1. During the rolling process in steel mills, the rolls undergo slight deformation due to heat, resulting in variations in the thickness of the rolled plate. Generally, the center is thicker than the edges. National regulations stipulate that the plate thickness should be measured at the center of the top edge.

2. Tolerances reflect market and customer requirements and are generally categorized as large tolerances, which allow for greater thickness variation, and small tolerances, which permit only minimal thickness differences. For example, plates may require a large tolerance of ±0.5 mm or a small tolerance of ±0.2 mm, depending on specific customer needs.

There are three main factors affecting the corrosion of stainless steel:

Generally speaking, steel with 10.5% chromium is less prone to rust. The higher the chromium and nickel content, the better the corrosion resistance. For example, 304 stainless steel contains 8-10% nickel and 18-20% chromium; it will not rust under normal conditions.

Large stainless steel mills with advanced smelting technology, equipment, and processes can ensure control over alloying elements, impurity removal, and billet cooling temperature, resulting in stable, reliable product quality, good internal quality, and reduced rust. Conversely, some small steel mills have outdated equipment and processes; impurities cannot be removed during smelting, and the resulting products are prone to rust.

Environmental conditions with high humidity, continuous rainfall, or high air acidity or alkalinity are more prone to rust. Even 304 stainless steel can rust if the surrounding environment is too harsh.

Use pickling paste or spray to help re-passivate the rusted areas, forming a chromium oxide film that restores corrosion resistance. After pickling, it is crucial to rinse thoroughly with clean water to remove all contaminants and acid residue. After all treatment, repolish with polishing equipment and seal with polishing wax. For minor rust spots, a 1:1 mixture of gasoline and engine oil can be used to wipe them away with a clean cloth.

Shot blasting, using glass or ceramic microparticles, shot blasting, burring, brushing, and polishing are all options. Mechanical methods may remove contaminants from previously removed materials, polishing materials, or burring materials. All kinds of contaminants, especially foreign iron particles, can become sources of corrosion, particularly in humid environments. Therefore, mechanical surface cleaning should ideally be performed under dry conditions. Mechanical methods only clean the surface and do not change the material’s inherent corrosion resistance. Therefore, it is recommended to re-polish with polishing equipment after mechanical cleaning and seal with polishing wax.

One of the most widely used austenitic stainless steels, suitable for manufacturing deep-drawn parts, acid pipelines, containers, structural components, and various instrument bodies. It can also be used to manufacture non-magnetic, cryogenic equipment and components.

Developed to address the severe intergranular corrosion tendency of 304 stainless steel under certain conditions due to Cr23C6 precipitation. Its sensitized state intergranular corrosion resistance is significantly better than that of 304 stainless steel. Except for slightly lower strength, its other properties are the same as those of 321 stainless steel. It is mainly used for corrosion-resistant equipment and components that require welding but cannot undergo solution treatment, as well as for manufacturing various instrument bodies.

An internal branch of 304 stainless steel, with a carbon mass fraction of 0.04%–0.10%. Its high-temperature performance is superior to that of 304 stainless steel.

An ultra-low carbon steel with good resistance to reducing media and pitting corrosion. It exhibits superior corrosion resistance compared to 304 stainless steel in seawater and other media, and is primarily used for pitting-corrosion-resistant applications.

An internal branch of 316 stainless steel with a carbon content of 0.04%–0.10%. Its high-temperature performance is superior to that of 316 stainless steel.

Superior resistance to pitting corrosion and creep compared to 316L stainless steel. Used for manufacturing equipment resistant to petrochemical and organic acid corrosion.

A titanium-stabilized austenitic stainless steel. The addition of titanium improves its resistance to intergranular corrosion and provides good high-temperature mechanical properties. It can be used as a substitute for ultra-low carbon austenitic stainless steel. Except for special applications such as high-temperature or hydrogen corrosion-resistant applications, its use is generally not recommended.

A niobium-stabilized austenitic stainless steel. The addition of niobium improves its resistance to intergranular corrosion. Its corrosion resistance in acidic, alkaline, and saline media is similar to that of 321 stainless steel. It has good weldability and can be used as both a corrosion-resistant material and a heat-resistant steel. It is mainly used in the thermal power and petrochemical industries, including the manufacture of containers, pipes, heat exchangers, shafts, furnace tubes for industrial furnaces, and furnace tube thermometers.

A super-austenitic stainless steel invented by Outokumpu of Finland. Its nickel content is 24%-26% and its carbon content is less than 0.02%. It has excellent corrosion resistance and is resistant to non-oxidizing acids, including sulfuric, acetic, formic, and phosphoric acids. It also has good resistance to crevice corrosion and stress corrosion. Suitable for sulfuric acid of various concentrations below 70℃, and exhibits excellent corrosion resistance in acetic acid and formic acid mixtures of any concentration and temperature under normal pressure. The original ASME SB-625 classified it as a nickel-based alloy, while the new standard classifies it as stainless steel. In China, only the approximate grade 015Cr19Ni26Mo5Cu2 steel is available. A few European instrument manufacturers use 904L stainless steel as a key material; for example, the measuring tube of E+H’s mass flow meter is made of 904L stainless steel, and the case of Rolex watches is also made of 904L stainless steel.

Martensitic stainless steel, the hardest among hardenable stainless steels, with a hardness of HRC57. Mainly used for making nozzles, bearings, valve cores, valve seats, sleeves, valve stems, etc.

Martensitic precipitation-hardening stainless steel, with a hardness of HRC44, offers high strength, hardness, and corrosion resistance, but cannot be used above 300℃. It exhibits excellent corrosion resistance in atmospheric conditions and in diluted acids or salts, comparable to that of 304 and 430 stainless steel. It is used in the manufacture of offshore platforms, turbine blades, valve cores, seats, sleeves, and stems.

In instrumentation, considering versatility and cost, the conventional selection order for austenitic stainless steel is 304-304L-316-316L-317-321-347-904L. 317 is less commonly used; 321 is not recommended; 347 is used for high-temperature corrosion resistance; and 904L is the default material for some components from certain manufacturers; it is generally not actively chosen in design.

In instrument design and selection, there are often situations in which the instrument material differs from the piping material, especially at high temperatures. Special attention must be paid to whether the instrument material selection meets the design temperature and pressure requirements of the process equipment or piping. For example, if the piping is made of high-temperature chromium-molybdenum steel, but the instrument is made of stainless steel, problems are likely to occur. It is essential to consult the relevant temperature and pressure gauges for the material.

In instrument design and selection, we often encounter various stainless steel systems, series, and grades. When selecting, we need to consider issues from multiple perspectives, such as specific process media, temperature, pressure, stress-bearing components, corrosion, and cost.