Stainless Steel 316 and 316L

Stainless Steel Grade 316 (UNS S31600) and Grade 316L (UNS S31603)

Chapter One – What is Stainless Steel 316?

Stainless steel is an alloy that contains a minimum of 10% chromium, which imparts its corrosion-resistant properties. The chromium forms a thin oxide layer on the surface of the metal, shielding it from corrosive elements.

A commonly used grade of stainless steel is 316, which typically includes 16 to 18% chromium, 10 to 14% nickel, 2 to 3% molybdenum, and a small amount of carbon. The addition of molybdenum enhances the corrosion resistance of stainless steel 316 compared to other grades. Further alloying improves its overall properties.

The properties and characteristics of stainless steel 316 make it the second-most widely used stainless steel grade after stainless steel 304. It is used in corrosive environments such as chemical plants, refineries, and marine equipment.

Stainless steel 316L features lower carbon content and is preferred for applications where there is a risk of sensitization. In contrast, stainless steel 316H has a higher carbon content, providing improved thermal stability and resistance to creep. Another popular variant is stainless steel 316Ti, which is stabilized to offer enhanced resistance to intergranular corrosion.

Stainless steel relies on the process of passivation, which makes the metal "passive" or resistant to oxidation from corrosive elements in the environment and process fluids. Passivation is achieved by exposing the stainless steel to air, allowing a layer of chromium oxides to form on its surface.

To improve the passive film formation, the stainless steel undergoes chemical treatment in acidic passivation baths, typically containing nitric acid. This process cleans the metal thoroughly, removing contaminants like free iron or iron compounds that could disrupt the passive layer.

Following acidic treatment, the metal is neutralized in an aqueous sodium hydroxide bath. Additionally, a descaling process is employed to eliminate oxide films formed during high-temperature processes such as hot-forming, welding, and heat treatment.

Chapter Two – How do stainless steel grades compare to stainless steel 316?

The key feature of stainless steel 316 is its molybdenum content, which significantly improves its resistance to corrosion. It is the second most widely used austenitic stainless steel, following grade 304. Austenitic stainless steels are distinguished by their nickel or nitrogen content, which imparts a unique crystalline structure to these materials.

Stainless steels are categorized based on their chemical composition, physical properties, metallographic structure, and functional characteristics. They are classified into four main families: ferritic, martensitic, austenitic, and duplex, with duplex stainless steels being combinations of the first three types, such as martensitic-ferritic or austenitic-martensitic. The matrix structure of these stainless steels defines their classification into these 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 adhere to its specified alloy composition. Any modification or addition to the alloy can significantly affect the performance of the stainless steel grade. When different families and grades are combined and identified, they are expected to exhibit a specific set of characteristics, properties, and performance traits.

Stainless steel grades vary in their levels of corrosion resistance, strength, toughness, and performance at high and low temperatures. The defining factor for each grade is its microstructure, which can be examined under a microscope at 25x magnification. The microstructure affects the material's physical properties, including strength, toughness, ductility, hardness, corrosion resistance, temperature stability, and wear resistance.

The microstructure of stainless steel 316 features cell structures with boundaries enriched with elements such as chromium, manganese, molybdenum, and niobium, which contribute to its enhanced corrosion resistance. This improved resistance results from the densification and fine cellular structures, as well as the enrichment of chromium and molybdenum at the boundaries of these structures.

• 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 316 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.

  

  
  
  • Stainless Steel 300 Series:
  • Austenitic stainless steel 300 series are designed to resist corrosion and wear with excellent formability and exceptional strength at any temperature. The defining element of the 300 series stainless steel is nickel content in percentages of 6% up to 20%, depending on the grade of 300 series stainless steel.
  • Series 304 - The most widely used of the 300 series of stainless steels is series 304, which is also the most widely used of all stainless steel alloys. It has a high tensile strength of 621 MPa or 90 Ksi with a maximum operating temperature of 1598°F (870°C). The many positive properties of series 304 stainless steel make it ideal for various applications.
  • Series 316 - After series 304, series 316 is the second most used stainless steel, with a tensile strength of 549 MPa or 84 Ksi and a maximum use temperature of 1472°F (800°C). Although series 316 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 316 is the presence of molybdenum in series 316 at percentages of 2% to 3%, which identifies series 316 as a Cr-Ni-Mo system. Adding molybdenum makes series 316 resistant to pitting caused by phosphoric acid, acetic acid, and dilute chloride solutions. The strength and toughness of molybdenum increase series 316’s heat and wear resistance.

