The contribution of machine-induced residual stress to the pitting susceptibility of an Iron-1 Wt Pct nickel binary allo
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a t i c and plastic deformation heating resulting in either phase transformations a n d / o r additional temp e r i n g heat treatment. This may contribute t o changing the surface mechanical, chemical and electrochemical properties. b) Introducing s u r f a c e irregularities such as flow laps and machining grooves which may act as localized s i t e s for c r e v i c e corrosion. These may develop t o give s e v e r e s t r e s s concentrations coupled with localized changes in environment composition such that s t r e s s corrosion c r a c k s may be initiated. 3's1-~ c) Generating high residual macrostresses and microstresses in the near surface regions resulting from inhomogeneous plastic working and associated t h e r m a l transients generated by processes described in a). a7-42 A machined s u r f a c e may influence the corrosion behavior by all of the above processes; however, any systematic evaluation of the contribution from each is difficult. The present work investigates the effect of residual macrostress, introduced by machining, on the localized corrosion behavior. Simultaneous contributions a r i s i n g from the surface topography are m i n i m i z e d by testing at elevated temperature in high concentration solutions where b o t h mass and ion transport are sufficiently r a p i d to limit the formation of occluded environmental c e l l s associated with surface i r r e g ularities. Furthermore, in o r d e r t o minimize m i c r o structural phase changes introduced by surface plastic flow and l o c a l heating a stable, single phase binary alloy was used in which metastable, a t h e r m a l phase transformations and precipitate coarsening processes cannot occur. T o this end the corrosion pitting behavior of a variously machined iron-1 wt pct nickel binary alloy in boiling 5 M sodium hydroxide solution has been investigated and correlated with the m e a s ured r e s i d u a l macrostress distribution introduced by machining and subsequent s t r e s s relieving heat t r e a t ment. Residual macrostresses and microstresses were m e a s u r e d u s i n g the X - r a y diffraction technique 3 7 , 4 3
2. EXPERIMENTAL P R O C E D U R E 2.1. Materials A high p u r i t y binary iron-1 wt pct nickel alloy was prepared by vacuum induction melting high p u r i t y base constituents followed by remelting in an electron beam furnace u n d e r a vacuum of < 1.3 mPa. Chemical analy-
ISSN 0360-2133/79/0411-0413500.75 l 0 © 1979 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 10A, APRIL 1 9 7 9 - 4 1 3
s i s g a v e i r o n 1.00 wt p c t n i c k e l c o n t a i n i n g l e s s t h a n 40 p p m c a r b o n a n d 100 p p m o x y g e n . T h e i n g o t w a s c o l d r o l l e d t o --6 m m t h i c k n e s s f r o m w h i c h s a m p l e s 2 x 2 x 0 . 3 c m a n d 50 x 5 x 3 m m w e r e m a c h i n e d f o r subsequent X - r a y s t r e s s measurement a n d corrosion t e s t i n g respectively. All s a m p l e s were v a c u u m ann e a l e d (< 1.3 m P a ) a t 923 K f o r 104s a n d f u r
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