Uniaxial ratchetting in steels with different cyclic softening/hardening behaviours

The cyclic deformation of three structural steels, SS316L stainless steel, 40Cr3MoV bainitic steel and 25CDV4.11 steel, were studied experimentally by uniaxial cyclic straining or stressing tests at room temperature. The cyclic softening/hardening behaviours of the steels were discussed first by cyclic straining tests; and then the effects of cyclic softening/hardening behaviours on the uniaxial ratchetting of the materials were investigated by asymmetrical cyclic stressing tests. It is concluded from the experimental results that the ratchetting greatly depends on the cyclic softening/hardening behaviours of the materials, as well as the loading history. Different ratchetting and failure behaviours are observed for the prescribed steels. It is also stated that the proposed unified visco‐plastic constitutive model can provide a fairly reasonable simulation of the uniaxial ratchetting of SS316L stainless steel and 25CDV4.11 steel; but cannot simulate the ratchetting of 40Cr3MoV bainitic steel since the dependence of cyclic softening behaviours on the applied inelastic strain amplitude cannot be reasonably described in the discussed constitutive model. Some significant conclusions are obtained, which are useful to construct constitutive model to describe the ratchetting of the materials with different cyclic softening/hardening behaviours.     

Cyclic behaviour of 12% Cr ferritic‐martensitic steel upon long‐term on‐site service in power plants

The strain‐controlled and stress‐controlled low‐cycle fatigue behaviour of served 12% Cr ferritic–martensitic steel is conducted at room temperature. Continuous softening is observed at both control modes, and the fitting results show that the fatigue properties of 12% Cr steel are not reduced significantly after 230 000 h service at 550 °C/13.7 MPa. Scanning electron microscopy has been employed to investigate the microstructure evolution after long‐term service. It is proved that the decomposition of martensite laths structure and the coarsening of carbides at grain/lath boundaries are the main reasons why the pipe bursts after 180 000 h service at 550 °C/17.1 MPa. The fracture under both control modes has been observed by using scanning electron microscopy, and it indicates coarse carbides along grain/lath boundaries are favourable sites for micro‐crack nucleation and the secondary cracks along the fatigue striations are formed by the connection of micro‐cracks nucleated during fatigue behaviour.     

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.     

Investigations of ferritic/martensitic super heat resistant 11‐12 %Cr steels for 650 °C power plants

The investigations of advanced ferritic/martensitic 11–12 %Cr steels for 650 °C power plant components focus on the improvement of high‐temperature creep properties with respect to chemical composition. The claim of the DFG research work was the development of new heat‐resistant 12 %Cr ferritic‐martensitic steels with sufficient creep and oxidation resistance for a 650 °C application by using basic principles and concepts of physical metallurgy on the basis of the state of art and to overcome the usual trial and error industrial alloy development. Efforts are focussed on a 100,000h creep strength of 100MPa at 650 °C in combination with a sufficient corrosion resistance by a Cr content of 12 % with contents 4‐5 %W, 3.4‐5,5 %Co, V, B and 1 %Cu as well as the choice of Ta or Ti instead of Nb. The results demonstrate that the aim is not to realize with the used alloying concept. In the long term range all 12 %Cr melts have a lower creep rupture strength than the advanced 9 %Cr piping steel P92. A high creep strength could be reached with a 0.06 % Ta alloyed 11 %Cr melt, which is in addition alloyed with a higher C and B content and as well as with lower W and Co portions. The results indicate in accordance with the finding of other steel researcher that a lower Cr content allows more effectiveness for the alloying partners respectively for the generation of more stable precipitates.     

Cyclic stress response and fatigue behavior of Cu added ferritic steels

Steels in which Cu content was altered from 0 to 1.5 mass% were subjected to various heat treatments to change the state of Cu. Concerning these respective steels, fatigue ratio by a stress controlled fatigue test and fatigue resistance by a strain controlled fatigue test thereof were compared. Furthermore dislocation substructure and surface defect during and after cyclic straining were investigated to clarify the effect of Cu on fatigue properties. The fatigue ratios at 2.0 × 10⁶ loading cycles of the Cu added steels after aged at 450 and 750 degrees C are 0.7, remarkably high as compared with those of the Cu added steels after aged at 550 and 650 degrees C, the Cu free steels and any other conventional steels whose fatigue ratio are approximately between 0.5 and 0.6. The fatigue resistance of the as-rolled Cu added steel maintains steady cyclic hardening until fracture. To the contrary the Cu added steel after heat treatment at 550 degrees C shows cyclic hardening to the peak stress and then shows a cyclic softening until fracture. The surface roughness of the Cu added steels after cyclic straining are relatively shallow compared with those of the Cu free steel. The internal substructure of the Cu free steel shows typical cell structure but those of the Cu added steels exhibit vein structure.     

