a~ 2;@ second international congress on hydrogen in … · a"~ 2;@ second international...

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INTERNATIONAL a"~ 2 ; @ SECOND INTERNATIONAL CONGRESS \ ON HYDROGEN IN METALS pAAi~ - FRANCE THE INFLUENCE OF MECHANICAL SURFACE TREATHENT TO HYDROGEN EMBRITTLEMENT OF A HIGH-STRENGTH STEEL. L'IWFLUENCE DE L'ETAT 3E SURFACE USINE SIJR LA FRAGILISATION PAR L'HYDROGESE D ' UN ACIER MARTESSITIQUE PENDANT LE TRAITEMENT ELECTIIOLYTZQUE. SCE-PIITT-THOX4S KH . G. - WOLLRAB P .41 HOFFMEZSTER E. INN, TECHNISCHZ UNIVERSITXT LIEBHERR-AEROTECHN IK PIIJNICH LINDENBERG ~~SuME2'rois surfaces differentes peuvent 6tre produites mecaniquement par meulage, sablage et grenaillage et ont &t& charg&s electrolytiquement en hydrog6ne. 'es propri&t&s topo- graphiques et les tensions residuel1.e~aux surfaces ont 5th &tudi&s i l'aide du microscope 5 balayage et de diffraction des Rayons-X. WMARy~hree different surfaces are produced mechanically by grinding, sandblasting and shot- peening and are electrolytically charged with hydrogen. These surfaces are investi- gated by SEX for topographic properties. The state of residual stress at the surfaces is studied by means of X-Ray-Diffraction. The invested material is a low-alloy steel. A siutable heat-treatment can bring it up to a tensile strength of 190 hbar. After annealing the steel it is necessary to remove the oxide. Especially the producers of aircraft-parts -e.g, the landing gear- use mechanical methods, before galvanizing those parts to protect them against corrosion and wear. The application of chemical methods, as for example pick- ling or etching is not possible for materials with the require for highest properties. Resides removing the oxid by mechanical surface-treatment there is great importance in inducing residual stress and in producing certain roughnesses. In our experiments we produced mechanically three different surfaces as they are in utilization for landing gears of aircrafts. These surfaces have been investi- gated by SEX for topographic properties. The state of residual stress has been studied by means of X-ray-diffraction. Electrolytically charging with hydrogen under defined conditions has been chosen to simulate the hydrogen absorption during electrolytical plating with Cadmium and Chromium. Tensile testing after the charging with hydrogen has shown, that the charging-time and the mechanical behaviour of the specimens can be assigned to each other. By means of SEN we could find out a relationship between the extent of hydrogen -embrittled microstructure and the facts mentioned before. 11.1. The high-strength steel with the french indication 35 NCD 16 has the percentage: 4.7 Ni 1.9 Cr 0.45 Mo 0.35 C 0.27 Si 0.20 Mn After the austempering at 875'~ and after the annealing at 200°C the tensile strength at 190 hbar and the yield strength at 155 hbar is attainable. Of course, before heat- treatment the specimens got their shape. We used tensile specimens, ses FIG. 6.

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INTERNATIONAL

a " ~ 2;@ SECOND INTERNATIONAL CONGRESS \ ON HYDROGEN IN METALS

p A A i ~ - FRANCE

THE INFLUENCE OF MECHANICAL SURFACE TREATHENT TO HYDROGEN EMBRITTLEMENT OF A HIGH-STRENGTH STEEL. L'IWFLUENCE DE L'ETAT 3E SURFACE USINE SIJR LA FRAGILISATION PAR L'HYDROGESE D ' UN ACIER MARTESSITIQUE PENDANT LE TRAITEMENT ELECTIIOLYTZQUE.

SCE-PIITT-THOX4S KH . G. - WOLLRAB P .41 HOFFMEZSTER E.

