Kocurek, R., & Adamiec, J. (2015). Evaluation Of Susceptibility To Hot Cracking Of Inconel 617 Nickel Alloy Welds In Transvarestraint Test.pdf

Solid State Phenomena Vol 226 (2015) pp 95-98 © (2015) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/SSP.226.95

Submitted: 2014-06-02 Revised: 2014-10-14 Accepted: 2014-10-14 Online: 2015-01-12

Evaluation of susceptibility to hot cracking of Inconel 617 nickel alloy welds in Transvarestraint test Robert Kocurek1,a, Janusz Adamiec1,b 1

Faculty of Material Science and Metallurgy, Silesian University of Technology, Krasinskiego 8, 40-019 Katowice, Poland a

[email protected], b [email protected]

Keywords: Inconel 617, nickel alloys, hot cracking, welding, transavarestraint test.

Abstract. Defining the susceptibility to hot cracking of Inconel 617 alloy welds is essential for assessment welding and pad welding technology. Because of that technological transvarestraint test was performed in the study. Test simulates strains that form in the material during welding. Transvarestraint test enables the assessment of susceptibility to hot cracking and resistance to hot cracking characterized by cracking threshold (εp) and critical strain speed (CSS). Performed investigations enabled to characterize the phenomena occurring in Inconel 617 during welds crystallization, which are important for engineers selecting the joining technology of Inconel 617. Introduction Nickel alloys, due to their unique mechanical properties at high temperatures, creep strength and oxidation resistance are widely used in the manufacture of elements in the petrochemical industry, aerospace and power generation. The main problems in the use of welding technology of nickel alloys are unacceptable cracks, especially hot cracks [1, 2]. These cracks can form in the weld and heat-affected zone of the welded joint. It is believed that the main cause of the intergranular cracking is a reduction in strength of metal in the Brittleness Temperature Range and deformation during crystallization. This phenomenon may occur due to a reduction in the strength of grain boundary areas by the presence of a liquid film or the accumulation of a large amount of crystal lattice defects in these areas. The liquid phase can occur in areas of the solid phase grain boundaries in the final phase of solidification, or as a result of these areas in the partially melted process of heating the metal in the solid state [3, 4]. Weldability can be very widely understood, there are various research techniques and evaluation criteria for the cracking tendency. Technological transvarestraint test is widely adopted. It allows the determination of the cracking threshold and the critical strain speed, which may be indicative of resistance to cracking of materials [5]. Material for research The base metal for study was heat-resistance Inconel 617 alloy. Research were carried out on plate prepared with dimensions: 150 mm x 80 mm x 5 mm. Technological transvarestraint test To the determine the hot-cracking tendency of nickel alloy Inconel 617 the transvarestraint test was carried out. It is done by rapid bending of the test specimen on the cylindrical die block to accurately reproduce the curvature. Bending takes place during the throughout welding using arc inert gas technology (141, TIG). During bending native material and the weld are deformed. The extent of deformation depends on the thickness of the bended sample (g) and the radius of curvature of the die block (R) – the bending mandrel. In order to ensure constant thermal conditions samples of equal size should be used. Changing the radius of the block changes deformation. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 131.215.225.9, California Institute of Technology, Pasadena, USA-27/05/15,18:22:26)

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The study was conducted using the test setup designed and built at the Institute of Materials Science, Silesian University of Technology. It is equipped with a hydraulic deformation module with a set of bending rods of following radius: 16, 22, 34, 50, 100 mm and a welding machine Lincoln V205 AC/DC. Autogenous weld was carried out in argon shield, constant current intensity 140 A. Autogenous welding speed was 0.95 mm/s. The process parameters were chosen to obtain the complete joint penetration of the specimen. As a criterion for assessing the susceptibility to hot cracking of Inconel 617 nickel alloy in transvarestraint test the cracking threshold εp and the critical strain speed (CSS) were adopted. Metallography The tests included: macroscopic observations carried out on the Olympus SZX9 stereoscopic microscope in the dark-field technique and microstructure observations taken on Olympus GX71 optical microscope in the bight-field technique by 50-500x magnifications. Research results and analysis Deformation value was determined according to the relationship [5]: (1) where: ε –deformation value [%], g –sample thickness [mm], R – the radius of curvature of the die block [mm]. In addition to the deformatiuon value the following parameters were determined: the length of the longest crack in the weld axis (Lmax), the sum of the lengths of all cracks (Limax) and the threshold of cracking (εp) as the minimum strain at which there are no cracks. Knowing the length of cracks formed in the center of the weld (Lmax) and the corresponding deformation and welding speed (vs) enables to calculate the crack development time (tmax) using the relationship: (2) where: tmax –crack development time [s], Lmax – the longest crack [mm], vs – welding speed [mm/s]. Table 1. Summary of measurements results and calculations for hot cracks formed in the technological transvarestraint test The sum of Radius Welding The longest Time crack The size of Specithe lengths of mandrel speed crack development deformation men all crack R [mm] Vs [mm/s] Limax [mm] tmax [s] ε [%] ΣLi [mm] P0 0 0.95 P1 100 0.95 0.24 0.88 0.25 2.5 P2 50 0.95 0.4 1.72 0.42 5 P3 34 0.95 0.68 4.32 0.72 7.35 P4 22 0.95 1.12 4.76 1.18 11.36 P5 16 0.95 1.2 5.4 1.26 31.25

