We usually referred to the stainless steel used for the production or storage of food containers and processing machinery as the food-grade stainless steel. The so-called “food-grade” generally refers to the material production of the product that meets the standard, or there are requirements for corrosion resistance and heavy metal precipitation(Lead, Chromium, Nickel, Cadmium, Arsenic). Type 304, 316 is the most commonly used food stainless steel material, then the question comes, 321 stainless steel can be used as food grade?
What’s 321 Stainless Steel?
Type 321 Stainless Steel is a Titanium-stabilized austenitic chromium-nickel alloy with excellent resistance to intergranular corrosion. It has a maximum of 0.70% titanium, making make it suitable for applications requiring increased strength and corrosion resistance. It exhibits excellent corrosion resistance in general atmospheric environments and good resistance to most inorganic and organic chemicals. 321 also has excellent weldability and cold working. But its ductility is not ideal for marine environments, and the alloy is not recommended for this purpose. For this reason, applications of 321 stainless steel are limited to specific industrial settings. Do you know steel 321 can be used for food industrial and is it food-grade stainless steel?
Stainless Steel Requirements For Food Industry
Corrosion resistance, weldability, sensitization, surface condition, treatment and roughness of stainless steel sheets and pipe will affect the application in the food industry such as beer brewing plants, dairy industry, sugar factories, pharmaceutical companies, biologics companies and so on. Most food and beverage machinery is heated by steam or cooled by water, a process that involves pasteurization and sterilization, often with problems such as stress corrosion cracks. Fatigue corrosion is similar to stress corrosion cracking failure mechanism. In beer brewing, containers and pipes may operate at low temperatures or as high as the boiling point. However, chloride cracking or corrosion fatigue may occur when used in Welds and heat-affected zones are often the primary source of corrosion, especially in breweries and other food industries, where weld defects such as inadequate penetration cause hygiene and bactericidal problems. Optimum surface condition is very important for hygiene, easy cleaning and corrosion resistance. The weld is also not allowed to have a pyrotechnic color, usually light yellow is acceptable. The proper weld geometry is also very important.
Can 321 Stainless Steel Be Used For Food Industrial?
Let’s begin with its chemical composition. The chemical composition of 321 stainless steel varies according to the type. the content of titanium making AISI 321 has excellent corrosion resistance and is equivalent to type 302 in the annealed condition. Generally, this grade is used in heavy-duty welded equipment operated between 800deg F and 1500deg F and then slowly cooled. The chemical composition of 321 stainless steel enables it to be welded together without deformation or cracking under most operating conditions. However, the chemical composition of 321 stainless steel is not suitable for use in high-temperature applications, and it does not polish well. This stainless steel is not practical for achieving cosmetic architectural features.
Weldability of 321 stainless steel
Its high titanium content provides excellent resistance to chromium carbide precipitation during welding. Type 321 is a good choice for high temperature applications, but its low weldability makes it unsuitable for use in parent plate materials. Type 321 has been supplanted by Type 304L in some applications. Type 321 is slightly harder than Type 304 stainless, producing stringy chips. It is also superior to 304 in post-weld annealing, and can withstand a wide range of temperatures. Its good corrosion resistance, despite its high carbon content, is reflected by its excellent ductility and ease of forming. But despite the superior corrosion resistance, 321 can be difficult to machine, which makes it an undesirable choice for many applications.
AISI 304 or 316 stainless steel with a carbon content of less than 0.08% is generally sensitized to exposure to 500 ~ 800℃ for a given period of time. This may occur during welding, therefore, welding can cause sensitization of the “heat affected zone” along the weld. Sensitization will form chromium carbide at the grain boundary and cause poor chromium at the grain boundary. This process is easy to cause intergranular corrosion of stainless steel in the case of thick pipe wall (> 2 ~ 3mm). To avoid this, “weldable grades” are often used:
L grade steel, such as 304L, 316L, their carbon content is less than 0.03%.
Thermal pyrochroma is caused by the difference in the thickness of the transparent oxide layer in which light is absorbed. Because the color refraction coefficient is different, the blue – looking oxide layer can only reflect blue light and absorb other light. A thicker oxide layer has more holes than a fully transparent thin oxide layer, so a thicker oxide layer will reduce the corrosion resistance and non-adhesion of stainless steel. For most standards, light pyrogenic colors are acceptable, and all other pyrogenic colors such as red and blue are unacceptable. Thermal pyrochromes are not permitted in the pharmaceutical industry.
Weld geometry should be as regular as possible. Qualified welds do not break the surface of the base metal itself. Corrosion often begins inside tiny pinholes at the beginning/end of a weld. In theory, there are no tiny pinholes, looseness or other bumps and depressions at the beginning/end, so good weld penetration is very important.
Surface finish of 321 stainless steel
A high-quality surface finish is essential for a range of applications, including marine, nuclear, and petroleum industries. Stainless steel has good sanitary properties and is widely used in the food and beverage industry, compared with other fields such as oil and gas. Hygienic and non-adhesive properties dictate that food equipment requires optimal surface condition. Type 321 can be stress-relief annealed within the carbide-precipitation range of 800 to 1500deg F (around 427 to 816deg C). This method does not reduce corrosion resistance significantly, but prolonged heating lowers it to a level where the material begins to corrode. The 321 steel is also abraded by SiC paper, which is used to remove surface roughness.
Surface roughness has a great influence on the hygienic properties and corrosion of stainless steel. Electropolished surfaces have the best corrosion resistance, followed by mechanically polished surfaces. This surface condition is used in the pharmaceutical industry and other industries that require high standards. Generally, the beer industry and the food industry do not specifically designate the use of polished surfaces (electrolysis). However, such surfaces are sometimes required to achieve excellent sanitary conditions and for easy cleaning. Most pipes are annealed bright during manufacture, so internal pickling of pipes is often not performed unless the material surface is severely pyrochromatic or contaminated with iron. 321 stainless steel sheets often processed 2B machined surfaces, which have good surface properties. The most common type used in breweries is 2B surface thin-wall direct-welded pipe, sometimes brushed or polished surface finish.
Therefore, we say 321 stainless steel can be used in the food industry, and other applications that depend on it meet specific industry standards or not, and the stainless steel used in the food industry is not necessarily food-grade stainless steel. Generally speaking, the most commonly used food-grade is 304 and 316 because of good economy performance. 321 stainless steel is mainly used for industrial purposes such as aircraft exhaust olds, heat Exchangers, high-performance boiler and furnace parts, expansion joints and so on,
