Rob Koltz and Dave Meyer discuss the ferritic (magnetic) and austenitic (nonmagnetic) features of welding stainless steel. Getty Images
Q: I am welding a tank made of 316 stainless steel, which is nonmagnetic. I’ve started welding the tank using ER316L wire and found that the welds are magnetic. Did I do something wrong?
A: You probably don’t have anything to worry about. It is normal for welds made with ER316L to attract a magnet, and it’s common for rolled 316 sheet and plate not to attract a magnet.
Depending on temperature and alloying level, iron alloys exist in several different phases, meaning the atoms in the metal are aligned differently. The two most common phases are austenite and ferrite. Austenite is nonmagnetic and ferrite is magnetic.
In ordinary carbon steels, austenite is a phase that exists only at high temperatures, and as the steel cools, the austenite transforms to ferrite. So, at room temperature, carbon steels are magnetic.
Several grades of stainless steel, including 304 and 316, are called austenitic stainless steels because their primary phase is austenite at room temperature. These stainless steels solidify as ferrite and transform to austenite as they cool. Austenitic stainless steel plate and sheet undergo controlled cooling and rolling operations that generally ensure that all ferrite has been transformed to austenite.
In the mid-20th century it was discovered that some ferrite present in the weld metal when welding austenitic stainless steels prevents microfissuring (cracking) that can occur if the filler metal is fully austenitic. To prevent microfissuring, most filler metals for austenitic stainless steel are designed to contain ferrite content of between 3% and 20%, so they will attract a magnet. In fact, the gauges used to measure how much ferrite is in a stainless steel weld also measure the level of magnetic attraction.
There are a few applications of 316 where it is crucial to minimize the magnetism of the welds, but this is rarely a requirement in a tank. I expect you are fine to continue welding without any worries.
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