Stainless steel is a material that has critical uses in industries ranging from aerospace to automotive. Metal alloys, including stainless steel, contain a mixture of several elements. In stainless steel, iron is mixed with other elements. The exact composition of stainless steel defines its properties, so you must know what it contains.
Let’s discuss it in detail.
An Overview of Stainless Steel’s Composition
Not every steel is a stainless steel alloy: stainless steel is characterized by a minimum of 10.5% of chromium.
Two other commonly used components are nickel and manganese. Apart from these, you’ll also find molybdenum, silicon, and carbon in varying amounts.
The presence and percentage of alloying elements affect the material’s properties. So, the extent of its heat resistance, strength, durability, and rust resistance depends on the grade you’re using.
Other important stainless steel properties include the extent of its weldability and machinability – the steel’s ability to be joined to another steel part or be cut, drilled, or milled by machines. All these characteristics depend upon the chemical composition of stainless steel.
Understanding the Alloying Agents in Stainless Steel
Stainless steel is primarily iron with chromium, nickel, and manganese, depending on its grade. Let’s discuss these in detail.
1.Iron
Iron is the chief constituent in any steel, providing the bulk of the composition. You’ll typically find it ranging from 50% to 70% in stainless steel, if not more.
Different stainless steel categories have the following iron composition:
- Austenitic stainless steel contains high iron levels at around 70%.
- Ferritic stainless steel has the highest iron content at around 70-75%.
- Martensitic stainless steel grades have a lower iron content at around 50%.
The iron content in stainless steel determines its corrosion resistance properties. Generally, the more iron there is, the less corrosion-resistant the steel is.
Too much iron causes a thicker passive oxide layer as it reacts with the chromium to form iron-chromium oxide, which will be less stable. It will also lead to a localized decrease in chromium in some areas, making it susceptible to corrosion. But less iron content will provide your steel with a thinner but stable oxide layer.
2.Chromium
This metal is of significant importance in stainless steel, as it provides steel with its corrosion resistance with its passive oxide layer. This chromium oxide layer is self-healing and can fix itself if damaged.
But chromium’s role in stainless steel isn’t just corrosion resistance: it also increases hardness and strength and improves weldability by reducing the steel’s tendency to crack when welding.
The chromium content is between 16-26% in austenitic stainless steel. In ferritic steels, it varies a lot but is always above 10.5%. And in martensitic stainless steel, it is at around 14-18%.
3.Nickel
Nickel is an essential constituent of stainless steel. It is an austenitic metal and provides austenitic steel with its characteristic crystal shape. So, austenitic steels are rich in nickel.
Nickel has two leading roles in stainless steel: corrosion prevention and strength. It also generates a protective oxide layer against corrosion.
The tensile strength of a steel alloy is proportional to its nickel content: adding nickel can make your steel harder and increase its tensile strength. So, you can use nickel-intensive grades in highly corrosive environments, like marine systems, and in industries requiring high strength, like aerospace.
You can use two types of nickel: nickel alloys and nickel pig iron. The former has other materials like molybdenum and copper. And nickel pig iron is a low-cost alternative to nickel alloys containing nickel, carbon, and iron.
4.Manganese
This essential stainless steel component reduces its brittleness and improves its mechanical properties.
Manganese does not directly affect corrosion prevention in stainless steel as it does not generate an oxide layer. However, it prevents pitting corrosion by stabilizing the austenitic crystal structure of steel.
Manganese also improves the metal’s hardness, strength, and durability, making it less likely to crack and fracture. This is useful as you can use manganese-based steels for applications that face a lot of stress, like bridge building.
You use either electrolytic manganese or ferromanganese for making stainless steel. Electrolytic manganese is the purer form of metal used in high-performance alloys. Ferromanganese is manganese mixed with iron to make lower-grade stainless steel alloys.
5.Other Elements
Stainless steel can have other components, depending on its grade.
Carbon
You’ll find carbon in stainless steel to make them harder, but its amount must be controlled. Too much carbon can cause carbide formation, interfering with metalworking processes and making the alloy less corrosion-resistant. Usually, the carbon content in stainless steel is 0.03% or lower.
Silicon
Silicon improves stainless steel’s high-temperature resistance and makes it less vulnerable to oxidation. It works similarly to chromium and nickel and forms a stable silicon oxide layer on the material’s surface.
Molybdenum.
This is another element you add to stainless steel to improve its corrosion resistance, particularly in harsh environments like seawater or acidic conditions. Molybdenum-containing steel is also tougher yet weldable.
Copper
Copper can also improve stainless steel’s corrosion resistance, specifically against chloride ions and sulfuric acid. It also protects against stress corrosion cracking.
Austenitic stainless steels can contain titanium to stabilize the austenitic structure at higher temperatures and resist pitting corrosion. Titanium-containing steel also has good weldability.
Nitrogen.
You can use nitrogen to improve this material’s strength and corrosion resistance. It mainly helps against pitting corrosion: localized corrosion where corrosive agents make small holes in the steel surface.
