Onsite Ferrite Measurement

Invariably all types of austenitic stainless steel contains ferrite with percentage depends mostly on the chemical composition and manufacturing condition. The common austenitic stainless steel Grade 304 which is known as 18/8 stainless steel contains ferrite around 0.5 to 2%. Stainless steel Grade 316/316L which is more stable than Grade 304 contains ferrite less than 1%. Ferrite in an unstable austenitic stainless steel known as Grade 301 varies between 2 to 30% depending on the cold work deformation to achieve hardness in the range of Rockwell C Scale 40 to 45. In austenitic weld, the ferrite is intentionally maintained between 4 to 7% to avoid hot tearing cracking. Ferrite has higher solubility of tramp element such as sulphur and phosphorus which are responsible for hot tearing cracking. Ferrite in the duplex stainless steel is found to vary between 30 to 60% to improve resistance to stress corrosion and tensile properties. The ferrite percentage in martensitic Grade 431 is restricted up to 2% and the precipitation hardenable 17-4PH in H1100 condition contains.

The amount of ferrite in stainless steel can be measured by metallographic method as per ASTM E562 and magnetic method or by magnetic permeability method as per Fischer Ferritscope. The metallography is a destructive method while Ferritscope is a non destructive method. In metallographic method sample is sectioned, or removed, polished and then electrolytically etched with 10% NaOH solution to reveal ferrite. The polished surface is examined under optical microscope using manual or automatic image analysis process to determine volume fraction of ferrite. This method is considered as referee method and does not require any calibration process. However the point count produces considerable error when the volume fraction is less than 2%.

Ferritscope method is anon destructive method can be used to determine ferrite in steel. This method is based on the principle of magnetic permeability. This method has high precision, high repeatability and accurate. This method requires calibration standard having known ferrite content.

According to ASTM A 800, the amount of ferrite can be estimated form chemical composition using the formula as given below . The ferrite forming elements such as Cr, Mo, Nb, Si etc are added together to determine chromium equivalent. Similarly forming element such C, Mn, Ni, N etc are added to determine Nickel equivalent. Ferrite is reduced by lowering the ferrite forming element. In cast structure the accuracy and repeatability of ferrite measurement by point count method is found questionable. The measurement of ferrite by Ferritscope is found to be accurate.
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