Dissociated Ammonia Atmosphere Research Articles

Investigation of Pressing and Sintering Processes of Powder Copper Samples for Equal Channel Angular Pressing

The pressing and sintering processes of copper powders produced by different methods are studied in the paper. The main research is based on PMS-1 powders, obtained by electrolysis. To find the easiest and the most reasonable sintering mode, the process is carried out in the open air and dissociated ammonia atmosphere. The samples parameters after each processing procedure were calculated, compression tests were carried out. Basing on preliminary experiments the masses and sintering modes of rectangular shape forms used for equal channel angular pressing were figured out. The possibility in principle to carry out the process of equal channel angular pressing of porous sintered samples without metal damage was shown.

Mechanical properties of diamond grinding discs brazed in a continuous-belt tunnel furnace or vacuum furnace

A new method for brazing diamond grains is presented. The brazing was conducted in a continuous-belt tunnel furnace under a dissociated ammonia atmosphere, which allows for a continuous large-scale production and greatly enhances productivity. To compare the brazing quality, identical samples were also brazed in a traditional vacuum oven. The surface morphology and interfacial microstructure were characterized, and the residual stresses, the static compressive strength, and the impact toughness were determined for both types of brazed diamond samples. Furthermore, grinding tests were conducted using the brazed diamond grinding discs. The results revealed that graphitization did not occur on the surface of the diamond grits. Cr3C2 and Cr7C3 were found at the interface between the diamonds and the Ni-Cr filler alloy brazed in the tunnel furnace. The residual stresses in the tunnel-brazed diamond grits were smaller than the stresses in the vacuum-brazed diamond grits in case of an identical measuring height, and the static compressive strength and the impact toughness of the etched tunnel-brazed diamond grits were 2316.7 MPa and 66.5 %, respectively, which was higher than the values obtained for the vacuum-brazed diamond grits (1865.6 MPa and 59.3 %). After grinding a granite test sample using the brazed diamond grinding discs, the fraction of abrasive grains pulled out from the surface of the tunnel-brazed grinding disc was slightly higher than the fraction of grains pulled out from the surface of the vacuum-brazed disc, whereas the overall abrasive grain fracture rate was lower at the same grinding length.

Investigation of Application Possibilities of Re-Ni Alloy Powder to Tungsten Heavy Alloys Production

The experiments results connected to possibilities of use of Re-Ni alloy powder for tungsten heavy alloys production were presented in the paper. This powder was obtained by nickel (II) perrhenate reduction under dissociated ammonia atmosphere. The optimal conditions for the thermal reduction for nickel (II) perrhenate were determined. The production process of investigated sinters contained, among others, cold isostatic pressing and two-staged sintering. Selected results of microstructure investigation and mechanical properties of obtained sinters were shown. Performed research indicated that the application of nickel (II) perrhenate to manufacture tungsten heavy alloys resulted in material with the microstructure typical of tungsten heavy alloys and characterized by high mechanical properties.

Effect of Heating Mode on Sinterability of Fe-Ni Steels

The present study examines the effect of heating mode on the densification, microstructure, and mechanical properties of iron-nickel steel with graphite and phosphorus addition. The compacts were sintered in conventional (radiatively-heated) and microwave (2.45 GHz, multimode) furnaces at 1120°C for 1 hour in forming gas (dissociated ammonia atmosphere, 95% N2-5% H2). The experimental results show that microwave sintered alloy has better properties compared with the conventionally sintered counterparts. Detailed analyses by using optical microscopy and scanning electron microscopy (SEM) reveal that microwave sintered sample has finer microstructure. SEM examination of the fractured surfaces indicate that a mixed mode fracture containing both, ductile and brittle types, is present in microwave sintered alloy, in contrast with the brittle fracture only in conventional sintered counterpart.

Sintering behaviour improvement of a low Cr–Mo prealloyed powder steel through Mn additions and others liquid phase promoters

Although its good hardenability, chromium has being avoided in powder metallurgy (PM) steels because of its high oxygen affinity and difficulties in sintering. For this purpose, a prealloyed iron powder Fe–3Cr–0.5Mo Astaloy CrM ® grade, with different carbon content and some alloying elements additions have been studied. Mixtures were uniaxially compacted at 700 MPa. Sintering process was carried out at 1120° in dissociated ammonia atmosphere for 40 min. Due to previous detected problems in sintering it at low temperature (1120 °C) when is not possible to control properly the atmosphere, the item was focused on the behaviour improvement of Astaloy CrM. Trying to enhance the density and amount and extension of particles contacts should be interested introduce a transient liquid phase on mixtures. Moreover, if the objective is reduce the oxide layer of Astaloy CrM particles, avoiding carbon losses, some manganese additions can perform as oxygen traps. Two aspects are critical in this case: to achieve a good distribution of the mixture and, because of the secondary pores, to control de-dimension stability. Based on these investigations a brief overview of sintered densities and mechanical properties was done, as well as thermal analysis (performed by simultaneous thermal analysis with simultaneous thermo gravimetric (DTA–TG) test) and microstructural study by LOM.

