Study of the effect of microstructural constituents on fatigue crack propagation of high-performance PM steels

Cette recherche est principalement planifiée pour améliorer les propriétés en fatigue des aciers MP afin qu'ils remplacent largement leurs contreparties corroyées, principalement en raison de leurs avantages en lien avec leur coût de production. L’atteinte de cet objectif est fait en déterminant la microstructure la plus efficace des aciers MP au cours de chargements cycliques. La microstructure la plus efficace est celle qui permet de réduire ou d'arrêter la propagation de fissures de fatigue par la combinaison appropriée de ses phases constitutives. Il existe quelques travaux sur ce sujet, cependant, ils n'ont pas atteint de conclusions cohérentes en raison du manque de données suffisantes et / ou une comparaison inappropriée. Par conséquent, l'effet de différentes phases constitutives d'une microstructure hétérogène d'un acier MP est encore ambigu et inconnu. Afin d'élucider cette question, il convient d'étudier différentes phases microstructurales et le comportement de propagation des fissures de fatigue dans celles-ci. Notre étude commence par choisir deux aciers MP communs fabriqués à l'aide de deux techniques d'alliage soient : pré-mélangeage et pré-alliage, afin de produire respectivement des microstructures hétérogènes et homogènes. Deux types de traitements thermiques ont également été utilisés pour fournir différentes phases microstructurales qui sont nécessaires aux fins de cette étude. Les échantillons ont ensuite été testés en utilisant la charge cyclique et quatre ratios de contraintes pour étudier l'effet des conditions de fatigue. Une analyse quantitative des surfaces de rupture, qui comprend l'étude détaillée du cheminement des fissures en OM et en SEM a ensuite été effectuée sur les échantillons fracturés. Les données de vitesse de propagation de fissures de fatigue dans différents aciers MP ayant des microstructures différentes avec les données quantitatives acquises quant à leur parcours préférentiel nous ont amené à des résultats intéressants sur l'effet des constituants de microstructure sur le comportement de propagation de fissure de fatigue. Il s'est avéré que la fissure de fatigue se propage plus rapidement à travers la phase la moins résistante parmi celles présentes dans la microstructure. Ainsi, la perlite n'était pas favorable à la propagation des fissures en compagnie de ferrite riche en Ni, alors qu’en présence de martensite, on a trouvé que les grains perlitiques étaient le chemin de fissure préféré. De plus, l'austénite résiduelle, qui a été identifiée dans la littérature comme étant une phase bénéfique pour le retard de fissure de fatigue, s'est révélée inefficace. Bien que les fissures de fatigue contournent ces régions, la déformation causée par ce changement de chemin de fissure n'a pas montré d'effet positif sur le retard de fissure de fatigue.This research is mostly planned to enhance the fatigue properties of PM steels inasmuch as they are extensively replacing their equivalent wrought steels due mostly to production cost benefits. This goal is going to be achieved through determining the most effective microstructure of PM steels in cyclic loadings. The most effective microstructure is the one that can reduce or stop the fatigue crack propagation through the proper combination of its constituent phases. There exists some researches on this topic, however, they did not reach consistent conclusions due to the lack of sufficient data and/or improper comparison. Therefore, the effect of different constituent phases of a heterogeneous microstructure of a PM steel is still ambiguous and unknown. In order to study this issue, diverse microstructures and the fatigue crack propagation behaviour through them should be studied. Our research begins by choosing two common PM steels manufactured using two alloying techniques of admixed and pre-alloyed to produce heterogeneous and homogeneous microstructures respectively. Two types of heat-treatments namely sinter-hardening and oil-quenching were also utilized to provide more microstructural phases that is needed for the purpose of this study. The samples were then tested in cycling loading using different R-ratio in order to study the effect of fatigue conditions as well. Quantitative analysis of the fracture surfaces, which includes the detailed study of the crack path in OM and SEM, were then performed on the fractured samples. The fatigue crack growth rate data in different PM steels having different microstructures along with the quantitative data acquired from their crack path led us to interesting results on the effect of microstructural constituents on fatigue crack propagation behaviour. It was found that the fatigue crack will propagate more rapidly through the weakest i.e. lowest strength phase among the ones present in the microstructure. Thus, pearlite was not favourable for crack propagation in the company of Ni-rich ferrite while in the presence of martensite, pearlitic grains were found to be the preferred crack path. Moreover, the retained austenite, which was identified in literature to be a beneficial phase for fatigue crack retardation, was found to be ineffective on the matter. Although fatigue cracks circumvented these regions, the deflection caused by this change of crack path did not show any positive effect on fatigue crack retardation

Understanding the fatigue notch sensitivity of high-strength steels through fracture toughness

This study presents an innovative approach for selecting high-strength materials for fatigue dimensioning parts, considering both fracture toughness and fatigue performance. Warm and hot forming processes enable the construction of high-strength parts above 1000 MPa with complex geometries, making them suitable for lightweight chassis in automotive and freight applications. This research reveals that high-strength steels can experience up to a 40% reduction in fatigue performance due to manufacturing defects introduced during punching and trimming. Fracture toughness has been proposed as a good indicator of notch sensitivity, with a strong correlation of 0.83 between fracture toughness and fatigue notch sensitivity. Therefore, by combining fracture toughness measurements and fatigue resistance obtained through the rapid fatigue test, it becomes possible to quickly identify the most fatigue-resistant materials to deal with defects. Among the nine materials analysed, warm-formed steels show promising characteristics for lightweight chassis construction, with high fatigue resistance and fracture toughness exceeding the proposed fracture threshold of 250 kJ/m2.Peer ReviewedPostprint (published version

