Creep crack growth characterization of Type 316 stainless steel using miniature specimens

Remaining life estimation of components which experience elevated-temperature service requires knowledge of creep-crack-growth characteristics of the service-exposed material. In most cases only a small amount of material from in-service components is available for testing; thus, there is a need to design and benchmark test methods that use miniature creep-crack-growth specimens. In order to address this problem, a creep-crack-growth test methodology was developed using miniature single edge-notched tension specimens. The material used in this work was from a well characterized heat of Type 316 stainless steel, for which creep-crack growth data were previously obtained using conventional compact-tension type and center-cracked-tension type specimens. Good correlation was obtained between the crack growth rate and C_j, where C_j is the experimental counterpart of C^*. The details of the analytic procedure for miniature specimens, particularly for small crack lengths, are presented. The current results agreed very well with those for large specimens as well as with creep-crack growth data obtained by other investigators.     

Strengthening of threaded connections of titanium and high-temperature nickel-chromium alloys by thermovibration treatment

The article presents results of a study of the strengthening of threaded connections M6 and M8 of alloys ÉI698, VT16, and VTZ-1 by thermovibration treatment to raise their creep resistance. Treatment regimes and empirical formulas for determining the index of strengthening (for the ÉI698 alloy) are discussed. Data on long-term strength of threaded connections in initial and post-treatment states, and on the effect of aging on retention of the strengthening effect, are presented along with some mechanical and fatigue characteristics.Translated from Problemy Prochnosti, No. 9, pp. 98–103, September, 1990.     

Creep of low-activated Fe-12Cr-20Mn-1W austenitic corrosion-resisting steel

Conclusions 1. Creep resistance of the Fe-12Cr-20Mn-1W steel, containing 0.1-0.26 wt. % C and also small additions of Ti, B, and P in the range 823–973 K is similar to the creep resistance of the Fe-Cr-NI steels of the 304 type, 800 alloy, and also the Fe-Cr-Mn-Ni steel of the ÉP838 type.2. An increase of the carbon content in the range 0.1–0.26 wt. % in the Fe-12Cr-20Mn-1W alloys slightly reduces the minimum creep rate 873 K and does not effect this parameter at 973 K.3. The efficiency of the effect of carbon on the minimum creep rate of the Fe-12Cr-20Mn-1W steels is evidently lower than the 800 alloy.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 2, pp. 81–85, March–April, 1993.     

Method of predicting endurance of structural alloys on large load bases from data of high-frequency tests

The article suggests a method of determining the parameters of the equation of the fatigue curve taking into account the effect of the frequency and asymmetry of the load cycles. The dependences of the parameters of the equation on the coefficient of asymmetry of the load cycles are established. The article demonstrates the efficiency of the suggested method of predicting endurance of smooth specimens and of specimens with stress raisers on larger bases of low-frequency loading from the results of high-frequency tests by comparing a large amount of experimental investigations of the fatigue limit of the alloys AMg6N, ÉP202, and VNS-25 with calculated data.Translated from Problemy Prochnosti, No. 6, pp. 14–20, June, 1990.     

Fatigue-Crack Propagation in Heat-Resistant Alloys under Thermomechanical Loading. Part 2. A Method of Accounting for the Influence of Thermomechanical Loading on the Material Resistance to Fatigue-Crack Growth

A method is proposed for a comprehensive consideration of the influence of thermomechanical-loading components on the variation of the fatigue-crack propagation rate. The method takes into account the variability of the values of the calculated stress intensity factors over the crack front and with the time of a thermal cycle. Experimental results obtained for alloys KhN70VMTYu (Eacute;I617) and KhN73MBTYu (Eacute;I698) are described. For two regimes of thermal cycling, near-threshold and medium sections of kinetic diagrams of fracture of these alloys are constructed.     

Fatigue-Crack Propagation in Heat-Resistant Alloys under Thermomechanical Loading. Part 1. Experimental Method and Results of the Investigation of Crack-Propagation Rates

We propose a calculation and experimental method for studying the influence of cyclic thermomechanical loading, wherein the object under investigation is subjected to the simultaneous action of a cyclically varying mechanical load and temperature varying with time and resulting in the occurrence of thermal stresses, on fatigue-crack propagation in specimens of triangular cross section. We present results of the calculation of the stress-intensity factors in elastic formulation for an angular mode-I crack in the case of loading by pure bending and temperature varying with time. We studied crack-growth resistance of alloys KhN70VMTYu (Eacute;I617) and KhN73MBTYu (Eacute;I698) at a constant temperature and under thermomechanical loading. Kinetic fracture diagrams da/dN vs K_I have been constructed for the alloys at a constant temperature and for two regimes of thermal cycling within a near-threshold and mean-amplitude portions of the curve da/dN vs K_I .     

