Microstructure and cleavage in lath martensitic steels

• Published in: Science and technology of advanced materials Vol. 14; no. 1; pp. 014208 - 9
• Main Authors: Morris, John W, Kinney, Chris, Pytlewski, Ken, Adachi, Y
• Format: Journal Article
• Language: English
• Published: United States Taylor & Francis 01.02.2013; IOP Publishing; Taylor & Francis Ltd; Taylor & Francis Group
• Subjects: 10.07; Austenite; Cleavage; Deflection; Focus; fracture; Grain size; lath martensitic steels; Martensite; Martensitic stainless steels; Martensitic transformations; Microstructure; Nickel; Nickel steels; Structural steels; Temperature; cleavage; microstructure; lath martensitic steels; 10.07; fracture
• ISSN: 1468-6996; 1878-5514
• DOI: 10.1088/1468-6996/14/1/014208

In this paper we discuss the microstructure of lath martensitic steels and the mechanisms by which it controls cleavage fracture. The specific experimental example is a 9Ni (9 wt% Ni) steel annealed to have a large prior austenite grain size, then examined and tested in the as-quenched condition to produce a relatively coarse lath martensite. The microstructure is shown to approximate the recently identified 'classic' lath martensite structure: prior austenite grains are divided into packets, packets are subdivided into blocks, and blocks contain interleaved laths whose variants are the two Kurjumov-Sachs relations that share the same Bain axis of the transformation. When the steel is fractured in brittle cleavage, the laths in the block share {100} cleavage planes and cleave as a unit. However, cleavage cracks deflect or blunt at the boundaries between blocks with different Bain axes. It follows that, as predicted, the block size governs the effective grain size for cleavage.