Effect of strain rate on spatio-temporal behavior of Portevin–Le Châtelier bands in a twinning induced plasticity steel

Uniaxial tensile tests at three strain rates are performed with the aid of the digital image correlation (DIC) technique to experimentally investigate the spatio-temporal behavior of PLC bands in a twinning induced plasticity (TWIP) steel. The whole strain fields of tensile specimens are acquired throughout the tests. Significant serration crests corresponding to band nucleation are observed on the true stress vs. true strain curves derived from DIC results beyond a critical true strain. The work hardening exponent (n-value) increases from ∼0.08 to ∼0.5 when true strain increases to the critical true strain, and beyond that, the n-value exhibits serrations with increasing true strain. Two typical nucleation modes of Type-A Portevin–Le Châtelier (PLC) bands are observed in all tests. Nucleation and propagation of PLC bands are described in details based on these two nucleation modes of Type-A PLC bands. The PLC band orientation, which indicates the angle between the normal direction of a PLC band and tensile direction, fluctuates during propagation, and the fluctuation amplitude increases during the development of a localized necking band from a PLC band before fracture. In particular, the effect of strain rate on the kinematics of Type-A PLC bands (band strain, band width and band propagating speed etc.) in the TWIP steel is quantitatively analyzed, and a new algorithm based on the DIC results is presented which includes the elongating effect of tensile specimens during deformation to show the actual kinematics of Type-A PLC bands.     

CONTINUOUS SEM OBSERVATIONS OF CREEP-FATIGUE DAMAGE PROCESSES

In order to examine the relation between damage evolution and changes in microstructure, e.g. from creep cavities, surface micro-cracks and dislocation structures at high temperature, strain controlled creep-fatigue tests were performed and interrupted at several damage levels on Types 304 and 316 stainless steels. The creep-fatigue tests on Type 304 stainless steel at a low strain level were conducted in a high-temperature fatigue testing machine combined with a scanning electron microscope, and the micro-crack initiation and growth behaviour were continuously observed to clarify the damage extension mechanism. It was found that even though many cavities were initiated and grew on the internal grain boundaries of the specimens during the strain-controlled tests, the failure life was governed by the propagation of surface cracks. On the other hand, micro-cracks of about the order of one grain size were initiated mainly along grain boundaries normal to the loading axis under low stress creep-fatigue, and the crack propagation rate of the micro-cracks was slow and random due to the nature of the microstructures. The micro-cracks gradually opened in the loading direction with increasing number of cycles and coalescence contributed to growth.     

High temperature cross-weld characterisation of steel weldments by microtensile testing

Abstract

Study of local material properties and damage mechanisms are undertaken in order to characterise weldments that show significant variation of properties across weldments. One of the methods to characterise the local variation of properties is microtensile (MT) testing of specimens machined out of specific narrow zones of weldments. The literature data, though limited, on microtensile specimen testing are reported at their low temperature behaviour. On the other hand, systematic study of crossweld local material properties at high service temperatures have not yet been reported. In the present study, MT tests are conducted across similar welds of P22 and P91 steels at 550 and 600°C, respectively. In order to study deformation mechanisms and the role of surface condition on properties, specimens with different surface conditions (i.e. machined, polished and electropolished surfaces) are tested. Two different loading rates of 0.2mm/min and 0.5mm/min are used to study the effect of loading rate on deformation and mechanical properties. Variations of material properties yield strength (R_p0.2) and ultimate tensile strength (R_m), for the weldments are presented as a function of surface conditions of specimens and loading speeds. Higher loading rates yield higher values of R_p0.2 and R_m, and specimens with machined and polished surfaces show consistent and higher values of R_p0.2 and R_m compared to specimens with an electropolished surface finish. Deformation behaviour is studied on the side surfaces of tested microtensile specimens using an SEM. Deformation is correlated to microstructural constituent that is observed on specimen side surfaces. The metallographic information is used to interpret the variation of mechanical properties determined in tension at high temperatures. The MT data are compared with standard tensile data obtained on specimens with simulated microstructures. The prospects of using MT tests for characterising the material at high temperatures and feasibility of use of data for assessment of components under service loading conditions are reported.     

