Austenite stabilizer
Austenite formers are alloying elements that stabilize the austenite lattice. They shift the transformation to lower temperatures, so that less hard martensite forms during cooling. This keeps the weld tough and less crack-sensitive.
Physical-Chemical Mechanism of Action
Nickel, manganese, carbon, and nitrogen dissolve in austenite and extend its stability range in the iron-carbon diagram. They lower the critical cooling temperature at which austenite transforms to ferrite or martensite. Thus, the fcc lattice (face-centered cubic) is retained even at room temperature. This phase is softer, tougher, and non-magnetic.
Metallurgical Relevance & Welding Risks
Austenite formers reduce hardness and crack risk in the heat-affected zone. However, an excessive amount (especially in austenitic Cr-Ni steels) can lead to hot cracking. Reason: Pure austenite structure solidifies coarse-grained and has low solubility for impurities like sulfur and phosphorus. These elements form low-melting films at grain boundaries, which tear under shrinkage stress.
Workshop Practice & Instructions
When welding austenitic steels, always aim for a small ferrite content (3–10%). This prevents hot cracking. Check the composition with the Schaeffler diagram: plot nickel equivalent (Ni + 0.5 Mn + 30 C) against chromium equivalent (Cr + Mo + 1.5 Si + 0.5 Nb). For unalloyed steels: keep carbon equivalent CEV below 0.45%, otherwise martensite formation due to too rapid cooling. Preheating and slow cooling reduce the risk.