sections of the casting. T e length of the solidifi cation process and the pa- rameters of the transformation needed to create the ausferritic microstructure must also be included in the analysis. Researchers fi rst developed a way
to achieve the as-cast ausferritic microstructure for a single alloy for a specifi c casting (steering knuckle). However, many automotive castings that are candidates for this technology
present geometries with signifi cant thickness variations and consequently diff erent cooling rates. T ese diff er- ences can complicate or make impos- sible the production of fully ausferritic as-cast parts by engineered cooling. In order to utilize engineered cool-
ing for a wider variety of parts in real- world applications, work was needed to develop an experimental model that defi nes the thickness window in
CASE STUDY: STEERING KNUCKLE In an early study on cre-
ating an ausferritic matrix through engineered cooling, sample steering knuckles were produced and their mechani- cal 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 section reached the tem- perature of 752F (400C). They then were introduced into the insulat- ing material and held there for 90 minutes. After- ward, they were air cooled to room temperature. Table A presents the mechanical properties of the prototype cast- ings. The tensile strength, ductil- ity and impact strength of the material are high, and they are between the two lowest strength
Fig. A. This steering knuckle’s thick and thin sections were investigated in the study.
which an ausferritic as-cast micro- structure can be achieved without the use of conventional austempering heat treatment by chemical composition adjustments.
Question Can a simple method be developed
to determine the process parameters necessary to produce as-cast ausferritic parts with given mechanical properties?
ADI grades following the ASTM A897/897M-06 (2011) (Grade 750/500/11 and Grade 900/650/09).
Some typical micrographs 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) Y.S. (MPa) 822
589
El. (%) 9.2
HB 271
Charpy KV room T (J) 13/9/12
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 cor- respond 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 ausfer- rite (86%), lower ausferrite (8%), martensite (2%) and ferrite (4%). The thick section presents upper aus- ferrite (89%), lower ausferrite (3%) and ferrite (8%). The thin and thick sections pres- ent a similar micro- structure, which shows the viabil- ity of the as-cast method to produce castings. ■
40 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2015
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