  Comparison of the Elements of Series 304 Stainless Steel and Series 316 Stainless Steel
  	Type 304	Type 316
  Carbon	0.08% Max	0.08% Max
  Manganese	2.00% Max	2.00% Max
  Phophorus	0.045% Max	0.045% Max
  Sulfer	0.030% Max	0.030% Max
  Silicon	1.00% Max	1.00% Max
  Chromium	18.00 - 20.00	16.00 - 18.00
  Nickel	8.00 - 10.50%	10.00 - 14.00
  Molybdenum	-	2.00 - 3.00%

• Ferritic Stainless Steels: As the name suggests, these stainless steels that have a ferritic microstructure. Its ferritic microstructure is present at all temperatures due to the addition of chromium with little or no austenite forming elements such as nickel. Because of this constant microstructure, like the austenitic stainless steel, they do not respond to heat treatment. They are more difficult to weld due to excessive grain growth and intermetallic phase precipitation, especially at higher chromium content. The result is lower toughness after welding which makes them unsuitable for structural materials. Ferritic stainless steels are designated as AISI 400 series. This designation is shared with martensitic stainless steels.

  

  
  

• Martensitic Unit Cell: These stainless steels have higher amounts of carbon that promotes a martensitic microstructure. Martensitic stainless steels are hardenable by heat treatment. When heated above its curie temperature, they have an austenitic microstructure. From an austenitic state, heating rapidly results in martensite, while cooling slowly promotes the formation of ferrites and cementite. Varying the carbon content results in a wide range of mechanical properties, making them suitable for engineering and tool steels. Increasing the carbon content makes the stainless steel harder and stronger, while decreasing it makes the alloy more ductile and formable. However, adding more carbon results in lower chromium to maintain a martensitic microstructure. Thus, higher strength is attained at the expense of corrosion resistance. They generally have lower corrosion resistance than ferritic and austenitic stainless steels.

  

  
  

• Duplex Stainless Steels: This type of stainless steel consists of a combination of austenitic and ferritic metallurgical structures, usually in equal amounts. It is created by adding more chromium and nickel to a standard martensitic stainless steel, promoting a duplex ferritic-austenitic microstructure. Since they do not have a constant ferritic and austenitic microstructure, they respond to heat treatment. Austenitic stainless steel is far superior to ferritic in terms of corrosion resistance and mechanical properties. However, they are highly susceptible to stress corrosion cracking. Stress corrosion cracking happens when a crack propagates when the material is subjected to a highly corrosive environment. This can lead to the sudden failure of ductile materials. A ferritic microstructure is resistant to stress corrosion cracking. By combining the ferritic phase with the austenitic phase, added resistance to stress corrosion cracking is obtained. Aside from improved corrosion resistance and mechanical properties, the price of duplex stainless steels is more stable than austenitic. This is attributed to the lower nickel content. The most common grade is the standard duplex 2205. Duplex stainless steels are not covered by AISI designation.

  

  
  

• Precipitation Hardening Stainless Steels: These are stainless steels that can further be modified by precipitation hardening. Initially, precipitation hardening stainless steels are supplied in a solution annealed condition. Manufacturers can perform an additional aging process to attain the desired mechanical properties. Note that this heat treatment has a different mechanism than hardening martensitic stainless steels. In precipitation hardening, precipitates or secondary phase particles are allowed to form at elevated temperatures usually lower than the curie temperature. The formation of these secondary phase particles is promoted by alloying elements such as copper, niobium, aluminum, and titanium. Their growth rate, size, and dispersion are controlled by temperature and time. These secondary phase particles act as dislocation sites to the crystal structure, improving the metal's overall toughness and strength. Moreover, unlike the martensitic varieties, they have comparable corrosion resistance with austenitic and ferritic stainless steels.

