PSI - Issue 13

Fidan Smaili et al. / Procedia Structural Integrity 13 (2018) 1347–1352 Fidan Smaili, Tomaž Vuherer / Structural Integrity Procedia 00 ( 2018) 000 – 000

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zones such: coarse grain heat affected zone (CG HAZ), fine grain heat affected zone (FG HAZ), inter critical heat affected zone (IC HAZ), over tempered heat affected zone (IC HAZ). For reliability of the weld joint or welded constructions it is crucial that each part of weld joint has adequate properties in order to prevent sudden breakage of the welded constructions [1-4]. Each of the mentioned sub-zones have different properties because of their structure arrangement. HAZ can be in many cases the critical part of the weld joint, usually failure of weld toe occur inside of this zone because of the defects, which can be present on microstructure of the material. If defect is detected by NDT methods inside of these zones as a designer we have to be able to predict and monitor crack growth rate (crack propagation) so that to discard the component from usage before catastrophic failure can occur. The Vickers indenter is used as artificially small defect in two different HAZ microstructures (coarse grain HAZ and fine grain HAZ) in this investigation.

2. Experimental procedure

Nickel-molybdenum alloy steel 17CrNiMo7 was used to prepare samples of FG HAZ microstructure. Chemical composition and mechanical properties of the steel as delivered condition are shown in Tables 1 and 2.

Table 1. Chemical composition of the steel (/ weight %) Chemical element C Si Mn

P

S

Cr

Ni

Cu

Mo

Al

Weight %

0.18

0.22

0.43

0.012

0.028

1.56

1.48

0.15

0.28

0.023

Two groups of the specimens with different microstructures, which are present in HAZ of weld toe (CG HAZ and FG HAZ) are simulated by using laboratory furnace. Heat treatments for artificially prepared CG HAZ and FG HAZ microstructure are presented in Figure 1. The goal of the heat treatment is to achieve lath martensitic CG HAZ and FG HAZ microstructure as it is in HAZ real weld toe where the average grain size 200 µm in CG HAZ and respectively 10 µm in FG HAZ with adequate hardness.

0 300 600 900 1200

0 300 600 900 1200

Ac 3 Ac 1

Ac 3 Ac 1

T / C

T / C

0 100 200 300

0

50

100

b

a

t /min

t /min

Fig. 1. Heat treatment in order to prepare specimens with HAZ microstructure; (a) CG HAZ, (b) FG HAZ

In order to obtain mechanical propertied of artificially made CG HAZ and FG HAZ microstructure following mechanical test were used: tensile tests, hardness measurements, instrumented impact tests, fatigue crack growth tests and fatigue tests on the specimens with artificial defect. Tensile test were performed according to standard EN ISO 6892 1 using B method. Standardized cylindrical specimens with diameter 12 mm and 10 mm were used for tensile testing. Tensile testing were performed by servo-hydraulic tensile machine Amsler 599/594. Hardness test were tested by diamond Vickers indenter by using force 98.1 N (10 kg). Before testing surface was prepared by fine grinding and polishing on water resistance papers with different granulations from 100 to 1200 grains/cm 2 . Hardness were measured according to EN ISO 9015 part 1 standard and 30 measurements were performed on CG HAZ and 30 measurements on FG HAZ artificially made microstructure. Instrumented impact tests were carried out on Instrumented Amsler Charpy Pendulum RPK 300 according to EN ISO 148-1 and ASTM E2290-15 standards. Standardized specimens with ISO-V notch were used for testing. Geometry of the Charpy specimen is presented in Figure 2a. Force versus time and energy versus time diagrams were recorded during the tests. The instrumented Charpy test enables to split the total energy into the energy for initiation and the energy for propagation what were used at all tested specimens. Testing were performed at room temperature. Fatigue crack growth test were carried out on 160 Nm RUMUL Cractronic machine and Fractomat equipment for fatigue growth measurement. The geometry of specimens was similar like Charpy specimen but the notch was different (1 mm in deep) and it was prepared by electro erosion machining. The strain gauge foil were attach on the side surface of the specimen, which it was used for crack length measurement during testing. Firstly, the Figure 2b shows geometry of the specimen. The Figure 3a represents the RUMUL Cractronic machine and Fractomat equipment for the test and finally Figure 3b is photo of detail of clamping of the specimen during the testing.