Materials Science Forum Vols. 534-536

Abstract: Most PM components are exposed to cyclic loading over long periods of time, and the endurable stress amplitude is an important factor considered in many applications. With the continuously growing volume of experimental test results, functional relationships can be derived which permit predictions on the effects of density ρ, external notches characterized by a stress concentration factor Kt, and static mean stresses σm for standardized test specimen geometries.

Abstract: Crack initiation and short crack propagation was studied on the polished notched surfaces of Cr-Mo prealloy sintered steels with 7.35 g.cm-3 sintered density. An ultrasonic resonance test system operating in push-pull mode at 20 kHz and R=-1 was used. It showed that crack initiation took place in several places, small cracks growing oriented to the local pore structure rather than to stress orientation. Their growth rate is markedly higher than the corresponding one of long cracks. Finally, several microcracks join to form a dominant crack.

Abstract: For attaining optimum fatigue resistance of PM steels, high density levels are necessary. In this work, sintered steels Fe-1.5%Mo-0.6%C and Fe-1.5%Cr-0.2%Mo-0.6%C were produced with density levels of 7.1 to 7.6 g.cm-3. Ultrasonic fatigue testing with 20 kHz was performed in push- pull mode up to 10E9 cycles. It was shown that the fatigue endurance strength is strongly improved by higher density levels, but also higher sintering temperatures are beneficial. The Cr-Mo steels proved to be superior to the plain Mo alloyed, due to a more favourable as-sintered matrix microstructure.

Abstract: Based on the comparison of structures and properties of the HS6-5-2 high speed steels made with the powder injection moulding method, pressureless forming, compacting and sintering, and commercial steels made with the ASEA-STORA method, fine carbides spread evenly in the steel matrix were found in the structure of all tested high-speed steels in the sintered state. The use of a nitrogen atmosphere in the sintering process, causes the formation of fine, spherical MX type carbonitrides, stable in high sintering and austenitizing temperatures. The steels made with the pressureless forming method are characteristic of the lowest sintering temperature and the highest density, resulting from the high carbon concentration coming from the binding agent degradation. Moreover, the higher carbon concentration causes an increase in the retained austenite portion and a lower hardness after quenching and tempering. The heat-treated injection moulded steel attains hardness comparable to the commercial ASP23 type one, demonstrating the well-founded reasons for using the powder-injection moulding method for manufacturing the high-speed steel. The powder-injection moulding makes manufacturing tools possible with their final shape, i.e., leaving out the plastic forming and machining which is necessary for instance in case of the ASP 23 type steel. Furthermore, the degradation and sintering process time of the injection moulded steels is approximately 10h shorter than for steels made with pressureless moulding, which is due to the use of a two-component binding agent.

Abstract: The efforts to increase the potential PM market [makes necessary to accept new challenges to develop new products. To address this question, we can consider modifying the pores system or the material composition and at same time, the cost of the alloying elements and the compromise between strength, tolerances and cost. The present study examines the sintering behavior and effect of manganese addition, both mechanically-blended and mechanically alloyed, on Cr-Mo low alloyed steels to enhance the mechanical properties. Mn sublimation during sintering results in some specific phenomena to occur which facilitate the sintering of alloying elements with high oxygen affinity. To benefit from the Mn sublimation effects, small Mn particles must be homogenously added in order to increase the specific surface available to sublimate. First, the milling time is optimized to attain a master alloy with 50% of Mn that is diluted in Fe-1.5Cr-0.2Mo water atomized prealloyed powder by normal mixing. These mixtures were pressed to a green density of 7.1 g/cm3 and sintered at 1120 °C in 90N2-10H2 atmosphere. The resulting mechanical properties and the microstructures are discussed considering the high energy stored in the master alloy which favors the mass transport mechanism during sintering.