  

  
  

Chapter Three – Who are the top producers of stainless steel 316?

Source 21, Inc.

Source 21 is a supplier of stainless steel in coils and strips, offering various grades including 316 and 316L. Their processing services for stainless steel coils include cutting to length, polishing, masking, and tempering. The finished products are available in custom boxes, export crates, or specialty skid mounts. Source 21 serves multiple industries, such as aerospace, roll forming, automotive manufacturing, and food processing. The company is committed to providing stainless steel solutions that meet the diverse needs of their customers, whether they require standard grades or specialized alloys.

Cleveland-Cliffs Inc.

Cleveland-Cliffs is a leading producer of flat-rolled stainless steel in North America, with iron ore mining operations in Michigan and Minnesota. The company supplies stainless steel to the automotive industry and is committed to steel recycling and sustainability. Cleveland-Cliffs manufactures all five main types of stainless steel, including grades 304 and 304L, and caters to both the automotive sector and other manufacturing industries.

Allegheny Technologies (ATI)

Allegheny Technologies is a manufacturer of a diverse array of metals and specialty metals, including various grades of stainless steel. The company provides a comprehensive range of stainless steels, including specialized forms such as superaustenitic and superferritic stainless steels. ATI produces stainless steel through hot rolling, offering a wide range of product sizes and thicknesses. The company is known for its ability to meet customer orders efficiently with short lead times.

Acerinox

Acerinox is a Spanish manufacturer specializing in various forms of stainless steel for industries including transportation, industrial equipment manufacturing, food processing, and environmental technology. The company produces stainless steel plates, hot and cold coils, sheets, strips, and discs. With a global presence, Acerinox provides its diverse range of stainless steel products to a broad spectrum of industries and customers worldwide.

Aperam

Aperam, based in Luxembourg, delivers a wide range of stainless steel products to 40 countries, with a production capacity of 2.5 million tons across five global facilities. The company offers stainless steel coils, sheets, tubes, discs, flat bars, strips, and heavy plates, and operates a service center in Sterling Heights, Michigan, specializing in flat stainless steel. Aperam is committed to innovation and high-performance solutions for technological and manufacturing challenges. All of its stainless steel products are recyclable and produced with a notably low CO2 footprint.

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Chapter Four – What are the composition and alloying elements of stainless steel 316?

As discussed earlier, stainless steel 316 is part of the austenitic family, where nickel acts as a stabilizer for the austenitic structure. The typical composition of stainless steel 316 includes 16–18% chromium, 10–14% nickel, 2–3% molybdenum, up to 2% manganese, up to 0.75% silicon, up to 0.10% nitrogen, up to 0.08% carbon, up to 0.045% phosphorus, up to 0.03% sulfur, with iron making up the remainder. Additional alloying elements like titanium and niobium may be included to produce other grades. The compositions of various stainless steel grades are summarized below.

Notes:

1. The minimum amount of titanium is calculated as 5 x (C + N). The maximum amount is 0.70%.
2. Columbium (Cb) is now known as Niobium (Nb). The minimum amount of niobium is calculated as 10 x (C + N). The maximum amount is 1.10%.

Effect of Alloying Elements

The alloying elements of stainless steel 316 and their effects on the properties of the alloy are listed below.

• Carbon: This is the main alloying element of steel. Iron alone has poor mechanical properties, but when alloyed with varying amounts of carbon imparts a wide range of hardness and strength. Adding carbon makes the steel harder and stronger but more brittle. Decreasing it improves ductility. Also, adding sufficient amounts of carbon allows the steel to respond to heat treatment. However, there is a certain limit to how much carbon can be added. For austenitic stainless steels, adding too much carbon promotes sensitization. Sensitization is the precipitation of chromium carbides at the grain boundaries that consumes the chromium from the adjacent regions. This makes the stainless steel susceptible to intergranular corrosion.

  

  
  
• Chromium: The addition of chromium makes steel become stainless steel. The minimum amount required is around 10.5%. Chromium at the surface reacts with oxygen to form a passive layer of chromium oxide which protects the metal from corrosion. Chromium has a ferrite stabilizing effect on steel. For austenitic stainless steels, the amount of chromium is balanced with other alloying elements that promote an austenitic microstructure.

