Fatigue assessment and fatigue life calculation of quenched and tempered SAE 4140 steel based on stress–strain hysteresis, temperature and electrical resistance measurements

In this paper, mechanical stress–strain-hysteresis, temperature and electrical resistance measurements are performed for the detailed characterization of the fatigue behaviour of quenched and tempered SAE 4140 steel used for many applications in the automotive industry. Stress-controlled load increase and constant amplitude tests (CATs) were carried out at ambient temperature on servo-hydraulic testing systems. The applied measurement methods depend on deformation-induced changes of the microstructure in the bulk material and represent the actual fatigue state. The plastic strain amplitude, the change in temperature and the change in electrical resistance can be equally used for an assessment of baseline fatigue properties in generalized cyclic deformation curves as well as in generalized Morrow and Coffin–Manson curves. On the basis of comprehensive experimental fatigue data, the physically based fatigue life calculation method ‘PHYBAL’ based on the generalized Basquin equation was developed. S–N (Woehler) curves calculated with ‘PHYBAL’ using data from only three fatigue tests agree very well with the conventionally determined ones.     

Micromechanism of cyclic plastic deformation of alloy IN 718 at 600 °C

In the present investigation, an attempt was made to understand the cyclic deformation micromechanism of gas turbine alloy Inconel 718 at 600 °C (i) by conducting low cycle fatigue and creep–fatigue interaction tests and (ii) by studying the microstructure evolution in the material during fatigue tests through extensive electron microscopy. Bilinear slope was obtained in the Coffin–Manson plot for all low cycle fatigue tests, and it was confirmed through transmission electron microscopic examination that microtwinning was the predominant mode of deformation at low plastic strain values, whereas slip and shearing of γ″ precipitates were the predominant mode of deformation at higher plastic strain values. Fatigue life was adversely affected when hold time was introduced at peak tensile strain during creep–fatigue interaction tests. Formation of stepped interface at microtwin boundaries and coarsening of niobium carbide precipitates were observed to be the major microsturctural changes during creep–fatigue interaction tests.     

Characterisation of the surface damage of X38CrMoV5 (AISI H11) tool steel at room temperature and 600 °C

A novel approach is used to characterise the surface damage of AISI H11 hot work tool steel. The fatigue crack growth rate (FCGR) experiments on thin specimens, considered representative of the surface of tool steels, are carried out. Single edge notched tension specimens of 8 mm width and different thicknesses are used in the study. Initially, the effect of thickness (scale) on the FCGR is investigated to establish if there exists a difference between bulk and near surface properties of the tool steel. Then, the effect of R value on the thin specimens is investigated. All the experiments are carried out at room temperature and 600 °C. These temperatures represent the limits of use of this steel. Paris curves are established. Effect of R ratio on the threshold value of propagation at elevated temperatures is investigated in detail. A special ascending ΔK experiment for establishing the threshold of propagation at elevated temperatures in small specimens is proposed, and the results are presented. The increase in R ratio increases the FCGR at low temperature, while it has no effect at 600 °C. A reduction in thickness shows a reduction in the FCGR. Increase in temperature increases the FCGR and dramatically increases the threshold of crack propagation. The sharp increase in threshold value is studied in detail. Scanning electron microscopy of the specimens is performed to explain some of the characteristics observed in the specimen testing.     

Frequency effect and influence of testing technique on the fatigue behaviour of quenched and tempered steel and aluminium alloy

Influences of testing technique and frequency on the fatigue behaviour of 50CrMo4 and EN AW-5083 were investigated. To clarify the effect of test frequency on the fatigue behaviour, tests with 20 kHz and f < 400 Hz were performed. The frequency effect can be caused by temperature, environment and strain rate. For the aluminium alloy, the influence of environment is responsible for the dependence of fatigue lifetime on the frequency. The fatigue lifetime of the steel showed in both environments similar frequency dependency, i.e. the strain rate is assumed to be responsible for the differences in fatigue lifetime.     

