This article takes an in-depth look at stainless steel grades.
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Stainless steel grades each consist of carbon, iron, 10.5%-30% chromium, nickel, molybdenum, and other alloying elements. It is a popular metal used in various products, tools, equipment, and structures that serve in many industrial, commercial, and domestic applications.
Corrosion resistance is the most valuable property of stainless steel. Chromium is the alloying element responsible for corrosion resistance. This element reacts with oxygen in air and water and forms a thin, stable chromium oxide film protecting the underlying metal surface. Molybdenum enhances corrosion resistance by protecting the film from pit formation. The protective layer is regenerated after slight abrasions. However, rust can still be formed when the layer is severely damaged after exposure to chlorides, strong cleaning agents, environments with high salinity and high humidity, and after high abrasion.
Aside from corrosion resistance, stainless steels are known for their excellent mechanical properties such as high strength, toughness, ductility, fatigue strength, and wear resistance. Stainless steels can withstand high temperatures, high pressures, and cryogenic environments. They are non-reactive to most chemical substances; hence, they are commonly used in chemical handling equipment and vessels. They also have an aesthetically pleasing, lustrous, and bright surface.
Stainless steel grades are divided into five main categories:
Austenitic stainless steel is the most common type of stainless steel. It is named after an English metallurgist named Sir William Chandler Roberts-Austen. It is the most widely used category of stainless steel, which comprises the 200 and 300 series.
Austenitic stainless steels possess a face-centered cubic (FCC) crystal structure by further adding nickel, manganese, and nitrogen. The FCC structure exists at all temperatures. Austenitic stainless steels moderate to high nickel content ranging from 8-12%. Their high manganese and nitrogen contents stabilize the austenitic structure. However, the addition of these elements makes them more expensive. Carbon is present in low concentrations.
Austenitic stainless steels cannot be hardened by heat treatment; hence, they are cold worked to increase their strength and hardness and improve their surface finish. They are non-magnetic and can be slightly magnetic after cold working. They possess high formability and weldability. Since their chromium content is also high (16-30%), austenitic stainless steels possess superior chemical resistance.
Ferritic stainless steels possess a body-centered cubic (BCC) crystal structure. They have high chromium content ranging from 11% to 27%, but they contain small amounts of nickel. The carbon content of ferritic stainless steel is low (about 0.03%), making it highly ductile and formable. However, like austenitic steels, they cannot be hardened by heat treatment.
Ferritic stainless steels are magnetic. They have high thermal conductivity; hence, they are suitable materials for constructing boilers, heat exchangers, and other applications involving heat transfer. They have a low coefficient of thermal expansion; thus, they are dimensionally stable in a wide range of temperatures. They also have excellent resistance to stress corrosion cracking due to their ferritic structure, which enables them to withstand chlorides, high humidity, and high temperature.
Martensitic stainless steels possess a body-centered tetragonal crystal structure. They consist of 11.5-18% chromium and 0.1-1.2% carbon. The relatively high strength and brittleness of martensitic stainless steels are attributed to their high carbon content. However, nickel present in martensitic stainless steel is low, which makes them less resistant to corrosion. Martensitic stainless steels are divided into two types based on their carbon content: low carbon martensitic steels (0.05-0.25% carbon) and high carbon martensitic steels (0.61-1.50% carbon). Low carbon martensitic steels provide better corrosion resistance, but high carbon martensitic steels have higher strength and are more brittle.
The hardness and strength of martensitic stainless steels can be increased by heat treatment processes including age hardening, annealing, quenching, and tempering. Martensitic stainless steels are magnetic and impact-resistant. However, they are difficult to weld and form due to their brittleness.
Ferritic and martensitic stainless steels comprise the 400 series.
Duplex stainless steels possess both austenitic and ferritic phases in their microstructures in approximately equal proportions. They are twice as strong as regular austenitic and ferritic stainless steel. Their toughness, ductility, and formability are greater than ferritic steels but never reach the level of austenitic steels. Duplex stainless steels have good resistance to stress corrosion cracking inherited from the ferritic side. Corrosion resistance varies greatly in duplex stainless steels since this property depends on the composition; resistance to pitting and crevice corrosion increases as the content of nickel, molybdenum, and nitrogen are increased. In terms of cost, duplex stainless steels remain a cheaper alternative to austenitic stainless steel.
Precipitation-hardened (PH) stainless steels contain small amounts of copper, aluminum, titanium, and molybdenum. After the stainless-steel elements have been alloyed, the stainless steel is subjected to age hardening treatment in order to precipitate these elements as hard intermetallic elements. The precipitate phase impedes the movement of dislocations, the defects in the crystal lattice structure, and imparts excellent strength and hardness to stainless steels. PH stainless steels have corrosion resistance comparable to austenitic stainless steels.