INN, TECHNISCHZ UNIVERSITXT LIEBHERR-AEROTECHN IK

PIIJNICH LINDENBERG

~~SuME2'rois surfaces differentes peuvent 6tre produites mecaniquement par meulage, sablage et grenaillage et ont &t& charg&s electrolytiquement en hydrog6ne. 'es propri&t&s topo- graphiques et les tensions residuel1.e~ aux surfaces ont 5th &tudi&s i l'aide du microscope 5 balayage et de diffraction des Rayons-X.

W M A R y ~ h r e e different surfaces are produced mechanically by grinding, sandblasting and shot- peening and are electrolytically charged with hydrogen. These surfaces are investi- gated by SEX for topographic properties. The state of residual stress at the surfaces is studied by means of X-Ray-Diffraction.

The invested material is a low-alloy steel. A siutable heat-treatment can bring it up to a tensile strength of 190 hbar. After annealing the steel it is necessary to remove the oxide. Especially the producers of aircraft-parts -e.g, the landing gear- use mechanical methods, before galvanizing those parts to protect them against corrosion and wear. The application of chemical methods, as for example pick- ling or etching is not possible for materials with the require for highest properties. Resides removing the oxid by mechanical surface-treatment there is great importance in inducing residual stress and in producing certain roughnesses.

In our experiments we produced mechanically three different surfaces as they are in utilization for landing gears of aircrafts. These surfaces have been investi- gated by SEX for topographic properties. The state of residual stress has been studied by means of X-ray-diffraction.

Electrolytically charging with hydrogen under defined conditions has been chosen to simulate the hydrogen absorption during electrolytical plating with Cadmium and Chromium. Tensile testing after the charging with hydrogen has shown, that the charging-time and the mechanical behaviour of the specimens can be assigned to each other. By means of SEN we could find out a relationship between the extent of hydrogen -embrittled microstructure and the facts mentioned before.

11.1. The high-strength steel with the french indication 35 NCD 16 has the percentage: 4.7 Ni

1.9 Cr 0.45 Mo 0.35 C 0.27 Si 0.20 Mn

After the austempering at 875'~ and after the annealing at 200°C the tensile strength at 190 hbar and the yield strength at 155 hbar is attainable. Of course, before heat- treatment the specimens got their shape. We used tensile specimens, ses FIG. 6 .

X E C k U I C h L SURFACE TREATXEST

11.2. To produce mechanically three different surfaces we used the dures:

Grinding d i s k 711 J O T Diameter of disk 400 mn Speed of grinding disk LAO0 R/mn Speed of specimen 2>0 R/mn

SLIND BLASTING

Sul>stance !<orindon that is c:,mposed of osids of the elements Ti, t\l, Si, Zr Par-ticLe-size 10 to 30 um (90 to 150 mash) Air-pressure 4 to h.5 bar B 1 as t ing- t inie 2 0 c:

SHOT PEENING

TI . ? . SEX-Inves tigation for topographic properties See FI! : . 1 . 2 and 3 .

Na3nif icat ion 200

i l o w i n g p roce -

YECHANiChL SURFACE TRE,lT>IENT

X a g n i f i c a t i o n 200

MECHANICAL SURFACE TREATMENT

1 1 . 3 . R e s i d u a l S t r e s s ?feasurements

The d e t e r m i n a t i o n of t h e s t a t e of r e s i d u a l s t r e s s a t t h e ground, b l a s t e d and sho t -peened s u r f a c e s was performed by means of t h e s1N2-Y-blet T h i s method i s b a s e d on a r a g g ' s Equa t ion n.A= 2 d s i n Q . The d e v i c e i s a S a - G o n i o r n e t e r b]. The o r i g i n of t h i s method i s t h e a s s u m p t i o n a l a y e r c l c s u r f a c e b e i n g inhomogeniously p l a s t i c a l l y deformed by m e c h a n i c a l t r e a t m e n t r e g i o n s of t n e m i c r o s t r u c t u r e a r e more o r l e s s i n t e n s i v l y deformed w i t h th quence of r e q u i r e d e l a s t i c s t r a i n t h a t i s measurab le by X - r a y - d i f f r a c t i o n . t h e e x i s t e n c e of e l a s t i c s t r a i n t h e coherence of t h e m i c r o s t r u c t u r e would p o s s i b l e .

