What is Stainless Steel 316?
Stainless steel is an alloy that includes chromium, at a bare minimum of 10%. Alloys of this nature owe their corrosion resistance to the protective layer of oxide film generated due to chromium, on the surface of steel.
At the ‘Stainless Steel’ level, 316 is prominent among others. Its composition typically contains 16% to 18% chromium, 10% to 14% nickel, and 2% to 3% Molybdenum, along with some carbon.
The addition of molybdenum and its further alloying with other elements improves the overall stainless steel 316’s attributes and enhances its corrosion resistance over other grades.
Due to its properties and advantages, it is the second most used stainless steel after 304 grade. Chemical plants, refineries, and marine applications are among the many corrosive environments where it works wonders.
When sensitization poses an issue, low carbon variant, stainless steel 316L is preferred. On the other hand, with more carbon comes better thermal stability and creep resistance, in stainless steel 316 H. Lastly, stainless steel 316Ti is a variant that has been stabilized for greater intergranular corrosion resistance.
Passivation greatly enhances the resistance of stainless steel to oxidation in a corrosive environment or process fluid. This is done by exposing stainless steel to air where a protective layer of chromium oxides forms on its surface.
Stainless steel is usually treated chemically with acid passivation baths that contain nitric acid. These baths help erode the impurities such as free iron or iron compounds which would otherwise damage the passive layer. The layer is referred to as a “passive film”.
The metal undergoes neutralization in a sodium hydroxide bath after the acidic treatment. The metal is additionally subjected to a descaling procedure during neutralization which removes oxide films formed during hot-forming, welding, and heat treatment processes.
How do stainless steel grades compare to stainless steel 316?
Molybdenum greatly enhances the resistance of stainless steel 316 to corrosion which is its defining characteristic. Stainless steel 316 is the second most used austenitic stainless steel and is only surpassed in use by grade 304.
The distinguishing trait of austenitic stainless steels is the addition of nitrogen and nickel which results in a unique crystalline structure to these materials.
The classification of stainless steels takes into consideration the alloys’ composition, physical properties, microstructure, and functionalities.
This classification includes ferritic, martensitic, austenitic, and duplex families. Duplex stainless steels are the combination of the first three types, for instance martensitic-ferritic or austenitic-martensitic. These families are distinguished by their primary matrix structure which defines their classification.
Stainless steels families are also subdivided into different grades that detail the properties of the alloys utilized in production. The older grades are identified with three-digit numbers given by the Society of Automotive Engineers (SAE).
Despite the use of three-digit identifiers, numerous other countries developed their own identification systems, with North America adopting a six-digit one by the American Society for Testing and Materials (ASTM).
Every grade of stainless steel, regardless of the identification criteria, must follow the designated composition for the specified alloys. Changes or addition to the alloy could greatly alter the performance of the stainless steel grade.
It is assumed that the hybrid of different families and grades, once combined and labeled, will perform in line with the predicted benchmark standards for characteristics, properties, and overall performance.
The differing grades of stainless steel are associated with different amounts of toughness, strength, resistance to corrosion, high and low temperature performance, and many more. Each grade has a defining factor and that is its microstructure, which can be seen under a 25x microscope.
The microstructure affects physical properties and its parameters include but are not limited to: strength, ductility, hardness, along with wear and corrosion resistance, static temperature, and stability, as well as dynamic energy.
Chromium, manganese, molybdenum, and niobium are some of the elements that corrode less and allow stainless steel 316 to resist corrosion. Their presence gives 316 cell structures in the boundary and gives cellular structures with boundaries.
The reason for this advanced resistance is the fine cellular structures that are formed along with densification and the increase of molybdenum and chromium at the boundary.
Austenitic Stainless Steels:
The largest and most used group of stainless steels are Austenitic stainless steels. They have low levels of carbon, but a more than sufficient amount of nickel and chromium. And most importantly, they are non magnetic.
