Stainless Steel Grades

Choosing stainless steel products can be difficult and confusing. We have developed this guide to assist and educate you in choosing the best stainless steel products to fit your needs.

16 Gauge VS 18 Gauge: What's the difference?
16 gauge stainless steel is a higher quality grade than 18 gauge. This is because 16 gauge is thicker and heavier making it stronger and more durable. 16 gauge is ideal for every day heavy duty use making it a worthwhile investment.

304 VS 430: What's the difference?
Type 304 stainless steel (also known as 18/8 stainless steel) is an austenitic stainless steel. It has higher nickel content than its ferritic 430 counterpart. Type 304 is non-magnetic, more resistant to corrosion than 430, and is more durable. Type 304 is immune to foodstuffs, sterilizing solutions, most organic chemicals and dyestuffs, and a wide variety of inorganic chemicals. Because 304 can withstand the corrosive action of various acids found in fruits, meats, milk, and vegetables, it is used for sinks, tabletops, stoves, refrigerators, and steam tables. It is also used in numerous other utensils such as cooking appliances, pots, pans, and flatware.

Type 430 has good resistance to a wide variety of corrosives including nitric acid and some organic acids. It attains its maximum corrosion resistance when in the highly polished or buffed condition. In general, its resistance to pitting and crevice corrosion resistance is close to that of Type 304. Stress corrosion cracking resistance of Grade 430 is very high, as it is for all ferritic grades.

Types of stainless steel
There are different types of stainless steels: when nickel, for instance is added the austenite structure of iron is stabilized. This crystal structure makes such steels non-magnetic and less brittle at low temperatures. For higher hardness and strength, carbon is added. When subjected to adequate heat treatment these steels are used as razor blades, cutlery, tools etc.

Significant quantities of manganese have been used in many stainless steel compositions. Manganese preserves an austenitic structure in the steel as does nickel, but at a lower cost.

Stainless steels are also classified by their crystalline structure:
* Austenitic stainless steels comprise over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy. A typical composition is 18% chromium and 10% nickel, commonly known as 18/10 stainless is often used in flatware. Similarly 18/0 and 18/8 is also available.

* Super austenitic stainless steels, such as alloy AL-6XN and 254SMO, exhibit great resistance to chloride pitting and crevice corrosion due to high Molybdenum contents (>6%) and nitrogen additions and the higher nickel content ensures better resistance to stress-corrosion cracking over the 300 series. The higher alloy content of "Super austenitic" steels means they are fearsomely expensive and similar performance can usually be achieved using duplex steels at much lower cost.

* Ferritic stainless steels are highly corrosion resistant, but far less durable than austenitic grades and cannot be hardened by heat treatment. They contain between 10.5% and 27% chromium and very little nickel, if any. Most compositions include molybdenum; some, aluminum or titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni.

* Martensitic stainless steels are not as corrosion resistant as the other two classes, but are extremely strong and tough as well as highly machineable, and can be hardened by heat treatment. Martensitic stainless steel contains chromium (12-14%), molybdenum (0.2-1%), no nickel, and about 0.1-1% carbon (giving it more hardness but making the material a bit more brittle). It is quenched and magnetic. It is also known as "series-00" steel.

* Duplex stainless steels have a mixed microstructure of austenite and ferrite, the aim being to produce a 50:50 mix although in commercial alloys the mix may be 60:40. Duplex steel have improved strength over austenitic stainless steels and also improved resistance to localized corrosion particularly pitting, crevice corrosion and stress corrosion cracking. They are characterized by high chromium and lower nickel contents than austenitic stainless steels.

Stainless Steel Grades
The AISI defines the following grades among others:
- 200 Series
Austenitic iron-chromium-nickel-manganese alloys
- 300 Series
Austenitic iron-chromium-nickel alloys
Type 301: Highly ductile, for formed products. Also hardens rapidly during mechanical working.
Type 303: Free machining version of 304 via addition of sulfur
Type 304: The most common; the classic 18/8 stainless steel.
Type 316: The next most common; for food and surgical stainless steel uses; Alloy addition of molybdenum prevents specific forms of corrosion. Also known as "marine grade" stainless steel due to its increased ability to resist saltwater corrosion compared to type 304. SS316 is often used for building nuclear reprocessing plants.

- 400 Series
Ferritic and martensitic alloys
Type 408: Heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.
Type 409: Cheapest type; used for automobile exhausts; ferritic (iron/chromium only).
Type 410: Martensitic (high-strength iron/chromium).
Type 420: "Cutlery Grade" martensitic; similar to the Brearley's original "rustless steel". Also known as "surgical steel".
Type 430: Decorative, e.g. for automotive trim; ferritic.
Type 440: A higher grade of cutlery steel, with more carbon in it, which allows for much better edge retention when the steel is heat treated properly.

