PSI - Issue 13

Frank Tioguem Teagho et al. / Procedia Structural Integrity 13 (2018) 763–768 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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Fig.2. Distribution of inter-carbide spacing of (a) Material A; (b) Material B and (c) Material C. The entire distributions are fairly well represented with a lognormal function (continuous lines).

3.2. Tensile properties From the results reported in Table 1, Material A exhibits the higher yield strength while the difference in yield strength between Materials B and C is about 100MPa. Globally, Material A was shown to be stronger than the other two, while Materials B and C displayed the same elongation after fracture and reduction area. In fact, fracture surfaces were fully ductile, with carbides observed inside the dimples. Many noteworthy studies have been proposed on the effect of carbides on tensile properties. Intercarbide spacing was proposed by Curry et al . (1979) as microstructural feature controlling the ductile fracture mechanism in high carbon steel. They showed an increase in the yield and tensile strength with the reciprocal square root of the intercarbide spacing. Moreover, Tomita (1990) proposed for tempered high carbon steel, a Hall-Petch-type correlation between the true fracture stress and the reciprocal square root of the mean carbide size. Our results tend to confirm that for high strength steel, there is an increase in the yield and tensile strength with decreasing both carbide size and intercarbide spacing, and that these tensile properties decrease while increasing the tempering temperature.

Table 1. Average microhardness and tensile properties (three specimens per condition) of the investigated materials. Material HV 0.3 hardness Yield strength (MPa) Tensile strength (MPa) Percentage elongation after fracture (%)

Percentage reduction area (%)

A B C

344±15 269±10 236±12

900 690 590

1050

16 21 22

56 63 63

840 720

3.3. Charpy impact properties Figure 3.a. presents the ductile-to-brittle transition temperature (DBTT) curves for Material A, B and C. The investigated temperature range was not sufficiently large to reach the lower shelf of Material B and Material C. Thus, the detailed comparison between the three microstructures only focused on the USE region. In the upper shelf domain, Material C was show to have higher absorbed energy. A difference of 10J was noticed between Material C and Material B, while Material A showed the lowest USE. These results confirm that the USE of quenched and tempered steels increases with tempering temperature. Salemi et al. (2008) already pointed out that the impact energy of a Ni-Cr-Mo-V steel could be improved by increasing tempering temperature. Takebayashi et al (2013) analyzed the effect of tempering temperature on USE of tempered martensitic steels. They suggested that USE did not change when tempering was realized from room temperature to 300 °C. However, the USE significantly increased when tempering was realized at temperatures higher than 300°C. These observations confirmed those by Briant (1989) on AISI 4340 steel. An increase in impact energy was accompanied by a decrease in yield and tensile