鉄系圧粉体の焼結時の寸法変化におよぼす脱ろう条件の影響

The influence of delubricating conditions on dimensional change of the compacts of the system Fe-2wt%.Cu-0.8wty.C-0.75wt%.Zn-st was examined under an atmosphere of endothermic gas. An abnormal expansion, which increases under slower heat pattern and atmosphere of lower dew point, occurred above about 500°C. As this expansion becomes larger, copper growth decreases, the shrinkage becomes larger and carbon content after sintering becomes higher.The diffusion of carbon is greatly influenced by delubricating conditions but that of copper is little. The delubrication in Ar, N2, H2 and dissociated ammonia atmosphere makes the sintering behaviors and sintered properties stable. In the case of endothermic gas, the similar tendency is observed at the rapid delubrication under the high dew point atmosphere.

Some effects of the plasma nitriding process on layer properties

The layer properties of AISI 722M24, AMS 6472 and AISI H13 materials plasma nitrided under various conditions were defined. The materials were plasma nitrided at temperatures between 510 and 590 °C for times ranging between 4 and 100 h in a dissociated ammonia atmosphere. The effects of duration, temperature, chemical composition and the shape of sample on the layer properties were determined. It was determined that the total case increased with treatment time and temperature increases the white layer thickness and the effective case depth. The increase in the white layer thickness is very sharp up to a total case depth of 0.3 mm, but it is very slow between 0.3 and 0.75 mm total case. The chemical composition of the material also affects the depth and hardness of the case. High alloy steels have higher hardness but thinner case than low alloy steels. Although the γ'-Fe 4N single-phase compound layer was formed on 722M24 steel, a thick and mixed ϵ- γ' (Fe 2−3N+Fe 4N) phase was produced on H13 steel. The hardness distributions from curved and flat surfaces to the core of the test material exhibit different profiles.

Corrosion Study of Industrially Sintered Copper Alloyed 316L Austenitic Stainless Steel

AbstractAI5I 316L type sintered stainless steel specimens were produced in a continuous industrial furnace at 1180°C using a dissociated ammonia atmosphere. Copper was used as alloying element in different percentages (2, 4, 9%). Compacting pressure and sintering time were varied to study their combined effect on the corrosion behaviour of sintered stainless steels in a sulphuric acid solution. In this environment, the electrochemical behaviour depends in particular on the interconnected porosity and the precipitation of chromium rich phases. The compaction pressure affects markedly the amount and morphology of the open porosity, reducing the total exposed area, whereas a sintering time increase induces, in the present experimental conditions, more intense chromium nitride precipitation. Alloying with copper shows some benefits for the corrosion resistance of the sintered stainless steel, significantly reducing chromium nitride precipitation. Further, if added in an amount sufficient to give some free cop...

Electrochemical Characterization of Copper-Alloyed Sintered Stainless Steel in Borate Solutions: Passivation and Passivity Breakdown

Abstract Type 316L (UNS S3T603) sintered stainless steels were produced in a continuous industrial furnace at 1180°C using a dissociated ammonia atmosphere. Copper was used as an alloying element i...

Magnetic susceptibility of sintered 30nm steel in the area of strong magnetic fields

In this work the differential magnetic susceptibility of specimens of sintered 30NM steel with variations in the weight content of carbon from 0.15 to 0.4% and in porosity from 6 to 14% was measured. The investigations of the magnetization and differential susceptibility curves were made in a closed magnetic circuit in an electromagnet in fields up to 3.2 kOe. Rectangular 10 x 10 x 55 mm specimens were prepared by pressing and sintering a mixture of powders of the original components in a dissociated ammonia atmosphere. The variations in carbon were provided by the graphite content in the powder mixture and the variations in porosity by the pressing pressure.

