PSI - Issue 16

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 16 (2019) 153–160 Hryhoriy Nykyforchyn et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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3

elongation δ . Another set of tests was loaded by slow strain rate tension (SSRT) at a strain rate of 10 – 6 с – 1 in the NS4 solution (g/l: 0.131 MgSO 4 ∙ 7 H 2 O; 0.181 CaCl 2 ∙ 2 H 2 O; 0.483 NaHCO 3 ; 0.122 KCl) saturated by CO 2 , pH 5.7 to determine SCC resistance of the studied steels. SCC resistance was evaluated by a factor λ = ( P NS4 – P air )/ P air 100%, where P NS4 and P air – the same characteristics, obtained during the tests in NS4 solution and in air correspondingly. The fractographic investigations were carried out by scanning electron microscope (SEM) EVO-40XVP.

Table 1. Chemical composition of the investigated steels, mass. % Steel code State of metal Duration of operation, years C

Si

Mn Сr

S

P

17H1S 17H1S

As-received

–

0.140 0.994 1.54 0.067

0.0290 0.008

After operation

30

0.176 0.488 1.28 0.0287 0.0042 0.0005

API Х60 API Х60 API X70 API X70

As-received

–

0.111 0.235 1.49 0.021 0.118 0.410 1.75 0.044 0.108 0.260 1.42 0.016 0.116 0.336 1.62 0.022

0.0072 0.0037 0.0041 0.005

After operation

25

As-received

–

0.004

0.0005

After operation

37

0.0037 0.0009

3. Tests results and discussion. 3.1. Tension tests in air

The mechanical properties of strength and plasticity for steels in as-received state are presented in the Table 2.

Table 2. The mechanical properties of the investigated steels in as received state. Steel code σ UTS (MPa) σ YS (MPa) Elongation δ (%)

Reduction in area ψ (%)

17H1S

565 585 615

483 485 521

26.0 23.5 22.3

78.0 82.0 73.4

API Х60 API X70

For analysis of changes in mechanical behaviour of metal caused by operation a factor λ was used. In this case it was considered as relation ( P op – P rec )/ rec 100%, where P op and P rec – the same characteristics of steels in operated and as-received states correspondingly. The analysis results are presented in Fig. 1. The slight increase of the strength characteristics σ UTS and σ YS for the all studied operated steels and noticeable decrease of the plasticity characteristics δ and ψ for the API X60 and 17H1S steels was revealed. Thus, both plasticity characteristics were changed in the highest degree for the 17H1S steel: δ decreased by 22% and ψ – by 18%. At the same time, no noticeable change of plasticity characteristics was observed for the API X70 steel.

1

10

2

3





0

 UTS

 YS

 , %

-10

-20

Fig. 1. A change of the mechanical characteristics of strength and plasticity of the 17H1S (1), API Х60 (2) and API Х70 (3) o perated gas pipeline steels in comparison with as-received state.