Microstructural stability of ODS steels in cyclic loading

The remarkable microstructural stability of high chromium steels prepared by powder metallurgy and strengthened by dispersion of nanometric yttrium oxides in cyclic loading at high temperatures is reported. Contrary to the continuous cyclic softening and profound changes in the microstructure during fatigue of common high chromium steels, the addition of 0.3 wt% Y₂O₃ stabilizes the microstructure and significantly reduces cyclic softening of investigated steels. The evolution of microstructure as a result of fatigue loading at room temperature, 650 and 750 °C, was examined by means of transmission electron microscopy. Only minor changes in the microstructure were detected. The stability of oxide particles after high‐temperature exposure was confirmed by energy dispersion spectroscopy chemical analysis. The microstructural features are discussed in relation to the cyclic behaviour of the oxide dispersion strengthened steels. The analysis of the hysteresis loop indicates that oxide nanoclusters are intersected and dissolved in slip bands of ODS Eurofer steel. This process contributes to cyclic softening.     

Nano-sized precipitates in 11 % Cr ferritic-martensitic steel after thermomechanical treatment

9 %–12 % Cr ferritic/martensitic steels with a good long-term creep strength at temperatures up to 650 °C and higher are being developed in order to increase steam temperature of coal-fired power plants.Thermomechanical treatment can effectively enhance the mechanical properties of high-Cr ferritic/martensitic steels mainly due to plenty of nano-sized precipitates produced by thermomechanical treatment. Nano-sized precipitates in an 11 % Cr ferritic/martensitic steel produced by a thermomechanical treatment, including warm rolling at 650 °C plus tempering at 650 °C for 1 h, were investigated by transmission electron microscopy. The average size of precipitates in the steel after the thermomechanical treatment was determined to be about 30 nm in diameter, which is only one-third of the average size of precipitates in the steel with the normalized and tempered condition. A large number of Cr-rich precipitates having an average diameter of about 25 nm in the steel produced by the thermomechanical treatment were identified as Cr-rich M₂C carbide with a hexagonal crystal structure, rather than M₂₃C₆ or MX phase. The plenty of nano-sized Cr-rich M₂C carbides were dominant phase in the steel after the thermomechanical treatment. The reason why prior precipitate phase formed in the steel during the thermomechanical treatment was Cr-rich M₂C carbide is also discussed.     

Ausscheidungs- und Versprödungsverhalten nickelhaltiger Superferrite

Precipitation and embrittlement behaviour of nickel containing Superferrites During the last years high chromium ferritic stainless steels socalled Superferrites have become of general interest since the mean disadvantages of these steel grades, namely cold brittleness and susceptibility to intergranular corrosion, can be avoided by means of new melting techniques. This group of steels show remarkable strength properties and excellent corrosion behaviour especially in chloride containing solutions. Because of the high content of alloying elements the ferritic solid solution is supersaturated, so that precipitation and embrittlement can occur. From 950° to 550 °C the brittle Chi- and/or Sigmaphase can be precipitated, at temperatures below 550 °C the material can fail by the socalled 475°-embrittlement. Additions of nickel and stabilising by titanium enhance the precipitation process well as the 475°-embrittlement, while a columbium stabilisation of the present carbon and nitrogen contents up to 150 ppm shows no influence. Within the investigated alloying range with 20 to 28% Cr, up to 5% Mo and up to 4% Ni the columbium stabilised steel with 28% Cr, 2% Mo and 4% Ni results as the optimal combination referring to its technological properties as well as to its precipitation and embrittlement behaviour.     

The effect of temperature on low-cycle fatigue behavior of prior cold worked 316L stainless steel

Low-cycle fatigue tests on cold worked 316L stainless steel were carried out at various temperatures from room temperature to 650 °C and tensile tests were conducted on the cold worked and solution-treated materials. At all test temperatures, the cold worked material showed the tendency of higher strength and lower ductility. Following initial cyclic hardening for a few cycles, cyclic softening behavior was observed to dominate until failure occurred during low-cycle fatigue deformation. The softening behavior strongly depends on temperature and strain amplitude. Several life prediction models were examined and it was found that it is important to select a proper life prediction parameter since stress and strain depend strongly on temperature. A phenomenological fatigue life prediction model is proposed to account for the influence of temperature on life. The model is correlated with the experimental results.     