PH stainless steels are categorized into three types: martensitic, austenitic, and semi-austenitic PH stainless steels. Austenitic PH steels retain their crystal structure at all temperatures. Semi-austenitic PH steels remain austenitic after solution treatment and quenching; the austenitic structure is converted into martensitic after cryogenic treatment or cold working.
There are over 150 stainless steel grades, but only around 15 of them are commonly used. The most popular stainless steel grading system was developed by the Society of Automotive Engineers (SAE). Other grading systems were developed, like the Universal Numbering System (UNS) for Metals and Alloys, and they can vary in different regions.
The stainless-steel grades and their properties are the following:
Grade 201 stainless steel is an austenitic stainless steel containing 16-18% chromium and 3.5-5.5% nickel. It is a substitute for grade 304 stainless steel and high nickel alloys. Due to its lower nickel content, it offers below-average protection from corrosion. However, it is cheaper than other stainless steel, and the concentration of manganese and nitrogen can be increased to improve its yield strength. The versatility of grade 201 is owed to its good formability and weldability.
Grade 202 stainless steel is an austenitic stainless steel containing 17-19% chromium, 4-6% nickel, and 7.5-10% manganese. It is one of the stainless-steel grades widely used for precipitation hardening and has good corrosion resistance, high hardness, toughness (even at lower temperatures), strength, and weldability. It almost has similar properties with grade 302, but grade 202 has lower yield strength. Grade 202 can be machined in an annealed condition. Like grade 201, grade 202 is also cost-efficient.
Grade 301 stainless steel is austenitic stainless steel. It offers the same corrosion resistance with grade 304 in mildly corrosive environments at ambient temperatures; however, the corrosion resistance of grade 301 is lower in extremely corrosive and high-temperature environments due to its lower chromium content (16-18%).
Grade 301 stainless steel possesses high strength and is available in annealed and cold-worked conditions. The annealed variation of this grade offers high formability, while the cold worked variation has higher strength. 301 stainless steel becomes magnetic when cold worked. It is suitable for welding. However, welded grade 301 parts need annealing to dissolve the precipitated chromium carbide induced by welding and prevent the intergranular attack.
Grade 301L stainless steel is a variant of grade 301 that has improved ductility and weldability. It does not need to be annealed for maximum corrosion resistance. Another variant is the grade 301LN stainless steel, which contains higher nitrogen and has a higher work hardening rate. Annealing is not necessary for these variants.
Grade 302 stainless steel is an austenitic stainless steel containing about 18% chromium and 8% nickel. Its chemical composition is almost the same as the grade 304, except for slightly higher carbon content. Grade 302 has high toughness, tensile strength, yield strength, and corrosion resistance. Grade 302 stainless steel is processed at low speeds and high feed volumes to overcome their work hardening tendencies. Due to gummy chip formation, it is advised to use chip breakers on all tooling. The recommended welding methods for grade 302 are resistance and shielded fusion welding. Post weld annealing must be performed.
Grade 304 stainless steel is the most popular austenitic stainless steel. It contains 18% chromium and 8% nickel: thus, deriving its old name 18/8. Grade 304 has excellent chemical, oxidation, and corrosion resistance in a wide range of temperatures. However, it is susceptible to pitting corrosion and stress corrosion cracking when exposed to chlorides. Low and high carbon content variants of grade 304 are available, which are the grade 304L and grade 304H, respectively.
Aside from excellent corrosion resistance, grade 304 stainless steels have high ductility. Hence, they can be easily formed and machined into varieties of products. They can be severely deep-drawn. Grade 304 has higher formability than grade 316. However, they require annealing after cold working to minimize work hardening.
Grade 305 stainless steel is austenitic stainless steel that contains 18% chromium, a minimum of 10% nickel, and a high amount of carbon. It has good chemical and corrosion resistance and high strength. It has a low rate of work hardening due to its elevated nickel content. The low rate of work hardening makes grade 305 suitable for deep drawing applications. Grade 305 can be welded with resistance and fusion methods; however, oxyacetylene welding is not recommended.
Grade 309 stainless steel is an austenitic stainless steel containing a minimum of 22% chromium, 12% nickel, and a low amount of carbon. It possesses excellent corrosion and oxidation resistance, high tensile and creep strengths maintained at high temperatures. It can withstand saline environments. Grade 309 stainless steels can be rolled, formed, stamped, and deep drawn readily; annealing is required after cold working to prevent work hardening and maintain its ductility.
Grade 316 stainless steel is the most popular and widely used stainless steel grade next to grade 304. It is an austenitic stainless-steel containing elevated amounts of molybdenum and high amounts of silicon, manganese, carbon, chromium, and nickel. High molybdenum concentration makes grade 316 more resistant to pitting and crevice corrosion than grade 304 in saline environments. Grade 316 has good forming and welding characteristics; annealing is usually not required after welding thin sections.
