Common Stainless Steel Grades and Their Properties in Instrumentation
304 Stainless Steel. It is one of the austenitic stainless steels with the largest application volume and widest usage range. It is suitable for manufacturing deep-drawn parts, acid-transporting pipelines, containers, structural components, and various instrument bodies. It can also be used to manufacture non-magnetic, low-temperature equipment, and components.
304L Stainless Steel. An ultra-low carbon austenitic stainless steel developed to address the severe intergranular corrosion tendency of 304 stainless steel due to the precipitation of Cr23C6 under certain conditions. Its resistance to intergranular corrosion in the sensitized state is significantly better than that of 304 stainless steel. Except for slightly lower strength, other properties are the same as 321 stainless steel. It is mainly used for corrosion-resistant equipment and components that need to be welded and cannot be subjected to solid solution treatment, and can be used to manufacture various instrument bodies.
304H Stainless Steel. A branch of 304 stainless steel with a carbon mass fraction of 0.04%-0.10%, it has better high-temperature performance than 304 stainless steel.
316 Stainless Steel. Molybdenum is added to the base of 10Cr18Ni12 steel, which gives the steel good resistance to reducing media and pitting corrosion. It has better corrosion resistance than 304 stainless steel in seawater and various other media, and is mainly used for materials resistant to pitting corrosion.
316L Stainless Steel. An ultra-low carbon steel with good resistance to sensitized intergranular corrosion, suitable for manufacturing thick-section welded components and equipment, such as corrosion-resistant materials in petrochemical equipment.
316H Stainless Steel. A branch of 316 stainless steel with a carbon mass fraction of 0.04%-0.10%, it has better high-temperature performance than 316 stainless steel.
317 Stainless Steel. It has better pitting corrosion resistance and creep resistance than 316L stainless steel and is used for manufacturing equipment resistant to organic acid corrosion in the petrochemical industry.
321 Stainless Steel. A titanium-stabilized austenitic stainless steel, with added titanium to improve intergranular corrosion resistance and good high-temperature mechanical properties. It can replace ultra-low carbon austenitic stainless steel. It is not generally recommended for use except in special occasions such as high-temperature or anti-hydrogen corrosion.
347 Stainless Steel. A niobium-stabilized austenitic stainless steel, with added niobium to improve intergranular corrosion resistance. It has the same corrosion resistance as 321 stainless steel in acids, alkalis, and salts, and has good welding properties. It can be used as both corrosion-resistant material and heat-resistant steel, mainly used in the fields of thermal power and petrochemicals, such as for making containers, pipelines, heat exchangers, shafts, furnace tubes in industrial furnaces, and furnace tube thermometers.
904L Stainless Steel. A super fully austenitic stainless steel invented by Finland's Outokumpu company, with a nickel mass fraction of 24%-26% and a carbon mass fraction of less than 0.02%. It has excellent corrosion resistance, especially in non-oxidizing acids like sulfuric acid, acetic acid, formic acid, and phosphoric acid. It also has good resistance to crevice corrosion and stress corrosion. It is suitable for various concentrations of sulfuric acid at temperatures below 70°C and has good corrosion resistance in any concentration and temperature of acetic acid and formic acid, as well as mixed acids of acetic acid and formic acid under atmospheric pressure. The original standard ASME SB-625 classified it as a nickel-based alloy, while the new standard classifies it as stainless steel. China only has an approximate grade 015Cr19Ni26Mo5Cu2 steel. A few European instrument manufacturers use 904L stainless steel for key materials, such as the measurement tubes of E+H's mass flow meters and the cases of Rolex watches.
440C Stainless Steel. A martensitic stainless steel, it is the hardest of the quenched and hardened stainless steels with a hardness of HRC57. It is mainly used for making nozzles, bearings, valve cores, seats, sleeves, valve stems, etc.
17-4PH Stainless Steel. A martensitic precipitation hardening stainless steel with a hardness of HRC44, it has high strength, hardness, and corrosion resistance, and should not be used above 300°C. It has good corrosion resistance to atmospheres and dilute acids or salts, with corrosion resistance comparable to 304 and 430 stainless steels. It is used for manufacturing offshore platforms, turbine blades, valve cores, seats, sleeves, valve stems, etc.
In the field of instrumentation, considering versatility and cost, the conventional selection order for austenitic stainless steel is 304-304L-316-316L-317-321-347-904L, with 317 being rarely used, 321 not recommended, 347 for high-temperature corrosion resistance, and 904L used only for specific components by some manufacturers, and generally not actively chosen in design.
In the design and selection of instruments, there are often occasions where the material of the instrument is different from that of the pipeline, especially under high-temperature conditions. It is important to pay special attention to whether the material selection of the instrument meets the design temperature and pressure of the process equipment or pipeline. For example, if the pipeline is high-temperature chromium-molybdenum steel and the instrument is selected as stainless steel, problems are likely to occur, and it is necessary to consult the temperature-pressure tables of the relevant materials.
In the design and selection of instruments, various systems, series, and grades of stainless steel are often encountered. The selection should be based on multiple perspectives such as the specific process medium, temperature, pressure, stressed components, corrosion, and cost.
