Key Engineering Materials Vols. 297-300

Abstract: This study was focused on the effects of pre-strain on the torsional fatigue properties of three kinds of medium carbon steels, including the fatigue strength, surface hardness, microstructure, and the crack initiation and propagation behaviors. The effect of pre-strain on the non-propagating cracks was also discussed. The main results obtained in this test are as follows: 1) the fatigue limits increase with the increasing of tensile pre-strain ratio for all kinds of the test materials; 2) under certain stress amplitude, with the increasing of tensile pre-strain ratio, the fatigue crack initiates a little earlier and propagates faster; 3) the length of non-propagating crack decreases with the increasing of tensile pre-strain ratio.

Abstract: The influence of hydrogen content on the microstructure and the tensile property of Ti-6Al-4V alloy was studied, and the phenomenon of minimum yield stress at certain hydrogen content was discussed. The results show that Ti-6Al-4V alloy can absorb hydrogen above 600°C and the different hydrogen contents can be achieved by changing the flow rate of hydrogen. With increase of hydrogen contents, the microstructure gradually transforms from the original near basket-type to the α clusters which consist of α plates and hydrides distributed in α plates, and then to the mixture of α, β and the large amount of hydrides. When the specimens tensioned at 600°C, their strength first decreases and then increases, but their ductility changes quite the contrary as increasing hydrogen contents. There is optimum hydrogen content at which the strength is the lowest and the plasticity is the highest for the specimens tensioned at 600°C. Ti-6Al-4V alloy may gain the higher tensile strength or better ductility at 600°C through appropriate hydrogenation treatment in comparison with samples untreated. With increase of hydrogen contents, the fracture type transforms from microvoid coalescence type to “cleavage like” type for specimens tensioned at 600°C.

Abstract: The application of regular blanking for brass under extra-low cycle rotating bending fatigue was studied. The experiments of rotating bending fatigue have been made in special equipment designed by us. Several problems, the relations of the cycle times of fracture, the strain amplitude near the tip of notch and fracture toughness of fatigue to the presetting deflection, the effects of depth and tip radius of notch to the cycle times of fracture, were discussed through the experiments. The formulae of stress amplitude at the tip of notch and fracture toughness of brass to presetting deflection were obtained. The suitable parameters of fracture in rotating bending fatigue for blanking of brass under extra-low cycle times have been proposed.

Abstract: A thermomechanical fatigue (TMF) life prediction model for ferritic stainless steel, used in exhaust manifold of automobile, was developed based on Tomkins’ two-dimensional crack propagation model. Low-cycle fatigue (LCF) and TMF tests were carried out in a wide temperature range from 200 to 650°C. New concept of plastic strain range on TMF was proposed. Effective stress concept was introduced to get a reasonable stress range in TMF hysteresis loop. The proposed model predicted TMF life within 2X scatter band. The experimental results reveal that TMF life is about 10% of isothermal fatigue life.

Abstract: The present study has investigated plasticity-induced martensitic phase transformation in fatigue of unnotched SUS304 plates. Martensitic phase transformation occurred in uunotched SUS304 plate specimens fatigued at room temperature in air. Volume fraction Va’ of a’ martensite in the uunotched portion of fatigued specimens was measured by ferrite scope. The relations between the maximum value of Va’, Va’max, and the number of load cycles N were represented by reverse sigmoidal curves for all the applied stress range Ds levels tested in this study. For the most portion of fatigue life, the value of Va’max remained almost constant. This value was increased with increase in the value of Ds. The spatial distribution of Va’ in the specimens varied with N: i.e., continued cycling of stress made a’ transformation localized near the central portion of specimens where the Va’ value reached as high as 35-40%. This value is more than doubled compared to the highest Va’ value found in the tensile tests of SUS304 at room temperature in air. Invisible cracks of 200µm in length were found in the high Va’ value region. These results imply that the measurement of Va’ in fatigued SUS304 components may detect crack initiation sites and may predict residual fatigue life.

