and excess cerium, among other factors. Te thermal centers of the castings (those areas located under risers) are especially susceptible to this degeneration because the metal is slow to solidify. In 2010, a solid cast insert with
equal parts bismuth and cerium proved successful in controlling the formation of chunky graphite in field trials at a number of metalcast- ing facilities across Europe. More recently, in a 7,800-lb. ductile iron torque arm casting for the wind energy industry, late in-mold inocu- lations prevented the formation of chunky graphite in thermal centers. By introducing the solid cast insert in a nobake pouring basin, the pro- cess window for chunk-free ductile iron is expanded.
Controlling Graphite Structures Chunky graphite occurs when,
instead of forming spherical nodules, the iron’s graphite forms branches and interconnected patterns (Fig. 1) that reduce mechanical properties signifi- cantly. Tensile properties can dimin- ish by as much as 22%, and elongation can be reduced by nearly 80% (Table 1), though research indicates hardness and yield strength remain unaffected. Glidewell Specialties Foundry
Company, Calera, Ala., a 150,000- sq.-ft. facility producing gray and ductile iron castings, experienced such difficulties when making a nearly 4-ton ductile iron torque arm for a wind turbine. Following shakeout, shot blasting and riser removal, Glidewell’s inspection department discovered dark spots near the base of an 8-in. neck-down riser designed to feed the heaviest section of the casting. An on- casting microstructure evaluation then confirmed the formation of
Ca Al
Fig. 1. A photomicrograph of chunky graphite shows irregular “branched” patterns of graphite instead of proper spherical formations.
chunky graphite. Such malforma- tions ensured the casting would not meet the wind energy industry’s demanding mechanical and impact properties. Chunky graphite often results from the accumulation of multiple
factors instead of one single cause, so its prevention must be a part of a metalcasting facility’s entire pro- cess control. Steps to avoid and/or discourage the formation of chunky graphite include: • Introducing a preconditioner when using high purity charge material.
• Keeping silicon levels below 2.2%. • Maintaining a carbon equivalent between 4.2 –4.4.
• Using chills to provide direc- tional solidification in the last areas to solidify.
• Controlling cerium levels, espe- cially with high purity charge and low tramp elements. • Improving nodule count.
Developing a New Approach During 2010 field trials in a
Fig. 2. The starter inserts are rammed up in the large pouring basin to inoculate uniformly.
Table 2. Nominal Analysis of Solid Cast Inserts Containing Bismuth and Cerium Si
Bi 70 – 75% 1.5% max 3.2 – 4.5% < 1%
Table 3. Nominal Analysis of Solid Cast Ductile Iron In-Mold Inoculant Si
Ca 70 – 78% 0.3 – 1.5% Ce < 1% Al 0.3 – 1.5%
number of European facilities, the addition of a solid cast insert with 0.1% bismuth and 0.1% cerium increased nodule counts and reduced chunky graphite. Bismuth promotes the formation of spheroidal graphite, though excessive amounts can result in irregular formations. To neutralize this adverse effect, the solid cast insert producers included an equal amount of cerium in the inoculant. Introducing the inoculant imme-
diately after magnesium treatment (or at ladle transfer, depending on
August 2013 MODERN CASTING | 33
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