Progress in Powder Metallurgy

Abstract: This work presents mechanical properties and corrosion resistance of duplex stainless steels obtained through powder metallurgy starting from austenitic X2CrNiMo17-12-2 (AISI 316L), martensitic X6Cr13 (AISI 410L) powders by controlled addition of alloying elements in the proper quantity to obtain the chemical composition of the structure similar to biphasic one. In the mixes preparations the Schaffler’s diagram was taken into consideration. Prepared mixes of powders have been sintered in a vacuum furnace with argon backfilling. After sintering rapid cooling was applied using nitrogen. Produced duplex stainless steels have been studied by SEM with EDS and light optical microscopy (LOM) and X-rays analysis to determine obtained structures. Corrosion properties have been studied through electrochemical methods in 1M NaCl.

Abstract: This paper investigates the possibilities of niobium using for the mechanical properties of the common low alloyed Cu-Ni-Mo-C structural sintered steels enhancing by a similar technique with that used in the classical HSLA steels obtaining. For this purpose, both thermodynamic and experimental investigations were carried out, their results showing that the added Nb to these steels form at sintering in a nitrogen containing atmosphere, rather then the other alloying elements, carbides, nitrides, carbonitrides. They are solved in austenite and precipitate at the sintered steels cooling / artificial aging, forming precipitates of an expected strengthening effect. The results for 0.2 ÷ 1.0 % fine Nb powder and 0.3 % graphite additions to Distaloy AB powder cold compacted and sintered in dissociated ammonia proved that such a strengthening effect, which leads to higher mechanical properties of the processed steels than of the Cu-Ni-Mo-C ones, can be achieved.

Abstract: Stainless steel has become increasingly used in the nuclear industry recently. Thus, this study is aimed at investigating stainless steel 316L with boron addition and the possibility of sinter these materials in nitrogen rich atmospheres. By analyzing the final product, the properties of the stainless steel 316L (good mechanical properties and high corrosion resistance) with the boron neutron absorption properties were found to unify. The P/M technologies enable higher boron quantities to be added to the steel. This was not possible with the solidification conventional technologies, as segregation is produced in the latter. Mixtures with 0.75 and 1.5% boron were prepared. Uniaxial compaction (at 700 MPa) was carried out to study the green density of compacted materials. The sintering atmosphere used was N2-10%H2-0.1%CH4, and was used to form boron nitrides instead of chromium nitrides. Although some boron nitride was formed, not all chromium nitride formation was avoided. The sintered samples were characterized through their physical properties (density and dimensional change), chemical analysis (carbon and nitrogen contents), mechanical behavior (bending strength and hardness) and wear behavior. To finish the materials characterization, a microstructural study is proposed. Lastly, the wear tracks were observed by SEM. Boron nitride has precipitated in grain boundaries, making more difficult the sintering of the material and reducing the properties of the stainless steel.

Abstract: 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.

Abstract: Tensile stress-strain and dynamic acoustic resonance tests were performed on Fe-C-Ni- Cu-Mo high-strength steels, characterized by a heterogeneous matrix microstructure and the prevalence of open porosity. All materials display the first yielding phenomenon and, successively, a continuous yielding behavior. This flow behavior can be described by the Ludwigson equation and developes through three stages: the onset of localized plastic deformation at the pore edges; the evolution of plastic deformation at the pore necks (where the austenitic Ni-rich phase is predominant); the spreading of plastic deformation in the interior of the matrix. The analytical modeling of the strain hardening behavior made it possible to obtain the boundaries between the different deformation stages.

Abstract: The stress corrosion cracking (SCC) susceptibility of Alloy 600 MA, Alloy 600 TT, Alloy 800, and Alloy 690 TT were investigated in a deaerated 0.01 M solution of sodium tetrathionate using reverse u-bend test samples at 340 °C. The results showed that SCC occurred in all alloys, excluding Alloy 690 TT. The SCC susceptibility of the alloys increased in the following order: Alloy 690 TT, Alloy 800, Alloy 600 TT, and Alloy 600 MA. The SCC susceptibility decreased with an increase in the chromium content of the alloys. The results of the deposits and spectra taken from an energy dispersive X-ray system confirmed the existence of a reduced sulfur causing SCC.

Abstract: The influence of porosity (P) on Young’s modulus (E) and Poisson’s ratio (υ) of sintered steels produced from four types of steel powders were investigated. The values of E and υ depends mainly on the value of P, and those were a little affected by alloying elements. The relationship between compacting pressure (CP) and P and the relationships between E, υ, and P were described as following equations: P = P1 + P2 exp(- CP / P3), E = E0 (1 - kEP)2, and υ = (υ0 - υsub)(1 - kυP)2 + υsub, where subscript 0 means P = 0, and P1 - 3, kE, kυ and υsub were empirical constants. It was clarified that if powder and sintering temperature is defined, the elastic moduli of sintered steel can be calculated as a function of compacting pressure.

Abstract: The effect of chemical composition of the sintering atmosphere on the density, microstructure and mechanical properties of Fe-3%Mn-(Cr)-(Mo)-0.3%C is described. Pre-alloyed Astaloy CrM and CrL, ferromanganese and graphite powders were the starting powders. Following the pressing in rigid dies, compacts (green density approx. 6.8-7.1 g/cm3) were sintered at 1120 and 1250°C in atmospheres with different H2 and N2 content. The dew point of the sintering atmospheres was below -60°C. Subsequently samples were furnace cooled to room temperature. Tensile and transverse rupture strengths, elongation, R0.2 yield offset, impact toughness and apparent surface hardness were examined. Following the mechanical tests, to investigate microstructure optical microscopy was employed. As the results show, sintering in nitrogen-rich atmospheres allow to achieve comparable properties of the specimens with those of the specimens sintered in hydrogen-rich atmosphere. It means that it is possible to produce sintered Fe-Mn-Cr-Mo-C PM steels in safe and cheaper nitrogen-rich atmosphere.

Abstract: The effect of different cooling rate after sintering Mn steels at 1120°C and 1250°C by adding Cr and Mo was investigated. Pre-alloyed Astaloy CrM and CrL, ferromanganese and graphite powders were used as the starting powders. Pressing was carried out in steel rigid dies to achieve the same density in the range of 6.8-7.2 g/cm3. During pressing rectangular and ISO 2740 specimens were prepared. Then, green compacts were sintered at 1120°C and 1250°C for 60 minutes in atmosphere with different H2/N2 content and cooled at a cooling rate 1.4°C/min (slow, furnace cooling) and 65°C/min (convective cooling). Convecitve cooled specimens were subsequently tempered at 200°C for 60 and 240 minutes. To investigate the mechanical properties: UTS, elongation A, R0.2 yield offset, TRS, impact toughness IT and hardness measurements results were examined. After the mechanical examination, the microstructure of Fe-Mn-Cr-Mo-C PM steels were studied by optical microscopy.