STAINLESS STEEL
Stainless Steels
What is stainless steel?
Stainless steels as we know them today were first discovered in 1913 by metallurgist Harry Brearley who was experimenting with different types of steel. He found one with a particular chemical composition that allowed it to be particularly resistant to corrosion, and thus stainless steel was born.
Essentially, stainless steel is an alloy of iron that has a minimum of 10.5% chromium. When this chromium is combined with oxygen it forms a thin, sealed, fixed-adhering, chromic-oxide layer – the so-called passive layer. It is this passive layer that give stainless steel its distinctive corrosion resistance. While not truly stainless, even the most basic stainless steel alloys have an ability to withstand the elements and maintain their shine.
Stainless steel also contains varying amounts of Carbon, Silicon and Manganese. Further to this, alloying elements such as Nickel, Titanium, Copper and Molybdenum can also be added. This then gives the metal enhanced useful properties such as formability, strength, cryogenic toughness and increased corrosion resistance. Non-metal additions, such as carbon and nitrogen, can also be made.
The main prerequisite for a stainless steel is that it needs to be resistant to corrosion within the context of the particular environment it is being used in. This means that specific types and grades of stainless steel have to be carefully selected before a product is made to ensure that it is suitable for the task assigned to it. Extra mechanical or physical properties may also be needed so that the service performance requirements are achieved.
Applications of stainless steel
The wide variety of stainless steels means that the metal can be used in literally thousands of applications. The following gives a flavour of the full range:
Domestic – cutlery, lights, watches, washing machine drums, razor blades
Architectural/Civil Engineering – cladding, handrails, door and window fittings, structural sections, reinforcement bar, lintels, masonry supports
Transport – exhaust systems, car grilles, road tankers, ship containers, ships chemical tankers, refuse vehicles
Chemical/Pharmaceutical– pressure vessels, process piping
Oil and Gas– platform accommodation, cable trays, subsea pipelines
Medical – Surgical instruments, surgical implants, MRI scanners
Food and Drink– Catering equipment, brewing, distilling, food processing
Water – Water and sewage treatment, water tubing, hot water tanks
General – springs, fasteners (bolts, nuts and washers), wire
Let’s specifically look at the chemical, oil and gas industries. In these markets, the machinery has to operate in demanding environments involving high heat and highly toxic substances. This means it has to be made with special compositions to ensure they have enhanced resistance to corrosion over a wider range of temperatures.
Because of its properties, stainless steel can often be found in piping systems, separators, scrubbers, pumps, manifolds, heat exchangers, flowlines, and storage tanks. It can also form essential components of most offshore oil rigs and marine environments. The corrosive nature of crude oil means that stainless steel is the perfect material to cope with it. Similarly, stainless steel is also highly sought-after in in sub-sea conditions such as deep-sea drilling because the equipment has to be both highly durable and resistant to corrosion.
Different types of stainless steel
Stainless steel is usually divided into 5 types:
Ferritic: Ferritic stainless steels are initially based on chromium and then have a small amount (usually less than 0.10%) of carbon. They are magnetic and have a microstructure that is comparable to carbon and low alloy steel and subsequently are usually only used in applications where thin sections are required as they have been found to have a lack of toughness in welds. When welding is not required, they can prove to be very useful. Ferritic steels have good ductility and are regularly chosen for their ability to withstand stress corrosion cracking.
Austenitic: Austenitic stainless steels are the most common type. They have added Nickel, Manganese and Nitrogen and their microstructure gives them both weldability and formability. Their ability to resist corrosion can be strengthened by adding Chromium, Molybdenum and Nitrogen.
Martensitic: Martensitic stainless steels are similar to ferritic steels as they are based on chromium, however, they do have higher carbon levels, up to 1%. This addition of carbon allows them to be hardened and tempered and therefore they can be used where high strength, but moderate corrosion resistance is necessary.
