PSI - Issue 16

Yevstakhiy Kryzhanivskyy et al. / Procedia Structural Integrity 16 (2019) 237–244 Yevstakhiy Kryzhanivskyy et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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We evaluated the impact toughness of the metal at room temperature and low temperatures on specimens with a cut, which was applied in different zones of the weld. Specimens from base metal (BM) and templates cut by the mechanical method (cutter) (W1) and plasma method (W2) were used in order to detect the influence of the cutting method on the impact toughness of the welds of the pipe steel (Tabatchikova et al. (2018)). Low temperatures were used to reduce the relaxation ability of the material (the formation of a rigid stressed state in the vicinity of the concentrator). Tests were performed on the RKP- 300 “AMSLER” impact testing machine with a maximum impact force of 300 J, and stress- strain diagrams were recorded in coordinates “load - time” and “load - specimen bending”. Patterns of deformation and fracture were investigated on fractograms of fracture surface of specimens, using the REM 106I scanning electron microscope.

3. Influence of temperature on the impact toughness of the base metal and the weld

3.1. Impact toughness

Pipe steel of the X70 strength class had high impact toughness KCV, indicating its conformity to the requirements. In particular, for the BM under test temperatures 20 °C; 0 °C; - 20 °C; - 40 ° C; -60 ° C, the KCV was 376 J/cm 2 ; 334 J/cm 2 ; 210 J/cm 2 ; 150 J/cm 2 ; 110 J/cm 2 , respectively. As is seen from Fig. 2, the impact toughness of the weld metal of the investigated pipes is also high enough in general.

400

1 2 3 4

350

300

150 KCV, J/cm 2 200 250

100

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-60

-40

-20

0

20

t, o C

Fig. 2. Effect of temperature on impact toughness of pipe steel (strength class X70): 1 – base metal (BM); 2 – weld (W); 3 – fusion zone (W1); 4 – fusion zone (W2).

For specimens cut from the weld (W), the impact toughness at temperatures - 40 °C and - 20 ° C was identical to that of the base material. Since the temperature - 40 ° C is recommended for comparative assessments, it can be stated that the quality of the weld was high. At room temperature, the impact toughness of the base metal pipe was 1.3 times higher than that of the weld. The impact toughness of the W1 and W2 fusion zones at a temperature of - 40 ° C was 1.14 and 1.5 times lower than that of the base metal, respectively. This difference increased with an increase in the test temperature and reached its maximum at 20 °C with the values of 1.9 (W1) and 1.5 (W2), respectively. However, despite such significant differences in the impact toughness of the investigated pipe steel in all of the investigated zones, it meets the requirements. This is primarily due to the use of modern welding materials. The optimal combination of alloying elements provides for the predominant formation of welds of ferrite structure with high ductile characteristics in the metal. Differences in the values of impact toughness in specimens taken from the central section of the weld and in the