The properties of stainless steel can vary based on its type due to different compositions, microstructures, and processing methods. And you need to understand them to choose the right alloy for your business.
Let’s discuss this material’s variations, types, and grades in detail so you can opt for the right variation.
An Overview of Stainless Steel.
This material is characterized by its minimum limit of 10.5% chromium and a small amount of carbon. Such components give it properties like excellent corrosion resistance, high strength, and durability.
You can use it for various applications, including automotive parts, medical equipment, kitchenware, and construction material.
Classifying Stainless Steel.
Let’s discuss the categories of this material in detail.
1. Austenitic.
Structure and Classes
Austenitic steels have an FCC crystal structure with more chromium than other steel categories. The high amounts of chromium make this structure unstable, so you need to add a nickel to stabilize it.
There are two main classes of this category:.
- nickel-chromium alloys
- nickel-chromium-manganese alloys
Stainless steels that primarily consist of nickel and chromium as alloying agents form the 300 series. If you add manganese in 300 austenitic stainless, you’ll further improve their mechanical strength, forming the 400 series.
The most commonly used grade in this category is the AISI 304 – the 18/8 steel. Another important grade is 316 steel, commonly used for food processing and medical applications. AISI 201 is another grade that can be hardened by cold working.
Properties and Uses.
Austenitic steels are tough and strong yet ductile. So, you can easily form them into different shapes. This category is also known for its superior corrosion resistance and non-magnetic properties, although you can make some grades magnetic after cold working.
Note that you cannot harden materials in this category with heat treatment.
Due to their excellent properties, you can use them for aerospace and food processing applications. Several grades, such as AISI 316L, are also used in the medical industry to manufacture orthopedic implants.
Commonly Used Grades
Let’s talk about some commonly used austenitic steel grades below that are used for various industrial applications. These include:
1. Grade 301
The 301 grade is composed of 17% chromium and 7% nickel, giving it a high strength-to-weight ratio and high ductility with good corrosion resistance. Due to higher strength and ductility, you can use this grade in welding, machining, and drawing operations.
2. Grade 303
This grade is commonly known for its superior machinability due to the presence of sulfur. However, this gives it a lower corrosion resistance than the 304 grade.
Due to its high machinability, you can use this grade in applications requiring heavy machinings, such as gears and fasteners.
3. Grade 304
This grade is extremely common and has 18% chromium and 8% nickel (18-8 steel).
You’ll find that it has high strength, excellent corrosion resistance, high-temperature resistance, and formability, so you can use it in kitchen equipment, food processing equipment, and medical devices.
4. Grade 316
This grade is commonly called the marine grade. Grade 316 has high corrosion resistance, especially in chloride environments, making it ideal for marine applications. However, it has a lower tensile strength than other grades, like 304.
5. Grade 321
Type 321 contains titanium, giving it better temperature resistance than grade 304. You can use this material for high-temperature applications such as furnace components, oil refineries, and aircraft exhaust systems.
6. Grade 347
This grade is commonly known for its high-temperature resistance granted by the addition of niobium and tantalum. So, you can use it for welding applications; you’ll also find it in various automobile applications.
Limitations
Materials in this category have low thermal conductivity and low high-temperature strength. They are also costly.
2. Ferritic.
Composition and Structure.
Ferritic stainless steels contain approximately 10.5-27% chromium. The amount of carbon is kept low, around 0.2%, to maintain toughness and ductility. Nickel is also absent in these steels, making them a relatively low-cost option, but you can add molybdenum and titanium to improve their toughness and creep resistance.
This category of steel has a Body Centred Cubic crystal structure, making most of its grades magnetic.
Classification
Ferritic stainless steel grades are often referred to as chromium grades. The 400 series classify the ferritic stainless steel, and the most commonly used grades include the AISI 409, AISI 430, AISI 439, and AISI 441.
The AISI 409 is a low-cost grade often used for automotive applications.
Properties and Applications.
This family provides good resistance to corrosion, oxidation, and stress corrosion cracking. However, they are less stable against corrosive forces. You cannot harden them through heat treatment, either.
You can use ferritic stainless steel where cost and properties are more important than aesthetics, for instance, in automotive exhaust systems. Other common applications include kitchenware and architectural trims due to their ease of cleaning and corrosion resistance. Due to their oxidation resistance, you can also them in boilers and heat exchangers.
Commonly Used Grades
Here are some of the most standard ferritic grades in the industry:
1. Grade 409.
The low chromium concentration of SS 409 makes it the least expensive. This gives it a lower corrosion resistance but it is suitable for mildly corrosive environments, such as mildly acidic or basic. You can use this material for automobile exhaust systems.
2. Grade 430.
This type exhibits moderate corrosion resistance and lower strength. Due to its low cost and good formability, you can use it to form various complex shapes for different applications.
3. Grade 434
This grade has higher strength and better higher temperature resistance as compared to SS 430. You can use it for high-temperature applications such as electric kettles and microwave oven elements.
4. Grade 436.
This grade has a similar composition to grade 434 but also has molybdenum, allowing you to use it for stretch-forming applications.
Grade 436 has a higher hardness, making it more abrasion resistant than grade 434.
You’ll find it in various automobile applications.
5. Grade 444.
SS 444 has good resistance to pitting and crevice corrosion due to its low carbon content and the addition of molybdenum. Its pitting resistance is comparable to grade 316.
