Retained austenite
Retained austenite is the untransformed austenite fraction in a martensitic matrix after quenching. It can transform to martensite under mechanical loading or at low temperatures.
Physical-Chemical Mechanism of Action
After welding and cooling of steels, a martensitic matrix with a remaining austenite fraction generally forms. This austenite fraction remains in a metastable state and can transform to martensite under mechanical stress or at low temperatures.
Metallurgical Relevance & Risks in Welding
The existence of retained austenite can lead to various problems. Under mechanical stress, the metastable state can cause transformation to martensite, which increases hardness and toughness but at the same time impairs dimensional stability and deformation properties. At low temperatures, the austenite fraction can lead to complete austenite transformation, which can reduce strength.
Workshop Practice & Instructions
To minimize the effects of retained austenite, it is important to ensure that the steel is cooled sufficiently slowly after welding. This can be achieved through controlled cooling or by using cooling temperature curves. For special applications where high hardness is required, post-weld heat treatment (PWHT) can be applied to transform the metastable austenite to martensite.