The Welding Metallurgy of Custom Age 625 PLUS Alloy The solidification behavior and weld properties of a new nickel-based alloy are compared to Alloys 625 and 718

BY M. J. CIESLAK, T. J. HEADLEY AND R. B. FRANK

ABSTRACT. The welding metallurgy (solidification behavior, mechanical properties of weld metal, and susceptibility to fusion zone hot cracking) of Carpenter Custom Age 625 PLUS®1 alloy has been determined. This alloy solidifies to a nickel austenitic matrix with the formation of a Nb-rich 7/Laves terminal solidification constituent. The low Si and C concentrations in combination with a lower Nb content (3.4 wt-%) in this alloy result in a relatively small volume fraction of eutectic constituent relative to that previously observed in Alloy 718. Because of the low volume fraction of eutectic constituent, this alloy has better weldability than Alloy 718. In addition to Laves phase, 7" was observed adjacent to Laves in the fusion zone microstructure of the new alloy. Weld metal strengths approximately equal to aged wrought product can be achieved by direct aging of welds.

(0.01 wt-% versus 0.10 max wt-%), and minor differences in chromium, molybdenum and iron concentrations. The alloy was critically balanced to provide excellent resistance to stress corrosion cracking, sulfide stress cracking, pitting and crevice corrosion at yield strength levels of 120 ksi and above (Refs. 2, 3). The purpose of this study was to determine the welding metallurgy of Alloy 625 PLUS, including the susceptibility to fusion zone hot cracking during autogenous welding.

Experimental and Analytical The chemical compositions of the alloys used in this study are given in Table 1. For the purpose of comparing the susceptibility to fusion zone hot cracking of commercial alloys, heats of Pyromet® 2 Alloys 625 and 718 were also examined and are listed in Table 1. All alloys were tested for weldability in the solution annealed condition (1900°F/1 h, air cool). The new alloy was also examined in the aged condition. The aging treatment subsequent to solution annealing is as follows: 8 h at 1350°F (732°C) followed by a furnace cool at 100°F/h (56°C/h) to 1150°F (621 °C) and an 8-h isothermal hold at 1150°F. The alloy is then air cooled to room temperature. This heat treatment

Introduction Custom Age 625 PLUS (UNS N07716) is a new highly corrosion-resistant nickelbased alloy that can be age hardened (7") to yield strengths of 120 ksi (827 MPa) and above without warm or cold working (Ref. 1). The corrosion resistance of the new alloy is similar to that of Alloy 625 and superior to that of Alloy 718 in many environments. Alloy 718 is also age hardenable to high strength levels while Alloy 625 must be deformation strengthened to obtain high strength levels because of its sluggish aging response. Compared to nominal Alloy 625, the new alloy has a higher titanium content (1.3 wt-% versus 0.4 max wt-%), a lower carbon content

2. Pyromet is a registered trademark of Carpenter Technology Corp.

1. Custom Age 625 PLUS is a registered trademark of Carpenter Technology Corp.

M. j. CIESLAK and T. j. HEADLEY are with the Materials and Process Sciences Directorate, Sandia National Laboratories, Albuquerque, N. Mex. R. B. FRANK is with Carpenter Technology Corp., Reading, Pa.

KEY W O R D S Alloy 625 PLUS 21Cr8MoNi-bal Alloy Precipitation Hardening Welding Metallurgy Fusion Zone Hot Cracking Stress Corrosion Differential Analysis Thermal Analysis DTA Analysis

