1
Background One of the key points to
achieve as-cast ausferritic microstructure is to define the minimum cooling rate.
Continuous Cooling Trans-
formation (CCT) diagrams were developed for three different alloys with chemistry in the range of 3-5% Ni, 0-0.2% Mo and 0.1-1% Cu by weight. The change of the minimum cooling rate to prevent the forma- tion of pearlite (pearlite would keep the ausferritic microstructure from forming) was linked to the con- tent of the main alloying elements
(nckel, molybdenum and copper). Shakeout and isothermal trans-
formation temperatures have a major infl uence on the fi nal microstructure. Isothermal transformation refers to the transformation of the iron’s microstruc- ture at constant temperature. Diff erent thicknesses in the same casting involve diff erent processing temperatures. To be successful, engineered cooling must provide a fully ausferritic microstruc- ture in all the sections of a casting or at least in the sections defi ned by the designing engineer. For this reason, the thickness window where completely as-cast ausferritic microstructures are
CASE STUDY: STEERING KNUCKLE
In an early study on creating an ausferritic matrix through engineered cooling, sample steering knuckles were produced and their mechanical properties evaluated to compare with those found in austempered ductile iron (ADI) grade 750/500/11. In the experiment, the
steering knuckle castings were shaken out once the thin sec- tion reached the temperature of 1,472F (800C) and air cooled until the thick sec- tion reached the tempera- ture of 752F (400C). They then were introduced into the insulating material and held there for 90 minutes. Afterward, they were air cooled to room temperature. Table A presents the mechanical properties of the prototype castings. The tensile strength, duc- tility and impact strength of the material are high, and they are between the two lowest strength ADI grades following the ASTM A897/897M-06
Fig. A. This steering knuckle’s thick and thin sec- tions were investigated in the study.
(2011) (Grade 750/500/11 and Grade 900/650/09).
Some typical micrographs corresponding to the tensile test specimen (a) and one of the Charpy specimens (b) are shown in Fig. B. The specimens have been machined from the arm and the bridle, so they corre- spond to a thin section (0.14 sq. in. [90
sq.mm], thermal modulus 0.22 in.) and a thick section (0.25 sq. in. [160
sq.mm], thermal modulus 0.4 in.). In the case of the thin section, the structure consists of upper ausferrite (86%), lower ausferrite (8%), martensite (2%) and ferrite (4%). The thick section presents upper ausferrite (89%), lower ausferrite (3%) and ferrite (8%).
The thin and thick sections present a similar microstructure, which shows the viability of the as-cast method to produce castings.
Fig. B. Micrographs of various sections of a steering knuckle.
Table A. Mechanical properties obtained from samples machined from the steering knuckles. U.T.S. (MPa)
822
Y.S. (MPa) 589
44 | MODERN CASTING October 2015
El. (%) 9.2
HB 271
Charpy KV room T (J) 13/9/12
obtainable must be clearly defi ned. T e goal of the research was to
develop an experimental model able to defi ne the thickness window where the as-cast ausferritic microstructures can be guaranteed. Additionally, the model was validated in a semi-industrial process for the chemical composition range. When the thermal moduli of a casting are in the range of the processing thickness window, the model defi nes the opti- mum processing parameters with the aim of obtaining mechanical proper- ties (such as ultimate tensile strength and hardness) that meet the require- ments of the ADI materials.
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