Finite element simulation of residual stresses and residual deformation in hardened martensitic steel sheet

Jessica Gyhlesten Back is a PhD-student within SIFOS research school and her project’s goal is to, simulate residual stresses and residual deformation in hardened martensitic steel strips. Involved in the project are SSAB in Borlänge, Dalarna University and Luleå University of Technology.

SSAB produces and develops new materials with increasing higher strength, among these duplex and martensitic steel grades. During manufacturing of these materials they are hardened by which the material is rapidly quenched from high temperatures. The diffusionless phase transformation from austenite to martensite leads to volume changes because carbon atoms are trapped in the crystal. As the quench process is not fully homogeneous the volume changes will be accompanied with an inhomogeneous deformation leading to flatness problems of the sheet. To obtain improved flatness, a cold-rolling operation is added after hardening. The extra process is costly and reduces the ductility and forming capability of the sheet. This project focuses on material modelling, material characterization and simulation of the hardening process in order to gain increased understanding of which parameters that are most important to control for obtaining a stable process and flat sheet.

Jessica’s research focuses on Hardox 450, a martensitic wear plate. Expected effects are, better understanding of variations in the process and understanding of how they can be inhibited, which will reduce flatness problems and in the end, reduce costs. Dilatometry, hardness tests and EBSD analyses have been done in order to characterize the microstructure. The results so far have raised many questions, thus, focus is now austenite grain size and the condition of the austenite. Also the experimental setup is under consideration. Modelling of the phase transformation with and without deformation have been done in Matlab.

The results so far have given;

  • A phase transformation model based on composition
  • Knowledge about the microstructure after hot rolling and quenching, and after dilatometry
  • Understanding about the substructure’s influence on hardness

 


The image shows the experimental setup during dilatometry. The sample is located between tubes of quarts and thermal elements are attached to the sample surface in order to steer the temperature. The sample is heated by induction heat and is cooled with helium gas.

 


Resulting dilatometry curves from various cooling rates (CR). CR 40 - CR 100 r
esult in 98 % martensitic structure.

 


Band contrast image showing martensitic microstructure of a Hardox 450 specimen subjected to dilatometry and quenching by 60 degrees per second. The contrast image is colored by using all Euler.