Remarks no cracks cracks cracks cracks cracks cracks

Based on the test results minimum threshold i.e. strain at which cracks disappear were determined, for the test of the Inconel 617 alloy it has a value of εp = 2.5%. The results of calculations were used to determine the time dependence of crack development as a function of strain tmax = f(ε). Designated critical strain rate CSS (Critical Strain Speed) as the tangent of the angle between tangent to the crack development curve and deformation axis (Fig. 1).

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The value of CSS can be adopted as one of the criteria for assessing the susceptibility to hot cracking during welding. For nickel alloy Inconel 617 evaluates CSS is 1.81 [1/s].

tgγ = CSS γ

Fig. 1. Dependence of crack development time as a function of strain tmax = f(ε) Macroscopic tests after transvarestraint test revealed hot cracks in the weld and the heat affected zone. Fig. 2 shows the face of weld specimen P1, where the total length of all fractures was the smallest – ΣLi = 0.88 mm. With the increase of the amount of deformation during the autogenous weld the increase in the sum of the lengths of cracks was observed, it was respectively ΣLi = 1.72 mm with ε = 5% and ΣLi = 4.76 mm with ε = 11.36% (Table 1). Fig. 3 shows the face weld of specimen P5 with visible hot cracks initiated at the edge of the weld pool, the longest was the value of Limax = 1.2 mm.

Fig. 2. Weld face after transvarestraint test, specimen P1

Fig. 3. Weld face after transvarestraint test, specimen P5

Microstructure study of specimen P5 revealed cracks in the weld and heat-affected zone, the longest observed in the center of the weld (Fig. 4). Observation of the microstructure showed that the crack extending into the seam ended with dendritic structure on the fusion line in native material, and proceeded to the grain boundaries (Fig. 5).

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Fig. 4. Autogenous macrostructure after transvarestraint test, specimen P5.

Fig. 5. The hot crack in face of weld, magnification 500x (LM).

Summary Technological transvarestraint test can be used to assess the tendency for hot cracking. Designated indicators include: the size of the longest cracks in the weld axis (Limax = 1.2 mm), the threshold of cracking (εp < 2.5%) and critical strain speed (CSS = 1.8 [1/s]) can be also used as a criterion for hot cracking resistance – Fig. 1. Analysis of the transvarestraint test calculations (Table 1) showed that increase in the amount of deformation in the range of 2.5-11.36% causes an increase in the sum of the lengths of all the cracks ΣLi = 0.88-5.4 mm. Metallographic examination revealed that hot cracks were initiated on the line of the weld pool (Figs. 2, 3). Cracks developed in the weld and heat affected zone. The longest cracks was observed in the center of the weld (Figs. 4, 5). The results of this test are used to determine resistance of the alloy to hot cracking during welding. This test can also be used to assess the weldability of the Inconel 617 nickel alloy. References [1] T. Bollinghaus, H. Herold, C.E. Cross, J.C. Lippold, Hot Cracking Phenomena in Welds II, Publishing house Springer, Berlin 2008. [2] E. Tasak, Metalurgia spawania (in Polish), Publishing house JAK, Cracow 2008. [3] T. Bollinghaus, H. Herold, Hot Cracking Phenomena in Welds, Publ. Springer, Berlin 2005. [4] J.N. DuPont, J.C. Lippold, S.D. Kiser, Welding Metallurgy and weldability of nickel base alloys. Ed. A John Wiley & Sons, New Jersey 2009. [5] P. Adamiec, J. Dziubiński, A. Gruszczyk, P. Makosz, M. Mazur, W. Mazur, A. Szymański, Metalurgia spawania (in Polish), Publishing house of Silesian University of Technology, Gliwice 1992.

Technologies and Properties of Modern Utility Materials XXII 10.4028/www.scientific.net/SSP.226

Evaluation of Susceptibility to Hot Cracking of Inconel 617 Nickel Alloy Welds in Transvarestraint Test 10.4028/www.scientific.net/SSP.226.95