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Title: Development of Ultra High Strength Steels for Engineering Applications
Authors: Kasana, Shivraj Singh
Supervisor: Pandey, O. P.
Keywords: Ultra high strength steel;Hydrogen flaking;Nano indentation;Delta ferrite;Sensitization
Issue Date: 19-Oct-2021
Abstract: The present work is focused on studies undertaken to improve the mechanical properties of different ultra-high strength steels (UHSS). The subject matter resulting out of the present study has been arranged in nine chapters in this Ph.D. thesis. The outline of the chapters is as follows: Ø Chapter 1: The chapter introduces the basic terms involved in the present research viz. steels, the classifications of steels, etc. This chapter discusses the need and benefit of UHSS steels and also presents the origin of the present research work. The outline of the complete thesis is also covered in this chapter. Ø Chapter 2: This chapter presents a detailed review of literature on the processing and mechanical properties of UHSS. This includes (i) AMS-4340 steels, (ii) 10.7CrMoVNbN steels, (iii) AISI321 steels, (iv) C250 steels, and (v) 300M steels on which the present work has been done. The chapter also brings forth the summary of the existing literature and also the main gaps in the existing literature in their field of work. Ø Chapter 3: The chapter presents a brief description of equipment used for testing and characterization of UHSS. This include X-ray diffraction (XRD), Optical microscopy, Scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS), and different Mechanical tests done for the processed UHSS to fulfill the basic objectives mentioned in this chapter. Ø Chapter 4: This chapter presents the processing methodology of AMS-4340 steels developed for defense applications viz. gun barrel and ballistic applications. Four different ESR heats E1, E2, E3 and E4 were prepared under different processing conditions. The microstructural investigations revealed that the hydrogen flaking was dominant factor in E1 heat. Because of this the ESR heat E1 was rejected as it could not achieve the desired yield strength. Although, ESR heats E2 and E3 have shown the desired mechanical properties but got rejected during the ultrasonic testing. ESR heat E4 exhibited better mechanical properties among these developed steels. This E4 steel was given longer hightemperature homogenization and two stage anti flacking treatment. The process route used for fabrication of ESR heat E4 showed that this steel is acceptable by the industries. Ø Chapter 5: This chapter presents the processing methodology of 10.7CrMoVNbN steels for turbine blades, nozzle partition and bucket for supercritical turbine applications. Four different heats viz, Heat 1, Heat 2, Heat 3, and Heat 4 were prepared under different processing conditions. FESEM-EDS analysis confirmed the presence of tempered martensitic matrix, carbides, and δ-ferrite. Field emission scanning electron microscopy and EDS investigations showed that various microstructure constituents are present in the matrix of martensite. It is observed that there is not much variation in hardness, yield strength (YS), ultimate tensile strength (UTS), and % elongation. However, the impact energy of Heat 3 and Heat 4 is much higher than that of Heat 1 and Heat 2. Lower impact in Heat 1 and Heat 2 are due to the presence of higher amount of δ-ferrite. Precipitation of delta ferrite is detrimental to mechanical properties. Further, Creq is a very important factor and hence, controlling the chromium equivalent to 12 maxima in this grade of steels has a significant effect on the mechanical properties. The Creq ratio of Heat 1 and Heat 2 was higher than 12 (12.99 and 12.47 respectively). Thus, lower impact strength was observed. Further, for Heat 3, Creq was 12.05 hence, higher impact strength was observed. Heat 4 presented superior mechanical properties. The Creq ratio observed for Heat 4 was lower than 12 (11.59). This indicated that lower Creq (lower than 12) improved the mechanical properties of the steel.Chapter 6: This chapter presents the different processing methodology adopted for AISI321 steels to get better mechanical properties. This particular steel is mainly used in nuclear reactors, heat exchangers and for boiler tubes applications. Two different types of titanium stabilized AISI 321 steels TB1 (without boron; 1 ppm) and TB30 (with boron; 30 ppm) were prepared. Microstructure analysis revealed that TB30 steel exhibits population of borides (Cr2B/Fe2B) along with TiN/TiCN precipitates. The number of precipitates is much higher compared to TB1 steel where only TiN and TiCN precipitates exist. For both room temperature and elevated temperature conditions the YS and UTS of TB30 steel in all conditions was higher than the YS of TB1 steel. This could be attributed to the precipitation strengthening of TB30 steel as the TB30 steel revealed a higher precipitate count in comparison to TB1 steel. In addition to this, addition of boron led to the formation of precipitates of Fe2B, Cr2B, TIN, and TICN which restricts the dislocation movement and causes an increase in yield strength of TB30 steel. Ø Chapter 7: This chapter presents the processing methodology of C250 steels for aerospace landing gear applications. Higher magnification microstructure shows the presence of lath martensite, retained austenite and fine distribution of precipitates such as Ni3Ti and Fe2Mo. The higher content of precipitates (Ni3Ti and Fe2Mo) leads to the improvement in the strength of C250 steels. The martensite laths observed for T0 were coarser in size. The equiaxed laths of martensite are clearly visible in the matrix phase. The YS and UTS of C250 steel in the solution annealed (SA) condition (T0) was minimum and the total elongation and impact strength were maximum. Beyond T14 cold working, a sudden drop in the percentage elongation and impact strength is observed. This could be attributed to the excessive cold working of the steel. The excessive cold work has the tendency to change the grain from crystalline to amorphous, thus beyond T14 cold work, the mechanical properties of the C250 steel resemble to that of a brittle material. Also, beyond T20 the dislocation density of the steel becomes constant thus a marginal change in YS and UTS is observed. Further, cold working leads to the alignment of grains along the cold working direction. From XRD spectra it can be observed that cold working leads to the change in crystal orientation and growth of crystals takes place in the new planes. This growth of crystals in T20 and T21 steel leads to a decrease in percentage elongation of the steel. Best results were observed for T14 cold work steel. The YS, UTS and hardness of T14 C250 steel were 1939.13 MPa, 2123.53 MPa and 56 HRC respectively. Ø Chapter 8: This chapter presents the processing methodology of 300M steels for automobile and aerospace applications. The microstructure of 300M steel consists of martensite, bainite and retained austenite (RA). Martensite mainly consists of laths parallel to each other while bainite is mainly observed as cross arranged bundles of different sizes. The best combination of mechanical properties was achieved by the IST heat treated 300M steel. IST steel exhibit comparatively better mechanical properties in terms of % elongation, % RA, impact energy with marginal loss in YS and UTS. It attributed good combination of hard martensite, tough bainite and soft retailed austenite microstructure of IST 300M steel. Heat treatment of 300M steel refined the grain size of bainite, martensite and retained austenite. The refinement of microstructure was higher for conventional route (CQT) and modified route (MAT) heat treated 300M steels. The higher refinement in the microstructure increases the grain boundaries in the microstructure. Grain boundaries act as a barrier to the dislocations and increase the resistance to deformation and increase the YS and UTS of steel. Ø Chapter 9: This chapter summarizes the entire work and discusses the main conclusions drawn from the present study. The main results and findings of the experimental work are included in this chapter. The study revealed that for developing different grade of UHSS different processing route has to be followed to achieve better properties. The UHSSdeveloped in the present study can be used in specific engineering applications. The mechanical properties achieved by the UHSS in the present study are exceptional and are well within the acceptable range set by the suppliers. Finally, this chapter also presents the major conclusions and recommendations from the present work and the scope for further research in this area.
Appears in Collections:Doctoral Theses@SPMS

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