Dependency of tensile strength of ductile cast iron on strain rate and temperature

The dependency of the tensile strength σ B smooth and the notch strength σ B notch on strain rate and temperature were investigated for conventional ferrite-pearlite type ductile cast iron (JIS-FCD500) to make clear the applicability of ductile cast iron to components for welded steel structures. High speed tensile tests were conducted on notched and smooth specimens with varying strain rate and temperature. Charpy absorbed energy was also evaluated on notched specimen with varying temperature. It is found that the tensile strength is in a good relationship with strain rate-temperature parameter R for the wide range of strain rate and temperature. With decreasing R parameter, both σ B smooth and σ B notch increase even when Charpy absorbed energy starts decreasing. It should be noted that the notch strength σ B notch is always larger than the tensile strength at room temperature σ B , RT smooth in the range of R parameter required for the welded structures. Therefore, the tensile strength σ B , RT smooth is confirmed to be useful for the structural design.


Introduction
Ductile cast irons are widely used as structural members for automobile, railway vehicle, machine tool, and so on. In some cases, they have replaced cast steel and forging products [1]. In recent years, ductile cast irons efficiently substitute for several welded components because of their manufacturing and engineering advantages, such as broad range of mechanical properties and easy fabrication of components with complicated shapes. Furthermore, the cast products show better fatigue property than welded parts [1].
Ductile cast iron consists of nodular graphite particles and iron matrix. Spheroidal graphite is obtained by adding 0.035～0.045% Mg to molten iron. Usually, the required strength of ductile cast iron is obtained by controlling the ferrite-pearlite ratio in the matrix. Typically, ferritic ductile cast iron exhibits higher elongation and lower tensile strength, which is approximately 350 MPa, while pearlitic ductile cast iron has lower elongation and higher tensile strength ranging over 800 MPa. So that various strength levels of ductile cast iron castings were utilized in many fields.
However, for designing of structural components, we should take into account the upper limit of strain rate and lower limit of temperature because the toughness and strength of ferrous materials  (Temperature of this table) 3524 Industrial field a strongly depends on strain rate and temperature. Table 1 shows the example of the strain rate and temperature range required for welded structural members [2,3]. The values for Charpy impact test is also given in Table 1 [4,5], indicating Charpy impact test is not suitable for evaluating the property of welded structural components because the impact speed does not correspond to that in real products failure. Therefore, for example, one of the authors studied on high-speed tensile testing with varying tensile speed to investigate the impact properties of engineering plastics [6][7][8]. In addition, not Charpy absorbed energy but tensile strength and yield strength have been used for designing ductile cast iron products. To design the castings for welded structural members such as for architectures, it is necessary to make clear the influence of strain rate and temperature on the tensile strength of ductile cast iron. Therefore, high speed tensile tests were conducted on smooth and notched specimens with varying strain rate and temperature to discuss the applicability of ductile cast iron to welded structural components. Table 1. Example of strain rate and temperature range acting on welded structural components.

Test material
Conventional ferrite-pearlite type ductile cast iron having a tensile strength of ≧ 500MPa (JIS-FCD500) was prepared with 300kW high frequency electric furnace. Ductile cast iron melt was prepared by the Sandwich Method and cast into sand molds for JIS TypeⅡY-shaped blocks [9]. All tests specimens are taken from the gray part of Y blocks as shown in Figure 1. Table 2 and Figure 2 show the chemical composition and typical microstructures of test specimens. The matrix structure is composed of 47% ferrite and 53% pearlite. Table 3 shows graphite structures of specimen evaluated according to JIS-G5502. Table 4 shows the results of tensile test on JIS No.4 test piece whose diameter = 14 mm and gage length = 50 mm. The test procedure meets JIS-Z 2241 standard [10]. Figure 3 shows the specimens for high speed tensile test. The smooth specimen was prepared to investigate influence of strain rate and temperature on the tensile strength  B smooth . Furthermore, to enhance sensitivity to strain rate and temperature, the notched round bar specimen was also prepared to obtain the notch strength  B notch . The notch has the same shape and dimension as the Charpy Vnotch specimen. The tensile test was carried out in the range of stroke speed, v = 8.5 × 10 -3 ~ 2.7 × 10 2 mm/s, at temperature T = -130 ~ 22 °C by using electrohydraulic servo testing machine. Then, the tensile strength  B smooth and the notch strength  B notch were calculated by using equation (1).

