EBSD and TEM investigation of the hot deformation substructure characteristics of a type 316L austenitic stainless steel

The evolution of crystallographic texture and deformation substructure was studied in a type 316L austenitic stainless steel, deformed in rolling at 900 °C to true strain levels of about 0.3 and 0.7. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used in the investigation and a comparison of the substructural characteristics obtained by these techniques was made. At the lower strain level, the deformation substructure observed by EBSD appeared to be rather poorly developed. There was considerable evidence of a rotation of the pre-existing twin boundaries from their original orientation relationship, as well as the formation of highly distorted grain boundary regions. In TEM, at this strain level, the substructure was more clearly revealed, although it appeared rather inhomogeneously developed from grain to grain. The subgrains were frequently elongated and their boundaries often approximated to traces of {111} slip planes. The corresponding misorientations were small and largely displayed a non-cumulative character. At the larger strain, the substructure within most grains became well developed and the corresponding misorientations increased. This resulted in better detection of sub-boundaries by EBSD, although the percentage of indexing slightly decreased. TEM revealed splitting of some sub-boundaries to form fine microbands, as well as the localized formation of microshear bands. The substructural characteristics observed by EBSD, in particular at the larger strain, generally appeared to compare well with those obtained using TEM. With increased strain level, the mean subgrain size became finer, the corresponding mean misorientation angle increased and both these characteristics became less dependent on a particular grain orientation. The statistically representative data obtained will assist in the development of physically based models of microstructural evolution during thermomechanical processing of austenitic stainless steels.

FEA comparison of high and low pressure tube hydroforming of TRIP steel

The increasing application of hydroforming for the production of automotive lightweight components is mainly due to the attainable advantages regarding part properties and improving technology of the forming equipment. However, the high pressure requirements during hydroforming decreases the costs benefit and make the part expensive. Another requirement of automotive industries is weight reduction and better crash performance. Thereby steel industries developed advanced high strength steels which have high strength, good formability and better crash performance. Even though the thickness of the sheet to form the component is reduced, the pressure requirement to form the part during expansion is still high during high pressure hydroforming. This paper details the comparison between high and low pressure tube hydroforming for the square cross-section geometry. It is determined that the internal pressure and die closing force required for low pressure tube hydroforming process is much less than that of high pressure tube hydroforming process. The stress and thickness distribution of the part during tube crushing were critically analysed. Further, the stress distribution and forming mode were studied in this paper. Also friction effect on both processes was discussed.

Influence of processing on the roll forming characteristics of recovery annealed steel

Roll forming (including corrugating) of high strength steel has for many years been treated as an art rather than a science. This work, by using analysis at both the microscopic and standard mechanical level, has demystified the production of these high strength steels, and has helped point the direction for further development of these efficient construction products.

Effects of aging bake hardenable steel in sheet metal forming

Bake hardening steels are being used as body panels in modern cars. These steels are stronger through an ageing process after baking in the paint ovens, which allows the thickness of the steel to be reduced without reducing the denting performance. The current study examined whether the steel ages at room temperature prior to forming which would deteriorate the formability. The work developed a new test and provided insight into acceptable levels of natural ageing which could be tolerated in the manufacturing process.

Electron backscattering diffraction analysis of a reconstructed wootz damascus steel blade

The historical impact and subsequent fame of wootz weaponry in the ancient world has created interest in what has come to be seen as an advanced material even by modern standards. Ancient wootz artifacts are classed as high carbon (hypereutectoid) crucible steels and are characterised by high strength, hardness and wear resistance, but especially by their attractive surface pattern.

An investigation of complex shapes during low pressure tube hydroforming

Hollow structures made of Advanced High Strength Steel (AHSS) are increasingly used in the automobile industry for crash and structural components. Generally high pressure hydroforming is used to form these tabular parts, which is a costly manufacturing process due to the high pressure equipment and large tonnage presses required. A new process termed low pressure hydroforming, where a pressurized tube is crushed between two dies, represents a more cost effective alternative due to the lower pressures and die closing forces required.

In this study the low pressure tube hydroforming of one simple and two different complex hollow shapes is investigated. The complexities of the pat1S compared to simple shapes are critically studied and the die filling conditions are investigated and discussed. FUl1hennore the thickness distributions over the circumference of the part during forming are analyzed.

Crash investigation of side intrusion beam during high and low pressure tube hydroforming

Advanced high strength steels (AHSS), in particular, are an attractive group materials, offering higher strength for improved energy absorption and the opportunity to reduce weight through the use of thinner gauges. High pressure tube hydroforming (HPTH) has been used to produce safety components for these steels, but it is expensive. Low pressure tube hydroforming (LPTH) is a lower cost alternative to form the safety components in the car. The side intrusion beam is the second most critical part after front rail in the car structure for passenger safety during crash. The forming as well as crash behaviour of a square side intrusion beam from both processes was investigated using numerical simulation. This paper investigated the interaction between the forming and crash response of these materials in order to evaluate their potential for use in vehicle design for crashworthiness. The energy absorption characteristics of the different tubes were calculated and the results from the numerical analyses compared for both hydroforming process.

