Large Forgings Research Articles

Improving the quality of forgings based on upsetting the workpieces with concave facets

We propose a forging method for forgings, which implies the upsetting of workpieces with concave facets. A procedure for the theoretical research has been devised aimed at studying the mechanism of closure of artificial axial defects in workpieces. The study was performed based on a finite element method. The key examined parameter was the depth of the concave facets in a workpiece. This parameter varied in the range 0.75; 0.85; and 0.80. The angle of the concave facets was 120°. The results of the theoretical study are the following distributions: deformations, temperatures, and stresses in the body of a workpiece in the process of upsetting the workpieces with concave facets. Based on these parameters, we established an indicator of the stressed state in the axial zone of the workpiece.In order to verify the theoretical results obtained, a procedure for experimental research has been developed. The study was performed using the lead and steel workpieces. The results of the theoretical study allowed us to establish that the effective depth of the concave facets is the ratio of diameters of protrusions and ledges equal to 0.85. At this ratio there occurs the intensive closure of an axial defect. This is due to the high level of compressive stresses when upsetting the workpieces with concave facets. We have established the effective degree of deformation at which the intensive closure of defects takes place. Also established are the distributions of deformations for the cross-section and height of the workpiece, as well as a change in the indicator of the stressed state in the process of upsetting workpieces with concave facets. The closure of axial defects has been confirmed by experimental study using lead and steel samples.The new technique for upsetting workpieces with concave facets has been implemented. The results of ultrasonic testing have allowed us to establish that the obtained parts do not have internal defects, which exceed the requirements of the European standard SEP 1921. Our research has led to the conclusion of the high efficiency of the proposed new method for upsetting workpieces with concave facets, which implies the improvement of quality of the axial zone of large forgings when using a given technique.

Manufacture of Large Disc and Ring Forgings in Alloy 600

Saarschmiede has focused on the development of large remelted Ni-based alloy forgings, typically used for research and development projects in the field of the power generation industry and the nuclear industry. Across these industrial sectors, there is a drive towards larger forgings with consistent mechanical properties, low levels of chemical segregation, and high requirements on the ultrasonic testability, for example, the increasing of the sensitivity. In light of this, Saarschmiede has focused on improving the maximum ingot sizes, ingot yield, hot working processes, and also the heat treatment cycles to meet the requirements of these highly regulated industries. Over the last three years, Saarschmiede has manufactured several large disc and ring forgings from Alloy 600 with a maximum forging weight of approximately 30 tons. These forgings were manufactured from several Electro Slag Remelted (ESR) ingots with a diameter of 1300 mm and a maximum ingot weight of 70 tons. This was, according to our knowledge, the worldwide biggest ingot in this grade ever produced at this time. This paper discusses the manufacturing route, the mechanical properties, the microstructure, and the Non-destructive Evaluation (NDE) for these disc and rings. It also includes a customer perspective on product quality.

Relation between the Nonuniformity of the Properties and the Structure of Large Forgings

Heterogeneous different-scale structures in large forgings made of a heat-hardenable Cr–Ni–Mo steel are compared. The effect of heterogeneity on the properties of the metal is evaluated.

Investigation of Materials and Welds for the Precompression Structure of the ITER Central Solenoid

The central solenoid (CS) is the backbone of the ITER magnet system. It consists of six independent coils held together by a vertical precompression structure that must react to tensile loads and provide sufficient preload to maintain coil-to-coil contact during all stages of plasma operation. Material selection and specifications applicable to the structural components of the precompression structure are particularly demanding. These include large forgings manufactured from a high-strength austenitic stainless steel (FXM-19) with a stringent specification in terms of microstructure and maximum allowed magnetic permeability. Stringent requirements are also imposed on all welded joints. In particular, the attachment welds of the cooling pipes to the structure are subject to challenging restrictions in terms of weld imperfections and geometry. They must induce limited distortion of the components and are submitted to inspections carried out in accordance with the most severe acceptance levels of applicable national and international standards. The results of a campaign of tests covering nondestructive and destructive evaluation of samples from different components and welds are summarized in this paper, particularly focusing on the quality achieved at the microstructural and macrostructural level. The influence of the microstructure on the final properties and on the inspectability of the material by nondestructive examinations is also discussed.

