Keywords: Haynes 214, Nickel based super alloy, Gas tungsten arc welding, Post-weld heat treatment.

Transcription

1 Advanced Materials Research Vol. 585 (2012) pp Online available since 2012/Nov/12 at (2012) Trans Tech Publications, Switzerland doi: / Effect of Post Weld Heat Treatment on Mechanical Properties and Microstructure of Nickel based Super Alloy Welds Agilan M *, T.Venkateswaran, D.Sivakumar, Bhanu Pant Heat Treatment and Welding Metallurgy Division, Materials and Metallurgy Group. Vikram Sarabhai Space Centre, Indian Space Research Organization Trivandrum, India * agilan103@gmail.com Keywords: Haynes 214, Nickel based super alloy, Gas tungsten arc welding, Post-weld heat treatment. Abstract In the present paper, Haynes 214 plate of 7mm thick was welded by Gas tungsten arc welding (GTAW) in two passes. The welded plates were subjected to post-weld heat treatment such as direct aging (DA) and solution treatment + aging (STA). The aging cycle was optimized using hardness by generating isothermal aging curve. The weld tensile properties, hardness test, microscopic analysis and fractographic studies were carried out in as-welded condition and at two different post-weld heat treatment conditions. The results show a significant increase in both yield and tensile strength after post weld heat treatment. 1. Introduction In launch vehicles, the combustion chamber parts in the engine experience relatively low stress and high temperature oxidizing environment. A nickel based superalloy, Haynes 214 is considered as the candidate material for that application because of its superior oxidation resistance. Above 955 C, tightly adherent Al 2 O 3 type oxide layer forms and protects the material from oxidation. Even at 1300 C it shows better oxidation resistance at short term exposure. When this alloy is exposed to temperature in the range of C, it responds to precipitation hardening by forming second phase, γ' (Ni 3 Al). The γ' precipitation results in volume contraction as well as increase in strength with decrease in ductility. Gas tungsten arc welding (GTAW) process is commonly used to fabricate this alloy to achieve high weld efficiency and weld quality. Haynes 214is normally put in service in as-welded condition and for high strength post-weld heat treatment (PWHT) is done. Strain age cracking may occur after welding due to Ni 3 Al formation and weld residual stress. It can be avoided by processing the materials with appropriate heat treatment and welding parameters. Welding studies on this specific alloy were very limited in the literature. Therefore, an attempt has been to weld and study the effect post weld heat treatment on microstructure and properties. 2. Experimental Details The chemical composition of the base material is given in Table 1. Hot rolled plates of 7 mm thick Haynes 214 were used for welding, which were solution treated at 1095 C prior to welding. 130x60x7mm plates, 60 V-groove with 1.6 mm root face were machined for weld coupons. Before welding, the surface oxide layer on the plates was cleaned thoroughly using rotary stainless steel wire brush followed by acetone cleaning with a lint free cloth. The coupons were Gas tungsten arc (GTA) welded in two passes, using ESAB make 180 A rated GTA welding machine. The welding parameters are given in Table 2. After welding, the coupons were subjected to two different post-weld heat treatments: a) weld + direct aging (DA) and b) weld + solution treatment + aging (STA). 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 TTP, (ID: , Pennsylvania State University, University Park, USA-13/01/15,03:25:07)

2 436 Advances in Materials and Processing Table 1: Chemical composition of Haynes 214 Nickel based super alloy. Element Cr Al Fe Mn Si Ti Zr C B Ni Comp, Wt% Bal To optimize the aging temperature and time, solution treated samples were aged at different temperatures (600 C, 650 C, 700 C, 750 C and 800 C) with different time (1hr, 3hr, 6hr, 9hr, 12hr and 15hr). The hardness was measured using Vickers hardness testing machine using 10Kg load. Welded coupons were cut transversely to the weld and polished using standard metallographic techniques. The polished surfaces were etched with aquaregia. The fusion zone microstructure was analyzed using optical microscope. The weld joints were cut into the required dimensions to prepare standard sub-scale tensile specimens and tests were conducted in Instron make 10 T Universal Testing Machine. The fractographic analysis of the tested specimens was done using Scanning Electron Microscope. 3. Results and Discussion 3.1 Base metal microstructure Welding Parameter Polarity Joint Configuration Current Speed Voltage Shielding Gas Heat Input Table 2: GTA Welding Parameters Selection DCEN 60 single V-groove, 2 pass 140 Ampere 60 mm/min Volts Argon (99.9% Pure) 1.6 KJ/mm Figure 1 shows the microstructure of solution treated Haynes 214 alloy, with fully recrystallized equi-axed grains with mean grain size of 80±15 micron. In addition, random distribution of carbides was also observed. Figure 1. Base metal microstructure of solution treated Haynes 214alloy 3.2 Aging cycle optimization The hardness of the alloy in the solution treated condition was 245 H V. Fig. 2 shows the isothermal aging curve for the alloy. For all the temperature, peak hardness was observed for aging of 9 hour. However, the hardness varied with the aging temperature. At 650 C, the hardness has increased from 245 to 273 Hv and a drop was observed thereafter. A similar trend was noticed for other aging temperature. Aging at 750 C for 9 hour yielded peak hardness of 320 Hv compared to that of 245 Hv in ST condition. An increase in hardness of 75 Hv was noticed after peak aging.

