Parameshwar Prasad Sinha

Bio: Parameshwar Prasad Sinha is an academic researcher from Vikram Sarabhai Space Centre. The author has contributed to research in topics: Maraging steel & Martensite. The author has an hindex of 7, co-authored 29 publications receiving 162 citations.

Austenite reversion in 18 Ni Co-free maraging steel

Abstract: The mechanism of austenite reversion in 18 Ni Co-free maraging steel (250 grade) has been established by conducting extensive X-ray diffraction (XRD) and transmission electron microscopy (TEM) under differently aged conditions. It has been proposed that contrary to the precipitate dissolution mechanism suggested for the initiation of austenite reversion in 18Ni-8Co-5Mo type maraging steels, the initiation of transformation of martensite to austenite in this type of maraging steel is due to the diffusion of Ni from matrix to the dislocations and other defect structures on prolonged/high temperature ageing. This results in local enrichment of Ni which lowers both A s and M s temperatures of the region. Lowering of these transformation temperatures is responsible for the early reversion of martensite to Ni-enriched stable austenite which, on subsequent cooling to room temperature, does not transform back to martensite.

Local Deformation Behavior of Inconel 718 TIG Weldment at Room Temperature and at 550°C

Abstract: The weld portion will have different mechanical properties in weld bead, HAZ and fusion boundary due to variation in the microstructure, chemistry and internal stresses that arise in the welding process. Inconel 718 TIG welded samples are tested to study the deformation behavior of the weld constituents with the help of non contact laser extensometer. Stress-strain curves for the individual constituents of weld, parent, fusion boundary and HAZ are evaluated at room temperature and at 550°C. Test results show that the welded samples exhibited 50% lower tensile properties compared to the parent material. Parent metal remains elastic through out the test. The proof stress of the weld alone is 100 MPa lower than the average proof stress of the welded sample when considered standard gauge length.

Metallographic investigations of the heat-affected zone II/parent metal interface cracking in 18Ni maraging steel welded structures

Abstract: During the fabrication of a large diameter pressure vessel out of 18 Ni maraging steel by manual TIG welding, microcracks were noticed at the heat-affected zone (HAZ)/parent metal interface. The location of these cracks was very different from those reported at the fusion zone/HAZ I interface due to “constitutional liquation”. Extensive optical metallography, scanning electron microscopy and energy dispersive X-ray analyses were carried out to identify the cause for the occurrence of these cracks. It is inferred from the experimental results that the microsegregation of titanium and nickel due to repeated thermal cycling during multipass welding led to the formation of TiC/Ti(CN) and stable austenite film on the grain boundaries. Under severe thermal stresses developed during welding, microvoids generated at the interface of TiC/Ti(CN) inclusions and austenite and further propagated intergranularly due to the premature failure of the austenite films.

Development of heat treatment parameters to improve fracture toughness and grain size of an embrittled maraging steel

Abstract: An embrittled 18 Ni maraging steel rolled ring was examined and samples cut from the ring were subjected to a wide range of heat treatments including high temperature solutioning and thermal cycling. The effects of these treatments on toughness were evaluated by measuring impact energy and plain strain fracture toughness. The microstructural analyses were carried out using extensive optical and scanning electron microscopy, and scanning electron fractography. It has been established that the ring was embrittled due to the combined effects of deformed structure and grain boundary precipitation of TiC or Ti (CN). Heat treatment parameters have been devised to improve the fracture toughness and grain size of the materials affected by these two types of embrittlement. It has been suggested that toughness and grain size can be improved by (a) annealing at 1223 K followed by water quenching in the case where deterioration in toughness is marginal and is caused by nonrecrystallized grains or deformation texture, and (b) solutioning at 1473 K followed by water quenching, and thermal cycling twice between room temperature and 1198 K with a holding time of 30 min at peak temperature in the case where the loss in toughness is considerably large due to excessive grain boundary precipitation of second phase particles.