You can use it for bending and stretching applications; however, it is difficult to weld.
Grade 444 is commonly used in automobile exhaust systems, heat exchangers, and hot water tanks.
Limitations
The most prominent limitations of this category are its values of ductility and toughness than austenitic stainless steel. Due to its magnetic properties, you cannot use ferritic stainless steel in electronic applications, as magnetic properties can interfere with electronic equipment.
3. Martensitic.
This is the least popular type of stainless steel, primarily used in applications requiring high tensile strength or impact resistance.
Composition and Structure
The microstructure of these steels consists of a metastable phase called martensite. This is a hard phase formed upon quenching from higher temperatures.
Martensitic stainless steel consists of around 11.5-18% chromium. They also contain a higher carbon amount than other stees: approximately 1.2%. So, you can make these materials harder by heat treatment.
Properties
You’ll find that these steels have extremely high strength and hardness due to their high hardenability. So, you will find them to have excellent wear resistance.
Martensitic stainless steels have good magnetic properties, too. However, their corrosion resistance is relatively moderate.
Classification and Applications
The 400 series has martensitic stainless steel grades, and due to these alloys’ high strength, you can make turbine blades and jet engine parts from them. They are also often also used to manufacture cutlery and different surgical equipment. You can also use them in applications requiring high wear resistance, such as bearings.
Commonly Used Grades
Some of the most commonly used grades in this category include:
1. AISI 410.
This grade has excellent resistance to corrosive forces and high strength. So, you can use it for manufacturing pump shafts and other parts like turbine blades.
2. AISI 440C.
This grade contains high amounts of carbon and chromium that contribute to its high hardness and wear resistance. You can use it for making parts like bearings and knife blades.
3. AISI 431.
This type contains high amounts of chromium, which gives it high corrosion resistance and strength, and you can use it for manufacturing airplane parts, pumps, and valves.
Limitations
Martensitic stainless steel has some major limitations as well.
These steels are not as corrosion-resistant, so you can not use them in chloride-rich or acidic environments. And their high carbon percentage decreases their weldability, whereas inadequate heat treatment can cause the steel to become extremely brittle.
4. Duplex.
This family combines austenitic and ferritic phases, usually in equal amounts.
Composition and Properties.
Duplex steel usually has the following components:
- 21-27% chromium.
- 2-8% nickel,
- 0.05-3%.copper.
- 0.05-5% molybdenum.
This category has grades with higher corrosion resistance, and they exhibit high mechanical strength and ductility. They‘re also highly weldable.
Common Grades.
You can use this steel’s different grades to address requirements like a high strength-to-weight ratio, high tensile strength, and high toughness.
Some commonly used grades include:
1.S32205.
This steel grade has 22% chromium, 5% nickel, and 3% molybdenum. You can use it in oil and gas plants and marine applications because of its high resistance to corrosive forces.
2.S31803.
This steel grade has lower levels of chromium and molybdenum compared to S32205. You can use this steel for structural applications due to the steel’s high strength.
3.S32760.
This grade contains 25% chromium, 7% nickel, and 3.5% molybdenum. It also has tungsten and copper in small amounts. You can use it in highly corrosive environments because of its high stability against corrosion.
Applications
You can use these materials in applications that have tough requirements, like high corrosion resistance and enhanced mechanical properties. They’re common in the oil and gas industry.
As duplex steel has high strength and corrosion resistance in chloride environments, you can use it in marine applications, too, like boat hulls and propellers.
You can also utilize this family of steel in the construction industry to construct bridges, tunnels, and seawalls.
Limitations
You cannot use duplex stainless steel above 250°C for extended periods. Intermetallic phases form above this temperature, and they have a negative impact on the stainless steel properties.
These harmful intermetallic phases can also be formed during welding, so you must use special methods for welding duplex stainless steel.
Another major issue is the low impact toughness at lower temperatures, limiting its usability.
5. Precipitation-Hardening.
This category goes through a process called age hardening which toughens them up.
Composition and Properties.
This category typically has 15-17.5% chromium and 3-5% nickel. They may also have trace components, like aluminum and copper.
The yield strength of PH stainless steel is very high: 3-4 times greater than austenitic stainless steel. They also have excellent corrosion resistance.
Common Grades and Applications
Some of the most commonly used precipitation-hardened stainless steel grades include:
1.17-4PH.
This grade contains 17% chromium and 4% nickel and copper. You can use this steel for aerospace applications and other high-end applications as it has high toughness and high resistance to corrosive forces.
2.17-7PH.
This type contains around 17% chromium and 7% nickel. Because of its high corrosion resistance and formability, you can use it for making electronic and automotive parts.
3.15-5PH.
This grade contains chromium and nickel in 15 and 5 percentages, respectively. You’ll find that it has high strength and is resistant to corrosive forces, so you can use it in chemical processing and nuclear industries.
4.13-8PH.
As the name identifies, this grade contains 13% chromium and 8% nickel. This grade also has high strength and good corrosion resistance, and you can use it to manufacture parts in the nuclear and aerospace industries.
Limitations
The main disadvantage of precipitation-hardening stainless steel is the high cost attached to the age-hardening process. Producing this steel is time-consuming and requires specialized equipment.
So, their use is limited to high-end applications.
Conclusion.
All stainless steel categories have their unique own properties, applications, and limitations. You should understand them in detail before finalizing a steel grade for your business or project.
For more information regarding your stainless steel needs, please contact us!
