Stainless steel is a low carbon steel containing chromium; It is this added chromium that endows ordinary steel with unique pollution and corrosion resistance. Chromium in steel can form a rough, invisible and corrosion-resistant chromium oxide film on the surface of steel. If the material is mechanically or chemically damaged, the film will heal itself (as long as oxygen is present). With the addition of chromium and other elements such as molybdenum, nickel and nitrogen, steel has higher corrosion resistance and other properties.
Stainless steel is generally divided into 5 categories. Each is identified by the alloying elements that affect its microstructure and is named after it.
Martensitic stainless steel
Ferritic stainless steel
austenitic stainless steel
Duplex (Ferritic Austenitic) stainless steel
Precipitation hardened (PH) stainless steel
Martensitic stainless steel grades are a group of corrosion-resistant and hardenable stainless steel alloys (using heat treatment). All martensitic grades are nickel free direct chromium steels. All these grades are magnetic. Martensite grade is mainly used where hardness, strength and wear resistance are required.
Type 410: a basic martensite with low alloy content. Its cost is relatively low, and it is a universal and heat treatable stainless steel. It is usually used in places with less severe corrosion (such as air, water, some chemicals and food acids). Applications of the product include components requiring bonding strength and corrosion resistance, such as fasteners.
Type 410S: maintains a lower carbon content than type 410, but provides improved weldability and lower hardenability. This is a general-purpose corrosion-resistant and heat-resistant chromium steel.
Type 414: this type increases nickel content (2%) to improve corrosion resistance. Typical applications include springs and tableware.
Type 416: phosphorus and sulfur added to this type can improve processability. Typical applications include screw mechanical components.
Type 420: adding this type of carbon helps to improve mechanical properties. Typical applications include surgical instruments.
431 type: increased chromium content, stronger corrosion resistance and good mechanical properties. Typical applications include high-strength components such as valves and pumps.
Type 440: further increase in chromium and carbon content helps to improve toughness and corrosion resistance of this type. Typical applications include surgical instruments.
Ferritic stainless steel is resistant to corrosion and oxidation while maintaining resistance to stress and cracking. Although these steels are magnetic, they cannot be quenched by heat treatment. Once annealed, these grades can be cold worked. They have higher corrosion resistance than martensite grade, but mostly lower than austenite grade. These grades are nickel free straight chromium steels, which are commonly used for decorative purposes, sinks, and certain automotive applications, such as exhaust systems.
Type 430: basic grade, with poor corrosion resistance compared with Type 304. This type has corrosion resistance, such as nitric acid, sulfur-containing gases, as well as many organic acids and food acids.
Type 405: this type has a low chromium content in combination with added aluminum. This chemical composition helps prevent hardening after high temperature cooling. Typical applications include heat exchangers.
Type 409: one of the cheapest grades of stainless steel due to reduced chromium content. This type can only be used for internal or external components in non critical corrosive environments. Typical applications include silencers.
Type 434: this type has increased molybdenum content and improved its corrosion resistance. Typical applications include automotive trim and fasteners.
Type 436: niobium is added to this grade for corrosion and heat resistance. The most typical applications include deep drawing parts.
Type 442: the anti scaling property is improved due to the increase of chromium content. Applications include furnace and heater components.
Type 446: higher chromium content is added to further improve the high-temperature corrosion resistance and scaling performance. It has good oxidation resistance in sulfuric acid environment.
Austenitic stainless steel is the most commonly used type of stainless steel. High chromium and nickel grades in this group provide superior corrosion resistance and very good mechanical properties. They cannot be quenched by heat treatment, but can be greatly quenched by cold working. None of the grades in this class is attractive.
The standard grade of austenitic stainless steel has a maximum carbon content of 0.08%; There are no minimum carbon emission requirements.
Low carbon grade (L grade)
Class “L” is used to provide additional corrosion resistance after welding. The letter “L” after the stainless steel grade indicates low carbon. The carbon content shall be kept at or below 0.03% to avoid carbide precipitation, which may lead to corrosion. Due to the temperature generated during welding (which may lead to carbon precipitation), class “L” is usually used. Generally, stainless steel manufacturers provide these stainless steel grades as double certification, such as 304/304l or 316/316l.