Thanks to nickel acting as an alloy, austenitic stainless steels contain a face-centered cubic (FCC) crystal structure with one atom at each corner of the cube and one in the center of each face. The microstructure of austenitic stainless steel also makes it more ductile and tough, even at cryogenic temperatures.
The strength of austenitic stainless steels is retained at high temperatures, thus they have very good formability and weldability.
As the austenitic structure is retained at all temperatures, they are not responsive to heat treatment. Their toughness, strength, hardness, and stress resistance are enhanced through cold working.
Nickel is the main constituent of all austenitic stainless steels and is found in all 300 series austenitic stainless steels including 316 and 316L. A stainless steel is no longer considered a 300 series if it has low nickel and high nitrogen, thus becoming a 200 series stainless steel.
Nitrogen is restricted in stainless steels due to its detrimental effects. Hence, low nickel and nitrogen stainless steels are classified as 200 series stainless steels.
- Stainless Steel 300 Series: Austenitic stainless steel 300 series combines increased resistance to corrosion and wear while sustaining formability and strength, especially at elevated temperatures. The 300 series stainless alloys nickel content is 6% to 20% depending on the grade of 300 series stainless steel.
- Series 304: Of all the stainless steel alloys, Series 304 stands out for its wide acceptance and use. Its alloys have a tensile strength of 621 MPa or 90 Ksi and a maximum operating temperature of 870 °C (1598 °F). Due to the numerous positive characteristics of series 304 stainless steel, it can be used in various applications.
- Series 316: Series 316 is the second most widely used stainless steel, with a tensile strength of 549 MPa (84 Ksi) and a maximum operating temperature of 800 °C (1472 °F). Like series 316 is preferred for applications involving chlorides and salt, it is known for having weaker tensile strength and temperature tolerance compared to series 304.
Other than resistance to chlorides, the sequel difference distinguishing 304 and 316 is the 2 to 3 percent addition of molybdenum in series 316, making it a Cr-Ni-Mo System, thus identifying it as cr nickel molybdenum steel.
The addition of molybdenum provides series 316 with enhanced resistance to pitting from phosphoric acid, acetic acid, and even dilute chloride solutions. Furthermore, the heat and wear resistance of Series 316 is increased with the addition of molybdenum, which enhances the strength and toughness of the metal.
Ferritic Stainless Steels:
As the name indicates, these stainless steels possess a ferritic microstructure. It is caused primarily by the addition of chromium with little or no austenite forming elements like nickel, so it is present at all temperature.
Because of this constant microstructure, as in the case of austenitic stainless steel, they do not respond to heat treatment. Due to excessive grain growth and intermetallic phase precipitation, especially at higher chromium content, they are more difficult to weld.
The outcome is reduced toughness after welding, which renders them unsuitable for structural materials. These ferritic stainless steels are classified as AISI 400 series and share this designation with martensitic stainless steels.
Martensitic Unit Cell:
This class of stainless steels has higher carbon content to support a martensitic microstructure. The martensitic stainless steels are hardened by heat treatment. Above curie temperature, they exhibit an austenitic microstructure.
Rapid heating from the austenitic state results in martensite formation whereas slow cooling promotes the formation of cementite and ferrites. These also have wide range of mechanical properties which make them suitable for sustaining tool and engineering steels.
The carbon content is directly proportional to the hardness and strength of the stainless steel, while reducing the carbon makes the alloy ductile and formable.
Increasing carbon content, however, necessitates lowering chromium to sustain martensitic microstructure which leads to lower chromium containing stainless steels.
Consequently, these gain higher strength at the expense of increased corrosion resistance. They are usually found to possess lower resistance to corrosion as compared to ferritic and austenitic stainless steels.
Duplex Stainless Steels:
Like other Duplex Stainess steels, a combination of austenitic and ferritic structures in equal proportions comprise this type of steel. More chromium and nickel is added into a standard martensitic stainless steel, it promotes a microstructure of ferritic-austenite, creating Duplex Stainless Steel.
Since they do not have a constant ferritic and austenitic microstructure, they respond to heat treatment. Corrosion resistance alongside mechanical properties of the steel is great provided its ferritic components are outranked by components of the austenitic structure.
