The Morphology of Microstructure Composed of Lath Martensites in Steels

Abstract

Synopsis Metallographic characteristics of lath martensite stTllcture were investigated ill detail in pure Fe-C alloys and plain carbon commercial steels containing liP to O.B % C, Fe-0.2 % C-X alloys (X: about 1% ifMn, Si, i, Cr or Mo) and an 1B% Ni maraging steel. In the Fc-C alloys and steels, both jJackets and blocks are clearly observed in the lath martensite structure containing less than about 0.2 ";o C. With increase in carbon content from 0.2 % to 0.5 % C, the blocks are gradually extinguished and finally become difficult to observe, although the packets are still fo und. When the carbon content is more than about 0.5 % C, not only the blocks but also tlze packets become dijJicult to observe and tlze lath s are irrationally arranged in a prior austenite grain. In the O.B % C alloy, lenticular mar-tensite plates are observed in addition to lath martensites. I n tlze IB % Ni maraging steel, packets and blocks are clearly defined, especially the blocks are well develojJed in IJarallel in a IJacket, whereas the blocks in the low carbon martensile do 1I0t develop distinctly and are generally in Ihe shape of wedge. Such a dijJerence in morphology ~r blocks suggests tlzat the basic microslructural unit which contributes to Ihe strength and fracture of steels having lalh martensite structure should be the packets in carbon steels and the blocks in IB ";o Ni maraging steel. The jJacket size of Fe-C lath martensite slightly decreases with increase il1 carbon content UIJ to about 0.5 % C. A linear relation is found between the austenite grain siz e and the martCllsite packet size in the Fe-0.2 % C alloy and the JB % Ni maraging steel. The block width of 1B% N i maraging steel also increases with increase in austenite grain size, although this change is very small. A ddition of alloying elements if about 1% to Fe-0.2 % C alloy has little effect on the mOlphology of lath martensile stTllcture and the martens£te packet size.