Advances in the mechanism of cleavage fracture of low alloy steel at low temperature. Part II: Fracture model

The authors discuss in detail the existing cleavage fracture model including the physical model and the statistical model. Based on the discovery that a minimum distance for initiating the cleavage fracture was revealed to have a definite physical meaning, that the stress triaxiality reaches the critical value Tc at this distance instead of that the presence of an eligible carbide crack within this distance is assured; a combined criterion for cleavage fracture, i.e. a critical plastic strain (εp ≥ εpc) for initiating a crack nucleus, a critical stress triaxiality (σ m/σ ≥ Tc) for preventing it from blunting and a critical normal stress (σyy ≥ σ) for its propagation has been proposed to substitute for the criterion of σyy ≥ σ over a ‘characteristic distance’ suggested by the RKR model. With regard to the statistical model the authors suggested that it is not necessary to describe the fracture stress distribution by Weibull function. The fracture probability at a given applied load was modified by multiplying a ‘remaining probability’ taking account of the effect of the preceding loading process. The authors also discuss the mechanism of transition from a fibrous to a cleavage crack within the toughness transition temperature range. This revised version was published online in August 2006 with corrections to the Cover Date.     

Advances in the mechanism of cleavage fracture of low alloy steel at low temperature. Part III: Local fracture stress σ

Local fracture stress values σ were measured accurately using 4PB, COD and Charpy V specimens. σ is the most stable parameter characterizing the fracture toughness of materials. It's scattering is analyzed to be determined only by the size distribution of the largest particles, accounting for less than 5 percent of the total particles of the weakest constituent, and is not subject to the influence of their locations. The toughness of a structurally heterogeneous zone such as a weldment could be characterized by a unique value of σ or σ/σy with a narrow band of scattering. σ or σ/σy is a potential engineering parameter used for assessment of fracture toughness of materials. The physical meaning of σ associated with the surface energy is discussed in detail. This revised version was published online in August 2006 with corrections to the Cover Date.     

On locations initiating cleavage fracture in precracked specimens of low alloy steel and weld metal

On the basis of a model of an `active zone' for initiating cleavage fracture proposed by authors, the distributions of cleavage initiation location in precracked specimens are explained, and the factors affecting cleavage initiation locations are analyzed. The change of the length of the active zone with applied load determines the distributions of cleavage initiation locations. With increasing temperatures, the distance X _f from precrack tip to locations initiating cleavage fracture and its scatter increase, and the lower boundary of X _f increases slowly, and the scatter is mainly caused by the rapid increase of the upper boundary. With decreasing the strength of the weakest constituent in steels and increasing their number, the minimum distance X _\min and the average distance for initiating cleavage fracture will decrease and the maximum distance X _\max will increase, and the corresponding toughness values will be decreased.     

Brittle intergranular fracture at elevated temperatures in low–alloy steel

Abstract

Recent studies of stress-relief cracking in low-alloy steels have focused attention on a novel mode of brittle intergranular fracture which occurs at elevated temperatures (300–650°C) in hard, coarse–grained heat–affected–zone microstructures. Fracture initiates at stress concentrators such as sharp cracks or inclusions, and can propagate under static loading at rates of 10^?11?10^?5 ms^?1 to produce intergranular facets with very little associated plastic deformation. The stress-intensity parameter K has been used to characterize crack growth, and three regimes of behaviour have been observed: (i) a threshold region at growth rates of 10^?11?10^?10 m S^?1, (ii)a plateau region, in which growth rates are independent of K between 10^?10 and 10^?8 m S^?1, and (iii) a region of highly K-sensitive crack growth between 10^?8 and ^?5 m S^?1. Independent Auger electron spectroscopy analyses have demonstrated that sulphur segregates locally to the high-temperature crack tip, giving rise to the embrittlement of a limited area of grain boundary. Together with other presegregated solutes, this enables brittle fracture to occur at high temperature, and the transfer of sulphur to the crack tip controls the rate of crack growth. Two models describing crack-tip sulphur segregation are currently proposed. In the first model, a quantitative analysis demonstrates that the crack-tip stress field will drive undersize solute atoms such as sulphur to the physical crack tip. In the second, the intergranular crack is modelled as a sharp cavity. Grain-boundary sulphides which are exposed by cavity formation become unstable and dissolve, saturating the cavity surface with sulphur, which is then drawn into the tip as part of the cavity growth process.

