Stainless Steel 316

Chapter One – What is Stainless Steel 316?

Stainless steel is a type of steel alloy containing a minimum of 10.5% chromium. Chromium imparts corrosion resistance to the metal. Corrosion resistance is achieved by creating a thin film of metal oxides that acts as protection against corrosive materials. A popular grade of stainless steel is stainless steel 316. Stainless steel 316 is generally composed of 16 – 18% chromium, 10 – 14% nickel, 2 – 3% molybdenum, and about 0.08% carbon. The added molybdenum makes this grade more corrosion resistant than the other types. Aside from those mentioned, other elements can be added to modify certain properties of the alloy. Stainless steel 316 is widely used in highly 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 allowing the stainless steel to be exposed 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 process 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 then neutralized in a bath of aqueous sodium hydroxide. Descaling is also done to remove other oxide films formed by high-temperature milling operations such as hot-forming, welding, and heat treatment.

Chapter Two – Overview of Stainless Steel Classifications

Stainless steels are available in various grades that are used for specific applications. Different grades have their degree of corrosion resistance, strength, toughness, high and low-temperature performance. Stainless steel grades are generally classified according to their microstructure. There are five main groups of stainless steel. These are austenitic, ferritic, martensitic, duplex, and precipitation hardening.

• Austenitic Stainless Steels: These are the largest group of stainless steels which comprise around two-thirds of all stainless steel production. Their austenitic microstructure allows them to be tough and ductile, even at cryogenic temperatures. Moreover, they do not lose their strength when subjected to high temperatures. These attributes result in excellent formability and weldability. Since the austenitic structure is maintained at all temperatures, they do not respond to heat treatment. Their hardness and high tensile strength are acquired through cold working. Austenitic stainless steels are further divided according to the austenite forming elements.
  
  

  

  
  
  • Stainless Steel 300 Series: These are stainless steels that achieve their austenitic microstructure through the addition of nickel. These are the largest subgroup and are considered general-purpose stainless steels. This sub-group includes stainless steel 316 and other popular grades such as 302, 304, and 317.
  • Stainless Steel 200 Series: These are austenitic stainless steels that use manganese and nitrogen to minimize the use of nickel. Alloying with nitrogen increases their yield strength by approximately 50% than 300 series stainless steel. However, lowering the nickel content reduces the corrosion resistance of the alloy.
• Ferritic Stainless Steels: As the name suggests, these are 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, cooling 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 which makes them suitable for engineering steels 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 which promotes 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 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 which improves the overall toughness and strength of the metal. Moreover, they have comparable corrosion resistance with austenitic and ferritic stainless steels, unlike the martensitic varieties.
  
  

  

  
  

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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 are also added such as titanium and niobium 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 which 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 which is another ferrite former aside from chromium.
  
  

  

  
  
• Molybdenum: Molybdenum is added to maintain the high-temperature toughness of stainless steel. When stainless steels are used at temperatures around 400 to 550°C, their toughness significantly decreases. 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. It is known that 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. This results 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 provides a better stabilizing effect while niobium ensures excellent weld strength and creep resistance.
• Silicon: Silicon is used as a deoxidizer in making 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 present as 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 have adverse effects 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 where there 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, in contrast with ferritic and martensitic grades. Ferritic grades tend to form intermetallic phases that contribute to embrittlement while martensitic generally have high carbon content making them intrinsically harder but 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 the welding of different stainless steel groups.
  
  

  

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

Grades of Stainless Steel 316 and Their Specific Properties

Below are the different grades of stainless steel 316. Basically, 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: Currently, 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 lesser formation of chromium carbide precipitates and less depletion of chromium at 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 which improves 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 of these 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 as 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 that has 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 which imparts corrosion resistance to the metal. Its corrosion resistance is achieved by creating a thin film of metal oxides that acts as protection 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 which comprise 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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