Evaluation of Hydrogen-Assisted Cracking Susceptibility in Grade T24 Steel

The delayed hydrogen cracking test was performed to evaluate the hydrogen-assisted cracking (HAC) susceptibility of Grade T24 steel base metal and the simulated coarse-grained heat-affected zone (CGHAZ). The base metal did not fail after testing for up to 672 h. In contrast, the CGHAZ sample failed after about 2 h when charged from all four sides, and 4 h when charged only from the internal diameter (ID) surface. The higher HAC resistance of the base metal compared to the CGHAZ was due to the microstructure difference. The tempered bainitic-martensitic microstructure in the base metal was more resistant to HAC compared to the untempered martensite microstructure in the CGHAZ. Fractography analysis indicated the decarburized zone on the ID surface delayed the development of the critical hydrogen concentration in the CGHAZ, thus improving the HAC resistance. The HAC cracking initiated with an intergranular fracture, then transitioned to quasi-cleavage and microvoid coalescence. The fracture behavior was explained using Beachem’s model.

Effective Yield Criterion Accounting for Microvoid Coalescence

An effective yield function is derived for a porous ductile solid near a state of failure by microvoid coalescence. Homogenization theory combined with limit analysis are used to that end. A cylindrical cell is taken to contain a coaxial cylindrical void of finite height. Plastic flow in the intervoid matrix is described by J2 theory while regions above and below the void remain rigid. Velocity boundary conditions are employed which are compatible with an overall uniaxial straining for the cell, a postlocalization kinematics that is ubiquitous during the coalescence of neighboring microvoids in rate-independent solids. Such boundary conditions are not of the uniform strain rate kind, as is the case for Gursonlike models. A similar limit analysis problem for a square-prismatic cell containing a square-prismatic void was posed long ago (Thomason, P. F., 1985, “Three-Dimensional Models for the Plastic Limit–Loads at Incipient Failure of the Intervoid Matrix in Ductile Porous Solids,” Acta Metallurgica, 33, pp. 1079–1085). However, to date a closed-form solution to this problem has been lacking. Instead, an empirical expression of the yield function proposed therein has been widely used in the literature. The fully analytical expression derived here is intended to be used concurrently with a Gursonlike yield function in numerical simulations of ductile fracture.

The Shear Response of a Thin Aluminum Layer

Aluminum layers, 10–50 μm thick, have been diffusion bonded to alumina blocks and subjected to simple shear in order to determine the sensitivity of shear stress versus strain response to layer thickness. No significant thickness effect on strength is observed and reversed loading tests indicate isotropic hardening. Final failure in shear is by microvoid coalescence within the sandwich layer with a void spacing comparable to the layer thickness. The significance of the results for strain gradient plasticity theory is discussed.

Effect of Deformation Temperature on the Hot Ductility of Non-Oriented Electrical Steels

The hot tensile ductility of solution treated Si-Al electrical steels was investigated at temperatures between 850 and 1150 °C. Samples for mechanical testing were obtained from continuous cast thin slabs and hot rolled strips. A continuous decrease in ductility was observed up to about 1000 °C. After that, the ductility was recovered in strip samples while in slab samples the ductility remained constant at RA<10%. This behavior was associated with the presence of large quantities of undissolved AlN particles formed during slow cooling of the slab. In the case of strip specimens, where the starting slab is not cooled to room temperature, the 1000 °C ductility minimum was attributed to strain localization at grain boundary nucleated ferrite grains. Rapid nucleation and growth of microvoids at AlN particles formed during cooling to test temperatures in the vicinity of Ae3 resulted in intergranular tensile failure by microvoid coalescence.

Effect of Heat Treatment on Microstructures and Properties of Mg-9Gd-4Y-0.3Zr Alloy

microstructures, aging hardness, mechanical properties of Mg-9Gd-4Y-0.3Zr alloys were investigated. The microstructure is a typical dendritic structure of as-cast sample, The aging test of extruded samples were carried at a temperature rang of 200-300°C and at a different aging time. The aging peak hardness is about 120HV, tensile strength was tested at temperature 25°C, 200°C, 250°C and 300°C, tensile strengths are 375 Mpa, 364 Mpa, 329 Mpa, 286 Mpa respectively, the maximum elongation is 13.32% at 300°C. The fracture mode is mainly microvoid coalescence fracture combination the brittle cleavage fracture at room temperature, and microvoid coalescence fracture at 200-300°C.

Influence of Hydrogen on Tensile Property of Ti-6Al-4V

The influence of hydrogen content on the microstructure and the tensile property of Ti-6Al-4V alloy was studied, and the phenomenon of minimum yield stress at certain hydrogen content was discussed. The results show that Ti-6Al-4V alloy can absorb hydrogen above 600°C and the different hydrogen contents can be achieved by changing the flow rate of hydrogen. With increase of hydrogen contents, the microstructure gradually transforms from the original near basket-type to the α clusters which consist of α plates and hydrides distributed in α plates, and then to the mixture of α, β and the large amount of hydrides. When the specimens tensioned at 600°C, their strength first decreases and then increases, but their ductility changes quite the contrary as increasing hydrogen contents. There is optimum hydrogen content at which the strength is the lowest and the plasticity is the highest for the specimens tensioned at 600°C. Ti-6Al-4V alloy may gain the higher tensile strength or better ductility at 600°C through appropriate hydrogenation treatment in comparison with samples untreated. With increase of hydrogen contents, the fracture type transforms from microvoid coalescence type to “cleavage like” type for specimens tensioned at 600°C.

In-situ observations of microvoid coalescence: Stacking fault energy effects

Although it has been well established that microvoid coalescence occurs during static or quasi-static fracture in ductile materials, the exact mechanism for microvoid formation is still unclear. It has been argued that microvoids initiate and grow from second phase particles. However this argument cannot be used to explain the existence of microvoids on the fracture surfaces of "pure" materials. An alternative mechanism for their formation in "pure" materials is that they initiate and grow along dislocation cell walls. If this premise is true; then the nature and extent of microvoid coalescence should be related to the stacking fault energy (SFE) of the material since the latter is a controlling parameter in the formation of dislocation cells. The relationship between microvoid coalescence and stacking fault energy may have some basis since absolute cell dimensions are of the same magnitude as the observed dimple sizes. The present study examines the effect of dislocation cell structures on the formation of microvoids as a function of the stacking fault energy of a given material through direct observation of the void formation and growth process within the TEM. The fundamental aspects of the work is to correlate the dislocation substructures, void initiation, growth, and coalescence to the resulting fracture surfaces.

Influence of Sulfur Content on the Fracture Toughness Properties of 2 1/4 Percent Cr-1 Percent Mo Steel

The effects of sulfur content on the fracture toughness properties of 2 1/4Cr-1 Mo steel were evaluated at test temperatures above, at, and below the nil ductility transition temperature (NDTT) of −23°C (−10°F). Small, 12.7-mm (0.5-in.) thick compact tension specimen results were combined with J-integral, Equivalent Energy, and Crack Opening Displacement analytical techniques to provide KIc results up to 22°C (72°F). It was found that the sulfur content of this steel has a large detrimental effect on KIc at the NDTT and above, where microvoid coalescence is the fracture mode. Sulfur has no significant effect at −73°C (−100°F) where cleavage occurs. These results also indicate that the higher Charpy V-notch energy at NDTT, shown by lower sulfur steels, is translatable into increased fracture resistance.