Due to its outstanding mechanical and chemical properties, stainless steel is suitable for numerous industrial applications. However, its properties vary based on its composition, processing conditions, and the heat treatment methods you use.
In this article, we will review stainless steel’s mechanical properties and the methods you use to test them. We will also discuss the various factors that affect these properties and how stainless steel applications depend on them.
Stainless Steel’s Mechanical Properties.
Stainless steel has much structural importance, so you should understand its mechanical properties and the factors affecting them.
1. Tensile Strength.
This is the maximum stress a material can bear without fracturing when it is under tension. Simply put, it is the maximum force that you can apply to a material without breaking it.
Stainless steel is well-known for its tensile strength, but it can vary depending on the composition and treatment methods. So, different classes of this material have different tensile strength values depending on their composition and processing.
The Martensitic and precipitation-hardening steels show the highest tensile strength among the various stainless steel classes, whereas duplex steels exhibit a good combination of strength and ductility. The tensile strength for various classes of stainless steel is presented in the table below:
| Class | Tensile Strength (MPa) |
| Austenitic | 600 |
| Ferritic | 500 |
| Duplex | 700 |
| Martensitic | 650 |
| Precipitation Hardening | 1100 |
You can carry out a tensile test to measure this property; ASTM A370 defines the testing procedure and the required sample preparation for the tensile testing of metals.
- To test your steel’s tensile strength, you prepare a sample of the required shape and size and place it in the jaws of the tensile tester.
- The tensile tester stretches the sample slowly and measures the stress required to deform the stainless steel sample.
- The test results are presented as a stress-strain curve.
From this curve, you can obtain necessary information such as tensile strength, yield strength, elongation, and toughness of stainless steel.
2. Yield Strength.
This is the stress at which a material permanently deforms. Once plastically deformed, it cannot revert back to its original shape, even if the load is removed.
In simple words, yield strength determines how stiff or malleable your material is. You can determine it using the stress-strain curve we discussed previously.
The austenitic stainless steel class usually exhibits a yield strength of around 40-45% of the tensile strength value. Cold working increases the yield strength to 80-90% of the tensile strength value.
The table below shows the yield strength of different stainless steel grades.
| Class | Yield Strength (MPa) |
| Austenitic. | 250. |
| Ferritic. | 280. |
| Martensitic. | 350. |
| Duplex. | 450. |
| Precipitation Hardening. | 1000. |
3. Hardness.
This property shows how well a material can withstand localized plastic deformation. Simply put, it’s the ability of the material to maintain its physical features without any permanent change to its shape when a force is applied.
These forces can include indentation, scratching, and penetration.
You can measure the hardness of stainless steel using an indentation test. Common indentation tests include the Rockwell, Brinnel, Knoop, and Vickers tests. These measure the resistance to deformation by pressing a sharp indenter on the surface of stainless steel. The size or diameter of the indent is measured, and hardness is calculated based on it.
Stainless steel shows high hardness; one of the most common grades, AISI 304, has a Rockwell hardness of 70B.
However, this property varies based on composition and heat treatment.
4. Fatigue Strength.
This shows the is the maximum stress a material can withstand without breaking when subjected to cyclic loading. And when any material experiences fluctuating forces repeatedly, it is prone to fatigue failure.
You can use a fatigue testing machine to determine the fatigue strength of stainless steel. In this, you repeatedly apply stress below the fracture point of the stainless steel. The fatigue strength will be the number of cycles required to break your stainless steel sample at a specific stress level.
The minimum stress required for failure is called the endurance limit.
Austenitic stainless steel has a high fatigue resistance. The commonly used AISI 304 and AISI 316 have endurance limits of 240 and 270 MPa-N/mm2, respectively. You can increase fatigue resistance by improving the surface finish: smooth surfaces exhibit higher fatigue resistance.
5. Creep Resistance.
This refers to the material’s ability to resist deformation when a force acts on it for a long period of time. In stainless steel, creep or slow deformation is usually observed at higher temperatures. You can determine its creep strength by subjecting it to tensile loads at high temperatures for long periods.
You can measure the degree of deformation and compare it against the original dimensions. Refer to ASTM E139 for creep testing standards for stainless steel and other metallic materials.
Austenitic stainless steel shows good creep resistance due to the high amount of chromium and nickel. You can use austenitic stainless steel like SUPER 304H at operating temperatures of 650-700 ºC.
You can improve the creep resistance of stainless steel by various heat treatment methods such as annealing.
6. Ductility.
This mechanical property measures your material’s capability to deform plastically without fracturing. You can also refer to it as the “ability of a material to be drawn into wires”. Materials with high ductility usually show higher amounts of elongation before fracture.
You can determine stainless steel’s ductility by conducting a tensile test. You have to measure the stainless steel specimen’s initial and final lengths (or area), and you’ll get ductility in terms of percentage elongation or percentage area reduction.
Annealed austenitic stainless steels exhibit very high elongations: you can typically achieve up to 60-70% elongation. This allows their use in applications where severe deformation is required.
7. Toughness.
This property defines the ability of the material to absorb energy without breaking when deforming plastically. Materials with higher toughness usually have a good combination of strength and ductility. You can test the toughness of stainless steel using a common method called the Charpy V-notch test.
The V-notch is an impact test in which a hammer swings from a large pendulum and strikes the sample. Here, you measure the amount of energy the hammer requires to break the sample.
You can also determine toughness by calculating the area under the dress-strain curve of stainless steel.
Austenitic stainless steels are tougher in a wide range of temperatures. Martensitic, ferritic, and duplex stainless steels show a transition from tough to brittle behavior at low temperatures.
The Impact of Composition on Mechanical Properties of Stainless Steel.
