During forging operations, strain can occur through three primary mechanisms: strain due to load applied through dies, strain due to thermal contraction, and strain due to creep. In materials behavior models, strain due to applied load and thermal contraction are directly considered and predictions are based on thermophysical properties and flow stress behaviors as inputs to the models. Strain due to creep after forging (during cooling) is often more difficult to predict and capture due to lack of materials data. In particular, data that capture the changing flow stress behavior during cooling (rather than from isothermal testing) are not commonly available. In this project, creep strain behavior during cooling was investigated by physical simulations using a Gleeble 3500. Standard cylinder-shaped Ti-6Al-4V samples with 10 mm diameter were heated to below β-transus temperature (1775°F) or above β-transus (1925°F), followed by constant cooling rates of 250°F/min and 1000°F/min with and without applied load during cooling to 1000°F. Total strain for the tests ranged from 2 – 6%. Characterization of prior microstructure and texture was carried out using XRD, optical microscopy, and SEM. The results provide insights on the relationship of flow stress behavior and microstructure as a function of temperature and cooling rate and are applicable to forging practices. These materials data can be used as input for future process modeling, potentially giving better prediction accuracy in industry applications.

Precision cold-forging processes are used to produce near-netshape parts that may then be carburized. During carburization thermal cycles, abnormal grain growth (AGG) after cold forging is known to develop microstructures which limit fatigue strength. In the present study, a small 0.04 wt.% Nb addition was made to a low-alloyed AISI 4121 steel containing 0.3 wt.% Mo. Subcritically annealed specimens were cold rolled (to simulate cold forging) at selected reduction ratios up to 50%, heated according to a simulated gas carburizing cycle at 930 °C, and water quenched to produce a final martensitic microstructure. The number density of abnormally grown grains increased rapidly as the cold rolling reduction ratio increased from 0 to 10%. With a further increase in reduction ratio, the extent of AGG decreased and was absent in samples subjected to the maximum reduction ratio of 50%. The evolution of fine (Nb, Mo)(C,N) precipitates at various stages of processing was characterized by thermodynamic calculations and electron microscopy and compared to the occurrence of abnormal austenite grain growth. The significance of these results for controlling AGG and thus optimizing fatigue performance in commercially-produced cold-forged and carburized components is discussed.

Forging processes include various steps to attain favorable material properties such as heat treatment, rapid quench, cold work stress relieving, and artificial aging. These steps, however, also contribute to bulk residual stress. Excessive bulk residual stresses cause a wide of problems, including part distortion during machining and in use, reduced crack initiation life, increased crack growth rates, and an overall reduction in part life. This paper summarizes recent work aimed at measurement-based assessment of bulk residual stresses in cold-compressed aluminum die forgings. The results show that forging process induced residual stress is a repeatable phenomenon with RMS repeatability less than 5% of yield.

In industrial applications, hot forging of aluminum alloy AA 6082 is carried out at 480 °C following a preheating process in an induction heater. The forged parts are then cooled down to room temperature, heated up again to apply conventional solution treatment followed by quenching and artificial aging processes. Repetitive heating/cooling steps are a significant cause of energy loss. The aim of this study was to provide time and energy efficiency by combining hot forging and solution treatment processes in a single high temperature process. To achieve this a new and improved heat treatment pattern was introduced. AA6082 parts were quenched immediately from a rather high forging temperature and artificially aged without any necessity for a second heating step and solution treatment. Mechanical properties of parts heat treated by this new pattern were than compared to the mechanical properties of parts heat treated conventionally. Heat treatment of AA6082 alloys were carried out for 30 minutes at three different temperatures (480, 510 and 540 °C) for comparison, followed by forging, water quenching and artificial aging (180°C, 8h). Mechanical properties of each sample were investigated using hardness and tensile tests. Elemental analysis and microstructural characterization were carried out using Energy Dispersive Spectrometry (EDS), Scanning Electron Microscope (SEM) and Optical Microscope (OM). Required minimum hardness for the samples after heat treatment was considered as 90 HB. This hardness value could not be obtained for the parts forged/solution treated at 480°C and 510°C. Hardness values of parts heat treated at 540°C, water quenched and aged at 180°C were higher than 90 HB.