Duplex: Duplex stainless steels were developed in the 1960s and get their name as a consequence of being made up of approximately 50% ferritic and 50% austenitic stainless steels. The resultant metal has better strength than either of them on their own. They are also resistant to stress corrosion cracking have better weight saving and often a better price stability as well.
Precipitation Hardening (PH): Precipitation Hardening – or PH – stainless steels can be developed to have extremely high tensile and yield strength when elements like copper, niobium and aluminium are added. When they undergo aging heat treatments, fine particles form in the matrix of the steel which subsequently gives it the extra strength. They can be machined into many different shapes and are regularly used in the oil and gas, nuclear and aerospace industries where a combination of high strength, corrosion resistance and a generally low but acceptable degree of toughness is required.
How to choose the right stainless steel
Stainless steel is one of the most durable materials and therefore is in high demand. However, as has been covered, there are many different types to choose from which all have different properties. If the correct stainless steel is chosen, with an appropriate surface finish, design and grade to its environment, and if it receives the correct maintenance then its appearance should remain virtually the same throughout its length of service, whether that be months or years.
Realistically, it will be a specialist engineer who will be making the final decision on what stainless steel is right for a project, but it is important to consider factors such as how corrosiveness and temperature of the environment, what strength is required, whether welding is required and what the required life-cycle is.
There are two factors that have a particular influence om what type of stainless steel should be used in a specific application, these being the grade of the metal and the surface metal.
What is stainless steel?
Stainless steels as we know them today were first discovered in 1913 by metallurgist Harry Brearley who was experimenting with different types of steel. He found one with a particular chemical composition that allowed it to be particularly resistant to corrosion, and thus stainless steel was born.
Essentially, stainless steel is an alloy of iron that has a minimum of 10.5% chromium. When this chromium is combined with oxygen it forms a thin, sealed, fixed-adhering, chromic-oxide layer – the so-called passive layer. It is this passive layer that give stainless steel its distinctive corrosion resistance. While not truly stainless, even the most basic stainless steel alloys have an ability to withstand the elements and maintain their shine.
Stainless steel also contains varying amounts of Carbon, Silicon and Manganese. Further to this, alloying elements such as Nickel, Titanium, Copper and Molybdenum can also be added. This then gives the metal enhanced useful properties such as formability, strength, cryogenic toughness and increased corrosion resistance. Non-metal additions, such as carbon and nitrogen, can also be made.
The main prerequisite for a stainless steel is that it needs to be resistant to corrosion within the context of the particular environment it is being used in. This means that specific types and grades of stainless steel have to be carefully selected before a product is made to ensure that it is suitable for the task assigned to it. Extra mechanical or physical properties may also be needed so that the service performance requirements are achieved.
Applications of stainless steel
The wide variety of stainless steels means that the metal can be used in literally thousands of applications. The following gives a flavour of the full range:
Domestic – cutlery, lights, watches, washing machine drums, razor blades
Architectural/Civil Engineering – cladding, handrails, door and window fittings, structural sections, reinforcement bar, lintels, masonry supports
Transport – exhaust systems, car grilles, road tankers, ship containers, ships chemical tankers, refuse vehicles
Chemical/Pharmaceutical– pressure vessels, process piping
Oil and Gas– platform accommodation, cable trays, subsea pipelines
Medical – Surgical instruments, surgical implants, MRI scanners
Food and Drink– Catering equipment, brewing, distilling, food processing
Water – Water and sewage treatment, water tubing, hot water tanks
General – springs, fasteners (bolts, nuts and washers), wire
Let’s specifically look at the chemical, oil and gas industries. In these markets, the machinery has to operate in demanding environments involving high heat and highly toxic substances. This means it has to be made with special compositions to ensure they have enhanced resistance to corrosion over a wider range of temperatures.
