Low Density Steel Pada Baja ,,,,,,,,,,,,,

Low Density Steel

1. PHASE TRANSFORMATION STUDY OF Fe-Mn-Al-C LIGHT WEIGHT STEEL BY Suman Sadhu Summer Trainee, CSIR-NML Metallurgical And Material Engineering, 3 rd year Jadavpur University BTTD-2017 Work Duration (2 nd june – 20 th june )

2. Why Light-Weight Steel Lesser The Weight of Vehicle Low Fuel Consumption Lesser Amount Of Green House Gas Emission Over 75% Weight of Car Body Made of Steel So low weight of Steel has greater importance on decreasing the weight of Vehicle CO2 Emission About 55% CO 2 emits from automobiles.

3. Why Light-Weight Stee l To compensate that weight requirement for battery motor or fuel cell in electric vehicle with alternative energy source. Due to its corrosion resistance it also can replace high cost chromium stainless steel. Competition from other composite material. battery system / motor / fuel cell / gas storage Additional weight require +250kg

4. High Strength to weight Ratio It has wide range of strength and ductility and even better than 2 nd generation TWIP steel.

5. Concept of low density steel Density of Al: 2.7gm/cc Density of Fe: 7.8gm/cc Density of light weight steel(8-10% Al): 6.8-7gm/cc Due to two reason decrease in density take place Lattice Expansion due to Al addition Lower atomic mass of substitutional Al atom. 1% Al addition leads to 1.3% decrease in density. So addition of 8-10 % Al will lead to 10-12 % density reduction. lattice expansion lower atomic mass of substitutional atom ● Specific weight of Fe-Al alloy ■ Specific weight of Fe-14~28Mn-Al-C [Frommeyer]

6. Research work in this area Lots of Fe-Al-Mn-C alloy developed by the combination of different Al-Mn %. And they are studied for different combination of Austenite, Ferrite and K-Carbide fraction Lots of Research done to control the formation of K- Carbide(fine ppt., coarse ppt.) in austenite or ferrite. K-carbide are causes embrittlement and therefore precipitation should be controlled to impart toughness and ductility.

7. Experimental Work : Analysis of As-Forged Alloy Taken up for study: Chemical Composition : Fe C Al Mn Si Rest 0.8-1 % 10-13 % 12-15% 0.4-0.5% Structure is deformed Consist of two phase structure possibly ferrite and austenite

8. Thermodynamic Analysis through thermocalc equilibrium phases present: Austenite, Ferrite and K-Carbide. H owever no kinetics data is available and w e don’t know how the phase fraction will change with the different cooling cycle So heat treatment at different cooling cycle were done and the microstructure was analyzed. We got an indication about what phases will be present.

9. Heat Treatment cycle: effect of cooling rate on phase fraction Heating At 1200 o C 3 Sample Soaking At 1200 o C (20 min) Air Cooled Forced Air Cooled Water Quenched Heat Treatment Cycle Optical Microscopy SEM-EDX EBSD 1 Sample Soaking At 1200 o C (1 hour) Water Quenched

10. Optical Microscopy Air Cooled from 1200 o C Forced Air Cooled from 1200 o C Water Quenched 1200 o C The microstructure consist of elongated ferrite grains and equiaxed austenite grains. Austenite contains extensive twining (Annealing Twin).

11. Carbide or something else? Some black phases with in the ferrite are seen for quenched sample. The question arises is are they carbide or something else? But those black phases are not visible for air cooled specimen, if they are carbide those phase must be present with in the air cooled sample also. Because for air cooled sample chance of diffusion is higher and there was much time for precipitate to occur. But, From SEM and EDX analysis of this quenched sample shows that they are some lenticular shaped phase whose composition is almost same as bulk composition of ferrite. Initially we though that they might be quenched in vacancy. Vacancy formed at high temperature might be retained during quenching.

12. Lenticular in shaped. Formed with in the ferrite phase and sub-grain boundary of the ferrite grains. Only for quenching heat treatment those phases/precipitate are formed. EDX analysis shows that the composition of those phases are same as bulk composition of ferrite Further work to characterize those phases are going on. Now we can only say that those are the phases having same composition as ferrite and formed with in the ferrite phase during the quenching of high Al light weight steel. Element Wt % At % C K 03.42 12.18 AlK 16.31 25.85 SiK 00.87 01.32 MnK 12.31 09.58 FeK 59.41 45.48 Element Wt % At % C K 02.13 07.91 AlK 16.04 26.59 SiK 00.14 00.22 MnK 13.47 10.97 FeK 59.83 47.92 Composition of the needle like phases. Bulk composition of Ferrite

13. Phase Analysis (EBSD) Phase fraction and phase map: Air Cooled from 1200 o C Water Quenched 1200 o C (20 min) Forced Air Cooled from 1200 o C Extensive twins are found in austenite grains No k-carbide type phase are detected in EBSD, it is more or less DUPLEX ferrite austenite structure at all cooling regimes. It is seen that for air cooled, forced air cooled, and water quenched there is not any considerable change in phase fraction. This proves that there is no phase transformation is occurring during Air cooling and forced air cooling. Because the phase fraction is same as of as quenched.

14. Water Quenched 1200 o C (20 min) Water Quenched 1200 o C (1 Hr ) Effect of soaking time : With soaking time volume fraction is changed, higher the soaking time higher the volume fraction of ferrite Grain growth has occurred due to holding for longer time at higher temperature.

15. Conclusion There is no considerable difference in phase fraction found with varying cooling regimes like air cooled, forced air cooled and water quenched. With increasing Soaking time the phase fraction of austenite is found to decrease. N o decomposition of austenite into k-carbide is detected during all the three cooling regimes (Air Cooled, Forced Air Cooled, WQ) Austenite grains contain extensive annealing twins. Some secondary phases are formed with in the ferrite grains for quenched sample, further work is being carried out.

16. Acknowledgements Dr. Sandip Ghosh Chowdhury, Chief-Scientist, CSIR-NML Mr . Biraj S ahoo , Scientist CSIR-NML Metallography lab personnel CSIR-NML

17. THANK YOU