A Finite Element Analysis of Creep-Crack Growth in Viscoelastic Media

In this paper, the effects of viscoelastic characteristics, on the creep-crack growth process are studied through a finite element approach. The general approach of an independent path integral is extended to crack propagation. Afterwards, fracture parameters are computed through a coupling process with an incremental viscoelastic formulation. Finally, numerical examples are presented in order to demonstrate the independence of the integration domain and the possibility of evaluating fracture characteristics which can be energetic (energy release rate) and local in the vicinity of the crack tip (stress and crack opening intensity factors).     

The microstructure of unidirectionally solidified Ag-Ge eutectic alloys

Ag-Ge alloys containing 15 to 22 wt% Ge were unidirectionally solidified to investigate the growth conditions for fully eutectic growth (the coupled region) in the range of growth rate 1.4 x 10^–4 to 1.1 cm sec^–1 and at a temperature gradient of 200° C cm^–1. Primary silver was not formed in the hypereutectic Ag-Ge alloys, implying that the coupled region of Ag-Ge alloys may be different from that of the other nf-f alloy systems such as Al-Si, Fe-C, Al-Ge and Al-Fe, whose coupled regions are usually skewed towards the faceted component. It was also observed that the morphologies of primary silver, primary germanium and eutectic structure were changed with increasing growth rate. Lamellar colonies were formed prominently in the fast-grown hypereutectic alloys. As the growth rate increased the tendency for branching in massive primary germanium was so pronounced that a lamellar colony was finally formed.     

Characterization of creep-fatigue crack growth behavior in 2.25 Cr-1 Mo steel using (C t)avg

Time-dependent creep-fatigue crack growth (CFCG) is an important consideration in the design and remaining life estimation of high temperature components. CFCG tests were carried out on compact type (CT) specimens of 2.25 Cr-1.0 Mo steel and its behavior, for hold times ranging from 10 seconds to 50 seconds, at 594°C (1100°F) was characterized using the average value of the C _t-parameter, (C _t)_avg. The trends in the creep-crack growth (CCG) data for this material are also compared with the CFCG data. The analytically estimated values of (C _t)_avg are compared with the experimental values of (C _t)_avg obtained from the measured values of load-line deflection rates. It is also shown that even in the absence of accurate creep deformation constants, accurate estimates of the measured values of (C _t)_avg can be obtained in CT specimens     

Oxygen enhanced crack growth in nickel-based superalloys and materials damage prognosis

This paper summarizes the results from a comprehensive multidisciplinary study to better understand the role of niobium and other strengthening elements in enhancing crack growth by oxygen in nickel-based superalloys at high temperatures, and considers its importance for materials damage prognosis and life cycle engineering in high temperature service. Three γ′ strengthened powder metallurgy (P/M) alloys, with 0, 2.5 and 5 wt pct Nb and comparable volume fractions (about 53 vol pct) of γ′′ precipitates, were specially designed for this study. Coordinated crack growth, microstructural and surface chemistry studies were conducted on the alloys. They were complemented by oxidation studies of Nb, Ni₃Nb, NbC, Ni₃Al and Ni₃Ti, and analyses of fracture surfaces of interrupted crack growth specimens by X-ray photoelectron spectroscopy (XPS). The findings taken in toto show that oxygen enhancement of crack growth is the result of the formation of a brittle film of surface oxides along grain boundaries and interfaces ahead of the crack tip by the preferential oxidation of Nb, Ti and Al in the Nb-rich carbides and Ni₃Al, Ni₃Ti and Ni₃Nb (in Inconel 718) precipitates. The results also showed that the oxidation of Nb-rich carbides alone can significantly enhance crack growth in oxygen. The findings are discussed in relation to the previously proposed crack growth mechanisms, and their applications.     

Studies on age-hardenable aluminium alloys by atom-probe field-ion microscope

Field-ion microscopy connected with the successive field-evaporation technique and the atom-probe analysis has been applied to studying precipitation processes, especially the early stage of precipitation, in age-hardenable aluminium alloys, such as Al-Cu, Al-Ag and Al-Sc alloys. In Al-Cu alloys, coexistence of the single-layer G.P.(1) zones and the multilayer G.P.(1) zones has been confirmed and the difference between G.P.(1) zones and G.P.(2) zones has been clearly recognized. In Al-Ag alloys, the octahedral shape of η-G.P. zones has been confirmed. In Al-Sc alloys, it has been found that the equilibrium Al₃Sc phase particles precipitate homogeneously from the beginning without any preprecipitation stage and coherency between the precipitate and the matrix is maintained even in an alloy aged at relatively higher temperature for prolonged periods of time.     

