Besides, for all engineering professionals working in steel plants who need to understand the basic principles of steel making, the text provides a sound introduction mxking the subject. Add 3 Items to Cart. Beginning with a brief introduction to the historical perspective and current status of steel making steel making by a k chakraborty with the reasons for obsolescence of Bessemer converter and open hearth processes, the book moves on to: Chakrabarti A K. Third Printing, Pages: Beginning with a brief introduction to the historical perspective chakrabroty current status of steel making together with the reasons for obsolescence of Bessemer converter and open hearth processes, the book moves on to: He taught and conducted research at IIT Kharagpur for nearly thirty years.
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Ground scrap powder after pre-sintering Figure Figure Mixture being cold pressed into pellet. Fillers like graphite or pigments normally do not melt or agglomerate nor interact with PTFE during sintering at atmospheric pressure but this is not the case with reprocessed PTFE when used alone or used as a filler with virgin PTFE Figure The surface became rough on sintering due to agglomeration of reprocessed particles and their migration to surface of the pellet.
Reprocessed PTFE is amenable to sintering only under pressure or under ram extrusion. Compression molding Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, while heat and pressure are maintained until the molding material has cured.
Its advantage lies in its ability to mold large, fairly intricate parts. Initially it was thought that the die and plunger could be heated during compression in the hydraulic press. A strip heater was wound around the die to heat the die with the help of a heating programmer and a thermocouple was inserted into a hole made specifically for this purpose in the die to measure the temperature during compression, The experimental arrangement is shown below.
Compression molding The strip heater wound around the die, however, could not heat it to the desired temperature even after several hours of operation as the heat capacity of the die made out of hardened steel was much high and so this attempt was discontinued. Instead, after cold pressing in the press, the die and plunger itself with the PTFE disc inside it was kept inside the high capacity air over under mechanical pressure.
Cold pressed pellet along with the die and plunger being pressurized with the help of a C-clamp Figure Repro filled PTFE Discs when sintered inside the die and plunger under C-clamp show discoloration which runs through its volume although good surface smoothness is seen and cracks disappeared totally. This was attributed to carbon production on reaction of volatiles with the die material.
Sample is white in color but surface is rough. The reason for the discoloration could be due to carbon generation on reaction of volatiles with the stainless steel die under pressure. This can be avoided if a pathway can be provided for the escape of volatile gases during sintering under pressure.
This will need fabrication of a new die and plunger with a series of holes. While high pressure to psi is required during cold pressing powder into pellet, a relatively lower pressure to psi should suffice during sintering. Further efforts were made by reducing the die pressure to psi and sintering without pressure.
This showed that mild pressure during sintering is a must. High pressures with clamp not only discolor the pellets but also fuse them with the die. So a compromise in pressure during cold pressing as well as provision to let out the volatile gases are necessary 6. Sintering treatment Sintering temperatures were varied from to deg C and duration from 15 min to 1h. From the points of view of polish, smoothness and strength, the best treatment was found to be deg C, 1h which is the same used for sintering virgin PTFE.
Lower temperatures resulted in poor strength due to under-sintering while higher temperatures resulted in poor strength as it reduced the polymer strength. The ram is then withdrawn, the die tube re-charged with powder and the cycle repeated. This way the powder is continuously fed into the heating section of the die tube where it is sintered and then it passes through a cooler section from which the finished products rods, tubes etc flow out continuously which are cut into desired lengths.
Apart from PTFE materials like ultra high molecular weight polyethylene as well as their compounds can be ram extruded.
Uniform distribution of powder into the cavity is essential. The powder should exhibit good flow properties. The heated length of the tube can vary from 44 to 90 cm. It is clear that the discoloration in ram extruded rods top made by us was due to the production of carbon Fig. As a result the extruded rod bottom exhibited biscuit color all thorough its volume without any black patches.
This is a significant result and confirms that the discoloration is caused by carbon production. The fluorine being a gas produced on disintegration of fluorocarbon perhaps has already escaped during the sintering while carbon in the absence of oxygen has deposited within the rod. Acid treatment before sintering has been found to improve the color further. The rods came out fine from the points of polish and strength but was dark in color with patches all through the length and volume due to carbon deposition.
Conclusions Among all plastics, PTFE has the least stability against ionizing radiation, a property which is used to break down carbon-carbon bonds in the polymer chain in the PTFE scrap and reduce its molecular weight which makes it very brittle and the end product is a white, free-flowing PTFE powder which was found to be useful as lubricant additive in other materials or system such as printing ink, thermoplastics, elastomers, coatings and other lubricants.
While the turnings of PTFE scrap before irradiation are tough and elastic, those after irradiation in air crumbles into a powdery material. The molecular weight of irradiated PTFE is in the range of a few tens of thousands to a few hundreds of thousands, compared to several million for the unirradiated resins. Unirradiated PTFE scrap could be successfully ground with the help of a commercial shredder and milling machine.
Suitable pre-heat treatments were arrived at to remove organic and other volatile impurities. Since normal sintering procedures used for molding virgin PTFE did not work with repro filled PTFE, sintering under pressure and ram extrusion techniques were tried to mold them. The discoloration can, however, be avoided if a pathway can be provided for the escape of volatile gases during sintering under pressure That will need fabrication of a new die and plunger with a series of holes.
While high pressure to psi and hence a solid die and plunger are required during cold pressing powder into pellet, a relatively lower pressure psi should suffice during sintering.
The rods came out satisfactorily from the points of polish and strength but was dark in color with patches all through the length and volume due to carbon deposition.
This is a significant result and confirms our view that the discoloration is caused by carbon production. Discoloration could be further reduced with acid treatment so off-white recycled PTFE rods could be made with ram extrusion.
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