钢的恒温超塑性固相焊接研究

在对结构钢 (40Cr)同材、结构钢 (40Cr)与工具钢 (T1 0A、GCr1 5 )异材实施恒温超塑性固相焊接可行性全面分析的基础上 ,通过对结构钢、工具钢待焊面表层实施高频淬火组织超细化处理 ,在非真空、无保护气氛下 ,系统地探讨了恒温超塑性固相焊接工艺参数对接头质量的影响 .试验结果表明 :只要选择适宜的焊接工艺参数 ,经短时间压接可使接头强度达到或接近 40Cr母材的强度 ,实现钢的同材和异材恒温超塑性固相焊接 .     

Void nucleation during tensile deformation of a C-Mn structural steel

Various methods have been used to reveal the voids arising from ductile fracture processes in a structural steel and the techniques compared. Using a series of the interrupted smooth bar tensile tests specimens were stopped at different plastic strains and nucleation was investigated. The material tested was a C-Mn structural steel with low sulphur content. The critical nucleation strain and the relative void volume at the critical nucleation strain were determined.Résumé On a fait appel à diverse sméthodes pour faire apparaitre les lacunes générées par un processus de rupture ductile dans un acier de construction, et les techniques utilisées ont été comparées.En procédant à une série d'essais de traction interrompus sur un barreau lisse, on a pu isoler différents spécimens de déformation plastique et étudier les nucléations de lacunes y associées. Le matériau soumis à l'essai était un acier de construction au C-Mn à faible teneur and S. On a pu déterminer une dilatation critique de nucléation, ainsi que le volume relatif de lacunes créées à la dilatation critique de nucléation.     

Dilatometric analysis of phase transformations in hypo-eutectoid steels

Dilatometry is a useful technique to obtain experimental data concerning transformation kinetics in ferrous alloys. This technique is commonly used in cooling experiments to study the austenite decomposition of hypo-eutectoid steel grades. In the standard analysis of the dilatation signal there are two factors that are normally neglected. During the pro-eutectoid ferrite formation the austenite enriches in carbon, resulting in a non-linear temperature dependence of the specific austenitic volume. Furthermore, the specific volume of the formed ferrite is considerably different from that of the formed pearlite. In total not taking into account these two effects can lead to an error in the determined fraction ferrite of up to 25%. A method is presented that takes into account the two above-mentioned factors. In order to determine both the fraction ferrite and the fraction pearlite, in the analysis the temperature range of the transformation is divided into a ferrite-formation range and a pearlite-formation range. Two possible criteria for this division are discussed, and it is shown that the choice does not have an essential influence on the results.     

Modeling of deleterious phase precipitation during isothermal treatments in superduplex stainless steel

The deleterious constituents σ, χ, and eutectoid γ₂ can precipitate during hot forming and welding operations in superduplex stainless steels. In this study the precipitation of these phases in superduplex UNS S32750 was investigated. Two materials with similar composition and quite different grain sizes were used in the study. Specimens were subjected to isothermal treatments in the 800–950 °C temperature range for time periods up to 240 min. Under these conditions, quantifications of deleterious phases by light optical microscopy were carried out in order to model the precipitation kinetics of these deleterious constituents. A sigmoidal equation, similar to the one developed by Johnson–Mehl–Avrami was fitted to all the analyzed temperatures. Precipitation velocity was evaluated so as to compare the kinetics of deleterious phases in the two materials. The activation energies for precipitation were calculated based on Arrhenius equation.     

Fatigue crack growth retardation following load reductions in a plain C-Mn steel

Transient fatigue crack growth effects following perturbations in applied loading conditions have been studied using a low strength plain C-Mn steel. Reductions in both the alternating and maximum components of the fatigue loading spectrum have been systematically investigated and evaluated using a linear elastic fracture mechanics approach. Results are discussed in terms of the residual stress concept, and a model based on an effective stress intensity concept is propossed to rationalise growth rates within the retardation affected zone. Using constant amplitude fatigue threshold and crack growth data obtained under similar conditions, the model is shown to be in good agreement with experimental data.     

Kinetics of isothermal phase transformations in a dental porcelain

The dissolution of leucite (KAlSi₂O₈) and the precipitation of sanidine (Na _x K_1–x AlSi₃O₈; 0.1<x<0.3), occurring during isothermal heat treatments of a dental porcelain for porcelain-fused-to-metal restorations, was studied. The identification of phases was performed by X-ray diffraction, optical and scanning electron microscopy. An isothermal time-temperature-transformation (TTT) diagram, from 800 to 1000°C, and for periods up to 1440 min, is proposed. No metastable cubic leucite was retained by air-quenching in any sample. No increase of leucite volume fraction was observed.     

