Papers by Keyword: PM Steel

Abstract: Local mechanical properties of high Cr-alloyed sintered and cast steels with the same chemical composition were investigated using instrumented indentation method. Standard loading/unloading mode was applied, the measurements were done in load range 1 – 500 mN. Load size effect was observed and its parameters were evaluated. Indentation hardness and elastic modulus were found slightly higher for the sintered material. Differences in indentation parameters were explained based on microstructure of materials.

Abstract: It is now fairly well established that to achieve low values of the Paris exponent for the growth of fatigue cracks in PM steels, high values of fracture toughness are required. Fracture toughness is related to other measures of toughness, such as impact tests and the mechanical work that the material can absorb before fracturing. All of these are functions of the basic ductility of the material. A coherent picture of all these inter-relationships is presented.

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.

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: Fe-4Ni-0.5Mo-1Cu powder was selected as raw material, pressed and sinter-hardened at 1135°C for 30 min with rapid cooling. The density varies in the range of 7.24-7.29 g/cm3. Its fatigue properties have been tested in axial loading of alternating tensile/compressive stress at R=-1 with a servo-pulse pump. The fatigue endurance limit was measured to be 260 MPa. The microstructure showed more homogeneous bainite and martensite. Neither Ni nor Mo rich areas were detected. Fractography displayed the fatigue cracks initiated from the pore areas near the surface. A nontypical ductile fatigue striation was found. More dimples occurred on fracture surface due to the plastic deformation, which can prohibit cracking propagation and improve its fatigue properties.

Abstract: The characteristics of residual stresses occurring in PM steel based nickel (FLN2-4405) was investigated. Residual stresses were measured by electrochemical layer removal technique. The values and distributions of residual stresses occurring in PM steel processed under various densities (6.8, 7.05, 7.2 and 7.4 g/cm3) and heat treatment conditions (sintered at 2050 ºF, sintered at 2300 ºF, quenching-tempered, and sinter-hardened) were determined. In most of the experiments, tensile residual stresses were recorded on the surface of the samples. The residual stress distribution on the surface of the PM steels was found to be affected by the heat treatment conditions and density. The maximum values of residual stresses on the surface showed sinter hardened condition and a density of 7.4 g/cm3. The minimum level of recorded tensile residual stresses are150 MPa and its maximum level is 370 MPa.

Abstract: The effect of tempering temperature and microstructure on dry sliding wear behavior of quenched and tempered PM steels was investigated. For this purpose, atomized iron powder was mixed with 0.3 % graphite and 1-2 % Ni powders. The mixed powders were cold pressed and sintered at 1200°C. The sintered specimens were quenched from 890°C and then tempered at 200°C and 600°C for 1 hr. Wear tests were carried out on the quenched+tempered specimens under dry sliding wear conditions using a pin-on-disk type machine at constant load and speed. The experimental results showed that the wear coefficient effectively increased with increasing tempering temperature. With increasing Ni content, the wear coefficient slightly decreased at all tempering temperatures due to the high amount of Ni-rich austenitic areas.

Abstract: Powder grades pre-alloyed with 1.5-3 wt% chromium are suitable for PM steel components in high performance applications. These materials can be successfully sintered at the conventional temperature 1120 °C, although well-monitored sintering atmospheres with low oxygen partial pressures (<10-17-10-18 atm) are required to avoid oxidation. Mechanical properties of the Cralloyed PM grades are enhanced by a higher sintering temperature in the range 1120-1250 °C, due to positive effects from pore rounding, increased density and more effective oxide reduction. A material consisting of Astaloy CrM, which is pre-alloyed with 3 wt% Cr and 0.5 wt% Mo, and 0.6 wt% graphite obtains an ultimate tensile strength of 1470 MPa combined with an impact strength of 31 J at density 7.1 g/cm3, after sintering at 1250 °C followed by cooling at 2.5 °C/s and tempering.

Abstract: Production scale inert atmosphere microwave sintering has not been successful yet, mainly because of lack of suitable equipment. For sintering in air microwave furnaces are available at different industrial scale, e.g., gas-microwave hybrid heated batch kilns or microwave “adiabatic casket” tunnel kiln [1]. Inert atmosphere furnaces require vacuum and gastight coupling of microwave energy. In the State of the Art technology, microwave-coupling windows represent a heat sink inside the sintering furnace. Although, radiation heat loss at temperatures > 1600 °C can be reduced by using a thermally insulating casket inside the microwave furnace, heat loss and temperature gradients caused by the microwave transparent windows are still not acceptable. Therefore, a new concept was developed assisted heating in inert atmosphere sintering furnaces, enabling high power microwave coupling by means of high temperature resistant antennas [2]. This paper presents microwave sintering results of commercial powder metallurgical (PM)-parts, details of the antenna system and the processing technology.