Ve o n l y r e c o r d e d macroscopic r e s i d u a l s t r e s s (Eigenspannungen 1. we know t h a t r e c o r d i n g microscopic r e s i d u a l s t r e s s (E igenspannungen 2 . A r t [ 3 , 4 ] w i l l b e n e c e s s a r y i n t h e f u t u r e , t o have more profund view of p l a s t i mat ion w i t h i n a r e a s l i k e g r a i n s .

The ~ ~ ~ ~ - y - ? l e t h o d c o n s i d e r a t e s t h e s h i f t of t h e B r a g g - a n g l e & & p l a s t i c d e f o r m a t i o n i n c o r r e l a t i o n t o a n g l e s v . y is t h e a n g l e between t h t beam and t h e s u r f a c e t o b e i n v e s t i g a t e d , s e e FIG. 4 .

FIG. 4 . : Compi l ing r e s u l t s by means of t h e SIN2Y-Method

The t e s t v a l u e s a t 3 p - a n g l e s of o", 2 5 " , 35" , 42" and f o r l a t e r t i o n s a t a d d i t i o n a l y - a n g l e s of 12 .5 ' , 3 0 " , 38 .5" s h o u l d be s i t u a t e d unde d i t i o n s o n t o a s t r a i g h t l i n e , The g r a d i e n t of t h i s l i n e p e r m i t s t o c a l c u l v a l u e and t h e p l u s o r minus s i g n of t h e r e s i d u a l s t r e s s .

The p o s i t i o n of t h e peak-masinium was d e t e r m i n e d by: p o i n t s of i n t e r s e c t i o n of t h e t a n g e n t l i n e s c a l c u l a t i n g t h e c e n t e r of t h e e n v e l o p e c u r v e s .

MECHANICAL SURFACE TREATXENT

11.4. Electrolytically charging with hydrogen

;41J nens to t h e Some conse- ithou: t be

t), but d 3.hr:: defor-

Tensile specimens (see FIG.6) have been charged with hydrogen in In-H SO at 2 4 a temperature of 55°C. This temperature is conform to the medium temperature requl-

red for certain Chromium-plating processes. The current density of 3 n'iJmm2 was gene- rated by a direct-current constanter. A net of Platinum of cylindric shape and of a diameter of 25 m was the anode (see FLG. 7). Times of 6s,30s, 60s,90s and 120s have been chosen for charging.

FIG. 6.: Tensile specimen DIN 50 125, do = 6mm, Lo = 30mm, dl = 8 mm

FIG. 7.: Charging-device 1 glass-container, 2 stirrer, 3 heater, 4 thermometer 5 glass-electrode, 6 anode , 7 specimen, 8 reference-electrode Immediately after charging the specimens have been cleaned to begin

tensile testing as soon as possible. The time between charging with hydrogen tensile testing was about 3 mn.

the and

11.5. Tensile Testing

Tensile testing has been executed at a deformation-speed of 0.35 mm/mn at room-temperature. A relatively low speed like this one is required for visualizing embrittled microstructure H. We could confirm this fact in our own experiments. The mechanical properties were recorded by the stress-strain diagram.

Most of the specimens have been loaded as high as their ultimate strength. All the planes of failure have then been prepared for SEN. Besides gener'il views to mahe obvious the macro-fracture appearance we tried to find some typical feature by application of higher magnification.

