Phosphorus as an alloy element is quite common in powder metallurgy, the contents industrially used being markedly higher than those present in wrought steels. However, embrittlement effects are reported also for sintered steels, in part depending on the alloy elements present. In this study, the influence of phosphorus addition on the mechanical properties of PM steels alloyed with Mo, as the most common VI group element in sintered steels, was investigated. PM steels of the type Fe-x%Mo-0.7%Cy% P were manufactured with varying contents of Mo and P, respectively. It showed that P activates sintering also in these materials and enhances Mo homogenization, but there is in fact a risk of embrittlement in these steels that however strongly depends on the combination of Mo and P in the materials: If a critical level is exceeded, embrittlement is observed. At low Mo contents, higher P concentrations are acceptable and vice versa, but e.g. in a material Fe-1.5%Mo-0.7%C-0.45%P, pronounced intergranular embrittlement occurs, further enhanced by sinter hardening effects. This undesirable phenomenon is more pronounced at higher sintering temperatures and in case of faster heating/cooling; it was observed both in materials prepared from mixed and prealloyed powders, respectively. This typical intergranular failure observed with embrittled specimens, in particular after impact testing, indicates the precipitation of brittle phases at the grain boundaries, apparently when exceeding the solubility product between Mo and P.
In situ characterization of the sintering process is a difficult task, in particular for systems without pronounced dimensional changes. Dilatometry is not too helpful in those cases, and therefore other properties have to be recorded. In the present study, sintering of ferrous powder compacts was studied in situ by measuring the thermal diffusivity a using a laser flash apparatus. This property is a measure to characterise the heat flow through a material; it depends on the contact area between the particles and thus reveals their change during sintering. It is shown that the change of a during sintering of ferrous compacts is much less pronounced than in the case of cemented carbides which is not surprising when regarding the widely differing porosity changes. The results are however in good agreement with expectations when considering some experimental limitations. The trend for the thermal conductivity λ. which can be calculated from a, the specific heat and the density, is in good agreement with that found for the electrical conductivity, both properties being linked through Wiedemann-Franz’ law.
Phosphorus as an alloy element is quite common in powder metallurgy, the contents industrially used being markedly higher than those present in wrought steels. In this study, the influence of phosphorus addition through different P carriers was investigated. PM steels of the type Fe-0.7%C-x%P (x = 0.0 … 0.8%) were manufactured by pressing and sintering in H2. It showed that Fe3P is the best phosphorus carrier, resulting in fine and regular microstructure and in high impact energy data at 0.3 … 0.45%P while red P and also Fe2P showed a tendency to agglomeration, with resulting secondary porosity. At high P levels the mechanical properties tend to drop, for the tensile strength at P > 0.60%P while for the impact energy the threshold is 0.45%P. The dimensional behaviour of Fe-C-P can be related to PM aluminium alloys, expansion by transient liquid phase being followed by shrinkage by persistent liquid phase, at least at higher temperatures. In contrast to the dimensional behaviour, degassing and reduction is hardly affected by the phosphorus content.
Sintering of Cr-prealloyed PM steels requires atmospheres with good quality – low oxygen potential – to achieve satisfactory sintering results. But during heating even the best atmospheres may be oxidizing, the system turns to reducing conditions only at high temperatures, which can be monitored by thermal analysis. During the dewaxing process, oxidizing conditions are favourable for effective dewaxing without sooting and blistering. However, this may result in some oxygen pickup during heating, and then the final properties of the produced parts may be strongly influenced by this intermediate oxidation. This study demonstrates the behaviour of artificially oxidized steels (Fe-C and Fe3Cr-0.5Mo-C) during the sintering process by stepwise sintering. Iron and steel powder were slightly oxidized and then pressed and sintered at different temperatures. In parallel, as a second approach, pressed samples were oxidized and then sintered. Density, hardness and impact energy were measured and dilatometry/MS was used for online monitoring of the sintering process. The starting oxygen content of 0.20 to 0.30 wt% is high enough to change the sintering behaviour of the materials, but still leads to rather good properties. Thermal analysis showed that most of the oxygen picked up was present as iron oxides on the surface which were reduced by hydrogen at rather low temperatures, confirming that these were iron oxides, which also holds for the Cr-prealloyed variant. The biggest influence on the final performance was exerted by the final carbon content and the microstructural development of the material.
Boron has been known to activate densification during sintering of ferrous powder compacts, though with risk of embrittlement. In the present study, specimens Fe-B and Fe-C-B prepared from standard atomized iron powder with addition of ferroboron Fe-21%B were sintered in different atmospheres, and the resulting microstructures and properties were studied. It showed that the activating effect of boron is observed during sintering in argon and in hydrogen while sintering in N2 containing atmospheres results in rapid deactivation of boron, through formation of stable BN. In hydrogen atmosphere, surface deboronizing was observed to considerable depth. Ar is chemically inert, but Ar trapped inside closed pores tends to inhibit further densification. The impact energy data indicated that the embrittling effect of boron is enhanced significantly by presence of carbon. In the fracture surfaces, transgranular cleavage fracture can be observed both at very low and high impact energy values.
Fused Layer Modelling (FLM) is one out of several material extrusion (ME) additive manufacturing (AM) methods. FLM usually deals with processing of polymeric materials but can also be used to process metal-filled polymeric systems to produce metallic parts. Using FLM for this purpose helps to save costs since the FLM hardware is cheap compared to e.g. direct metal laser processing hardware, and FLM offers an alternative route to the production of metallic components.
To produce metallic parts by FLM, the methodology is different from direct metal processing technologies, and several processing steps are required: First, filaments consisting of a special polymer-metal composition are produced. The filament is then transformed into shaped parts by using FLM process technology. Subsequently the polymeric binder is removed (”debinding”) and finally the metallic powder body is sintered. Depending on the metal powder used, the binder composition, the FLM production parameters and also the debinding and sintering processes must be carefully adapted and optimized.
The focal points of this study are as following:
1. To confirm that metallic parts can be produced by using FLM plus debinding and sintering as an alternative route to direct metal additive manufacturing.
2. Determination of process parameters, depending on the used metal powders (steel and copper) and optimization of each process step.
3. Comparison of the production paths for the different metal powders and their debinding and sintering behavior as well as the final properties of the produced parts.
The results showed that both materials were printable after adjusting the FLM parameters, metallic parts being produced for both metal powder systems. The production method and the sintering process worked out well for both powders. However there are specific challenges in the sintering process that have to be overcome to produce high quality metal parts. This study serves as a fundamental basis for understanding when it comes to the processing of steel and copper powder into metallic parts using FLM processing technology.
For powder metallurgy products, high density is an essential requirements to obtain maximum mechanical properties. Here, supersolidus liquid phase sintering (SSPLS) is an effective means to attain high sintered density, as known from PM high speed steels. In the present work it is shown that this technique can also be applied to Cr prealloyed low alloy steel grades. Supersolidus sintering through indirect heating requires precise control of temperature and also the atmosphere, to avoid uncontrolled changes of the carbon level. Higher C contents are beneficial here since they enable lower temperatures and result in wider temperature windows for sintering. The temperatures necessary for SSLPS at moderate C levels are fairly high for standard sintering furnaces, therefore induction sintering was studied in this work. It showed that, as was to be expected, also here precise temperature control is required, but for any carbon level tested a sintering temperature could be identified that yielded high sintered density and good shape retention. The high density attained, in combination with the very high temperatures, results in pronounced grain growth, this process no more being inhibited by the presence of pores, which is undesirable but can however be remedied by suitable heat treatment.