Effects of peak temperatures and cooling rates on delta ferrite formation and mechanical properties for heat affected zones in 9Cr-RAFM steel

Highlights: • The morphologies of delta ferrite depend on the variation of cooling rates. • The delta ferrite coarsen with the increasing of peak temperature. • The impact energies are low when the samples experiencing at high peak temperatures. • The martensite lath widths influence the mechanical properties. Delta ferrite can easily form in the reduced activation ferritic/martensitic (RAFM) steels after welding, leading to a change in mechanical properties. With the aim of clarifying the delta ferrite formation and its influence on the mechanical properties for the heat affected zones, the enlarged heat affected zones samples with 8–10 mm wide uniform temperature zone were prepared by the Gleeble 3800 thermal-mechanical physical simulator. Different peak temperatures and cooling rates were adopted to prepare the samples with different volumes and shapes of delta ferrite. Results show that the pebble-like delta ferrite in the RAFM steel for this study was produced at low cooling rates (1 and 10 °C/s), while the stripe-like delta ferrite and the block-like delta ferrite occurred at medium cooling rates (30, 50 and 100 °C/s) and high cooling rate (200 °C/s), respectively. The impact energies decreased gradually with the increase of peak temperature and thus, the fracture types changed from ductile fracture to brittle fracture. The impact energies existed at a relatively low level overall for the samples containing delta ferrite at high peak temperatures. Besides this, the martensite lath widths affected the mechanical properties for the samples with low peak temperatures.