Is martensite a brittle steel?

• "Martensite is strong but brittle" that's what is commonly thought. Yet, martensite like every other bcc-type steel microstructure fails by ductile fracture at sufficiently high temperature. The transition to brittle failure occurs however often at higher temperature than for softer steel microstructures and ductile toughness absorbs relatively less energy. This short tutorial explains how martensite can be made tougher using grain refinement.

• For ductile failure to appear, yielding must occur. In martensite, yield strength is a priori very high. Yield strength is further increasing with decreasing temperature and by grain refinement (Hall-Petch relationship). The grain size effect on yield strength can be determined experimentally.

• When the yield strength becomes larger than the cleavage stress, plastic deformation cannot anymore occur under either static or dynamic loading conditions, thus the steel will fail brittle. This scenario will definitely happen below some temperature, which is known as the ductile-to-brittle transition temperature (DBTT).

• Cleavage stress is assumed not to depend on temperature. However, it also increases with decreasing grain size according to a Hall-Petch type relationship. This effect can be estimated theoretically using Griffith theory and by knowing the energy required to generate a crack surface.

• The decisive question is, on which of the two stresses the grain size effect is bigger? Yield stress increases with around 24 MPa∙mm^-1/2 while cleavage stress increases with about 160 MPa∙mm^-1/2, respectively. Therefore, one can always make the cleavage stress being higher than the yield stress by adjusting a sufficiently fine grain size. For martensite the prior austenite grain size (PAGS) is the relevant parameter in this respect. Of course, refining PAGS has technical limitations so that at some low temperature yield stress overcomes cleavage stress and brittle fracture still does occur.

• A simple diagram allows estimating the acceptable maximum PAGS for different strength classes (carbon content) of martensite. While 0.2%C martensite requires PAGS to be less than 20µm, 0.35%C martensite requires it to be below 10µm. This analysis should be done for the lowest expected operating temperature of the steel in its application.

• Needless to say, niobium micro-alloying is a most efficient way for reducing PAGS either by active grain refinement during austenite rolling (TMCP) and/or by obstructing grain coarsening during heat treatment (precipitate particle pinning).