In-situ Martensite Formation

Martensite is of pivotal importance in steel metallurgy as its presence in steel’s microstructure directly influences the mechanical properties. Martensitic transformations in steels are usually considered to be athermal, implying that the transformation is instantaneous. On the other hand, time dependent, i.e. thermally activated martensite formation has also been reported in some iron-based alloys and steels. Typically, such transformations are observed in the sub-zero Celsius temperature regime. Still, the latter is not widely accepted concept in steel metallurgy. Clearly, despite extensive research, the precise mechanism of martensitic transformations remains elusive. In-situ, time resolved investigation can assist in unravelling the peculiarities of martensitic transformations as such observations provide real-time information under controlled conditions.

A martensitic grade of steel with extensive industrial application and suitability for additive manufacturing is 17-4 precipitation hardenable (PH) stainless steel. Nitrogen uptake during additive manufacturing is expected to stabilize austenite and hence affect martensite formation in this steel quality.

In this PhD project, in-situ magnetometry and synchrotron-based dark field X-ray microscopy were utilized to monitor the kinetics of martensite formation in deliberately nitrogen-alloyed 17-4 PH stainless steel. The in-situ observations were supplemented with ex-situ X-ray and electron back-scatter diffraction and transmission electron microscopy techniques. The results show that the addition of nitrogen significantly affects the transformation temperature and the morphology of martensite. A higher nitrogen content lowers the temperature where martensite starts to form. For nitrogen contents above 0.09 wt.% the morphology of martensite changes from lath to plate type. The applied thermal cycle particularly affects the plate and lenticular morphology. Lath type martensite can in principle be suppressed by fast cooling. Both lath and plate martensite can form isothermally at a temperature above their respective martensite start temperatures. Dark field X-ray microscopy provides direct evidence of time-dependent martensite formation in the bulk of a specimen, demonstrating a new approach to investigate martensitic transformations in steels.