Mechanical properties of the Sn-Zn eutectic alloys

The structure and mechanical properties of Sn-Zn unidirectionally frozen eutectic alloys have been examined over the growth range 5 to 4000mm h^–1. The structure is predominantly broken-lamellar below 750mm h^–1 but becomes increasingly fibrous at higher growth rates. The yield and ultimate strengths when tested in tension and compression were found to increase monotonically with growth rates up to 1000 mm h^–1 above which they assumed near constant values. This behaviour is attributed to some loss of axial growth at higher growth rates. The hardness measured on transverse sections increased over the entire growth rate range. Annealing at near eutectic temperatures followed by quenching increased the strength of alloys grown at less than 750 mm h^–1 and decreased that of those grown at higher rates. Similar behaviour was observed in selected Cd-Zn eutectic alloys. The increase in strength is attributed to solid solution hardening and the reduction to structural degradation during annealing. The Sn-rich matrix in this broken-lamellar eutectic appears to contribute significant strengthening to the composite.     

Small-punch technique for measurement of material degradation of irradiated ferritic alloys

The yield stress and fracture strain of irradiated ferritic alloys were measured using a small-punch test. The characteristics of hardening and intergranular embrittlement induced during neutron irradiation (0.94×10²³ n m^–2) at 395C in ferritic alloys doped with Cu, P and/or C were investigated. The effect of neutron irradiation on hardening was found to be greater in alloys doped with Cu and/or P than in C-containing alloys. The neutron irradiation produced a more substantial increase in the ductile-brittle transition temperature in the Cu-doped alloy compared with the other alloys and cause a significant decrease in the ductility of the Cu-doped alloy. Intergranular and transgranular fracture occurred in the alloys doped without and with C, respectively. The neutron irradiation did not alter the fracture mode.     

Strength of Wheels of High-Pressure Hydrogen Pumps

With the example of an oxygen–hydrogen liquid-propellant rocket engine with afterburning for the launch vehicle Énergiya, we consider the structural strength of impellers of a hydrogen pump. It is shown that the specific strength of the structural materials of impellers in liquid hydrogen is considerably influenced by their plasticity. On the basis of the data of experiments, the admissible peripheral speeds are recommended for impellers made of cast steels and titanium alloys. The method of hot isostatic pressing for producing the impellers of titanium alloys provides a number of advantages over other techniques. This method makes it possible to ensure an admissible peripheral speed of impellers up to 630 m/sec in the nominal mode and 690 m/sec for short periods in the maximum operational modes of the liquid-propellant rocket engine.     

Grain growth behaviour and consolidation of ball-milled nanocrystalline Fe–10Cr alloy

Nanocrystalline iron–chromium alloys may provide considerable corrosion resistance, even at low chromium contents. However, processing of such alloys could be a challenge. This paper describes successful synthesis of nanocrystalline Fe–10%Cr alloy by ball-milling route. In the absence of suitable hot compaction facility, the alloy powder could be successfully compacted close to the desired density, by employing a step of prior annealing of the powder. Grain growth behaviour of Fe–10%Cr nanocrystalline alloy was investigated at 500, 600 and 700 °C. At 500 °C, no appreciable grain growth was observed, after the initial grain growth. However, sudden and rapid grain growth was observed after 90 min at 600 °C, and 30 min at 700 °C.     

Design and development of powder processed Fe–P based alloys

The present investigation deals with designing Fe, Fe–P binary and Fe–P–Si ternary alloys produced by an in-house developed powder metallurgical technique based on ‘Hot Powder Preform Forging’. Proper soaking of preforms at high temperature (1050 °C) eliminates iron-phosphide eutectic and brings entire phosphorus into solution in iron. Attempting hot forging thereafter completely eliminates hot as well as cold shortness and thereby helps to form these preforms (alloys) into very thin sheets of 0.5 mm. The use of costly hydrogen atmosphere during sintering has been eliminated by the addition of carbon as a reducing agent to form CO gas within the compact by reacting with oxygen of iron powder particles. The glassy ceramic coating applied over the compact serves as a protective coating to avoid atmospheric oxygen attack over the compact held at high temperature. These alloys so formed were subjected to density examination at various stages. Microstructural study has been carried out to estimate the grain size, volume percentage of porosity in the alloys, and uniform distribution of phosphorus and silicon in an iron matrix. X-ray diffraction studies of these alloys revealed the presence of only ferrite as product phase. Addition of alloying elements such as P and Si has improved the resistivity and magnetic properties of iron. Fe–0.07C–0.2O–0.3P–0.5Si alloy showed a resistivity as high as 31.7 μΩ cm. Coercivity values of the alloys ranged from 0.51 to 1.98 Oe. The total magnetic loss of Fe–0.07C–0.2O–0.3P–0.5Si alloy was the lowest (2.03 W/kg) amongst the alloys developed owing to its high resistivity combined with its low coercivity. These alloys which are drawn to thin sheets could find their possible application in the manufacturing of transformer cores.     

