The composition of both steels, AISI 1020 and AISI 8620, were within allowed specification limits as shown in Table 1. Grain size is influenced by the presence of aluminum together with nitrogen. The AISI 8620 steel contained 0.03% of aluminum which was ten times greater than the amount contained in AISI 1020 steel.
Optical microscopy showed that there was no internal oxidation or were there observed any microcracks within the carburized case. It is reported that if the silicon content is greater than 0.09%, there is increased potential for the formation of internal oxidation (Ref 9). However, this was not observed.
Table 3 summarizes the grain sizes obtained for the three carburized steel test specimens used for this study. The ASTM grain sizes for carburized AISI 1020, whether conventionally or intensively quenched, were 6.5. The carburized low-alloy AISI 8620 steel yielded a smaller grain size than the carburized AISI 1020 steel (9.5 versus 6.5 ASTM). Comparison of the as-received steels (Table 1) with carburized steels (Table 2) showed that the ASTM grain size for AISI 8620 was essentially the same both before and after carburizing and conventionally quenching (ASTM 10 and 9.5, respectively). However, there was a significant difference in ASTM grain sizes observed before and after carburizing and quenching, whether conventionally or intensively quenching of ASTM 1020. The as-received steel ASTM grain size was 8 (Table 1) and 6.5 after carburizing (Table 2).
Parrish reported that the presence of nickel and molybdenum helps in preserving the grain size during austenitization at relatively high temperatures (Ref 9). Moreover, chemical analysis of the two steels showed that the AISI 8620 contained 0.03% of aluminum. Aluminum reacts with nitrogen forming a second phase of aluminum nitride particles. This reaction was established by plotting the solubility curves shown in Fig. 2. The difference in the grain sizes shown in Table 3 is substantial and can lead to a large difference in mechanical properties. The average grain size of the carburized and quenched AISI 1020 steel is three times greater than the grain size of the carburized and conventionally quenched AISI 8620 test specimens.
Figures 8, 9, and 10 show the microstructures obtained for the different carburized test specimens. A high-carbon martensitic microstructure is present in the near-surface carburized layer. The microstructure varies through the carburized case to the core. There is a transition zone between the carburized layer and the core. The conventionally quenched carburized AISI 8620 (Fig. 9) and the intensively quenched carburized AISI 1020 (Fig. 8) test specimens contain martensite as the primary microstructure only varying from a high-carbon martensite in the carburized case to a low-carbon martensite at the core. However, the conventionally quenched AISI 1020 specimens (Fig. 10) exhibit a high-carbon superficial martensitic structure, and as the core region is approached, there is a transition region with a mixed microstructure of martensite, bainite, pearlite, and ferrite. At the core, the microstructure is a mixture of low carbon martensite and acicular ferrite.
Figure 11 shows the microhardness profiles of the carburized and conventionally quenched AISI 8620 and the carburized and intensively quenched AISI 1020 steels. The case depth, conventionally defined as the depth to a hardness of 550 HV is between 0.7 and 0.9 mm for both carburized steel grades. The superficial hardness is approximately 850 HV, and the core hardness is approximately 450 HV. The carburized carbon steel AISI 1020 which was subjected to intensive quenching exhibited nearly the same hardness profile as the low-alloy AISI 8620 steel test specimens subjected to conventional quenching. The use of intensive quenching would appear to allow for the replacement of low alloy steel with a less expensive carbon steel because the heat transfer is sufficiently intense to attain the maximum hardness. This is consistent with observations made earlier by Kimura (Ref 10).
In Fig. 12, the Vickers microhardness profiles are shown for the carburized and conventionally quenched AISI 1020 steel and the carburized and intensively quenched AISI 1020 steel. The carburized cases do not exhibit large differences. However, with intensive quenching, there is the preservation of martensite throughout the core region. The depth of the effective carburized layer is between 0.7 and 0.9 mm for both samples. There is an oscillation of hardness values at the core of the conventionally quenched carburized AISI 1020 test specimen. This may be due to the existence of regions of martensite and regions of ferrite.
