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Transcript of Martensite Transformation In Sandvik influence the martensite transformation [5]. Later on, the...

  • Under supervision of: J.P.M. Hoefnagels T. Nulens V.G. Kouznetsova M.P.F.H.L. van Maris Eindhoven, September 2008

    Martensite Transformation In Sandvik Nanoflex

    Influence of heat treatments on strain induced transformation

    T.A.J.M. van de Ven MT 08.38

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    Abstract In this report the metastable austenitic steel Sandvik Nanoflex is investigated. This material is interesting because it possesses the capability of combining good deformability with high strength. The final microstructure and associated mechanical properties strongly depend on the thermal and mechanical loading history of the sample. The chemical composition of Sandvik Nanoflex is 12Cr- 9Ni-4Mo-2Cu %wt. The microstructure changes from austenite to martensite during straining or isothermal cooling. The martensite transformation is a irreversible plastic deformation. This is why it is called a TRIP steel (TRansformation Induced Plasticity) During the research on the transformation austenite to martensite a remarkable material property was discovered, i.e. an unexpected increase of the yield and tensile strength of the material was observed after a heat treatment. Therefore the goal of this project is to study if there is a relation between the development of the martensite fraction and the development of the stress-strain curve. This is done by determining the martensite fraction at different strains. It was found that the evolution of the stress-strain curve of the samples heat treated at 1000°C and 1100°C can be explained with the evolution of the martensite fraction during the tensile test. For the 1000°C samples the martensite fraction develops linearly which explains the nearly constant stress in the measured area. The relaxation effect observed in the 1100°C samples can be explained by the observed increase transformation rate during the tensile test. Also the 1100°C samples were expected to show the most martensite because they are stronger but this is not the case. The samples heat treated at 1000°C are showing the most martensite. In all the samples heat treated at 1000°C a martensite band is formed. Martensite band are bands with a very high density of martensite. These bands explain why the samples heat treated at 1000°C have the most martensite in it.

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    Table of contents Abstract.................................................................................................................1 Table of contents ..................................................................................................2 Table of contents ..................................................................................................2 1 Introduction....................................................................................................3

    1.1 Philips .....................................................................................................3 1.2 Sandvik Nanoflex....................................................................................3 1.3 Motivation and Goal of this project .........................................................3 1.4 Structure of the report.............................................................................4

    2 Heat treatment ...............................................................................................5 3 Experimental procedure for the initial tensile tests.........................................6

    3.1 Introduction.............................................................................................6 3.2 Cutting out the samples..........................................................................6 3.3 Tensile tests ...........................................................................................7

    4 Results of the initial tensile test......................................................................8 4.1 Introduction.............................................................................................8 4.2 Results....................................................................................................8 4.3 Hypothesis..............................................................................................9

    5 Experimental procedure for determining the martensite fraction..................10 5.1 Introduction...........................................................................................10 5.2 Precipitation treatment..........................................................................11 5.3 Embedding in a resin............................................................................11 5.4 Grinding and polishing..........................................................................11 5.5 Etching..................................................................................................11 5.6 Images..................................................................................................12 5.7 Image analyses ....................................................................................12

    6 Experimental results ....................................................................................14 6.1 Graphs and images ..............................................................................14 6.2 Results of the as-received samples......................................................19 6.3 Resuls of the 1000°C samples .............................................................19 6.4 1100°C samples ...................................................................................19 6.5 Standard deviation................................................................................20

    7 Conclusion and recommendations...............................................................21 7.1 Conclusions ..........................................................................................21 7.2 Recommendations................................................................................21

    8 References ..................................................................................................23 9 Appendices..................................................................................................24

    9.1 Martensite fractions ..............................................................................24 9.2 As-received sample images..................................................................25 9.3 1000°C sample images ........................................................................26 9.4 1100°C sample images ........................................................................27

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    1 Introduction

    1.1 Philips

    Philips uses the stainless steel Sandvik Nanoflex for the production of razorblades and caps for their electrical shavers. The material properties are very well suited for this application. In the austenitic phase the material is reasonably soft and easy to form into a razorblade. After deformation the razorblade becomes much harder and less deformable due to the martensite transformation induced by the deformation during forming. The use of Sandvik Nanoflex comes with some problems because there is no accurate micro scale model for this material. For each batch of this material Philips is receiving they have to determine the material properties. A change in the material properties of that batch Philips may require the alteration of the production process parameters.

    1.2 Sandvik Nanoflex

    Sandvik Nanoflex is a TRIP steel. TRIP stands for TRansformation Induced Plasticity. In the as-received state the material has an austenite phase. During thermal or mechanical loading the austenite will transform to martensite. This transformation comes with a volume change. The volume will increase with approximately 3 to 6 percent. Due to the transformation the material will deform inelastically [2]. The transformation is not reversible with removal of the mechanical load. To turn the martensite back into austenite is has to be heat treated. In the austenite phase this material has a FCC (Face Centered Cubic) microstructure. In this state the material is soft and ductile. During the transformation martensite is formed. Martensite has a BCT (Body Centered Tetragonal) microstructure and has a higher yield strength and hardness than austenite The chemical composition of the material is given in Table 1.1. The large amounts of chrome, nickel and molybdenum are making this material corrosion resistant. Element Cr Ni Mo Cu Ti Al Mn Si C+N

    Wt-% 12,0 9,0 4,0 2,0 0,9 0,4 0,3

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    becomes more clear when different material behavior occurs even when the samples where cut out of the same plate. During the study on this material more and more material properties that are hard to explain where discovered. One of those material properties was discovered by Tom Nulens [3]. During his study he discovered that the material becomes stronger during a heat treatment. A heat treatment in general enlarges the grain size. A material with larger grains normally becomes weaker because dislocations have to travel through fewer grains boundaries. It is observed, however, that a sample of Sandvik Nanoflex that is heat treated becomes stronger. The apparent yield point becomes higher and the heat treatment also changes the shape of the stress-strain curve. The aim of this project is to study if there is a link between the increase in strength after a heat treatment and the martensite fraction in the material. To research this, heat treatments and tensile tests were performed. To make the m