PIECHANICAL SURFACE TREATMENT

111. RESULTS H e a s u r i n g t h e s t a t e o f r e s i d u a l s l r e s s r e n d e r s t h e f o l l o w i n g v a l u e s :

Ground s u r f a c e 1 6 . 0 Rbar ( r e s i d u a l t e n s i l e a n d c o m p r e s s i v e s t ~ x s s )

S a n d - b l a s t i n g s u r f a c e -17 .4 h b a r ( r e s i d u a l c o m p r e s s i v e s t r e s s )

S h o t - p e e n i n g s u r f a c e -35 .5 h b a r ( r e s i d u a l c o m p r e s s i v e s t r e s s )

To e x p l a i n t h e r e s u l t s o f t e n s i l e t e s t i n g we u s e F I G . 8 and F I G . 9

*SAND-BLASTING ASHOT-PEENING GRINDING

W/THOU T SURFACE - TREA THEN T F I 6 . 8 . : T e n s i l e s t r e n g t h v s . c h a r g i n g - t i m e

The r e l a t i o n s h i p o f t h e t e n s i l e s t r e n g t h t o t h e p e r i o d o f t h e cha rg ing- t ime i s o b v i o u s . Xore r e m a r k a b l e i n o u r o p i n i o n i s t h e l i n e a r i t y o f r e l a t i o n t o b e con- c l u s i v e f o r t h e g round and s h o t - p e e n e d s u r f a c e s . C l e a r l y t o b e s e e n i s t h e f a v o r a b l e i n f l u e n c e of t h e s a n d - b l a s t i n g p r o c e s s t o a v o i d h y d r o g e n e m b r i t t l e m e n t . I s s u e i n g fro" a p e r c e n t a g e o f t h e d e c r e a s e o f t e n s i l e s t r e n g t h we c a n s a y t h a t t h e r e i s a v a r i a t i o n

of a b o u t 20% d u r i n g t h e f i r s t 6 0 s d e p e n d i n g upon t h e s u r f a c e s . F o r l o n g e r of c h a r g i n g w i t h h y d r o g e n t h e r e i s a r a n g e o f a b o u t 15% t o 35X.

MECHANICAL SURFACE TRFATMENT

-' + SAND- BL ASTING A SHOT- PEENING GRINDING '- WITHOUT SURFACE-TRTA TMENT

FIG.9.: LOG of elongation vs. charging-time

Plotting LOG 6 (%) versus charging-time, we found out a rather close linearity for the ground: sand-blast and shot-peened surfaces. Only specimens with- out any defined surface-treatment cannot be approximated to linearity.

Onto the fracture planes of the tensile specimens a general view gives characteristic features of the macro-fracture of the investigated high-strength steel (FIG. 10, 11).

The typical 'cap and cone'-fracture (specimens without hydrogen) and the typical brittle fracture are examples of the interaction of hydrogen in steel. The image of the brittle fracture appears almost without any deformation and without any necking and the plane of fracture is orientated vertically to the direction of tensile stress. The embrittled microstructure is conspicous located into an area that is limited by two concentric circles. The depth of penetration of embrittled microstruc- ture is about 700 pm.

FIG. 12,13,14 show fracture planes of specimens with ground surfaces. One has been charged with hydrogen for a period of 30 s, one for 60 s, and another for 120 s.

b e e n charged ' i t loll 20

XTCHX?!IChL SURF.ICE TREATXENT

: ' . 1 2 . : Fractiire i w g e of a ground tensile spe- FIG. 13. : Fracture image of a ground tensi- cin~en, hydrogen-enibrit t Led micros truc- l e specimen, hydrogtn-enhritiled ture in tile external zone, charging- microstructure in the external tinc 31k , by ST-!!, magnification 150 zone, charging-time 6Os, by SEN,

magnification 150

XECHANICAL SURFACE TREATYEST

C l l ~ r g i n g - t i m e 30 s : d e p t h of p e n e t r a t i o n a b o u t 2&0 pm Clhcirging-time 60 s: d e p t h of p e n e t r a t i o n a b o u t 380 ,urn C h a r g i n g - t i m e 120 s : d e p t h of p e n e t r a t i o n a b o u t 480 pm

F o r t h e s e and a l l t h e o t h e r i n v e s t i g a t e d s p e c i m e n s t h e r e was n o d i f f i c u l t y i n d i s t i n g ~ i i s h i n g i n t e r c r y s t a l l i n e f r o n t r a n s c r i s t a l l i n e a p p e a r e n c e of f r a c t u r e .

