Optimization of the process-route of a Nickel-base alloy : Investigation of Sigma-phase precipitation in heat treatment

Sammanfattning: The focus of this master’s thesis is on the heat treatment of Ni-base alloys, specifically the risk of intermetallic σ-phases during different stages of heat treatment. The alloy studied is Sanicro®28, a super-austenitic stainless steel produced by Alleima AB. The problem at hand is that the quench-annealing stage is in high demand at the manufacturing facility, and the goal is to investigate if it can be removed from the manufacturing route. During forging, the outer surface and bar-ends can reach low temperatures, posing a high risk of σ-phase precipitation. Additionally, a necklace structure with large grains surrounded by fine re-crystallization is often observed at the surface of forged superalloys/Ni-base alloys. Today, this forged structure is re-crystallized and σ-phase dissolved during the quench-annealing stage. An alternative to quench-annealing after forging is to re-heat the bar using a Car Wagon Furnace(CWF). The thesis includes two laboratory experiments simulating two stages of heat treatment, the CWF and induction furnaces/soaking. The samples subjected to simulated CWF treatment showed re-crystallization throughout the entire structure. Annealing in CWF removes the large grains in surface positions. The time in the CWF also showed to be sufficient to dissolve σ-phase present from forging. Samples heated to the induction furnace set temperature do not contain precipitates, while temperatures below the induction set temperature induce σ precipitation to varying degrees. The key findings of the thesis are as follows: • Re-heating in a CWF right after forging is enough to dissolve σ-phase at half-radius and surface locations. • Quench-annealing stage could be removed by changing the route to a CWF after forging. • If temperatures fall below the σ-maximum stability temperature during induction furnace heating cycles, σ-phase precipitation occurs.