MSTj77     

A new statistical local criterion for cleavage fracture in steel. Part II: application to an offshore structural steel

In the first part of this paper, a new model for cleavage fracture in steel was presented, based on a new statistical local criterion, which expresses the necessity of simultaneously fulfilling the conditions for both cleavage microcrack nucleation and propagation. In this second part, the assumptions and predictive capabilities of the new model are assessed using a modern offshore structural steel plate (Grade 450EMZ). It is shown that the model assumptions are consistent with the cleavage fracture behaviour of the steel and that the new model has the potential of correctly quantifying the effects of size, constraint, temperature and strain rate on cleavage fracture risk.     

Transgranular fracture in low temperature brittle fracture of high nitrogen austenitic steel

Morphological features of transgranular fracture facets in low temperature brittle fracture of 18Cr–18Mn–0.7N high nitrogen austenitic steel have been studied by means of scanning electron microscopy and their formation mechanisms have been discussed. The transgranular fracture facets are fairly coarse compared with intergranular fracture facets and annealing twin boundary fracture facets. There are parallel steps and river patterns on the facets. Dual-surface observation indicated that these patterns are parallel to {111} planes and of strict crystallographic features. Microstructure observation shows that there are a lot of planar deformation structures formed prior to low temperature transgranular fracture. Transgranular fracture originates from microcracks formed at the intersections of the deformation structures on different {111} planes. These microcracks propagate toward adjacent microcracks on different {111} planes, forming transgranular fracture facets with steps and river patterns.     

Corrosion fatigue behaviour and fracture mechanisms in nitrided low alloy steel

Rotary bending fatigue tests have been performed in 3%NaCl aqueous solution using specimens of a low alloy steel (Cr–Mo steel) with different nitride case depths. The effect of case depth on corrosion fatigue strength, the fracture process and mechanisms were studied. The corrosion fatigue strengths of the nitrided materials increased compared with the untreated material and increased with increasing thickness of the compound layer, but tended to saturate above a certain thickness. All the materials showed lower fatigue strength in 3%NaCl aqueous solution than in laboratory air and the reduction of fatigue strength decreased with increasing thickness of the compound layer, but remained nearly constant above a certain thickness. Corrosion pits were seen underneath the compound layer, from which cracks initiated. The corrosion fatigue strengths of the specimens whose compound layer was completely removed by electropolishing were almost the same as that of the untreated material, indicating a very significant role of the compound layer in improving corrosion fatigue strength. Because of the porous nature of the compound layer, particularly in the surface‐adjacent part, the solution penetrated the compound layer and reached the base steel, thus the corrosion fatigue strength of the nitrided materials was controlled by the penetration of corrosive media.     

Surface cracking mechanism of continuously cast low carbon low alloy steel slabs

Abstract

The present state of understanding of surface cracking in low C low alloy steel slabs in the continuous casting (CC) and direct rolling (DR) processes is outlined. Hot cracking of the CC slab surface can be explained in terms of carbide and/or nitride precipitation behaviour. In addition to γ grain boundary precipitation, the matrix strengthening owing to dynamic precipitation and the existence of softer layers along the boundaries such as grain boundary allotriomorphs of ferrite or precipitate free zones play animportant role in intergranular ductile fracture. The origin of hot cracking during the DR process lies also in the precipitation of carbides and/or nitrides, and is not related to the severe embrittlement caused by a similar mechanism with dynamic precipitation of sulphides, which is observed usually in the high strain rate deformation after reheating at higher temperatures. Furthermore, a well known effect of C on hot cracking susceptibility in both CC and DR processes, attaining a maximum in the range 0·10–0·15 wt.–%C, is found to arise mainly from γ grain growth during solidification in the mould. Some methods to prevent surface cracking are also discussed.

MST/1226     

Influence of temperature on impact fracture behavior of an alloy steel

In this paper, the influence of temperature on impact toughness and fracture behavior of alloy steel (AISI Classification 8320) is presented and discussed. Impact toughness decreased with a decrease in test temperature. The extrinsic influence of temperature on impact toughness–fracture resistance relationships is rationalized in light of the conjoint and mutually interactive influences of intrinsic microstructural features, local stress states and macroscopic fracture behavior.     

Influence of austempering temperature on the mechanical properties of a low carbon low alloy steel

In this investigation, a new low alloy and low carbon steel with exceptionally high strength and high fracture toughness has been developed. The effect of austempering temperature on the microstructure and mechanical properties of this new steel was examined. The influence of the microstructure on the mechanical properties and the fracture toughness of this steel was also studied.Test results show that the austempering produces a unique microstructure consisting of bainitic ferrite and austenite in this steel. There were significant improvement in mechanical properties and fracture toughness as a result of austempering heat treatments. The mechanical properties as well as the fracture toughness were found to decrease as the austempering temperature increases. On the other hand, the strain hardening rate of steel increases at higher austempering temperature. A linear relationship was observed between strain hardening exponent and the austenitic carbon content.     

