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The paper presents the results of modeling and testing of a heavy weight part made of Cr-Mo, which was V-modified ultrahigh strength steel grade AISI 4140, processed through a novel open-die forging program and two alternative routes of twostage heat treatment cycles designed to meet requirements of high-duty components for energy sector. By using unconventional forging conditions based on the assumption of large feed and reduction ratio and modifying the chemical composition, better control of the austenite grain was achieved to minimize abnormal grain growth and/or strain uniformity problems. Using the Finite Element Modeling, the multi-stage sequence of upsetting and the cogging strain distribution were optimized to minimize the strain variation along the length to a range 2.2÷2.7, and correlated with the microstructure generated at each main stage on the large cross-sections of the shaft. Mechanical and thermal processing cycles designed using the finite element method were fully verified physical modeling using a 16 ton forging block, including two alternative quenching strategies: oil vs. water spray and air. The material was studied in the as-forged, normalized and heat-treated states to observe the behavior of the hot-formed material and the effects of cooling conditions on the microstructure during the final heat treatment. It was found that the use of large feed ratios on cogging and varied cooling allowed to suppress the adverse effects of the inevitable abnormal grain growth, resulting in 1-2 ASTM in forged condition and reaching 6 ASTM and 8/9 ASTM after quenching in oil and water spray, respectively, which allowed a corresponding notched impact strength of 44÷48 and 85÷122 J/cm2 in the critical region of the forged shaft after tempering. [ABSTRACT FROM AUTHOR]

Abstract AD730™ is a newly developed Ni-based superalloy for turbine disk applications produced by cast and wrought processes. Conventional ingot-to-billet conversion is an expensive and very complex operation. Because of the difficulties to achieve a uniform strain for recrystallization, large unrecrystallized grains are retained along this process. Heterogeneities of grain size have a negative impact on subsequent forging processes as well as ultrasonic inspectability of the destination part. The main aim of this work is to analyse the impact of both forging direction (billet, cogging) and strain (ε = 0.3–2) on the microstructural evolution of AD730 at subsolvus temperatures from a semi-finished product (billet) under conditions representative of both cogging and hot forging operations. Special attention to the presence of large unrecrystallized grains was paid. Double truncated cones (DTCs) were hot forged at subsolvus temperatures followed by air cooling. SEM and EBSD analysis were conducted in the as-received (billet) and the as-forged conditions. AD730 alloy presents a complex microstructure characterized by the presence of large unrecrystallized grains, aligned in the billet direction, with remarkable differences in γ′ precipitates distribution as compared to recrystallized structures. The fine distribution of primary γ′ precipitates (pinning effect) plays a key role on grain size control but also on the recrystallization behaviour. Continuous dynamic recrystallization (CDRX) mechanism was found to be operating in the large unrecrystallized grains, promoting the formation intragranular DRX grains and the gradual recrystallization of these grains. Strain presents a strong effect on the microstructural evolution of AD730, increasing the recrystallization fraction and refining the structure. By contrast, no significant effect associated to the forging direction was found. Conditions representative of cogging operations (ε ≤ 0.6) at subsolvus temperatures were translated into large fractions of unrecrystallized structures (strain accumulation). Highlights • Large unrecrystallized grains present a fine distribution of intragranular γ′ precipitates; • Intragranular precipitates play a key role on strain distribution and subsequent recrystallization behaviour for AD730; • Continuous dynamic recrystallization (CDRX) promotes the gradual consumption of large unrecrystallized grains. • Undissolved secondary precipitates play a key role on the microstructural evolution of AD730 • No significant effect associated to the forging direction on the recrystallization behaviour of AD730 was found. [ABSTRACT FROM AUTHOR]

The hot deformation behavior of Waspaloy alloy has been investigated by two-pass hot cogging process. The paper presents theoretical end experimental analysis of deformations and microstructural evolutions. The results of a thermo-mechanical simulation for the spatial hot cogging process on the shaped anvils with the application of the three–dimensional finite element method, are presented. The numerical calculation gave an assessment of the effective strain, mean stress and temperature distributions in the work-piece. Models for predicting the evolution of microstructure were developed for dynamic recrystallization and grain growth phenomena. The influence of shape of the anvils on the grain size after dynamic recrystallization was analyzed. The numerical analysis was performed using a commercial program "DEFORM 3D" with thermo-mechanical and microstructural evolution coupled. The results are compared with the experimental data, a good agreement between the predicted and experimental results was obtained.

• The isothermal compression of four typical microstructures for Ti-6Al-4V is studied. • A modified H-S CE considering the alpha/beta structure is proposed and calibrated. • Compared with the H-S and S-C-A CEs, the W and Δ% of the modified CE are smaller and more concentrated. • The modified H-S CE is embedded in the finite element code, and good agreement with the experimental results. The isothermal compression of four typical microstructures in the cogging process for Ti-6Al-4V was conducted at strain rates ranging from 0.01 to 1 s−1 with varying temperatures. The flow behavior of different microstructures was compared. The Hansel-Spittel (H-S) and strain compensated Arrhenius (S-C-A) constitutive equations (CEs) were calibrated. To improve prediction ability, a modified H-S CE considering the evolution of microstructure was proposed and embedded in the finite element code. The original, modified H-S, and S-C-A CEs were evaluated based on the average absolute error (W) and relative error ratio (Δ%). Compared with the original H-S and S-C-A CEs, the W (−2.5%~2.5%) of the modified CE was found to be smaller and the Δ% was more concentrated. Furthermore, hot forging experiment and finite element simulation (FES) at different speeds were performed. The average relative error (AARE) of load-stroke for the H-S and modified H-S CEs is 13.51% and 3.67%, respectively. The comparisons between the FES and hot forging experiment indicate that the modified CE is more accurate for the simulation of the cogging process. [ABSTRACT FROM AUTHOR]

Abstract: Microstructural changes during the cogging of an Alloy 718 ignot processed by (vacuum induction melting)/(vacuum arc re-melting) were predicted using the combined approach of three-dimensional finite element simulation and modeling for the recrystallization and grain growth. In order to establish recrystallization and grain growth models for Alloy 718 ingot, hot compression and isothermal heat treatment tests were carried out at different temperatures and strain rates. The constitutive equations for metadynamic and static-recrystallization behavior were generated using the experimental data collected from various references. From the experimental results, it was found that the dissolution temperature of δ-phase (Ni3Nb) played an important role in the microstructural refinement of Alloy 718 ingot. The predicted grain size and its distribution were compared with the microstructures of the cogged billet, and the reliability of the prediction was discussed. [Copyright &y& Elsevier]