Our p r o c e d u r e s o f s u r f a c e - t r e a t m e n t c o r r e s p o n d t o t h o s e of p r o c e d u r e s o f h i g h - s t r e n g t h c o n s t r u c t i o n u n i t s f o r a i r p l a n e s . B e s i d e s t h e s t r u c t u r a l p r o p e r t i e s we payed a t t e n t i o n t o t h e s t a t e of r e s i d u a ! s t r e s s . A compac t s u r f a c e i s a s w e l l o f i n t e r e s t a s a s u r f a c e f u l l o f f i s s u r e s r e g a r d i n g t h e amount o f h y d r o g e n a b s o r p t i o n . The a d v a n t a g e of n s h o t - p e e n i n g s u r f a c e i s on t h e one hand s i d e t h e low n o t c h e f f e c t i n r e l a t i o n t o t h e g round o r s a n d - b l a s t i n g s u r f a c e . On t h e o t h e r hand s i d e t h e f a v o u r a b l e i n f l u e n c e of t h e n e e d l e e f f e c t e s p e c i a l l y o n t o t h e s a n d - b l a s t i n g s u r f a c e h , ~ s t o b e r s g a r d e d . The n e e d l e e f i e c : ? r a m s t z s h y d r o g e n r e c o n b i n a t i o n b e f o r e a tomic hyrii-ogen b e i n g a b l e t o m i g r a t e i n t o t h e r , e t a l l a t t i c e [6,7].

.., l!ie g r i n d i n g - t r e a t m e n t was p ~ r p o s e l y m a n u f a c t u r e d a t two n e a r l y v e r t i c a l d i r e c t i o n s t o p r e v e n t as f a r a s p o s s L 3 l e a- i n f l u e n c e of t e x t u r e i r8]. The g , - inding- p r o c e d i i r e n e e d s s p e c i a l c a r e o f t e m p e r a t u r e e f f e c t s . G r i n d i n g w i t h o u t l o c a l i n c r e a s e of t s m p e r a t u r e seems t o b e i m p o s s i b l e e v e n u n d e r a t t e n t i v e c o n d i t i o n s . The t empera - t u r e - i n d u c e d d e g r a d a t i o n of r e s i d u a l s t r e s s makes t h e s c a t t e r i n g of r e s u l t s n e a r b y Z e r o e s p l i c a b i e . The p o s s i b i l i t y o f h;:drogen a b s o r p t i o n e v e n by g r i n d i n g h a s n o t been t a k e n i n t c a c c o u n t . The i n f l u e n c e of ti12 c m ~ l i n g a g e n t [lg m i g h t b e t h e s u b j e c t mat- t e r of f u r t h e r i n v e s t i g a t i o n s .