Effect of nitrogen content on the compressibility of Kh25 stainless steel powder

During the production of high-chromium steel powders in a dissociated ammonia atmosphere the powders undergo nitriding. To obtain powders of negligible nitrogen content, heating during diffusional impregnation should be performed at the highest rate; with an incompletely gas-tight liquid seal, the nitrogen concentration in the powder sharply rises, particularly at low cooling rates. The compressibility of high-chromium steel powders produced by diffusional impregnation in a dissociated ammonia atmosphere is determined by their nitrogen content and cooling rate; the effect of the latter is strongest at nitrogen contents in excess of 0.2–0.4%. High nitrogen contents and rapid cooling after diffusional impregnation have a deleterious effect on the compressibility of the powder, which is attributable to changes in the microstructure and microhardness of its particles.

Sintered 20Kh13 steel

1. The diffusional impregnation technique enables Kh13 chromium steel powders of satisfactory processing characteristics to be produced in a dissociated ammonia atmosphere. 2. The degree of densification in the sintering of the chromium steel depends on the latter's graphite content, and is determined by the influence exerted by carbon on the structural state of the steel at the isothermal heating temperature. The mechanism of this process is not yet fully understood. 3. The mechanical properties of the sintered steel can be substantially improved by heat treatment. 4. The corrosion resistance of sintered 20Kh13 steel in a humid air atmosphere is comparable with that of the cast steel. 5. Parts made of sintered 20Kh13 steel may be successfully employed in instruments and machines operating under severe climatic conditions. 6. Further researches are necessary in order to optimize the conditions of manufacture of sintered stainless steels of the martensitic-ferritic class and impart to them better properties by increasing their density and improving their structure.

THE EFFECT OF DEFORMATION ON TENSILE AND IMPACT PROPERTIES OF HOT PM-FORMED NICKEL-MOLYBDENUM STEELS

Hardenability, tensile, and impact properties have been determined for two nickel-molybdenum powder-formed steels. The compositions of the two steels were Fe-1·8% Ni−0·5% Mo and Fe −0·5% Ni −0·6% Mo; carbon contents studied for both materials were 0·2, 0·3, and 0·4%. Preforms (50 × 50 × 127 mm) were pressed and then sintered for 1 h in a dissociated ammonia atmosphere at 1120°C, to provide a density of 6·5 g/cm3. Preforms were hot formed to full density by upsetting (plane strain) at 980°C or by re-pressing (uniaxial strain) at 1040°C.Standard 25 mm-dia. × 100 mm-long Jominy end-quenched tests were made to determine hardenability. Room-temperature tensile properties were determined using 6·4 mm-dia. specimens with a 25 mm gauge-length. Standard Charpy V-notch specimens were used to determine the impact properties at −40, 0, 22, and 100°C. Triplicate tests were made in the as formed as well as the quenched-and-tempered conditions.The additional flow that was realized by upsetting provided higher tensile and impact strengths than re-pressing. Tensile ductility was the same for both processing conditions. Tensile yield strength (heat-treated to ∼26 HV) increased with increasing carbon content for the slack quenched material and levelled off for fully martensitic structures. Room-temperature impact strength also increased with increasing carbon content owing to a decrease in the amount of ferrite in the slack quenched structure. Increasing carbon content caused a decrease in impact strength once fully martensitic structures were achieved.Impact strength at −40°C ranged from ∼20 to 40 J. Room-temperature impact strength ranged from ∼20 to 50 J. These properties were obtained for materials heat-treated to a UTS level of ∼850 N/mm2. At the lower carbon contents the hardenability curve was found to fall within the AISI ‘H’ band for 4620 wrought steel. At the higher carbon level (0.4% C) the Jominy curves were close to the lower limit of the ‘H’ band for the 4640 wrought steels.

The thermal conductivity, electrical resistivity, and mechanical properties of a sintered iron-copper alloy. Part 2

1. It was established in this investigation that, when 15% nickel is added to an iron-copper-graphite alloy containing after sintering 1.1% C and 10% Cu, the hardness of the alloy increases by 20–25%, but, at the same time, its mechanical strength, ductility, and impact strength decrease compared with the same properties of a similar nickel-free alloy. 2. Other conditions being equal, sintering in a dissociated ammonia atmosphere reduces the mechanical and impact strengths by approximately 10–15% and slightly decreases the ductility compared with the properties of the same alloy sintered in a hydrogen atmosphere. 3. The tensile and impact strength tests of specimens at 400°C revealed a slight decrease in the mechanical and impact strengths at that temperature. 4. The thermal conductivity increases linearly with increasing temperature. The Lorenz function is greater than its theoretical value, and decreases with increasing temperature. This indicates that heat conduction in this alloy is due to conduction electrons and crystal-lattice conduction. 5. The coefficient of linear thermal expansion at first shows an insignificant increase, but at 400°C attains values characteristic of cast austenite.