PSI - Issue 41

Abdelmoumene Guedri et al. / Procedia Structural Integrity 41 (2022) 564–575 Abdelmoumene Guedri et al. / Structural Integrity Procedia 00 (2022) 000–000

566

3

Fig. 1. Initial microstructure of specimens.

The contents of addition elements of this material were determined after precipitation treatment (% C= 0.075; % S=0.011; % P=0.017; % Al=0.047; % Si=0.22; % Mn=1.67; % V= 0.051; % Nb=0.062; % Ti=0.045). Dilatometry measurements were performed on our materials to determine allotropic transformation points AC1 and AC3. We used a dilatometer equipped with a programmable heating system with imposed speed equal to 50 °C per hour. The heating of our samples was done under argon. It should be pointed out that the phase transformation points obtained by dilatometer were confirmed by differential thermal analysis (AC1=705°C and AC2=850°C). The samples taken for the tensile tests were subjected to heat treatment beforehand. The objective is to cause precipitation before deformation. This heat treatment is intended to gradually precipitate the additive elements after homogenization of the structure by heating at 1300 ° C, followed by quenching with water see Figure 2. The specimens machined in the quenched state are fixed in the tensile device where they undergo a solution treatment at 1200°C. Then they are cooled to the deformation temperature which is between 700°C and 1150°C, thereby causing progressive precipitation of the additive elements. The hot formability of a metal is the greater or lesser ability of the latter to undergo hot plastic flow shaping without the appearance of defects. This is a characteristic of the metal. It can be investigated by hot tensile tests on the basis of a ductility criterion called "rupture necking or fracture area Z" defined as follows:   0 u S S The deformation device used in our tests was realized and is mounted on a INSTRON machine. The deformation specimens have a conical head, as shown in Figure 3, which allows obtaining a very fast quenching. The tests were all carried out under argon in order to avoid any oxidation of the test specimen during hot traction. The gas is swept in a sealed chamber (quartz tube) inside which the traction device is located. The heating of the specimen is provided by an image furnace. The thermal cycle is programmed at the beginning of the manipulation and the temperature control is done by means of a thermocouple welded on the useful part of the specimen. The interval between the breaking moments is the immersion in water was estimated at about 0.1 seconds. Our tests were carried out over a range of deformation temperatures ranging from 700 °C to 1200°C, and for deformation rates ranging between 10 -2 s -1 and 5.10 -4 s -1 . 0 Z 100 S   (1) with S 0 = area of the initial section of the calibrated part and S u = area of the minimum section of the test piece after deformation.