K. Narasimhan

Effect of heat treatment of preform on the mechanical properties of flow formed AISI 4130 Steel Tubes—a theoretical and experimental assessment

Abstract Flow forming is an advanced eco-friendly chipless metal forming process, which employs an incremental rotary point deformation technique. Flow forming offers a remarkable increase in tensile properties due to strain hardening and provides excellent dimensional accuracy and surface finish for the formed part. This paper presents the results of a study on the effect of heat treatment of preform material on the mechanical properties of the flow formed part and the validity of using empirical relations in predicting the properties of the flow formed components. The strength coefficient K and strain hardening exponent n of equation σ=Ken are determined from stress–strain curves under different heat treatment conditions for AISI 4130 steel. The effect of cold work on mechanical properties of flow formed part for a given reduction in area (RA) is predicted using empirical relations published in literature. In order to validate the hypothesis of prediction of cold worked material properties after flow forming, a few flow forming experiments are carried out. The preform dimensions are worked out based on constant volume principle. A three pass flow forming sequence was followed and the properties are measured for about 88–90% thickness reduction. Comparison of the experimental results and the properties of the flow formed tubes predicted using empirical relations shows that empirical relations can be used for predicting the properties of flow formed tubes with reasonable accuracy. Preforms processed through hardening and tempering route give better performance. These results are used for design of performs for manufacturing of large number of high strength pressure vessels successfully.

A new criterion to predict necking failure under biaxial stretching

Abstract Localized necking limits useful forming of sheet materials. Several theoretical models predict limit strains under biaxial stretching conditions. Models that assume infinite-length defects (M-K approach) or finite compact defects (FEM approach) have been successful in predicting limit strains. In both these approaches, a failure criteria is required to predict the limit strain during simulation. Failure criteria based on comparing the principal strains, strain rates in the defect and the bulk region of the deforming sheet are reviewed. A new failure criterion is developed, which is defined in terms of thickness gradients that develop during biaxial stretching. This new criterion can be used under a wide range of forming conditions to predict limit strains. FLDs predicted using the new criterion are compared to: (a) FLDs predicted by other existing criteria and (b) Experimental FLDs. The effect of material parameters on the shape of the FLDs is predicted using the new criterion.

Experimental studies on bursting pressure of thin-walled flow formed pressure vessels

Abstract The design of pressure vessels for critical high-pressure applications has to consider two failure modes. The first mode of failure can occur when deformation becomes excessive and permanent deformation might occur. The second type of failure occurs at higher magnitudes of pressure resulting in bursting of tubes and catastrophic failure. Hence predicting the bursting pressure of thrust chambers is an important consideration in their design and gives a more realistic indication of the factor of safety than the value that can be provided based on the yield pressure value. The burst pressure gives an indication of the margin of safety available over the maximum expected operating pressure (MEOP). The assessment of burst pressure is very important for pressure vessels which are used for critical applications. In this paper an attempt has been made to study the suitability of the burst pressure prediction formula available in literature and its modification for thin-walled high strength pressure vessels manufactured by a flow forming technique.

Numerical simulation of the influence of air bending tool geometry on product quality

Abstract The quality of a bend is determined by the accuracy and consistency of the angle along the length of the part. This accuracy is directly related to the penetration of the tool in the die. Small variations in tool penetration can cause relatively large changes in bend angle. Such variations are caused by changes in the repeatability of the tool stopping point, by deflections of the press brake beams and by distortions of the tooling. Earlier studies [Int. J. Mech. Sci. 22 (1980) 583; J. Mater. Process. Technol. 35 (1992) 129; Precision bending of sheet metal, Master’s Thesis, University of Twente, 1992; A finite element simulation of free bending, in: Proceedings of the Second International Sheet Metal Conference, SheMet’94, University of Ulster, 1994, pp. 201–211] on finite element modelling of the air bending process assume that the tooling is perfectly rigid. The present 3D finite element model of press brake air bending tools allows calculation of elastic distortions in typical tools and relates the distortion to the accuracy of the formed part. The finite element model has been validated by experiments conducted on a commercial press brake. Some suggestions are made for improvements to the direct and cross stiffness of the tools which should lead to improved alignment of the tool and die and, in turn, to improved product accuracy.

Measurement of strain history during the stretching of forming-grade steel sheets

Abstract It is well known that the strain history (the strain paths) greatly affects the forming limit strains of sheet metals. It is usually assumed that the strain paths remain linear during the in-plane stretching of sheets using a flat cylindrical punch. The present results show that the nature of the strain path even in such simple benchmark tooling actually varies depending upon the strain ratio. The strain path in the negative minor strain domain shows a curvilinear behaviour, but in the biaxial stretching region the paths are nearly linear.

Finite element based optimization study on hydroformed stepped tube

Tube hydroforming process is an advanced manufacturing process in which tube is placed in between the dies and deformed with the help of hydraulic pressure. A sound tube hydroformed part depends upon die conditions, material properties and process conditions. In this work, a finite element study, along with response surface methodology (RSM) for designing the simulation, has been used to construct models with loading path, friction, anisotropic index, strain hardening exponent and tube thickness. The responses studied are the die corner radius filling and strain non-uniformity index (SNI) chosen in each step of the tube with maximum 30% thinning as stopping criteria. The factors effect and their interactions on each response were determined and analysed.

Effect of Geometric Parameters on Formability and Strain Path During Tube Hydrforming Process

Forming limit diagram (FLD) is an important tool to measure the materials formability for metal forming processes. In order to successfully manufacture a component through tube hydroforming process it is very important to know the effect of material properties, process and geometrical parameters on the outcome of finished product. This can be obtained by running a finite element code which not only saves time and money but also gives a result with considerable accuracy. Therefore, in this paper the mutual effect of diameter as well as thickness has been studied. Firstly the finite element based prediction is carried out to assess the formability of seamless and welded tubes with varying thickness. Later on, effect of varying diameter and thickness on strain path is predicted using statistical based regression analysis. Finally, the mutual effect of varying material property alongwith varying thickness and diameter on constraint factor is studied.

Study of Fracture Behaviour and Strain Path During Tube Hydrforming Process

Forming limit diagram (FLD) is a tool which is widely used to measure the material formability of hydroforming process. It is well known that strain based FLD is dependent on strain path which the material undergoes during the course of deformation. This work is based on understanding the deformation and fracture behaviour of as-received (AR) tube and annealed tube during hydroforming process. The strain path is being measured at different locations of the AR tube as well as annealed tube and is co-related with the microhardness value to understand the localization and fracture behaviour. The annealed tube tends to fracture suddenly from the weld line and fracture surface study reveals sudden dominant brittle mode of fracture indicating the impact of weld notch on the fracture process and thus giving lower limiting strains and hardness value.

Microstructure Evolution in Three FCC Materials During Limited Dome Height Test

AA 1050 Aluminium, AISI 304L and AISI 316L austenitic stainless steel were deformed at different strain and strain path. Deformed microstructure in AISI 304L and AISI 316L austenitic stainless steel shows significant amount of deformation twin and Strain Induced Martensite (SIM). AA 1050 Aluminium shows grain interaction between neighbouring grains. In this study effort has been made to understand these microstructural developments. It has been found that biaxial strain path and high strain shows higher amount of deformation twins in AISI 316L stain less steel, strain induced martensite in AISI 304L stainless steel and grain interaction in AA 1050 Aluminium.