As-quenched martensitic steels exhibit a unique yielding behavior characterized by very low elastic limit. This phenomenon deeply relates to the behavior of mobile dislocations which have been introduced by martensitic transformation. In order to clarify the effect of mobile dislocations on the yielding behavior of martensitic steels, tensile testing, measurement of dislocation density by X-ray diffractometry and TEM observation were carried out in an ultralow carbon martensite (Fe-18% Ni alloy). And then the effect of dislocation strengthening on the yield stress was also discussed in terms of Bailey-Hirsch relationship. Elastic limit and 0.2% proof stress are very low in the as-quenched martensite, however slight pre-straining to the martensite has caused a marked increase in the both properties despite of a significant decrease in dislocation density. It is also confirmed that slight pre-straining to the martensite causes a clear microstructural change from randomly distributed dislocations to cellar tangled dislocations. These results suggest that mobile dislocations in martensite can easily disappear through the dislocation reaction and remained dislocations form cellar tangled structure in the early stage of tensile deformation. The Bailey-Hirsch plot for 0.2% proof stress indicated that the data of pre-strained martensite are just on the line extended from the data of cold-worked ferritic iron, while that of as-quenched martensite is obviously lower than the value expected from the Bailey-Hirsch relationship. This means that mobile dislocations in martensite do not contribute to dislocation strengthening but only make the yield stress lower through the generation of plastic strain.