Effect of retained austenite stability on cyclic deformation behavior of low‐alloy transformation‐induced plasticity steels

The retained austenite (RA) characteristics of Al‐containing TRIP700 steels have been manipulated using varying bainitic isothermal transformation (BIT) processing. The microstructural evolution was investigated using optical microscopy and quantitative image analysis, while the amount of transformed RA was evaluated with the saturation magnetization (SM) technique. Cyclic behavior is found to depend on the applied strain amplitude and stability of RA. At strain amplitudes with comparable elastic and plastic strain components, cyclic softening prevails, facilitated by more stable RA microstructures and Low Cycle Fatigue (LCF) performance benefits from a lower RA stability, which controls the amount of cyclic softening rate. With increasing plastic strain component, a transition to cyclic hardening is observed, and the transition strain increases with increasing RA stability. LCF performance deteriorates because of excessive cyclic strain hardening promoting martensitic transformation. The effect is accompanied by a transition from mixed dimple/cleavage to cleavage‐type fracture characteristics.     

Precipitates change of ferritic‐martensitic 11 % chromium steel under short‐term creep

A ferritic‐martensitic (FM) 11 % chromium steel with final heat treatment was subjected to a short‐term creep test at a stress of 150 MPa and 600 °C for 1100 h in order to study the change of precipitates in the steel during the creep test. Except for Nb‐rich metall carbides (MC, M₂₃C₆) and Laves phases, Fe‐W‐Cr‐rich M₆C (based on Fe₃W₃C) carbides forming during the creep test were also identified in the crept steel by electron diffraction and x‐ray diffraction in combination with energy dispersive x‐ray analysis of extraction carbon replicas. The identified M₆C carbides have a fcc crystal structure, a metallic element composition of approximately 44Fe, 32 W, and 20Cr in atomic %, and large sizes ranging from 100 nm to 300 nm in diameter. The M₆C carbides are a dominant phase in the crept steel. M₆X precipitates are generally not easy to form during high temperature creep, even if it is a long‐term creep, in ferritic‐martensitic 9–12 % chromium steels with a final heat treatment. The present work provides the evidence for the M₆C carbides forming during short‐term creep in ferritic‐martensitic high chromium steels. The formation of the M₆C carbides was discussed.     

Foreword – Materialwissenschaft und Werkstofftechnik 10/2011

Load controlled fatigue tests were performed up to 10⁷ cycles on flat notched specimens (K_t = 2.5) under constant amplitude and variable amplitude loadings with and without periodical overloads. Two materials are studied: a ferritic‐bainitic steel and a cast aluminium alloy. These materials have a very different cyclic behaviour: the steel exhibits cyclic strain softening whereas the Al alloy shows cyclic strain hardening. The fatigue tests show that, for the steel, periodical overload applications reduce significantly the fatigue life for fully reversed load ratio (R_σ = –1), while they have no influence under pulsating loading (R_σ = 0). For the Al alloy overloads have an effect (fatigue life decreasing) only for variable amplitude loadings. The detrimental effect of overloads on the steel is due to ratcheting at the notch root which evolution is overload's dependent.     

Cyclic deformation and microstructural behaviour of reduced activation ferritic–martensitic steels

Abstract

The present paper presents results about cyclic behaviour and the evolution of the dislocation structure of reduced activation ferritic–martensitic steels and commercial martensitic steels AISI 410 and 420. The variation of the free dislocation density within subgrains and subgrain size was mainly analysed during the cyclic softening of EUROFER 97 steel. From the analysis of the flow stress components, the friction and back stresses, and the information of the evolution of the dislocation structure, it could be concluded that the softening of tempered martensitic steels at 20°C is produced by the contribution of the friction stress and aided later by the back stress.     

Influence of reinforcement morphology and matrix strength of metal–matrix composites on the cyclic deformation and fatigue behaviour

The cyclic deformation behaviour of three metal–matrix composites, namely AA6061-T6 reinforced with 20 vol.% alumina particles and short-fibres, respectively, and pure aluminium reinforced with 20 vol.% short-fibres, has been investigated at temperatures between T=−100°C and T=300°C in total strain controlled symmetrical push–pull fatigue tests. The cyclic stress response exhibits initial cyclic hardening, subsequent saturation and cyclic softening, depending on the test parameters for temperatures lower than T=150°C. Initial cyclic hardening is less pronounced with increasing temperature and decreasing applied strain amplitude. Short-fibre reinforced composites — both with alloyed and unalloyed aluminium matrix — harden cyclically more than the particulate-reinforced composite. The comparison of the cyclic with monotonic stress–strain curves indicates that, depending on the testing conditions, both cyclic hardening and cyclic softening can occur.     

