SURFACE MODIFICATION OF CASE HARDENABLE AISI 8620 STEEL USING TIG ARCING PROCESS

Abstract

Surface modification of steel surface is conventionally done by thermal treatment using various thermo-chemicals heating such as flame heating, induction heating, laser heating and electron beam heating. The rapid advancement in the field of surface engineering, conventional techniques for surface treatment like carburizing and flame hardening have been often replaced by the techniques using advanced high energy heat sources such as plasma, laser, and electron beam. These techniques use high energy concentrated beam on the workpiece which generates steep thermal gradients leading to rapid solidification and quick phase transformation in the matrix of limited depth at the substrate surface. The electron beam process is widely used for surface modification of steel, but a shallow case depth (≤ 1.5 mm), need of vacuum and a limited use for relatively small components make this process non-versatile for application at site. The laser beam process also has certain disadvantages like high cost of investment in the equipment, poor laser light absorption in the metal, radiation problem and highly skilled operator requirement. However, a relatively high energy concentration to produce a steep thermal gradient for effective surface treatment by modification of the matrix morphology of metal can also be effectively obtained by using a versatile process of arcing. In view of the above, the use of readily available autogenous tungsten inert gas arcing (TIGA) process, which requires relatively lower cost of investment, is considered as a new alternative for surface modification of materials. The easy availability at the site, requirement of less skilled worker, support to high heat absorption in metal and less environmental pollution makes TIGA process most favourable for surface modification of steel. In this process, the heat energy is provided by an electric arc maintained between a tungsten electrode and workpiece where a high quality partial melting is ensured under a good inert gas shielding of the molten pool. The TIGA process operates in two modes. The first mode is known as Continuous current TIGA (C-TIGA) and second mode is known as pulse current TIGA (P-TIGA). In C-TIGA process continuous arc is acted over the surface of base plate to modify its surface, whereas in the P-TIGA process the pulsing of arc at a regular frequency creates interruption in arc energy which modifies the substrate surface through controlled melting and solidification. As compared to C-TIGA process the P-TIGA process may be proved as a better process for surface modification because of using pulse current instead of continuous current facilitates that gives more precise control of energy input along with its distribution in the system. The P-TIGA has number of advantages that primarily include intended operation at relatively low ii heating, less distortion of the substrate and more precise control of isotherm and thermal cycle leading to controlled necessary phase transformation and its morphology. The variable parameters investigated in C-TIGA process were arc current (I), arc voltage (V) and travel speed (S) whereas in P-TIGA process, the operating parameters are frequency (f), pulse time (tp), base time (tb), base current (Ib), and pulse current (Ip). The control of pulse arc is comparatively more complicated than control of continuous arc due to the involvement of a large number of simultaneously interactive pulse parameters in it. So, the difficulty in use of pulse parameters can be effectively resolved by using a summarized influence of pulse parameters proposed earlier and defined by a dimensionless hypothetical factor b b p I f t I     where, 1 b p t t f         derived on the basis of the energy balance concept of the system. The primary objective of this research is to understand the comparative effect of CTIGA and P-TIGA processes for industrial application in order to produce more effectively modified surface layer with enhanced hardness and wear properties in AISI 8620 steel substrate. The AISI 8620 steel is the chromium, molybdenum, nickel based case hardenable low alloy steel that gives superior case/core properties after surface modification. The complete work has been carried out in seven parts. 1. The single pass C-TIGA parameters are optimized with the help of studies on influence of C-TIGA parameters on the thermal behaviour of surfacing including the modified FZ and HAZ geometry. The single pass P-TIGA is applied on the AISI 8620 steel surface at the optimized parameters of the C-TIGA to compare both the processes in terms of modified geometry (Depth of penetration, Width of FZ, HAZ width and FZ area). 2. An analytical thermal model is used to predict the isothermal curve, thermal cycle and cooling rate of the single pass C-TIGA and P-TIGA processes. In order to check the effectiveness of the thermal model, the estimated results (Isothermal curve, thermal cycle and cooling rate) are validated with the experimental results. The effect of single pass C-TIGA and P-TIGA processes on the cooling rate are also compared with respect to each other in this part. 3. Microstructural studies are carried out at optimized parameters of both single pass C-TIGA and P-TIGA processes and correlated to their characteristics cooling rate of the matrix. 4. Effect of both the C-TIGA and P-TIGA parameters on the matrix hardness is studied and compared. 5. Multi-pass TIG arcing process is performed over a larger substrate surface to analyse the effectiveness of both the C-TIGA and P-TIGA processes for industrial application at optimized parameters. 6. The residual stress analysis is performed on the modified zone of both the single and multi-pass TIGA processed surface. 7. The wear analysis iii is done to see the effect of C-TIGA and P-TIGA processing on the substrate surface. The characteristics of surface modification obtained by C-TIGA have been compared to those of the surface modified by P-TIGA process in order to find out the utility of the processes used. After finding the potentials of P-TIGA process to produce improved surface modification, effort has been made to develop knowledge of critical application of pulse parameters of autogenous P-TIGA process for optimum surface hardening of case hardenable AISI 8620 steel substrates, better than that produced by using C-TIGA process. The knowledge includes appropriate control of pulse parameters primarily in order to manage the thermal cycle and isotherm of the heating zone giving rise to desired surface modification of the substrate primarily defined by its geometry and microstructure. In this regard the relatively low heating characteristics of the pulse current arcing process is also kept under consideration for comparatively lower distortion and residual stresses of the substrate than that observed in case of the conventional non pulse arcing process. The innovation in surfacing is established after carrying out comparative studies using the C-TIGA for the same purpose. The use of P-TIGA process gives 110 % increment in hardness of the modified zone with request to that of base metal with adequate depth of modified zone of 2.5±1 mm. The use of P-TIGA process increases 30% hardness in the modified zone as compared to that observed in case of C - TIGA process at the similar heat input, arc current and travel speed. The use of P-TIGA process gives 25±5 % more depth of the modified zone as compared to that happens in case of using C-TIGA process. However nominal difference in compressive stresses of about 5±5 % is established by both of the processes in the modified zone. The increment in hardness with sufficient case depth makes P-TIGA process most suitable for bearing industry. The typical hardness requirement for large case hardenable bearing is 450-650 VHN with the case depth of 10 % of the cage thickness as per the ASTM standards. The use of C-TIGA and P-TIGA processes fulfils the ASTM hardness requirement for large roller bearing with sufficient case depth. The use of P-TIGA process at appropriate ɸ enhances considerably the depth of hardened layer and hardness with moderate compressive residual stresses in the modified matrix which are observed in case of using the C-TIGA process. The variation in ɸ and pulse frequency has been found to significantly affect the thermal behaviour of fusion and consequently the width and penetration of the modified region along with its microstructure, hardness and wear characteristics. It is found that P-TIGA is relatively more advantageous over the C-TIGA process, as it leads to relatively higher depth of penetration, higher hardness, improved wear resistance, and a better control over surface characteristics. The uses of surface modification iv processes develop residual stress over the surface. It is experimentally found that use of TIGA process produces compressive stresses over the surface in case of both the single and multi-pass surface treatment. The development of compressive stresses over the substrate surface is beneficial to improve surface properties like resistance to wear and fatigue.