Silicon-Bearing High-Chromium Heat-Resistant Steels

Abstract

The addition of silicon in the steel can increase the fraction of the δ ferrite. δ ferrite reduces the strength but does not decrease the toughness. The matrix continuity is interrupted by the stripe-like δ ferrite distributed along the rolling direction, which causes delamination fracture upon impact. However, the δ ferrite can be eliminated by increasing the carbon content to 0.25 %. Carbides in the steel are dissolved greatly above 980 °C. Additional expansion occurs on the dilatometry curve. When normalised at 1030–1100 °C, large-size Cr₂₃C₆ carbides containing silicon precipitate along the grain boundaries. Tensile properties of the steel increases with normalising temperature below 1030 °C and remains almost unchanged at higher normalising temperatures. The toughness of the steel decreases with normalising temperature. The large-size chain-like Cr₂₃C₆ carbides along grain boundaries are the key factor to reduce the toughness. The toughness of the steel decreases greatly when tempered at 450–600 °C, the hardness reaches a maximum and the steel exhibits morphology of intergranular plus quasi-cleavage fracture. The grain boundary weakening caused by the precipitation of chain-like carbides along grain boundaries and the secondary hardening produced by fine dispersed carbides are the main causes for the temper embrittlement.