Living with creep damage… outside the creep range

Abstract

The present paper addresses the effects of creep cavitation damage on other mechanical properties – chiefly those that affect behaviour outside the creep range. Such effects seem not to have been systematically studied, yet they are significant for the understanding and prediction of component integrity and life. The paper presents results obtained mainly as byproducts of research programmes on low alloy steels for both fossil and nuclear power plant and seeks to rationalise the findings to generate an overall picture of the effect. It is seen that a simple loss of effective section model is adequate to describe many of the phenomena observed, but that other factors may also need consideration.     

Classic contributions: cryogenic treatment Influence of deep cryogenic treatment on microstructure,mechanical properties and dimensional changes of vacuum heat-treated high-speed steel

Abstract

In this contribution, originally published in 2002, it is shown that the microstructure of AISI M2 high-speed steel can be modified substantially by a vacuum heat treatment combined with a deep cryogenic treatment to optimise the ratio between hardness and fracture toughness. This ratio is affected significantly by the volume fractions of retained austenite and undissolved eutectic carbides, as well as by the mean distance between these carbides. Values calculated using a newly developed semiempirical equation prove that, for given vacuum heat treatment conditions, the volume fraction of retained austenite and the hardness have a significant effect on the fracture toughness. Analysis of vacuum heat-treated modified 'Navy C' ring specimens confirms that the dimensional changes can be controlled. The main factors that influence the dimensional changes are the volume fraction of retained austenite and the steel's hardness.     

Toughness and fatigue behaviour of eutectic and hypereutectic Al–Si–Cu–Mg alloys produced through lost foam and squeeze casting

Abstract

The strength and toughness of four high silicon content Al–Si–Mg–Cu alloys have been studied at room temperature (RT), 200°C and 300°C. The fatigue behaviour has also been investigated. The alloys were produced using two very different processing routes: lost foam and squeeze casting. In the tensile tests, the ductility was low for alloys produced via both routes irrespective of the testing temperature. The strength was similar at RT and 200°C, but at 300°C it fell abruptly. The toughness followed the same trend with testing temperature. Direct observation of fatigue cracks revealed that the brittle silicon and intermetallic particles broke ahead of the crack tip; the fatigue crack advanced by linking the main crack with cracks formed ahead of it. The T6 thermal treatment improved fatigue resistance in the squeeze cast material, especially at high D K values.     

Study of Ni3 Al–Cr7 C3 coating fabricated by self-propagating high temperature synthesis casting route

Abstract

A coating on carbon steel was fabricated by a self-propagating high temperature synthesis (SHS) casting route. The process is described in detail. The phases in the coating were examined using X-ray diffraction (XRD). The microstructures of the coating and the substrate were analysed using electron probe microanalysis (EPMA) and optical microscopy, respectively. The wear behaviour and microhardness of the coating were determined. The experimental results show that the coating was composed of Ni₃ Al and Cr₇ C₃ phases and had a dense microstructure. Metallurgical bonding had formed between the coating and the substrate. The grain size of the substrate near the interface was coarsened. The wear resistance of the coating is better than that of a bearing steel or a Ni₃ Al coating. The microhardness of the coating is higher than that of a Ni₃ Al coating or the substrate.     

Quasi-static fracture toughness testing of a single CT specimen at two test conditions

Abstract

It is proposed that a single CT specimen can be used for determining J ₀.₂ at two testing conditions, provided it can be ensured that the crack tip plastic zones for the two tests do not interfere. This is achieved by extending the crack at the end of the first fracture test by fatigue cycling at ambient temperature to obtain the starting crack for the second test. This method has been validated by testing thermally aged CT specimens of modified 9Cr - 1Mo steel at 653 K and 803 K. The J-Δa values obtained by a multispecimen method at a specific temperature were on a single curve irrespective of whether the data were generated from the first test or second test on that sample. Also, the J-Δa curves obtained using a single specimen normalisation method from data on first and second tests were within the expected specimen to specimen variation.     

