Väitös

Naiara Vázquez: Understanding the microplasticity of the TRIP steels is the key to provide better steels

Naiara Vazquez Fernandez
Both strain rate and temperature play an important role on the microplasticity of TRIP steels and its mechanical properties. While the effect of temperature on the TRIP steels can be easily analyzed at low strain rates, deforming the material at high strain rates induces a temperature increase that must be carefully considered. The results of this work show that it is possible to uncouple the effects of the strain rate and the adiabatic heating on the steel and that the strain rate has a major influence on the mechanical response of the steel.

Steel is a well-known material that is used in a wide range of applications. We can find steel all around us. The cutlery we use is made of steel, the sink and the tap in our kitchens might be made of steel, even some pens we write with have a spring made of steel! Steel is also used to build cars, trains and even larger structures, such as buildings and beautiful bridges like the Laukonsilta in Tampere or the Isoisänsilta in Helsinki.

The steel industry has been challenged to develop steels with higher ductility and stronger strain hardening capabilities. This has been accomplished by producing steels with complex microstructures where the combination of the different phases provides the material with the desired mechanical performance. This is the case of steels containing both austenite and martensite in the microstructure, where austenite is a soft and easy to deform phase while martensite is strong and hard. Transformation Induced Plasticity (TRIP) steels undergo a phase transformation while being deformed where some of the initial austenite can turn into martensite during the forming process, resulting in a steel with the required mechanical properties for a certain application.

The TRIP effect is highly influenced by both the temperature and the speed at which the material is deforming. Increasing either the temperature or the strain rate will reduce the TRIP effect, resulting in a lower amount of hard martensite in the steel. Thus, the strength and ductility of the steel will be decreased. Deforming a material at high strain rates usually induces a temperature increase due to adiabatic heating. The temperature increase observed at high strain rates has been considered the culprit of the reduction of the TRIP effect. However, the effect of adiabatic heating and strain rate on the TRIP effect should be uncoupled to fully understand the microplasticity of these steels and take advantage of the possibilities that TRIP steels can offer.

The thesis aimed at answering what are the individual effects of both strain rate and adiabatic heating on the martensitic phase transformation. The thermo-mechanical behavior of the steels was analyzed in tension at strain rates ranging from 10-4 s-1 up to 1500 s-1. Full-field imaging techniques were used to measure the strain and temperature of the specimens. The results presented in show that the decrease of the TRIP effect at higher strain rates is not only a consequence of the temperature increase from the adiabatic heating.

"In fact, the strain rate itself has an important effect on the formation of the martensite in these steels. At low strain rates, the martensite nucleates in the two most preferred variants of the initial austenite, whereas the nucleation of martensite is favored in one variant only when the strain rate is increased. This knowledge helps the steel industry to predict the microstructure that would result from a certain process and optimize the forming velocities to obtain the desired microstructures for each application", says Vázquez.

The doctoral dissertation of M.Sc. Naiara Vázquez in the field of Materials Science titled Effects of Strain Rate and Adiabatic Heating on the Strain-Induced Martensitic Phase Transformation in Austenitic Stainless Steels will be publicly examined in the Faculty of Engineering and Natural Sciences at Tampere University at 12 o’clock on 13th of November 2020 in the auditorium FA032 Pieni Sali 1 of the Festia Building, Korkeakoulunkatu 8, Tampere. The Opponents will be Professor Lorenzo Peroni from Politecnico di Torino, Italy, and Professor Jari Larkiola from University of Oulu, Finland. The Custos will be Associate Professor Mikko Hokka from the Faculty of Engineering and Natural Sciences, Tampere University, Finland.

The dissertation is available online at the http://urn.fi/URN:ISBN:978-952-03-1749-2.

The public can follow the event via remote connection.