances where the surface will require further polishing operations to a finer finish after subsequent fabrication or forming.
No. 4 Finish
This is a ground unidirectional finish obtained with 150 grit abrasive. It is not highly reflective, but is a good general purpose finish on components which will suffer from fairly rough handling in service (eg restaurant equipment).
No. 6 Finish
These finishes are produced using rotating cloth mops (Tampico fibre, muslin or linen) which are loaded with abrasive paste. The finish depends on how fine and abrasive is used, the uniformity and finish of the original surface. The finish has a non-directional texture of varying reflectiveness. "Satin Blend" is an example of such a finish.
No. 7 Finish
This is a buffed finish having a high degree of reflectiveness. It is produced by progressively using finer and finer abrasives and finishing with Buffing compounds. Some fine scratches (grit lines) may remain from the original starting surface.
No. 8 Finish
This is produced in an equivalent manner to a No. 7 Finish, the final operations being done with extremely fine buffing compounds. The final surface is blemish free with a high degree of image clarity, and is the true mirror finish.
The finer polished finishes (No. 4, No. 6, No. 7 and No. 8) are generally only produced one side of the sheet, the reverse side being either a 2B or No. 3 Finish.
Commonly Asked Questions?
What gauge of stainless steel should you use?
We recommend 16 gauge stainless steel for all our residential countertops. 14 and 12 gauge material is available for industrial or harsh applications.
Is stainless steel more expensive than laminate?
Yes. On average, stainless steel is 3-4 times more costly than laminate but unlike laminate, will last almost indefinitely.
Is stainless steel more expensive than solid surface material?
No. Stainless steel is competitively priced compared to solid surface materials and is less that marble and granite.
Stainless Steel Care
Do's and Don'ts that all users of stainless steel countertops and sinks should heed to:
- Clean your stainless steel sink and countertop frequently with warm water, soap, or ammonia using a sponge or soft cloth. Thoroughly rinse with water, and wipe dry with a soft cloth to eliminate the possibility of water stains.
- Household cleaning products that contain chlorides such as Chlorine bleach or acids can stain and corrode the surface of stainless steel if not thoroughly rinsed with clean water.
- Never leave acidic foods lay in a sink or on a countertop for long periods of time as the acid will stain the surface.
- Always use non-abrasive type pads, a sponge or soft cloth when cleaning stainless steel, as steel wool pads can leave steel particles on the surface and quickly form rust deposits.
- When scouring a sink or countertop with an aggressive pad or mild cleaner, rub in the same direction as the grain to avoid cross scratching.