What is Austenite in Welding?

Austenite, also known as gamma phase iron, is a non-magnetic allotrope of iron that is characterized by its face-centered cubic crystal structure. At room temperature, austenite exists in equilibrium with ferrite and cementite, but it can also be formed during welding or other high-temperature processes.

What is the meaning of austenitic steel? What is austenite used for?

Austenitic steels are widely used in the food and beverage industry due to their superior corrosion resistance. However, they are also susceptible to weld decay and sensitization, which can lead to cracking and corrosion.

When austenitic steels are cooled rapidly, they transform into martensite, a hard and brittle microstructure. This transformation can be undesirable in some applications, so heat treatment is often used to stabilize the austenite. Austenitic steels are also used in cryogenic applications, as they retain their ductility at low temperatures.

There are many different types of austenitic steels, each with its own unique set of properties:

The most common types are 304 and 316 stainless plates of steel, which are considered to be the standard grades for general purpose applications.
Other popular austenitic steels include 321, 347, and duplex steels.
Austenitic steels are generally more expensive than other types of steel due to their higher nickel content. However, they offer superior corrosion resistance and mechanical properties.

What is austenitic welding?

Austenitic welding is a type of welding that uses an austenitic filler material, such as 316 stainless steel. This welding process is used to join austenitic steels together, as well as other metals, such as nickel alloys.

Austenitic welding is characterized by its high heat input and slow cooling rate. This welding process is often used for critical applications, such as in the food and beverage industry, where corrosion resistance is paramount.

Austenitic stainless steel welding problems

Can occur due to weld decay or sensitization. Weld decay is the result of too much heat input, which can cause the formation of martensite in the weld zone.

Sensitization, on the other hand, is the result of too little heat input, which can lead to chromium carbide precipitates forming in the weld zone. These carbides can make the weld zone susceptible to corrosion.

Austenitic welding is a challenging process, but it offers superior corrosion resistance and mechanical properties. When done properly, it can provide a long-term service life in critical applications.

What is the ferrite number?

Ferrite number (FN) is a measure of the austenite content in the weld metal. It is determined by the amount of iron in the austenitic phase.

The higher the FN, the higher the austenite content, and the greater the corrosion resistance. However, too much austenite can make the weld metal susceptible to weld decay.
The ideal FN for a particular application depends on many factors, such as the type of steel being used, the welding process, and the desired properties of the weld metal.