Strengthening mechanism in ultra low carbon martensitic steel

In this paper, the strengthening mechanism of martensite was investigated by using an ultra low carbon Fe-18%Ni alloy to eliminate the confusion caused by carbon. In addition, Fe-18%Ni-C alloys were used to compare with the data of Fe-C alloys already reported and the effect of solute Ni was discussed in terms of solid solution strengthening in martensitic steels. The results obtained are as follows: 1) Microstructure of lath martensite is markedly refined by lowering the solution treatment temperature, nevertheless no large difference is found on the nominal stress-strain curves, especially on the yielding behavior. 2) Low elastic limit of ultra low carbon martensite is due to the existence of high density of mobile dislocation which has been introduced during martensitic transformation. Extra mobile dislocations can be eliminated by charging small amount of pre-strain more than 0.6% and this leads to an increase of 0.2% proof stress. 3) Substantial yield strength of carbon free martensite is determined by only the mechanism of dislocation strengthening without the contribution by grain refinement strengthening depending on microstructural parameters such as prior austenite grain size, packet size and block size. 4) Solid solution strengthening by Ni does not appear in martensitic steels which contains high density of dislocation, even though it is significant in ferritic steels.