Phosphorous and Sulfur
Phosphorus and sulfur are generally considered contaminants in stainless steel manufacturing, so you confine them to low levels.
Too much phosphorus causes the formation of brittle phosphides, and too much sulfur causes the formation of sulfides. They cause cracking and corrosion and also lead to lower strength. But in smaller amounts, sulfur and phosphorus can improve the machinability of stainless steel.
Factors Affecting Stainless Steel Composition
Several factors can affect stainless steel’s composition. Some of these include:
1.Impurities
The common impurities in the stainless steel production process are sulfur and phosphorus. Sometimes, nitrogen can also be an impurity. These impurities affect the chemical composition of stainless steel.
For example, they cause the formation of sulfides and phosphides and reduce the overall strength of stainless steel. They also reduce the steel’s ductility, and their presence reduces the overall percentage of other elements in the alloy too.
2.Processing
The processing method, such as hot rolling and cold rolling, affect stainless steel composition at a microstructure level by causing the locking of crystals in a particular shape. The choice of elements you add at the processing stage also impacts the final composition of stainless steel.
3.Heat Treatment
You can use heat treatment processes, such as annealing, quenching, and tempering, to change the material’s microstructure and provide it with better corrosion and heat resistance. For example, if you quench austenite steel, its carbon stays in and forms a martensitic stainless steel microstructure.
Heat treatment affects the product’s corrosion resistance, heat resistance, strength, and ductility by affecting its composition.
What are the Different Grades of Stainless Steel and Their Properties?
Stainless steel exists in different series, which help categorize its properties:
- Series 200: Series 200 has lower corrosion resistance than the other series. However, it is cheap and mainly used where hardness and cost considerations are important. For example, household uses of steel, such as kitchen cutlery, use series 200.
- Series 300: This series, also known as higher-grade austenitic steel, is one of the best steel grades for industrial applications. It mainly contains high chromium and nickel contents, and some grades may also contain molybdenum and manganese. You use it when you require excellent corrosion resistance, such as in food or chemical processing.
- Series 400: Series 400 is mainly martensitic stainless steel, and you use it where you require extra durability, wear resistance, and hardness, like in pump parts. It generally contains chromium and carbon.
- Series 600: This series consists of duplex stainless steel. It is premium-quality stainless steel, has excellent corrosion resistance, and is used in jet engines and gas turbines.
What are the Most Common Applications of Stainless Steel and Why?
The industrial applications of stainless steel depending upon its composition. Common ones include:
- Aerospace: This industry requires stainless steel grades that can withstand high temperatures and stress levels. Consider grades like 15-5 PH and 17-4 PH for aerospace uses.
- Automotive: Automobiles also require high-temperature resistance as the engine can significantly heat up. You also need good impact resistance for automotive use. Stainless steel 409 and 430 are good candidates here.
- Oil and Gas: This industry requires high-pressure and high-temperature-withstanding steel. Stainless steel 310 is a popular choice.
- Chemical Processing: You require steel grades with excellent chemical resistance, such as molybdenum-containing grades, for use in chemical processing tankers.
- Food Processing: Here, you require corrosion-resistant stainless steel, preferably the series 300. Grades 304 and 316 are popular here.
How Stainless Steel’s Composition Affects Its Sustainability
The composition of stainless steel can impact how sustainable it is. Some alloys are easier to recycle than others, so your composition determines the steel’s recyclability. For example, steels containing higher nickel content are less recyclable, as nickel recycling has significantly high costs.
Likewise, the environmental impact stainless steel manufacturing process depends upon its composition. As chromium has a high melting point, steel grades with more chromium content use more energy and have more emissions than steel grades with less chromium content.
Conclusion
Stainless steel is an alloy of iron with at least 10.5% chromium content, but the specific composition of stainless steel is variable. Each steel grade has a different composition in terms of the elements used and their proportion.
Generally, the most common elements you’ll find in this material are chromium, nickel, and manganese. Other elements, like molybdenum, copper, titanium, carbon, and silicon, are also used. The composition of stainless steel affects its properties, applications, and sustainability.
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FAQs
1.Can stainless steel withstand high temperatures?
The ability of stainless steel to withstand high temperatures depends upon its grade. Generally, stainless steel is more heat resistant than other types of steel, and many stainless steel grades can withstand temperatures up to 870 degrees Celsius without much damage. Austenitic steels are particularly temperature resistant.
2.Is stainless steel weldable?
Stainless steel is generally weldable, but the exact welding properties depend upon its composition and grade. Generally, austenitic stainless steel is considered to have the most weldability. Usually, you will require pre- and post-weld treatments to ensure your steel does not crack during the welding process.
3.How easy is it to machine stainless steel?
The machinability of stainless steel depends upon its grade and composition, but most stainless steels are conveniently machinable. The bigger, harder, and more complex the steel, the more difficult machining it will be. Generally, austenitic stainless steel has excellent machinability.
More resources:
Descaling Stainless Steel – Source: KDM
Stainless steel finishes – Source: KDM
