Interactions Between Dynamic Softening and Strengthening Mechanisms During Hot Forging of a High-Strength Steel

Open-die forging is a critical step in the manufacture of large numbers of components used in the transportation and energy industries. Dynamic recrystallization, dynamic transformation, and dynamic precipitation take place during the hot deformation process and significantly affect microstructure conditioning, which ultimately influences the service properties of the component. In the present work, using a Gleeble 3800 thermomechanical simulator, the open-die forging of a large-size ingot made of a modified AISI 6140 medium carbon high-strength steel is investigated. Deformation temperatures ranging from 950°C to 1,250°C and strain rates ranging from 0.01 to 1 s−1, representative of the actual process, are considered in the analysis. The generated true stress–true strain curves are used as a basis for the development of a constitutive model predicting the occurrence of softening and strengthening phenomena as a function of thermomechanical conditions. The corresponding activation energy is determined to be about 374 kJ mol−1 and is compared against the values reported in the literature for other high-strength steels. Dynamic recrystallization kinetics is studied using the t50 model, and the influence of temperature and strain rate is quantified and discussed. The interaction between dynamic precipitation and dynamic recrystallization is discussed, and the deformation conditions under which such interactions occur are determined. The thermomechanical results are validated by microstructure examination, including laser confocal microscopy, field emission scanning electron microscopy, transmission electron microscopy, and energy-dispersive spectroscopy. The present study focuses on reproducing the deformation cycle applied during the open-die forging process of a vanadium-containing high-strength steel used in the industry with special attention to the interaction between dynamic recrystallization and precipitation processes.

Case Study on Large Ingots Cracking Failure

Use of Metal and its alloys have become the need of this world. high strength pipes and equipment are required for the extraction of bituminous oil from soil. For this High strength Low Alloy Steel (Micro-alloyed steels) can be used. HSLA required large size ingots for its manufacturing therefore in recent years, large size ingots demand is increased. Large size ingots are subjected to cracking while manufacturing during open die forging process. Optical Scanning Electron Microscopies and Energy dispersion Spectroscopy techniques are carried out for the investigation of root causes of the cracking during forging of large size ingots. Microstructure of large size ingots sample are reviewed at different locations and grain boundaries. Results of these chemical techniques shows the enrichment of chromium and oxygen content at the boundaries which clearly differentiated a cracked boundary and non-cracked locations. This writing comprises the literature study, test procedures and analyzing the results to review that abnormal grain growth was the cause of cracking of large size ingots during forging process. The report also reviewed, alternate heat treatments and possible solution.

Effect of Direct Powder Forging Process on the Mechanical Properties and Microstructural of Ti-6Al-4V ELI

The study of powder metallurgy processing methods for titanium represents a promising avenue that can respond to a growing demand. This work reports the feasibility of direct powder forging (DPF) as a method to process large spherical Ti-6Al-4V powder into wrought products with noteworthy properties and physical characteristics. Direct powder forging is a thermomechanical process that imparts uniaxial loading to an enclosed and uncompacted powder to produce parts of various sizes and shapes. Stainless steel canisters were filled with prealloyed Ti-6Al-4V powder and consolidated through a multi-step open-die forging and rolling process into wrought DPF bars. After DPF, annealing was performed in the upper α+β phase. The results show that full consolidation was achieved and higher mechanical properties than the Ti-6Al-4V grade F-23 requirements in annealed conditions were obtained. The results also show that direct powder forging of spherical titanium powder could produce wrought mill products and exhibit some potential for further investigation for industrial applications.

Investigation of the Influence of Open-Die Forging Parameters on the Flow Kinetics of AZ91 Magnesium Alloy

This paper presents the results of numerical tests of the process of forging magnesium alloy ingots (AZ91) on a hydraulic press with the use of flat and proprietary shaped anvils. The analysis of the hydrostatic pressure distribution and the deformation intensity was carried out. It is one of the elements used for determining the assumptions for the technology of forging to obtain a semi-finished product from the AZ91 alloy with good strength properties. The aim of the research was to reduce the number of forging passes, which will shorten the operation time and reduce the product manufacturing costs. Numerical tests of the AZ91 magnesium alloy were carried out using commercial Forge®NxT software.

Successful Pass Schedule Design in Open-Die Forging Using Double Deep Q-Learning

In order to not only produce an open-die forged part with the desired final geometry but to also maintain economic production, precise process planning is necessary. However, due to the incremental forming of the billet, often with several hundred strokes, the process design is arbitrarily complicated and, even today, often only based on experience or simple mathematical models describing the geometry development. Hence, in this paper, fast process models were merged with a double deep Q-learning algorithm to enable a pass schedule design including multi-objective optimization. The presented implementation of a double deep Q-learning algorithm was successfully trained on an industrial-scale forging process and converged stable against high reward values. The generated pass schedules reliably produced the desired final ingot geometry, utilized the available press force well without exceeding plant limits, and, at the same time, minimized the number of passes. Finally, a forging experiment was performed at the institute of metal forming to validate the generated results. Overall, a proof of concept for the pass schedule design in open-die forging via double deep Q-learning was achieved which opens various starting points for future work.

Subsurface Crack Initiation and Propagation in an Open Dieforged High Strength Steel: A Microstructural Analysis

This paper presents an experimental investigation with the objective to determine the root causes for the cracking of a large size bar made of a medium carbon low alloy steel after open die forging and heat treatments operations. The cracks were observed below the surface during the machining step. In order to understand the mechanisms of crack initiation and propagation, micro-CT tomography and scanning electron microscope (SEM) were employed. Microstructural damage analysis revealed oxidation of different alloying elements, more specifically manganese, chromium and silicon. The presence of defects in the form of cavities and porosities were also observed at the grain boundaries. Some of the above defects were observed along the crack path, while others were on both sides of the cracks without any connection to them and finally, a third group completely isolated from any crack. The characteristics of the defects were thoroughly analyzed and it was found that the crack initiation could be attributed principally to the porosities/cavities formed during solidification. The analysis also showed that crack propagation occurred during solidification and/or forging and heat treatment steps.