G. Silva

Study of the SCC Behavior of 7075 Aluminum Alloy After One-Step Aging at 163 C

For the past many years, 7075 aluminum alloys have been widely used especially in those applications for which high mechanical performances are required. It is well known that the alloy in the T6 condition is characterized by the highest ultimate and yield strengths, but, at the same time, by poor stress corrosion cracking (SCC) resistance. For this reason, in the aeronautic applications, new heat treatments have been introduced to produce T7X conditions, which are characterized by lower mechanical strength, but very good SCC behavior, when compared with the T6 condition. The aim of this study is to study the tensile properties and the SCC behavior of 7075 thick plates when submitted to a single-step aging by varying the aging times. The tests were carried out according to the standards and the data obtained from the SCC tests were analyzed quantitatively using an image analysis software. The results show that, when compared with the T7X conditions, the single-step aging performed in the laboratory can produce acceptable tensile and SCC properties.

The Influence of Notch Root Radius and Austenitizing Temperature on Fracture Appearance of As-Quenched Charpy-V Type AISI 4340 Steel Specimens

Charpy-V type samples either step-quenched from 1200 °C or directly quenched from the usual 870 °C temperature, fractured by a slow bend test procedure, have been fractographically examined. Their notch root radius,ρ, ranged from almost zero (fatigue precrack) up to 2.0 mm. The fracture initiation process at the notch differs according to root radius and heat treatment. Conventionally austenitized samples withρ values larger than 0.07 mm approximately (ρeff) always display a continuous shear lip formation along the notch surface, whereas specimens with smaller notches do not exhibit a similar feature. Moreover, shear lip width in specimens withρ >ρeff is linearly related to the applied J-integral at fracture. In high temperature austenitized samples similar shear lips are almost nonexistent. The above findings, as well as overall fractographic features, are combined to explain why blunt notch AISI 4340 steel specimens display a better fracture resistance if they are conventionally heat treated, whereas fatigue precracked samples show a superior fracture toughness when they are step-quenched from 1200 °C. Variations of fracture morphologies with the notch root radius and heat treating procedures are associated with a shift toward higher Charpy transition temperatures under the combined influence of decreasing root radii and coarsening of the prior austenitic grain size at high austenitizing temperatures.

Microstructural and mechanical characterisation of some sinter hardening alloys and comparisons with heat treated PM steels

Abstract Four grades of sinter hardening materials have been compared, using industrial equipment. Three powder types were completely prealloyed; the last one was a hybrid, combining prealloying and diffusion bonding. Different amounts of Cu have been added by mixing. The lubricated mixes, containing 0.6% graphite, have been compacted at different pressures, to form gears at green densities ≥7.0 g cm−3 ; the compacts have been sintered at 1120°C, under endogas from methane and fast cooled (at least 7 K s−1 within the range 850–400°C). The final step has been stress relieving, at 180°C, for 1 h. Material properties have been investigated, focusing on porosity, pore shape, hardness, microhardness, microstructure, local chemical composition and mechanical properties. For comparison, other gears, compacted in the same tool and at the same density level, but manufactured according to a more conventional cycle, i.e. starting from less alloyed powders and adding carbonitriding, quenching and stress relieving, have been used. The analysis of the different experimental results enabled the authors to find out and outline some criteria suitable for selecting sinter hardening materials and for choosing more reliable manufacturing conditions to fulfill specific application requirements.

Influence of Notch Root Radius on Ductile Rupture Fracture Toughness Evaluation with Charpy-V Type Specimens

Abstract The blunt notch fracture toughness of four types of carbon-manganese steel (ASTM A516 grade 70) has been determined by J -integral tests on Charpy-V type samples with different values of notch root radius, ρ. J-ρ plots, determined using specimens with a notch depth to width ratio, a/w, equal to 0.5, have shown the existence of a limiting ρ value ( ρ e ff ) below which applied J -intergral values at fracture initiation are constant. These ρ eff values have been seen to depend only on second-phase particle distribution and not on their volume fraction or on the steel ferritic grain size. The procedure for deriving J -integral values at the onset of stable crack growth from J resistance curves in the case of notches has also been discussed. Experiments with Charpy specimens with a/w = 0.2 do not allow the derivation of meaningful J - ρ plots. In all cases, a ductile fracture criterion based on the constancy of the notch tip strain at rupture initiation has been proved when ρ > ρ eff .

Sinter hardening of low-alloy steels: influence of part geometry and physical properties of the material

Assuming that the cooling rate of parts depends on thermal conductivity of the material and on the surface/volume ratio, a two-ways study has been carried out. Firstly, the isothermal surfaces have been calculated by a numerical method on parallelepipeds of constant cross section and varying height and weight. Microstructures, depending on isothermal surfaces predicted by the numerical model, agree with microhardness and mechanical properties. A maximum weight of about 200 grams has been observed to be apt to a full sinter-hardening process and the surface/volume ratio seems to be the critical driving factor. Numerical analysis and experimental tests show that the thermal diffusivity of sintered steels increases as porosity increases.

