PSI - Issue 68

Swastik Soni et al. / Procedia Structural Integrity 68 (2025) 513–519 S. Soni et al. / Structural Integrity Procedia 00 (2025) 000–000

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Nomenclature σ Stress σ o Ɛ Strain ̇ ϕ σ UTS

Yield strength

Ultimate tensile strength

Strain rate

Strain energy density Strain hardening exponent

n k

Material constant

In the ductile-to-brittle transition (DBT) regime, Charpy impact testing and the master curve approach are commonly employed to assess material behavior. However, quantitative fractography has also been utilized to measure the stretch zone width, which provides further insight into fracture characteristics in this transitional regime. While the Charpy impact test offers information on the energy absorbed during fracture and the master curve helps predict fracture toughness across a range of temperatures (Johannes 2017). The tensile toughness which the area under the stress-strain diagram can also be used for characterizing DBT regime. However, the properties obtained from tensile tests have been demonstrated to not show significant scatter in the DBT regime. Quantitative fractography allows for a detailed examination of the fracture surface. This method allows for the calculation of stretch zone width, and as demonstrated by Hohenwarter et al. (2010), it shows that stretch zone width is affected by factors such as grain size and testing temperature, during the transition from ductile to brittle failure. The ductile to brittle transition bheaviour of ferritic/martensitic steels are extensively examined for its fracture behavior including the influence of strain rate and prior ductile tearing by Tiwari and Singh (2018) and Tiwari et al. (2018). In this study, quantitative fractography is used to analyze the fracture surfaces of Modified P91 (mod 9Cr-1Mo) steel within the ductile-to-brittle transition (DBT) regime, under different strain rates and temperatures. Tensile tests were conducted in a controlled environment, with liquid nitrogen being circulated to precisely regulate the temperature. This approach allows for a detailed investigation of how varying strain rates and low temperatures influence the fracture behavior of the steel. By employing quantitative fractography, the study aims to provide deeper insights into the microstructural mechanisms governing fracture in the DBT regime, which is crucial for predicting material performance in high-stress, low-temperature environments. Quantitative fractography methods such as measuring the average dimple diameter and calculating the fractal dimensions of the fracture surfaces provide insights into the microscopic mechanisms governing ductile and brittle fracture modes. 2. Experimental Procedure 2.1. Material The material used in this study was Modified P91 (Mod 9Cr-1Mo) ferritic/martensitic steel. This steel variant, known for its high strength and creep resistance at elevated temperatures, is commonly used in high-temperature applications such as power generation equipment. Detailed chemical composition data for Modified P91 steel is provided in Table 1, while Figure 1 illustrates its microstructure, highlighting key features such as the distribution of carbide precipitates and the tempered martensitic phase.

Table 1. Chemical composition of Mod. 9Cr-1Mo steel (wt.%). Element Cr Mo V

Nb

C

Si

Ni

N

Value (%)

9.37

0.911

0.189

0.08

0.085

0.336

0.097

0.02