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Reaction of Unalloyed and Cr-Mo Alloyed Steels with Nitrogen from the Sintering Atmosphere

Abstract

Nitrogen is usually regarded as an inert sintering atmosphere for PM steels; however, this cannot be taken for granted in particular for steels alloyed with nitride forming elements. Among those elements, chromium has become more and more important as an alloying element in sintered low alloy structural steels in the last decade due to the moderate alloying cost and the excellent mechanical properties obtainable, in particular when sinter hardening is applied. The high affinity of Cr to oxygen and the possible ways to overcome related problems have been the subject of numerous studies, while the fact that chromium is also a fairly strong nitride forming element has largely been neglected at least for low alloy steel grades, although frequently used materials like steels from Cr and Cr-Mo prealloyed powders are commonly sintered in atmospheres consisting mainly of nitrogen.

In the present study, nitrogen pickup during sintering at different temperatures and for varying times has been studied for Cr-Mo prealloyed steel grades as well as for unalloyed carbon steel. Also the effect of the cooling rate and its influence on the properties, of the microstructure and the composition have been investigated. It showed that the main nitrogen uptake occurs not during isothermal sintering but rather during cooling. It could be demonstrated that a critical temperature range exists within which the investigated CrM-based steel is particularly sensitive to nitrogen pickup.

Open access
Sintered Structural Steels Containing Mn, Cr And Mo – The Summary of the Investigations

Abstract

The paper is presented the development and method of production of modern, Ni-free sintered structural steels which contain carbide forming alloying elements (Cr) with high affinity for oxygen (Cr, Mn) and the much smaller additive of an expensive alloying element (Mo), enabling the production of structural sintered steels in commercial belt furnaces, using safe sintering atmospheres. The investigations reported deal with the analysis of microstructure and mechanical properties of these sintered structural steels produced in different processing conditions, especially modification of chemical composition of sintering atmosphere and also the connections between the microstructure of sintered material and its mechanical properties. This analysis was done to propose the appropriate chemical composition of sintered Ni-free steels with properties which are comparable or even better than those of sintered structural steels containing rich and carcinogenic nickel. The investigations of PM Mn- Cr-Mo steels were preceded by those on Mn steels.

Open access
Static Corrosion Test of Porous Iron Material with Polymer Coating

Abstract

At present biodegradable implants received increased attention due to their use in various fields of medicine. This work is dedicated to testing of biodegradable materials which could be used as bone implants. The samples were prepared from the carbonyl iron powder by replication method and surface polymer film was produced through sol-gel process. Corrosion testing was carried out under static conditions during 12 weeks in Hank’s solution. The quantity of corrosion products increased with prolonging time of static test as it can be concluded from the results of EDX analysis. The degradation of open cell materials with polyethylene glycol coating layer was faster compared to uncoated Fe sample. Also the mass losses were higher for samples with PEG coating. The polymer coating brought about the desired increase in degradation rate of porous iron material.

Open access
Analytical Model of the Anisotropic Dimensional Change on Sintering of Ferrous PM Parts

Abstract

This work proposes an analytical model developed from experimental data to describe the anisotropic dimensional change on sintering. Axial-symmetric iron parts differing for geometry and sintering conditions have been investigated, aiming at highlighting the influence of geometry. The specimens were measured in the green and sintered state by a coordinate measuring machine (CMM). The dimensional changes of height, external diameter and internal diameter were derived from measurement results. The anisotropy of the dimensional variations has been studied with reference to the isotropic dimensional change derived from the change in volume of the parts. The influence of geometry and sintering temperature was highlighted. To properly describe the dimensional variations in the compaction plane, the dimensional change of the external diameter versus the dimensional change of the internal one has been analysed. By means of the experimental data, a reliable analytical relationship has been found, dependent on the parts geometry. An anisotropy parameter has been identified, which allows relating the dimensional change in the compaction plane and in the axial direction to the isotropic dimensional change. This parameter depends both on geometry and on sintering conditions. By means of the anisotropy parameter an analytical model for the anisotropic behaviour has been developed.

Open access
Copper Bronze Powder Surface Studied by XPS and HR SEM

Abstract

The state of the powder surface represents one of the main interests in the whole cycle of components’ production using powder metallurgy (PM) route. Large specific surface area of the powder in combination with often alloying with oxygen sensitive elements results in oxidation of the powder surface in most of the cases. The information about surface chemistry of the powder is of vital importance for further consolidation and sintering steps. Surface sensitive analytical techniques – X-ray photoelectron spectroscopy (XPS) and high-resolution scanning electron microscopy combined with energy dispersive X-ray analysis (HR SEM+EDX) were used for surface chemical analysis of the 60Cu-40Sn bronze powder. Determination of the compositional profiles and estimation of the surface oxide layer thickness was done by altering of ion etching and XPS analysis. The results showed tin oxide enrichment and presence of copper hydroxide on the surface of the powder particles. The impurities of P, Zn and Ca were also detected on the top surface of the powder in trace amounts.

Open access
The Effect of Transient Liquid Phase on the Joining Process of Aluminum Foam Core Sandwiches

Abstract

Despite recent developments in sandwich panels production technology, there are some difficulties in joining core sandwiches. Liquid Phase Sintering is a conventional method to increase the density of powder metallurgy parts. In this paper, we applied LPS as a joining process between Al-foam and Al-metal by using Al-mixture powders with different compositions as the interlayer. At first stage, Al-Zn powder mixture was used and the possibility of this process was investigated. At later stages, we tried to increase the joint bonding strength with different Al-mixture powder compositions. 3-point bending test was applied and by using mathematical relations, bonding strengths were calculated. The highest bonding strength was obtained, about 9 kPa, when Al-Zn-Mg was used as the interlayer. Also energy dispersive spectrometry (EDS) was used to investigate the diffusion of additive elemental powders to Al-mixture powders.

Open access
Phosphorus in Sintered Steels: Effect of Phosphorus Content and P Carrier in Sintered Steel Fe-C-P

Abstract

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.

Open access
Phosphorus in Sintered Steels: Interaction of Phosphorus with Mo

Abstract

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.

Open access
Powder Metallurgy Progress
Journal of Science and Technology of Particle Materials
Open access