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, and the crystal structure helps to define an alloy's physical and mechanical properties. This article covers ferrite content generally and in austenitic stainless steels; duplex and super duplex grades target a different, much higher ferrite content - see what is ferrite content in duplex stainless steel? for that case.
A ferritic microstructure in a stainless steel is usually associated with high strength and resistance to chloride stress corrosion cracking. It is also magnetic, which can be used to measure the ferrite content in duplex and super duplex stainless steels using a Ferritescope. This is useful because it works in-situ, assessing the quality of welds or reviewing incoming stock without cutting test samples from the components.
Predicting ferrite content
Several methods predict ferrite content, the most common being the DeLong diagram. Ferrite reduces the tendency for the steel to solidification crack while cooling. It is not uncommon for 304 castings to contain between 8% and 20% ferrite, and the cast ingot composition of wrought 304 stainless is balanced to have 1% to 6% ferrite, which reduces the chance of cracking during hot working or forging.
Solution annealing dissolves most of the ferrite retained from ingot solidification. Because cast products are balanced to a higher ferrite content, 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 ferrite that could be present in a weld deposit made from the wrought product. Plotting the composition of the wrought plate or bar 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.
Measuring ferrite content
Ferrite content in the actual product can be determined through laborious metallographic examination, or by using sensitive magnetic instruments in a laboratory. In most cases, comparative calibrated magnets can establish the approximate ferrite content to within 0.5% to 1%. Testing solution-annealed Alloys 304, 316L (UNS S31603, 1.4404), 309 and 310 yields values of 2% or less, with ferrite contents typically less than 0.5%.
| Product form | Typical ferrite content |
|---|---|
| 304 castings | 8-20% |
| Wrought 304 (cast ingot, before annealing) | 1-6% |
| Solution-annealed 304, 316L, 309, 310 | 2% or less |