3 Advanced Materials Research Vol C 700 C 750 C 800 C Hardness, VHN Time (hours) 3.3 Weld microstructure Figure 2. Isothermal aging curve of Haynes 214alloy. Figure 3: Weld fusion zone microstructures a) as-welded b) Weld + aged (DA) c) Weld + solution treatment +aged (STA). Weld fusion zone microstructure of Haynes 214 at as-welded and post-weld treated conditions are shown in Figure 3. Microstructure show solidified dendritic structure and the growth of columnar grains towards weld centre. Weld microstructure after direct aging (DA) showed dendritic features similar to that in as-welded condition. Weld after STA condition, the dendritic structure was transformed considerably and the grain boundaries were visible after solution treatment and precipitation of Ni 3 Al phases takes place as result of aging. 3.4 Effect of post-weld heat treatment on mechanical properties The parent material and weld tensile properties at different heat treatment conditions are given in Table 3. Parent material tensile results show 999 MPa tensile strength, 667 MPa 0.2% yield strength and 16% ductility. The weld tensile strengths (as-welded, DA, STA condition) are lower than the parent material strength because of as-solidified dendritic structure in the weld fusion zone. All weld tensile specimens failed in weld metal and the weld joint efficiency was calculated as 85 percent. As-welded condition is similar to solution treated condition, during weld metal cooling no time was given for the formation of γ' (Ni 3 Al) strengthening phase. Hence, the tensile and yield strength of as-welded condition are lesser than post weld heat treatment conditions.

4 438 Advances in Materials and Processing Table 3: Tensile properties of base material and weld at different conditions Condition Parent ST as-welded weld+aged (DA) weld+st+aged (STA) UTS (MPa) % YS (MPa) Elongation 33% 21% 16% 25% Failure Location weld weld weld Weld+ direct aging (DA) condition shows minimum ductility, 16% elongation, due to the formation γ' (Ni3Al) phase without relieving weld thermal stress. STA treatment results in reasonable improvement in weld tensile properties compared to DA condition. Figure 4: Fractographs of weld tensile specimen a) as-welded b) weld + aged (DA) c) weld + solution treatment +aged (STA) The fracture features of weld tensile specimens at different conditions are shown in Figure 4. Fractography show completely dimple features in all the three conditions. Weld fracture surface in DA condition (Figure 4b) confirms slight decrease in ductility by exhibiting a dendritic pattern. It evidence that fracture occurred preferentially along the inter-dendritic regions. Figure 5: Hardness values across the weld at different conditions Hardness values show that weld metal hardness in as-welded condition is similar to parent material at solution treated condition. After PWHT the hardness improved considerably from 250 HV to 310 HV (Figure 5). Weld metal shows slightly higher hardness in DA condition than STA condition. Hardness values in the weld metal, heat affected zone, the base material of as-welded condition show that precipitation of Ni3Al (γ') phase did not take place during weld metal cooling. PWHT resulted in increase hardness due to Ni3Al (γ ) precipitation.

5 Advanced Materials Research Vol Conclusions A peak hardness of 320 H V was obtained on aging at 750 C for 9 hours in the parent material. Columnar dendritic was observed in fusion zone of as-weld and direct aged (DA) condition whereas a dendrite free structure was noticed in STA condition. Weld tensile properties are lower than parent metal properties in all the conditions. An improvement in both yield and tensile strength of weld was observed after post weld heat treatment. Direct aged (DA) sample showed better strength than solution treatment + aging (STA) sample. Hardness and Tensile values follow similar trend in the weld at all conditions. 5. References [1] Matthew J. Donachie, Stephen J. Donachie, Superalloys, A Technical guide, Second edition, ASM International, Ohio, March [2] John N. Dupont, John C. Lippold, Samuel D.Kiser, Welding Metallurgy and Weldability of Nickel-base Alloys, John Wiley & Sons Inc, New Jersey, [3] G.D.Janaki Ram, Effect of laves phase on mechanical properties of Inconel 718 welds. Ph.D Thesis, IIT Madras, November [4] Haynes 214, High temperature alloys, Information brochure, Haynes International Inc, [5] Q.Wang, D.L.Sun, Y.Na, Y.Zhou, X.L.Han, J.Wang, Effects of TIG Welding Parameters on Morphology and Mechanical Properties of Welded Joint of Ni-base Superalloy. Procedia Engineering, 2011 (10), pp

6 Advances in Materials and Processing / Effect of Post Weld Heat Treatment on Mechanical Properties and Microstructure of Nickel Based Super Alloy Welds /