The Effect of Microstructure on the SCC Behavior of Alloy 718 - TMS

THE EFFECT OF MICROSTRUCTURE ON THE SCC BEHAVIOR OF ALLOY

I.L.W.

Wilson

and

M.G.

Westinghouse Electric Pittsburgh, PA.

718

Burke

Corporation 15235

Abstract The

susceptibility

has been In order

evaluated

the

the

a detailed using

presence

treated

in simulated

to determine

susceptibility, performed

of thermally

of Laves

pressurized

microstructural characterization

analytical

Alloy

electron

phase

in the

718

to

and

boiling

feature(s) of the

microscopy. microstructure

stress

corrosion

water

which

thermally

and

the

718, 625 and Various Derivatives Edited by Edward A. Im-ia The Minerals, Metals & Materials Society, 1991

631

was SCC

(SCC)

environments.

associated

treated

alloy.

Superalloys

reactor

are

A correlation

cracking

materials observed performance

with has

SCC been

between of the

INTRODUCTION Alloy 718 is a high strength, precipitation-hardened Ni-based alloy which has been The heat treatments which used for certain critical applications in nuclear reactors. had previously been used for Alloy 718 were based upon those which had been developed for aerospace applications rather than light water reactor service. In reactor service applications, Alloy 718 has provided good service, and incidents have laboratory data, however, not seriously affected reactor operation [I]. Available indicate that Alloy 718 is resistant to stress corrosion crack (SCC) initiation, but that relatively small defects could lead to rapid crack growth 12-41. No major variation in cracking behavior with heat treatment has yet been observed [3-51, but Corrosion fatigue data, however, no systematic study has been performed to date. have suggested that significant improvements in performance could be gained by employing heat treatments that avoid the precipitation of Laves phase at the grain boundaries [6]. Moreover, recent combined AEM/APFIM studies have clarified the detailed chemistries of the component phases, thereby facilitating the development of optimized microstructures via controlled thermal treatments [7,8]. This study, funded by the Electric Power Research Institute, was conducted to determine if increased resistance to crack growth could be obtained in Alloy 718 while maintaining its resistance to crack initiation through the use of various thermal treatments.

EXPERIMENTAL The composition of the Alloy 718 used for this investigation is listed in Table I. The solution heat treatment of 1038’C for 1 h was selected based upon optical microscopy examination of the material which showed that this temperature was sufficient to dissolve most second phase particles without promoting excessive grain Aging treatments, subsequent to the solution heat treatment, were growth. systematically selected to produce a range of 7”, 7’. 6. and Laves phase structures within the 7 matrix. The major variation in the heat treatments involved an intermediate aging step at either 982’C or 87l’C. The individual aging treatments are detailed in Table II. The measured hardness values of the heat-treated samples are also included in Table II. TABLE I OF ALLOY

COMPOSITION

Ni

Cr

Fe

Ti

Al

Nb

50.3

19.4

19.6

0.95

0.43

718

(wt.%)

Ta

MO

Mn

5.20

0.11

3.00

0.16

Mg

Zr

S

B

Cu

C

Co

Zn

v

0.001

0.005

0.05

0.054

0.26

0.002