The composition of stainless steel advantages many applications where strength, corrosion resistance, and surface quality are key. But what exactly is it that leads stainless steel to provide the protection and aesthetic appeal we’ve come to expect?
It’s helpful to think about stainless steel as any steel alloy that contains at least 10.5% Chromium. The composition of stainless steel may also include Carbon, Silicon, Manganese, Phosphorus, Sulfur, Nickel, Molybdenum, and Titanium. Each of these alloying elements boost particular metal qualities. With many different stainless steel grades and various formats including stainless steel sheet, stainless steel plate, stainless steel bar, and stainless steel tube, the manifold compositions of stainless steel result in an expansive list of possibilities.
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What specific characteristics do these alloys enhance?
The properties that exhibit in response to certain alloying elements include toughness, magnetism, strength, corrosion resistance, stress resistance, durability, workability, and the ability to be heat treated. With over 150 different stainless steel grades, countless alterations in composition result in various combinations of any of these properties.
What are the compositions of stainless steel families?
The ratio of the metals that makeup the alloy determine which family the stainless steel grade belongs to. Understanding the five different families of stainless steel— austenitic, ferritic, austenitic-ferritic (duplex), martensitic, and precipitation hardened—is one way to remember how the composition of stainless steel impacts mechanical properties.
- Austenitic: The amount of chromium, molybdenum, and nickel are higher in austenitic stainless steels. This improves corrosion resistance, strength, and produces a stainless steel that’s easily bent and formed.
- Ferritic: One of the distinguishing factors for ferritic stainless steel is that it’s magnetic. The composition of ferritic stainless steels have elevated chromium levels and lower carbon levels. This results in greater stress resistance.
- Austenitic-Ferritic (Duplex): Duplex stainless steels combine austenitic and ferritic properties. This enhances the metals protection against corrosion. The composition of duplex stainless steels make it a great choice for marine environments. Duplex steels demonstrate superior resistance to stress chloride cracking which occurs when a corrosive environment (like ocean exposure) is coupled with tensile stress. In addition, it’s often seen in the oil and gas industries.
- Martensitic: the compositional signature of martensitic stainless steels is increased carbon. Like ferritic steels, they’re magnetic. However, the increased carbon makes martensitic steels less resistant to corrosion.
- Precipitation-Hardening: The composition of precipitation hardening stainless steels includes 15-17.5% chromium and 3-5% nickel. Trace amounts of aluminum, copper, and niobium are also present. This combination makes for a terrifically strong, durable, hard, corrosion-resistant stainless steel.
What are some of the austenitic stainless steel grades?
- 201 stainless steel – The composition of 201 stainless steel includes carbon, manganese, phosphorus, sulfur, silicon, nickel, nitrogen, and 16-18% chromium. You’re likely to see it the food service industry for things like cooking equipment and commercial appliances, as well as the architecture and automotive industries. Kloeckner Metals supplies 201 stainless steel plate, 201LN stainless steel plate, 201 stainless steel sheet, and 201LN stainless steel sheet.
- 301 stainless steel – The composition of 301 austenitic stainless steel is similar to 201. It includes carbon, manganese, phosphorus, sulfur, silicon, nickel, nitrogen, and 16-18% chromium. However, it has a much lower manganese level and almost double the amount of nickel than 201. You’re most likely to see 301 in components needed in the aerospace industry. Kloeckner Metals supplies 301 stainless steel sheet.
What are some of the ferritic stainless steel grades?
- 409 stainless steel – This general purpose ferritic stainless steel has a significantly lower chromium content than either 201 or 301 at less than 12%. The main difference is the addition of titanium. Titanium stabilizes the steel contributing to excellent protection against corrosion at high temperatures. One common application is for exhaust systems in both the automotive and agricultural industries. Kloeckner Metals supplies 409 stainless steel sheet, 409 stainless steel plate and 409 stainless steel tube.
- 441 stainless steel – The two main differences in chemical composition between 409 and 441 stainless steel is the addition of niobium and a whopping chromium content between 17.5 and 19.5%. Together, these differences result in a stainless steel with high performing oxidation and corrosion resistance at even higher temperatures. Kloeckner Metals supplies 441 stainless steel sheet.
How do you know which composition of stainless steel is best for your project?
Now that we’ve learned that there are both different families of stainless steel, determined by a combination of microstructure and the ratio of alloying elements, that contain slight to major chemical variances, how do you know which composition of stainless steel is right for your project?
Don’t worry, no one expects you to become a metallurgical expert or Periodic Table Aficionado. The truth is there’s no one-size-fits-all perfect answer to this question. But, there are a few helpful tips to keep in mind as a guide. Perhaps the top tip is to consider how the primary effects of the main alloying elements present in the composition of stainless steel match up to your projects needs.
- Chromium (CR): Chromium is the most important elemental contribution in stainless steel. Remember how in order for a steel to be classified stainless it must have a minimum of 10.5 – 11% chromium. When you look at the chemical composition of the various stainless steel grades, the grades that have higher chromium content will demonstrate better protection against corrosion. So, prioritize higher chromium levels for highly corrosive environments, especially at high temperatures.
- Nickel (Ni): Nickel improves corrosion resistance and strength, and enhances formability. Of these three effects, formability will increase the most dramatically as percentage of nickel present in a stainless steel goes up.
- Molybdenum (Mo): Molybdenum increases resistance to atmospheric corrosion. For example, 316 stainless steel is one of the most popular grades and it excels in temperature intense environments that are known to cause rust, pitting, or strain.
- Carbon (C): In general, stainless steels have much lower carbon content than other steel products, usually less than 2%. As carbon increases, resistance to corrosion decreases, while hardness and strength might increase.
- Nitrogen (N): Pitting occurs when metal is exposed to a highly localized corrosive environment. The metal develops holes which can expand to cracks. Most projects will want protection against pitting, and that’s where nitrogen can help. It greatly improves pitting, crevice resistance, and inter-granular corrosion.
- Manganese (Mn): Manganese has a stabilizing effect, especially when nitrogen is present in the composition of stainless steel. Other effects include increased tensile strength and hardenability.
- Silicon (Si): Silicon is a ferrite stabilizer that deoxidizes. Deoxidation benefits corrosion resistance.
- Titanium (Ti): Titanium also improves stabilization. Two of the key effects titanium will have in stainless steel is to minimize inter-granular corrosion and increase weldability.
The above is just a guide. If you’re unsure which composition of stainless steel will best satisfy the specific needs of your project, Kloeckner Metals’ team of experts are more than ready to assist.
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