CRACK INITIATION MECHANISMS IN TORSIONAL FATIGUE

Abstract— The development of fatigue damage in Co45Ni specimens during push—pull and reversed torsion tests, performed inside a scanning electron microscope, was observed and the different stress states compared. It appeared that transgranular crack initiation and development is delayed and intergranular crack initiation promoted under torsional loading. This was explained in terms of reduced surface distortion at the emergence of persistent slip bands (PSBs) and smaller compatibility stresses at the PSB-matrix interfaces. The influence of the mechanical strength of grain boundaries on the difference between tensile and torsional fatigue lives is discussed.     

STATISTICAL INVESTIGATION OF THE BEHAVIOUR OF SMALL CRACKS AND FATIGUE LIFE IN CARBON STEELS WITH DIFFERENT FERRITE GRAIN SIZES

Abstract— In order to study the relation between the scatter characteristics of small crack growth behaviour and fatigue life, rotatory bending fatigue tests of smooth specimens were carried out using 0.21% carbon steels of different ferrite grain sizes. Fifteen to eighteen specimens were fatigued at each stress amplitude, and the initiation and propagation behaviour of the cracks which led to the final fractures were examined for all the specimens. The physical basis of scatter in fatigue life was investigated, based on the successive observation of fatigue damage on the surface using the plastic replica technique, followed by an analysis of the data assuming a Weibull distribution. A statistical investigation of the physical basis of scatter in relation to the ferrite grain size was performed, i.e. the distributions for crack initiation life, crack propagation life, fatigue life and growth rate of small cracks. Finally, the fluctuation of crack growth rate was studied in relation to the application of a crack growth law for microstructurally small cracks.     

A study of microcrack formation in multiphase steel using representative volume element and damage mechanics

Multiphase steels have become a favoured material for car bodies due to their high strength and good formability. Concerning the modelling of mechanical properties and failure behaviour of multiphase steels, representative volume elements (RVE) have been proved to be an applicable approach for describing heterogeneous microstructures. However, many multiphase steels exhibit inhomogeneous microstructures which result from segregation processes during continuous casting. These segregations lead to a formation of martensite bands in the microstructure causing undesirable inhomogeneities of material properties. The aim of this work is to develop an FE evaluation procedure for predicting a microcrack formation provoked by banded martensitic structures. A micromechanism based damage curve was applied as a failure criterion for the softer ferritic matrix in the microstructure in order to simulate the propagation of cracks resulting from the failure of martensitic bands. The parameters of the damage curve were determined by in situ miniature bending tests and tensile tests with notched samples. The presented approach provides the basis for an assessment criterion of the component safety risk of multiphase steels with inhomogeneous microstructures.     

The effect of dynamic strain aging on fatigue properties of dual phase steels with different martensite morphology

Dual phase (DP) steels with network and fibrous martensite were produced by intercritical annealing heat treatment cycles. Some of these steels were deformed at dynamic strain aging temperatures. Room temperature tensile tests of specimens deformed at 300 °C showed that both yield and ultimate tensile strengths for both morphologies increased, while total elongation decreased. Fatigue test results before and after high temperature deformation showed that dynamic strain aging has a stronger effect on fatigue properties of dual phase steels with fibrous martensite. Cracks in DP steels with fibrous martensite propagate in a tortuous path in soft ferrite phase, while they pass of both hard and soft phases in DP steels with network martensite.     

Effect of processing parameters on shear bands in non-isothermal hot forging of Ti -6Al -4V

Abstract

The alloy system Ti- 6Al- 4V is the prominent Ti alloy system for aerospace and biomedical applications, as a result of its mechanical property balance and biocompatibility. Since the mechanical characterisation of Ti- 6Al- 4V is strongly sensitive to processing parameters there is relationship between processing variables, i.e. strain rate and temperature, microstructure, and properties under different loading conditions. Two phase (α + β) titanium alloys undergo flow instabilities and are susceptible to shear bands or regions of localised deformation crossing many grains during hot forging under non-isothermal conditions (dies and workpiece at different temperatures). Under such conditions shear bands can be generated even in materials without flow softening attributes. This occurs if the forging parameters lead to large amounts of heat transfer between the dies and the workpiece. This study investigates the occurrence of shear bands during non-isothermal, hot forging of Ti -6Al- 4V in order to evaluate the process parameters that generally lead to shear bands in conventional hot forging of metals. Upset compression tests on cylindrical specimens were conducted in a mechanical press and lateral side pressing tests on long, round bars were performed in either a mechanical press or a hydraulic press. The tests ranged from axisymmetric to plane strain compression. In upset specimens shear bands occurred at an angle of 45° to the compression axis and bands of intense deformation separated chill zones from the deforming bulk. Observation also demonstrated that the fracture might be owing to microvoids nucleated at weak points in sections of the shear surfaces. For plane strain deformation, shear bands were found to initiate along zero extension directions in a manner analogous to the formation and propagation of shear bands in isothermal hot forging. Although the shear band features at hot forging temperatures were similar to each other, there was a difference in the hardness and thickness of the shear bands depending on deformation mode, amount, and temperature.     