Effects of dendritic arm coarsening on macroscopic modelling of solidification of binary alloys

Abstract

A pure macroscopic two-dimensional numerical model has been developed, capable of capturing the effects of dendritic arm coarsening on the transport phenomena occurring during a binary alloy solidification process. The general continuum conservation equations are aptly modified to take into account shrinkage induced fluid flow. Simultaneously, the effective permeability of the mushy zone is numerically modelled according to the microscopic coarsening kinetics. Moreover, a new nodal latent heat updating algorithm is proposed that takes into account dendritic arm coarsening considerations. The numerical results are first tested against experimental results reported in the literature, corresponding to the solidification of an Al-Cu alloy in a bottom cooled cavity. It is concluded that dendritic arm coarsening leads to an increased effective permeability of the mushy region as well as an enhanced eutectic fraction of the solidified ingot. Consequently, an enhanced macrosegregation is predicted, compared with that dictated by shrinkage induced fluid flow alone. Physical insights are also developed regarding the effects of various parameters on the overall macrosegregation.     

Fracture toughness of structural aluminium alloy thick plate joints by friction stir welding

Abstract

The fracture toughness in a friction stir welded joint of thick plates of structural aluminium alloy type A5083-O is investigated. A joint between two 25 mm thick plates is fabricated by one sided, one pass friction stir welding. The Charpy impact energy and critical crack tip opening displacement (CTOD) in the friction stir weld are much higher than those in the base metal or heat affected zone, whereas mechanical properties such as stress–strain curve and Vickers hardness are not conspicuously different. The effects of the microstructure on crack initiation and propagation are studied in order to clarify the difference in fracture toughness between the stir zone and base metal. The analyses of the fracture resistance curves and the diameters of dimples in the fracture surface after both tensile and bending tests show that the fine grained microstructure in the stir zone helps to increase ductile crack initiation and propagation resistance. It is found that the high fracture toughness value in the stir zone is affected by the fine grained microstructure in friction stir welds.     

Fracture toughness of ceramic moulds for investment casting with ice patterns

Abstract

Ice patterns can be used to make ceramic investment moulds for metal castings. Owing to the characteristics of ice, the ceramic mould must be made at subzero temperatures and consequently, requires a different formulation than shells built at room temperature. Success of this process depends greatly on the fracture toughness of mould materials. The present paper describes the experimental results of fracture toughness of mould materials processed from different compositions. The Taguchi method was used to reduce the trial runs. The parameters considered included the ratio of fibre containing fused silica and aluminosilicate powders, the volume of binder and the volume of catalyst. The microstructure and green fracture surface of test bars were also examined to understand the underlying mechanism. While conducting the four point bend test on ceramic mould samples, some samples had exceedingly low strengths appearing as outliers in the Weibull analysis. Examination of these low strength ceramic samples improved understanding of failure of mould materials. Sound moulds have been made for the investment casting process with ice patterns based on the analysis of experimental results. The casting of an M8 bolt is used to demonstrate that metal castings of complex geometry can be fabricated using ice patterns. The measured tolerances are within the required tolerance range.     

Improvement in fracture toughness of austempered ductile iron by two-step austempering process

Abstract

Ductile cast iron samples were austenitised at 900°C and subjected to two types of austempering called as conventional austempering and two-step austempering. Five different temperatures, 280, 300, 320, 350, 380 and 400°C, with an austempering time of 2 h, were chosen for conventional austempering. For two-step austempering process, the first step temperatures were 280, 300 and 320°C. The samples were austempered at each of these temperatures for different times, i.e. 10, 20, 30, 45 and 60 min, and then upquenched to higher temperature of 400°C for 2 h. Fracture toughness and tensile studies were carried out under all these austempering conditions. During conventional austempering, the fracture toughness initially increased with increasing austempering temperature, reached a peak value of 63 MPa m^1/2 and dropped with further increase in temperature. During the two-step austempering, fracture toughness was found to increase with increasing first step time. The curve shifted to higher values of fracture toughness as the first step temperature was decreased and the maximum value of 78 MPa m^1/2 was obtained. The results of the fracture toughness study and the fractographic examination were correlated with microstructural features such as bainitic morphology, the volume fraction of retained austenite, and its carbon content. Ferrite lath size and stability of the retained austenite were found to influence the fracture toughness.     

Microstructure and properties of semisolid hypereutectic high chromium cast iron prepared by slope cooling body method

Abstract

To improve the toughness of hypereutectic high chromium cast iron, the morphology of the primary carbides were improved by the slope cooling body method, a semisolid forming process. The semisolid specimens were prepared under different forming techniques, and the microstructure, impact toughness and wear resistance were studied. The results indicate that the morphology of primary carbide is improved, impact toughness initially increases then remains constant as the semisolid forming temperature increases, impact toughness was improved, but wear resistance was reduced.