Stainless steel is an alloy that has a composition of different metals. When you mix these metals, they have a higher strength and durability than in their separate form. Over the years, engineers have been able to create many varieties of the stainless steel alloy by the addition and subtraction of different metals. Today, stainless steel is a unique material with multiple applications in many industries. If you want to understand stainless steel grades, it is necessary to have a refresher course on the terms and elements of this unique alloy.
Various Terms & Elements of Stainless Steel
Stainless Steel Terms
Stainless steel is an alloy that’s known for its durability and strength. As an affordable metal, it has multiple uses in different industries. It doesn’t corrode and can retain its integrity for long periods without degradation.
Corrosion is a natural process where a metal breaks down into a stable form. A well-known byproduct of corrosion is rust. Amongst other types of steel, stainless steel is inherently resistant to corrosion and rust. However, the alloy eventual corroded if left in an environment that encourages the process. Some of the ways to speed up the corrosion of stainless steel are:
- Exposure to salt
- Harsh acids
- Excessive moisture
An alloy is a combination of different metals for a unique purpose. Stainless steel is not a naturally occurring metal but is created by mixing iron with carbon and other metals.
Element is a term in chemistry to describe atoms with an equal number of protons. Elements are chemical substances in their most basic form, and you can combine them to form alloys. Some of the common elements on earth include carbon, iron, nitrogen, and oxygen.
Stainless Steel Elements
If you want to stabilize a metal and increase its strength, you add nickel. Nickel also gives stainless steel that luster, making it shiny and smooth.
If you want to make your stainless steel resistant to smears and stains, you add chromium. It is also an element that reduces the rate of corrosion.
The introduction of carbon to stainless steel makes it hard, gives it strength, and reduces wear and tear.
Gauges of Stainless Steel
Most people do not understand the above term. As with many things, we believe the higher the number, the better. But that’s not true when it comes to stainless steel. The gauge of stainless steel relates to its thickness. And for stainless steel, the lower the number, the thicker the steel. That’s why a 15 gauge stainless steel is far superior to a 22 gauge stainless steel. You do not want to underestimate the importance of the gauge when choosing stainless steel for your application. For instance, using lighter gauge steel may be the best fit for your kitchen sink, but for an industrial sink, you are more likely to require something thicker.
Grades of Stainless Steel
Now you know the thickness of stainless steel is known as the gauge, the grade of stainless steel refers to the various materials and elements that makes the steel stainless. There are three terms to you need to understand:
One of the most commonly used grades of stainless, Austenitic steel contains chromium and nickel. Some grades may also replace the nickel content with manganese and nitrogen. One unique feature of austenitic steel is its non-magnetic properties. This quality makes it an ideal material to work within the foodservice industry.
Unlike Austenitic Steel, ferritic steel is magnetic. It may not have the durability or corrosion-resistant properties of the austenitic grade, but it still ranks as one of the most used grades of stainless steel second only to austenitic steel.
You’ll hardly find this grade of steel in food service, but martensitic is a magnetic stainless that has little amounts of nickel content and a moderate amount of carbon.
Many people do not consider duplex as a distinct grade of stainless steel. It is seen as hybrid steel because it has two phases: ferrite and austenite.
The grade of stainless steel is further defined by the Society of Automotive Engineers (SAE) below:
Also known as UNS S30100, SAE 301 is highly resistant to corrosion and has extremely low levels of chromium and nickel. The stainless steel forms and welds easily. Some of the industries where SAE 301 is found include:
- Aircraft parts
- Trucks and trailers
- Home and Industrial
- Conveyor Belts
- Mass transit
For stainless steel to fall under the SAE 301 category, it must have the following composition:
- Cr 16-18%
- Ni 6-8%
- C 0.15% maximum
- Mn 2% maximum
- Si .75% maximum
- P 0.40% maximum
- S .030% maximum
SAE 304, A2
SAE 304-A2 is arguably the most widely used and versatile stainless steel on the planet. As austenitic steel, it is highly resistant to corrosion, and ideal for high-temperature applications because of its low carbon content. You can find SAE 304,2 in the following industries:
- Food and Beverage
- Domestic tools
SAE 304, 2 also has unique welding and forming properties. It has a nice lustre, easy to clean and fabricate. For a stainless steel alloy to fall under the SAE 304 category, it must have the following composition:
- Cr 18-20%
- Ni 8-10%
- Mn 2%
- Si 0.75%
- N .10%
- C .08%
- P .045%
- S 0.030%
SAE 430 is the most used ferritic steel with non-hardenable properties on the market. It is resistant to corrosion, heat, and doesn’t oxidize easily. SAE 430 has a higher corrosion resistance when it’s surface is polished. And because it has high heat resistance, you must weld at higher temperatures. On the plus side, the steel readily bends, forms and machines with ease.