At the same time, it’s important to note that their alloys do not undergo change in phase or temperature, making them vulnerable to sress corrosion cracking.
The development of a crack on a material when placed in a very corrosive environment is called stress corrosion cracking. Which, in turn, may result in brittle fracture in otherwise ductile structures.
Stress corrosion cracking is proved to be effective with the ferritic structure. By combining the ferritic phase with the austenitic phase, added resistance to stress corrosion cracking is obtained.
Duplex Stainless Steels are believed to be priced lower than austenitic, but their mechanical properties and resistance to corrosion are greatly enhanced.
Other grades comprise standard duplex 2205, which is further most popular. Not covered by the AISI designation, duplex stainless steels do not carry an identifier.
Precipitation Hardening Stainless Steels:
These are stainless steels that can be further enhanced with precipitation hardening. Precipitation hardening stainless steels are supplied in solution annealed form initially.
An additional aging step can be executed to increase the mechanical properties of the material. It should be noted that this heat treatment is different from the mechanism associated with hardening martensitic stainless steels.
During precipitation hardening, precipitates or secondary phase particles are allowed to form at higher temperatures, typically just below the curie temperature.
The alloying elements copper, niobium, aluminum, and titanium serve to advance the rate of formation of these secondary phase particles. The evolution rate, dimensions, and distribution are dictated by the duration and temperature.
These secondary phase particles provide more sites for dislocation in the crystal lattice increasing the toughness and the strength of the metal. In addition, these particles are more resistant to corrosion compared to martensitic variants and are similar to austenitic and ferritic stainless steels.
What are the composition and alloying elements of stainless steel 316?
Stainless steel 316 is classified to the austenitic family and as previously mentioned, it has nickel as one of its primary constituent which is austenite stabilizer.
The constitution of stainless steel 316 generally contains 16–18% chromium, 10–14% nickel, 2–3% molybdenum, up to 2% manganese, 0.75% silicon, 0.10% nitrogen, 0.08% carbon, 0.045% phosphorus, 0.03% sulfur with the remainder being iron.
Some other grades may require additional alloying materials like titanium and niobium.
#1. Carbon.
This Carbon alloying element is the principal for steel. Iron has mechanical attributes, and its application in industry alone doesn’t lend itself to significant value.
But when alloyed with varying amounts of carbon, iron has a wide scope of hardness and strength. It boils down to managing how much carbon is added to ameliorate-sharpness and strength to the steel, while making it brittle.
Lowering these levels increases ductility. Also, adding certain levels of carbon permits steel to be subjected to heat treatment. However, there is a limit to the optimal amount of carbon.
For austenitic stainless steels, having overly abundant carbon causes sensitization. Sensitization is the precipitation of chromium carbides at the grain boundaries that consumes the chromium from the adjacent regions, in subsequent steps this stainless steel will be more vulnerable to intergranular corrosion.
#2. Chromium.
The steels become stainless upon the inclusion of chromium. At minimum, over 10.5% is required. With better regard to its surface, chromium works like a double edged sword, rod and chain. Steel, using chromium at the surface literally becomes corrosion proof while also transforming into oxide.
Chromium has a ferrite stabilizing effect on steel. Equivalently important for austenitic stainless steel, the amount of other alloying elements is balanced for chromium that fosters austenitic microstructure.
#3. Nickel.
The role of nickel in stainless steel is to help retain an austenitic microstructure or, at the very least, maintain one at room and low temperatures.
Nickel is needed in at least 8 to 9% amount to stabilize an austenitic microstructure, whereas in austenitic stainless steels, 10 to 14% is needed due to the addition of molybdenum, another ferrite former aside to chromium.
#4. Molybdenum.
Molybdenum is added to stainless steels to provide high-temperature toughness. The temperature range where these stainless steels begin to lose toughness is between 752 to 1022°F (400 to 550°C).
This effect is called temperature embrittlement. Other than preserving toughness, molybdenum adds to the pitting corrosion resistance of the stainless steel.
