Effects of hydrogen charge on microscopic fatigue behaviour of annealed carbon steels

The effects of hydrogen charge on cyclic stress–strain properties, slip band morphology and crack behaviour of annealed medium carbon steels (JIS‐S45C) were studied. The total strain range of the stress–strain hysteresis loop in the hydrogen‐charged specimen was smaller than that in the uncharged specimen. Localized slip bands were observed in the hydrogen‐charged specimen, while the slip bands were widely and uniformly distributed in the uncharged specimen. It is presumed that the decrease in the total strain range of the hysteresis loop is due to the slip localization caused by the hydrogen charge and cyclic loading. The sites of fatigue crack initiation were mostly at grain boundaries in the uncharged specimen. The sites of crack initiation in the hydrogen‐charged specimen were not only at grain boundaries but also at slip bands inside ferrite grains. These results imply that hydrogen enhances dislocation mobility along slip bands and results in slip localization. These slip bands then attract hydrogen. This mechanism of hydrogen–slip band interaction may play an important role in the hydrogen‐influenced metal fatigue.     

Model of hydrogen behaviour in enamelling grade steels

A model has been developed of the behaviour of hydrogen in enamelling-grade steels in relation to the delayed defect of blow-off of enamelled surface (fishscaling). The model is based on current theories concerning reversible and irreversible trapping of hydrogen in metallic materials. It leads to the establishment of a free hydrogen parameterC _L which can be used to assess the susceptibility of a steel to fishscaling following the usual enamelling processess. The model can also be used to study the effect of both thermomechanical steelmaking cycles and enamelling processes on resistance to the defect.Nomenclature E _aD Activation energy of hydrogen diffusion through normal lattice - E _s Saddle-point energy - E _B Trap binding energy - E _aT Trap activation energy=E_s+E_B - A Trapping site - B Normal lattice site - v ₀ Vibration frequency of hydrogen at a normal lattice site - v ₁ Vibration frequency of hydrogen at a trapping site - N _L Density of normal lattice sites for hydrogen - N _T Density of trapping sites for hydrogen - C _L Concentration of hydrogen on lattice sites - C _T Concentration of hydrogen captured on traps - k Probability of trapping=v₀ exp(–E _s/R T) - p Probability of detrapping =v ₁ N _L exp (–E _aT/R T) - n Fraction of trapping sites occupied with hydrogen atoms among the total trapping sites=C _T/N _T - t Time - T Temperature - R Gas constant     

Warm rolling behaviour of low carbon steels

Abstract

Warm (ferritic) rolling can be a low cost method of producing sheet steel products. However, for steels containing solute carbon, microstructural development during processing is affected by dynamic strain aging (DSA). This can significantly weaken the {111} texture formed during annealing, thus resulting in products with poor formabilities. It is known that the DSA behaviour can be modified by the addition of elements such as boron and chromium. Experimental low carbon (LC) steels with various additions of chromium, boron and phosphorus were warm rolled and their behaviour compared with that of a standard LC material. It was found that these additions promote the formation of shear bands under warm rolling conditions, thus resulting in a stronger {111} recrystallisation texture than that of the unmodified LC steel.     

Comparative study on biaxial low-cycle fatigue behaviour of three structural steels

In this study the uniaxial/biaxial low‐cycle fatigue behaviour of three structural steels (Ck45 normalized steel, 42CrMo4 quenched and tempered steel and AISI 303 stainless steel) are studied, evaluated and compared. Two parameters are considered for estimating non‐proportional fatigue lives: the coefficient of additional hardening and the factor of non‐proportionality. A series of tests of uniaxial/biaxial low‐cycle fatigue composed of tension/compression with cyclic torsion were carried out on a biaxial servo‐hydraulic testing machine. Several loading paths were carried out, including proportional and non‐proportional ones, in order to verify the additional hardening caused by different loading paths. The experiments showed that the three materials studied have very different additional hardening behaviour. Generally, the transient process from the initial loading cycle to stabilized loading cycle occurs in a few cycles. The stabilized cyclic stress/strain parameters are controlling parameters for fatigue damage. A factor of non‐proportionality of the loading paths is evaluated based on the Minimum Circumscribed Ellipse approach. It is shown that the microstructure has a great influence on the additional hardening and the hardening effect is dependent on the loading path and also the intensity of the loading.     

