Laser melting of plasma-nitrided steel samples

Surface and Coatings Technology, 45 (1991) 399 402

399

Laser melting of plasma-nitrided steel samples M. B. K a r a m i s and B. S. Yilbas Erciyes University, Engineering Faculty, 38090 Kayseri (Turkey)

Abstract Laser surface melting has been widely used as a rapid screening technique for the evaluation of potential new rapidly solidified alloy compositions. The present study is an examination of the metallurgical and mechanical properties of nitrided EN-40-B steel surfaces subjected to Nd YAG laser melting. It was found that mixing of the surface layer with the underlying material occurs at a depth of 0.25 mm below the surface and the hardness of the laser-melted zone varies considerably when compared with the nitrided zone.

1. I n t r o d u c t i o n A l a s e r beam, w h e n used as a h e a t source, c a n affect the s u r f a c e of a m e t a l in two ways: (i) It c a n c a u s e t r a n s f o r m a t i o n h a r d e n i n g , w i t h o u t m e l t i n g or signific a n t l y r o u g h e n i n g the surface. (ii) It c a n m e l t a t h i n u n i f o r m s m o o t h film on the surface. T h e second process c a n be used to c r e a t e a l a y e r of a l m o s t a n y alloy on m a n y b a s e m a t e r i a l s by simply a d d i n g the n e c e s s a r y a l l o y i n g e l e m e n t s to the m e l t e d w o r k p i e c e surface. T h e t e c h n i q u e c a n also be used for e l i m i n a t i n g s u r f a c e defects in p o w d e r m e t a l p a r t s a n d c a s t i n g s [1]. S u r f a c e h e a t i n g by lasers e m e r g e d w i t h the use of p u l s e d lasers. A f t e r i r r a d i a t i o n by a pulsed N d - Y A G laser, t h i n foils of n e a r l y p u r e i r o n were f o u n d to e x h i b i t a defect s t r u c t u r e w i t h a h i g h d i s l o c a t i o n density a r r a n g e d in a c e l l u l a r n e t w o r k t y p i c a l of low c a r b o n m a r t e n s i t e [2]. T h e h a r d n e s s i n c r e a s e a s s o c i a t e d w i t h a m a r t e n s i t i c t r a n s f o r m a t i o n was o b s e r v e d in carbon steels a f t e r e x p o s u r e to a p u l s e d laser. T h e d e p t h s of the h e a t - a f f e c t e d zones for v a r i o u s l a s e r p o w e r s a n d p r o c e s s i n g speeds h a v e b e e n d e t e r m i n e d e x p e r i m e n t a l l y for v a r i o u s m a t e r i a l s [3]. H e a t c o n d u c t i o n d u r i n g l a s e r heating h a s b e e n f o r m u l a t e d for g a u s s i a n , r e c t a n g u l a r g a u s s i a n a n d u n i f o r m l y i n t e n s e r e c t a n g u l a r or c i r c u l a r b e a m s [4]. The r e m a r k a b l e m e c h a n i c a l p r o p e r t i e s of p l a s m a - n i t r i d e d l a y e r s - - w e a r resistance, antiscuffing and tribological properties, ductility and fatigue s t r e n g t h - - w h i c h are f r e q u e n t l y s u p e r i o r to t h o s e of c o n v e n t i o n a l l y n i t r i d e d s u r f a c e s [5, 6] derive directly f r o m t h e s t r u c t u r e of the p l a s m a - n i t r i d e d layer. G e n e r a l l y , a f t e r p l a s m a nitriding, E N 4 0 - B t y p e steels h a v e h a r d n e s s v a l u e s of 780-800 HV, 0.3 0.4 m m case d e p t h a n d c o m p o u n d l a y e r t h i c k n e s s e s of 5 1 0 p m [5]. H o w e v e r , a t o t a l case d e p t h of up to l m m on E N 40 B steel c a n be p r o d u c e d [7]. Elsevier Sequoia/Printed in The Netherlands

400' er Head

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H i g h p o w e r p u l s e d N d - Y A G lasers c a n be used to modify the c h e m i c a l c o m p o s i t i o n of alloy s u r f a c e s to d e p t h s r a n g i n g from 0.01 to 0.8 m m [8]. I n the p r e s e n t s t u d y n i t r i d e d E N 40 B steel s a m p l e s w e r e e x a m i n e d a f t e r l a s e r s u r f a c e melting. N i t r i d i n g w a s a c h i e v e d u s i n g a p l a s m a - n i t r i d i n g u n i t while a n N d - Y A G l a s e r w a s used to m e l t the n i t r i d e d surfaces. S u r f a c e a n d c r o s s - s e c t i o n a l m i c r o s t r u c t u r e s a n d h a r d n e s s d e p t h profiles are p r e s e n t e d .