Grade 316 has two variants: grade 316L and grade 316H stainless steel. Grade 316L has lower carbon content and is insusceptible to chromium carbide precipitation. Hence, grade 316L is preferred when a project requires much heavy welding. Meanwhile, Grade 316H has higher carbon content; therefore, it has higher tensile and yield strength and is more suitable in high-temperature applications.
Grade 321 stainless steel is a titanium-stabilized austenitic stainless steel with good corrosion resistance. It has excellent resistance to chromium carbide precipitation when exposed to high temperatures. It also has higher creep and stress rupture properties than grade 304. All these properties are maintained in high temperatures.
Grade 321H is a modification of grade 321. It has an enhanced creep resistance, and its higher carbon content increases its strength at higher temperatures.
Grade 347 stainless steel is a niobium-stabilized austenitic stainless steel. Like grade 321, grade 347 is also insusceptible to chromium carbide precipitation and has excellent intergranular corrosion resistance. It has higher creep and stress rupture properties than grade 304. It has good corrosion resistance and offers oxidation resistance slightly better than grade 321.
Grade 409 stainless steel is ferritic stainless steel stabilized by the presence of titanium and/or niobium. It has good mechanical properties and corrosion resistance that are maintained at high temperatures. It has good formability and welding characteristics. It can be welded by various methods such as arc, resistance spot, and seam welding. However, grade 409 welding requires preheating to 150-2600C and post-weld annealing to improve ductility. It is not suitable for aesthetic applications as it tends to form light surface rusting.
Grade 410 stainless steel is a heat-treatable, general-purpose martensitic stainless steel containing 11.5% chromium. Processes such as hardening, tempering, and polishing are performed in grade 410 stainless steels to elevate wear resistance, corrosion resistance, and overall mechanical properties. It is compatible with water, air, hot gases, food products, and most chemicals such as mild acids, nitric acid, concentrated sulfuric acid, dilute acetic acid, and naphtha. However, grade 410 is only suitable for mildly to moderately corrosive environments due to their lower nickel content.
Grade 410 can be welded by most conventional welding methods. To mitigate the risk of cracking, it should be preheated between 150° and 260°. Post welding annealing should also be performed. This grade maintains its ease of machinability after tempering and annealing processes. Grade 410 steels are used in refinery equipment, turbines, pumps, valves, and as a material for machine elements such as bolts, screws, nuts, and bushings.
Grade 420 stainless steel is martensitic stainless steel with high carbon content of 0.15 to 0.45% and a minimum chromium content of 12%. It is a higher form of grade 410. Its strength significantly increases by about 1000 MPa when hardened and stress relieved. Its corrosion resistance is lower than austenitic and ferritic stainless steel, but it is enough to withstand mild acids, alkali, freshwater, normal atmospheric conditions, and food products. It has good ductility in its annealed state but has poor corrosion resistance; hence, it is utilized in the fully hardened, surface grounded, or polished condition.
Grade 420 can be machined easily, but its machinability decreases with increasing hardness. Grade 420 steels with hardness greater than 30 HRC are difficult to machine. Grade 420 is widely used in making cutting tools (e.g., knives, dies), cutlery, needles, surgical tools, and bushings.
Stainless steel grade 420HC has higher carbon content than grade 420 but is considered to be a softer steel when compared to other steel types and is rated as a mid range steel. Its higher carbon content gives 420HC higher hardness. Grade 420HC is ideal for the manufacture of cutting tools since it has good edge retention and sharpens easily. The significance of edge retention refers to the ability to maintain a sharp edge. Grade 420HC is more expensive than grade 420 and has lower corrosion resistance.
Knives made using 420HC are popular with hunters and fishermen since 420HC holds up well in hostile weather conditions. Since it is less expensive than steel, it is used to make lower cost knives that are sold by large retailers. Stainless steel grade 420HC sharpens easily but does not hold its edge like a steel blade, which requires that it be sharpened frequently.
Stainless steel grade 420J2 is inexpensive and is corrosion resistant in mild atmospheres such as domestic and industrial environments. Additionally, grade 420J2 is resistant to dilute nitric acid, carbonic acid, ammonia, crude oil, detergent solutions, vinegar, food acids, several petroleum products, and steam.
What differentiates stainless steel grade 420J2 from other steels is its strength and impact resistance in conditions of hardening and tempering. It can be turned, drilled, and bent. Grade 420J2 can be annealed at 730° C up to 790° C or 1346° F up to 1451° F.
Stainless steel grade 420J2 is used to manufacture surgical instruments, daggers, swords, budget knives, diving knives, domestic scissors, and hair cutting scissors.
