STAINLESS STEEL: ITS TYPES AND USE IN PHARMACEUTICAL INDUSTRY
Pharmaceutical applications, in common with food and beverage, demand that the materials of construction maintain the integrity of the structure (ie are corrosion resistant and sufficiently robust to withstand the service environment) and, once again, inert surfaces are required (ie insignificant release of contaminants into the product). Stainless steels are widely used in the pharmaceuticals industry because of their resistance to corrosion, inert (easily cleaned) surfaces and ease of fabrication. Although grade 1.4401 (AISI 316) and its derivatives are the most widely used stainless steels in pharmaceutical plant and are considered by many as the industry standard, materials for each application are selected on the basis of their resistance to corrosion in a specific service environment. The selection of a suitable grade of stainless steel must also include consideration of the cleaning regime and cleaning agents used in the plant. Furthermore, the operation of the plant (whether continuous with a “clean in place” system or batch operation with shutdowns to clean the plant) may also influence the choice of material. The austenitic stainless-steel grade 1.4031 (AISI 304) and its derivatives are used in mild environments where the chloride content is less than 200 mg/l, while grade 1.4401 (AISI 316) and its derivatives may be used with chloride contents up to 500 mg/l. At higher chloride contents and especially if combined with increased operating temperatures, the duplex stainless steels grades 1.4462 (2205) and 1.4362 (2304) are used for their resistance to stress-corrosion-cracking. For more aggressive service environments, the superaustenitic (e.g. 1.4547) and superduplex (e.g. 1.4410) grades may be required. Typical examples of stainless steel applications in pharmaceutical production are processing and reaction vessels, storage tanks/vessels, pumps, pipelines and tubes, heat exchangers, scrubber units, taps and valves.
Stainless steel is usually divided into 5 types:
- Ferritic:- These steels are based on Chromium with small amounts of Carbon usually less than 0.10%. These steels have a similar microstructure to carbon and low alloy steels. They are usually limited in use to relatively thin sections due to lack of toughness in welds. However, where welding is not required they offer a wide range of applications. They cannot be hardened by heat treatment. High Chromium steels with additions of Molybdenum can be used in quite aggressive conditions such as sea water. Ferritic steels are also chosen for their resistance to stress corrosion cracking. They are not as formable as austenitic stainless steels. They are magnetic.
- Austenitic:- These steels are the most common. Their microstructure is derived from the addition of Nickel, Manganese, and Nitrogen. It is the same structure as occurs in ordinary steels at much higher temperatures. This structure gives these steels their characteristic combination of weldability and formability. Corrosion resistance can be enhanced by adding Chromium, Molybdenum, and Nitrogen. They cannot be hardened by heat treatment but have the useful property of being able to be work hardened to high strength levels whilst retaining a useful level of ductility and toughness. Standard austenitic steels are vulnerable to stress corrosion cracking. Higher nickel austenitic steels have increased resistance to stress corrosion cracking. They are nominally non-magnetic but usually exhibit some magnetic response depending on the composition and the work hardening of the steel.
- Martensitic:- These steels are similar to ferritic steels in being based on Chromium but have higher Carbon levels up as high as 1%. This allows them to be hardened and tempered much like carbon and low-alloy steels. They are used where high strength and moderate corrosion resistance is required. They are more common in long products than in sheet and plate form. They have generally low weldability and formability. They are magnetic.
- Duplex:- These steels have a microstructure which is approximately 50% ferritic and 50% austenitic. This gives them a higher strength than either ferritic or austenitic steels. They are resistant to stress corrosion cracking. So called “lean duplex” steels are formulated to have comparable corrosion resistance to standard austenitic steels but with enhanced strength and resistance to stress corrosion cracking. “Superduplex” steels have enhanced strength and resistance to all forms of corrosion compared to standard austenitic steels. They are weldable but need care in selection of welding consumables and heat input. They have moderate formability. They are magnetic but not so much as the ferritic, martensitic and PH grades due to the 50% austenitic phase.
- Precipitation hardening:- These steels can develop very high strength by adding elements such as Copper, Niobium and Aluminum to the steel. With a suitable “aging” heat treatment, very fine particles form in the matrix of the steel which imparts strength. These steels can be machined to quite intricate shapes requiring good tolerances before the final aging treatment as there is minimal distortion from the final treatment. This is in contrast to conventional hardening and tempering in martensitic steels where distortion is more of a problem. Corrosion resistance is comparable to standard austenitic steels like 1.4301 (304).
Austenitic Stainless Steel:- These are chromium and nickel containing stainless steels with very low carbon content. They are non-magnetic, but can become slightly magnetic when cold worked. Cold working also enhances their strength. Austenitic Stainless Steels have excellent corrosion resistance; good formability; good weldability, and excellent mechanical properties over a wide range of temperatures. In addition, these steels are easy to clean, which enhances their use in applications in hygienic and sterile environments.
Typical application for the various grades include:
304/304L:- Tanks, storage vessels and pipe work for corrosive liquids. Process equipment in the mining, chemical, cryogenic, food, beverage and pharmaceutical industries. These stainless steels are also used for manufacturing hollowware, cutlery, architectural products and sinks.
309/310:- These grades have a higher chrome and nickel content that the 304 grade. As a result of their high oxidation resistance, these steels are used for high temperature applications such as furnace, kiln and catalytic converter components.
318/316L:- Tanks, pressure vessels, pipe work and components for more aggressive conditions and specialized applications, such as the manufacture of tank containers for bulk transportation of chemicals and corrosive liquids. The molybdenum content enhances the corrosion resistance.
321/316Ti:- These are the “stabilized” grades. They are resistant to sensitization and thus the possibility of Inter Granular Corrosion. In addition, they are used in components, which require elevated temperature strength and corrosion resistance, such as afterburners, super heaters, compensators and expansion bellows.
Ferritic Stainless Steels:- Ferritic Stainless Steel are plain chromium stainless steels, usually with a low carbon content. They are magnetic and have good ductility and resistance to corrosion and oxidation. They are generally resistant to stress corrosion cracking. However, there are weldability limitations which restrict their use to thinner gauges. 3CR12 is a special grade, developed and patented by Columbus Stainless which largely overcome this problem.
Typical applications for the different grades include:
409:– Automotive exhaust tubing and catalytic convertor casings
430:– Kitchen sinks, wash troughs, cutlery, kitchen and catering equipment and utensils.
1.4509 (441):- This grade is specially produced by Columbus Stainless for used in automotive components. Its superior mechanical strength at elevated temperatures (up to 850OC) makes it the ideal material for the front end (close to the engine) of an exhaust system. It can also be used for fabrication of heat exchanger tubes.
AISI 444:- AISI Grade 444 has a very similar PRE (Pitting Resistance Equivalent) to Grade 316, meaning that its corrosion resistance is similar in aggressive outdoor environments, e.g. at the coast. Grade 444 tubing can be polished, bent and welded by conventional methods and tubing will be marked clearly on the inside of the tube. PRE is defined as %Cr + (3.3%Mo) + (16%N) which gives an indication of the steels resistance to pitting corrosion.
3CR12:- This is a price competitive, corrosion resisting, weldable, utility ferritic stainless steel with particular advantages in wet abrasive applications. Unlike other ferritic stainless steels, it can be welded in thicknesses of up to 30mm. It is extremely used in the mining, materials handling and sugar industries due to its resistance to atmospheric corrosion and wet abrasive corrosion.
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