Mechanical and Fatigue Properties of Heavy Section Solution Strengthened Ferritic Ductile Iron Castings

Abstract

Recently, standardized solution strengthened ferritic ductile irons have met the interest of the industrial world since their improved mechanical properties and workability compared to standard ferritic-pearlitic ductile irons. For this reason, mechanical and fatigue properties of heavy section solution strengthened ferritic ductile irons castings are examined. Mechanical tests are carried out on specimens taken from the center of castings with long solidification times. Metallographic analyses are performed by using optical microscopy. Fracture surfaces of fatigue specimens are investigated in order to evaluate crack initiation and propagation zones. Finally, fatigue curves of the analyzed alloy are compared with those obtained from pearlitic ductile iron with the same casting geometry.

Novel method for the fatigue strength assessment of heavy sections made by ductile cast iron in presence of solidification defects

“…In particular, the microstructure of the GJS 400-18 LT (casting F-F) is characterized by spheroidal graphite nodules within a ferritic matrix, with areas of pearlite due to the segregation of undesired carbide promoting element present in the alloy. 32,34 Tensile test results confirmed, as reported in Borsato et al, 12 that ferritic ductile irons are less affected by the wall thickness and cooling rate, compared with pearlitic ductile irons. Finally, the solution strengthened ductile iron (S-I) shows a fully ferritic matrix with limited traces of pearlite.…”

“…It is also important to note that, due to the presence of chunky graphite, the ultimate tensile strength and more markedly, the elongation at failure, showed a considerable reduction compared with chunky-free castings. 32,34 Tensile test results confirmed, as reported in Borsato et al, 12 that ferritic ductile irons are less affected by the wall thickness and cooling rate, compared with pearlitic ductile irons.…”

Influence of solidification defects on the fatigue behaviour of heavy‐section silicon solution–strengthened ferritic ductile cast irons

“…The fracture surfaces of broken specimens have been examined using a scanning electron microscope in order to identify crack initiation (Figure A,B) and propagation zones (Figure C,D). As reported in other works, all the samples showed shrinkage porosities, some of them of large dimensions, near the surface that have been identified as crack initiation points.…”

Effect of the porosity on the fatigue strength of metals

“…Similarly, Kainzinger et al 33 observed that micro‐shrinkage pores were the dominant sites for fatigue crack nucleation in specimens extracted from a wind turbine hub made of DCI. Moreover, under tension‐compression cyclic loading, the fatigue crack nucleation process in ferritic‐pearlitic DCI specimens was considerably accelerated by the presence of micro‐shrinkage pores near the specimen subsurface, according to the experimental evidences reported in Nový et al 34 Similar results have been obtained also by Borsato et al 35,36 regarding the fatigue behavior of heavy section pearlitic DCI components. Furthermore, they argued that a combined effect between micro‐shrinkage pores and non‐nodular graphite particles could exist if they were both present in the DCI.…”