- Do Just C D 1 (56.23 KiB) Viewed 7 times
(b) When a Fe-0.4 wt% C steel is austenitised at 1000°C for 30 minutes and is slow cooled to room temperature we expect eutectoid alpha-iron, pro-eutectoid alpha- iron and cementite to form. If instead a Fe-1.3 wt% C steel is austenitised at 1000°C for 30 minutes and is slow cooled to room temperature we expect alpha- iron, eutectoid cementite and pro-eutectoid cementite to form. Use the equilibrium phase diagram together with the lever rule to describe as quantitatively as possible the key difference in terms of phases, phase fractions and microstructure between these alloys. Include sketches which illustrate (i) the stages in the development of the microstructures and (ii) the mechanisms of the phase transformations. [13 marks] (c) If the steel described in (b) is quenched rapidly from 1000°C to room temperature, describe the martensitic phase and microstructure which will be formed and explain why this is different from that formed in any of the cases above. [3 marks] (d) If reflected light optical microscopy were used to confirm the formation of martensite (e.g., lath type or plate type or mixed) a sample of the steel in part (c) would be polished flat to a sub-micron surface finish, etched with sodium bisulfate solution and examined. (1) Explain the reason for the polishing and etching steps during sample preparation. [2 marks] (ii) Typically what type of objective lens would you need to use in terms both magnification and numerical aperture, in order to observe the fine martensitic microstructure? [2 marks] (iii) Depending on the exact martensitic microstructure, a variety of defects could be observed such as dislocations and twins. Describe the general form of both these defects and indicate how they might be formed. [3 marks] (iv) If not all the austenite had transformed to martensite, using an additional technique, how could you determine the crystallographic orientation relationship (i.e. the orientation of one crystal lattice relative to another) between the retained austenite and the martensite? [2 marks]