Stainless Steel 316

Chapter One – What is Stainless Steel 316?

Stainless steel is a steel alloy that contains at least 10% chromium, making it corrosion resistant. The addition of chromium to stainless steel creates a thin film of metal oxide over the metal's surface that protects against corrosive materials.

A popular grade of stainless steel is stainless steel 316 is generally composed of 16 to 18% chromium, 10 to 14% nickel, 2 to 3% molybdenum, and a small percentage of carbon. Adding molybdenum to stainless steel 316 increases its corrosion resistance compared to other grades of stainless steel. The addition of other alloys further enhances its 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 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 316 is the stabilized 316Ti. Stainless steel 316Ti 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 316

The predominant characteristic of stainless steel 316 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 316 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 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 oF (870 oC). 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 oF (800 oC). 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.

  
  

  

  
  
• 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 – Stainless Steel 316 Composition and Alloying Elements

As mentioned in the previous chapter, stainless steel 316 belongs to the austenitic group in which nickel is added as the austenite stabilizer. The standard composition of stainless steel is 16–18% chromium, 10–14% nickel, 2–3% molybdenum, 2% manganese (maximum), 0.75% silicon (maximum), 0.10% nitrogen (maximum), 0.08% carbon (maximum), 0.045% phosphorus (maximum), 0.03% sulfur (maximum), and iron as the balance. Other alloying elements, such as titanium and niobium, are added to make other grades. The compositions of different 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

Enumerated below are the alloying elements of stainless steel 316 and their effects on the alloy‘s properties.

• 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 Four – Stainless Steel 316 Grades and Properties

Stainless steel 316 is the second most widely used stainless steel grade next to 304. 316 is preferred due to the presence of molybdenum, which makes it suitable for applications with is a higher risk of a chemical attack, especially from chloride solutions. Aside from the alloying of molybdenum, most of its desirable properties are attributed to its austenitic microstructure.

General Properties

Summarized below are the general properties of stainless steel 316 and its variants. Most of these properties describe their advantages over 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

Below are the different grades of stainless steel 316. These are modifications of the standard 316 composition wherein the amount of carbon is decreased or alloying elements called stabilizers are added. 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.

• 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.