Carburizing is a method of enhancing the surface properties of components, primarily made from low to medium carbon steels, such as shafts, gears, bearings, etc. Carburized parts are generally quenched and tempered before being put into service; however, after quenching of carburized parts further annealing and hardening treatments can be employed before final tempering. This work analyses the impact of the two aforementioned heat treatment approaches on the development of subsequent microstructures and mechanical properties of hot forged 18CrNiMo7-6 steel. Moreover, this study aims to understand the impact of normalizing treatments prior to the two aforementioned heat treatment routes. Microstructural and mechanical tests were conducted on four as forged flat cylinder components that received a combination of the abovementioned heat treatments. In general, better microstructure refinement, in terms of prior austenite grain size (PAGS), was obtained for carburized parts that received the intermediate annealing and hardening treatments after quenching and prior to the final tempering. Additionally, further refinement of the martensitic pockets/blocks was observed for parts that did not receive a normalizing treatment prior to carburization. The studied heat treatments appear to have a negligible effect on the mechanical properties of the hot forged flat cylinder components.

For over two decades, heat treat modeling has progressed from an academic concept to a mature production tool. This presentation will discuss many barriers that have been mitigated with a wide range of developments. Early limitations included solver speed and robustness, material data, specialized heating and the requirement to include microstructure development models over a series of dissimilar operations and processes. Solver improvements ranging from parallel processing to specialized iteration methods allow models with millions of elements to run on a personal computer. Additional degrees of freedom have greatly improved solution accuracy. Meshing techniques allow users to identify critical regions for a finer mesh, such as the surface of gear teeth that will be carburized. Rotational (and other) symmetry is frequently used to further refine many models. Driven by the demand for modeling data, sources for quality material properties have increased over the years. Additionally, tools to approximate required data based on chemistry are available and maturing. Radiant, convective, electrical resistance and induction heating effects are incorporated into heat treat simulation systems. Integrated simulation systems also include large deformation behavior to capture the effects of forging, coining or other mechanical processes on the microstructure. A vision of the future will include the use of Design of Experiments (DOE) and optimization in heat treat simulation. How companies will model the entire process chain to build a more accurate fatigue model for the part in service will be discussed. In terms of TRL (technology readiness level), heat treat simulation was in the 2 – 3 range in the 1990’s. Today it is in the 7 – 8 range and moving quickly.

The distortion behavior of carburized and fully heat treated Ni-Cr-Mo martensitic steel (S156) has been experimentally evaluated. Dimensional measurements of Navy C-ring distortion coupons during interrupted heat treatment process for parts manufactured from two forming routes, hot forging and machined from as received bar, was performed. Metallurgical analysis was carried out to attempt to relate the observed microstructural characteristics with measured process induced distortion. The carburization process was found to be the most severe in terms of inducing distortion. It was found that additional heat treatments during the process results in a larger final distortion. Machining parts from forgings results in higher distortions than that of those machined directly from as received bar due to the added thermal processing history. A finite element simulation of the carburization process for a C-ring coupon is presented.

High frequency welding is a thermo-mechanical process that relies on precise heat input as well as mechanical control as strip edges are heated and forged together to result in a seam weld. Heat input can be defined as a way of characterizing the temperature distribution at the strip edges prior to forging them together. Heat input is affected by several process variables ranging from raw material properties to welder settings and weld area setup. These are summarized in this paper, with special attention on the effects of welder frequency, welder power, line speed, and steel alloy composition on heat input and the resulting weld quality. Frequencies in the range of 100 – 800 kHz are considered. Data from tube mills (including general data and controlled on-the-mill experiments) and laboratory evaluations are included in this paper.

An attempt was made to characterize microstructure, mechanical properties and cleanliness of continuous cast as rolled billets versus microstructure, mechanical properties and cleanliness of the forging in normalized condition, upset forged from AISI 41B30 modified chemistry billets. Two forgings were compared, one in as forged condition and one in normalized or heat treated condition. Upsets were produced by upsetting only one end of the billet by hydraulic press. Samples from cold portion of the forgings, near the flange location and from flanges were taken and examined. Results of microstructure, mechanical properties and hardness are presented. Normalizing cycle did not improve mechanical and impact properties. Low impact and ductile properties are results of Widmanstätten structure and continue to be present in the final product. Low impact and ductile properties of this structure might not be the best solution for dynamically loaded parts.

Extensive steady-state flow boiling heat transfer data have been collected for both water and oil using a test rig that allows for control of the primary variables of fluid velocity, fluid temperature, surface temperature, and surface orientation. Using the resulting database, functional relationships for boiling heat transfer trends have been determined. These boiling heat transfer relationships have been incorporated into the Ansys-Fluent commercial CFD package using a user-defined function (UDF). This software combination provides a simulation tool capable of approximating the quenching of metal parts by applying a situationally correct heat flux at each point in the part surface throughout the quench cycle. No manipulation of model tuning parameters is required. Simulation predictions are compared to test data collected for a cylindrical forging.