Because of its properties, stainless steel can often be found in piping systems, separators, scrubbers, pumps, manifolds, heat exchangers, flowlines, and storage tanks. It can also form essential components of most offshore oil rigs and marine environments. The corrosive nature of crude oil means that stainless steel is the perfect material to cope with it. Similarly, stainless steel is also highly sought-after in in sub-sea conditions such as deep-sea drilling because the equipment has to be both highly durable and resistant to corrosion.
Different types of stainless steel
Stainless steel is usually divided into 5 types:
Ferritic: Ferritic stainless steels are initially based on chromium and then have a small amount (usually less than 0.10%) of carbon. They are magnetic and have a microstructure that is comparable to carbon and low alloy steel and subsequently are usually only used in applications where thin sections are required as they have been found to have a lack of toughness in welds. When welding is not required, they can prove to be very useful. Ferritic steels have good ductility and are regularly chosen for their ability to withstand stress corrosion cracking.
Austenitic: Austenitic stainless steels are the most common type. They have added Nickel, Manganese and Nitrogen and their microstructure gives them both weldability and formability. Their ability to resist corrosion can be strengthened by adding Chromium, Molybdenum and Nitrogen.
Martensitic: Martensitic stainless steels are similar to ferritic steels as they are based on chromium, however, they do have higher carbon levels, up to 1%. This addition of carbon allows them to be hardened and tempered and therefore they can be used where high strength, but moderate corrosion resistance is necessary.
Duplex: Duplex stainless steels were developed in the 1960s and get their name as a consequence of being made up of approximately 50% ferritic and 50% austenitic stainless steels. The resultant metal has better strength than either of them on their own. They are also resistant to stress corrosion cracking have better weight saving and often a better price stability as well.
Precipitation Hardening (PH): Precipitation Hardening – or PH – stainless steels can be developed to have extremely high tensile and yield strength when elements like copper, niobium and aluminium are added. When they undergo aging heat treatments, fine particles form in the matrix of the steel which subsequently gives it the extra strength. They can be machined into many different shapes and are regularly used in the oil and gas, nuclear and aerospace industries where a combination of high strength, corrosion resistance and a generally low but acceptable degree of toughness is required.
How to choose the right stainless steel
Stainless steel is one of the most durable materials and therefore is in high demand. However, as has been covered, there are many different types to choose from which all have different properties. If the correct stainless steel is chosen, with an appropriate surface finish, design and grade to its environment, and if it receives the correct maintenance then its appearance should remain virtually the same throughout its length of service, whether that be months or years.
Realistically, it will be a specialist engineer who will be making the final decision on what stainless steel is right for a project, but it is important to consider factors such as how corrosiveness and temperature of the environment, what strength is required, whether welding is required and what the required life-cycle is.
There are two factors that have a particular influence om what type of stainless steel should be used in a specific application, these being the grade of the metal and the surface metal.
Grade: The grade of stainless steel has a major influence on its performance and needs to be matched to the environment. Broadly speaking, the different grades simply represent a difference is composition. As an example, the only practical difference between 304 / 316 and 304L / 316L is carbon content; with the [L] representing the lower carbon content. These lower carbon content variants were developed to overcome the risk of intercrystalline corrosion
Surface Finish: A smooth surface is often essential in helping to reduce the risk of corrosion as it reduces the ability of corrosive substances to adhere to a surface. A smooth surface finish makes it difficult for contaminants to stick and also increases the effectiveness of manual and heavy rain-washing, therefore reducing staining. The grade of stainless steel has a major influence on its performance and needs to be matched to the environment. Broadly speaking, the different grades simply represent a difference is composition. As an example, the only practical difference between 304 / 316 and 304L / 316L is carbon content; with the [L] representing the lower carbon content. These lower carbon content variants were developed to overcome the risk of intercrystalline corrosion
Surface Finish: A smooth surface is often essential in helping to reduce the risk of corrosion as it reduces the ability of corrosive substances to adhere to a surface. A smooth surface finish makes it difficult for contaminants to stick and also increases the effectiveness of manual and heavy rain-washing, therefore reducing staining.