- 600 Series
Martensitic precipitation hardening alloys
Type 630: Most common PH stainless, better known as 17-4; 17% chromium, 4% nickel

Stainless steel finishes
Standard mill finishes can be applied to flat rolled stainless steel directly by the rollers and by mechanical abrasives. Steel is first rolled to size and thickness and then annealed to change the properties of the final material. Any oxidation that forms on the surface (scale) is removed by pickling, and the passivation layer is created on the surface. A final finish can then be applied to achieve the desired aesthetic appearance.

No. 0 - Hot Rolled Annealed, thicker plates
No. 1 - Hot rolled, annealed and passivated
No, 2D - cold rolled, annealed, pickled and passivated
No, 2B - same as above with additional pass through polished rollers
No, 2BA - Bright Anealed (BA) same as above with highly polished rollers
No. 3 - coarse abrasive finish applied mechanically
No. 4 - fine abrasive finish*
No. 6 - matt finish
No. 7 - reflective finish
No. 8 - mirror finish

*DLF Products are #4 finished

Stainless Steel Properties
Stainless steels have higher resistance to oxidation (rust) and corrosion in many natural and man made environments, however, it is important to select the correct type and grade of stainless steel for the particular application.

High oxidation resistance in air at ambient temperature is normally achieved with additions of more than 12% (by weight) chromium. The chromium forms a passivation layer of chromium (III) oxide (Cr2O3) when exposed to oxygen. The layer is too thin to be visible, meaning the metal stays shiny. It is, however, impervious to water and air, protecting the metal beneath. Also, when the surface is scratched this layer quickly reforms. This phenomenon is called passivation by materials scientists, and is seen in other metals, such as aluminum. When stainless steel parts such as nuts and bolts are forced together, the oxide layer can be scraped off causing the parts to weld together. When disassembled, the welded material may be torn and pitted, an effect that is known as galling.

Stainless steel's resistance to corrosion and staining, low maintenance, relative inexpense, and familiar luster make it an ideal base material for a host of commercial applications. There are over 150 grades of stainless steel, of which fifteen are most common. The alloy is milled into sheets, plates, bars, wire, and tubing to be used in cookware, cutlery, hardware, surgical instruments, major appliances, industrial equipment, and building material in skyscrapers and large buildings.

Stainless steel is 100% recyclable. In fact, over 50% of new stainless steel is made from re-melted scrap metal, rendering it a somewhat eco-friendly material.

Corrosion
Even a high-quality alloy can corrode under certain conditions. Because these modes of corrosion are more exotic and their immediate results are less visible than rust, they often escape notice and cause problems among those who are not familiar with them.

Pitting Corrosion
Passivation relies upon the tough layer of oxide described above. When deprived of oxygen (or when another species such as chloride competes as an ion), stainless steel lacks the ability to re-form a passivating film. In the worst case, almost all of the surface will be protected, but tiny local fluctuations will degrade the oxide film in a few critical points. Corrosion at these points will be greatly amplified, and can cause corrosion pits of several types, depending upon conditions. While the corrosion pits only nucleate under fairly extreme circumstances, they can continue to grow even when conditions return to normal, since the interior of a pit is naturally deprived of oxygen. In extreme cases, the sharp tips of extremely long and narrow pits can cause stress concentration to the point that otherwise tough alloys can shatter, or a thin film pierced by an invisibly small hole can hide a thumb sized pit from view. These problems are especially dangerous because they are difficult to detect before a part or structure fails. Pitting remains among the most common and damaging forms of corrosion in stainless alloys, but it can be prevented by ensuring that the material is exposed to oxygen (for example, by eliminating crevices) and protected from chloride wherever possible.

Pitting corrosion can occur when stainless steel is subjected to high concentration of chloride ions (for example, sea water) and moderately high temperatures.

Weld decay and knife line attack
Due to the elevated temperatures of welding or during improper heat treatment, chromium carbides can form in the grain boundaries of stainless steel. This chemical reaction robs the alloy of chromium in the zone near the grain boundary, making those areas much less resistant to corrosion. This creates a galvanic couple with the well-protected alloy nearby, which leads to weld decay (corrosion of the grain boundaries near welds) in highly corrosive environments. Special alloys, either with low carbon content or with added carbon "getters" such as titanium and niobium (in types 321 and 347, respectively), can prevent this effect, but the latter require special heat treatment after welding to prevent the similar phenomenon of knife line attack. As its name implies, this is limited to a small zone, often only a few micrometers across, which causes it to proceed more rapidly. This zone is very near the weld, making it even less noticeable. Modern steel making technologies largely avoid these problems by controlling the carbon content of stainless steels to <0.3% and historically such grades were referred to as "L" grades such as 316L; in practice most stainless steels are now produced at these low carbon contents.