• Nickel: Nickel is added to stainless steel to form or retain an austenitic microstructure at room and low temperatures. The minimum amount required to stabilize an austenitic microstructure is around 8 to 9%. In austenitic stainless steels, 10 to 14% is required due to the addition of molybdenum, another ferrite former aside from chromium.

  

  
  
• Molybdenum: Molybdenum is added to maintain the high-temperature toughness of stainless steel. The toughness of these stainless steels significantly decreases when used at temperatures around 752 to 1022°F (400 to 550°C). This phenomenon is known as temperature embrittlement. Aside from maintaining toughness, adding molybdenum increases the stainless steel‘s resistance to pitting corrosion.
• Manganese: Manganese is added with nitrogen to decrease the amount of nickel required to maintain an austenitic microstructure. Substituting manganese and nitrogen for nickel reduces the impact of nickel price volatility and lowers its cost. Moreover, it reacts with sulfur forming manganese sulfide, which prevents the formation of the more unstable compound, ferrous sulfide. At the same time, manganese sulfide inclusions reduce sulfur brittleness and improve the machinability of stainless steel.
• Nitrogen: Nitrogen is added with manganese to promote the formation of an austenitic microstructure. Nitrogen is more powerful in forming austenite than nickel, manganese, and even carbon. Alloying nitrogen produces effects similar to carbon but with additional benefits. Nitrogen has a lesser tendency to react with chromium. Thus, its amounts can be increased to improve the strength of the stainless steel with lesser susceptibility to sensitization. This, in turn, increases its resistance to intergranular corrosion. Moreover, when alloyed with molybdenum, it increases the stainless steel‘s resistance to pitting corrosion.

• Titanium: Titanium is a stabilizer added to standard or straight 316 stainless steels to form the 316Ti variant. Titanium is a stronger carbide-former than chromium. At high temperatures, chromium tends to react with carbon and precipitate at grain boundaries. In stainless steel 316Ti, titanium reacts with carbon instead of chromium. This maintains the amount of chromium present within the austenite, resulting in the high-temperature stability of 316Ti. By lessening the formation of precipitates, intergranular corrosion resistance is improved.

  

  
  
• Niobium (Columbium): Like titanium, niobium is a stabilizer in stainless steel, particularly the 316Cb grade. They are sometimes used in conjunction with titanium. Titanium stabilizes better while niobium ensures excellent weld strength and creep resistance.
• Silicon: Silicon is a deoxidizer in steel and is present in alloys as minor residues. The presence of small amounts of silicon improves the strength of stainless steel. In high amounts, it tends to form intermetallics at elevated temperatures that cause embrittlement.
• Phosphorus: Phosphorus is also a residue from the manufacture of carbon steel. High amounts of phosphorus increase vulnerability to temper embrittlement and are more detrimental than silicon.
• Sulfur: Sulfur is naturally present in raw ores and slags. Like silicon and phosphorus, sulfur is present in stainless steel as residues from production. High levels can cause sulfur embrittlement and adversely affect on weldability and high-temperature performance. Moreover, it decreases resistance to corrosion, particularly pitting resistance. When added in controlled conditions, sulfur improves the machinability of stainless steel. In this case, manganese is used to counter the negative effects of sulfur.

Chapter Five – What are the different grades of stainless steel 316, and what are their properties?

Stainless steel 316 is the second most commonly used stainless steel grade after 304. It is favored for its addition of molybdenum, which enhances its resistance to chemical attacks, particularly from chloride solutions. Beyond the role of molybdenum, many of its beneficial properties are due to its austenitic microstructure.

General Properties

Below is a summary of the general properties of stainless steel 316 and its variants. These properties highlight its advantages compared to other types of stainless steel.

• Corrosion Resistance: All stainless steel 316 grades have molybdenum as an alloying element that further improves corrosion resistance, particularly pitting corrosion. Pitting is a highly localized type of corrosion that creates shallow holes on the surface of the metal. This takes place in the presence of solutions containing chloride ions, such as seawater. High resistance to pitting corrosion makes stainless steel 316 recommended for marine applications. Molybdenum, together with chromium and nitrogen, is one of the factors in determining the pitting index or pitting resistance equivalence number.

