The following information will provide you with a comprehensive understanding of the different grades of stainless steel, making it easier for you to choose the appropriate type for your needs.
Presently, the most widely used stainless steels are 304 and 316.
In terms of cost, 304 stainless steel is significantly less expensive than 316 stainless steel.
You can select the appropriate type of stainless steel based on your specific requirements.
Stainless Steel Grades
200 series stainless steel
Contain chrome, nickel, and manganese, which belongs to austenitic stainless steel.
300 series stainless steel
Contain chrome, and nickel, which also belongs to austenitic stainless steel.
It has good malleability and is applied in forming products.
It can also be quickly hardened by machining.
The abrasion resistance and fatigue strength are superior to 304 stainless steel.
302 stainless steel
The corrosion resistance is the same as that of 304 stainless steel, however, it has higher strength because of its high carbon content.
303 stainless steel
The 303 stainless steel can be machined more easily than 304 stainless steel by adding a small amount of sulfur and phosphorus.
304 stainless steel
It belongs to the general model, which is 18/8 stainless steel. GB grade is 0Cr18Ni9.
309 stainless steel
It has better temperature resistance than 304 stainless steel.
316 stainless steel
After 304 stainless steel, 316 stainless steel is the second most widely used steel, primarily in the food industry and in surgical equipment.
The addition of the molybdenum element gives it a special anti-corrosive structure.
Due to its better anti-chloride corrosion resistance compared to 304 stainless steel, it is also referred to as “Marine steel.”.
SS316 is commonly used in devices for recovering nuclear fuel, and grade 18/10 stainless steel is also often used in this application level.
In addition to the risk of the corrosion of the weld joint is reduced because of adding the titanium element, the other performance is similar to 304 stainless steel.
400 series stainless steel
Belongs to ferrite stainless steel and martensitic stainless steel.
408 stainless steel
Good heat resistance, weak corrosion resistance, 11% Cr, 8% Ni.
409 stainless steel
The cheapest model (UK & US), commonly used as a car exhaust pipe, is ferritic stainless steel (chrome steel).
410 stainless steel
Martensite stainless steel (high-strength chromium steel), has good wear resistance and poor corrosion resistance.
416 stainless steel
The processing properties of the material are improved by adding sulfur.
420 stainless steel
“Blade grade” martensitic steel, the earliest stainless steel, similar to Brinell high chromium steel.
It is also used for surgical cutting tools, which can be very bright.
430 stainless steel
Ferritic stainless steel, for the purpose of decorative, for example, car accessories.
The good forming property, but poor temperature endurance and corrosion resistance.
440 stainless steel
The most common application of 440 stainless steel is the razor blade.
There are three commonly used models: 440A, 440B, 440C, and 440F stainless steel (easily processed)
500 series stainless steel
Belongs to heat-resistant chromium alloy steel.
600 series stainless steel
Belongs to martensite precipitation hardening stainless steel
630 stainless steel
The most commonly used type of precipitation hardened stainless steel, also been called 17-4;
It contains 17% Cr, 4% Ni.
Stainless Steel Classification
The main chemical composition of stainless steel can be divided into several categories, including chromium stainless steel, chromium-nickel stainless steel, chromium-manganese-nitrogen stainless steel, chromium-nickel-molybdenum stainless steel, ultra-low carbon stainless steel, high molybdenum stainless steel, and high-purity stainless steel.
Classification based on steel properties and application includes nitric acid (nitric grade) stainless steel, corrosion-resistant stainless steel, stress stainless steel, high-strength stainless steel, among others.
In terms of functional characteristics, stainless steel can be divided into low-temperature stainless steel, non-magnetic stainless steel, easy-cutting stainless steel, and ultra-plastic stainless steel.
It is also classified based on its metallographic structure, which includes ferrite (F) stainless steel, martensite stainless steel (M), austenitic stainless steel, austenitic-ferritic duplex stainless steel (A-F), austenite-martensite duplex stainless steel (A-M), and precipitation-hardening stainless steel (PH).