Abstract: Globalization enables P/M part makers to choose powders from different sources. Raw materials produced by a given process and having equal chemical composition are supposed to be equivalent. The differences in sintering behavior, in industrial equipment, have been investigated on P/M steels obtained from four diffusion-bonded powders (Fe + Ni + Cu + Mo) on atomized iron base, at the same alloy contents. Two levels of carbon and two sintering conditions have been investigated. Dimensional changes, C content, hardness, microhardness pattern, universal hardness, fractal analysis, pore features, microstructure features, and rupture strength have been compared, to characterize different raw materials. An index of homogeneity of microstructures, based on a specific statistical approach, does not agree completely with observed microstructures and pore feature. The results show that the claimed equivalence is not confirmed by experimental data. Analyses of microhardness and microstructure distributions seem powerful tools to rate the real equivalence of so claimed powders. P/M part makers may use the proposed approach to assess detectable differences on performances among powders that are declared as “true” substitute, so confirming or contradicting any even not negligible cost difference. The comparison methods here described and applied can also help powder producers to improve the suitability of their products to sintering processes.

Abstract: A Mn-containing master alloy (MA) has been specially designed, through thermodynamic and metallurgical criteria, for obtaining high performance low alloy PM steels by SPSS or DPDS. This MA exhibits improved characteristics with respect to ferromanganese and other Mn carriers for alloying PM steels preventing oxidation, keeping a high compressibility of the powder mixture and providing opportunities for low temperature processing. The improved sinterability through the formation of a transient liquid phase leads to dimensional stability and high reproducibility of mechanical properties after sintering at 1120°C. The microstructural development of the PM steels was studied during the sintering cycles. The final microstructure of these PM steels, after defined sintering cycles, was characterised by LOM while the mechanical properties of the consolidated materials were determined by tensile testing.

Abstract: Powder forging is used for heavy-loaded parts (rings of rolling-contact bearings, gears etc.) production. Rolling contact fatigue is material property values of which characterize possibility of practical utilization of such parts. Rolling contact fatigue of some steels obtained out of prealloyed powders Astaloy CrM, Atomet 4601, Atomet 4901 and powder blends iron-carbon-nickel by hot forging is studied in the present paper. Effect of various kinds of heat and thermomechanical treatment on rolling contact fatigue is determined. Thermomechanical treatment provides optimal values of rolling contact fatigue. In this case steel structure contains up to 40% of retained metastable austenite which is transformed to martensite on trials. Thus typically crack is generated on residual pores and non-metallic inclusions instead of martensite zones in wrought steels.

Abstract: A new molybdenum hybrid-alloyed steel powder has been developed. The powder is based on a molybdenum prealloyed steel powder to which molybdenum powder particles have been diffusion bonded. The sintered compact made of the developed powder has a finer pore structure than that of the conventional molybdenum prealloyed steel powder, because the ferritic iron phase (α-phase) with a high diffusion coefficient is formed in the sintering necks where molybdenum is concentrated resulting in enhanced sintering. The rolling contact fatigue strength of the sintered and carburized compacts made of this powder improved by a factor of 3.6 compared with that of the conventional molybdenum prealloyed steel powder. The improvement in the rolling contact fatigue strength should result from the fine pore structure without coarse pores acting as stress concentration points.

Abstract: Several chemicals were studied to suppress the damage due to stress corrosion cracking (SCC) of steam generator (SG) tubes in nuclear power plants. The polarization curves showed that the electrochemical properties on the surface of Alloy 600 MA changed with the addition of inhibitors. The SCC tests were conducted by using a m-RUB specimen in a 10% NaOH solution at a temperature of 315°C. The effects on the SCC of the compounds, TiO2, TyzorLA and CeB6, were tested for several types of SG tubing materials. The test with the addition of TiO2 (P25) and CeB6 showed an effect in decreasing the SCC for the SG tubing material. However, CeB6 caused some more SCC for Alloy 800. The penetration property into a crevice of the inhibitors was investigated by using Alloy 600 specimens with different gap sizes and an AES analysis was performed on the oxide layer of the specimen.