Temperature dependence of mechanical properties in martensitic-austenitic stainless steels obtained by quenching and partitioning (Q&P) processing

• Applicant: Professor Javad Mola, Ph.D.
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2014 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 254704745

Quenching and partitioning (Q&P) processing is a method of stabilizing austenite in the microstructure of steels in order to enhance the strength-ductility combination. With the aid of a simplified Q&P processing consisting of quenching to room temperature (RT) and partitioning at 450 °C, different austenite fractions were adjusted in the microstructure of high C stainless steels containing 13 % Cr and additions of Co, Si, Mn, and N. Tensile properties were subsequently determined in the temperature range of 20 °C - 200 °C. Due to the insufficient C enrichment of austenite and the low stability of austenite against the deformation-induced martensitic transformation, tensile elongations were very low at RT. As the temperature was raised to 200 °C and the austenite stability increased, favorable combinations of strength and ductility were achieved (e.g. an ultimate tensile strength (UTS) of 1520 MPa and a uniform tensile elongation (UTE) of 25 % for the Si-added steel). An important objective of the second phase of the project is to design Q&P processing conditions associated with UTS x UTE products exceeding 30,000 MPa.% at RT. This will be done by applying subzero quench temperatures to the steels used in the first phase of the project so as to increase the fraction of martensite. Since martensite supplies the C required for the partitioning, an increase in its fraction will enhance the C enrichment of austenite. The effect of martensite fraction on the extent of partitioning will be subsequently studied by estimating the C enrichment of austenite by means of X-ray diffraction. Cementite precipitation in the martensite is one of the processes competing with the C enrichment of austenite in the Q&P steels. TEM investigations indicated that this undesirable reaction was absent in the Si-added steel. Nevertheless, the C enrichment of austenite in the Si-added steel was not higher than that in the Si-free steel where cementite precipitation had clearly occurred. Therefore, the status of C in the Si-added steel will be studied by high-resolution analytical techniques such as the atom probe tomography. The thermal history of austenite in Q&P steels results in the development of compressive stresses in the partitioning step. Since the C enrichment of austenite is associated with an expansion, the presence of compressive stresses in the austenite is considered to be a barrier against the C enrichment of austenite. Accordingly, it will be examined if the application of external tensile stresses during the partitioning step aids with the partitioning of C. Finally, the pre-straining of austenite is known to influence the kinetics of martensitic transformation. Since the austenite in the steels processed according to the simplified Q&P processing readily transformed to martensite at RT, the influence of pre-straining in the temperature range of 200 °C - 400 °C on the kinetics of deformation-induced martensitic transformation at RT will be investigated.

• DFG Programme: Research Grants