Formation of Cleavage Facet in Brittle Fracture for Duplex Stainless Steel Weldment

Abstract

Relation between microstructures in the heat affected zones (HAZ) and impact strength was investigated for SAF2205 duplex stainless steel on the basis of three-dimensional fractography.
Simulation of thermal cycles occurring in the HAZ was performed. The specimens were heated at various peak temperatures from 1373 K to 1623 K using Gleeble simulation tester, and then impact-tested in the temperature range from 77 K to 273 K.
The morphologies of the fracture surfaces tested at 77 K were like shear fracture in case of the base metal specimen and the specimen heated at 1473 K. The impact strengths were relatively high, even at liquid nitrogen temperature, because of the presence of austenite. However, the morphologies of the fracture surfaces with the microstructures heated to higher than 1573 K such as the bond region showed cleavage fracture of ferrite. The impact strengths were much lower than those of the as-received metals, because of the grain growth of ferrite, the dissolution of austenite and precipitation of chromium nitrides.
The orientations of cleavage facets observed in the specimen heated at 1623 K and tested at 77 K were measured using three-dimensional topography. As a result, it was found that there were groups of facts having almost same orientations. It is considered that the group of facets is corresponding to one ferrite grain and each facet is separated by intragranular austenite. It was found that intragranular austenite subdivided a ferrite grain into smaller facets and suppressed the degradation of bond toughness of duplex stainless steel.