Multi-scale modelling of DP590 steel

Dual Phase (DP) steel one of the Advanced High Strength Steels (AHSS) has a two phase microstructure where soft and hard phase acts together to offer a high strength composite effect. The high strength, however, must be balanced with ductility so that complex parts and designs can be manufactured from AHSS sheets. However, during forming certain grades of DP steel a sudden crack can occur without any intimation of necking. Thus, due to this abnormal forming behaviour, is difficult to accurately predict because most classical modelling approaches are not designed for such micro-structurally heterogeneous materials. These modelling approaches are generally based on an average representation of the material behaviour in a continuum mechanics formulation. This works for materials that are homogenous, or at least could be assumed to be homogenous at scales lower than the naked eye can see. However, for a material like AHSS, the microstructure plays a significant role in dictating the mechanical behaviour at the macro-scale. This paper studies the multi-scale modelling ofDP590 steel. It is found that the sufficient accuracy can be achieved from multi-scale modelling while comparing with the experiments.

Modelling of tooling wear in trim dies for AHSS

Using Advanced High Strength Steels (AHSS) in forming and cutting dies generates higher loads in tools. Tool wear is an issue when applying AHSS. This study investigates the effect of process parameters such as clearance, material properties and punch/die bluntness on contact pressure values and tool wear. Some desirable process parameters that minimise wear have been found.

Numerical simulations on warm forming of stainless steel with trip-effect

In steels with TRIP-effect, a phase transformation from the retained-austenite to martensite occurs during forming, and it significantly affects hardening behaviours. Such an effect is sensitive to the amount of strain as well as the temperature variation. For materials with a strong TRIP-effect, new forming techniques are needed to develop that can lead to lighter and stronger components in automotive industry. This paper presents a coupled thermo-mechanical finite element modelling and simulation of a warm deep drawing of austenitic stainless steel (including a TRIP-effect) using LS-DYNA and temperature effect on forming process of such materials is investigated.

Deformation and frictional heating in relation to wear in sheet metal stamping

In the automotive industry, press production rates often need to be reduced in order to minimize tool wear issues and successfully stamp advanced high strength steels. This indicates that heating affects may be important. This paper examines friction and deformational heating at the die radius during sheet metal stamping, using finite element analysis. The results show that high temperatures, of up to 130°C, can occur at the die radius surface. Such behavior has not been previously reported in the literature, for what is expected to be ‘cold’ sheet metal stamping conditions. It will be shown that the temperature rise is due to the increased contact stresses and increased plastic work, associated with stamping AHSS. Consequently, new insights into the local contact conditions in sheet metal stamping were obtained. The outcomes of this work may impact the wear models and tests employed for future tool wear analyses in sheet metal stamping.

Constitutive modeling of TWIP steel in uni-axial tension

High Mn steels demonstrate an exceptional combination of high strength and ductility due to their high work hardening rate during deformation. The microstructure evolution and work hardening behavior of Fe18Mn0.6C1.5Al TWIP steel in uni-axial tension were examined. The purpose of this study was to determine the contribution of all the relevant deformation mechanism : slip, twinning and dynamic strain aging. Constitutive modeling was carried out based on the Kubin-Estrin model, in which the densities of mobile and forest dislocations are coupled in order to account for the continuous immobilization of mobile dislocations during straining. These coupled dislocation densities were also used for simulating the contribution of dynamic strain aging on the flow stress. The model was modified to include the effect of twinning.

Surface alloying of metals using fluid bed reactor

The use of fluidised bed reactors for surface alloying is reviewed. Research at Deakin has includes the use of chemical vapour deposition to form chromium rich layers on ferrous substrates, including stainless and tool steel grades. These layers can be modified to carbide or nitride if required by the end application. The deposition of aluminium and silicon has also been successfully achieved.

Fatigue characterisation of HSS and AHSS

The thesis investigated the fatigue behaviour of modern automotive steels, which are stronger and more ductile than traditional steels. The failure mechanisms of these steels under fatigue were identified, enabling researchers to further improve the material properties, which will ultimately lead to a reduction in overall vehicle mass.

The use of Fe-30% Ni and Fe-30% Ni-Nb alloys as model systems for studying the microstructural evolution during the hot deformation of austenite

The development of physically-based models of microstructural evolution during thermomechanical processing of metallic materials requires knowledge of the internal state variable data, such as microstructure, texture, and dislocation substructure characteristics, over a range of processing conditions. This is a particular problem for steels, where transformation of the austenite to a variety of transformation products eradicates the hot deformed microstructure. This article reports on a model Fe-30wt% Ni-based alloy, which retains a stable austenitic structure at room temperature, and has, therefore, been used to model the development of austenite microstructure during hot deformation of conventional low carbon-manganese steels. It also provides an excellent model alloy system for microalloy additions. Evolution of the microstructure and crystallographic texture was characterized in detail using optical microscopy, X-ray diffraction (XRD), SEM, EBSD, and TEM. The dislocation substructure has been quantified as a function of crystallographic texture component for a variety of deformation conditions for the Fe-30% Ni-based alloy. An extension to this study, as the use of a microalloyed Fe-30% Ni-Nb alloy in which the strain induced precipitation mechanism was studied directly. The work has shown that precipitation can occur at a much finer scale and higher number density than hitherto considered, but that pipe diffusion leads to rapid coarsening. The implications of this for model development are discussed.