produced a tensile yield strength of 131 ksi (903 MPa), an ultimate tensile strength of 188 ksi (1296 MPa) and an elongation of 36% in the strip samples used for weldability testing. Typical mechanical properties for solution annealed material are 55 ksi (379 MPa) tensile yield strength, 120 ksi ultimate tensile strength and an elongation of 60%. Quantification of critical solidification events (liquidus, solidus and eutectic reaction temperatures) was accomplished by means of differential thermal analysis (DTA). A Netsch thermal analyzer STA 429 was used in this investigation. Samples weighed approximately 0.8 grams and were tested in high-purity alumina crucibles. Heating and cooling rates of 2 0 ° C / min (36°F/min) were employed. Tungsten was the reference material and all tests were run in a flowing helium environment. Reaction temperatures were taken as deviations from the local baseline in a manner consistent with that suggested by Flemings and coworkers (Ref. 4). Conventional metallographic techniques were used to reveal the microstructure in the fusion zone and to prepare samples for scanning electron microscopy (SEM). Elemental segregation resulting from dendritic solidification was quantified with a Cameca MBX microprobe. Samples were examined at an accelerating potential of 15 kV and a beam current of ~ 2 0 nA. K a x-ray lines were used for the analysis of all elements of interest except for Mo and Nb, where the La lines were used. Raw counting data were reduced to weight percentages with a (p,Z) correction algorithm (Ref. 5). Analytical electron microscopy (AEM) was performed on a JEOL 2000FX microscope equipped with a Tracor Northern EDS detector/spectrometer. Selectedarea electron diffraction was used to identify the crystal structures of the phases observed. Tensile mechanical properties were obtained for as-welded, welded and direct double aged (age at 1350°F/8 h, furnace coolat100°F/hto1150°F,ageat1150°F/

WELDING RESEARCH SUPPLEMENT 1473-s

CARPENTER CUSTOM AGE 625 PLUS

Fig. 1-DTA 625 PLUS.

thermogram (20°C/min)

of Alloy

Table 1—Alloy Compositions (wt-%)

Element

C Mn Si P

S Cr Ni Mo

Nb Ti Al B Fe

Alloy 718

Alloy 625

0.040 0.08 0.14 0.010

0.039 0.03 0.10 0.005 0.002 22.14 62.03

0.002 18.33 52.30

3.12 5.44 1.00 0.60 0.0042 18.33

8.79 3.86 0.26 0.18 0.0021 2.54

Alloy 625 PLUS

0.009 0.03 0.03 0.007 0.002 21.03 60.87 7.96 3.39 1.31

0.18 0.0035 5.18

8 h, air cool) a n d w e l d e d , solution a n nealed ( 1 9 0 0 ° F / 1 h) a n d d o u b l e - a g e d material. Samples w e r e p r e p a r e d f r o m w e l d e d (0.46-in./11.7-mm-thick, 60 deg V - g r o o v e , n o r o o t opening) plate o f t h e n e w alloy. T h e w e l d s w e r e m a d e w i t h the gas tungsten arc (CTA) process using w e l d i n g w i r e ( 0 . 0 4 5 - i n . / 1 . 2 - m m diameter) f r o m t h e heat o f A l l o y 625 PLUS listed in Table 1. T h e w e l d i n g parameters used w e r e 2 5 0 A, D C , 12.5 V arc v o l t a g e a n d a travel speed o f 7 i n . / m i n (178 m m / m i n ) . Shielding gas w a s 25 f t 3 / h (53 L/h) He + 15 f t 3 / h (32 L/h) A r w i t h a backshield o f 20 f t 3 / h (42 L/h) Ar. T h e w e l d w a s fabricated in 13 passes w i t h an interpass t e m p e r a t u r e o f 3 0 0 - 3 5 0 ° F ( 1 4 9 - 1 7 7 ° C ) . Each b e a d w a s cleaned w i t h a stainless steel w i r e brush prior t o the subsequent pass. All w e l d metal tensile samples (0.252 i n . / 6 . 4 m m g a u g e diameter) w e r e o b t a i n e d b y machining along the w e l d i n g d i r e c t i o n . Susceptibility t o fusion z o n e hot cracking w a s established b y using the V a r e straint test (Refs. 6 , 7 ) . Test samples 0.125in. (3.2-mm) thick b y 1.0 in. (25.4-mm) w i d e b y 6.5-in. (165-mm) l o n g w e r e m a chined f r o m 0.163-in. (4.1-mm) thick strip stock. A u t o g e n o u s C T A w e l d i n g p a r a m e ters used d u r i n g the test w e r e 100 A current ( e l e c t r o d e negative) at 12 V a n d 8 i n . / m i n (203 m m / m i n ) travel s p e e d . Arg o n w a s used as the shielding gas (20 f t 3 / h). These parameters p r o d u c e d w e l d s app r o x i m a t e l y 4.2 m m (0.17 in.) w i d e . Both m a x i m u m crack length (MCL) and total crack length (TCL) w e r e used as measures of susceptibility t o h o t cracking.