Tensile test results are shown in Fig. 13. A comparison of conventionally quenched carburized AISI 8620 and intensively quenched carburized AISI 1020 showed that the AISI 8620 steel exhibited superior mechanical resistance compared with AISI 1020. The rupture stress of the AISI 1020 sample was 954 MPa, while the rupture stress of the AISI 8620 steel is 1510 MPa. From the literature, it is expected that a steel that is intensively quenched should exhibit superior tensile strength to a steel that was conventionally quenched (Ref 2, 4, 11-13) because intensive quenching produces “superhardening” (Ref 14, 15). However, as previously reported, the grain size may counterbalance this effect because of the more refined structure exhibited by the carburized AISI 8620.
In order to compare the effects of different quenching processes, additional tensile tests were performed. Carburized AISI 1020 steel after intensive quenching yielded better mechanical resistance than the carburized AISI 1020 steel after conventional quenching, as seen in Fig. 14. However, the intensively quenched AISI 1020 tensile test specimen yielded a superior toughness compared with the conventionally quenched test specimen. This result confirms that intensive quenching produces better mechanical resistance relative to conventional quenching.
Using the Wohler curves shown in Fig. 15 as reference, it was possible to verify that the conventionally quenched carburized AISI 8620 yielded performance relatively superior to the intensively quenched carburized AISI 1020 steel. This is attributed to the large difference in grain size between the two types of steel which counters any potential advantage that may be afforded by intensive quenching, and this would be expected to result in better mechanical resistance and consequently better fatigue resistance. Moreover, it is assumed that any additional compressive residual stresses from intensive quenching would not be realized because of the non-optimal low diameter which would not be expected to yield the optimum hardened case depth after intensive quenching.
The surfaces of some of the fractured samples were inspected using low magnification. Macroscopically, two regions were distinct in all samples: an external layer that represents the fractured region in the carburized layer; and a central region, at the core, which exhibits a rougher fractured surface. The carburized AISI 8620 samples exhibited a more refined fracture characteristic compared with the carburized AISI 1020 samples because of the smaller grain size.
For the carburized 8620 steel, test specimen numbers 8, 10, and 11 did not exhibit a primary region of crack origin as seen in Fig. 16. Compared with other samples, these three fractured with fewer cycles. As the cycle number is increased, the appearance of the fracture possesses a small subsurface circle, which is referred to as the “crack origin.” This type of fracture is seen in Fig. 17 and is related to samples that passed through a medium number of cycles.
However, test specimens 2, 4, and 12 correspond to high-cycle testing. Figure 18 shows that the fracture in the sub-superficial region increases noticeably in size. These facts are in accordance with those reported by Reguly (Ref 16). These failures of medium and high cycles probably originate because of the presence of inclusions. Yin and Fatemi (Ref 17) also indicated that the high-cycle regime and low stress failures have their origin in the subsuperficial carburized layer.
In the microfractographies obtained by SEM, as seen in Fig. 19, the sub-superficial origin of failure can be observed which contains a cavity that probably indicates the presence of an inclusion (Test Specimen 2). All samples of the carburized AISI 8620 steel exhibited a transgranular characteristic (Fig. 20a). In the central region, extensive dimples in all samples were observed (Fig. 20b).
For the carburized AISI 1020 steel samples, a larger grain size is visible at the fracture, as well as an irregular fractographic surface, as seen in Fig. 21. This shows a clear region of fatigue initiation only for Test Specimen 2 which can be seen in Fig. 22. In this case, it is possible to observe the crack origin. In all other samples, there were no failure initiation regions.
In all the samples, a carburized layer with only intergranular characteristics was seen as illustrated in Fig. 23(a). The core region exhibited a mixture of dimples with cleavage as shown in Fig. 23(b).