Effect of 475°C embrittlement on the low cycle fatigue behaviour of lean duplex stainless steels

Lean duplex stainless steels (LDSSs) with lower nickel and molybdenum are less susceptible to suffer spinodal decomposition than standard duplex stainless steels. It is the purpose of this work to study the effect of thermal embrittlement on the low cycle fatigue behaviour of 2 LDSSs with different Cr_eq and Ni_eq. The correlation between the fatigue behaviour and the dislocation structure is attempted. Transmission electron microscopy was used to observe the dislocation microstructure. Additionally, STEM‐EDS technique in conjunction with Vickers microhardness measurements was used to characterize the amplitude of the spinodal decomposition. The results show that the LDSS with lower Cr_eq and Ni_eq values exhibits improved fatigue properties in the as received and aged conditions. Furthermore, it is important to emphasize that with an adequate volume fraction of phases in LDSSs, the ageing treatment leads to an increase in strength without causing a great detriment in low cycle fatigue life.     

Ratcheting‐fatigue behaviour of bainite 2.25Cr1MoV steel with tensile and compressed hold loading at 455°C

The ratcheting behaviour of a bainite 2.25Cr1MoV steel was studied with various hold periods at 455°C. Particular attention was paid to the effect of stress hold on whole‐life ratcheting deformation, fatigue life, and failure mechanism. Results indicate that longer peak hold periods stimulate a faster accumulation of ratcheting strain by contribution of creep strain, while double hold at peak and valley stress has an even stronger influence. Creep strains produced in peak and valley hold periods are noticeable and result in higher cyclic strain amplitudes. Dimples and acquired defects are found in failed specimen by microstructure observation, and their number and size increase under creep‐fatigue loading. Enlarged cyclic strain amplitude and material deterioration caused by creep lead to fatigue life reduction under creep‐fatigue loading. A life prediction model suitable for asymmetric cycling is proposed based on the linear damage summation rule.     

Residual strength at −40 °C of a precracked cold‐formed rectangular hollow section made of ultra‐high‐strength steel – an engineering approach

This study investigated the residual strength of a precracked cold‐formed rectangular hollow section made of novel ultra‐high‐strength steel. The primary goal was to experimentally discover the residual strength of the structure when used in low temperature service conditions. The secondary goal was to predict the residual strength by using a J‐integral approach with nonlinear finite element calculations and to compare these predictions with measured results. The experimental tests were carried out with a beam in four‐point bending loading. The test specimens were taken from a cold‐formed rectangular hollow section fabricated from direct quenched (untempered) ultra‐high‐strength steel S960 QC omitting the annealing in the fabrication process. The tests for final failure were carried out at ?40 °C, with the exception of the first pilot test. There were two kinds of tests: (1) the beam was cyclically loaded until the final fracture or the fatigue precrack was first introduced and (2) the specimen was then subjected to a quasistatic bending loading condition until it failed. The new experimental results matched well with our predictions, and both confirmed the high toughness of ultra‐high‐strength steel in beam construction studied, even at a low ambient temperature.     

Influence of hydrogen content and microstructure on the fatigue behaviour of steel SAE 52100 in the VHCF regime

The fatigue life of the bearing steel 52100 (100Cr6) in bainite and martensite conditions was investigated up to 2 × 10⁹ cycles. The tests were performed under cycling tension (R = 0.1) and tension–compression (R = −1) on a piezo-electric ultrasonic testing equipment. The specimens are designed with a cylindrical part in the highly stressed centre. Due to grinding, compressive residual stresses are found at the surface, hence crack initiation solely occurs subsurface. Prior testing half of the specimens was charged with hydrogen. The hydrogen content varies from 0.6 as initial condition to 3 ppm after charging. The increased hydrogen content decreased the endurance limit to nearly half of the value of uncharged conditions and crack initiation changed: Conditions with low hydrogen content failed from chromium carbides, titanium nitrides or slag agglomerations. Conditions with 3 ppm hydrogen failed from slag agglomerations, often in combination with aluminium magnesium oxides, and manganese sulphides. Next to the inclusions a fine granular area (FGA) could be observed in some cases, and nearly all fractured surfaces showed a fisheye surrounded by an Optically Bright Zone (OBZ) with the crack initiating inclusion in its centre. Furthermore, selected specimens where analysed using secondary ion mass spectroscopy in a time of flight setup (ToF-SIMS) to ascertain the local hydrogen content. From the results it is assumed that hydrogen accumulates in the cavity at inclusions or bonds to the inclusion if it contains silicon.     

