Stainless Steel 316L Sheet Plate Widely Used in Marine And Highly Acidic Environments, Underwater Equipment, Surgical Tools, Food and Pharmaceutical Applications

● Stainless steel 316L has a lower carbon content and is used in applications that subject the metal to risks of sensitization. The higher carbon variant is stainless steel 316H, which offers greater thermal stability and creep resistance. Another widely used grade of stainless steel 316L is the stabilized 316LTi. Stainless steel 316LTi offers better resistance to intergranular corrosion.

● Stainless steel utilizes the principle of passivation, wherein metals become "passive" or unreactive to oxidation from corrosive compounds found in the atmosphere and process fluids. Passivation is done by exposing the stainless steel to air, where it builds chromium oxides on its surface.

● To enhance the formation of the passive film, the alloy is introduced to a chemical treatment where it is thoroughly cleaned by submerging it in acidic passivation baths of nitric acid. Contaminants, such as exogenous iron or free iron compounds, are removed to prevent them from interfering in creating the passive layer.

● After cleaning with an acidic bath, the metal is neutralized in a bath of aqueous sodium hydroxide. A descaling process removes other oxide films formed by high-temperature milling operations, such as hot-forming, welding, and heat treatment.

Chapter Two – Stainless Steel Grades Compared to Stainless Steel 316L

● The predominant characteristic of stainless steel 316L is its molybdenum content, which enhances its corrosion resistance. It is the second most important of all austenitic stainless steel after stainless steel grade 304. Austenitic stainless steels are differentiated from other stainless steels by their nickel or nitrogen content, which gives austenitic stainless steel a unique crystalline structure.

● Stainless steels are divided by their chemical content, physical properties, metallographic structure, and functional characteristics. Their mechanical properties are classified into four families: ferrite, martensite, austenite, and duplex, stainless steel that includes combinations of the first three families, such as martensite-ferrite or austenite-martensite. The matrix structure of the different stainless steels determines the four classifications or families.

● The families of stainless steel are further divided into grades describing the properties of the alloys used to produce them. Older grades are designated by three-digit numbers established by the Society of Automotive Engineers (SAE). Although three-digit identifiers are common, many countries have their own systems, with North America using a six-digit system established by the American Society for Testing and Materials (ASTM).

● Regardless of the numbering system, each grade of stainless steel must comply with its predetermined combination of alloys. Each change, adjustment, or addition to an alloy impacts the performance of a grade of stainless steel. A specific set of characteristics, properties, and performance qualities are expected when families and grades are placed together and identified.
The different grades of stainless steel have various degrees of corrosion resistance, strength, toughness, and high and low-temperature performance. The specific determining factor for the various grades is their microstructure, which is observed using a microscope set at 25 times magnification. The microstructure of any material influences its physical properties, such as strength, toughness, ductility, hardness, corrosion resistance, temperature behavior, and wear resistance.

● The microstructure of stainless steel 316L has cell structures with boundaries enriched with chromium, manganese, molybdenum, and niobium elements, which enhances its corrosion resistance. The corrosion resistance is improved due to densification, the fine cellular structures, and the enrichment of chromium and molybdenum at the interfaces.

● Austenitic Stainless Steels: Austenitic stainless steels are non-magnetic with high levels of chromium and nickel and low levels of carbon. They are the largest and most used group of stainless steels.
Austenitic stainless steels have a face-centered cubic (FCC) crystal structure with one atom at each corner of the cube and one in the center of each face, a grain structure formed due to nickel being added as an alloy. The microstructure of austenitic stainless steel makes it tougher and more ductile, even at cryogenic temperatures.

● When subjected to high temperatures, austenitic stainless steels do not lose their strength, which gives them excellent formability and weldability. Since the austenitic structure is maintained at all temperatures, they do not respond to heat treatment. Instead, they are cold-worked to improve their toughness, strength, hardness, and stress resistance.

● The principle alloy for all austenitic stainless steels is nickel, which is used for all series 300 austenitic stainless steels, including grades 316L and 316L. When a stainless steel has a low nickel and high nitrogen content, it is no longer a 300 series stainless steel. The presence of nitrogen in stainless steels is limited since it can have very negative effects. Stainless steels with a low nickel and nitrogen content are classified as series 200 stainless steels.

● Series 316L - After series 304, series 316L is the second most used stainless steel, with a tensile strength of 549 MPa or 84 Ksi and a maximum use temperature of 1472 oF (800 oC). Although series 316L has lower tensile strength and temperature tolerance than series 304, it has better resistance to chlorides, like salt, which makes it the preferred choice for applications involving chlorides and salt.
Aside from its resistance to chlorides, the main difference between series 304 and series 316L is the presence of molybdenum in series 316L at percentages of 2% to 3%, which identifies series 316L as a Cr-Ni-Mo system. Adding molybdenum makes series 316L resistant to pitting caused by phosphoric acid, acetic acid, and dilute chloride solutions. The strength and toughness of molybdenum increase series 316L’s heat and wear resistance.

● Construction encasement, doors, windows and armatures, offshore modules, cisterns and pipes for chemical tankers, production, warehousing and overland transportation of chemicals, food and beverages, pharmacy, synthetic fibre, paper and textile plants and pressure vessels. Due to the low C-content, the resistance to intergranular corrosion is also guaranteed in the welded condition.