Figure 2 presents the distribution of 184 tensile yield strength (TYS) observations separated into the four Heats and the two grain orientations. Each bin is over a range of 4 MPa, and each point is centered at the median of the bin.
The transverse and longitudinal tensile results show that H1, H2, and H4 follow a normal probability distribution. The transverse TYS data from H1 do have a larger variance compared with H2 and H4. H3 shows a bimodal distribution. In the longitudinal orientation, the split on H3 appears to be values ≥1730 MPa (n = 10) and values ≤1730 MPa (n = 14). Likewise in the transverse orientation, the split on H3 appears to be values ≥1727 MPa (n = 14) and values ≤1727 MPa (n = 10). The mean TYS for H3 is similar to H2 and H4 in each orientation but the variance is greater. H1 has a lower mean TYS than the other three Heats; however, the difference is small at 22 and 21 MPa in the longitudinal and transverse orientations respectively.
Figure 3 shows the distribution of the 184 ultimate tensile strength (UTS) observations in the similar manner as the TYS observations. Each bin is over a range of 4 MPa. The UTS for each dataset shows the same form of distribution as seen for the TYS. The bimodal distribution in H3 is split at values ≥1816 MPa (n = 10) and values ≤1816 MPa (n = 14) in the longitudinal orientation, and at values ≥1806 MPa (n = 14) and values ≤1806 MPa (n = 10) in the transverse orientation. The mean UTS for H3 is similar to H2 and H4 but H1 has a lower mean than the other three Heats in each orientation. There is a strong positive correlation between the TYS and UTS data with a correlation coefficient of 0.91.
The results show that H2, H3, and H4 have consistent tensile strengths with similar means. H3 has a significantly larger standard deviation than H2 and H4. All the H3 tensile coupons were heat treated in the same batch and there is no correlation between location within the billet and strength. A possible explanation for the greater standard deviation and a bimodal distribution is variation in furnace heat zone during aging or variation in furnace set point during aging. However, the difference in means of the two H3 distributions is small (between 22.7 and 28.2 MPa). It is unlikely that metallographic or fractographic analysis of the failed samples would reveal any differences due to the observed small differences in the mean values. A normal distribution can still be used to describe all 24 longitudinal or transverse coupons of H3.
H1 shows a lower distribution for tensile strength than the other three Heats. H1 has the largest billet diameter and the smallest reduction ratio from the 30″ ingot. Less strain is imparted during press forging of the larger H1 billet, possibly accounting for the small reduction in strength. However the decrease in TYS and UTS between H1 and the other Heats is sufficiently small for the difference to be considered within expected variance Heat to Heat.
The observations from the four Heats were combined into a single dataset. For the 92 observations for each grain orientation, the following are the average values and the sample standard deviations: longitudinal TYS of 1723 ± 13 MPa, transverse TYS of 1722 ± 12 MPa, longitudinal UTS of 1812 ± 12 MPa, and transverse UTS of 1809 ± 11 MPa. There is no significant difference in average TYS and UTS for the different grain-orientated specimens (from ANalysis Of Variance (ANOVA) at the 5% level).
Compressive and Shear Strength
The average compressive yield strength (CYS) at 0.2% for the 13 test coupons is 1817 ± 43 MPa. There is a linear positive correlation between the average CYS and TYS values for each heat. A decrease in TYS of 25 MPa corresponds with a decrease in CYS of 100 MPa. This results in the large standard deviation for the average CYS.
The average shear strength (SS) for the 10 test coupons is 1118 ± 6.5 MPa. The range of SS test data is only 21 MPa, 1.9% of the average value.
The distribution of % ELongation (%EL) data of the 184 coupons separated by Heat and grain orientation is plotted in Fig. 4. The bin size is 0.4%. For all four Heats in the longitudinal and transverse directions the %EL data appear to follow a normal probability distribution. H4 shows a slightly lower mean %EL in both orientations compared with the other three Heats. The %EL standard deviations for each orientation and Heat are all similar, in contrast to the tensile properties. Although H4 has the highest average longitudinal UTS, H2 has a similar average value and the largest average longitudinal %EL. A similar observation can be made by comparing the transverse datasets. H2 has the highest average transverse UTS and a greater average transverse %EL than H4. For the data presented here, there is no correlation between UTS and %EL, and no correlation between ductility and billet diameter.
The distribution of % Reduction in Area (%RA) data for the 184 coupons separated by Heat and grain orientation is plotted in Fig. 5. Again, for all four Heats in the longitudinal and transverse directions the %RA data approximately fit the normal probability distribution. There is a positive correlation between the distribution of %EL and %RA. For example the distribution of H4 transverse data is shifted lower to the other three Heats for %EL and %RA. The average %RA between the four Heats shows good reproducibility and similar standard deviation. As with %EL there is no correlation between tensile strength and %RA, and between %RA and billet size.
For H3, the mean %EL and %RA for the two separate distributions observed in the tensile distributions (n = 10 and n = 14) in Fig. 2 and 3 (per grain orientation) were calculated. There is no difference between the two %EL means in the longitudinal orientation and a very small difference of 0.39% in the transverse orientation. For %RA, there is a small difference between the two means of 1.4% in the longitudinal orientation and 1.07% in the transverse orientation. These small differences are within expected variation and are not significant.
Combining the observations from the four Heats, the following are the average values and the sample standard deviations from 92 observations: longitudinal %EL of 10.33 ± 0.47%, transverse %EL of 9.86 ± 0.49%, longitudinal %RA of 55.6 ± 1.9%, and transverse %RA of 51.5 ± 1.8%. In contrast to the tensile strength, the difference in ductility between longitudinal and transverse grain-orientated coupons is significant (from ANOVA at the 5% level). Transverse grain orientation has a lower ductility.