Effect of surface roughness on low-cycle fatigue life of Cr---Mo---V steel at 550 °C

Low-cycle fatigue tests have been carried out on Cr---Mo---V steel specimens with two different modes of surface roughness at 550 °C. The fatigue life of specimens with a rough surface is approximately half of that of specimens with a smooth surface over the plastic strain range investigated. From interrupted tests, it is seen that low-cycle fatigue behaviour is largely influenced by the crack initiation process even for a high strain range and the decrease of fatigue life in specimens with a rough surface is mainly due to a reduction in the number of cycles for crack initiation.     

Monotonic and low cycle fatigue behaviour of 2024‐T3 aluminium alloy between room temperature and 300 °C for designing VAWT components

In the present study, the results of the monotonic tension tests and low cycle fatigue tests performed on aluminium alloy EN AW‐2024‐T3 under various operating temperatures are presented in order to assess the fatigue behaviour of the aluminium alloy under evaluated temperatures. Monotonic tests were performed to determine the influence of temperature on mechanical properties of the material. The aim of cyclic tests was to acquire the parameters required for Manson–Coffin equation in order to plot strain–fatigue life curves. Moreover, stress–strain behaviour of the alloy and the cyclic hardening behaviour were evaluated using Ramberg–Osgood equation. Finally, P_SWT‐damage parameters for each temperature have been calculated for further investigation of the effects of the temperature on fatigue life using acquired data while taking the account of mean stress effect into calculations. Variations in the experimental data due to various test temperatures are presented for both monotonic and cyclic tests.     

The influence of environment and stress ratio on the low frequency fatigue crack growth behaviour of two medium-strength quenched and tempered steels

The results of low frequency corrosion fatigue crack growth tests on HY 130 and Q1N steels are reported and compared. The similarities and differences in the response of the two steels to variations in environment and stress ratio are highlighted and discussed in terms of some recent theories of corrosion fatigue crack growth and the impurity element embrittlement of grain boundaries.     

In‐phase and out‐of‐phase thermomechanical fatigue behavior of 4Cr5MoSiV1 hot work die steel cycling from 400 °C to 700 °C

The hysteresis loops, stress and strain behavior, lifetime behavior and fracture characteristic of 4Cr5MoSiV1 hot work die steel at a wide range of mechanical strain amplitudes (from 0.5% to 1.3%) during the in‐phase (IP) and out‐of‐phase (OP) thermomechanical fatigue (TMF) tests cycling from 400 °C to 700 °C under full reverse strain‐controlled condition were investigated. Stress‐mechanical strain hysteresis loops of 4Cr5MoSiV1 steel are asymmetric, and stress reduction appears at high‐temperature half cycles owing to a decrease in strength with increasing temperature. 4Cr5MoSiV1 steel always exhibits continuous cyclic softening for both types of TMF tests, and the cyclic softening rate is larger in OP loading condition. OP TMF life of 4Cr5MoSiV1 steel is approximately 60% of IP TMF life at the same mechanical strain amplitude and maximum temperature. Lifetime determined and predicted in both types of TMF tests is adequately described by the Ostergren model. Fracture surfaces under IP TMF loading display the striation and tear ridge, showing quasi‐cleavage characteristics, and the cracks are less but longer. However, fracture surfaces under OP TMF loading mainly display the striation and dimple characteristics, and the cracks are more and shorter.     

