Effect of Microstructure Degradation on Fracture Toughness of 20MnMoNi55 Steel in DBT Region

1. Introduction

Reactor pressure vessel (RPV) of any nuclear power plant is the most critical component. The critical load of this component and its degradation is assessed by measuring the fracture toughness of the material on temperature scale. Due to the continuous neutron irradiation, the RPV material degrades drastically over small temperature range. The embrittlement (i.e., loss of ductility and fracture toughness) is reflected through the shift in fracture toughness vs. temperature curve along the temperature axis (International Atomic Energy Agency, 2005; Wallin, 2002). The ductile to brittle transition (DBT) of RPV steels with decreasing temperature remains a fundamental issue among researchers. Many studies have been conducted to find the effect of microstructure on fracture toughness of RPV steel. High toughness and low DBT temperature of RPV material is achieved by thermal treatment resulting in a bainitic microstructure. Also the harder auto–tempered martensitic microstructures have higher fracture toughness values than the softer bainites. The strength and ductility of steels are also improved by intercritical heat treatment. Higher cooling rate during quenching process increases the fracture toughness of the quenched and tempered steel. The type of microstructure plays an important role on mechanical properties including toughness of the material (Haverkamp, Forch, Piehl, & Witte, 1984; Ahn, Kim, Byun, Oh, Kim, & Hong, 1999).

A bainitic MnMoNi low alloy steel is commonly used as the material for the pressure vessels in the nuclear reactor. In bainitic low alloy RPV steels, the DBT temperature behaviour is a primary concern due to irradiation embrittlement during operation. Above the DBT temperature the bainitic steels are characterized by a ductile dimple fracture accompanied by high energy (upper shelf energy) absorption. Below the DBT temperature the fracture mechanism is transgranular cleavage and only little energy (lower shelf energy) is absorbed (Karlík, Nedbal, & Siegl, 2003; Yang, Lee, Oh, Huh, & Hong, 2004). Many researchers have shown that the effect of microstructure on the cleavage fracture strength and DBT region of low carbon steel are strongly affected by alloy content. Therefore, the selection of appropriate microstructure for RPV steels depends on both toughness and ductile to brittle transition temperature. The master curve approach proposed by Kim Wallin with the ASTM E1921–02 prescribes a three parameter Weibull distribution for the cumulative probability of cleavage fracture toughness in terms of K_JC at each temperature over the DBT region. The master curve can be used to measure the loss of ductility by measuring T₀ in ductile to brittle transition temperature. This T₀ is defined as the temperature at which median fracture toughness for 1T (one-inch-thick) specimen equals 100 MPa√m (Wallin, 2010). In this method, T₀ is the only fracture toughness characterizing parameter that can be evaluated using different methods like single temperature and multi-temperature methods and using specimens having different size, shape, crack depth and loading type. The fracture toughness value in DBT range is not a single valued parameter. Rather it is temperature dependent and even at a particular temperature, it follows Weibull distribution. Hence to assess the quality of a particular microstructure, the value of reference temperature T₀ should be evaluated. Higher fracture toughness at room temperature and a lower T₀ value is the preferred combination for the selection of RPV steels.