13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- an average value was used in the toughness versus temperature or DBT curves. For impact toughness testing, standard sample with a dimension of 50x10x10mm (SS-EN 10045-1) was used. For CTOD sample, SENT type of sample with a dimension of 90x20x10mm was used and tested according to BS 7448, 1991. In this paper, two methods have been used to determine a ductile to brittle transition temperature (DBTT). One is to determine the transition temperature, T50, by Eq. 1 with 50% probability from a ductile to brittle transition curve [18, 19]. ( ) ( ) ( ) min 50 50 min max 2 1 exp 2 exp KV C T T C T T KV KV KV + ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ × − + ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ × − × − = (1) Where KV is the toughness at temperature T, KVmax is the maximum toughness, KVmin is the minimum toughness, and T50 is the temperature with 50% probability of brittle fracture, C is a constant. Another is though fracture analysis using a scanning electron microscopy (SEM). DBTT is the temperature where no cleavage could be observed on the fracture of an impact sample. In this investigation, no cleavage could be observed on the fracture of the sample with impact toughness higher than 90 joules. Therefore, T90J is another definition of DBTT for this material. In order to study the influence of factors on cleavage behaviors in DSS, some special tests were arranged. One is the effect of spinodal decomposition. The samples were aged at 450°C for up to 300 hours. The second is the effect of intermetallic phase, mainly sigma phase. The samples were tempered at 850°C for up to 10 minutes. The third is the effect of hydrogen or hydrogen induced stress cracking, HISC. The experimental details are described in reference [21]. The effect of other defects such as inclusion and Al nitride were also investigated. 3. Results and discussion 3.1 Influence of temperature on DBT behavior Figure 2 shows the DBT curves from the CTOD and the impact toughness tests. As expected, both CTOD and impact toughness decrease with decreasing temperature. The modeling curves using Eq. 1 with C=35 were used to determine T50 and T90J. Table 2 shows a summary of the influences of the factors such as amount of ferrite, cold deformation and phase size on the DBTT. The influence of phase size on the DBTT is relatively small. This result is quite different from that of the weld material where the DBTT (T27J) increases significantly with increasing grain size [20]. An increase in ferrite content will raise the DBTT. This is due to the fact that it is the ferrite that causes a DBT. Cold deformation that introduces dislocations in the material can cause a decrease of impact toughness and consequently increase the DBTT since cold deformation increases both the strength of the material and the density of dislocation, which will promote the tendency for cleavage [21].
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