Substitute alloys as bearing materials

MODERN engineering depends in many instances upon bearings, components which allow relative motion to occur between the members of a mechanism. Bearings may take many forms but the most widely used types are plain and rolling bearings.The continuous demand for better power-to-weight ratios for increased mech-anical efficiency leads to a constant demand for bea- ring materials of improved strength to operate at increased speeds under heavier loads at higher temp-eratures.Whilst the materials of construction should be as cheap as possible, an important consideration in developing countries, particularly India, is the availability of the raw materials, thus the metall- urgist must not only search for improved bearing materials but also for suitable substitute indigenous materials

Surface treatments for nickel and nickel-base alloys

Surface treatments of nickel and nickel alloys by diffusion coating, electroplating, explosive hardening, peening, and other method

Effect of Pre-strain and Pre-corrosion on Ratcheting Behavior of ASTM A668 Class D Steel

Ratcheting can be considered as one of the serious problems as this phenomenon causes premature failure of structural components. Ratcheting deformation can be regarded as the stress controlled low cycle fatigue (LCF) response of a material which occurs due to accumulation of plastic strain during asymmetric cyclic loading. Ratcheting is dependant on various parameters like material chemistry, stress states and environmental issues. The aim of this investigation was to study the effect of pre-strain (0%, 2%, 4% and 8%) on ratcheting in different heat treated conditions (normalized and hardened-tempered). The effect of pre-corrosion (7, 14 and 21 days in 3.5% NaCl solution) on the nature and extent of strain accumulation was also studied. Ratcheting tests were carried out at room temperature on cylindrical specimens having 12.5 mm gauge length and 6 mm gauge diameter. The results include reduced strain accumulation with increasing pre-strain strain level due to work hardening of the pre-strained samples. Further cyclic hardening takes place during ratcheting deformation. Pre-corroded samples showed more amount of strain accumulation compared to the samples without corrosion. Post ratcheting tensile strength increases in all the tested samples owing to cyclic hardening during ratcheting. The factographic studies indicated typical dimple features

Fatigue of a spring steel with varying levels of non-metallic inclusions

Plain specimens of two batches of the commercial spring steel BS 251A58 of nominally identical processing but significantly differing levels of non-metallic inclusion levels, have been tested in rotating bending. The data produced has been analysed against some recent methods for the prediction of fatigue properties in high strength materials containing defects. The materials tested were specifically selected for their disparate cleanliness levels, yielding specimens with differing inclusion distribution and maxima. The morphology of critical inclusions is identical between the two material batches. Material microstructure is tempered martensite with ultimate tensile strength of approximately 2000 MPa, which renders it well above the strength level where sensitivity to defects causes variability in fatigue behaviour. Models have been selected from the literature for the prediction of fatigue limit using characterisation of the local microstructural state and the size and critical position of non-metallic inclusions. These models have been validated by the analysis of specific failures after fractographic analysis. It has been shown that these models are acceptably accurate and generally conservative. Difficulties in experimental work have precluded the planned measurement of crack growth rates during the current test work. These difficulties have yielded a superimposed mean stress to the rotating bend test. This mean stress has been quantified for each test and the result coupled with a parameter for mean stress correction. The validity of the mean stress correction has been proven in this work to be valid. More consistent results are observed for the mean stress corrected data. A statistical method for the prediction of maximum non-metallic inclusion size for a given number of specimens or components from small sample microsection analyses has yielded good results when compared to the fractographic observations. This work has investigated the effect of varying magnification level and number of fields surveyed on the accuracy of prediction and recommendations are made for the method for obtaining best accuracy. A 'unified' crack propagation life model from the literature has been applied which combines long and short crack growth regimes. The model has shown good correlation to the current data but only after fitting of constants and only within the low cycle regime. Relationships presented in the literature between constants and the material ultimate tensile strength were found to be inapplicable to the current material at this strength level

キョウセキコウ ノ ジツヨウ セイノウ コウジョウ ニカンスル ケンキュウ

The present research consists of several studies that assess the effect of pre-strain on fatigue properties of eutectoid steel and to evaluate ways to improve the practical　performance of eutectoid steel by cold rolling and ion-nitriding methods. The ult

Fatigue life of machined components

A correlation between machining process and fatigue strength of machined components clearly exists. However, a complete picture of the knowledge on this is not readily available for practical applications. This study addresses this issue by investigating the effects of machining methods on fatigue life of commonly used materials, such as titanium alloys, steel, aluminium alloys and nickel alloys from previous literature. Effects of turning, milling, grinding and different non-conventional machining processes on fatigue strength of above-mentioned materials have been investigated in detail with correlated information. It is found that the effect of materials is not significant except steel in which phase change causes volume expansion, resulting in compressive/tensile residual stresses based on the amounts of white layers. It is very complex to identify the influence of surface roughness on the fatigue strength of machined components in the presence of residual stresses. The polishing process improves the surface roughness, but removes the surface layers that contain compressive residual stresses to decrease the fatigue strength of polished specimens. The compressive and tensile residual stresses improve and reduce fatigue strength, respectively. Grinding process induces tensile residual stresses on the machined surfaces due to high temperature generation. On the other hand, milling and turning processes induce compressive residual stresses. High temperature non-conventional machining generates a network of micro-cracks on the surfaces in addition to tensile residual stresses to subsequently reduce fatigue strength of machined components. Embedded grits of abrasive water jet machining degrade the fatigue performance of components machined by this method