#5. Manganese.
Manganese, along with nitrogen, is introduced in order to reduce the amount of nickel needed to maintain an austenitic microstructure. Replacing nickel with manganese and nitrogen lowers the impacts of nickel’s market volatility, thus lessening overall costs.
Additionally, manganese also aids in the form of a compound known as manganese sulfide, which protects it from forming a more unstable compound called ferrous sulfide. Manganese sulfide inclusions, therefore, lessen sulfur brittleness while improving machinability of stainless steels.
#6. Nitrogen.
Nitrogen is used with manganese to aid an austenitic microstructure formation. In fact, nitrogen is more effective than even nickel, manganese, and carbon in forming austenite.
The alloying effect of nitrogen is similar to that of carbon but is more advantageous. This is due to the fact that nitrogen is less likely to bond with chromium. Therefore, the nitrogen amount can be increased to strengthen the stainless steel without the sensitization problem.
This also improves the intergranular corrosion resistance. In addition, it improves the pitting corrosion resistance of stainless steel when alloyed with molybdenum.
#7. Titanium.
Titanium is a stabilizing element for standard or straight 316 stainless steels. They transform to the 316Ti variant. Titanium forms stronger carbides than chromium. At elevated temperatures, chromium has a tendency to react with carbon and precipitate over grain boundaries.
In the case of stainless steel 316Ti, it is titanium that reacts with carbon instead of chromium. Doing so enables retaining chromium in the austenite thus ensuring high-temperature stability of 316Ti. Reduction in precipitate formation improves the intergranular corrosion resistance.
#8. Niobium (Columbium).
With stainless steel grade 316Cb in particular, niobium is known to serve as a stabilizer. It is noted that niobium is used in conjunction with titanium on occasion. While titanium performs better as a stabilizer, niobium provides excellent weld strength as well as resistance to creep.
#9. Silicon.
Silicon is known to serve as a deoxidizer in steel and is found as an alloy in lesser amounts. The value of stainless steel is enhanced due to the presence of small amounts of silicon.
On the other hand, large quantities tend to form intermetallics at elevated temperatures causing the steel to become brittle.
#10. Phosphorus.
Phosphorus may be regarded as an impurity which comes from carbon steel. With phosphorus, the weakening of temper resistance becomes more of a problem and is known for being more detrimental than silicon.
#11. Sulfur.
As with silicon and phosphorus, sulfur comes from ore and slag. It can be found naturally. Moreover, like silicon and phosphorus, sulfur can be found in stainless steel as a byproduct of production. SbE can occur, negatively impacting weldability at high temperatures.
In addition, sulpher at high quantities can reduce the ability to resist corrosion, especially pitting, greatly. However, when added under precise conditions, sulpher enhances the ability for stainless steel to be shaped. In this case, mangnese counters the adverse impacts of sulpher.
What are the different grades of stainless steel 316, and what are their properties?
316 stainless steel is the second most popular grade of stainless steel, after 304. It’s preferred because of the addition of molybdenum that makes it more resistant to chemical attack, especially from chloride solutions.
Apart for the role of molybdenum, many its advantageous characteristics are because of its austenitic microstructure.
General Properties
Below is a compilation of general properties of stainless steel 316 and its variants. These properties in comparison to the alloys make stainless steel 316 stand out amongst other varieties of stainless steel.
Corrosion Resistance:
All 316 grades of stainless steel contains molybdenum alloyed which further complicates the grade’s corrosion resistance to also include pitting corrosion. Pitting is form of localized corrosion that is very severe and leaves shallow cavities on the surface of the metal.
This happens with solutions containing chloride ions, for example seawater. Due to the high resistance to pitting corrosion, stainless steel 316 is highly suggested for marine applications.
Molybdenum along with chromium and nitrogen are factors in the pitting index or pitting resistance equivalence number (PREP).
Toughness:
Unlike the ferritic and martensitic grades, stainless steel 316 can maintain, over a broad range of temperatures, its toughness due to its austenitic microstructure.