  

  
  
• Toughness: Because of its austenitic microstructure, stainless steel 316 can retain its toughness over a wide range of temperatures, unlike ferritic and martensitic grades. Ferritic grades tend to form intermetallic phases that contribute to embrittlement, while martensitic grades generally have high carbon content, making them intrinsically harder but more brittle.

• Weldability: Austenitic stainless steels experience fewer negative effects from welding. They can retain their toughness and impact strength since they do not transform to martensite. They are less susceptible to cold cracking as encountered in martensitic stainless steels. Because of these, they are suitable for welding fillers even in welding different stainless steel groups.

  

  
  
• Hardenability: As mentioned earlier, austenitic stainless steel is not hardenable by heat treatment. Hardness can be obtained through cold-working. Compared with ferritic stainless steels, austenitic types respond better to cold working.

Specific Properties of Grades of Stainless Steel 316

Listed below are the various grades of stainless steel 316. These grades are modified versions of the standard 316 composition, where the carbon content is reduced or stabilizing alloying elements are added. These modifications enhance or maintain mechanical properties and corrosion resistance, particularly after welding. Variants with higher carbon and nitrogen contents are utilized for their improved hardness and creep resistance.

• 316L: As of now, this is perhaps the most widely used variant compared to the standard and the 316Ti grade. Originally, low carbon grades were more expensive and difficult to produce until the introduction of the production process known as Argon Oxygen Decarburization (AOD). This grade of stainless steel 316 has a lower carbon content to reduce the effects of sensitization.

  Lower carbon content means less formation of chromium carbide precipitates and less depletion of chromium in regions near the grain boundaries. This improves the retention of toughness and corrosion resistance of the stainless steel after welding.

• 316H: This grade contains higher amounts of carbon, improving its thermal stability and creep resistance. Its corrosion resistance is comparable to 316L. However, due to the high carbon content, it is prone to sensitization which makes welding joints vulnerable to corrosion.

  

  
  
• 316Ti and 316Cb: These are referred to as stabilized stainless steels. Instead of lowering the carbon content, titanium and niobium are added to reduce the effects of sensitization. Titanium and niobium are strong carbide and nitride formers, which helps prevent chromium from being consumed. Both grades have increased resistance to intergranular attack at the welded regions.
• 316N: This is a less popular grade of stainless steel 316 which has higher amounts of nitrogen. Nitrogen-rich stainless steels are usually considered highly alloyed, super austenitic grades in which higher amounts of chromium and molybdenum can be added. Alloying nitrogen in stainless steel 316 creates similar properties as adding more carbon, which results in improved hardness and higher strength.
• 316LN: This is a 316 variant with a lower carbon but higher nitrogen content. Similar to 316L, having a lower carbon content enables it to have better corrosion resistance in the welded condition. To compensate for the loss of carbon, the amount of nitrogen is increased to improve its mechanical properties.

Conclusion

• Stainless steel is a type of iron alloy containing a certain percentage of chromium that imparts corrosion resistance to the metal. Its corrosion resistance is achieved by through a thin film of metal oxides that protects against corrosive materials.
• The primary alloying elements of stainless steel 316 are 16–18% chromium, 10–14% nickel, and 2–3% molybdenum. The added molybdenum makes this grade more corrosion-resistant than other types.
• There are five main groups of stainless steel. These are austenitic, ferritic, martensitic, duplex, and precipitation hardening.
• Stainless steel 316 belongs to the austenitic group. It is the largest group of stainless steel comprising around two-thirds of all stainless steel production. Their austenitic microstructure imparts desirable characteristics such as low-temperature toughness, high-temperature stability, good formability, and weldability.
• Besides carbon, chromium, nickel, and molybdenum, other elements are added to modify the properties of the alloy. This is to improve or retain its mechanical properties and corrosion resistance after welding. The high carbon and high nitrogen variants are used for their increased hardness and creep resistance.

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