On the void growth in C-Mn structural steel during plastic deformation

A procedure has been established to measure the relationship between void growth, plastic strain and stress triaxiality during the fracture of a C-Mn structural steel. A calibration factor for void growth was determined using the smooth tensile specimen tests and pre-cracked side-grooved three-point bend bar tests. The factor was expected to be applicable over a rather wide range of the constraint generating geometries.The experimental void growth relation shows a real void growth rate in the structural steel which is smaller than that determined by the Rice-Tracey's void growth theory for stress triaxialities, larger than about 1.2. However, when the value is smaller than about 1.2, the void growth predicted by the Rice-Tracey's theory tends to slightly underestimate the void growth in the steel. The variations of the plastic strain and relative void volume have been measured and evaluated ahead of the blunting crack tip in the pre-cracked side-grooved three-point bend specimens at the point where the ductile tearing initiated. These results are related to theories for the level of triaxial stresses.

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Résumé On a mis sur pied une procédure pour mesurer la relaxation liant la croissance des lacunes, la dilatation plastique et l'état de triaxialité des contraintes au cours de la rupture d'un acier de construction au C-Mn. La croissance des lacunes a été étalonnée en recourant à des essais de traction sur éprouvettes lisses et à des essais de flexion en trois points de barreaux entaillés latéralement et préfissurés. Le facteur d'étalonnage tiré de ces essais devrait être applicable à une large gamme de géométries produisant une concentration de contraintes.L'expérience a montré que, dans l'acier de construction étudié, la croissance des lacunes est plus faible que ne le laisse prévoir la théorie de croissance de Rice-Tracey, dans le cas de triaxialité de contrainte a. 16 supérieure à environ 1,2. Par contre, lorsque cette triaxialité est inférieure à 1,2, la croissance de lacunes est légèrement sous-estimée par le théorie de Rice-Tracey.On a mesuré et évalué, dans les éprouvettes de flexion pré-fissurées, les variations de déformation plastique et de volume relatif de vide en avant de l'extrémité d'une fissure en cours d'ouvertures et ce dans la zone d'amorçage de l'arrachement ductile.Les résultats sont en relation avec les théories fixant le niveau de triaxialité d'une sollicitation.

     

Effect of specimen size and geometry on ductile crack growth resistance in a C-Mn steel

The effect of specimen size and geometry on the ductile crack growth resistance of a C-Mn steel has been investigated. The resistance, expressed in the form of J-R curves, was measured using the conventionally calculated J(J_r);(b/J×dJ/da) values. The results show that for specimens of given thickness the effect of specimen geometry was due to a change in the shear lip and not to the flat fracture contribution. This conclusion is consistent with the geometry invariance found in measurements of the crack opening displacement at the growing crack tip in the flat fracture region. In addition, it has been shown that J _m can characterise crack growth resistance over a wider range of conditions than J _r.

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Résumé On a étudié les effets de la taille et de la géométrie d'une éprouvette sur la résistance à la croissance d'une fissure ductile dans un acier au C-Mn. Exprimée sous la forme de courbes J-R, la résistance a été mesurée par des valeurs de J calculées par voie conventionnelle (J _r) et par la méthode de Ernst modifiée (J _m), pour une gamme de géométries d'éprouvettes sollicitées en flexion et en traction. La résistance globale d'éprouvette ne présentant pas d'entaille latérale a été divisée en deux contributions, qui s'experiment par les zones de ruptures plates et de lèvres de cisaillement, que l'on trouve sur les surfaces de rupture.Grâce à une telle approche, on a ramené à des grandeurs rationnelles la taille et la géométrie d'une éprouvette, et on a déterminé les limites de croissance d'une fissure régies par J _r et J _m en termes de l'extension maximale admissible d'une fissure, et des valeurs minimales de (b/j×dJ/da).Les résultats indiquent que, pour des éprouvettes d'épaisseur donnée, l'effet de la géométrie d'une éprouvette est associé à une modification de la contribution des lèvres de cisaillement, et non de la portion plate de la rupture. Cette conclusion est compatible avec la constance de la géométrie que l'on observe dans des mesures du déplacement d'ouverture de la fissure à la pointe de la fissure en progrès dans la zone de rupture plate. En outre, on a montré que J _m peut caractériser la résistance à la croissance d'une fissure, sur une gamme plus large de conditions que ne le fait J _r.

     

On the characteristic distance and minimum fracture toughness for cleavage fracture in a C-Mn steel

This study makes a further investigation on the characteristic distance, minimum fracture toughness and its temperature dependence for cleavage fracture in a C-Mn steel by the detailed finite element analysis combined with experimental observation and measurement. Results show that there is a minimum active zone for cleavage initiation, and the minimum fracture toughness of steel results from the minimum active zone necessary. Corresponding to the minimum fracture toughness, the cleavage fracture ahead of a crack tip can only initiate in a distance range from the minimum distance X_fmin determined by the lower boundary of the active zone to the maximum distance X_fmax determined by its upper boundary. The reason for the occurrence of the minimum active zone and the factors influencing it are analyzed. The temperature dependence of the characteristic distance and minimum fracture toughness and its mechanism are also discussed.     