Stainless Steel Mechanical Properties
Comparison of mechanical properties of stainless steel
The above classification only considers the matrix structure.
In addition to the three basic types of stainless steel, it also includes composite stainless steel, such as martensite-ferrite and austenite-ferrite, as well as precipitation-hardening stainless steel, such as martensite-carbide stainless steel.
Detailed Introduction to Stainless Steel
Low-carbon chromium stainless steel with a chromium content of more than 14%, chromium stainless steel with a chromium content of 27% and above, and with additional elements such as molybdenum, titanium, niobium, silicon, aluminum, tungsten, and vanadium.
In the chemical composition, elements that form ferrite hold a dominant position, and the matrix structure is primarily iron-based.
This type of steel is known as ferritic, with a quenched (solid solution) form, and small amounts of carbide and intermetallic compounds can be observed in the structures of annealing and aging.
Examples of such steels include Cr17, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, and Cr28.
Ferritic stainless steel is relatively corrosion-resistant and oxidation-resistant due to its high chromium content, but it has poor mechanical properties and processability.
It is mostly used in anti-acid structures and as an antioxidant steel.
This type of steel is in the Y+A (or δ) phase at high temperatures, and transforms to the Y-M phase when it approaches cold conditions.
It retains ferrite and exists as martensite and ferrite at normal temperatures.
The amount of ferrite in the structure can vary from a few percent to several tens percent, depending on the composition and heating temperature.
Examples of this type of steel include 0Cr13, 1Cr13, 2Cr13 with chromium near the upper limit and carbon near the lower limit, Cr17Ni2 steel, Cr17W4 steel, as well as many modified 12% chromium hot-strength steels based on 1Cr13 (which are also known as heat-resistant stainless steels), such as Cr11MoV, Cr12WMoV, Cr12W4MoV, 18Cr12WMoVNb, etc.
Ferritic-martensitic steel can exhibit partial hardening and obtain mechanical properties, but these are greatly influenced by the content and distribution of ferrite.
The chromium content in this type of steel is typically between 12-14% and 15-18%.
The former has the ability to resist atmospheric and weak corrosive media, and has good damping and a small linear expansion coefficient.
The latter type has comparable corrosion resistance to ferritic acid steel with the same chromium content, but still retains some of the disadvantages of high chromium ferritic steel.
Under normal quenching temperatures, martensitic steel is in the Y phase, but this phase only remains stable at high temperatures. The M phase is commonly stable around 300℃ and transforms into martensite upon cooling.
This type of steel includes 2Cr13, 2Cr13Ni2, 3Cr13, and some modified 12% chromium hot-strengthened steel, such as 13Cr14NiWVBA and Cr11Ni2MoWVB steel.
The mechanical properties, corrosion resistance, process performance, and physical properties of martensitic stainless steel are similar to those of 2-14% chromium ferrite-martensitic stainless steel.
Because there is no free ferrite in the structure, its mechanical performance is higher than the aforementioned steel, but its thermal sensitivity to heat treatment is lower.
Fe-C alloy contains 0.83% carbon.
In stainless steel, the S points are shifted to the left due to chromium. Steel with 12% chromium and 0.4% or more carbon, as well as steel with 18% chromium and 0.3% or more carbon, belong to hypereutectoid steel.
This type of steel is heated at normal quenching temperature, and the secondary carbide cannot be completely dissolved in austenite, so the hardened structure is composed of martensite and carbide.
There are not many grades of stainless steel that fall into this category, but some stainless steels with higher carbon, such as 4Cr13, 9Cr18, 9Cr18MoV, and 9Cr17MoVCo steel.
If quenched under low temperature, the 3Cr13 steel with carbon close to the upper limit may also have such a structure.
