Rob Koltz and Dave Meyer discuss the ferritic (magnetic) and austenitic (non-magnetic) characteristics of weldable stainless steels. Getty Images
Q: I am welding a non-magnetic 316 stainless steel tank. I started welding water tanks with ER316L wire and found that the welds were 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 rolled sheets and 316 sheets very often do not attract magnetism.
Iron alloys exist in several different phases depending on temperature and doping level, which means that the atoms in the metal are arranged in different ways. 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 turns into ferrite. Therefore, at room temperature, carbon steel is magnetic.
Some 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 harden to ferrite and turn to austenite when cooled. Austenitic stainless steel plates and sheets are subjected to controlled cooling and rolling operations which generally convert all of the ferrite to 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 microcracks (cracking) that can occur when the filler metal is completely austenitic. To prevent microcracks, most filler metals for austenitic stainless steels contain between 3% and 20% ferrite, so they attract magnets. In fact, the sensors used to measure the ferrite content in stainless steel welds can also measure the level of magnetic attraction.
316 is used in some applications where it is essential to minimize the magnetic properties of the weld, but this is rarely required in tanks. I hope you can continue soldering without any problems.
WELDER, formerly called Practical Welding Today, represents the real people who make the products we use and work with every day. This magazine has been serving the welding community in North America for over 20 years.
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