M.K. Karthikeyan

Aging Behavior in 15-5 PH Precipitation Hardening Martensitic Stainless Steel

15-5 PH stainless steel is strengthened by precipitation of copper rich phases in a low carbon body centred cubic (b.c.c) lath martensitic matrix. Microstructures developed in aerospace grade 15-5PH precipitation-hardened stainless steel with different aging heat treatments have been studied. An attempt has been made to correlate the microstructural observations with the mechanical properties in different aging regimes. The overaging treatment imparts excellent ductility with a compromise in strength in peak aged condition to overaged condition the tune of 300 MPa in both tensile and yield strength. Hardness and tensile strength showed a similar decreasing trend with increase in aging temperature.

Electron Beam Welding Studies on Nb-Hf-Ti Refractory Alloy

Niobium, a refractory metal is mainly used as alloying addition in steels, superalloys, titanium and copper alloys. Being lightest refractory metal with high melting temperature, niobium based alloys are developed for high temperature applications of aerospace systems. However, poor oxidation resistance at elevated temperature limits its fabrication options and also requires oxidation protection in service. Among the fabrication methods, electron beam welding has been found to be a realistic option and the same has been studied in the present work. The paper presents the details of the Electron Beam Welding study carried out in developing the welding procedure for this alloy. An attempt has been made to correlate the weldment microstructure with the mechanical properties.

Comparative Study on Tempering Response of Martensitic Grade AISI-420 Stainless Steels with Varying Carbon Content

Martensitic grade stainless steels are being extensively used in aerospace, defence and nuclear sectors for structural applications. Specialized applications require close control in chemistry and heat treatment parameters. Control of carbon in AISI-420, alloying content has typical service advantages. To study the effect of tempering temperature with carbon content on mechanical properties, various heat treatment cycles were devised, avoiding the regime of temper embrittlement. This paper presents the tempering response of medium carbon stainless steel AISI-420 grade with respect to change in carbon content from 0.2% to 0.3%. It is observed that, by varying the tempering temperature, the grade can be tailored to obtain wide range of mechanical properties. With increasing carbon content, martensite is found to be changing in morphology from lath to lath & plate (mixed) and the amount of retained austenite also increases. An attempt has been also made to arrive at a structure-property correlation in this grade of stainless steel.

Development of High Nitrogen Stainless Steel for Cryogenic Applications

Austenitic stainless steels are extensively used as structural materials for various aerospace systems. Nitrogen containing stainless steels have special role due to their austenite stabilization tendency down to subzero temperatures, improved strength and resistance to sensitization. Primary processing of nitrogen containing cryogenic grade stainless steel 202 has been carried out through two different melting routes viz. (1). conventional melt route of electric arc furnace (EAF) melting followed by vacuum oxygen decarburization (VOD) & vacuum degassing (VD) and other one through (2). vacuum induction melting (VIM) followed by ESR. Chemical analysis and macrostructure analysis was carried out on the samples drawn from these billets. Homogenization and thermomechanical processing parameters were selected and the same were followed for the ingots made through both the melt routes. Mechanical properties evaluation (including tensile properties at subzero temperature of 77K) and micro structure characterization of the products realised from all the two melt routes were carried out. It is observed that, both the melt routes could result in achieving the required aerospace quality of alloy with respect to the chemical composition, metallurgical and mechanical properties. This paper confirms that any of the melt routes studied herein can be adopted according to availability of the facilities. The process development and characterization of the steels processed by conventional EAF+ VD & VOD and VIM+ESR melt routes is presented in this paper.