The s u r f a c e s o n t o wtiici! t h e h i g ? , e s t v a l u e s o f r e s i d u a l c o m p r e s s i v e s t r e s s i s m e a s u r n b l e a r e o w i n s t o s h o t - p e e n i n g . Si - io i -peening c a u s e s c o l d w o r k - h a r d e n i n g wi . th in t i ic p l a s t i c r a n g e of t h e t e n s i l e s t r z n g - i h . T h i s v e r i f i c a t i o n c a n b e t a k e n f o r g r a n - t e d F o ] . A p r o f i l e of r e s i d u a l s t r s s s is r e c o r d a b l e by e t c h i n g i n s t e p s of a b o u t 10 urn s e e EL(:. 1 5 . The i n t e r a c t i o n d i s l i . c a t i o n s - a t o m i c h y d r o g e n i n s t e e l [14 ,15 ,1h , 1 1 , 1 8 7 s i i a l l n o t be d i s c u s s e d , thou; i a n i n c r e a s e o f t h e d e n s i t y of d i s l o c a t i o n s i s t o bed p r e m i s e d . A n o t h e r p o i n t o f vie!,; i s t h e q u e s t i o n . w h e t h e r t h e r e i s any i n f l u e n c e of t!ie p a r a m e t e r s o f s h o t - p e e n i n g t o t h e i n t e n s i t y of r e s i d u a l s t r e s s o r w h e t h e r the re h a p p e n s p r i m a r i l y s h i f t i n g o f t h e p o s i t i o p of maximum r e s i d u a l s t r e s s r e l a t i v e t o the c l l s t n n c e f r o m t h e s u r f a c e . T h i s o p i n i o n f i l l ~ n c l u d s s n e a r l y c o n s t a n t v a l u e s o f r e - s i d u , i l s t r e s s and was made f i r m by o u r oc 1 e x p e r i m e n t s .

7

B e f o r e s t a r t i n g o u r i n v e s t i g a ~ i o n s we e x p e c t e d s h o t - p e e n i n g t o b e t h e most e f f i c i e n t way t o r e d u c e h y d r o g e n - e m b r i t t l e a e n t d u r i n g e l e c t r o l y t i c a l p l a t i n g . So we w e r e s u r p r i s e d f i n d i n g o u t s a n d - b l a s t i n g t h e f a v o u r a b l e s u r f a c e - t r e a t m e n t of h i g h - s t r e n g t h l o w - a l l o y s t e e l . N e v e r t h e l e s s o u r r e s u l - t s a r e i n good c o n f o r m i t y w i t h some o t h e r a u t h o r s [?0,?1]. R e g a r d i n g t h e f i r s t 60 s o f e l e c t r o l y t i c a l p l a t i n g we s h i l l b e a b l e t o e s t , i b l ~ s ! - i a compac t l a y e r of Ci-irs:nium o r Cadmium o n t o t h e s u r f a c e t o a c t a g a i n s t h y d r o g e r ~ d i f f u s i o n d u r i n g t h e whole p e r i o d o f p l a t i n g . On t h i s f i e l d of r e - s e a r c h t h e r e e s i s t some i n v e s t i g a t i o n ~ n c t a l l l e a d i n g t o t h e same recommenda t ion

The c l a s s i f i c a t i o n o f f r a c t u r e a r e a s i s a v e r y s i g n i f i c a n t f a c i l i t y f o r f a i l u r e an2 L y s i s [19] . : \ l s o f o r b a s i c r z s e a r c h m i c r o f r a c t o g r a p h y ha become g r e a t e r i m p o r t a n c e . I n o u r i n v e s t i g a t i o n s we were a b l e t o f i n d o i i t a r e l a t i o n be tween t h e c h a r - g i n g - t i m e and t h e d e p t h o f p e n e t r a t i a n of t h e e r n b r i t t l . e d m i c r o s t r u c t u r e .

.LtECHAY ICAL SURFACE TREATMENT

FIG. 15.: Residual stress vs. distance from the surface, Plus: tensile stress Xinus: cnEpressivs stress

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Z. Xetallkunde, (1964), 626. r3] Wolfstieg C . - Xacherauch "c ., Lrsachen und Bewertung von Eigenspannungen, Chemie-

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genspannungen, HT?! 28, (l973), 201 - 211. [5! Bowker P. - Hardie D., L1~ydrog$ne dans 1es X6.taus 11, Paris, (1972), 284-289. [61 Kim D.H. - Clark D.H., Corrosi.on, NACE 30, (1974), 7 177 Altona P.L.- Casarini G. - Demrnini W., LthydrogGne dans les k1Etaux 11, Paris,

(l972), 520-526 [sf Faninger G. - \Galburger H., Anonalien be hen Ermittlung von

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