Strength of Alloys Based on Iron,Nickel, and Titanium in High-Pressure Hydrogen

The influence of high-pressure hydrogen at room temperature on the static and fatigue properties of corrosion-resistant materials based on iron and nickel is compared for different types of loads. The sequence of characteristics can be arranged in the order of increase in the influence of hydrogen as follows: fatigue limit, ultimate strength of specimens with concentrators, relative elongation of smooth specimens, fracture pressure for a membrane under biaxial tension, percentage reduction of the area of smooth specimens, percentage reduction of the area of specimens with concentrators, and low-cycle durability. The dependences of the intensities of true stresses on the intensities of true strains (in uniaxial and biaxial tension) reveal the difference between the curves plotted in air and in hydrogen. The diagrams of hydrogen resistance taking into account the strain rates are presented. The sequence of investigated types of steel and alloys can be arranged in order of decrease in the low-cycle hydrogen resistance as follows: annealed stable austenitic and iron-nickel alloys, nitrided Cr-Ni-Mn steels, titanium alloys of the Ti-Al-Sn and Ti - Al -V systems, steels with nonstabilized austenite, high nickel alloys, maraging steels, and ferritic steels. The micromechanism of hydrogen fracture is characterized by the presence of a great number of microstructural fracture sites (bulk damage) and the completeness of local plastic relaxation. New procedures aimed at increasing the hydrogen resistance of steels and alloys are proposed, namely, the thermocyclic annealing with shot-term overheating, additional compression (with an optimal value of about 20%), and changing the temperature of aging.__________Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 6, pp. 7–18, November–December, 2004.     

Electro-magnetic collapse of thick-walled cylinders to investigate spontaneous shear localization

We present an electro-magnetic (EM) setup in order to collapse thick-walled cylinders, for the investigation of spontaneous formation of multiple adiabatic shear bands. The EM setup is based on a pulsed current generator using a capacitor bank system. The cylindrical specimen is part of an assembly of coaxial cylinders, where the inner and outer cylinders, each attached to an opposite pole, are short-circuited. Upon capacitor discharge, a high current flows through the cylinders, in opposite directions, creating repulsive magnetic forces between them. The outer cylinder is driven outwards and the inner cylinder is driven inwards - in a collapsing manner. This work presents the design procedure of the specimens’ geometry using numerical simulations, and some preliminary experimental results for SS304L steel specimens. The spatial distribution of the multiple adiabatic shear bands in these specimens is in good agreement with that reported in the literature for explosively driven experiments with the same material. Our numerical simulations of the collapsing cylinder show good agreement with the experimental results for both global behavior and shear band distribution.     

Investigation of Surface Damage in Forming of High Strength and Galvanized Steel Sheets

Powdering/exfoliating of coatings and scratching are the main forms of surface damage in the forming of galvanized steels and high strength steels (HSS), which result in increased die maintenance cost and scrap rate.In this study, a special rectangular box was developed to investigate the behavior and characteristics of surface damage in sheet metal forming (SMF) processes.U-channel forming tests were conducted to study the effect of tool hardness on surface damage in the forming of high strength steels and galvanized steels (hot-dip galvanized and galvannealed steels).Experimental results indicate that sheet deformation mode influences the severity of surface damage in SMF and surface damage occurs easily at the regions where sheet specimen deforms under the action of compressive stress.Die corner is the position where surface damage initiates.For HSS sheet, surface damage is of major interest due to high forming pressure.The HSS and hot-dip galvanized steels show improved ability of damage-resistance with increased hardness of the forming tool.However, for galvannealed steel it is not the forming tool with the highest hardness value that performs best.     

Effects of ferrite grain size and martensite volume fraction on dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels

Abstract

Effects of ferrite grain size and martensite volume fraction on quasistatic and dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens that had different ferrite grain sizes and martensite volume fractions, using a torsional Kolsky bar, and then the test data were compared in terms of microstructures, tensile properties, fracture mode, and adiabatic shear band formation. Under dynamic torsional loading, maximum shear stress and fracture shear strain increased with decreasing ferrite grain size and increasing martensite volume fraction. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that adiabatic shear bands of 5 to 15 μm in width were formed along the shear stress direction, and that voids or microcracks initiated at ferrites or martensite/ferrite interfaces below the shear band. The width of the shear band decreased as the ferrite grain size increased or the martensite volume fraction decreased. These phenomena were then analysed by introducing concepts of theoretical critical shear strain.     