SAE 430 is found in the following industries:
- Furnace combustion
- Oil and gas
- Restaurant equipment
There are so many other grades of stainless steel, and they’ll be mentioned briefly.
Things to Consider When Choosing A Stainless Steel Grade
Stainless steel is known for its resistance to corrosion. The versatility of the stainless steel alloy allows it to be used in many industries. That said, there are different grades of stainless steel used for various purposes and depending on the task at hand.
Before you choose a stainless steel grade, below are a few factors to keep in mind:
- What’s the formability?
- Will you need to weld?
- Is machining part of the process?
- What degree of corrosion resistance do you need?
- Does it require heat treatment?
- What strength do you need?
- What’s the application?
What’s the formability?
If the task at hand requires a stainless steel alloy with good formability, you should look for an austenitic grade like the 304, or try out the 430 which is a ferritic grade. The martensitic grade of stainless steel is known for its brittle qualities and do not form easily. If you want formable stainless steel, austenitic remains the ideal choice.
Will you need to weld?
Carbon steel and stainless steel are on two different sides of the spectrum when it comes to welding. Stainless steel has inherent welding problems. They are prone to hot cracking, stress corrosion cracking, and intergranular corrosion. However, if you really have to weld stainless steel then go for SAE 304L, or 347. That’s because the most weldable grade of stainless is in the austenitic category. Ferritic grades like the SAE 430 and 439 are also good, likewise the duplex stainless.
Is Machining part of the process?
If you want to machine stainless steel, you must watch out for work hardening. Your precision tools must be kept in excellent working condition. You can also add sulfur to enhance machinability. SAE 303 is suitable for machining because it contains sulfur.
What degree of corrosion resistance do you need?
Stainless steel has inherent corrosion resistance, but different grades of this steel have varying degrees of corrosion resistance. For instance, austenitic steel is the most ideal for corrosion-resistant applications. The high quantity of chromium in its composition makes it so. Ferritic and martensitic stainless is less resistant because of lesser concentrates of chromium.
Does it require heat treatment?
If you have to subject the stainless steel to heat treatment, consider the martensitic grades like the SAE 440C. Ferritic and austenitic, when treated with heat, are non-hardenable.
What strength do you need?
You can achieve tremendous strength with the martensitic steels such as the SAE 440C. Austenitic grades like the SAE 316 provide high strength although not in the same category with the martensitic grades. Duplex stainless give you ferritic properties. It also retains the toughness and ductility that’s seen in the austenitic grade.
Stainless Steel Grades and Their Applications
Below is a list of stainless steel grades and their industrial applications. The list is not exhaustive.
SAE 304L: All SAE 304 applications with welding requirements
SAE 316: Chemical applications
SAE 316L: same as above but with welding requirements
SAE 409: heat exchange, and automotive industry
SAE 416: fastener applications
SAE 430: Food equipment industry
SAE 439: Automotive industry
SAE 410: Kitchen Utensils, hands tools, knives
SAE 440C: Surgical and dental instruments, valves, pivot pins
Duplex 2205: saltwater applications and pressure vessels
Duplex 2507: chemical applications, power industry, offshore platforms, saltwater application
Duplex 2304: pulp and paper, mining, food industry, construction, water treatment
More details please check below video.
We cannot underestimate or overemphasize the importance of stainless steel. It is a robust alloy that’s highly resistant to corrosion and environmentally friendly.