High speed tensile test
Where Pmax = maximum load (N), d = test specimen diameter (mm). The strain rate  smooth and  notch were calculated for the smooth specimen and notched specimen respectively by using equation (2).  Table 2. Chemical composition of test specimens (mass %).     Architectural structure, Where u(t) = stroke displacement (mm), t = time (s), l = gauge length, 40 mm. Strain rate concentration factor K tε̇ [6][7][8] was used to calculate  notch because measuring the strain rate at the notch root is too difficult. The K tε̇ value is 9.49 for the notched specimen in Figure 3.

Charpy impact test
Charpy absorbed energy was evaluated on V-notched specimens at 213 ～ 353 K using a Charpy impact machine with the maximum energy capacity 300 J [11]. The impact speed is estimated as 5.18 × 10 3 mm/s. The total absorbed energy in the fracture process E is determined according to JIS-Z 2242 standard [12].

Tensile strength
Based on Bennett and Sinclair's theory on the influence of strain rate and temperature on yield phenomena of BCC metals [13], strain rate-temperature parameter, R parameter given by the following equation (3), has been introduced to explain the combined influence of strain rate and temperature on the yield stress of steel and ductile cast iron [14][15][16]. A good correlation between tensile strength and R parameter was also reported on these materials [16,17].
Where T = temperature (K), A = 10 8 s -1 [13,16], ̇ is strain rate (s -1 ); ̇ =  smooth for smooth specimen, ̇ =  notch for notched specimen. As shown in Figure 4, there is a good correlation between  B smooth and R parameter. In Figure 4, the range of R parameter required for the welded structures is also shown as the hatched lines. This required range of R parameter was calculated from the strain rate and temperature for the welded toes of beam-column in Table.1. It is seen that  B smooth of JIS-FCD500 continuously increases with the decrease in R parameter over the range of R parameter required for welded structures. Here, decreasing R parameter means increasing strain rate or decreasing temperature, or both. The result of ductile cast iron is consistent with previous research reports on steel [18].

Notch strength
The dependency of notch strength  B notch of JIS-FCD500 specimens on R parameter is shown in Figure   5. With decreasing R parameter,  B notch keeps increasing until R value of 2500, and then it starts decreasing. The  B notch values is 100～130 MPa higher than  B smooth over the R parameter range required for architectural structure shown by hatched lines. As the higher strain rate and the lower temperature also give similar effects on the toughness of ductile cast iron [19,20], we tried to correlate Charpy absorbed energy E to R parameter [11]. Figure 6 shows the relationship between E and R parameter for JIS-FCD500 specimens. The absorbed energy E starts dropping at the critical R parameter indicated by the black arrow. However, it is clear that

Conclusions
High speed tensile test was carried out on the notched and smooth ductile cast iron specimens with 47%ferrite-53%pearlite matrix in the range of stroke speed v = 8.5 × 10 -3 ~ 2.7 × 10 2 mm/s, at temperature T = -130 ~ 22 °C. The experimental data were correlated to R parameter, and following conclusions are obtained.
(1) With decreasing R parameter, the tensile strength of both smooth and notched specimens,  B smooth and  B notch , continuously increases over the range of R parameter required for architectural structures. Therefore, ductile cast iron has wide application potentiality to components for welded steel structures.
(2) The notch strength  B notch is consistently larger than the room temperature strength  B, RT smooth in the range of R parameter required for architectural structures. Therefore,  B, RT smooth can be safely used for structural design of architectural components.