IMPROVEMENT OF CUTTING TOOLS BY TiN PVD HARD COATING

A review of recent results obtained in the field of cutting tools improved with a TiN physical vapor deposition (PVD) ion-plated hard coating is presented. Optimization of the tool material, tool surface morphology and interface problems between the TiN coating and the tool surface are discussed in view of their importance in optimum performance tests and the resulting workplace surface quality. The high effectiveness of the TiN PVD coating is demonstrated by selected data from our own data bank and from other sources on six groups of cutting tools, made of high-speed steels (HSS), powder metallurgical high-speed steels (PM-HSS) and of WC based hard metals. We also describe recently introduced, novel hard coatings (TiCN and TiAIN) for cutting tools, and new applications of cutting tools, improved by PVD hard coatings.     

The effect of salt bath cementation on mechanical behavior of hot-rolled and cold-drawn SAE 8620 and 16MnCr5 steels

In this study, the effect of salt bath cementation on mechanical behavior of SAE 8620 and 16MnCr5 cementation steels, which are widely used in industry, was investigated. The experiments were carried out with hot rolled and cold rolled specimens. The cementation processes were performed in NaCN salt bath at 920 °C temperature for 1, 2, 3 and 4 h. Abrasive wear tests of specimens were conducted with Wolfram Carbide (WC) ball for 1 h. After cementation processes, a martensite phase on the surface of specimens was confirmed by X-ray diffraction analysis. After cementation processes carried out with different times, a different surface hardness and effective cementation depth values were obtained. Experimental results showed that an effective cementation depth increased with increasing the cementation time. Wear tests showed that the wear resistance of specimens increased by the cementation processes. Experimental results revealed that the surface hardness of specimen affects the wear resistance of specimens.     

Cavity mediated strain localization and overall ductility in eutectic tin-lead alloy

A combined experimental and numerical study is undertaken to examine the effects of pre-machined holes on strain localization and overall ductility in eutectic tin-lead alloy. Thin-sheet specimens with equal-sized holes aligned in the tensile loading direction are used. The tensile tests were performed at room temperature with a nominal strain rate of 0.001 s⁻¹. The specimen, containing one hole, showed a significant reduction in ductility compared to the control (no-hole) specimen. With an increasing number of holes, however, the overall strain-to-failure increases and fracture tend to follow shear bands generated locally from the hole edges. Finite element analyses, taking into account the viscoplastic response, were carried out to provide a mechanistic rationale to corroborate the experimental findings. The dispersion of plastic deformation and the effect of hole interaction are both found to contribute to the observed behavior. The local maximum equivalent plastic strain decreases with increasing number of holes, resulting in more delayed fracture. Plastic deformation becomes more intense inside the shear band when the holes are spaced more closely, which explains the increasing propensity of fracture along the shear bands in specimens containing more pre-machined holes.     

Modeling ductile to brittle fracture transition in steels—micromechanical and physical challenges

Both scientists and engineers are very much concerned with the study of ductile-to-brittle transition (DBT) in ferritic steels. For historical reasons the Charpy impact test remains widely used in the industry as a quality control tool to determine the DBT temperature. The transition between the two failure modes, i.e. brittle cleavage at low temperature and ductile fracture at the upper shelf occurs also at low loading rate in fracture toughness tests. Recent developments have been made in the understanding of the micromechanisms controlling either cleavage fracture in BCC metals or ductile rupture by cavity nucleation, growth and coalescence. Other developments have also been made in numerical tools such as the finite element (FE) method incorporating sophisticated constitutive equations and damage laws to simulate ductile crack growth (DCG) and cleavage fracture. Both types of development have thus largely contributed to modeling DBT occurring either in impact tests or in fracture toughness tests. This constitutes the basis of a modern methodology to investigate fracture, which is the so-called local approach to fracture. In this study the micromechanisms of brittle cleavage fracture and ductile rupture are firstly shortly reviewed. Then the transition between both modes of failure is investigated. It is shown that the DBT behavior observed in impact tests or in fracture toughness specimens can be reasonably well predicted using modern theories on brittle and ductile fracture in conjunction with FE numerical simulations. The review includes a detailed study of a number of metallurgical parameters contributing to the variation of the DBT temperature. Two main types of steels are considered : (i) quenched and tempered bainitic and martensitic steels used in the fabrication of pressurized water reactors, and (ii) modern high-toughness line-pipe steels obtained by chemical variations and optimized hot-rolling conditions. An attempt is also made to underline the research areas which remain to be explored for improving the strength-toughness compromise in the development of steels.     