Fracture toughness and microstructure of multipass welds in a Q.T. C-Mn microalloyed steel

An assessment of the fracture toughness above the ductile-brittle transition temperature and microstructure was made for welded joints in a Q.T. C-Mn steel in the “as welded” condition. J integral and COD values were determined for two different M.M.A. electrodes and for a wire-flux combination for S.A. welding. A comparison was made between J integral results obtained using the standard ASTM and Sumpter-Turner procedures and J-COD relationships were also analyzed.     

Cyclic plastic deformation behavior in SA333 Gr. 6 C-Mn steel

Low cycle fatigue, ratcheting and dependence of their deformation response on previous deformation histories are investigated at room temperature laboratory environment for SA333 C-Mn steel. It is noticed that cyclic hardening/softening response of low cycle fatigue is considerably influenced by applied strain amplitude, monotonic pre-straining and high to low step loading sequences. Results indicate that ratcheting under high to low step loading has significant effect on its subsequent ratcheting response. However, ratcheting under low to high step loading has very little effect on successive ratcheting responses.     

Size and geometry effects on ductile rupture of notched bars in a C-Mn steel: experiments and modelling

The aim of this work was to investigate the effect of specimen size and geometry on ductile fracture of a C-Mn steel with high sulphur content. Uniaxial tensile tests were conducted at 300°C on axisymmetric notched specimens having different sizes and geometries. Geometry effects were studied using specimens with various notch radii, thus inducing different stress triaxiality levels. Size effects were evidenced using homothetic samples having the same geometry. Results show that ductility is reduced on specimens with sharp notches (which is a common observation). As specimen size increases, mean ductility as well as scatter are reduced (showing a clear size effect). In order to predict rupture, locally coupled (post-processing type) and fully coupled (continuum damage mechanics) Finite Element models were used. They are based on the plastic criteria introduced by Gurson and Rousselier. In order to model size effect (decrease of ductility and scatter), initial distribution of inclusion volume fractions, measured by quantitative metallography, was accounted for in the simulations. Comparison of experiments with simulations showed that both model types could predict mean values of ductility and scatter. This revised version was published online in August 2006 with corrections to the Cover Date.     

Free nitrogen effect on creep failure and creep crack growth of C-Mn steel

Different chemical compositions and heat-treatments of the C-Mn steels were investigated to characterise a susceptibility to cracking on cold bent tube. The C-Mn steels were categorised in terms of the content of free nitrogen. Ultimate tensile strengths, yield strengths and elongations were measured from the tensile testing at ambient, 250 and 360°C. Significant increase in ultimate tensile strength and decrease in elongation were observed in high free nitrogen material at 250°C the temperature at which free nitrogen is most active. From the uniaxial creep testing on high free nitrogen materials at 360°C increase in creep property, for example, lower minimum creep rate, was observed, however there was a decrease in creep ductility. This lower ductility of the high free nitrogen material has provided higher susceptibility to cracking in the creep crack growth tests at 360°C. Cracking in the high free nitrogen material A was approximately three times faster than the low free nitrogen material C at the same C* value. The creep cracking and rupture life in the high free nitrogen materials were more sensitive to the material condition, for example, pre-straining and/or heat treatment because of the role of the free nitrogen in the steels.     

Microstructural modification during isothermal ageing of a low nickel duplex stainless steel

In order to reduce the cost of duplex stainless steel (DSS), the Ni could be partially substituted by Mn and N, to maintain the alpha/gamma microstructure, the corrosion resistance and mechanical strength similar to the common Ni–Cr–Mo DSS. In this paper the microstructure stability, impact strength and the general corrosion behaviour of a low Ni grade, 22Cr–4Mn–0,2N DSS, were examined. In the solution annealing condition the corrosion resistance is quite similar to the austenitic grade. A moderate precipitation of nitrides and carbides was evidenced after isothermal treatment in the range 600–900 °C, while no dangerous topologically close packed phases (TCP-phases) were detected. The precipitation affects the impact strength, which decreases to about 50 J, while the corrosion resistance is less markedly affected.     

Dilatometric study of the sintering mechanism of nanocrystalline cemented carbides

Two types of WC-Co powders, nanometer particle size and micron particle size, were studied by means of dilatometry. The experiments showed that the sintering behavior of the nanocrystalline specimens is different from that of the micron grain-size specimen. While the sintering of the micron particle sized powder to full density is mainly done in the liquid phase, the nanoparticle sized powder can be sintered in the solid phase, at a lower temperature.