Because fatigue failures generally occur after a significant period of time has elapsed, fatigue behaviour can be degraded significantly by environments which accelerate crack initiation and growth. Figure 3.33 illustrates the reduction in fatigue strength resulting from exposure to water spray environments consisting or water and aqueous solutions of borax, sodium carbonate, and a soluble oil. In the most aggressive environment, borax, fatigue strength was reduced by 28 per cent. In accord with the time-dependent nature of corrosion-assisted fatigue, the effect of the corrosive environments decreased with decreasing fatigue life. Only potassium chromate, an inhibitor, prevented any significant loss in fatigue strength due to exposure to an aqueous environment. Chromate solutions are now considered to be toxic, and a combination of 0.5% sodium nitrate and 1% sodium silicate has been shown to be equally effective. Table 3.2 shows that spray coatings of zinc and aluminium provide excellent protection against corrosion fatigue of Ductile Iron by water and brine spray environments. Uncoated samples showed fatigue strength reductions of 1. 2 and 5.8 times respectively in water and brine sprays, while zinc- and aluminium-coated samples showed no loss of fatigue strength.

Table 3.2 Effect of environment and coatings on corrosion fatigue strength of pearlitic Ductile Iron.

 

 

In bending and torsional fatigue, in which cyclic stresses attain maximum values at the component surface, fatigue behaviour is strongly dependent upon surface geometry, residual stress conditions and material properties in the surface layer of the component. The use of adequate fillet radii, shot peening, surface rolling, flame and induction hardening and nitriding can significantly increase the fatigue limit of Ductile Iron components. These treatments, enhance fatigue resistance by 20 to 100 per cent by increasing the tensile strength and inducing compressive stresses in the surface layer of the component. In addition to improving surface stress conditions, shot peening also reduces the stress concentration effects of surface roughness.

Figure 3.34 illustrates the effect of different levels of shot peening intensity on the fatigue strength of pearlitic Ductile Iron with as-cast surfaces. Shot peening at the highest intensity level developed fatigue properties of the as-cast surfaces to within 6 per cent of those with defect-free machined surfaces.

Figure 3.35 illustrates the influence of surface rolling on the bending fatigue properties of ferritic and pearlitic grades of Ductile Iron. This Figure shows that v-notched samples, strengthened by rolling with a roller contoured to the notch geometry, had fatigue strengths from 58 to 73 per cent higher than the unnotched samples of the pearlitic and ferritic grades respectively. Table 3.3, which compares the reversed bending fatigue properties of different Ductile Iron crankshafts, confirms the significant strengthening effect of fillet rolling. Fillet rolling of ascast crankshafts increased fatigue strength from 30 ksi (207 MPa) to 83-97 ksi (572-669 MPa), an increase of 175-225 per cent over the as-cast pearlitic iron. This Table also documents the even greater benefits accruing from austempering and fillet rolling

 

 

 

Surface hardening by flame or induction heating is used to improve the resistance of Ductile Iron to both normal and pitting fatigue failures. Conventional fatigue strength is improved by a combination of high surface hardness and compressive surface stresses, while pitting fatigue is reduced by the increased surface hardness. Molten salt cyaniding produces a two-layer "case" on Ductile Iron components which can result in increases in fatigue strengths from 63 to 80 per cent, as shown in Figure 3.36.

 

The design stress for fatigue should not exceed one-third of the fatigue limit measured under conditions that suitably replicate the stress environment of the application. That is, notched data should be used when unavoidable stress concentrations are present in the component, and bending, torsional and push-pull fatigue data should be used according to the type of cyclic stress encountered by the component. The fatigue strength of Ductile Iron is frequency sensitive, and test frequencies should not exceed those encountered when the component is in service. The fatigue strength of Ductile Iron, like many other cast materials, is also influenced by both the cast section size and the specimen size. Both of these factors should be considered when extrapolating laboratory fatigue data to actual components, although the one-third safety factor may be sufficient to compensate for any degradation in fatigue strength due to size factors. The fatigue strength of Ductile Iron can be optimized through a combination of production and design practices which result in the following component characteristics.

 

Thermal fatigue is a special type of fatigue in which thermal cycling produces stress/ strain cycles in the component through differential expansion and contraction resulting from temperature gradients. The severity of thermal fatigue increases with increased temperature, increased range over which the temperature is cycled and increased rates of heating and cooling. Material properties which contribute to good thermal fatigue resistance are: high thermal conductivity, low modulus of elasticity and high strength and ductility. For severe thermal fatigue conditions, the high thermal conductivity and low modulus of high carbon Gray Iron make this material superior to both conventional and alloyed ferritic Ductile Irons and Compacted Graphite (CG) Iron.

For medium severity thermal fatigue, ferritic Ductile Iron and CG Iron provide superior cracking resistance but may fail by distortion. Pearlitic and alloy Ductile Irons provide the best performance for low severity thermal fatigue conditions. Figure 3.37 shows the increasing superiority of ferritic, pearlitic and alloy Ductile Irons in the Buderus Test in which thermal fatigue resistance is ranked by measuring the number of cycles between 650^oC (1200^oF) and room temperature required to produce bridge cracking between two holes in the test specimen. Performance of exhaust manifolds follows closely the ranking shown in this Figure. Ferritic Ductile Iron exhaust manifolds have been used widely due to a combination of good thermal fatigue strength and resistance to graphitization. Recent demands for increased service temperatures have resulted in the use of "Si-Mo" Ductile Irons containing 4-5% Si and up to 1% Mo. The increased strength and oxidation resistance of these alloys have resulted in excellent performance at service temperatures up to 750^oC (1380^oF).

 

Fracture Behaviour

Ductile Iron, like most ferrous materials, exhibits fracture behaviour which varies according to composition, microstructure, temperature, strain rate, and stress state. At low temperatures, brittle failure occurs by the formation of cleavage cracks, producing a facetted, shiny fracture surface. Very little deformation is associated with this type of fracture, resulting in low absorption of energy and low toughness. As the temperature increases, producing a decrease in flow stress, failure occurs by plastic deformation, primarily by the formation, growth and coalescence of voids. The resultant fracture surface will be dull gray, and the energy absorbed will be high, meaning very good fracture toughness. Fracture in ferrous materials traditionally has been characterized according to appearance and absorbed energy, with a Nil-Ductility-Transition (NDT) temperature quoted to indicate the change from brittle to ductile behaviour. In addition to transition temperature, upper shelf energies were quoted to define toughness in the ductile fracture region.

 

 

 

 

 

 

DUCTILE IRON DATA FOR DESIGN ENGINEERS ENGINEERING DATA Figure 3.33 Effect of different water spray environments on fatigue strength of pearlitic Ductile Iron.

 

 

 

 

DUCTILE IRON DATA FOR DESIGN ENGINEERS ENGINEERING DATA Figure 3.34 Effect of different shot peening intensities on the fatigue strength of pearlitic Ductile Iron with as-cast surfaces.

 

 

DUCTILE IRON DATA FOR DESIGN ENGINEERS ENGINEERING DATA Figure 3.35 Influence of surface rolling on the v-notched fatigue strength of ferritic and pearlitic Ductile Iron.