PSI - Issue 23

Aleš Materna et al. / Procedia Structural Integrity 23 (2019) 425–430 Author n me / Structural Int grity Procedia 00 (2019) 000 – 000

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Fig. 1. A schematic view on pressed side of rectangular specimen with locations of samples for microstructure observation and hardness measurement after annealing

2.3. Sample and surface preparation

Each rectangular specimen was cut to small approximately 6 mm thick samples according to Fig. 1 for microstructure observation as well as for annealing and consequent hardness measurement. All analyzed surfaces of samples were grinded and polished. Surfaces for metallographic observation were also etched using mixture of hydrochloric and nitric acid. 2.4. Annealing of 08Ch18N10T steel Total of 12 samples from 08Ch18N10T steel were annealed at 700 °C for 1, 2, 4 and 6 hours , and consequently, quenched in water to stop diffusion.

2.5. Hardness measurement

Each value of Vickers hardness HV5 on pressed side of rectangular specimen was averaged from 5 single indents located in a row oriented along the longer edge of samples. The distance between indentations and the distance from the indentation to the edge of the specimen was approximately 1 mm.

2.6. FEM model of cold-working

All numerical simulations were performed in FEM software MSC.Marc 2015. The purpose of the simulation of the cold-forming of the rectangular specimens is to quantify a local distribution of plastic strains on the specimen ’s face, which leads to uneven hardening across the measured area. 1/8 of symmetrical deformable specimen was pressed by a rigid contact surface. Material of the specimen was elastic-plastic with model yield stress 240 MPa and linear hardening. The slope of the strain hardening was 2000 MPa and the coefficient of friction between the loading plate and the specimen 0.2 was tuned to match real pressure forces and the final deformed shape of the specimen, respectively. The effect of local hardening of the specimen ’s s urface on the hardness was simulated by simplified 2D axisymmetric FEM model of material section size 12×12 mm. The model was pre-deformed with frictionless rigid flat contact plate to various values of plastic strains, which was followed by the indentation with a rigid indenter (Fig. 2). The real pyramidal shape of Vickers indenter was replaced by the rigid equivalent cone with semi-apical angle 70.3° , which has the same projected contact area to penetration depth ratio. Hardness of variously pre-deformed material was evaluated from the indenter load – depth curve using the Oliver-Pharr method. 2.7. FEM model of hardness test