Rouging
Stainless steel can actually rust quite rapidly if it fails to form its protective oxide layer. This tends to happen when the stainless has had carbon steel forced into its surface, as by being dragged over carbon steel during installation, brushing with carbon steel, grinding with a contaminated wheel, or temporary welds to carbon steel.

Inter-granular corrosion
This is a largely historical problem related to the high carbon contents of steels from the past, for modern steels it is very rarely an issue.

Some compositions of stainless steel are prone to inter-granular corrosion when exposed to certain environments. When heated to around 700 °C, chromium carbide forms at the inter-granular boundaries, depleting the grain edges of chromium impairing their corrosion resistance. Steel in such condition is called sensitized. Steels with carbon content 0.06% undergo sensitization in about 2 minutes, while steels with carbon content under 0.02% are not sensitive to it.

It is possible to reclaim sensitized steel by heating it to above 1000 °C and holding at this temperature for a given period of time dependent on the mass of the piece, followed by quenching it in water. This process dissolves the carbide particles, and then keeps them in solution.

It is also possible to stabilize the steel to avoid this effect and make it welding-friendly. Addition of titanium, niobium and/or tantalum serves this purpose; titanium carbide, niobium carbide and tantalum carbide form preferentially to chromium carbide, protecting the grains from chromium depletion. Use of extra-low carbon steels is another method and modern steel production usually ensures a carbon content of <0.03% at which level inter-granular corrosion is not a problem. Light-gauge steel also does not tend to display this behavior, as the cooling after welding is too fast to cause effective carbide formation.

Crevice Corrosion
In the presence of reducing acids or exposition to reducing atmosphere, the passivity layer protecting steel from corrosion can break down. This wear can also depend on the mechanical construction of the parts, e.g. under gaskets, in sharp corners, or in incomplete welds. Such crevices may promote corrosion, if their size allows penetration of the corroding agent but not its free movement. The mechanism of crevice corrosion is similar to pitting corrosion, though it happens at lower temperatures.

Stress corrosion cracking
Stress corrosion cracking is a rapid and severe form of stainless steel corrosion. It forms when the material is subjected to tensile stress and some kinds of corrosive environments, especially chloride-rich environments (sea water) at higher temperatures. The stresses can be a result of the service loads, or can be caused by the type of assembly or residual stresses from fabrication (e.g. cold working); the residual stresses can be relieved by annealing. This limits the usefulness of stainless steel for containing water with higher than little parts per million of chlorides at temperatures above 50 °C.
Stress corrosion cracking applies only to austenitic stainless steels and depends on the nickel content.

Sulphide stress cracking
Sulphide stress cracking is an important failure mode in the oil industry, where the steel comes into contact with liquids or gases with considerable hydrogen sulfide content, e.g. sour gas. It is influenced by the tensile stress and is worsened in the presence of chloride ions. Very high levels of hydrogen sulfide apparently inhibit the corrosion. Rising temperature increases the influence of chloride ions, but decreases the effect of sulfide, due to its increased mobility through the lattice; the most critical temperature range for sulphide stress cracking is between 60-100 °C.

Galvanic corrosion
Galvanic corrosion occurs when a galvanic cell is formed between two dissimilar metals. The resulting electrochemical potential then leads to formation of an electric current that leads to electrolytic dissolving of the less noble material. This effect can be prevented by electrical insulation of the materials, eg. by using rubber or plastic sleeves or washers, keeping the parts dry so there is no electrolyte to form the cell, or keeping the size of the less-noble material significantly larger than the more noble ones (eg. stainless-steel bolts in an aluminum block won't cause corrosion, but aluminum rivets on stainless steel sheet would rapidly corrode.

Contact corrosion
Contact corrosion is a combination of galvanic corrosion and crevice corrosion, occurring where small particles of suitable foreign material are embedded to the stainless steel. Carbon steel is a very common contaminant here, coming from nearby grinding of carbon steel or use of tools contaminated with carbon steel particles. The particle forms a galvanic cell, and quickly corrodes away, but may leave a pit in the stainless steel from which pitting corrosion may rapidly progress. Some workshops therefore have separate areas and separate sets of tools for handling carbon steel and stainless steel, and care has to be exercised to prevent direct contact between stainless steel parts and carbon steel storage racks.

Particles of carbon steel can be removed from a contaminated part by passivity with dilute nitric acid, or by pickling with a mixture of hydrofluoric acid and nitric acid.