Fatigue properties of transformation-induced plasticity and dual-phase steels for auto-body lightweight: Experiment,modeling and application

The substitution of conventional high strength steels (HSS) with advanced high strength steels (AHSS), e.g., low-alloy multiphase transformation-induced plasticity steel (TRIP steel) or dual-phase steel (DP steel), for body lightweight brings about increased stress of notched components. Thus the fatigue properties of TRIP and DP steels and the fatigue life of notched lightweight design are important considerations for reasonable material selection during the design stage of auto-body. For the mentioned issue, cyclic strain-controlled fatigue properties of TRIP and DP steels with equivalent grade and lightweight result were investigated experimentally. Different cyclic behaviors of TRIP and DP steels were observed due to different interior microstructures. The cyclic stress behavior of TRIP steel is characterized by cyclic hardening followed by stable at lower strain amplitudes, and softening at higher strain amplitudes; however cyclic softening followed by stable occurs consistently for DP steel throughout entire strain amplitude range of test. TRIP steel possesses enhanced fatigue life and cyclic stress at the same strain amplitude than DP steel. Furthermore, local strain-life models of two steels were developed by linear regression of experimental data, to predict and compare the fatigue life of notched body structures made of them by finite element method. The simulation result illustrates that TRIP steel can provide more beneficial potential than DP steel for the lightweight design of notched body structures from the viewpoint of fatigue resistance.     

Fatigue-creep interaction performance of Incoloy 825 nickel-based superalloy at 650 °C

The fatigue-creep interaction performance of Incoloy 825 nickel-based superalloy at 650 °C was investigated through introducing the tensile, compressive, and tensile-compressive strain hold time at the controlled total strain amplitude Δϵ_t = 0.3 %∼0.7 %. The results show that the Incoloy 825 nickel-based superalloy exhibits the cyclic hardening behavior, the cyclic hardening behavior followed by cyclic softening behavior and the cyclic hardening behavior followed by cyclic stability during the cyclic deformation with tensile strain hold time, while the alloy exhibits the cyclic hardening behavior and the cyclic hardening behavior followed by the cyclic stability during the cyclic deformation with compressive and tensile-compressive strain hold time. The relationship between both plastic and elastic strain amplitudes with reversals to failure for the alloy shows a single slope linear behavior, which can be described by the Coffin-Manson and Basquin equations, respectively. The deformation mechanism of the alloy under three loading condition of fatigue-creep interaction is mainly the planar slip. In addition, under three loading condition of fatigue-creep interaction, the cracks initiate and propagate in the transgranular mode.     

800MPa级低合金高强度钢的循环软硬化特性

在不同应变级别控制下,研究了800MPa级低合金高强度钢10CrNiSMoV轴向加载低周循环软硬化特性,研究结果表明：应变加载级别不同,材料表现出不同的循环软硬化特性,当应变幅在0．4％～1．0％范围变化时,材料表现出循环软化的特性,且应变幅越大,循环软化速率越高;当应变幅在0．30％-0．40％范围变化时,材料表现出先硬化,再趋于循环饱和,再轻微循环软化直至断裂的特性。采用透射电镜观察了不同应变级别下试样断口的位错形态,用位错理论解释了产生上述现象的原因。     

Low cycle fatigue behaviour of duplex stainless steel: influence of isothermal aging treatment

The low‐cycle fatigue behaviour of a duplex stainless steel was comparatively studied in standard heat‐treated and isothermally aged (at 475 °C for 100 h) conditions by mechanical testing, scanning and transmission electron microscopy. It was established that fatigue life is longer in the aged condition as compared to the annealed condition at lower values of total strain amplitude Δ?/2 (Δ?/2 = 4.0 × 10^?3 and 6.0 × 10^?3) and becomes similar in annealed and aged conditions at Δ?/2 = 8.0 × 10^?3. Fatigue resistance of the material converges with increase in Δ?/2 values as a result of rapid cyclic softening of the ferritic phase in the aged condition.     

Cyclic stress–strain characteristics of two microalloyed steels

Abstract

The cyclic stress–strain behaviour of two microalloyed steels with different microstructures has been characterised at room temperature under strain controlled low cycle fatigue. The cyclic stress–strain curve in the double logarithmic plot shows a linear relation for both steels. A transition of the cyclic stress–strain curve from softening to hardening with increasing strain amplitude has been observed with respect to the corresponding tensile curve. The strain amplitude for the onset of cyclic softening to hardening transition has been found to be dependent on grain size. The strain lifetime behaviour, estimated from modified universal slopes equation, shows similar trends as Nb or V bearing microalloyed steels. The cyclic characteristics of the two microalloyed steels have been compared with corresponding predeformed state carried out under stress controlled conditions. While, cyclic saturation was observed in case where the extent of predeformation was within the Lüders strain, cyclic softening occurred when it exceeded the Lüders strain. It has been attempted to provide a mechanistic understanding of the differences in the cyclic behaviour of the two steels owing to the microstructure and predeformation.