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Rings from titanium alloy Ti6Al4V used for making pressure vessels for launch vehicles are studied after annealing and after solution treatment and aging. The mechanical characteristics of the rings after quenching and aging do not always have the specified values, especially in thicker sections. The studied rings are of two sizes and have a final wall thickness of 30.5 and 17.5 mm. The effect of the temperatures of solution treatment and aging, of the quenching rate, and of the section thickness on the mechanical properties is studied, and the process is updated to provide the required properties. INTRODUCTIONTitanium alloy Ti6A14V is a promising material for the aerospace industry due to its high specific strength. The alloy is used for making fuel tanks of launched vehicles and satellites. The tanks are fabricated by welding domes and rings that are delivered in two conditions, namely, after annealing or after quenching and aging. The process of production of rings in annealed condition is relatively simple and well developed. The process of production of rings in quenched and aged condition is more complex and requires optimization with respect to several parameters in order to provide the specified mechanical properties, especially in the case of large sections. For this reason, quite a number of works have been devoted to determining the optimum conditions for the production of rings starting with melting of the alloy and ending with the heat treatment of ready articles [1,2]. The final mechanical properties of titanium alloys are determined by the morphology of the products of the martensitic transformation and of the decomposition of a¢-martensite during aging. Due to the low thermal conductivity of titanium the occurrence of these processes and hence the formation of the specified properties depends substantially on the sizes of the section.The aim of the present work was to optimize the heat treatment process (solution treatment and aging) in the production of rings from titanium alloy Ti6Al4V. METHODS OF STUDYWe obtained titanium alloys by the vacuum arc remelting (VAR) process with a cold compacted electrode. We used several remelting operations to control the content of gases and impurities in the ingot. The ingot was forged and rolled to the required ring sections. The preform was subjected to deformation (upsetting and drawing) to provide good results after the heat treatment. The rolled rings were subjected to a heat treatment involving solid solution treatment and aging. We studied rings of two sizes having a diameter of about 1.2 m and a wall thickness h = 17.5 and 30.5 mm. A typical production cycle included double VAR, forging, pancaking and piercing, saddling, ring rolling, solution treatment, measurement of the mechanical properties of specimens and rings, ultrasonic inspection, proof machining, and final inspection.The chemical analysis of the alloy was performed by the method of optical emission spectroscopy. The content of the gases (oxygen) was analyzed by the Leco method. The mechanical...

This paper presents the details on the development of a processing map from the isothermal compression test results of cobalt-free (CF250 grade) maraging steel over a wide range of temperature (900-1200 °C) and strain rate (0.001-100 s −1 ). Processing maps identifying 'stable' and 'unstable' regions during hot working were developed. Two safe processing windows with high efficiency of power dissipation and strain rate sensitivity can be noticed in the power dissipation and maps of strain rate sensitivity parameter at low strain rates (10 −3 s −1 ). However, a stable higher strain rate processing window occurring in the temperature range of 1050-1150 °C and strain rate range of 10 1 -10 2 can be used for increased productivity. Based on the high values of the efficiency of power dissipation, microstructural observations, and EBSD results, dynamic recrystallisation was found to be the softening mechanism occurring in this material. The measured flow stress data were used for the development of constitutive model for hot deformation behavior of this alloy. The activation energy for deformation of CF250 maraging steel was calculated to be 458.8 kJ mol −1 .

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Nickel-based superalloy Inconel 625 is widely used in aeronautical, aerospace, chemical, petrochemical and marine applications due to its good mechanical properties, weldability and resistance to high temperature corrosion on prolonged exposure to aggressive environments. It is a solid solution strengthened medium strength superalloy, which contains chromium, molybdenum and niobium as alloying additions. Considering the chemistry and specification requirements of the alloy, it was processed through vacuum induction melting (VIM) process followed by electro slag remelting (ESR) route to obtain alloy with controlled gas and inclusion contents. Homogenisation cycle was selected and was carried out at 1170°C temperature to obtain uniformity in chemistry and microstructure. Chemical homogeneity was confirmed through analysis of samples from top, middle and bottom of the secondary ESR ingot. Hot working range was decided considering the flowability of superalloy and the same was carried out under close monitoring of temperature and with specified amount of reduction per stroke. Intermediate reheating and reduction during forging was noted to be an important aspect so to avoid cracking during forging. Processing parameters were established to obtain forgings of different thicknesses with sound ultrasonic quality. Microstructure analysis revealed single phase austenitic grain structure with ASTM grain size no. 4-7, confirming that material has undergone sufficient amount of mechanical working. Mechanical testing was carried out and the mechanical properties were found to be meeting the requirement. Present paper provides details of melting process selection, thermomechanical processing and characterization of the superalloy to achieve the targeted mechanical properties.

The ultrahigh strength (2,400 MPa) 18Ni maraging steel (M350 grade) is widely used for critical structural applications, such as aircraft landing gears, in which the strength–toughness balance is the essential criterion for material selection. Microstructure control during thermomechanical processing is the key to obtain the desired mechanical properties on a repeatable basis in a manufacturing environment. This involves thorough understanding of the hot deformation behavior under a wide range of temperatures and strain rates to map the microstructural evolution as a function of process parameters to obtain defect-free products. Towards achieving this goal of optimization of hot workability with a view to control microstructure for M350 grade maraging steel, hot deformation processing maps have been developed and correlated to the microstructure evolved. Further, analysis of stress–strain curves was carried out to obtain fine prior austenite grain (PAG) size via discontinuous dynamic recrystallization, and the same was verified experimentally by the microstructures evolved through hot isothermal compression tests on cylindrical specimens subjected to different strain levels. A single peak DRX type σ − ε curve was selected for analyses. The theoretically determined critical strain value was verified experimentally for initiation of DRX (DRXI) and transition from DRX dominant region to grain growth dominant region (DRXT). Hot isothermal compression tests have been conducted at T = 950°C and ε˙=0.01 s−1 and obtained PAG size of 3.14 µm in the specimen deformed to theoretically determined optimum strain of 0.74, thereby validating the used models.