Experimental research on bolted joint strength and dispersion in steel lattice transmission tower legs under low temperatures (−20°C to −45°C)

Cold regions with subzero temperatures (?20°C to ?45°C) have important impacts on the mechanical properties of structural steel used in lattice steel towers of overhead transmission line systems. The results from regular material tests are not appropriate for the accurate analysis of joint strength in cold weather conditions. This paper presents the tensile test results of 18 coupons of steel material and eight groups (18 specimens per group) of bolted joints with Q345 and Q420 steel under temperatures of 20°C, ?20°C, and ?45°C. The results show that the yield‐to‐ultimate strength ratio of the joints under low temperature conditions is beyond the range of 0.60 to 0.75 for Q420 structural steel. Suggestions are made on how to improve the accuracy of joint design for both the partial resistance factor and the design value of joint yield strength in cold regions.     

Embrittlement of a superduplex stainless steel in the range of 550–700 °C

Duplex stainless steels are very attractive materials, combining high mechanical properties with improved corrosion resistance. However, these steels present technical limitations because they can experience embrittlement as a consequence of thermal cycles. Moreover, the higher level of alloying elements (Cr and Mo) in the new duplex generation, called superduplex, accelerates the precipitation kinetics of harmful intermetallic phases, which are the responsible of embrittlement. This fact raises the question of the influence of these cycles on the actual performance of duplex components and structures and on its possible failure during service.

In order to investigate this question, heat treatments in the range of 550–700 °C, with different exposure times and cooling rates, have been made on a superduplex stainless steel type EN 1.4507. The evolution of mechanical properties has been followed by means of hardness measurements, impact and fracture toughness tests, whereas microstructural changes have been identified by using optical and transmission electron microscopy analysis. A correlation between the degree of embrittlement and the different types of precipitates has been established.     

Anomalous Tensile Strength and Fracture Behavior of Polycrystalline Iridium from Room Temperature to 1600 °C

The nature of the brittleness of Iridium crystal is still unclear. The aim of this study is to explore the mechanism of ductile‐to‐brittle transition (DBT) and the fracture behavior in polycrystalline Iridium. Tensile tests are conducted from room temperature to 1600 °C. Furthermore, fracture morphology and deformation substructures are characterized by OM, SEM, and TEM. The results show that the tensile strength increases anomalously below 600 °C and then decreases with the increasing temperature. The elongation increases slowly from room temperature to 700 °C, and it then changes sharply from 9.88% at 700 °C to 31% at 800 °C. Below 700 °C, the polycrystalline Iridium exhibits intergranular and partial transgranular cleavage fracture pattern. In contrast, the ductile fracture morphologies associated with microvoids coalescence are observed between 800 and 1600 °C. Massive tangling screw dislocations form at 700 °C and less tangles appear when stretching at 900 °C, manifesting that the DBT is around 800 °C in polycrystalline Iridium.

     

Effect of temperature on the fracture behaviour of heat‐treated Al–Cu–Li alloy laser welds under low‐cycle fatigue loading

The paper presents the results of the studies of the effect of temperature on the fracture behaviour of Al–Cu–Li alloy laser welds under low‐cycle fatigue loading. The mechanical properties and the microstructure of the welded joints without and after postweld heat treatment (PWHT) were investigated. The tensile strength and the low‐cycle fatigue resistance of the welded joints were studied at various test temperatures (20°C, 85°C and ? 60°C). It was been found that heating up to 85°C and cooling down to ?60°C reduced the maximum number of loading cycles of the welded joints after PWHT by 1.5–2.0 times compared with that at a test temperature of 20°C.     

The mechanism of the low-temperature hot corrosion of pure iron and manganese at 600–800°C

The corrosion of pure iron and manganese in simulated combustion gases at 600–800°C in the presence of sodium sulfate deposits is faster than the same reaction without salt deposits. This effect is due to the formation of a liquid solution between sodium sulfate and the sulfates of the corroded metals. The nature of these liquid solutions is examined together with the possible mechanisms for the transport of matter through the liquid and the dissolution-reprecipitation of solid metal compounds in the liquid salt. Finally, the reasons for the formation of sulfides in mixture with oxides in the inner solid region of the scale and the effects produced by the presence of sulfides on the overall reaction rate are considered.