PSI - Issue 30

M.Z. Borisova / Procedia Structural Integrity 30 (2020) 17–22 Borisova M. Z. / Structural Integrity Procedia 00 (2020) 000–000

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al. (2000). In addition, the use of SPD methods provides to obtain high-strength states on cheap low-alloy steels, which will allow to saving significantly on material costs in the future. Such low-carbon low-alloy steels currently occur the highest production volume with the lower cost. These materials have wide range of applications but are limited to low strength. Grain refinement during SPD methods are able to improve the strength and toughness of steel without changing their chemical composition as pointed by Shin et al. (2002). Based on the preceding, the objective of this study is the determination of the influence of the ECAP process on low-alloy steel and the evaluation of its properties to identify the possibility of further practical use.

Nomenclature SMC submicrocrystalline SPD severe plastic deformation ECAP equal channel angular pressing

2. Experimental Experiments were conducted on low-carbon steel, with follow chemical composition (see Table 1). The low carbon steel was treated at 400°C for 2 passes of ECAP. The cylindrical specimens, with diameter of 20 mm and length of 90 mm, were pressed through a special die containing two channels, with equal cross-sections. The ECAPed specimens were tempered at 910°C for 8 minutes with oil quenching and subsequent short term tempering at 680°C. The initial microstructure composed of a ferrite matrix with pearlite grains in it with initial grain size of ~10 μ m.

Table 1. Basic chemical composition of steel.

C

Si

Mn 1,26

P

S

Cr

Ni 0,1

Al

Cu

V

Nb

Ti

0,09

0,64

0,007

<0,003 0,08

0,02

0,14

<0,002 0,01

0,013

To determine tensile strength of the samples for stretching of a dog-bone-type with dimensions of 2,5 x 10 x 80 mm were used. Tensile properties were measured on Zwick Roell Z600 Tensile Test Machine. Impact toughness was defined on Charpy V-notch samples with dimensions of 5x10x55 mm on impact test machine Amsler/RKP 450 at two different temperatures: +20°C and –20°C. The microfracture mechanisms were systematically studied with SEM observation on JEOL JSM-7800F. 3. Results and Discussion After 2 ECAP passes, quenching and tempering, the ferrite grains with initial size of ~10 μ m were refined to 0.2~0.3 μ m with uniformly dispersed fine cementite particles. Figure 1 shows the room temperature stress-strain curves before and after ECAP. The yield strength and ultimate strength of the steel after ECAP is higher than those in initial state about 2 and 1,5 times accordingly. It is known that the destruction of metal materials under the influence of external forces is preceded by an increase in the density of defects in the crystal structure such as dislocations, deformation vacancies, etc. Collective movements of such defects on different structural levels in the scale range from nano- to macrolevel are the effects of the plastic deformation as shown by Zhang (2019). The destruction process itself is essentially the final stage of the plastic deformation. The increase in the strength of the steel after ECAP is due to a decrease in the grain size. This makes it difficult for dislocations to move freely through a metal. The movement of dislocations is hampered by grain boundaries. The higher the grain boundaries percentage, the more difficult it is for the dislocations to move. In the results of that the metal is stiffer, stronger, and harder. But in addition to increasing strength, there is a decrease in plasticity (see Fig. 1).