Ferritic grades are prone to the formation of intermetallic phases that lead to the embrittlement of the material, whereas martensitic grades are typically high in carbon content which makes them harder, and more brittle.
Weldability:
Negative impact of welding is less severe on austenitic stainless steels. The reason being they do not transform to martensite, which means that toughness and impact strength can be retained.
They are less prone to cold cracking as encountered in martensitic stainless steels. For these reasons, they are appropriate for use as welding fillers even when welded to different groups of stainless steels.
Hardenability:
As stated above, austenitic stainless steel is not haredable by heat treatment. Hardness is achievable through cold-working. When compared with ferritic stainless steels, austenitic types have better responsiveness to cold working.
Specific Properties of Grades of Stainless Steel 316
The following are all of the types of stainless steel 316. These grades represent modifications to the standard 316, where the carbon content is either lowered or stabilizing alloying elements are added that in turn alter the properties of the steel.
These changes, especially post-welding, improve or at least maintain the mechanical properties as well as the corrosion resistance. The variants that possess higher amounts of carbon and nitrogen are used for their enhanced strength and creep resistance.
- 316L: Currently, this is arguably the most popular variant in comparison to both the standard and 316Ti grade. In the past, the low carbon grades were costly and challenging to manufacture due to the stringent production processes. However, with the introduction of the production process known as Argon Oxygen Decarburization (AOD), things changed. This grade of stainless steel 316 does have a lower carbon content to mitigate the effects of sensitization.
By lowering the carbon content, the amount of chromium carbide precipitates as well as the depleting of chromium in the vicinity of grain boundaries is significantly reduced. Hence, the retention of toughness and corrosion resistance of the stainless steel post-welding is enhanced.
- 316H: This grade has higher amounts of carbon which improves its thermal stability and creep resistance. Its corrosion resistance, however, is similar to 316L. Because of the high carbon content, it becomes prone to sensitization which makes corrosion of weld joints susceptible to damage due to the increased porosity of the weld.
- 316Ti and 316Cb: These are classified as stabilized stainless steels. Rather than lowering carbon content, the carbon is replaced with titanium and niobium to increase resistance towards sensitization. Because titanium and niobium are powerful carbide and nitride formers, they assist in preventing chromium from being consumed. Both have better intergranular attack resistance than the other two grades at the welded areas.
- 316N: This is a less popular grade of stainless steel 316 which has higher amounts of nitrogen. Stainless steels rich in nitrogen are often regarded as highly alloyed, super austenitic grades where more var chromium and molybdenum can be integrated. Adding nitrogen into stainless steel 316 provides similar effects to adding in carbon which improves hardness and increases strength.
- 316LN: This is a 316 variant with slightly lower carbon content, but higher nitrogen. Similar to 316L, having lower carbon content allows it to possess greater corrosion resistance in the welded condition. In order to reduce the carbon, nitrogen content is increased to improve mechanical properties.
Applications of 316 Stainless Steel
Such numerous advantages and applications which range from food preparation and lab equipment to agriculture and shipbuilding demonstrates why 316 Stainless Steel is so widely used. Specifically, its applications include but are not limited to:
- Marine architectural deck panels, steps, and railings.
- Equipment used in laboratory benches.
- Concentrated hydrochloric acid containers.
- Boat fitting equipment.
- Storage tanks for different chemicals.
- Used extensively in the solar thermal industry for mirrors.
- Rotating shovels used for salt mining.
- Acetic acid reactors.
- Used for maintaining the peroxide corrosion on the nuclear power plants servicing equipment.
- Nuclear plant parts.
- Special pumps.
- Paper manufacturing and pulp plants to decrease the contamination. Contaminated paper means a lower quality paper, which is not acceptable for Western civilization.
- Steel floats used for civil constructions.
- Pressure vessels and their covers.
- Stone and Concrete components for structural herculean constructions.
- Medical devices.
- Applications in Architecture.
- Medical implants such as orthopedic total hip and knee replacements, spinal instrumentation, screws, and pins.