Nonmetallic Inclusion Distribution within Ingots for Power Generation Engineering Forgings

Results are provided for a study of the distribution and location of nonmetallic inclusions in large forgings of steel 38KhN3MFA weighing 24.2 and 23.52 tons with a normal configuration and with a changed shape of the bottom part (“convex” bottom) cast in a vacuum. It is shown that the change in ingot bottom section geometry leads to an increase in temperature gradient and as a consequence to an increase solid phase advance intensity. In turn, this leads to more uniform distribution of oxysulfide inclusions. The sulfide inclusion content in the ingots compared increases from the periphery to the ingot axis. Dependences are obtained for metal ductility properties, i.e., relative elongation, relative reduction of area, and impact strength on sulfide inclusions distribution over ingot levels, and their unfavorable effect on these properties is established. It is shown that a reason for the reduction in ductility properties is sulfide phase location in the form of films of dendrite boundaries that occurs under conditions of a shortage of oxygen in the melt. This leads to a reduction in oxysulfide content and formation of a sulfide component in an unfavorable form distributed over cast grain boundaries. The ingots are destined for domestic power generation engineering forgings.

A method for measuring the thermal geometric parameters of large hot rectangular forgings based on projection feature lines

The online dimensional measurement of large hot forging is an important procedure in the forging process. Because of different production demands, the final geometrical shapes of large forgings are usually different (e.g. cylindrical columns, rectangular prisms). Forgings of different geometric shapes need to be measured along different dimensions. For rectangular forgings, the lengths primarily need to be measured. A generalized measurement system for different geometric shapes of forgings cannot provide the accuracy of measurement systems targeted at measuring a known shape. Based on the characteristics of the rectangular forgings, a thermal dimensional measurement system is proposed in this paper. The localization, rapid extraction of feature points and method for measuring the dimensions of rectangular forgings are presented. The proposed methods can easily and efficiently extract the feature points of the forging. The experiment results show that the method proposed in this paper has the advantages of high precision and high efficiency, which is appropriate for online measurement.

Determination of heat transfer coefficients for large scale steel forgings quenched in polymer solutions

The inverse heat conduction method is used to calculate surface heat transfer coefficients (HTCs) as a function of temperature and location during quenching of a complex geometry large steel forging in a static polymer solution. Experimental temperature-time data extracted from the piece were used as input data to calculate the HTC. The geometry was divided into seven different zones (surfaces). An individual HTC, with a high level of experimental confidence, was calculated for each zone by using the inverse heat transfer module of the commercial software DEFORM 2D/3D. These HTCs were then used to predict the through thickness cooling behaviour of the component with a high degree of replication. This method thus appears be useful for further understanding the quenching process on large steel forgings, in general, but could be critical for obtaining accurate cooling behaviour in forgings with non-simple shapes, where one HTC may not be sufficient to describe local cooling behaviour.

Analysis on the design of steel wire wound extrusion containers for hot extrusion process

Extrusion container is the most important tooling for steel hot extrusion process. Conventional design using large castings and forgings is very difficult to execute due to high cost and risk. Steel wire wound containers have many advantages over conventional designs. However, conventional wire wound containers are developed for use at room temperature which are not applicable to steel hot extrusion process. In this article, the impacts of preheating on the design of steel wire wound containers are discussed in detail. A finite element model was established to examine the preheating temperature distribution, and a 1:10 scaled extrusion container was manufactured to verify the effectiveness of the finite element model. Based on the finite element model–computed temperature field, thermal stress analysis was performed. The thermal impacts on the stress of extrusion container and steel wire were obtained. Results showed that insufficient stability of internal cylinder and greatly enhanced steel wire stress may lead to the failure of extrusion container. To solve the problems, an improved design was put forward by increasing the stability factor of internal cylinder, reducing the prestress factor and lowering the allowable stress of steel wire. Results showed that the improved design can meet the requirements and counteract the thermal impacts.

Common Defect Analysis for Large Section Special Steel Forging

In order to Figure the common defect in large section special steel forging and find the solution, systematic study was carried out on hundreds of large section special steel forgings in a domestic famous steel mill. The steels included: low-carbon steel Q345D/E, medium-carbon steel 27SiMn, high-carbon steel GCr15SiMn, stainless steel 20Cr13. Both the amount and type of all the defect in the above steel were calculated and analyzed. The results showed that the common defects of the steel were slags, inclusions, loose (cavity) and inner cracks. The evolution of the cavity in the ingot during forging process was simulated by a numerical simulation software Deform-3D. The inner cracks in Q345D/E and 27SiMn initiated after A→F+P transformation The cracks in GCr15SiMn formed after the precipitation of net-like proeutectoid carbides. The cracks in 20Cr13 formed after the precipitation of net-like carbides. The internal cause of the cracks was relevant to composition segregation and internal stress in the forging. The external cause was connected with effect of slow cooling. Based on the above study, a set of new process was proposed and put into industrial application, with the result that the qualified ratio of flaw inspection in the above steel mill was improved from 20% to above 87%.