Stainless steel is an alloy of iron and carbon with major additions of chromium, and smaller amounts of elements like nickel, molybdenum, and manganese. These additions are responsible for corrosion resistance, mechanical properties, and other important characteristics.
Let’s discuss how they affect the material’s mechanical properties.
1. Carbon
Carbon is an essential alloying addition to stainless steel, as it increases the yield strength of stainless steel. However, you cannot add it in large amounts as it reduces the corrosion resistance and toughness.
2. Manganese.
You can add this element to improve the hot ductility of stainless steel and increase the solubility of nitrogen. Note that adding nitrogen increases the mechanical strength but reduces the toughness of stainless steel.
3. Sulfur
You can add sulfur to stainless steel to improve its machinability. However, the ductility and formability of stainless steel are slightly reduced by adding sulfur. It also reduces corrosion resistance.
4. Nickel.
This element is usually present in austenitic stainless steel, and increasing its amount makes the steel more ductile. In precipitation-hardening steel, nickel enhances the strength of the material by forming intermetallic compounds.
How Processing Techniques Impact Stainless Steel’s Mechanical Properties
The mechanical properties of stainless steel are affected by its processing methods, like metalworking processes and heat treatment methods. Let’s discuss how:
1. Hot Working
Hot working is a form of mechanical working that is performed at elevated temperatures. At these temperatures, you do not need as much force to deform and shape your metals, and the degree of work hardening reduces. The resultant stainless steel has a lower strength but excellent ductility and toughness.
The self-annealing and recrystallization of stainless steel during hot working further increase the ductility.
So, you can hot work stainless steel when you need high amounts of deformation. You can also remove any compositional irregularities, defects, or stresses by hot working, improving the fatigue resistance of stainless steel.
2. Cold Working
Cold working is a form of mechanical working where stainless steel is deformed below its recrystallization temperature. You can significantly increase the hardness and strength of stainless steel by cold working. Strain hardening also occurs during cold working, where dislocations are introduced in the stainless steel microstructure.
However, the ductility and toughness of stainless steel are significantly reduced in this method – it decreases the material’s tolerance of deformation before fracture. The introduction of dislocations and defects in the microstructure increases the stresses in the steel. As a result, its fatigue resistance reduces.
3. Heat Treatment.
This process significantly impacts the mechanical properties of stainless steel, affecting its microstructure and the types and amounts of phases present.
Annealing involves heating the stainless steel to high temperatures and then slowly cooling it in the furnace. Normalizing is a similar process, but here, you air-cool the metal. And in hardening, you rapidly cool your metal by quenching it in a liquid.
Normalized stainless steel has higher tensile and yield strength and lower ductility than annealed stainless steel. The differences arise due to the formation of relatively finer grains in normalized steel. Fine grains usually result in higher strength and reduced ductility and toughness.
However, hardened stainless steel has extremely high strength and hardness compared to annealed or normalized stainless steel. The higher strength is due to the formation of a metastable phase called martensite.
Applications of Stainless Steel Based on Mechanical Properties
This material’s excellent mechanical properties make it popular in various industries, including:
1. Automotive
Due to its high toughness, fatigue resistance, and corrosion resistance, stainless steel is increasingly used in automobile suspension systems. You can also use stainless steel in braking systems because of properties like high strength and high-temperature resistance. These also allow for its use in structural components such as body panels.
2. Medical Devices
The high strength, fatigue resistance, corrosion resistance, and flexibility of stainless steel allow you to use it for various medical applications. Due to its good load-bearing capacity, you can use stainless steel for various orthopedic applications such as plates, nails, and screws.
Stents are used to open up blocked arteries, and the high flexibility of stainless steel allows for its use in stents. You can easily guide the stents through the arteries and inflate them to the size of the artery. It is also commonly used to manufacture various surgical instruments.
3. Construction
You can use stainless steel in different structural applications, such as beams and braces, due to its high strength-to-weight ratio and durability. Stainless steel is also used as reinforcement in the construction of bridges due to its high strength. Other important applications include commercial and industrial building claddings and roofings.
4. Marine Applications
The marine industry utilizes stainless steel due to its ability to withstand corrosive forces. Mechanical properties, such as high strength and durability also help. Stainless steel wire ropes are commonly used for lifting heavy loads due to their high tensile strength.
Due to the high strength and corrosion resistance in seawater, you can also use stainless steel to manufacture propellor shafts for large ships. The hulls of large ships are also made using stainless steel, along with different marine hardware such as pulleys and shackles.
5. Aerospace.
You can use stainless steel for manufacturing aircraft structures. These structures experience a high magnitude of stress, and the high strength and toughness of stainless steel make it ideal for this application. Due to good high-temperature properties, stainless steel can also be used in jet engines.
Endnote
Stainless steel has various desirable mechanical properties, including high tensile and yield strength, ductility, toughness, and fatigue resistance.
The mechanical properties of stainless steel are largely dependent on various factors. These factors include the composition of the alloy and the processing methods employed. You can obtain a wide range of mechanical properties by adding different alloying elements.
The versatile mechanical and corrosion properties make this material suitable for various applications. This includes industries like automobiles, marine, aerospace, and construction. You can also use it to manufacture medical devices, such as implants.
Finding the right kind of steel for your application can be complex, but don’t worry! We are here to help you: contact us for further details or queries!
FAQs
1. How can corrosion affect the mechanical properties of stainless steel?
When stainless steels experience corrosion, mechanical properties such as strength, ductility, and toughness are decreased.
2. How do stainless steels compare with titanium in terms of mechanical properties?
Stainless steels have a lower strength-to-weight ratio, strength than titanium. They show excellent corrosion resistance like titanium but can experience stress corrosion cracking in specific environments.