Results DTA The D T A t h e r m o g r a m Alloy 625 PLUS is s h o w n in Fig. 1 . The o n - h e a t i n g p o r t i o n of the t h e r m o g r a m s h o w s that the solidus is r e a c h e d at 1 2 5 7 ° C (2295°F) a n d the liquidus occurs at 1 3 5 6 ° C (2473 °F). O n c o o l -

Fig. 2 — SEM micrograph of hot cracked region in GTA weld metal of the new alloy.

474-s | DECEMBER 1989

ing f r o m 1 4 0 0 ° C (2552°F), 2 6 ° C (47°F) o f u n d e r c o o l i n g b e l o w the liquidus is e x p e r i e n c e d w i t h nucleation of t h e solid o c c u r ring at 1 3 3 0 ° C (2426°F). Solidification c o n cludes w i t h a terminal constituent f o r m i n g at 1 1 2 6 ° C ( 2 0 5 9 ° F ) . Microstructural Analysis In this p a p e r , only the microstructures of the n e w alloy will b e r e p o r t e d . A n analysis o f t h e microstructures i n v o l v e d w i t h the solidification of arc w e l d s a n d t h e f o r m a t i o n o f fusion z o n e h o t cracks in b o t h Alloys 7 1 8 a n d 625 has b e e n p r e sented e l s e w h e r e (Refs. 8 - 1 1 ) . Figure 2 is an SEM image of the fusion z o n e of the n e w alloy in t h e vicinity of an i n d u c e d h o t crack. In a d d i t i o n t o t h e darker-appearing nickel austenitic matrix, a b r i g h t e r - a p p e a r i n g m i n o r constituent is o b s e r v e d b o t h along the edges o f the h o t crack a n d in isolated interdendritic p o c k ets scattered t h r o u g h o u t the field of v i e w . Figure 3 s h o w s this m i c r o c o n s t i t u e n t in finer detail a n d a coarse lamellar m o r p h o l o g y is o b s e r v e d . T o m o r e fully characterize this constitu e n t , an e l e c t r o n m i c r o p r o b e trace w a s taken across this constituent. T h e e x t e n t of this profile w a s such that several d e n drites w e r e t r a v e r s e d d u r i n g the analysis p r o v i d i n g a d e t e r m i n a t i o n of the segregation p a t t e r n of the various alloying elements in this alloy d u r i n g G T A w e l d solidification. Figure 4(A-E) includes a series o f x-ray maps taken in a r e g i o n near a h o t crack that contains a locally high v o l u m e fraction of t h e m i n o r constituent. N b , M o , and Ti are f o u n d t o segregate t o t h e region w h e r e t h e m i n o r constituent a p pears. T h e r e is n o e v i d e n c e of C partitioning t o t h e m i n o r constituent. Figure 5 quantifies the segregation p a t t e r n of the various alloying elements. Local minima in N b , M o a n d Ti c o n c e n t r a t i o n s are t h e p o sitions of t h e d e n d r i t e centers a n d local maxima f o r these elements are i n t e r d e n -

Fig. 3 — Higher magnification SEM micrograph of constituent with hot cracks in GTA weld metal.

associated

dritic regions. An examination of the fusion zone microstructure in greater detail employed analytical electron microscopy as described above. Figure 6 is a representative thin foil micrograph showing distribution of the minor constituent in the fusion zone. Selected-area electron diffraction experiments indicated that the crystal structure of this phase was consistent with a hexagonal Laves phase. Figure 7A shows a thin foil EDS spectrum obtained from the Laves phase. For comparison purposes, a

during the thin foil analysis of weld metal.

similar spectrum is shown (Fig. 7B) for a random point in the weld matrix far removed from the Laves. With both spectra scaled to an approximately constant Ni intensity, it is obvious Mo, and especially Nb, partition very strongly to Laves phase. Closer examination of the matrix microstructure adjacent to the Laves phase revealed a dense population of a fine precipitate phase, as shown in Fig. 8. Electron diffraction patterns from this region revealed the identity of the precipitates as 7". No other constituents were observed

Mechanical Properties Table 2 lists the mechanical properties obtained during room temperature tensile testing of the weld metal from the new alloy. Both the double aging and solution anneal plus double-aging heat treatments result in the development of strengths similar to those observed in double aged wrought product.

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