Analysis on the Free Forging Process of the Heavy Rack for Ship Lift

Heavy racks are the main load-bearing components and play more important position in great engineering equipments. Along with the requirement of the equipment efficiency promotion, the large-scale trend of rack is obvious. Among them, the rack for the Chinese Three Gorges ship lift total up to 5000mm. At present, the large rack mainly made by casting. Compared with the casting, the forging rack can greatly improve the mechanical properties and the service life. The aim of this work is feasibility analysis of free forging for large rack, as large T-shape section forging. Four possible forging schemes were simulated and compared by the finite element method, and the possible defects of all the schemes and their application scope were explored. Finally, it was proved that the free forging of heavy rack was realized by simple dies.

Effect of Macrosegregation on the Microstructure and Mechanical Properties of a Pressure-Vessel Steel

Macrosegregation refers to the chemical segregation, which occurs quite commonly in the large forgings such as nuclear reactor pressure vessel. This work assesses the effect of macrosegregation and homogenization treatment on the mechanical properties of a pressure-vessel steel (SA508 Gr.3). It was found that the primary reason for the inhomogeneity of the microstructure was the segregation of Mn, Mo, and Ni. Martensite, and coarse upper bainite with M-A (martensite-austenite) islands have been obtained, respectively, in the positive and negative segregation zone during a simulated quenching process. During tempering, the carbon-rich M-A islands decomposed into a mixture of ferrite and numerous carbides which deteriorated the toughness of the material. The segregation has been substantially minimized by a homogenizing treatment. The results indicate that the material homogenized has a higher impact toughness than the material with segregation, due to the reduction in M-A island in the negative segregation zone. It can be concluded that the microstructure and mechanical properties have been improved remarkably by means of homogenization treatment.

Microstructural characterization of an α+β type Ti-5.5Mo-7.2Al-4.5Zr-2.6Sn-2.1Cr alloy during recrystallization annealing

Abstract An α + β type titanium alloy was subjected to various periods of recrystallization annealing and the resulting microstructural evolution was characterized. The results revealed that the recrystallization rate of the β phase was higher than that of the α phase, owing to the high strain energy storage of this phase during large deformation forging. Moreover, the recrystallized fraction of both the α and β phases increased with increasing holding time at 740 °C. The recrystallization fraction of the β phase accounted for 70.13% after annealing for 5 h. However, the average grain size remained constant when the recrystallization fraction reached approximately 54%, indicating that further grain refinement was prevented, owing to the high degree of recrystallization. The spatial microstructure which consisted of a globular, homogeneous, and equiaxed α phase dispersed in sub-structured β matrix grains, was characterized via a novel three-dimensional electron backscatter diffraction technique. The grain orientation and morphological parameters, including the equivalent-sphere diameter and number of neighboring grains, were calculated and discussed.

Automatic measurement method for the size of large forgings based on scattering on rough surface

The size measurement of large hot forgings plays an important role during the process of production. In order to realise the size measurement of large forgings in thermal state, an automatic size measurement method for large forgings based on the scattering of rough surface is proposed in this study. In the method, the theory of random facets is applied to research the characteristic of laser scattering on the surface of hot forgings. The model of laser scattering on the random rough surface of cylinder is built. According to the model, the edges of forgings can be detected and the size of forgings can be got. First, the charge coupled device (CCD) camera and the green laser are driven by the motor to move on the guide rail. The real-time images of forgings are captured by CCD camera and the positional information of laser is acquired by pulse coder. Then according to the change of the intensity of the scattered light, whether the laser line is on the edge of forgings is decided. Second, the positional information of two edges is got by pulse coder and the size of forgings is figured out by the positional information. Finally, the feasibility of the method is verified by the experiment of size measurement.

A Comparison of Input Data Used to Represent Phase Transformations during the Quenching of a Large Nuclear Forging

The present work explores the importance of model parameters and input variables when simulating the quenching of thick sectioned nuclear forgings. The modelling approach adopted uses values of specific heat capacity, containing latent heat release, to simulate cooling curves; rather than calculating transformation kinetics based upon a mathematical model. Termed the effective specific heat (Cpeff), two different methods were used to establish values: differential scanning calorimetry (DSC) and thermos dynamic predictive software. Values were then included in finite element (FE) models to simulate the characteristic cooling at the mid-wall position in a thick section forging and were validated against production thermocouple data. The investigation found that the formation of ferrite, bainite and martensite or lower bainite were all represented by the data established using DSC and critical formation temperatures were comparable with others in the literature. Conversely, values calculated using the thermodynamic software failed to represent ferrite formation and predicted different critical transformation temperatures for bainite. The simulated cooling curve that used the software predicted Cpeff data was comparable to the thermocouple data either side of the bainite transformation, however during the transformation the effects of latent heat on cooling rate were over predicting leading to disparities. The equivalent DSC cooling curves produced a near exact match.

