Soundness Discrimination in Ferrite Ductile Irons Through Tensile Data Analysis

Abstract

Ferritic ductile iron (DI) GJS 400 and high-silicon strengthened ductile irons (HSSDIs) with 3.5 and 4.5 wt% of silicon contents were produced with different cooling rates, and the microstructures were analyzed to find the main microstructure parameters. The Y-blocks GJS 400 presented good nodularity, while the heavy section GJS 400 presented some graphite degeneracy with a lower nodularity. The 3.5 wt% HSSDI shown a good nodularity, while with increasing silicon content to 4.5 wt%, significant graphite degeneracy occurred with the appearance of chunky graphite. Samples were tensile tested and the tensile data were analyzed through the physical-based constitutive Voce equation, and the Voce parameters were plotted to produce a matrix assessment diagram (MAD). Y-blocks and heavy section GJS 400 data lied on two distinct lines in MAD, and they had, however, positive and negative intercepts which meant indeed two contradictory plastic behaviors. The intercept of the best fitting line of the GJS 400 Y-blocks data was positive and so consistent with the physical meaning of Voce equation, while the intercept of the best fitting line of the GJS 400 heavy section data was negative, which meant an unexpected opposite plastic behavior. Same behavior was reported for the investigated HSSDIs, resulting in positive intercept of the best fitting line in MAD for the 3.5 wt% HSSDI data and negative intercept for the 4.5 wt% HSSDI data. The uniform strain energy density (SED U ) that is the area below the tensile flow curve up to the uniform strain, i.e., the strain where necking begins, was also investigated. SED U resulted to be almost constant for all the GJS 400 Y-blocks tensile flow curves and 3.5 wt% HSSDI, which was typical of a sound material, while in GJS 400 heavy section and 4.5 wt% HSSDI, SED U changed significantly in a systematic way, indicating that metallurgical defects had affected the plastic behavior. So it was concluded that in the MAD the best fitting line of the tensile data with positive intercept was a possible indication of the material soundness, while the negative intercept was indication of defected material. The results suggested that the MAD analysis produced from tensile Voce parameters can be a useful and easy tool for industry not only to classify the production routes of DIs (Si content mainly and heat treatments), but also to identify possible microstructure poorness within a single DI grade.

An Insight into the Defects-Driven Plasticity in Ductile Cast Irons

“…The tensile specimens from the different castings were so taken: prismatic heavy section casting GJS400_P—two tensile specimens called C_1 and C_2 were machined off from the core of the block; two additional specimens, named S_1 and S_2, were taken from close to the external surface of the block, so materials from different cooling conditions could be tested; cylindrical heavy section casting GJS400_Ce—10tensile specimens were machined off from a 25 mm thick slice at about the 200 mm height of the cylindrical block, with codes ranging from Nr 11 to Nr 20; 50 and 75 mm blocks GJS400_Y—details of the specimen selection and tensile testing are provided in [ 30 , 35 , 36 ]. …”

“…The analysis of the tensile strain hardening at room temperature in defective materials using the dislocation-density-related constitutive equation of Voce, which is based on physical principles, can provide valuable information regarding the defective nature of DIs [ 19 , 20 , 30 ]. The Voce constitutive equation is: where σ and ε P are the true stress and plastic strain, respectively; σ V is the saturation stress or maximum stress; ε C is the critical plastic strain that defines the rate with which the saturation stress is achieved; and σ o is the back-extrapolated stress to zero plastic strain, close to yield stress.…”

“…In a plot of d σ /d ε P vs. σ , called a Kocks and Mecking plot [ 19 , 20 , 30 , 31 ] (here on called K-M plot), data linearity can be found at high stresses well in the plastic regime corresponding to Stage III of strain hardening, and fitted with Equation (1b), so finding the constant Voce parameters Θ o and 1/ ε c . Stage II of strain hardening is found only in single crystal experiments, while Stage II cannot be found in tensile tests, because deformation localization starts before the transition from Stage II to Stage IV [ 31 ].…”

“…In addition to conventional techniques, micro-Computer Tomography (μCT) was employed to classify graphite aggregates, quantify the volume fractions where spheroidal nodules were present, and determine the volume fractions where CHG was dominant. The Voce approach [ 19 , 20 , 30 ] was used to analyze the tensile strain hardening behavior of castings, and the Voce equations parameters were used for the material quality assessment based on the MAD and IAD. New insights into the correlation between defective DIs and MAD are presented in this study, and the latest findings are used to propose a new quality index.…”

Graphite Nodularity Evaluation in High-Si Ductile Cast Irons

“…The deviation, using a sphere as a reference of graphite particles, was noticeably increased by Si-alloying (from 2.5 up to 5.5%Si) when a characteristic of the graphite particles appeared to be a larger perimeter, resulting in a large category [IV, V, VI forms, ISO 945] [ 8 , 9 , 10 , 11 ]. According to [ 12 , 13 , 14 ], the 3.5% Si ductile iron showed good nodularity. In contrast, with increasing silicon content to 4.5%, significant graphite degeneracy occurred, with the appearance of chunky graphite, negatively affecting the mechanical properties.…”