Zinc is an increasing threat because increasingly more automotive is being galvanized. In cupola melting, most of the zinc is volatilized, complicating the effluent disposal, but leaving little detriment to the iron. In induction melting, zinc has penetrated the lining and damaged In , troubles have been experienced from graphite deterioration from excessive zinc in the iron. Much is yet to be learned as threats of zinc contamination increase. ALUMINUM - (AI) Aluminum is present in amounts in alloys and is beneficial in inoculants. These do not cause excessive concentration unless supplemented by other inadvertent sources. But, if all cumulative sources of aluminum exceed .04%, graphite nodularity can be damaged. Fig. 46 shows deteriorated graphite from .13% aluminum. Aluminum is present in killed steels, and aluminum metal can be attached to steel scrap. ZIRCONIUM (Zr) Zirconium .01% can distort nodular graphite shape. SELENIUM (Se) similar to sulfur, causes flake graphite and should be below .01%. Combinations of these trace elements can be additive in effect. However, cerium can precipitate out and counteract marginal amounts of these deleterious elements and extend safe concentration levels. However, it is safer to avoid these contaminating elements than to depend on cerium protection. There is yet much to be learned about trace element combinations and their neutralization. UNDESIRABLE CARBIDE FORMERS - (B) Boron is a most powerful carbide former at trace levels of .002% and must be avoided. can be absorbed from new induction furnace linings with borate binder. Other sources can be vitreous enamels, some steels, malleable iron scrap, etc. CHROMIUM - (Cr) Chromium is a very potent carbide and pearlite stabilizer. In ferritic grades, chromium should be kept below .05%. In pearlitic grades, chromium may be tolerated up to .10%. Beyond .10%, chromium increases carbide tendency strongly and retards annealing. For wear and abrasion resistance, chromium is intentionally added to produce some wear resistant carbides in the microstructure.
APPENDIX IX IMPACT STRENGTH - FRACTURE TOUGHNESS One attractive property of ductile iron is ductility, the ability to bend and stretch as measured by percent elongation in the slow loaded tensile test. An equally attractive property of ductile iron is impact strength, the ability to resist the shock of high velocity stresses. This impact property is more difficult to test and measure because it is influenced by temperature, velocity of impact, size of specimen, and the effect of design angles, casting flaws, etc. Good service performance of many ductile iron verifies its serviceable impact resistance. Many ductile iron castings in service have withstood shock loads that caused failure of gray iron parts. Ductile iron pipe have admirably withstood earthquakes and the "water hammer" impact of valve closings. But, laboratory testing of impact qualities has been more difficult. CHARPY IMPACT TEST An earl y and popular impact test is the Charpy test in which a specimen is broken by a swinging pendulum and energy absorbed in breakage calculated. Small machined specimens are tested either unnotched or with a special V notch. Impact strength is calculated in foot pounds or in Joules and various metric designations in different countries. The Charpy test is easiest to perform, but a disadvantage is the specimen too s mall to represent service performance and the full section. TRANSITION TEMPERATURE Above a certain temperature, irons and steels break with a ductile fracture. Upper shelf energy is higher and increases little with further increases in temperature. a certain low temperature, the fracture is a brittle cleavage fracture and the energy level is much lower. The midpoint temperature where half the fracture is ductile (gray) and half brittle cleavage (white crystalline) is considered the transition temperature. In cold climates, a low transition temperature is desirable where impact is anticipated. CHARPY V NOTCH IMPACT STRENGTHS