EFFECT OF TIG SURFACING ON FATIGUE PROPERTIES OF AISI 4340 STRUCTURAL STEEL

Abstract

Surface modification can be achieved by using advanced high energy heat sources such as plasma, laser, and electron beam. These techniques use high energy concentrated beam on the work piece 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) of modification, requirement of high 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 requirement of highly skilled operator. In view of the above the use of readily available autogenous tungsten inert gas (TIG) arcing process, which requires relatively lower cost of investment but provides concentric high energy arc heat source, is considered as a new alternative for surface modification of metal. The TIG arcing process is having some more advantages such as its easy applicability at site, requirement of less skilled worker, support to high heat absorption in metal and operation with relatively less environmental pollution. It makes TIG arcing process quite 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. The TIG arcing process operates in two modes. The first mode is known as continuous current tungsten inert gas (C-TIG) arcing and second mode is known as pulse current tungsten inert gas (P-TIG) arcing. In C-TIG arcing process continuous arc conventionally acts over the surface of base plate to modify its surface, whereas in P-TIG arcing process a pulsing of arc at regular frequency operates on substrate surface for its modification under interrupted arc energy which works more precisely by controlled melting and solidification. The variable parameters in C-TIG arcing are the arc current (I), arc voltage (V) and travel speed (S) whereas in P-TIG arcing the operating parameters are frequency (f), pulse time (tp), base time (tb), base current (Ib), and pulse current (Ip). Thus, the control of pulse arc is comparatively more complicated than control of continuous arc due to involvement of a large number of simultaneously interactive pulse parameters. The difficulty in control of pulse parameters is effectively resolved by using a summarized influence of pulse parameters proposed earlier and defined by a dimensionless ii 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 work is to understand the effect of single and multi-pass C-TIG arcing under different heat input on surface modification of AISI 4340 structural steel characterized by its microstructure, hardness, tensile properties and fatigue behaviour. At optimum condition of C-TIG arcing parameters the characteristics of the single and multi-pass modified surface are also compared to the same produced by using P-TIG arcing at the same heat input of C-TIG arcing. The pulse parameters of the P-TIG arcing process are taken as an optimum one found in an earlier work for providing optimum surface modified properties of steel. The AISI 4340 steel is chromium, molybdenum, nickel based high strength low alloy steel that gives superior case properties on a good back-up core properties after surface modification. The complete work has been carried out in eight parts. 1. Optimization of process parameters for single-pass C-TIG arcing process. It is done on the basis of the studies on thermal characteristics, geometry of fusion modified zone (depth of penetration and zone width) and heat affected zone (HAZ). 2. An analytical modelling to estimate the thermal characteristics known as isothermal curve, thermal cycle and cooling rate of single-pass C-TIG arcing process. The estimated results of analytical model are compared to the experimental results for validation of the model. 3. Microstructural analysis and studies on hardness profile of the surface modified substrate prepared at different process parameters and correlated with cooling rate for the single-pass C-TIG arcing process. 4. Microstructural characterization of different zones of modification and analyse the hardness profile in surface modified substrate prepared by multi-pass C-TIG arcing process with 50 % overlaps. 5. Analysis of mechanical properties using uniaxial tensile test of surface modified substrate and three-point bend test applied (tensile) on modified surface of the substrate prepared by C-TIG arcing process. 6. Residual stress analysis of the modified surface by using hole drill method. The magnitude, type and distribution of stresses in the surface modified substrate are correlated with the procedural aspects (single and multi-pass) of modification and heat input of the arcing process. 7. Characterization of single and multi-pass C-TIG processed substrate for fatigue life (S-N curve) under dynamic uniaxial tensile and bending load conditions. 8. Comparative studies on various characteristics of the single-pass and multi-pass surface modified substrate prepared by C-TIG and P-TIG arcing processes applied at given heat input. iii After finding the potentials of TIG arcing process to produce improved surface modification, effort has been made to develop knowledge of critical application of autogenous TIG arcing process to obtain optimum surface modification of AISI 4340 structural steel substrate. The knowledge includes appropriate control of process parameters primarily in order to manage the thermal cycle and isotherm of the heating zone, giving 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 use of single-pass C-TIG arcing process gives 175 % and 50 % increment in hardness of fusion zone (FZ) and HAZ respectively of the modified zone with respect to that of base metal with adequate depth of modified zone of 2 ± 1 mm. The use of single-pass P-TIG arcing process further increases 5 to 10 % hardness in the modified zone as compared to that observed in case of C-TIG arcing process at the similar heat input, arc current and travel speed. It is observed that the P-TIG arcing is relatively more advantageous over the C-TIG arcing process, as it leads to relatively higher depth of penetration, higher hardness, and a better control over surface characteristics. The increment in hardness with sufficient case depth makes P-TIG arcing process more suitable for bearing industry. The mechanical properties have been significantly affected by single-pass C-TIG and P-TIG arcing process. In case of tensile properties, C-TIG arcing process improves yield strength and ultimate strength to 600 and 900 MPa respectively, but the ductility and toughness are decreased to 6 % and 38 J/mm3 respectively as compared to those of the base found as 395 and 705 MPA and 19.24 % and 119 J/mm3 respectively. Similar behaviour was also observed in flexural properties of bend test of modified surface of the substrate produced by C-TIG arcing process. In case of single-pass PTIG arcing, further reduction in ductility (% strain) is observed. The effect of C-TIG and P-TIG arcing process was discussed with different parameters and variables along with the residual stresses. Multi-pass C-TIG and P-TIG arcing process was performed at the optimized parameter of CTIG and P-TIG arcing processes used for optimum properties of single-pass surface modification. A back tempering effect was observed by the subsequent pass during multi-pass C-TIG and P-TIG arcing. Due to back tempering significant changes are occurred in hardness, tensile, bending and fatigue properties. These properties are studied in detail. The use of surface modification processes develops residual stress over the surface. It is found that the use of multi-pass C-TIG and P-TIG arcing process produces compressive stresses in longitudinal and transverse direction. The development of compressive stresses over the surface is found beneficial primarily to improve the fatigue properties of modified matrix of the substrate.