Issue 41

G. G. Cunto et alii, Frattura ed Integrità Strutturale, 41 (2017) 332-338; DOI: 10.3221/IGF-ESIS.41.44 334 The hot tensile tests were performed at temperature of 288 °C in according to the standard ASTM E8/E8M-13a and ASTM E21-09. The evaluation of the hot tensile tests includes the yield strength (σ 0 ) tensile strength (σ R ), flow stress (σ f ) and uniform elongation determination. The analysis includes also the determination of the Stress-Strain curve by means of the Ramberg-Osgood Eq. 1, as following:                0 0 0 n (1) where: σ = stress value; ε = strain value; σ 0 = reference stress; ε 0 = reference strain (σ o /E); α = parameter from curve fitting of data; and n = strain-hardening exponent. The RO parameters were obtained using the engineering stress-strain curve, fitting the data in the range between 0.1 % strain and the strain corresponding to 80 % of the ultimate strength. The fracture toughness tests were performed in accordance to the ASTM E1820-13 standard at temperature of 288 °C. The evaluation of the toughness tests includes the determination of the material resistance J-R curve, the initiation of stable crack growth J IC and the material power law formula for the J-R data, obtained using a linear regression analysis, as following:     m mat J C a (2) where: J mat = J Deformation in units of kJ.m -2 ; Δa = crack extension in mm; C = material constant; and m = exponent. The hot tensile and toughness tests were performed considering the three-component weld metal, base material and HAZ piping zones. Tested specimens quantity and orientation are showed in Tab. 1. Once only circumferential weld is used, it was considered that in the weld and HAZ zones only circumferential cracks could exists, so the transverse specimens orientation for the hot tensile test and C-L for the toughness test were considered only for the base material. Test type Specimen orientation Pipe zone Base Material AISI 316LN HAZ Weld AISI 316L Hot tensile test Longitudinal 3 3 3 Transverse 3 -- -- Hot Toughness test L-C 3 3 3 C-L 3 -- -- Table 1: Quantity of specimens for hot tensile and toughness tests Determination of the critical through-wall cracks size and the leakage cracks size The sizes of critical through-wall cracks and the leakage crack were determined using net section collapse (limit load) analysis. For the critical cracks found in these analyses, it was verified if fail would occur by tearing collapse using elastic-plastic J- integral analysis. The computer program PICEP [3] was used for the limit load analysis for determination both the size of critical cracks and the size of leakage cracks. For a given set of input conditions, including operation conditions, applied loads, crack