What is ferrite content? Stainless Steels
Ferrite content is a measure of the amount of ferrite in a steel. Ferrite is one form of microstructure – the internal crystal structure – that can exist within steel. The crystal structure helps to define the physical and mechanical properties of an alloy.
A ferritic microstructure in a stainless steel is usually associated with high strength and resistance to chloride stress corrosion cracking. It is also magnetic in nature, which can be used to measure the ferrite content in duplex and super duplex stainless steels by using a ‘Ferritescope’. This is a useful technique as it can be used ‘in-situ’, to assess the quality of welds or reviewing incoming stock without the need for test samples to be cut from the components.
Predicting Ferrite Content
There are a number of different methods of predicting the ferrite content with the most common being the DeLong diagram. Ferrite reduces the tendency for the steel to solidification crack whilst cooling. It is not uncommon for 304 castings to contain between 8% and 20% ferrite. The cast ingot composition of wrought 304 stainless is also balanced to have 1% and 6% ferrite as this reduces the chance of cracking during hot working or forging.
Solution annealing will dissolve most of the ferrite that was retained due to the ingot solidification. Because cast products are balanced to have a higher amount of ferrite, annealing will not transform all of it to austenite. In an annealed wrought product little or no ferrite remains.
The DeLong diagram also predicts the amount of ferrite that could be present in a weld deposit made from the wrought product. Plotting the composition of the wrought plate or bad product indicates how much ferrite could be present if the material were re-melted. This is not a valid indicator of the ferrite content in the annealed product.
Ferrite content in the actual product can be determined through laborious metallographic examination or by using sensitive magnetic instruments in a laboratory setting. In the majority of cases, the use of comparative calibrated magnets may establish the approximate ferrite content within 0.5% to 1%. Testing solution annealed Alloys 304, 316L (UNS S31603,1.4404), 309 and 310 yields value of 2% or less, with the ferrite contents typically less than 0.5%.
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