ON LINE OXYGEN ACTIVITY MEASUREMENTS TO DETERMINE OPTIMAL GRAPHITE FORM DURING COMPACTED GRAPHITE IRON PRODUCTION.
F. Mampaey Sirris, Gent, Belgium
D. Habets, J. Plessers and F. Seutens Heraeus Electro-Nite Intl., Houthalen, Belgium
Copyright © 2010 American Foundry Society Abstract
Lately, the present authors published a study where oxygen activities were measured using a commercial sensor, which became recently available. In ductile cast iron melts with ferritic and pearlitic structure, optimal properties occur for a well-defined oxygen activity. Castings poured in these circumstances present maximal nodularity, elongation and ferrite content combined with lowest hardness. Additionally, the first results for compacted graphite cast iron were published. The present contribution examines in much more detail the effect of sulfur and oxygen activity on several phenomena important during production of compacted
Introduction
During the past decades, European legislation (Euro 0-V) has pushed manufacturers of cars and trucks to improve the performance of engines. However, further improvement of engines is limited because the present day engine materi- als, aluminum alloys and lamellar graphite cast iron, have reached their mechanical limits. In the family of cast iron alloys, compacted graphite cast iron offers a possibility to substantially increase mechanical properties. Unfortunately, the production window of compacted graphite cast iron is extremely narrow. As a result, only a few engines using this material are produced today. These engines produce less toxic exhaust gases and are more energy efficient.
Background
An improvement of the fuel economy requires higher peak pressures during combustion.1
Increasing the peak pressure
with 10 bar raises the specific performance of the engine with 6,7 kW per liter engine cylinder capacity. Additionally, higher peak pressures also diminish the harmful components in the exhaust gases. At present, a maximal pressure up to 180 bar is possible in series production of cars. Unfortunate- ly, the current aluminum alloys and lamellar graphite cast iron — the present day two materials used to manufacture engines — have reached their mechanical limits. For exam- ple, in series production, the limit for aluminum alloys is 160
International Journal of Metalcasting/Spring 10
graphite cast iron. These phenomena are the limit for which mechanical properties as defined in ISO16112 are met, the transition from compacted graphite to lamellar graphite and the point at which 20 percent nodularity occurs.
Taking into account that the oxygen activity measurement is obtained in about 12 seconds, the sensor seems to be very promising for compacted iron process control.
Keywords: oxygen activity, compacted graphite iron, image analysis
bar.1
With higher combustion pressure, the internal cylinder temperature goes up from 200°C (392F) to 260°C (500F).2 The present day aluminum alloys considerably lose strength above 200°C (392F). Even for cylinder heads, which are now produced in aluminum alloys, temperatures above 230°C (446F) become critical with respect to the fatigue strength. On the other hand, cast iron room temperature mechanical properties remain the same up to 400°C (752F).
In comparison to lamellar graphite cast iron, which has a tensile strength of 250 MPa, compacted graphite cast iron with a pearlitic matrix has a proof strength of 350 MPa. Be- cause of the higher strength, the wall thickness of the engine may be reduced, giving 15 percent less mass.
The improved properties of compacted graphite cast iron results from a change in the graphite shape within the iron. Because lamellae are thin and long, the strength of the iron matrix drops substantially from about 800 – 1000 MPa in a graphite free matrix to 150 – 250 MPa in lamellar graphite cast iron. Additionally, this material does not present mea- surable elongation. One of the favorable features is the high thermal conductivity which is needed in engines to remove the heat from the combustion chamber through the cylinder wall. Soon after the invention of ductile iron, one noticed that in the case of insufficient magnesium addition, an in- termediate graphite structure resulted. Graphite appeared as vermicular particles. However, because the material could
25
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89