Transactions, SMiRT 21, 6-11 November, 2011, New Delhi, India Div-II: Paper ID# 238 1 APPLICATION OF MASTER CURVE METHODOLOGY IN THE DUCTILE TO BRITTLE TRANSITION REGION FOR THE MATERIAL 20MnMoNi55 STEEL S. Bhowmik 1 , A. Chatterjee 1 , S.K. Acharyya 1 , P. Sahoo 1 , S. Dhar 1 , J. Chattopadhyay 2 1 Department of Mechanical Engineering, Jadavpur University, Kolkata – 700032 2 Reactor Safety Division, Bhaba Atomic Research Centre, Trombay, Mumbai - 400085 E-mail of corresponding author: sanjib_acharyya@hotmail.com, skacharyya@mech.jdvu.ac.in ABSTRACT Fracture toughness is an important material property to assess the critical load for structural integrity of reactor pressure vessel steel. In this paper, Master Curve method proposed by Kim Wallin is used to estimate the fracture toughness of 20MnMoNi55 steel in the ductile to brittle transition regime. Reference temperature (T 0 ) is evaluated using both single temperature and multi-temperature method for one inch thick compact tension (1T-CT) and 1/2-CT specimens. Reference temperature (T 0 ) is also determined from Charpy V-notch test data and compared. Effect of selection of temperature range and number of test temperatures on the value of T 0 is also studied. The correction proposed for thickness adjustment has been verified. It is observed that Charpy test results yield lower values of unirradiated T 0 compared to 1T-CT specimen tests. It is also observed that most of the fracture toughness values fall between 5% and 95% boundary of fracture toughness curves for all the evaluations. INTRODUCTION Both the scatter of fracture toughness in the transition zone and temperature dependence of fracture toughness is captured and expressed through Master curve method [3]. The master curve methodology is based on modelling cleavage fracture toughness at a fixed temperature in the transition with a 3-parameter Weibull distribution and the proposition that the shape of the fracture toughness vs. temperature curve is identical for all ferritic steels but having difference in absolute position in the temperature axis. The particular position of the curve for a particular steel is given in terms of the fracture toughness reference temperature (T 0 ) which is defined as the temperature at which the median fracture toughness for a 1T (one inch thick) compact tension (CT) specimen equals to 100 MPa√m [4]. Thus the master curve defines both the variation of the median value of fracture toughness with temperature and the scatter of fracture toughness about this median value. The master curve together with a reference temperature (T 0 ) value define the complete transition fracture toughness curve in a manner appropriate for use in both probabilistic and deterministic analysis. The master curve method is also used to construct a bounding curve on the fracture toughness. Typically a bounding curve with a 95% degree of confidence is used as lower bound on the fracture toughness. In this paper Master Curve method proposed by Kim Wallin is used to estimate the fracture toughness of the 20MnMoNi55 material in the ductile to brittle transition regime. Transition zone for the material is identified by Charpy impact test results. Then tensile properties are measured at different temperatures in the transition zone. Fracture toughness of 1T-CT and 1/2-CT specimens of the 20MnMoNi55 material is measured at different temperatures covering upper shelf to transition. For Cleavage fracture in the transition zone Weibull Parameters (K min , K 0 ) are estimated from fracture toughness results obtained from the experiments. Then both the single temperature and multi-temperature analysis are used to determine the reference temperature for master curve of the material. The correction proposed for thickness adjustment has been verified. All the propositions described in Master curve methodology are verified for this particular steel and found to be satisfactory. Proposition regarding K min is validated. Single and multi-temperature reference temperature estimation algorithm yield equivalent estimates. It is found master curves obtained from single temperature analysis fall between 5% and 95% bound fracture toughness curves obtained from multi-temperature evaluation. The effects of specimen thickness, a/w, testing temperature range and no. f test temperatures used to compute T 0 on the value of T 0 are also studied. MASTER CURVE ANALYSIS Master Curve Analysis for Charpy Test Data Wallin showed that the brittle fracture probability P f for a given temperature in the transition region is described by a three parameter Weibull model in the following form