J.F. Knott

Mechanisms of fatigue crack growth in low alloy steel

Abstract A study has been made of fatigue crack propagation in a low alloy steel which is subject to temper embrittlement. Effects of mean stress on the growth rate have been examined and comparisons between temper embrittled and unembrittled conditions have been made. Whereas fatigue crack propagation rates were found to be insensitive to mean stress in the unembrittled steel, growth rates in the embrittled condition were significantly faster and were strongly influenced by the level of mean stress. The effects observed are ascribed to the presence of “static” modes of fracture which occur in association with classical fatigue striations. It is shown that similar static modes can account for effects of mean stress and for the enhanced growth rates observed in a variety of materials of low fracture toughness.

Effects of microstructure on cleavage fracture in pressure vessel steel

The paper describes studies made on a wide range of microstructures in A533B pressure-vessel steel, to explore the relationships between microstructural parameters and toughness, as characterized by both the critical stress intensity factor, KIC, and the microscopic (local) cleavage fracture stress, σF∗. Large variations in toughness are obtained as a function of microstructure. The results show that auto-tempered martensites possess toughnesses superior to those for mixed lower-and-upper bainities, or for upper bainites. The carbide size distribution is found to be the most important single microstructural feature that controls cleavage fracture in these heat-treated conditions. The coarsest carbides in the distribution are the most deleterious to toughness. The variation of σF∗ with carbide size may be predicted to a first approximation by a modified Griffith energy balance for carbide sizes greater than 340 nm. The measured temperature independence of σF∗ for ferritic, bainitic and martensitic microstructures provides strong support for a tensile-stress-controlled fracture criterion.

A study of stress relief cracking in 214Cr 1 Mo steel—I. The effects of P segregation

Abstract Stress relief cracking behaviour has been studied in controlled purity and (540 ppm) phosphorus-doped casts of 2 1 4 Cr 1 Mo steel, using a test developed to simulate conditions in the coarse-grained region of a weld heat-affected-zone (HAZ) during post-weld heat-treatment (PWHT). Two distinct cracking regimes were observed in the phosphorus-doped alloy during the PWHT cycle. At low temperatures (700–750 K), a smooth, intergranular failure mode was produced, whereas at higher temperatures (800–875 K) the cracking mode changed to a ductile intergranular fracture, with voids centred on MnS inclusions at the grain boundaries. The controlled purity alloy exhibited a similar type of ductile intergranular cracking at the higher temperatures but no cracking was observed at lower temperatures. Auger electron spectroscopy (AES) revealed a maximum in the grain boundary concentration of phosphorus during the stress-relief heating period of the phosphorus-doped alloy, confirming a prediction based on segregation thermodynamics and kinetic theory. The temperature at which this peak occurred was coincident with that of the low temperature cracking regime.

Micromechanisms of failure in CMn weld metals

Abstract Observations on the influence of reheating and preheat temperature on the microstructure and toughness of Cue5f8Mn weld metals are collated, and implications for failure micromechanisms are assessed. Both microvoid coalescence and cleavage micromechanisms are strongly influenced by the weld-metal non-metallic inclusion population. In particular a model for cleavage fracture in Cue5f8Mn weld metals is further developed and discussed. This involves initial plasticity in grain boundary ferrite; strain induced inclusion cracking; propagation of the crack into the ferrite matrix under a critical tensile stress; and preferential, though not exclusive crack propagation through grain boundary ferrite.

Crystallographic fatigue crack growth in aluminium alloys

Abstract A study has been made of the crystallographic nature of fatigue crack propagation in pure Al-Cu alloys. For a wide range of crack growth rate and microstructure the fatigue fracture surfaces observed appear indicative of ‘classical’ cleavage fracture. X-ray diffraction studies indicate that the crystallographic facets lie on {100} planes, consistent with failure by cleavage in this system. However, the results of environmental and mean stress experiments show conclusively that a true cleavage mechanism cannot be operating. A restricted-slip mechanism is proposed which successfully accounts for the observed plane of fracture.

Micromechanical modelling of fracture toughness

Abstract The paper describes studies made on a wide range of microstructures in ASTM A533B pressure vessel steel, to explore the relationships between the critical stress intensity factor, K i c , and the microscopic (local) cleavage fracture stress, σ F ∗ . The measured temperature dependence of the fracture toughness has been compared with that predicted by the Ritchie, Knott and Rice (RKR) micromechanical model. For most microstructures the fracture toughness predictions are poor but represent a lower bound to the experimental values. It is concluded that the fracture toughness at very low test temperatures, − 196°C, is controlled by the mean carbide size, while at higher test temperatures the fracture toughness is controlled by carbides of coarser size. The effect of temperature is therefore to change the sizes of carbides that can provide active (potent) microcrack nuclei. A limiting temperature, above which cleavage cannot occur, is predicted when the stress intensification in the process zone ahead of the crack tip is insufficient to propagate a micro-crack from the coarsest carbide.

