A model based creep equation for 9Cr–1Mo–0·2V (P91 type) steel

Abstract

The isothermal constant stress creep tests data for a 9Cr–1Mo–0·2V (P91 type) steel were submitted for a phenomenological analysis in order to obtain the relevant creep equation for such steel. Namely, the minimum creep strain rate of P91 type steel cannot be described by the simple Arrhenius type power law constitutive model. The incorporation of the threshold stress concept in the analysis of creep data leads to a modified power law, which satisfactorily describes the creep behaviour of the examined P91 steel. However, the threshold stress is not a good material parameter, as it often varies with temperature and/or applied stress. This adds uncertainty to the extrapolation of the creep rates into ranges where experimental data are not available. Besides the fact that the physical foundation for a threshold stress is questionable from a scientific point of view, this is a serious practical limitation of the modified power law creep equation. The second creep equation proposed in the present paper is the improved stress dependent energy barrier model. The improvement of the standard model is based on two assumptions: first, on the hypothesis that the application of a stress also affects the energy barrier to be overcome when a local region transitions from the initial to the final state, and second, by applying a simple power function of stress instead of a hyperbolic sin function in the model based equation. The obtained value of stress exponent, n=5·5, is too high for entirely climb controlled creep. The apparent activation energy of approximately 510 to 545 kJ mol^?1, which is considerably higher than the activation energy for lattice diffusion, is the stress dependent activation energy of the slowest, dominant rate controlling process of the supposed multiple creep mechanisms.     

Effects of hot deformation and accelerated cooling on microstructural evolution of low carbon microalloyed steels

Abstract

Using a Gleeble 1500 hot simulator, the effects of hot deformation parameters and accelerated cooling conditions on the microstructural characteristics of low carbon microalloyed steels were investigated by means of compression tests. It was found that the grain refinement effect of single pass reduction in the recrystallisation or unrecrystallisation temperature ranges is weaker than that of two pass reduction in the recrystallisation and unrecrystallisation temperature ranges. However, four pass deformation in the recrystallisation and unrecrystallisation temperature ranges could result in rather fine grained microstructures and, when coupled with moderately high cooling rate, partially acicular ferrite microstructure could be obtained. With the increase of cooling rate, the microstructure becomes finer and the content of acicular ferrite increases. Under similar deformation and cooling conditions, the specimens with relatively high carbon content have more refined microstructures.     

Microstructure and properties of carbide free bainite railway wheels produced by programmed quenching

Abstract

A kind of carbide free bainite steel was used to produce railway wheels. The alloy design, manufacture process, microstructure and behaviours are summarised. The novel steel is distinctively superior in excellent combination of strength and toughness compared with the traditional pearlite type wheel steels. The details of the microstructure of novel steel have been investigated in nanoscale.     

Fracture toughness testing for advanced technical ceramics: internationally agreed good practice

Abstract

The development of robust methods for determining fracture toughness and for interlaboratory assessment of results is discussed with respect to procedural standardisation. Acceptable methods are based on meeting several criteria, including ease and cost-effectiveness of testpiece manufacture, the method of introducing a sharp precrack and the acceptability of appropriate fracture toughness analysis. The four simplest methods meeting these criteria are reviewed in detail, together with information on their reproducibility gathered during international round robin exercises. Not all these methods are effective on all materials, and users need to be aware of their limitations. Details of calibration equations and some experimental nuances are given.     

Nickel or cobalt doped zinc oxide varistors

Abstract

Zinc oxide containing either Ni or Co dopants in various proportions has been investigated. Only zincite phase was found by XRD analysis. Incorporation of Ni or Co into the ZnO structure resulted in a marked shift in the XRD peaks. Remarkable microstructural uniformity was obtained by the addition of 1 mol-% of either Ni or Co oxides. Densification was achieved through atomic diffusion along grain boundaries. All samples showed non-linear I–V dependence. Ni concentration had a non-monotonic effect on breakdown voltage and the non-linear coefficient decreased with increasing Ni content. In contrast, breakdown voltage and non-linear coefficient increased with increasing CoO content.     

A Test for Liability to Electrostatic Charging—Reply

An investigation is described in which exhaustion exceeded 99.7 % when triphenyltin chloride was applied to wool in a hot dyebath. The associated residues were readily decomposed by persulphate, which indicated that it should be possible to mothproof wool with triphenyltin chloride without risk of contaminating the aquatic environment. Only small amounts of the triphenyltin were extracted from treated wool by human perspiration or saliva.     

Two body type abrasion wear behaviour in hypoeutectic 16%Cr cast irons with Mo

Abstract

Hypoeutectic 16%Cr cast irons, both Mo free and 1–3%Mo containing specimens were prepared to investigate their abrasion wear behaviour. Annealed specimens were hardened at 1323 K and then tempered at three temperatures from 673 to 873 K for 7·2 ks, the temperature giving the maximum hardness (H_Tmax) and the lower and higher temperature, (L-H_Tmax, H-H_Tmax). The abrasion wear behaviour was investigated using a two body type Suga abrasion wear tester. A linear relation was obtained between wear loss and wear distance. The highest wear resistance or the lowest wear rate (R _W) was obtained in H _Tmax specimens except for the Mo free specimen. The lowest wear resistance or the highest R _W was obtained in H-H_Tmax specimens. The R _W was decreased with an increase in macrohardness. The lowest R _W appeared around 25% retained austenite (V _γ ). The R _W was decreased with an increase of Mo content, and the V _γ value at the minimum R _W shifted to the high V _γ side.     

Comparative studies on wear behaviour between as cast AZ91 and Mg97Zn1Y2 magnesium alloys

Abstract

The wear behaviour of as cast magnesium alloys, Mg₉₇Zn₁Y₂ and AZ91, was investigated under dry conditions in load ranges of 20–380 and 20–240 N respectively, using a pin on disc wear testing machine. The microstructure, thermal stability and elevated temperature tensile properties were characterised by optical microscopy, X-ray diffraction, differential thermal analysis and tensile testing respectively. The wear behaviour can be divided into three successive phases in terms of surface temperature induced by frictional heat, i.e. ambient temperature to eutectic temperature or precipitate dissolution temperature, eutectic temperature or precipitate dissolution temperature to the liquidus temperature and above the liquidus temperature. The Mg₉₇Zn₁Y₂ alloy exhibited good wear resistance compared with the AZ91 alloy for applied loads in excess of 80 N, which has been explained in terms of thermal stability of the intermetallic phase and elevated temperature mechanical properties of the two materials tested, by surface temperature analysis and subsurface observation.     

Effect of solution annealing on weldability of aged alloy 800/25Cr–35Ni steel dissimilar welds

Abstract

Dissimilar welding of the aged alloy 800 and the as cast 25Cr–35Ni (wt-%) heat resistant steel was investigated. Microstructures, mechanical properties and weldability of the dissimilar welds were characterised using optical microscopy, scanning electron microscopy and transition electron microscopy equipped by energy dispersive X-ray spectroscopy, and Varestraint test. Since such dissimilar welding was susceptible to crack formation in the heat affected zone of the aged part, the effects of a preweld solution annealing, heat input, interpass temperature and type of filler metal on the weldability of two alloys were investigated. It was found that during the solution treatment, the precipitates produced in the service stage were decomposed and that TiC was formed. In addition, tensile strength and hardness were reduced, but ductility and toughness increased. It was concluded that the most important step to improve weldability and to reduce cracking susceptibility was solution annealing. A suitable annealing treatment was then proposed. The best weldability was found under conditions of solution annealing, low heat input, low interpass temperature and using Inconel 82 or 617 for filler metals.