What is austenite?
It is the most important structural constituent of austenitic steels and alloys, which is normally present in the metastable state. Besides the structural constituent, there is also the austenitic phase. This is cubic-face-centered and is the main component of the microstructure.
Austenite is characterized by the typical twin boundary. This can be seen in the reflected light microscope. The alloy composition can be embedded in the austenitic matrix. In this case, however, the formation of non-metallic influences must be favored.
What are the properties of austenite?
The structural constituent has the following properties:
- The strength properties of austenite are low. The value of the 0.2% proof stress for austenitic standard steels at room temperature is between 200 – 2005 N/mm2. The tensile strength reaches a value of about 600 N/mm2.
- It exhibits high toughness values. At 40 – 50 %, the elongation values at break are approximately twice as high as those of ferritic steels.
- The thermal conductivity of austenite is low.
- The coefficient of thermal expansion on the other hand is relatively high.
- A massive increase in strength can be achieved by cold forming. However, this leads to poor machinability of austenitic steels.
- Due to the low stacking-fault energy, austenite has high high-temperature strength.
What is the machinability of austenite?
Austenitic steel is characterized by comparatively poor machinability. Problems can therefore arise during milling, drilling or turning operations. Compared with other structural constituents, such as martensite or cementite, austenite has high formability and medium tensile strength and hardness.
Due to its high deformability, it repeatedly causes stuck cutting edges during machining. Likewise, the formation of built-up edges often cannot be prevented. The tendency to adhesion and the formation of long band and tangled chips is particularly pronounced.
As a result of the high physical deformation of austenite, the new surface is already work-hardened during machining. Therefore, additional cutting forces act on the materials during further processing.
The low thermal conductivity of austenite also has a negative effect on the machining of the material. As a result, the heat generated cannot be transferred to the chip and the cutting edge of the tool is subject to high thermal stress.
Weldability of austenitic steel
Austenitic steels are characterized by very good weldability. There is no danger of cold cracking or coarse grain formation. However, chromium carbides can form due to the welding heat generated. For steels with a carbon content of more than 0.07%, it is possible for these carbides to cause intergranular corrosion. This can be counteracted with welding consumables with a low carbon content.