The Effect of Inclusion Type on the Toughness of 4340 Steel

Abstract

The fracture toughness of ultra high strength steels can be improved for a constant inclusion volume fraction by increasing the inclusion spacing or by making the inclusion particles more resistant to void nucleation. One can increase the inclusion spacing by adding rare earth elements to form large and widely spaced inclusions. One can make the sulfide particles more resistant to void nucleation by adding small amounts of titanium to form small, closely spaced particles of titanium carbosulfide (Ti2CS). The fine particles of Ti2CS particles are much more resistant to void nucleation than other sulfide types, and this increased resistance to void nucleation can result in significant improvements in toughness. In this work, we considered three heats of 4340 steel in which the sulfur has been gathered in different ways. In one heat the sulfur has been gathered as particles of MnS. In a second an addition of lanthanum has been made to getter the sulfur as particles of La2O2S. In the third heat an addition of titanium was made and the sulfur was gathered as particles of Ti2CS. All three heats had yield strengths of about 1430 MPa. The fracture toughness was 87 MPa√m, 99 MPa√m, and 108 MPa√m for the manganese modified heat, the lanthanum modified heat, and the titanium modified heat, respectively.