Grain boundary crystallography in two Fe-C-P alloys

Abstract

Misorientation, grain growth and brittle fracture were investigated in two iron - carbon alloys containing 0.06 wt-% phosphorus (0.06P) and 0.12 wt-% phosphorus (0.12P) after selected heat treatment schedules. A 'fracture surface serial sectioning' technique was devised and combined with misorientation measurements to reconstruct specimens after fracture. Anomalous grain growth occurred in the 0.06P specimen only, after 1000°C annealing. This was attributed to the inhomogeneous distribution of phosphorus at the interfaces. No evidence was found for the direct influence of misorientation angle distributions or coincidence site lattice distributions on anomalous grain growth. The proportion of Σ3s increased greatly after annealing at 1000°C, attributed to the twinning that developed in the austenite range. There was strong evidence that Σ3s were in general more resistant to brittle fracture than were random boundaries. It is suggested that alloys of this type could be 'grain boundary engineered' to improve fracture resistance.     

Design and mechanical characterization of a Zr–Nb–O–P alloy

In this study, Sn-free Zr–1.5Nb–O–P alloys were manufactured and their mechanical properties were characterized. The ultimate tensile strength (UTS) of cold rolled Zr–1.5Nb–O–P alloy with 160 ppm phosphorous (680 MPa) were close to that of a commercially available Zr–1Nb–1Sn–0.1Fe alloy (720 MPa), achieving a good mechanical strength without the addition of Sn, an effective solution strengthening element. The UTS of recrystallized Zr–1.5Nb–O–P alloy with 160 ppm phosphorous (533 MPa) was far greater than that of a commercially available Zr–1Nb–O (323 MPa) because of the strengthening due to higher Nb and oxygen content combined with phosphorous strengthening. The activation volumes for the cold rolled Zr–1.5Nb–P alloys were not much different from those of annealed Zr–1.5Nb–P alloys despite the higher dislocation density in the cold rolled alloys. Insensitivity of the activation volume to the dislocation density and the decrease of the activation volume with the addition of phosphorous support the suggestion linking the activation volume with the activated bulge of dislocations limited by segregation of oxygen and phosphorous atoms.     

Cyclic endurance of alloy ÉP718ID after ion-beam treatment

A study has been made by electron Auger spectroscopy, x-ray structural analysis, and fractographic methods of the physico-chemical state of the surface layers of specimens and articles of alloy ÉP718ID after vibration-abrasive, heat, and ion-beam treatments. It has been shown that fatigue properties may be improved by means of implanting nitrogen and palladium ions.S. Ordzhonikidze Aviation Institute, Moscow. Translated from Problemy Prochnosti, No. 11, pp. 72–78, November, 1989.     

Coupled influence of microstructure and atmosphere environment on fatigue crack path in new generation Al alloys

The fatigue crack propagation behavior of new generation Al alloys developed for aeronautical applications is studied at moderate ΔK and in the near threshold domain. The crack growth rate and the crack path are shown to depend on alloy composition, aging condition and atmosphere environment, and to be governed by the slip morphology. In absence of environment assistance, a crystallographic (1 1 1) faceted cracking leads to a slow stage I-like propagation in Al-Li-Cu alloys and underaged Al-Cu-Mg alloys with microstructure consisting of shareable precipitates or solute cluster structures that promote heterogeneous slip-band formation, which is in contrast with a ductile transgranular featureless stage II crack path in overaged Al-Cu-Mg. In air at moderate ΔK, an adsorption assisted propagation mechanism is assumed to prevail in both Li and Mg bearing materials, water vapor assistance inducing a transgranular stage II regime associated to homogeneous slip generating a flat-facet and step-like features; in the near threshold domain, the same mechanisms is operating for Al-Cu-Mg alloys while even more accelerated growth rates and lower effective threshold for Al-Cu-Li alloys are attributed to an assistance of hydrogen produce from the dissociation of adsorbed water vapor.