Submonolayer adsorbate reference material based on a low alloy steel fracture sample for Auger electron spectroscopy Part 1 Characterisation

Abstract

A homogeneous set of Cr–3·5Ni low alloy steel samples has been made to enable laboratories studying grain boundary segregation by Auger electron spectroscopy to interrelate their studies. This steel has P, Sn, and Ni segregated at the grain boundaries. Data are provided of the effect of variation of signal intensities across a grain, on one grain as a function of the electron emission angle, on many grains, etc. for both unidirectional (concentric hemispherical) and multidirectional (cylindrical mirror) spectrometers. Reference spectra recorded from the sample allow the spectrometer transmission function to be calibrated so that data from all instruments may be interrelated. After correction for the transmission function, the Auger electron peak-to-peak height ratios, for a 5 eV Savitzky and Golay differentiation, are found to be P₁₂₀/Fe₇₀₃=0·33±0·06 and Sn₄₃₀/Fe₇₀₃=0·12±0·02. Calibrations from the literature show these signals can be attributed to 33 and 31% of a monolayer of P and Sn, respectively.

MST/1684a     

Analysis of the unstable fracture behaviour of a high strength low alloy steel weldment

Local brittle zones (LBZ) cause the unstable fracture behaviour of weld metals. This threatens the safe service of welded structures and makes structural assessment procedures difficult. Therefore, the unstable fracture behaviour of an overmatched high strength low alloyed steel weldment was experimentally investigated. It showed that any interaction between two adjacent weld metal matrix and soft weld metal inclusions produces LBZ, causing local unstable fracture behaviour. The formation of a low hardness region is attributed to the multipass welding reheating process between Ac₁ and the self-tempering temperature. The presence of partly solid metallic inclusions with a high content of alloying elements and pro-eutectoid ferrite microstructure were found to be additional causes for the local unstable fracture behaviour of the weld metal. Local strength mis-match induced the yielding and strain hardening in the soft weld metal inclusions, contributing significantly to unstable fracture behaviour. Thus, a significantly different scatter of experimental results can be obtained. In the cases of specimens with through-the-thickness crack, not only is the scatter significantly lower, but the toughness itself.     

GH4169合金的低温冲击试验

对GH4169高温合金边缘和中心部位进行了低温冲击试验,比较晶粒大小对合金冲击功的影响,进一步进行室温冲击试验,比较室温冲击和低温冲击冲击功的大小。同时还对径锻试样和热连轧试样低温冲击所需要的冲击功进行对比,并用SEM观察了冲击断口的表面。结果表明:晶粒越细小,所用的冲击功越大;低温冲击和室温冲击所用的冲击功几乎一样;热连轧试样低温韧性要远远好于径锻试样,冲击端口为韧窝断口。     

Static crack extension prediction in aluminium alloy at low temperature

The evaluation of the crack initiation and crack extension under different environmental conditions is very important for many engineering applications. Several crack extension criteria have been proposed in the last decades, but each criterion can be employed only for particular materials, loading configurations, environmental conditions. In the present paper, the R-criterion (minimum extension of the core plastic zone) is modified in order to take into account the temperature dependence. The modified criterion is herein employed to predict the crack path and the crack extension force for an edge-cracked finite plate under tension, by using a simplified procedure to determine the stress-intensity factor (SIF) for different initial crack configurations. Then, results concerning some experimental tests performed on edge-cracked aluminium alloy sheet specimens at different temperatures are reported. Finally, the theoretical results are compared (in terms of crack extension force and crack path) with the experimental data.     

Submonolayer adsorbate reference material based on a low alloy steel fracture sample for Auger electron spectroscopy Part 2 Interlaboratory tests

Abstract

A batch of low alloy reference material for the study of grain boundary segregations of P and Sn has been studied using eight Auger electron spectrometers in six laboratories. The instruments include both cylindrical mirror and concentric hemispherical analysers. Measurements in the differential mode using 5 eV Savitzky and Golay differentiation show that peaks close in energy, such as the ratio of the Sn 430 eV and Fe 651 e V peak-to-peak heights, can be compared directly to demonstrate a reproducibility of 8%. For peaks more widely separated, the scatter of intensity may exceed a factor of 2. However, using reference spectra from the material, all the spectrometers can be calibrated for their intensity/energy response function. After this calibration, the P and Sn peak-to-peak height ratios, with respect to Fe, show scatter of only 15%. In addition to this reduction in scatter, the mean values for the ratios can be corrected by up to a factor of 2. This work shows that the reference materials and methodology are suitable for relating the segregation spectra for P and Sn in low alloy steels from laboratory to laboratory on an absolute and traceable basis.

MST/1684b