Experimental Investigation of Microstructural and Mechanical Properties of New Steels for Large Blooms

In the automotive world, more and more components are produced by polymer injections using steel moulds reaching very large sizes, up to 1 m × 1 m sections or even more. These steels should have good thermal conductivity, good weldability, high machinability (especially for deep drilling), and good disposition to polishing and photoengraving. Moreover, high toughness, hardness, wear, and fatigue resistance across the whole thickness of the mould are strongly required. For satisfying these demands, chemical composition and heat treatment cycles of the steel shall be properly designed and optimized. In this paper two mould steels, which were recently developed, were experimentally investigated. Samples were machined from the surface and the core of each bloom and a full mechanical and microstructural analysis was carried out. The tensile properties at room temperature and at high temperature were investigated, together with the fracture toughness. All the obtained results were compared with the traditional and reference ISO 1.2738 mould steel. Moreover, dilatometric tests were performed to investigate the specific microstructural transformations during continuous cooling and isothermal treatments. The obtained results show good mechanical strengths for the whole thickness of the blooms associated with adequate toughness parameters. Finally, results were related to microstructural.

Stable and Unstable Crack Growth in Chromium Pre-Alloyed Steel

Powder metallurgy processing of steels typically results in a material characterized by residual porosity, whose dimension and morphology, together with the microstructure, strongly affect the fatigue crack growth behaviour of the material. Prismatic specimens were pressed at 7.0 g/cm3 density from Astaloy CrM powder and sintered in different conditions, varying the sintering temperature and the cooling rate after sintering. Optical observations allowed to evaluate the dimensions and the morphology of the porosity and the microstructural characteristics for all the investigated conditions. Fatigue tests were performed at R-ratio equal to 0.1 to investigate the threshold zone and to calculate the Paris law coefficients. All the tests were carried out according with the compliance method, and the crack length has been evaluated during whole the test. Moreover KIc tests were performed in order to complete the investigation. Both on fatigue and KIc samples a fractographic analysis was carried out to investigate the crack path and the fracture surface features. The results show that the Paris law crack growth exponent is around 6.0 for 1120°C sintered and around 4.7 for 1250°C sintered materials. The same dependence to process parameters is not found for KIth. Values close to 6 MPa√m are here found for all variants.

Observation of Austenitic Grain Growth in Some Alloy Structural Steels

In order to satisfy the demands coming from the thermal and thermo-chemical treatments, higher treatment temperature values are required to shorten the soaking time or to increase the structurally modified thickness, or both. Toughness and mechanical strength of a steel are strictly related to the presence of small grain dimension microstructure, which can be obtained through the presence of precipitates at the grain boundaries. The refinement of the microstructure is connected to the chemical composition and dimension of the precipitates, which dissolve partially or totally above a critical temperature. In this paper, two steels for mechanical application have been studied. In the supply conditions these steels are characterized by a precipitation condition due to the thermal treatment carried out in the steelmaker plant. By metallographic techniques, the grain growth has been studied, varying temperature and soaking time during thermal treatments. The dilatometric tests have been useful to follow the transformations able to contain the increase of the austenitic grain size and TEM analysis has contributed to confirm the nature of these transformations.

Steels for Plastic Molding: Relationship between Mechanical and Microstructural Properties

In the automotive world, many components are produced by polymer injection using steel molds with very large sizes, up to 1 m × 1 m sections or even larger. These steels should have good thermal conductivity, good weldability, high machinability, and good disposition to polishing and photoengraving. Moreover, high toughness and high mechanical properties across the whole mold thickness are strongly desired and appreciated. To satisfy these demands, chemical composition and heat-treatment cycles shall be properly designed and optimized. The traditional tensile and impact tests are useful for a basic characterization of the material, whereas the fracture toughness is related to an advanced and deeper analysis. Nevertheless, traditional tests are easier to be carried out and, hence, they are often preferred instead of the more complicated fracture-toughness tests. In this paper, the authors considered different mold steels as an attempt to correlate the fundamental mechanical parameters (i.e., the yield stress, the UTS, the elongation at rupture, and the absorbed impact energy) and the microstructures to the fracture toughness. Surface and core sampling position will be considered. The analysis of the available data resulted in very good correlations among the impact energy and the mechanical properties and among the fracture toughness and the impact energy.

Effect of shot peening on the fatigue behavior of some sintered steels

Shot peening is a mechanical treatment usually performed on metals for improving the mechanical properties of the surface layers. The application of such treatment to sintered steels causes a local surface densification and the birth of residual compression stresses. In this paper, the effect of different shot peening cycles, selected after a preliminary optimization, has been investigated focusing especially on the fatigue resistance measured by bending tests and on the surface residual stresses measured by XRD technique. Two high performances PM steels were considered: the former was sintered starting from a diffusion bonded powder, whereas the latter is based on a pre-alloyed powder. Two different nominal densities were considered, 6.9g/cm3 and 7.1g/cm3.