Double influence of hydrogen on fatigue crack growth in heat-resistant steels

We investigate fatigue crack growth in cast heat-resistant steel pipes of reforming furnaces in a vacuum, in air, and in gaseous hydrogen in the temperature range 20 – 800°C. It is shown that the character and intensity of hydrogen-induced effects depend on temperature and loading amplitude. For the crack resistance threshold, we discovered the phenomenon of temperature inversion of these effects. Namely, the value of K _th in hydrogen increases with temperature up to 400°C and then decreases. Under high-amplitude loading, the influence of hydrogen manifests itself only in the acceleration of crack growth. The ambiguity in the influence of hydrogen on the plastic strain resistance of the material at the crack tip is analyzed on the basis of well-known physical concepts of the influence of hydrogen on the processes of generation and displacement of dislocations. The effects discovered in this work are explained by the realization of different fracture mechanisms and different types of hydrogen-induced effects under different conditions. Thus, at low temperatures (up to 400°C) and high K, one observes a decrease in the tearing strength; the case of low temperatures and low K is characterized by the shear fracture mechanism and the strengthening effect of hydrogen; for high temperatures ( 400°C) and low K, the shear fracture mechanism is combined with a decrease in the plastic strain resistance under the influence of hydrogen.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No, 4, pp. 7–15, July – August, 1994.     

Effect of carbon concentration on tensile behaviour of pearlitic steels

Abstract

A quantitative relationship between flow stress and microstructure is studied for pearlitic steels incorporating 0.39 - 0.77 wt-% carbon. The distribution of true lamellar spacing (S ₀) is determined. It is found that S ₀ depends on carbon concentration and pearlite transformation temperature accompanying a considerable distribution. The 0.2% proof stress is described as a function of the averaged S ₀ but the influence of the accuracy in S ₀ measurement precludes satisfactory prediction of the 0.2% proof stress. High work hardening corresponds to the generation of phase stress caused by misfit plastic strain between ferrite and cementite. The stress partitioning behaviour between ferrite and cementite is verified by in situ neutron diffraction during tensile deformation.     

Hydrogen permeation behaviour and associated phase transformations in annealed AISI304 stainless steels

The effective diffusion coefficient and subsurface concentration of hydrogen in annealed AISI304 austenitic stainless steels have been measured by the electrochemical permeation method. The effects of different cathodic current densities on the effective diffusion coefficient, hydrogen concentration beneath the cathodic surface and steady state permeation current density have been studied. The value of the effective diffusion coefficient for the permeated specimens increases with increasing charging current density. The hydrogen subsurface concentration and steady state permeation current density first increase with increasing charging current density, then decrease with increasing current density. X-ray diffraction analyses were used to investigate the phase transformation during hydrogen charging. The results revealed that cathodic charging resulted in the formation of a considerable amount of ε and α′ martensites, which will increase with charging time.     

Hydrogen permeability of annealed and deformed carbon steels with a granular cementite structure

Hydrogen permeability of St.40, U8, and U12 steels with granular cementite structures was studied. It was established that polytherms of the rate of hydrogen permeation obtained for annealed specimens of these steels have deflection points at t_cr=400°C. The permeability parameters (E, p₀) of these steels for t < t_cr increase to a lesser extent than in the case of specimens consisting of lamellar pearlite. After plastic deformation followed by annealing at various temperatures, substantial changes in the permeability are also observed but only for t < t_cr; the effect is smaller, however, than that observed for the same steels with a lamellar pearlite structure. The results obtained were attributed to the influence of ferritecarbide phase boundaries whose adsorption power is changed as a result of plastic deformation and subsequent annealing.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 5, No. 5, pp. 588–591, September–October, 1969.     

Hydrogen permeability of annealed and deformed carbon steels with a lamellar pearlite structure

Tests were carried out on specimens of carbonyl iron and St. 20, St. 40, U8, and U12 carbon steels with a lamellar pearlite structure. It was established that the structure sensitivity of hydrogen permeability of steel is relatively low at elevated temperatures (t>t_cr) and extremely high at low temperatures (tcr). As a result, curves representing the temperature dependence of annealed iron and steel specimens have deflection points at t_cr=400° C.Plastic deformation and subsequent annealing also produce substantial changes in hydrogen permeability only at tcr. Substantial differences in the laws of recovery of the rate of hydrogen permeation through iron and steel were observed; they were attributed mainly to the two-phase structure of steel and, especially, to the presence of interphase boundaries between ferrite and cementite.     

Recrystallisation and dilatometric behaviour of low carbon and ultralow carbon steels

Abstract

During continuous heating of a cold rolled low carbon steel, a dilatometric anomaly has been detected before the well known ferrite to austenite transformation. The detailed study of the processes occurring during the heating and comparison with the case of an ultralow carbon steel showed that this anomaly is related to the change in dislocation density during recrystallisation.     

