In order to cover the complete range of compacted graph- ite iron, the transition from spheroidal graphite containing about 50 percent nodules, through compacted graphite and finally lamellar graphite has been examined. Basically, oxy- gen activity is measured while holding a magnesium treated melt at constant temperature in the furnace. During holding, magnesium gradually evaporates resulting in changes of cast iron properties. These changes are monitored by regularly pouring standard test bar molds.
Because the intent was to compare the new experimental results obtained for compacted graphite cast iron with the previous results valid for ductile iron, the base charge was left unchanged. For cast iron with a ferritic matrix, the charge (240 - 280 kg) consists entirely of Sorel iron. For compacted graphite iron with a pearlitic matrix, the same charge was deliberately kept in order to minimize chang- es of the process variables. Here, the melt composition was adjusted using manganese, copper and tin. Charges have been melted in an induction furnace of 300 kg. The magnesium treatment uses FeSiMg wire (7.5% Mg) at about 1480ºC (2696F). Once the magnesium reaction is over, the furnace is powered again in order to maintain a constant melt temperature in the furnace of about 1420ºC (2588F). At regular intervals, a standard Y-block (25 mm) is poured using a ladle of 20 kg liquid metal. During fill- ing of the ladle, 0.3% proprietary inoculant is added in the stream. Several inoculants have been added:
a zirconium - based inoculant (75% Si, 2.5% Ca, 1.4% Al, 1.6% Zr),
a zirconium - based inoculant – preconditioner (62-69% Si, 0.6-1.9% Ca, 3-5% Al, 3-5% Zr),
a lanthanum - based inoculant (50% Si, 1.55% Ca, 2.15% La) and
an inoculant (70-75% Si, 2.5% Ca + Al). A Ce-Bi inoculant has been tested previously.9
The goal was
to examine a possible effect of different types of inoculants on the oxygen activity transition data, e.g. the transition from compacted graphite to lamellar graphite.
Specimens for chemical analysis are taken from the Y- blocks. In this way sampling time is the same as for the Y-blocks. Maintaining the melt at a constant temperature, gradually lowers the magnesium content. The regular pouring of Y-blocks continues until gray iron finally re- sults. Apart from the magnesium content and the sulfur content, other elements vary only a little during holding in the furnace. In Figures 1, 5 and 9, time equal to zero corresponds to the end of the magnesium reaction in the furnace.
In many cases, after the first series, 140 kg low silicon Sorel iron is added to the melt remaining in the furnace. The procedure is repeated. In the previous research for ductile iron, a second magnesium treatment using
International Journal of Metalcasting/Spring 10
NiMg(15%) was carried out. However, the results in the compacted graphite iron range differed considerably in case nickel was present in the composition. Consequent- ly, the second magnesium treatment also employs the same magnesium wire. Moreover, NiMg is not applied in industry anymore. The same exact procedure is main- tained after the second magnesium addition as after the first treatment.
In the previous research for ductile iron, it was envisaged to examine the whole magnesium range in ductile iron. Consequently, a considerable amount of magnesium was added giving a relatively, high initial residual magnesium content. Here, much less magnesium has been added. In some experiments, when oxygen activity measurements indicated that the compacted graphite iron range was left, small additions of magnesium wire were added. The in- tent was to go back into the favorable compacted graphite production window and to examine if several small mag- nesium wire additions give the same result as one large magnesium wire.
Only one Y-block has been poured per mold (EN-1563, sili- ca sand, hand molded). The castings cooled overnight in the molds before shake out. Standard test bars (DIN-50125-B14) were machined from the Y-blocks. Mechanical testing was done according to the European Standard EN-10002-1, giv- ing proof strength Rp0,2, tensile strength Rm and elongation A5. A stress - strain curve was obtained using a video cam- era and real time computer analysis of the digitized pictures. In accordance with the EN Standard, the stress – strain curve is used to determine Rp0,2. Sirris is certified for mechanical testing. In ASTM E8-00b proof strength Rp0,2 is denoted as yield strength (offset = 0.2%), however in this paper proof strength will be used. Brinell hardness was measured as HBW 10/3000. After mechanical testing, a specimen for metallographic examination was taken from the test bar at about 5-8 mm distance from the crack. This procedure was used for all graphite types, ductile, compacted and gray iron. The normal procedure was a repetitive polishing and etching (5-8 times) before taking the pictures. The magnifications used were 200X for the analysis of the graphite structure and 100X for the measurement of the ferrite content. Ferrite content is calculated as the average of 4 pictures, taken after nital etching. (The sum of ferrite, pearlite and graphite is 100 percent). The four pictures are taken close to the edge, regularly spaced in the four directions of a compass. After polishing, 10 successive photographs are taken of each spec- imen from the edge to the center (tif format, 2088 x 1550 pixels). Each photograph is examined by image analysis and the results are saved. Finally, average results for all 10 pictures are calculated. Particles with a size smaller than 10 µm are not used for examination. Average results are pixel weighted. This technique implies that the influence of small particles becomes less important. In the present experiments as well as the previous ones8,9 never found in the test bars.
, carbides were 29
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