PSI - Issue 8

Matteo Loffredo / Procedia Structural Integrity 8 (2018) 265–275 M. Lo ff redo / Structural Integrity Procedia 00 (2017) 000–000

267

3

Equivalent plastic strain.

p p σ

Maximum tensile equivalent plastic strain.

Stress vector.

2. Experimental campaign

2.1. The material

This study examined a commercial Quenched and Tempered AISI 4140 steel whose chemical composition is listed in Tab. 1. The material was furnished as a 26 mm diameter round bar. Metallographic examination was carried out on material samples surfaces normal to the extrusion direction. Surfaces were grinded using SiC papers up to 800 grit and then mirror polished. The observation was performed using a standard optical microscope (LEICA DMI 3000 M, Leica Microsystems, Wetzlar-Germany).

Table 1. Material chemical composition Composition C Si Mn P

S

Cr

Ni

Mo Cu Sn

Al

Ti

V Nb

C.E.

(%)

0 . 44 0 . 31 0 . 90 0 . 020 0 . 021 1 . 12 0 . 08 0 . 20 0 . 25 0 . 014 0 . 026 0 . 012 0 . 004 0 . 003 0 . 88

Optical examination of the AISI 4140 in the as-received condition revealed a homogeneous microstructure over the bar cross section normal to the extrusion direction. The microstructure consists of tempered martensite matrix including a fine distribution of particle inclusions (Fig. 1). These evidences are consistent with commercial QT AISI 4140 microstructures.

(a) Unetched micrograph of the as-received AISI 4140 showing inclu sions and carbides.

(b) Etched appearance of steel revealing tempered martensite (Nital 2%).

Fig. 1. Material microstructure.

In Tab.2 the main mechanical properties of the AISI 4140 are reported, as declared by supplier, while in Fig.2 the engineering stress-strain curve for two tensile tests is reported (there is good agreement between measured and declared data). The results presented thereby show that the material exhibits a discontinuous yielding followed by an hardening after approximately 5% of total strain.