Effect of austenitizing temperature and cooling rate on the structure and properties of a ultrahigh strength low alloy steel

Abstract: A 0.3C-CrMoV(ESR) steel is being developed primarily for making pressure vessels used for aerospace applications. Since it is important to understand the range of microstructures and mechanical properties that will be obtained in the heat affected zone of welds, the steel has been subjected to different austenitizing treatments (temperatures ranging from 925°C to 1250°C) followed by cooling at various rates to room temperature. It has been shown that the austenite grain size increased from about 10 to 250 μm as the austenitizing temperature is increased from 925°C to 1250°C (1 hr) and that the hardness, YS, UTS,% elongation and% reduction in area as well as CVN energy for 450°C tempered condition decrease as the austenitizing temperature is increased for all cooling rates (furnace cooling, air cooling, oil quenching, quenching and tempering at 450°C). This is attributed mainly to the increase in austenitic grain size. The ranges of microstructures that can be obtained in the heat-affected zone are massive ferrite, fine pearlite, upper as well as lower bainite and martensite. The Charpy impact energy for the oil-quenched steel tempered at 200°C, however, did not vary significantly with austenitizing temperature.

American Society for Testing and Materials

Abstract: The American Society for Testing and Materials (ASTM) is an independent organization devoted to the development of standards.

Microstructural evolution and low temperature impact toughness of a Fe-13%Cr-4%Ni-Mo martensitic stainless steel

Abstract: The microstructural evolution of a low carbon Fe-13%Cr-4%Ni-Mo (wt.%) martensitic stainless steel has been studied using transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP). The as-quenched sample has a typical lath martensite structure, but carbon atoms were found to form ultra-fine clusters, or carbon-enriched regions, in the martensite. After a single-stage tempering at 680 degrees C. the steel mainly consists of the martensite and a certain amount of fine M(23)C(6) carbides, which act as heterogeneous nucleation sites of the reversed austenite formed in the two-stage tempered (680 degrees C x 4 h + 600 degrees C x 4 h) samples. 3DAP investigations have revealed that the Ni, Cr and Mn atoms are enriched in the reversed austenite as contrasted with the martensite. Phosphorus atoms were uniformly distributed only in martensite. carbon was not detected in either martensite or austenite, and there was no segregation at the martensite-austenite interfaces. Those partitioning behaviors of the alloying elements were thought to be a factor contributing to the stability of the reversed austenite. High impact toughness at low temperature was obtained due to the existence of the reversed austenite. (c) 2009 Published by Elsevier B.V.

The influence of tempering temperature on the reversed austenite formation and tensile properties in Fe–13%Cr–4%Ni–Mo low carbon martensite stainless steels

Abstract: The influence of tempering temperature on the reversed austenite formation and tensile properties are investigated in Fe-13%Cr-4%Ni-Mo low carbon martensite stainless steel in the temperature range of 550-950 degrees C. It is found that at the temperatures below 680 degrees C, the reversed austenite formation occurs by diffusion. Amount of the reversed austenite is determined I:IN the tempering temperature and the holding time. The segregation of Ni is the main reason for the stability of the reversed austenite. When the temperatures are above 680 degrees C, the reversed austenite formation proceeds by diffusionless. The reversed austenite will transform back to martensite after cooled to room temperature. The tensile properties are most strongly influenced by the amount of the reversed austenite obtained at room temperature. The excellent combination of good strength and ductility is at 610 degrees C. (C) 2011 Elsevier B.V. All rights reserved.

Effect of retained austenite on the microstructure and mechanical properties of martensitic precipitation hardening stainless steel

Abstract: The effect of retained austenite (γ) on the microstructure and mechanical properties of a martensitic precipitation hardening stainless steel was experimentally investigated, whose chemical composition was Fe-1.8Cu-15.9Cr-7.3Ni-1.2Mo-0.08Nb-low C, N (mass %). The microstructures of all specimens consist of a typical lath martensite with interlath films of the retained γ, which is not reverted with aging. Cu-rich precipitates which may contribute to precipitation hardening can not clearly be observed. The tensile properties and Charpy absorbed energy are linearly approximated to the amount of retained γ as follows: 0.2% Y.S. (MPa) = 1192.3 − 13.6 × γ%, T.S. (MPa) = 1250.1 − 9.3 × γ%, El. (%) = 12.16 + 0.43 × γ%, R.A. (%) = 64.25 + 0.14 × γ%, and A.E. (J) = 72.5 + 0.8 × γ%. The introduction of retained γ is not beneficial to the fatigue limit. An excellent combinations of strength, ductility and toughness obtained in the present work is attributed to the introduction of retained γ and also to the chemical composition of the specimen used.