Wire + Arc Additive Manufacturing

Depositing large components (>10 kg) in titanium, aluminium, steel and other metals is possible using Wire + Arc Additive Manufacturing. This technology adopts arc welding tools and wire as feedstock for additive manufacturing purposes. High deposition rates, low material and equipment costs, and good structural integrity make Wire+Arc Additive Manufacturing a suitable candidate for replacing the current method of manufacturing from solid billets or large forgings, especially with regards to low and medium complexity parts. A variety of components have been successfully manufactured with this process, including Ti–6Al–4V spars and landing gear assemblies, aluminium wing ribs, steel wind tunnel models and cones. Strategies on how to manage residual stress, improve mechanical properties and eliminate defects such as porosity are suggested. Finally, the benefits of non-destructive testing, online monitoring and in situ machining are discussed.

Ultrasonic Defect Characterization in Heavy Rotor Forgings by Means of the Synthetic Aperture Focusing Technique and Optimization Methods.

Ultrasonic nondestructive testing of steel forgings aims at the detection and classification of material inhomogeneities to ensure the components fitness for use. Due to the high price and safety critical nature of large forgings for turbomachinery, there is great interest in the application of imaging algorithms to inspection data. However, small flaw indications that cannot be sufficiently resolved have to be characterized using amplitude-based quantification. One such method is the distance gain size method, which converts the maximum echo amplitudes into the diameters of penny-shaped equivalent size reflectors. The approach presented in this contribution combines the synthetic aperture focusing technique (SAFT) with an iterative inversion scheme to locate and quantify small flaws in a more reliable way. Ultrasonic inspection data obtained in a pulse-echo configuration are reconstructed by means of an Synthetic Focusing Technique (SAFT). From the reconstructed data, the amount and approximate location of small flaws are extracted. These predetermined positions, along with the constrained defect model of a penny-shaped crack, provide the initial parametrization for an elastodynamic simulation based on the Kirchhoff approximation. The identification of the optimal parameter set is achieved through an iteratively regularized Gauss-Newton method. By testing the characterization method on a series of flat-bottom holes under laboratory conditions, we demonstrate that the procedure is applicable over a wide range of defect sizes. To show suitability for large forging inspection, we additionally evaluate the inspection data of a large generator shaft forging of 0.6-m diameter.

Overview of 50 Years of Development in Electroslag Remelting in Austria

The paper deals with the development work in the field of Electroslag Remelting (ESR), which has been executed in Austria since 1964 by Gebr. Bohler & Co. AG and since 1973 by INTECO Internationale Technische Beratung GmbH. Electroslag Remelting has initially been executed in open air independent from plant concept i.e. static crucible–single electrode or collar mould–electrode change. Difficulties in controlling oxygen and hydrogen in larger size ingots finally led to protective gas remelting from 1996 on. Specific plant concepts were developed for the production of high nitrogen steels—Pressure-ESR (PESR) and for the continuous production of small size billets at economic meltrates—Electroslag Rapid Remelting (ESRR®). Due to the need for larger and larger forgings, ESR ingot sizes and weights have been increased up to 2600 mm ingot diameter and 260 t in weight.

Use of controlled heat treatment to predict mechanical properties in steel components

This paper investigates the non-destructive, experimental determination of mechanical properties, at any location within large steel forgings, through the use of controlled heat treatment. Industrial time–temperature data from heat treatment furnaces have been used as the surface temperature boundary condition in finite element models. The models have been used to determine the thermal history at specific locations of interest within a 95 tonne forging. Application of the predicted thermal history to a laboratory sized sample of material, using controlled heat treatment, results in macroscale material that has an identical thermal history to the points of interest within the forging. This sample material has been mechanically tested and compared to mechanical test results from the forging itself.

Elimination of Coarse-Grain Property in Steel 25KhN3MFA After Overheating to Different Temperatures

The effect of the temperature of overheating in the range of 950 – 1250°C on the texture and grain size of steel 25KhN3MFA subjected to subsequent heating at 700 – 1100°C at a rate of 100 K/h is studied. The effect of the initial texture and grain size on the temperature of disappearance of large grains is determined. Recommendations are given on application of the results obtained for choosing heat treatment regimes for large forgings.

Residual Stresses Control for Large Scale Open Die Forged Parts

This paper gives an overview of residual stresses problem after heat treatment of large scale forged parts as well as numerical study and simulations of different methods to reduce stresses. DEFORM software was used for Finite Element Method (FEM) simulations as one of the most capable simulation tools combining both forging and heat treatment processes analysis. The study is performed on the sample of large scale airspace part of aluminum AK6 (AlMgSiCu) alloy. The comparison of different quenching media is presented. Practical conclusions and recommendations are made to optimize the process and reduce residual stresses after the heat treatment. This paper provides numerical simulation and theoretical conclusions and needs to be amended by experimental measurement of residual stresses by one of available methods as part of the future work.