Rosette graphite
Rosette graphite forms in pearlitic cast iron during undercooling as star- or rosette-like aggregates. It represents a transition between lamellar and compact graphite.
Physical-Chemical Mechanism of Action
Rosette graphite forms when, during undercooling in pearlitic cast iron, graphite needles assemble into rosette-like structures. This process occurs when the temperature and composition of the iron are such that both lamellar and compact graphite formation are possible.
Metallurgical Relevance & Welding Risks
Rosette graphite can cause difficulties in welding processing. It can impair the toughness of the material and lead to hot cracks or splinters, especially if the iron is not properly pretreated. The weld is prone to porosity and may be less robust than with pure lamellar or compact graphite.
Shop Practice & Action Guidelines
To handle rosette graphite, the following measures should be taken:
1. Pretreatment of the iron: Make sure the iron is properly conditioned and tempered before welding.
2. Set welding parameters: Use a slow welding speed and a high gas pressure to minimize porosity.
3. Post-weld treatment of the weld: Perform a post-weld heat treatment (PWHT) after welding to stabilize the microstructure and increase toughness.