What does tensile data say about the strength and ductility of ductile iron as opposed to cast steels? The data suggests that the two alloys are quite similar. However, when we look at standard notched Charpy tests, it appears that ductile iron has significantly lower fracture toughness (energy values 16-24 joules) compared to cast steels (60-75 joules).
But before we disqualify ductile iron from critical applications based on its supposedly inferior toughness, we should look at the Charpy test with an assessing eye considering current fracture mechanics information to determine toughness:
- Fracture mechanics samples are pre-cracked and measure resistance to crack propagation under quasi stress conditions
- Charpy notches are relatively blunt and measure both propagation and initiation and involve impact loading
- The large sample size of fracture mechanic test samples can produce plane strain conditions
- The Charpy test involves plane stress. This is confirmed by the “shear lips on fractured steel Charpy samples tested in the upper shelf region” 1
What are the implications of the bulleted information above?
- Firstly, the significant difference between the Charpy behaviour of ductile iron and cast steel is because of the formation of shear lips. In fact, “the shear lips developed by the steel are responsible for a considerable fraction of its upper shelf energy.” 2
- Contrast this to ductile iron which, regardless of any condition, will not exhibit shear lip formation.
- This confers a significant advantage to steel during the ‘similar’ plane stress conditions of the Charpy test as the shear lip formation of steel produces a substantially higher upper shelf fracture energy than ductile iron.
- However, “under plane strain conditions that could be expected in many component failures, the ‘shear lip advantage’ of steel would be absent, with dramatically lower fracture toughness.” 3
So how can we eliminate the differences in upper shelf fracture mode between ferritic ductile iron and cast steel? Modify the Charpy test. Pre-cracked and side-grooved samples were used to provide plane strain conditions at the initiation of crack growth. “Using the J-integral method, the dynamic stress intensity factor KID was calculated for both materials over a temperature range including both brittle and ductile fracture modes.” 4
- At temperatures above 32 degrees Celsius (90F) cast steel did have superior fracture toughness to that of ferritic ductile iron, but much less than that suggested by the Charpy test. See this figure.
- Ductile iron showed superior fracture toughness at temperatures below 32 degrees celcius (90F)
- From www.ductile.org – “…fracture toughness of good quality ferritic Ductile Iron is excellent to temperatures as low as -80oF (-62oC), giving a KID of 37.5 ksi (square root) in. (41 MPa (square root) m), which corresponds to a critical flaw size of 0.5 in. (1.25 cm) for a design stress equal to the yield stress, applied under static fracture conditions. Above 0 oF (- 18oC), the KID is 80 ksi (square root)in. (87 MPa (square root) m) giving a critical flaw size of 1.5 in. (3.75 cm). Both flaw sizes can be detected and prevented by the quality assurance and production procedures practiced by competent Ductile Iron foundries. Assuming such flaws can be avoided, ferritic Ductile Iron can be considered sufficiently tough to resist unstable crack propagation at temperatures as low as -80o F (-62oC).” See this figure. 5
- Also from www.ducitle.org – “Figure 3.48 illustrates the relationship between fracture toughness and nodule count for pearlitic Ductile Iron tested at room temperature. This level of fracture toughness, at a temperature well below the transition temperature for pearlitic irons, (see Figure 3.41) indicates that these irons are tougher than indicated by the notched Charpy test and have good flaw tolerance at temperatures at which they are labeled “brittle” by the Charpy test. The relationship between fracture toughness and nodularity indicates that the nodules are playing a role in determining fracture toughness, possibly through the relaxation of triaxial stresses through void formation at the crack tip. 6