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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1. Introduction

The development of a microstructural-based predictive tool of impact toughness of quenched and tempered (QT) martensitic steels has become a key for the design of new grades or to adapt the heat treatment cycle in this steel family. It seems important to understand which microstructural constituents control the tensile fracture behavior as well as the upper self-energy (USE) of the material during Charpy impact testing. Much has been written on the effect of microstructure on USE of quenched and tempered steels. Hong et al. (2016) showed that the USE of a Mn-Mo-Ni low alloy, bainitic steel is higher and its ductile-to-brittle transition temperature (DBTT) is lower, when homogenously distributed fine carbides are present. The effect of carbide precipitation on the impact properties of low carbon Mn-Ni-Mo bainitic steels was also stated by Im et al. (2001). The USE was increased by substituting M 3 C cementite carbide into fine M 2 C carbide through increasing molybdenum content. Takebayashi et al. (2013) underlined the effect of prior austenite grain (PAG) on impact properties of C-Si-Mn martensitic steels for given tempering conditions. Coarser PAG allows precipitation of coarser carbide that negatively affects the DBTT and the USE. Yet, there has been little reported work focusing on how cementite carbide could affect the USE during impact tests. The present study proposes a new and simple approach of the effect of carbide size and intercarbide distribution on the USE of medium carbon steels. Instrumented Charpy impact (ICI) curves in the USE domain were systematically investigated to separate contributions of crack initiation and propagation to the absorbed energy. The results are discussed in view of the effect of intercarbide spacing on the ductile fracture mechanism process.

Nomenclature QT

Quenched and tempered

USE Upper self-energy ICI Instrumented Charpy impact DBTT Ductile-to-brittle transition temperature F max

maximum load on instrumented Charpy impact curve

S m

displacement at the maximum load in instrumented Charpy impact curve

2. Experimental procedure

2.1. Material and microstructural investigation procedures The chemical composition of the investigated steel was 0.4C – 1.25Cr – 0.32Mo – 1.14Mn – 0.27Si – 0.2Ni – 0.018Al (wt.%) with low phosphorus and sulfur levels. The as-received 180-mm-dia. cylindrical bar had been austenitized at 875°C and water quenched then tempered at 600°C and air cooled. All specimens were taken at 25 mm from the skin of the bar in order to get a homogeneous microstructure. In order to keep a similar martensite matrix while varying the carbide precipitation state, an additional tempering was realized by reheating blanks of the as-received material. In the following, the three tested microstructures are respectively denoted Material A (as-received), Material B (A + 690°C, 1h) and Material C (A + 720°C, 4h). Metallographic samples perpendicular to the bar axis were polished down to 1µm diamond pastes then etched in 4% nital. To make carbon extractive replicas, they were coated with 50-nm-thick carbon film by using an evaporator. This thickness was shown to keep carbides embedded within the carbon film, preventing them from further moving. The thin carbon films were released from the sample by immersion in 10% nital solution for 20 min. Bulk samples and replicas of the three materials were observed in a field emission gun scanning electron microscope (ZEISS Sigma 300) operated as follows: high voltage of 5 kV, spot size of 60 µm and magnification 10,000. The polished samples and replicas were respectively observed with in-lens secondary electron and backscattered electron imaging. The image size was set to 1024  768 pixels for statistical analysis on ImageJ software.