Experimental analysis of bending fatigue strength of plain and notched case-hardened gear steels

Fatigue tests were performed on two commercial case-hardened steels using plain and notched specimens and results were compared with data available in the literature for similar steels. The purpose of the paper was to compare performances of different materials under the same testing conditions. Moreover, the results obtained may form the basis to apply ISO Standard methods mentioned in the Annex A, which estimate the fatigue life of gears starting from experimental data generated from specimens. Even though ISO Standard suggests primarily the use of experimental results generated from testing gears, specimens’ testing is faster and cheaper to evaluate different materials during the design process. From this point of view, specimens’ based approaches seem promising. Plane bending as well as axial fatigue tests were carried out, along with static tensile tests. Fatigue design coefficients relevant to the notch support effect, surface finish, mean stress effect in bending and load type (axial or bending) were derived from the experimental test results and, whenever possible, were compared with ISO Standard recommendations. Microstructures, hardness profiles, residual stresses and fracture surfaces were analyzed as well. To evaluate the accuracy of specimens-based approaches mentioned in the Annex of ISO Standard, additional work is needed in order to compare theoretical estimations with experimental results obtained from gears.     

Adiabatic shear banding under pure shear loading Part II: fractographic and metallographic observations

A metallographic and fractographic investigation was performed of dynamically loaded torsion specimens of VAR 4340 steel (HRC 40). Macroscopically, it was found that the shear bands propagated on several distinct planes around the circumference of the specimen. Microscopically, scanning electron microscope observations of the fracture surfaces reveal that microvoid nucleation and growth is the mechanism leading to shear band fracture. Patches of highly smeared and knobbly material, produced by sliding of the fracture surfaces against each other, were also found at several locations and suggest very high local temperatures. Metallographic cross sections revealed white-etching material, characteristic of transformed bands, only at limited locations, corresponding to regions of the fracture surface exhibiting a highly smeared or knobbly texture.These results indicate that microvoid nucleation and growth play a significant role in adiabatic shear banding and that one of the mechanisms for the formation of transformed bands is the extensive sliding and rubbing of the shear fracture surfaces against each other. Thus transformed bands may be, at least in some cases, a consequence rather than a cause of the final shear band fracture.     

Numerical modelling of damage behaviour of laser-hybrid welds

The effect of laser-hybrid welds on deformation and failure behaviour of fracture mechanics specimens is investigated in order to provide quantitative prediction of damage tolerance and residual strength. The simulation of crack initiation and crack extension in hybrid welds is performed by applying GTN damage model. The identification of damage parameters requires combined numerical and experimental analyses. The tendency to crack path deviation during crack growth depends strongly on the constraint development at the interface between base and weld metal. In order to quantify the influence of local stress state on the crack path deviation, the initial crack location is varied. Finally, the results from fracture mechanics tests are compared to real component, beam-column-connection, with respect to fracture resistance.     

Fatigue of a nickel base superalloy with bimodal grain size

Room temperature fatigue tests in the form of four point bending were performed at a frequency of 20 Hz and a load ratio R= +0·1 on electropolished Waspaloy specimens taken from a forged turbine disc. Samples, which had a partially recrystallised microstructure with a bimodal grain size, were removed from the outer rim of the disc. The S-N curve was similar to that reported for a fully recrystallised structure with a coarse grain size, from the same turbine disc. The dominant crack initiation sites were found to be inclusions with a subsequent stage I crack growth generally along slip bands. Other crack initiation sites observed included slip bands, annealing twins, and grain boundaries. Fatigue deformation occurred by the propagation of planar slip bands. Slip band cracks formed more frequently in coarse grains than expected from their volume fraction. Fluctuations in short crack growth rate were observed and were associated with microstructural features such as grain boundaries and twin boundaries, both of which acted as barriers in the early stages of crack growth. The short crack growth rate versus stress intensity range graph was similar to those from uniform fine grained Waspaloy.

MST/3413