Stainless Steel Composition

As you know that stainless steel materials are composed of various metal and non-metal elements – of course, we are talking about austenitic stainless steel here. So today we will briefly introduce stainless steel composition and the performance of each element.

a) Chromium – Basic Element in Stainless Steel

Chromium is the most basic element to determine the corrosion resistance of stainless steel. In the oxidizing medium, chromium can form an insoluble and chromium-rich oxidation film to prevent the corrosion medium passing through. This film is very dense, and it is firmly combined with the metal, so that protecting the steel from further oxidation by the external medium. Besides, chromium can also help to increase the electrode potential of steel. When the atoms in the chromium exceeds 12.5%, the electrode potential of steel can be changed from negative potential to positive. Therefore, the corrosion resistance of steel can be significantly improved. If there is more chromium content, better corrosion resistance the steel will be. When the atoms in chromium content reaches 25% and 37.5%, the second and third mutations will come, making the steel form higher corrosion resistance.

b) Nickel – Cannot Form Stainless Steel by itself

Nickel has influences on the corrosion resistance of stainless steel, but its effectiveness can fully be achieved only with chromium. If low carbon nickel steel requires a pure austenite structure, its content of nickel needs to reach 24%. Or if there is a need to change the corrosion resistance of steel in some media, the nickel content must exceed 27%. Therefore, nickel cannot form stainless steel by itself. However, under normal temperature, when 9% nickel is mixed with 18% chromium steel, the steel can obtain a single austenite structure, which can improve the corrosion resistance of the steel to the non-oxidizing medium (such as dilute, phosphoric acid, etc.), and can improve the welding and cold bending process performance of the steel.

c) Manganese and Nitrogen – Replacement of Nickel

Compared with nickel, manganese and nitrogen performs similar function in stainless steel. For manganese, it has just half effectiveness than nickel. As for nitrogen, its effectiveness is 40 times better than nickel. Therefore, manganese and nitrogen can be used to replace nickel in the single austenite structure. However, the corrosion resistance will be reduced if adding manganese in low chromium content stainless steel. Besides, the austenitic steel with is difficult to process. Therefore, nickel will not be fully replaced by manganese in stainless steel.

d) Molybdenum and Copper – Corrosion Resistance Improvement for Stainless Steel

Molybdenum and copper can improve the corrosion resistance of stainless steel to some corrosive media such as acetic acid. For molybdenum, it can also significantly improve the corrosion resistance of chloride-containing media like organic acids. However, the stainless steel containing molybdenum cannot be applied to nitric acid because the corrosion speed of stainless steel containing molybdenum will be doubled if putting it into the boiling 65% nitric acid. Besides, if adding copper into Cr-Mn stainless steel, the intergranular corrosion of the stainless steel will be accelerated.

Molybdenum has an adverse effect on single austenite structure in steel. Therefore, in order to make the molybdenum-contained steel have a single austenite structure after heat treatment, the content of nickel in manganese and other elements should be increased accordingly.

e) Silicon and Aluminum – Oxidation Resistance Improvement of Stainless Steel

Silicon can significantly improve the oxidation resistance of chromium steel. For the steel with 5% chromium and 1% silicon, its oxidation resistance is equal to 12% chromium steel. If the steel is required to be resistant to oxidation at 1000℃, there shall add 22% chromium for 0.5% silicon contained steel. However, if silicon content reach 2.5% to 3%, there just requires 12% chromium. The data also shows that adding 2.5% silicon into Cr15Ni20 chrome-nickel steel, its oxidation resistance is equivalent to Cr15Ni60 nickel chromium alloy.

Besides, adding aluminum to high chromium steels will also improve the oxidation resistance. The function of aluminum is like silicon.

The purpose of adding silicon and aluminum to high chromium steel is to further improve the oxidation resistance of the steel and to save chromium. Adding silicon and aluminum can greatly help with the oxidation resistance, but it also has many disadvantages. The most important one is that it will increase the grain coarse and brittleness tendency.

f) Tungsten and Vanadium

Adding Tungsten and vanadium to steel mainly helps with improving thermal strength of steel.

g) Boron

Adding 0.005% boron into the high chromium ferritic stainless steel (Cr17Mo2Ti) will improve the corrosion resistance of steel in boiling 65% acetic acid. And if adding a trace amount of boron (around 0.0006 ~ 0.0007%), the hot ductility of austenitic stainless steels will be improved. Boron has a good effect on improving the thermal strength of stainless steel, which can significantly improve the thermal strength of stainless steel. Besides, the Cr-Ni austenitic stainless steels containing boron also have special applications in the atomic energy industry.

However, adding boron in stainless steel will reduce the plasticity and impact toughness of the steel.

h) Others

Apart from the above listed elements, adding some rare metal elements or rare earth elements will also improve the performance of stainless steel pipe and fittings.