Rob Koltz and Dave Meyer discuss the ferritic (magnetic) and austenitic (non-magnetic) characteristics of welded stainless steels.Getty Images
Q: I am welding a tank made of 316 stainless steel, which is non-magnetic.I have started welding water tanks with ER316L wire and found that the welds are magnetic.Am I doing something wrong?
A: You probably have nothing to worry about.It is normal for welds made with ER316L to attract magnetism, and it is very common for rolled 316 sheets and sheets to not attract magnetism.
Iron alloys exist in several different phases depending on temperature and alloying level, which means that the atoms in the metal are arranged differently.The two most common phases are austenite and ferrite.Austenite is non-magnetic while ferrite is magnetic.
In ordinary carbon steel, austenite is a phase that exists only at high temperatures, and as the steel cools, austenite transforms into ferrite.Therefore, at room temperature, carbon steel is magnetic.
Several grades of stainless steel, including 304 and 316, are called austenitic stainless steels because their main phase is austenite at room temperature.These stainless steels solidify to ferrite and transform to austenite when cooled.Austenitic stainless steel plates and sheets undergo controlled cooling and rolling operations, which usually ensure that all ferrite has been transformed into austenite.
In the mid-20th century, it was discovered that when welding austenitic stainless steels, the presence of some ferrite in the weld metal prevents microcracking (cracking) that can occur when the filler metal is completely austenitic.To prevent microcracking, most filler metals for austenitic stainless steels are designed to contain 3% to 20% ferrite, so they attract magnets.In fact, the gauges used to measure the ferrite content in stainless steel welds can also measure the level of magnetic attraction.
316 has some applications where minimizing the magnetic properties of the weld is critical, but this is rarely required in tanks.I hope you can continue soldering without any worries.
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