Characteristics of stainless steel and performance of various grades

Characteristics of stainless steel and performance of various grades

1. Types and definitions of corrosion
Stainless steel can provide satisfactory corrosion resistance in many industrial applications. According to the experience of use, in addition to mechanical failure, the corrosion of stainless steel is mainly manifested in: A serious form of corrosion of stainless steel is local corrosion (i.e. stress corrosion cracking, pitting corrosion, intergranular corrosion, corrosion fatigue and crevice corrosion). Failure cases caused by these local corrosion account for more than half of the failure cases. In fact, many failure accidents can be avoided through reasonable material selection.
Stress corrosion cracking (SCC): It is a general term for the mutual failure of stressed alloys due to the expansion of cracks in a corrosive environment. Stress corrosion cracking has a brittle fracture morphology, but it can also occur in materials with high toughness. The necessary conditions for stress corrosion cracking to occur are the presence of tensile stress (whether residual stress or applied stress, or both) and a specific corrosive medium. The formation and expansion of the cracks are roughly perpendicular to the direction of the tensile stress. The stress value that causes stress corrosion cracking is much smaller than the stress value required for the material to break when there is no corrosive medium. Microscopically, cracks that pass through grains are called transgranular cracks, and cracks that extend along grain boundaries are called intergranular cracks. When stress corrosion cracking extends to a certain depth (here, the stress on the cross-section of the material bearing the load reaches its fracture stress in the air), the material breaks according to normal cracks (in ductile materials, usually through the aggregation of microscopic defects). Therefore, the cross-section of a part that fails due to stress corrosion cracking will contain characteristic areas of stress corrosion cracking and "tough dimple" areas associated with the aggregation of microscopic defects.
Pitting corrosion: It is a form of local corrosion that causes corrosion.
Intergranular corrosion: Grain boundaries are disordered boundaries between grains with different crystallographic orientations, so they are favorable areas for segregation of various solute elements in steel or precipitation of metal compounds (such as carbides and δ phases). Therefore, it is not surprising that in some corrosive media, grain boundaries may be corroded first. This type of corrosion is called intergranular corrosion, and most metals and alloys may exhibit intergranular corrosion in specific corrosive media.
Crevice corrosion: a form of localized corrosion that may occur in crevices where the solution is stagnant or in shielded surfaces. Such crevices can form at the junction of metal and metal or metal and non-metal, for example, at the junction with rivets, bolts, gaskets, valve seats, loose surface deposits and marine organisms.
General corrosion: a term used to describe corrosion phenomena that occur in a relatively uniform manner on the entire alloy surface. When general corrosion occurs, the material gradually becomes thinner due to corrosion, and even the material fails due to corrosion. Stainless steel may show general corrosion in strong acids and alkalis. The failure problem caused by general corrosion is not very worrying, because this corrosion can usually be predicted by simple immersion tests or by consulting corrosion literature.
2. Corrosion resistance of various stainless steels
304 is a general-purpose stainless steel that is widely used to make equipment and parts that require good comprehensive performance (corrosion resistance and formability).
301 stainless steel shows obvious work hardening when deformed and is used in various occasions requiring higher strength.
302 stainless steel is essentially a variation of 304 stainless steel with a higher carbon content, which can be cold rolled to obtain higher strength.
302B is a stainless steel with a higher silicon content, which has a higher resistance to high temperature oxidation.
303 and 303Se are free-cutting stainless steels containing sulfur and selenium respectively, and are used in applications where free cutting and high surface finish are the main requirements.
303Se stainless steel is also used to make parts that require hot upsetting because it has good hot workability under such conditions.
304L is a variation of 304 stainless steel with a lower carbon content, which is used in applications where welding is required. The lower carbon content minimizes the precipitation of carbides in the heat-affected zone near the weld, which may cause intergranular corrosion (weld erosion) of the stainless steel in certain environments.
304N is a stainless steel containing nitrogen, which is added to increase the strength of the steel.
305 and 384 stainless steels contain higher nickel, have lower work hardening rates, and are suitable for various applications where high cold formability is required.
308 stainless steel is used to make welding rods.
309, 310, 314 and 330 stainless steels have relatively high nickel and chromium contents in order to improve the oxidation resistance and creep strength of the steel at high temperatures. 30S5 and 310S are variants of 309 and 310 stainless steels, the only difference being that they have a lower carbon content in order to minimize the precipitation of carbides near the weld. 330 stainless steel has particularly high resistance to carburization and thermal shock.
316 and 317 stainless steels contain aluminum, so their resistance to pitting corrosion in marine and chemical industrial environments is much better than that of 304 stainless steel. Among them, 316 stainless steel has variants including low-carbon stainless steel 316L, high-strength stainless steel 316N containing nitrogen, and free-cutting stainless steel 316F with a high sulfur content.
321, 347 and 348 are stainless steels stabilized with titanium, niobium plus tantalum, and niobium, respectively, suitable for welding components used at high temperatures. 348 is a stainless steel suitable for the nuclear power industry, which has certain restrictions on the combined amount of tantalum and cobalt.