PSI - Issue 60

B Shashank Dutt et al. / Procedia Structural Integrity 60 (2024) 690–699 Author name / StructuralIntegrity Procedia 00 (2019) 000 – 000

695

6

Table 3. Fracture toughness results of P91 steel

Temperature, o C

Fracture toughness J

1c , kJ.m

Equivalent K

Validity limit (thickness) to meet plane strain condition

j1c MPa.m

-2

0.5

300 350 400 450 500 550

239 ±7 246 ±6 200 ±5 195 ±4 254 ±5 296 ±3

227 ±4 228 ±5 203 ±6 198 ±4 223 ±3 237 ±4

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The tensile test results and fracture resistance are briefly summarized as follows. In this study, decrease in yield strength, (%)Total elongation (Table 2), plastic strain energy density (Figure 2), fracture toughness and tearing resistance (Table 3 and Figure 3) were observed in the temperature range 300-400 °C. Dutt et al. (2018) have summarised that dynamic strain aging was one reason for decrease in ductility in this temperature range. Similar observations (Roy et al. 2009; Verma et al. 2015) were made by other investigators for P91 steels.

3.2 Estimation of fracture resistance

In the present investigation, an attempt was made to estimate K 1c from tensile properties for P91 steel. Estimation of K 1c was also carried out for a different grade (SA 333 type) of steel as mentioned in the introduction (section 1). The estimation was carried using equation 5 and based on a methodology by Baskes (1975). Modulus of toughness (I) values were estimated using equation 6. Equation 6 was previously applied for various types of steels (Oh 2022).

2

EI

K



(5)

2

(1

)

1

 

c

Where E = Youngs Modulus, I= modulus of toughness determined from area under stress- strain curve, ν = Poissons ratio (0.3)

) ( YS UTS 

(

 )

I



(6)

2

Where I = Modulus of toughness, YS = yield strength, UTS = ultimate tensile strength, ϵ = total elongation (%)

For various steels, estimated K 1c values are plotted in Fig.4. Estimated K 1c and determined Kj 1c values of P91 steel are mentioned in Table 4. For P91 steels, estimated K 1c values are within 7-28% of the determined values (Table 4). Estimation of K 1c was carried out for similar P91 steels tested at three (room temperature, 380 and 550 °C) temperatures by Dutt et al. (2011). For P91 steels, difference between estimated K 1c and determined fracture resistance was in the range 1-3% (Table 5). From Table 4, it is observed that decrease in fracture toughness (both Estimated and determined values) occurs in the temperature range 400-450 °C. Increase in fracture resistance (Table 4) was observed with increase in the test temperature range 450-550 °C. In the temperature range 400-450 °C (Table 4) no significant changes in the estimated Modulus of toughness (73 and 74 MJ.m -3 ) values was observed followed by increase in modulus of toughness in the test temperature range 450-550 °C. From Table 5, it is observed that estimated Modulus of toughness and fracture toughness values at 380 °C, were lower than the corresponding values at 550 °C. Dutt et al.(2018) and Dutt et al .(2011) have reported decrease fracture resistance of P91 steels in the temperature range 300-450 °C. Decrease in fracture toughness (Dutt et al. (2018) was explained based on the mechanism of dynamic strain aging (DSA) in the temperature range of 300-450 °C. Based on observations by Dutt et el. (2018), it can be inferred that decrease in estimated fracture toughness ( K 1c ) in the temperature range 400-450 °C, compared to the estimated values in the test temperature range 500-550 °C, is due to DSA.