Scope and Objectives

There is an urgent need for improvement of the material due to significant rise in haulage, high speed and frequency of operation in recent times. Currently, Indian railways are using carbon steels (Rail and elastic rail clips) and they are in the urgent need of improved materials. By modifying the chemistry of the steel and controlling microstructures by applying appropriate heat-treatment, the mechanical properties of all the above components, can be enhanced significantly. An example is current world wide effort to develop bainitic, multi-phase and nano-pearlitic steels for rails. Long term usage of these components in the harsh Indian environment also requires better corrosion and wear resistance. Here, we envisage integrated development of cost effective steels with control of composition, microstructures, processes and design of the final load bearing components, rails and elastic rail clips. It mainly caters to looking into improvement of strength, fracture toughness, wear and corrosion resistance of the above components.

Deliverables

1. Cost effective Chemical composition, thermo-mechanical processing route for microstructure control and mechanical properties including tensile, fatigue and fracture toughness. 2. Improved corrosion and wear resistance 3. Improved design of the components for better performance 4. Finally, improved steels for rails and elastic clips for high speed trains in India

Videos

Scientific Output

In the current ERC material, spring steels are austenitised and then directly quenched into oil to produce martensitic microstructure. Since, martensite is hard but brittle, an additional heat-treatment process of tempering is carried out to produce optimum combination of strength and ductility. This heat treatment technique has been used in other applications as well. Currently, extensive work is going on understanding and potential applications of bainitic microstructure. Usually, austenite is allowed to transform to bainite isothermally below bainitic start temperature or by cooling below the bainitic bay region. The present work has successfully produced bainitic microstructure in both the existing and newly designed compositions and therefore obviates the conventional quenching and tempering process. Due to complex features, the mechanism of formation of bainitic microstructure has not been concluded yet. According to the theory of shear transformation, bainite forms by diffusionless shear similar to martensite. Based on this theory, the austempering temperature is kept just above the martensitic start temperature which is the de-facto lowest transformation temperature of bainite. The new compositions are also designed to have lowest possible martensitic start temperature. According to the theory, transformation at a low temperature results in better strengthening owing to high dislocation density and fine plates of bainite. The present work has reported significant enhancement of tensile properties by applying lowest transformation temperature. Apparently, the observations of the current work is consistent with the theory of shear transformation, though the theory of diffusional transformation of bainite should also be examined in order to conclusively establish the mechanism of bainite transformation.

Results and outcome till date

a) The objective of this work is to produce fine bainitic microstructure along with stable retained austenite. Austempering the steel above martensitic start temperature for sufficient time results in lower bainite. Using this principle, properties of bainite obtained in the current elastic rail-clip (ERC) material are studied. The current ERC materials is austenitised at 950 C followed by austempering at 300 C for 4 hours. The austempering is carried out in molten LiNO3-KNO3-NaNO3 (12:70:18) salts. The bainitic ERC has 16% higher hardness, 11% higher yield strength and 18% higher tensile strength as compared to the current ERC. b) Calculation and design of new compositions for bainitic rail and clip are done. The compositions are melted at NML, Jamshedpur. One of the compositions is austenitised at 950 C followed by austempering at 250 C for 4 hours and the resultant bainitic microstructure is reported to have hardness of nearly 30% higher than the current ERC. Another composition of steel, austempered at 300 C for 2 hours, has resulted in 18% increase in hardness, 23% increase in yield strength and 27% increase in tensile strength. Currently, studies on corrosion of these newly developed bainitic steels and rail clip are being carried out. c) A steel having 0.61wt.%C, 1.71wt.%Si, 0.86wt.%Mn, 0.07 wt.%Cr, 0.01wt.%Mo and 0.04wt.%Ni steel is austenitised 1040oC followed by continuous cooling for 20 seconds and then isothermal holding at 300 C for 10 min has resulted significantly fine bainitic plate (165 +/-20 nm). The microstructure predominantly consists of ferrite (25%), bainite and retained austenite (75%). Such steel has 22% higher yield strength and 50% higher tensile strength in comparison to the current ERC material. d) By applying a cooling rate of 0.5 C/min after austenitisation at 1050 C for half an hour on a newly developed steel, it has been possible to reduce the interlamellar spacing by 40 nm.

Societal benefit and impact anticipated

Indian railways has always been the backbone of Indian economy. Fast movement of goods and passengers is evidently a characteristic of a developed society which India is determined to achieve. In order to address this issue, Govt. of India has taken a few initiatives for running high-speed train on the tracks. The maximum speed that Indian Railways has achieved till date is around 160Km/hour (Gatiman express). However, it is still less than that of a typical high speed train by international standards. The current materials being used in Indian railways has been an impediment to synchronise with the demand of growing economy in India. The bainitic materials being developed under the current project can certainly address this issue. In UK, bainitic steels are introduced in rail. In other countries, work is going on extensive application of bainitic steel in rail and other engineering applications. Significant enhancement in properties obtained by design of new compositions and processing techniques in commercially viable routes can be translated into realisation of net improvement in the basic infrastructure in the country.

Next steps

Currently, talks are going on with RDSO for possible manufacture of the prototype of elastic rail clip with material developed under this project. A contact person has been taken as consultant, who is facilitating contact with RDSO. This is in line with the suggestion received from the review in August 2018.

Publications and reports

1. Title: Corrosion behaviour of newly developed high toughness multi-phase steels (to be communicated) Abstract: Electrochemical techniques and immersion tests are applied to study the corrosion resistance of high strength and high toughness steel with various microstructures consisting of bainite, ferrite and pearlite obtained by continuous cooling followed by isothermal holding in the bainitic range. It has been observed that corrosion rate is a strong function of heat-treatment and microstructure. In conclusion, we found that the corrosion resistance of this steel is much better than that of existing rail clips with tempered martensitic microstructure. 2. Title: A comparative study on the corrosion behaviour of high silicon spring steel with multiphase microstructures (to be communicated) Abstract: A high silicon spring steel, used as the existing elastic rail clip (ERC), is austempered to produce bainitic microstructure and a detailed structure-property-processing correlations are established and then compared with the existing ERC having the microstructure of tempered martensite. Out steels show significant improvment in mechanical properties and corrosion resistance as compared to the existing rail clip.

Patents

Title: A high strength bainitic spring steel elastic rail clip (To be submitted) Abstract: The present invention of bainitic spring steel for application as elastic rail clip does not require environmentally hazardous heat-treatment of quenching and tempering. The clip should be austenitised at 950 C for sufficiently long time so that the formation of austenite is complete, followed by isothermal holding at 250 C for 4 hours. Such a low transformation temperature imparts strengthening due to high dislocation density as well as fine plate of bainite. Additionally, the stable retained austenite formed imparts TRIP ductility.

Scholars and Project Staff

IIT Kanpur: 1. Avisor Bhattacharya, Project Executive Officer, from 10.10.2017 onwards 2. Deepak Kumar Pal, Senior Project Mechanic, 28.05.2018 onwards 3. Neetu, Ph.D (ongoing). 4. Jayesh Zambre, M.Tech (ongoing). 5. Shubhendu Garg, M.Tech (completed). IIT Madras: 1. S. N. Manoj, Senior Research Fellow,01.01.2018-30.03.2018. 2. Subhrakanti De, Junior Research Fellow, 11.04.2018 onwards. 3. Arpan Arora, Junior Research Fellow, 26.04.2018 onwards.

Challenges faced

Currently, talks are going on with RDSO for possible manufacture of the prototype of elastic rail clip with material developed under this project.

Other information