Heat Treatment Studies on 50CrV4 Spring Steel

50CrV4 spring steel is a tough, shock resisting, shallow hardening chromium vanadium steel having high fatigue and impact resistance in the heat treated condition. It is used extensively in gears, pinions, springs, shafts, axles, pins, bolts, etc., which require high modulus of resilience. The alloy was realised through conventional melt route of electric arc furnace (EAF) followed by ESR. The application of the alloy is limited to a section thickness of 15mm [1]. Hence obtaining optimum mechanical properties becomes a challenging task. In this study, the hardening as well as tempering operations were limited to 15mm thickness. The samples from the alloy were subjected to hardening at 860°C for 1.25 h. and oil quenching to room temperature followed by tempering at four different temperatures of 250, 300, 370 & 450°C for 3 h. each with oil quenching to room temperature. It was found that the alloy exhibited good combination of strength and ductility when tempered at 450°C. Microstructural study revealed the presence of fine tempered lath martensite along with the presence of a very small amount of delta ferrite along prior austenitic grain boundaries.

Development and Characterization of 15Cr-5Ni-1W Martensitic Precipitation Hardening Stainless Steel for Aerospace Applications

15Cr-5Ni-1W precipitation hardening (PH) stainless steel is a martensitic PH stainless steel finding extensive use in semi-cryo engine applications. The alloy was developed through Vacuum Induction Melting (VIM) + Electro slag refining (ESR) under argon cover route. The alloy contains heavy elements like Mo, Nb, V and W totalling ~ 2 % by weight. Since the alloy is martensitic, stringent gas levels were also specified. Hence it was a challenging task to realise it without any segregation and stringent gas levels. The alloy was successfully melted through two different melt routes – (C). Electric Arc melting followed by Vacuum Oxygen decarburization (VOD) - vacuum degassing (VD) followed by secondary melting by ESR and also by melt route (V) vacuum induction melting (VIM) + ESR route. It was then forged into bars, rods and rings. The samples from the alloy were subjected to two different heat treatment cycles. Both the heat treatment cycles involved hardening at 1000°C for 2 hrs followed by air cooling to room temperature. In one of the cycle, sub-zero heat treatment at-70 °C was done prior to tempering while in the other cycle; direct tempering was carried out after hardening operation. Tempering was carried out at 2 different temperatures of 490 and 500 °C to achieve the specified mechanical properties. It was found that the alloy could meet the specified strength and ductility with both the heat treatment cycles mentioned above. However samples subjected to subzero heat treatment showed marginally higher strength with slight compromise in ductility. The alloy also exhibited similar impact toughness in both the heat treatment conditions. Delta ferrite was also found to be within 2% for both the heat treatment cycles employed in this study. The alloy also exhibited excellent strength and ductility at elevated temperature of 500 °C with just 25% reduction in yield strength compared to room temperature yield strength without much change in ductility.

Equal Channel Angular Pressing of Al Alloy AA2219

Severe plastic deformation processes (SPD) are gaining importance as advanced materials processing techniques and hold immense potential in obtaining ultra fine-grained high strength materials. Among the SPD techniques, Equal channel angular pressing (ECAP) has its own merits to produce materials with ultra fine grains in bulk with better mechanical properties. The material deforms with high level of plastic strain inside the channel resulting in grain refinement of the output material with improvement in mechanical properties. A very viable die configuration was conceptualized and die was made with 1200 channel angle. Processing of 25 mm dia. of Al alloy AA2219 at room temperature was successfully carried out and grain refinement was observed. The mechanism of grain refinement has been studied using optical and transmission electron microscopy (TEM). It was observed that low energy dislocation structure (LEDS) forms concurrently with sub-grain structure due to dislocation rearrangements, which provide stability to the evolving sub-grain structure. Dislocation mobility is hindered by the presence of precipitates and / or intermetallic dispersoids present in the matrix and results in presence of dislocations in grain interiors. The pile up of dislocations at intermetallic dispersoids was confirmed from the dark field TEM micrographs. Present paper describes the experimental procedure and followed to attain severe plastic deformation through ECAP. Increase in hardness as well as refinement in the grain size after 5-passes have been discussed in light of extensive optical and TEM. The mechanisms of grain refinement to achieve nano-grained structure and strengthening accrued from the grain refinement through ECAP has been discussed.