Abstract: Averaging the anisotropy of each crystal, the macroscopic behavior of polycrystalline materials is isotropic and homogenous in terms of elastic deformation. However, the anisotropic property of each crystal influences on the local stress field ahead of a crack tip if the crack size is not large enough in comparison with the grain diameter. This brings about the change in the crack driving force (CDF) such as stress intensity factors. In the present study, in order to investigate the cause and magnitude of the change in the CDF, the finite element analysis is performed. The calculations are carried out for a single crystal model, a bi-crystal model, and a polycrystal model containing a transgranular or an intergranular semi-circular crack. The results implied that the magnitude of CDF is dependent not only on the crystal orientation but also on the deformation-constraint caused by the difference in elastic modulus of grains near the crack tip. The statistical scatter of CDF due to the random crystal orientation in a polycrystal is examined by a Monte Carlo simulation. The variation in the SIF becomes small as the crack size increases.

Abstract: In this study, both mechanical and electrical fatigue properties of electrorheological (ER) materials whose global characteristics can be controlled by an external electric field are experimentally evaluated. In order to investigate the mechanical fatigue property, a linear reciprocating apparatus is devised and operated by the hydraulic unit. Two important characteristics of methylcellulose based ER material: the field-dependent yield stress and current density are investigated as a function of the operating cycle. The electrical fatigue property is investigated by applying high voltage to the ER material domain through the electrode gap. The voltage is imposed in on-off manner for the specific cycles by changing the field intensity. The yield stress and current density of the ER material are evaluated at each specified cycle and surface roughness of the electrode is observed as well.

Abstract: 1.4 %C ultra high carbon steel (UHCS) was prepared in order to study the structure of martensite transformation and mechanical properties. Ultra-fine spherical carbide and ultra-fine austenite grain size were obtained. A great deal of lath martensite was observed after quenching. The phenomenon does not agree with the traditional knowledge that the lath martensite would disappear when carbon content is in excess of 0.8% in austenite. The strength, fatigue properties and fracture toughness have been measured. A good combination of strength, toughness and fatigue properties come from fine and uniform distributed carbide particles and ultra-fine austenite grain size. Fracture strength increases by 48%, yield strength increases by 15% and plasticity keep the same comparing with that of hardened and tempered 40CrNiMo. The carbon content of ultrahigh carbon steels (UHCS) is in the range of 1.0-2.1% [1, 2]. Traditional heat treatments for normal steels will cause the microstructure of UHCS to be coarse and do not produce optimal properties. With controlled rolling and special heat treatment, UHCS can be in ferrite, pearlite, bainnite or martensite structures, which all have different mechanical properties. The yield stress of a 1.8%C, 1.6%Al ferrite UHCS can reach 1500MPa, which is much higher than that of high strength and plain alloy steels [3]. The tensile strength of a 1.25%C-1.5%Cr pearlite UHCS can reach 1810Mpa and its elongation can be 18%. When it is treated into martensite, its compression strength reached to 4690Mpa and compression strain reached to 26% [1, 4], which is comparable to WC-12Co. Such good mechanical properties can be ascribed to the ultra fine grain sizes because of the undissolved carbide particles which resist growth of austenite grain during heating. Another reason could be the lath martensite structures. O.D.Sherby [4] had reported that there was a lot of lath martensite in quenched UHCS. The UHCS was considered not only as tool steels but also as good structure materials. Fracture and fatigue properties are important for structure materials. However, they have rarely been studied. The present paper is going to study the martensite structure and mechanical properties of a prepared 1.4% C UHCS.

Abstract: According to the non-equilibrium grain-boundary segregation (NGS) kinetics curve of phosphorus, a series of Charpy impact tests was performed in an industrial steel 12Cr1MoV, at solution temperature 1050oC, with isothermal holding temperature 540oC, on specimens of different holding time (at the segregation process, the critical time, and the desegregation process respectively). The DBTT values were measured and a temper embrittlement kinetics curve was given. The reverse temper embrittlement NGS mechanism for steel 12Cr1MoV was proposed.