Martensitic Phase Transformation in Sandvik metastable austenite transforms isothermally to...

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Transcript of Martensitic Phase Transformation in Sandvik metastable austenite transforms isothermally to...

  • Under supervision of: V.G. Kouznetsova D. San Martin M.P.H.F.L. van Maris Eindhoven, April 2007

    Martensitic Phase Transformation in Sandvik Nanoflex

    Experimental research on isothermal and

    strain induced transformation behavior

    L.C.N. Louws

    MT07.11

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    Abstract Metastable steels, such as austenitic steels, have attracted a lot of interest for a number of applications due to their capability to combine high formability and high strength. The final mechanical properties of these steels depends strongly on the thermo-mechanical processing routes and are usually attained by process of martensite formation and, in some grades, followed by a final step of precipitation hardening. In this project, the steel termed Sandvik Nanoflex is investigated. This 12Cr-9Ni-4Mo- 2Cu metastable austenitic stainless steel has, in the as-received state, an austenitic microstructure that can be transformed to martensite either by isothermal cooling or by straining the material. Even at room temperature it has been observed that the metastable austenite transforms isothermally to martensite. After transformation of the martensite, a final strength of 2000 MPa can be obtained in this steel by a

    precipitation step at 450-550 °C. It has been observed that the martensitic transformation behavior in this steel is quite inhomogeneous from heat to heat, from sample to sample within the same heat and, also, along the thickness within the same sample. Compositional segregation of substitutional alloying elements in the microstructure has not been detected and, so far, the problem remains unsolved. It is known that grain size, grain orientation or grain boundary type can affect this transformation. For this reason, in this work, the influence of the crystallographic features of austenite on the isothermal and strain induced transformation behavior of this steel is studied. First, an approach to this problem has been designed and then put into action successfully. The combination of Orientation Imaging Microscopy (OIM) and in-situ optical microscopy with the aid of micros-tensile/cooling stages made it possible to achieve this goal. The formation of martensite comes with a volume increase that results in the martensite plates popping out of the polished surface, which makes their observation under optical microscopes much easier. The general experimental procedure carried out is described as follows: 1) samples are electropolished; 2) by using a micro-hardness tester some indentations are marked on the polished surface to reference the square area under analysis; 3) The roughness of the surface is examined by confocal microscopy; 4) the crystallographic features of austenite are analyzed by OIM; 5) the

    formation of martensite is stimulated (either by isothermal cooling at -40 °C or by straining) using micro-tensile/cooling stages. The formation of martensite is studied in-situ, on the polished surface, by optical microscopy. Pictures are recorded periodically while the phase transformation is taking place; 6) the same area is analyzed again by OIM and confocal microscopy after the transformation; 7) OIM recorded data and the optical images are compared. Combining the OIM recorded data with the optical images makes possible to investigate the nucleation and growth of martensite at grain level. The analysis of the data is very time consuming and only the first stages of the transformation have been analyzed so far. Several comments can be extracted from this research: 1) Nucleation and growth of martensite induced by straining or isothermal cooling takes place in different ways; by straining, nucleation takes place at random, while isothermal martensite grows continuously as a front in very localized areas. 2) No grain orientation influence has been found during both tests. Isothermal martensite was more difficult to study, because many plates were sometimes growing at the same time. 3) Nucleation of martensite plates by straining seems to occur at grain boundaries and stop also at grain boundaries (either at twin boundary or not). Nucleation takes place first in bigger grains for the staining test while this was not the case for the isothermal cooling test where the grain size has no influence on the

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    transformation behavior. 4) Comparing the total boundary length before and after for both tests showed a reduction in the relative amount of twin boundaries after the tests. This observation indicates a preferential nucleation at twin boundaries. These results are preliminary and more analysis of the data is necessary. This investigation is going on and the final results are intended to be published in International Conferences and SCI journals.

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    Table of contents Abstract..................................................................................................................... 1 Table of contents....................................................................................................... 3 1 Introduction ............................................................................................................ 4

    1.1 Philips .............................................................................................................. 4 1.2 Steel Sandvik Nanoflex .................................................................................... 4 1.3 Motivation and aim of this project ..................................................................... 6

    2 Experimental equipment......................................................................................... 7 2.1 Automatic electropolishing machine ................................................................. 7 2.2 Vickers microhardness tester ........................................................................... 7 2.3 Confocal microscope........................................................................................ 8 2.4 Electron microscope......................................................................................... 9

    2.4.1 Orientation Imaging Microscopy............................................................... 10 2.5 Optical microscope......................................................................................... 10 2.6 Cooling/heating stage..................................................................................... 11 2.7 Tensile stage.................................................................................................. 12

    3 Experimental procedure ....................................................................................... 13 3.1 Polishing of the samples ................................................................................ 14 3.2 Recording of reference indentations............................................................... 14 3.3 Confocal microscopy and OIM measurements ............................................... 14 3.4 Tensile test: strain induced martensite ........................................................... 14 3.5 Cooling test: Isothermal formation of martensite............................................. 15

    4 Experimental results and discussion..................................................................... 17 4.1 Electropolishing.............................................................................................. 17 4.2 Tensile test..................................................................................................... 20

    4.2.1 Confocal imaging ..................................................................................... 20 4.2.2 Optical microscopy (OM).......................................................................... 20 4.2.3 OIM measurements.................................................................................. 22 4.2.4 Combination of OM and OIM measurements ........................................... 32 4.2.5 Conclusions from tensile test ................................................................... 36

    4.3 Cooling test .................................................................................................... 37 4.3.1 Confocal imaging ..................................................................................... 37 4.3.2 Optical microscopy................................................................................... 38 4.3.3 OIM measurements.................................................................................. 40 4.3.4 Combination of OM and OIM measurements ........................................... 45 4.3.5 Conclusions from isothermal cooling test ................................................. 48

    5 Conclusions.......................................................................................................... 49 6 Recommendations for future research.................................................................. 50 7 References........................................................................................................... 51 8 Appendices .......................................................................................................... 52

    8.1 Appendix A..................................................................................................... 52 8.2 Appendix B..................................................................................................... 53 8.3 Appendix C .................................................................................................... 54

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    1 Introduction

    1.1 Philips

    This research