Serrated yielding and the localized shear failure mode in aluminium alloys

A study has been made of serrated yielding in two commercial Al-Zn-Mg alloys in the as-quenched condition. The different serration types produced in the two alloys and the shear failure mechanism observed in both notched-bend and tensile testing are related to the mechanisms of dynamic strain ageing occurring during the test. An estimate of 19.7 kJ/mole for the activation energy for exchange of a solute atom and a vacancy in Al-6.2 wt% Zn, 2.5 wt% Mg has been made.

A study of stress relief cracking in 214 Cr 1 Mo steel—II. The effects of multi-component segregation

Abstract The examination of stress-relief-cracking in the coarse-grained heat-affected-zone microstructure of 2 1 4 Cr 1 Mo steel has been extended from the study of controlled-purity and (540 ppm) phosphorus-doped alloys [reported in Part I. Acta metall. 28, 869 (1980)] to (500 ppm) tin-doped and commercial-purity alloys. Auger-electron-spectroscopy, electron microscopy, energy-dispersive X-ray analysis and potential-drop crack measurement techniques have been employed to monitor the development of grain-boundary segregation, precipitation microstructure, and crack propagation in each alloy during post-weld heat-treatment. In the controlled-purity alloy an ‘intergranular micro void coalescence’ stress-relief-cracking mode of fracture was observed at temperatures > 880 K. In contrast, an additional ‘low-ductility intergranular-fracture’ mode was observed at lower temperatures in the phosphorusdoped, the tin-doped and commercial alloys. The cracking temperature range was dependent upon material composition. The controlled-purity alloy exhibited a relatively low propensity to stress-reliefcracking, while the susceptibility of the commercial alloy was significantly higher than that for both of the impurity-doped alloys. Fractographic, chemical and microstructural analyses enabled the major processes contributing to each mechanism to be identified, and it was possible to rationalise the overall stress-relief-cracking behaviour in terms of the effects of residual impurities on (i) the relative extent of mechanisms operating in each alloy, and (ii) the rate-determining processes contributing to each mechanism. On this basis, multi-component segregation was found to be severely deleterious, promoting crack-propagation by low-ductility intergranular-fracture at relatively low stress intensities (e.g. 15 MNm −3 2 for the commercial alloy).

Effects of impurity segregation on sustained load cracking of 214 Cr-1Mo steels—I. Crack initiation

Abstract The paper describes experiments carried out to determine the element(s) responsible for “Low Ductility Intergranular Fracture” (LDIGF) at high temperatures in 2 1 4 Cr-1Mo steels. Isothermal testing at 500°C was conducted on both single-notched and double-notched bend bars under sustained load for commercial purity, high purity, and phosphorus doped alloys. Both the initial load and notch geometry were varied to control the position of peak stress ahead of the blunt notch. Microcracks ahead of the blunt notch were observed by optical and scanning electron microscopy. Auger and LIMA surface analyses revealed that elemental sulphur was responsible for the brittle intergranular microcracks produced under load at 500°C. The results strongly suggest that the stress field ahead of a blunt notch promotes the segregation of sulphur to grain boundaries prior to crack initiation.

Effects of segregation on brittle fracture and fatigue crack growth in coarse-grained, martensitic A533B pressure vessel steel

Abstract The susceptibility of coarse-grained A533B to “one-step-temper-embrittlement”, when tempered at 290°C, has been assessed using fracture toughness testing at −120°C. Auger spectroscopy and fractography. Phosphorus is the main embrittling element. The majority of the phosphorus segregation is found to occur in the austenite region, although support is also given to the mechanism of “carbide rejection” during tempering. The fracture toughness is reduced by tempering at 290°C, with or without an increase in the number of intergranular facets seen on the fracture surface. Fatigue crack growth at room temperature, and at 290°C has also been investigated. The crack growth rates are not affected by the occurrence of intergranular facets in the fatigue regions, even when they are the dominant mechanism of fatigue crack growth. The presence of intergranular facets in the fatigue regions does not result from phosphorus segregation. A possible explanation of the observed effects involves the dynamic segregation of sulphur to the growing fatigue crack tip.