PSI - Issue 13

Valeriy Lepov et al. / Procedia Structural Integrity 13 (2018) 1201–1208 Valeriy Lepov et al/ Structural Integrity Procedia 00 (2018) 000 – 000

1205

5

50 mkm

50 mkm

50 mkm

a

b c Fig. 3. (a) Microstructure of the weld area; (b) HAZ and (c) base metal in the St3sp probe, ×250.

Analysis of the microstructure of the base metal, weld metal and heat affected zone of the sample numbered by 7 was performed using metallographic optical microscope. Composition of pearlite and ferrite was seen with the print of the indenter of the micro hardness test. The formation of pearlite and ferrite in base metal is composed of 20/80 respectively. For weld zone and HAZ it changes due to thermal processes. So the microstructure analysis shows that the base metal is a ferrite and pearlite having an average grain diameter about 7 microns (see Fig. 3c). The structure of the weld metal is also made up of ferrite and pearlite (see Fig. 3a) with columnar crystals of cast metal. The HAZ is made up of Widmanstätten figures (see Fig. 3b). The width of the HAZ zone is about 1,5 mm. In different areas of heat affected zone is observed fine-grained ferrite-pearlite structure with a high degree of dispersion. Fig. 3b shows a micro crack with the length 1.7 mm in the HAZ of sample number 7. In the weld zone of this probe the micro crack 1,2 mm length was revealed also. The un-notched smooth tensile specimens were prepared to evaluate transverse tensile properties of the joints such as tensile strength and yield strength. Also the un-notched smooth cyclic loading specimens were prepared to evaluate fatigue properties under low cyclic loading. Two points were obtainable from the tensile test results like the average tensile loading Rm and the average Rp 0.2. An interval between these two values serve us as maximum starting point for the fatigue loading for each specimen. This cyclic loading was constant on the specimens under a constant frequency of 5 Hz until fracture and complete rupture occurs on the specimen. Steel St3sp after the long usage (about 40 years) in a pipeline at low temperature conditions was investigated for recycling by severe plastic deformation technology also. Steel was hardened by equal-channel angular pressing. Reference samples of steel was taken from an emergency stock of pipelines. The hardening technology allows to double the strength of steel but the plasticity has gotten a little worse. In that case steel after long usage has more high plasticity and strength. It can be explained only by changing the deformation mechanism changing. The probes were investigated by known technique (Lepov et al, 2008). In Fig. 4 the 2D and 3D ASM-images of deformation surfaces near the rapture has been presented. The deformation surface profiles change from slipping along grain boundary mechanism to viscous flow. As shown in Fig. 4 (e, f) nevertheless of fact that ductile deformation mechanism remains, the plastic flow terrain has changed because of the grinding grains. In last case (Fig. 4, f) the deformation has firm heterogeneity. More detail information about this can be get the morphological and fractal analysis. In Fig. 5 the results of morphological and fractal analysis have been presented. Fractal dimension of surface images was increased from in as-received condition steel probes to hardening material and steel after long usage and hardening processed. So the initial phase and structure inhomogeneity of steel could influence the mechanical properties after the hardening by severe plastic deformation process.

3. Modeling results and discussion

The modeling of damage accumulation processes should consider the complex effects of high-cycle fatigue and low-cycle impact loading and also friction damage. The impact toughness as shown in Table 3 greatly depends on the test temperature. So the overall damage  could differentiate for high-cycle fatigue damage  F and low-cycle impact damage  L and contact wear damage  Fr :

N

K

M

1

1

1

f i     l k 

,

(1)

 

     

fr j

F

L

Fr

N

K

M

i

j

k

1

1





1