Tensile fatigue behaviour of tightly woven carbon/carbon composites

The tensile fatigue behaviour of a tightly woven carbon/carbon composite was investigated as a function of stress level. Load-controlled fatigue tests were performed in tension-tension mode with a stress ratio, R, of 0.1 under ambient laboratory conditions. Results of composite behaviour are discussed in terms of the relationship of the stress/strain behaviour to the fatigue life of these composites as well as the effects of applied stress levels. It is shown that these composites exhibit good resistance to cyclic loading. No fatigue failures were obtained after 10⁶ cycles when the maximum tensile load in the fatigue cycle is less than or equal to 80% of the static tensile strength. Evidence of textural changes related to fatigue was observed in the matrix region of these composites.     

Mechanism of the effect of hydrogen on fatigue crack propagation in structural steels

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 3, pp. 39–42, May–June, 1988.     

Thermal degradation of pearlitic steels: influence on mechanical properties including fatigue behaviour

With the aim to predict the durability of railway wheels, thermomechanical damage was studied for two steels with different alloying levels of silicon and manganese in the temperature range of 500–725°C. Softening caused by cementite spheroidisation in pearlite leads to changes in the mechanical behaviour and an accompanying decrease in fatigue lifetimes. It was found that higher contents of Si and Mn lead to better resistance to softening of both virgin and plastically deformed material. Correspondingly, the high Si-Mn alloyed steel loses much less in fatigue lifetime than the lower alloyed steel.     

Effect of hardness on multiaxial fatigue behaviour and some simple approximations for steels

Constant-amplitude in-phase and 90° out-of-phase axial-torsional fatigue tests were conducted on tubular specimens made from a medium-carbon steel with three hardness levels obtained from normalizing, quenching and tempering and induction hardening to find the effect of hardness on multiaxial fatigue behaviour. In addition, the same loadings were applied on the normalized solid specimens to investigate the effect of specimen geometry on multiaxial fatigue life. Similar fatigue life variation as a function of hardness was found for in-phase and out-of-phase loadings, with higher ductility beneficial in low-cycle fatigue (LCF) and higher strength beneficial in high-cycle fatigue (HCF). Multiaxial fatigue data were satisfactorily correlated for all hardness levels with the Fatemi–Socie parameter. Furthermore, in order to predict multiaxial fatigue life of steels in the absence of any fatigue data, the Roessle–Fatemi hardness method was used. Multiaxial fatigue lives were predicted fairly accurately using the Fatemi–Socie multiaxial model based on only the hardness level of the material. The applicability of the prediction method based on hardness was also examined for Inconel 718 and a stainless steel under a wide range of loading conditions. The great majority of the observed fatigue lives were found to be in good agreement with predicted lives.     

Mechanical fatigue behavior of chromium and nickel plain carbon steels

The influence of chemical composition on fatigue behavior and fatigue crack growth rates is variegated. Thus, chromium and nickel affect the fatigue limit but have no influence on the mean values of crack growth rates described by the Paris equations. The effect of carbon on the fatigue limit and the propagation of fatigue cracks is utterly different: It affects the exponentm on the right-hand side of the Paris equation, and the higher the content of carbon, the more pronounced its effect. Carbon also affects the dependencesm=f(Tr),m=f(HB),m=f(R), and logC=f(m) by changing the microstructure of the steel and, hence, the average fatigue crack growth rates. If the microstructure of a steel contains a ferrite phase or interlamellar ferrite in pearlite colonies, as observed in 0.2% and 0.4% carbon steels tempered at temperatures of 400°C and 600°C, then the mechanism of crack propagation is mainly connected with the formation of striations through the ferrite phase. In this case, the values ofm lie in the range of 2–4. This conclusion is made on the basis of data presented in [9, 10, 14, 15]. For a martensite or tempered martensite microstructure, as in 0.4% carbon steels as-quenched and tempered at 200°C, the predominant mode of fracture is intergranular separation and void coalescence, and the values ofm lie in the range of 4–6. For intermediate values of K, low-carbon steels (0.2%) are weakly sensitive to the mean stresses. This agrees with results obtained for other materials with a ductile mode of crack propagation. For 0.4% carbon steels tempered at 600°C, the exponentm increases from 3.5 to 5 asR increases from 0.1 to 0.85. Most likely, this is explained by an increased role for intergranular separation.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 62–67, January–February, 1995.