Due to its high carbon content, even though the above three grades of steel contain more chromium, their corrosion resistance is only equivalent to that of stainless steel with 12-14% chromium.
This type of steel is mainly used for parts that require high hardness and good wear resistance, such as cutting tools, bearings, springs, and medical instruments.
This type of steel has a high concentration of stabilizing elements and a broad Y-phase zone at high temperatures.
Upon cooling, the Ms point falls below room temperature, resulting in an austenitic structure at normal temperatures.
This category includes chrome-nickel stainless steel such as 18-8, 18-12, 25-20, and 20-25Mo, as well as low-nickel stainless steel that uses manganese instead of some nickel and nitrogen, including Cr18Mn10Ni5, Cr13Ni4Mn9, Cr17Ni4Mn9N, and Cr14Ni3Mn14Ti steel.
Austenitic stainless steel has many benefits, including the ability to be strengthened by cold deformation methods through strain hardening, despite poor heat treatment properties.
However, it is also susceptible to intercrystalline corrosion and stress corrosion, which can be mitigated through the use of alloy additives and process measures.
Due to the limited amount of stable austenite elements, the steel does not have a pure austenitic structure at room temperature or high temperatures, resulting in an austenitic-ferritic phase state. The composition and amount of ferrite can vary greatly depending on heating temperature.
Many types of stainless steel fall into this category, including low-carbon 18-8 nickel-chrome steel, 18-8 nickel-chrome steel with titanium, niobium, and molybdenum, with ferrite being particularly visible in the structure of cast steel.
Other examples include chromium-manganese stainless steel with more than 14-15% chromium and less than 0.2% carbon (such as Cr17Mn11) and most of the chromium-manganese-nitrogen stainless steel that have been studied and applied in industry.
Compared to pure austenitic stainless steel, this type of steel has several advantages, including higher yield strength, increased resistance to intergranular corrosion, reduced sensitivity to stress corrosion, lower tendency for hot cracking during welding, and good casting fluidity.
However, it also has several disadvantages, such as poor pressure processing performance, high susceptibility to pitting corrosion, and tendency to exhibit c-phase brittleness and weak magnetism under strong magnetic field conditions.
These advantages and disadvantages are directly related to the presence of ferrite in the structure.
The Ms point of this steel is lower than room temperature, making it easy to form and weld for austenite after solid solution treatment.
Martensitic transformation can usually be achieved through two processes.
- After the solid solution treatment, heating at 700-800℃ causes the austenitic body to transform into a metastable state due to the precipitation of carbonized chromium. The Ms point then rises above room temperature, resulting in the transformation of the austenite into martensite during the cooling process.
- Direct cooling between the Ms and Mf points after the solid solution treatment results in the transformation of the austenite into martensite as well.
The second method provides better corrosion resistance, but the solid solution treatment and cryogenic interval time must not be too long, otherwise, the cold strengthening effect will be reduced due to the aging stability of the austenite.
After the treatment, an aging process at 400-500 degrees is performed to enhance the intermetallic compound.
Examples of steel grades that fall into this category include 17Cr-7Ni-A1, 15Cr-9Ni-A1, 17Cr-5Ni-Mo, and 15Cr-8Ni-Mo-A1.
Austenite-martensitic steel, also known as austenitic-maraging stainless steel, is a new type of stainless steel developed and applied starting in the 1950s.
It is also referred to as half austenitic precipitation-hardening stainless steel due to the presence of ferrite in addition to austenite and martensite in its structure.
These steels are characterized by their high strength (C can reach 100-150) and good heat strengthening performance, but their corrosion resistance is lower than that of standard austenitic stainless steel due to the low chromium content and chromium carbide precipitation during heat treatment.
The high strength is obtained by sacrificing some of the corrosion resistance and other properties, such as non-magnetism.
Austenite-martensitic steel is primarily used in the aviation and rocket missile industries, but is not widely used in machinery manufacturing and is sometimes classified as a type of ultra-high strength steel.