What is the strongest stainless steel alloy?
Stainless Steel Grades
Stainless steels can be divided into three major categories: austenitic, ferritic, and martensitic.
Austenitic Grades
G primarily uses 304 and 316 stainless steels from the 300 series. 304 and 316 are the most important members of the austenitic stainless steel family and are the most widely used stainless steels. Austenitic stainless steels are generally non-magnetic and can only be hardened by cold working. However, they can also have a small amount of magnetism, especially in formed parts. They can be softened by the "annealing" heat treatment.
Ferritic Grades
Suncor Stainless does not generally use ferritic stainless steels. Ferritic stainless steels are magnetic and are not hardenable.
Martensitic Stainless Steels
Martensitic stainless steels are hardenable by heat treatment and are magnetic. Stainless steels such as grade 440 are primarily used in products that are resistant to atmospheric oxidation, mildly corrosive chemicals, and dry and wet corrosion. These stainless steels contain up to 1% carbon. Suncor also uses this type of stainless steel in some of its products, such as quick-connect wedges, but these parts are plated to prevent further corrosion.
Typical Composition of Stainless Steels
Nickel – Stabilizes austenite structure, improves formability, increases ductility, high temperature strength and corrosion resistance (especially in industrial and marine environments, chemical, food and textile processing industries).
Silicon – Improves oxidation resistance and high temperature carburization resistance.
Manganese – Improves hot workability, increases yield strength and tensile strength (above 2%), partially replaces nickel and stabilizes austenite structure.
Molybdenum – Improves corrosion resistance (especially in sulfite, sulfate, acetic and acetate solutions and salt water environments), enhances creep resistance, increases high temperature strength, extends the passive range and counteracts pitting tendency.
Titanium, Niobium and Tantalum – Prevent intergranular corrosion by stabilizing carbon as titanium carbide or niobium carbide instead of chromium carbide, produces finer grain size, and reduces tensile strains in 430 steel during drawing and forming.
Sulfur, phosphorus and selenium – improve machinability.
Additional additions of chromium – improve resistance to spalling, wear and corrosion, and increase tensile strength.
Types and grades of stainless steel
Austenitic stainless steel
Type 301: Lower chromium and nickel content than type 302, more work hardening.
Type 302: The basic type of the 300 series, with 18% chromium and 8% nickel. It is the famous 188 stainless steel and the most widely used of the chromium-nickel stainless steels and heat-resistant steels.
Type 303: Phosphorus and sulfur are added to improve machinability. Corrosion resistance is slightly lower than type 302/304.
Type 303Se: Selenium and phosphorus are added to improve machinability.
Type 304/304L: Increased chromium and nickel content, reduced carbon content to reduce carbide precipitation and improve corrosion resistance. Type 304L has the lowest carbon content and is suitable for welding applications.
Type 305: Increased nickel content to reduce work hardening.
Type 309/309S: Chromium and nickel are added to improve corrosion resistance and high temperature oxidation resistance. 309S has a lower carbon content to minimize carbide precipitation.
Type 310S: Higher nickel content than 309/309S to further improve oxidation resistance. 310S has a lower carbon content than 310 to minimize carbide precipitation.
Type 316 (cast 316 is called CF8M) and 316L (cast 316L is called CF3M): Large amounts of molybdenum are added to improve corrosion resistance (especially pitting resistance), creep resistance, and high temperature strength. 316L has a lower carbon content for welding and reduced corrosion.
Type 321: Titanium is added to bind carbon and avoid chromium carbide precipitation during welding.
Type 330: Extra high nickel content to provide the best corrosion resistance in most furnace environments. This grade has a low coefficient of expansion, excellent ductility and high strength.
Type 347 – 348: Niobium and tantalum are added to bind carbon and avoid the precipitation of chromium carbides during welding. The applicable temperature is 800 to 1,650 degrees Fahrenheit (about 400 to 850 degrees Celsius).
Ferritic Steels
Type 405: Contains 12% chromium and aluminum is added to prevent hardening.
Type 430: Is the basic type in the ferritic steel group, with good ductility and excellent atmospheric corrosion resistance. Its oxidation resistance is higher than that of Type 302 under intermittent service conditions, and slightly lower under continuous service conditions.
Type 430F/430Se: Sulfur and selenium are added respectively to improve machinability.
Type 442: Chromium is added to improve oxidation resistance.
Type 446: Higher chromium content (27%), more resistant to oxidation, the highest chromium content of the standard pure chromium types. Alloys with more than 30% chromium become too brittle and difficult to machine.
Martensitic
Type 410: Is the basic martensitic type. It is a general purpose corrosion and heat resistant chromium stainless steel. It can be hardened by heat treatment to obtain a variety of mechanical properties. It can be annealed to soften for cold drawing and forming. This grade is always magnetic.
Type 403: Is a special high quality steel used for making blades and moving blades for steam turbine and jet engine compressors. This grade is very suitable for making highly stressed parts. This material is magnetic under all conditions.
Type 416/416Se: Is a modified version of Type 410, an easy to machine, non-seizure, non-galling alloy. These properties are obtained by adding sulfur or selenium to Type 410 stainless steel. This is a heat treatable stainless steel grade with corrosion resistance and other properties very close to Type 410 stainless steel.
Type 420: is a chromium stainless steel that can be heat treated to a maximum hardness of about 500 Brinell hardness. It has maximum corrosion resistance only in the fully hardened state. Type 420 is magnetic under all conditions.
Type 431: is a nickel (1.25-2.00%) chromium stainless steel that can be heat treated to obtain high mechanical properties. It is magnetic under all conditions of use. Its corrosion resistance is superior to that of Types 410, 416, 420, 430 and 440 stainless steels.