Fracture Toughness (K
The average K
1C values for the 21 tests per grain orientation in the longitudinal and transverse orientations are 83.9 ± 4.5 and 83.4 ± 6.8 MPa √m, respectively. There is no effect of grain orientation on the combined average value of K
1C, although the standard deviation is greater for the transverse orientation. Figure 6 presents the relationship between K
1C and average TYS values for each Heat of C465 and grain orientation. H2, H3, and H4 show a tight distribution of K
1C values. H1 has similar K
1C values but at a lower mean TYS as observed in Fig. 2 ANOVA was conducted hypothesizing that there is no correlation between K
1C and TYS at the 5% level. The hypothesis is not rejected at the 5% level for the longitudinal K
1C values. The hypothesis is rejected at the same level for the transverse K
1C values but this is due to the high influence of H1. Removing H1 from the analysis reverses the result of the test. This analysis indicates that over this small range of TYS (1705 to 1731 MPa) K
1C can be considered to be independent of TYS and billet diameter.
Salt Spray Corrosion
The minimum time to observe a corrosion point with red rust was 240 h from a H1 coupon. The red rust was only staining on the surface with no evidence of a pit. The most extensive corrosion was after 312 h on a H2 coupon (Fig. 7a) although the corrosion was over less than 1% of the surface. Six coupons were left in the test cabinet for 1000 h, of which five coupons showed no corrosion and the other a single corrosion point with red rust. The ASTM B117 test on 300M cones results in over 80% of the surface covered in red rust after 200 h (Fig. 7b).
Stress Corrosion Cracking (K
The RSL procedure was first used to determine a K
1C value for the 3.5″ bar. An average of three samples gave 90.8 MPa √m, slightly higher than the average from the four Heats. The K
1SCC value from two RSL tests polarized at −0.350 V (SCE) was 82.4 MPa √m (91% of K
A total of 24 coupon test results were obtained from H1 to H4 combined. No variation in grain size was found across the four Heats. Also, no variation in grain size between the core of a billet and the outer radius was found. From the combined H1 to H4 grain size measurements, the average ASTM grain size was 5.4, from a data range between 5 and 6.
Figure 8 presents the strain-controlled fatigue data from all four Heats with maximum strain plotted against cycles to failure. The data are subdivided into datasets representing the three different strain ratios tested; the symbols detail the Heat and grain orientation. Arrows indicate a run-out specimen that did not fail after 106 cycles. For a given maximum strain, the order of fatigue resistance with strain ratio is 0.1 > −1 > −2. At each target life and strain ratio, the data from the four Heats are randomly distributed with no systematic difference across the four Heats.
The majority of strain-controlled fatigue tests were carried out using longitudinal grain-orientated coupons. As with other material properties, the grain orientation may affect the fatigue life of C465. At Rε = −1.0, the grain orientation of the coupons has no effect on fatigue life. At Rε = 0.1, the transverse coupons tested at 0.009-0.01 strain show a slightly lower fatigue life. However, across the whole dataset there is no systematic difference between the fatigue life of longitudinal and transverse grain-orientated coupons. The analysis was carried out using all the data collected; 120 C465 observations including three run outs.
Fractography of the fracture surface was done on the 117 observations that failed. Of those, 109 observations showed initiation at a cleavage zone, a titanium carbonitride particle (Ti-C-N), or an initiation attributed to both. The remaining eight coupons did not show any clear features. The primary initiation site was found to be the edge of the specimen except for six coupons where initiation was sub-surface. The sequence of failure for the coupons was fatigue mode then static mode. Examples of initiation at a cleavage zone and Ti-C-N particle are shown in Fig. 9 along with the accompanying EDX spectra of the particle.
Figure 10 details the type of initiation of the fatigue crack for the 109 C465 coupons vs. fatigue life. In general, multiple initiation points are prevalent at lower life (<10,000 cycles). Two coupons tested at very high strain failed by single initiation at a cleavage zone. There is an increasing prevalence of initiation starting at Ti-C-N particles (either isolated or part of a cleavage zone) at longer life. For coupons with a negative Rε, initiation at a cleavage zone is dominant below 10,000 cycles. However, at Rε = 0.1, all three types of initiation seem equally probable below 100,000 cycles with only Ti-C-N initiation occurring above 100,000 cycles.
The size of cleavage zones and Ti-C-N particles was recorded for the 109 observations. There are 44 fatigue coupons where initiation was associated with a Ti-C-N particle. The average particle area is 133 µm2 with all particles having a diamond geometry. The range of particle area is large from a few µm2 to hundreds of µm2 representing a change in length scale from 1 to ~20 µm. A three-factor ANOVA was conducted to test the hypothesis that there is no significant difference, at the 5% level, in the main effect of the three factors: billet size, life, and strain ratio on the size of Ti-C-N particles. Table 3 summarizes the analysis. The null-hypothesis is not rejected for the main effect of life, and strain ratio. The hypothesis is rejected at the 5% level for the main effect of billet size. By inspection the mean of the H1 sample is significantly different to the other three. Repeating the three-factor ANOVA with the H1 coupons removed results in accepting the hypothesis for all three main effects (Table 3).
There are 78 fatigue coupons where initiation was associated with a cleavage zone. For coupons where multiple cleavage zones were identified an average size was calculated for the coupon. Cleavage zones are typically rectangular with an average area of 0.012 mm2. The range of cleavage zone area is large, 480 µm2 to 0.076 mm2. A three-factor ANOVA was conducted to test the hypothesis that there is no significant difference, at the 5% level, in the main effect of the three factors billet size, life, and strain ratio on the size of the cleavage zone. Table 3 summarizes the analysis. The hypothesis, that there is no difference in means across billet size, life, or strain ratio is not rejected at the 5% level.