High carbon grade (grade H)
Stainless steel grade “H” has a minimum carbon content of 0.04% and a maximum carbon content of 0.10%. A higher carbon content helps maintain strength at extreme temperatures. These grades are indicated by the letter “H” after the stainless steel grade number. Use this name when the end use involves extreme temperature environments.
304: one of the most commonly used (austenitic) stainless steel grades. The high content of chromium and nickel makes it the first choice for processing equipment in the chemical (mild chemicals), food / dairy and beverage industries. This grade has a good combination of strength, corrosion resistance and fabric properties.
Type 316: This stainless steel grade has 18% chromium, 14% nickel and molybdenum; These combine to increase its corrosion resistance. In particular, the use of molybdenum helps to control pitting corrosion. This grade will not scale at 1600 degrees Fahrenheit. Type 316 is used in chemical processing, pulp and paper industry, food and beverage processing and dispensing and in more corrosive environments. It is also used in the marine industry due to its corrosion resistance.
Type 317: molybdenum content is higher than 316. It is used in highly corrosive environment. The molybdenum content of this grade must be greater than 3%. A scrubber system typically used in air pollution control devices to remove particulates and / or gases from industrial exhaust gases.
Type 321: contains titanium additives with at least five times the carbon content. This addition is to reduce or eliminate chromium carbide precipitation due to welding or exposure to high temperatures. Used in aerospace industry.
Type 347: in the strong oxidation environment, the corrosion resistance of type 347 is slightly higher than that of Type 321 stainless steel. Type 347 shall be considered for applications requiring intermittent heating between 800 º f (427 º C) and 1650 º f (899 º C) or for welding under conditions that prevent post weld annealing.
Duplex is a combination of austenitic and ferritic materials. The strength of these grades is about twice that of austenite and ferrite grades. Although they do have better toughness and ductility than ferrite grades, they do not reach the level of austenite grades. The corrosion resistance of two grades is very close to that of austenitic grades, such as 304 and 316. Class 2205 is the most widely used duplex class.
Type 2205: duplex 2205 is very suitable for high pressure and high corrosive environment. Compared with austenite, it has higher corrosion and erosion fatigue properties, lower thermal expansion and higher thermal conductivity. The service temperature of this grade shall be limited to less than 315 ° C, because long-term high-temperature exposure will lead to brittle materials.
Type 2304: duplex 2304 is generally used in the same applications as alloys 304 and 316L. Its corrosion resistance is very close to or slightly better than that of Austenitic 304 and 316, but its yield strength is almost twice that of the original. It is suitable for use at temperatures between -50 ° C and 300 ° C. This grade has high mechanical strength and high resistance to stress corrosion cracking. It has good weldability, machinability and is easy to manufacture.
Type 2507: duplex 2507 is a super duplex stainless steel. It is suitable for applications requiring special strength and corrosion resistance, such as chemical process, petrochemical and seawater equipment. This grade has excellent resistance to chloride stress, corrosion cracking, high thermal conductivity and low coefficient of thermal expansion. High chromium, molybdenum and nitrogen levels provide excellent resistance to pitting, crevice and general corrosion.
Precipitation hardened stainless steels can be strengthened and hardened by heat treatment. This provides designers with a unique combination of fabric capabilities, strength, ease of heat treatment, and corrosion resistance of any other category of material. These grades include 17cr-4ni (17-4PH) and 15cr-5ni (15-5PH).
Type 17-4: Alloy 17-4 is a chromium copper precipitation hardened stainless steel for applications requiring high strength and moderate corrosion resistance. It has high strength and good corrosion resistance under all heat treatment conditions. This grade can be heat treated at various temperatures; This results in a wide range of finished product properties. This grade shall not be used for temperatures above 300 ° C or very low.
Type 15-5: This is a variant of the earlier 17-4 chromium nickel copper precipitation hardening martensitic stainless steel. 15-5 alloy is designed to have greater toughness than 17-4. Compared with other similar martensite grades, it is used in applications requiring better corrosion resistance and transverse properties.