Grade 430 stainless steel is ferritic stainless steel primarily composed of chromium and iron; the concentrations of carbon, nickel, and other alloying elements are very low, making grade 430 stainless steels less expensive. It has good heat and corrosion resistance which can handle organic acids and nitric acids. Being ferritic stainless steel, it has excellent stress corrosion cracking resistance. Grade 430 has good machinability and formability and a low work hardening rate. However, it has a low ductility rate and is prone to galling.
Grade 434 stainless steel is the most common non-hardenable ferritic stainless steel, which contains high molybdenum to increase its corrosion and heat resistance. Its performance is close to that of grade 430.
As a very widely used ferritic stainless steel, grade 434 is capable of withstanding temperatures of up to 1500°F. Due to its exceptional resistance to high temperatures, it cannot be hardened by heat treatment and is cold formed like low carbon steel. The main use of stainless steel grade 434 is as auto trim.
Grade 440C stainless steels are high carbon steels with high hardness, wear resistance and strength after heat treatment. The high hardness of grade 440 makes it suitable for applications such as valve components and ball bearings. Grades 440A and 440B have similar properties as 440C with lower percentages of carbon in grade 440A.
All grades of 440 stainless steel are commonly used with grade 440C being more readily available. Grade 440F is a machining type of grade 440 stainless steel with a carbon content that is similar to grade 440C.
Grade 440 stainless steels should not be used in high temperature applications due to the risk of being over tempered. All sub-grades of grade 440 have similar properties with grade 440C having similar corrosion resistance as stainless steel grade 304.
Due to its excellent mechanical properties and corrosion resistance, stainless steel can be found in a variety of products, equipment, and structures. The applications of the stainless-steel grades include but are not limited to the following:
Grade 201 stainless steel is used in structural members, decorative materials, household items, cooking utensils, windows, doors, automotive parts, railroad cars, and trailers.
Grade 202 stainless steel is formed into sheets, coils, and plates in order to manufacture household items, kitchenwares, structural members, automotive trim, hose clamps, and railway cars.
Grades 301 and grade 301LN stainless steels are used in aircraft components, structural components for railcar, automobiles, trucks, trailers, appliances, cooking utensils, kitchenwares, wiper blade clips and holders, screen frames, and curtain walls.
Grade 301L stainless steels are used in pressure vessels, door and drainage parts, utensils and appliances parts, and structural components for railcars.
Grade 302 stainless steels are commonly used in food handling equipment, cooking utensils, kitchenware, pressure vessels, and medical tools. They are widely sold in spring (conical compression springs), screen, cable, and wire forms.
Grade 304 stainless steels are used to manufacture deep-drawn sinks, pans, pots, and troughs. Grade 304 is also used in mechanical fasteners (nuts, bolts, and screws), industrial plant equipment (pressure vessels, storage vessels, tubing), kitchenware, cutlery, cooking utensils, decorative pieces, and architectural paneling.
Grade 305 stainless steels are formed into deep drawn parts, cups, pans, bowls, eyelets, rivets, and screws.
Grade 309 stainless steels are used to construct boilers, furnaces, ovens, heat exchangers, kilns, aircraft engines, and automotive exhaust parts.
Grade 316 stainless steels are commonly used in constructing chemical storage tanks, heat exchangers, reactors, furnaces, and other process equipment. Grade 316 is preferred in marine environments and in the refinery industry. It is also used in architectural panels and railings.
Grade 321 and grade 347 stainless steels are used in aircraft and aerospace components, expansion joints, bellows, furnace parts, heat exchangers, heating element tubing, and chemical process equipment.
Grade 409 stainless steels are originally developed for automotive exhaust systems, but their application has expanded to catalytic converters, mufflers, fuel filters, heat exchangers, and agricultural machinery.
Grade 410 stainless steels are used to manufacture cutlery, knives, flatware, cutting tools, bolts, bushings, and nuts. They are used to construct petroleum fractionating equipment, gas turbines, pumps, shafts, and valves.
Grade 420 stainless steels are commonly found in cutlery, surgical and dental instruments, scissors, blades, and other cutting tools. Grade 420HC and grade 420J2 are used in economical knives and cutting tools.
Grade 430 stainless steels are used in kitchen utensils, sinks, dishwasher linings, pipes, tubes, and automotive trim.
Grade 434 stainless steels are used in automotive trim, furnace chambers, range hoods, gas burners, steam iron bases and flatware, and chemical process equipment.
Grade 440 stainless steels are used in valves, valve seats, ball bearings, and cutlery.
Stainless steel screws have superior mechanical properties and corrosion resistance, making them a suitable fastener for a variety of environments.
Self-drilling screws are screws that can tap its hole as it is drilled into the material. They require a pre-drilled or pre-punched hole. They are used in softer and thinner materials.
Self-tapping screws are screws that can drill their own holes in the material. They can be used in wood, plastic, and metal substrates.
Thread cutting screws are screws that drill threads in predrilled holes on a material.
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