2. E x p e r i m e n t a l details T h e e x p e r i m e n t a l set-up is s h o w n in Fig. 1. An Nd YAG l a s e r d e l i v e r i n g pulses w i t h a m a x i m u m w i d t h of 1.48 ms a n d a m a x i m u m o u t p u t e n e r g y of 21.5 J was used. A f o c u s i n g lens of 51 m m n o m i n a l focal l e n g t h w a s used to focus the l a s e r beam. A v a c u u m cell was u s e d to p r e v e n t o x i d a t i o n d u r i n g the s u r f a c e m i x i n g p r o c e s s and, in o r d e r to e n s u r e t h a t o x y g e n w a s t o t a l l y e l i m i n a t e d , a r g o n w a s e m p l o y e d to p u r g e the cell of air before e v a c u a t i o n . E N 4 0 - B s a m p l e s w e r e selected as w o r k p i e c e s .

3. D i s c u s s i o n and r e s u l t s T h e c o m b i n a t i o n of t e c h n i q u e s e m p l o y e d in the p r e s e n t w o r k includes p l a s m a n i t r i d i n g a t a t e m p e r a t u r e of 570 °C for 9 h p r o c e s s i n g t i m e as a first stage, a n d l a s e r - b e a m m e l t i n g as a s e c o n d stage. I n v e s t i g a t i o n s u s i n g light m i c r o s c o p y w e r e m a d e on u n e t c h e d a n d n i t a l - e t c h e d or p i c r a l - e t c h e d crosssections. F i g u r e 2 shows a c r o s s - s e c t i o n of a n i t r i d e d E N - 4 0 - B steel sample. The c o m p o u n d l a y e r is visible a n d its t h i c k n e s s is a b o u t 9.7 pm. Some p r e c i p i t a t i o n is also visible a little w a y b e l o w the s u r f a c e of the sample. F i g u r e 3 shows the h a r d n e s s of the E N - 4 0 B steel samples. It c a n be seen t h a t the h a r d n e s s of the m a t e r i a l is c o n s i d e r a b l e in the s u r f a c e region. In Fig. 3, t h e h a r d n e s s t e s t r e s u l t s are also g i v e n for the s a m p l e s a f t e r l a s e r melting.

401

Fig. 2. The microstructure of a sample after plasma nitriding at 570 °C for 9 h (original magnification x 500).

900 8OO 500 ~500 z: 500

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Fig. 3. The variation of hardness with depth in plasma-nitrided (O) and laser-melted (D) samples. The h a r d n e s s h a s d i m i n i s h e d in the l a s e r - m e l t e d r e g i o n w h e n c o m p a r e d w i t h the n i t r i d e d - o n l y region. This m a y be due to m i x i n g of the s u r f a c e l a y e r w i t h the m a t e r i a l b e l o w the surface. F i g u r e 4 shows selected o p t i c a l m i c r o g r a p h s w h i c h d e m o n s t r a t e the p r e s e n c e of l a y e r f o r m a t i o n in t a p e r - g r o u n d s a m p l e s a f t e r p l a s m a n i t r i d i n g c o m b i n e d w i t h l a s e r - b e a m melting. I n the a r e a of the l a s e r - b e a m m e l t e d l a y e r the t r a n s f o r m a t i o n p r o d u c t s are finer in s t r u c t u r e , b e c o m i n g m o r e i r r e g u l a r a t h i g h l a s e r p o w e r intensities. G e n e r a l l y , the m e l t e d r e g i o n s a r e m o r e homogeneous after treatment.

402

(a)

(b)

Fig. 4. Microstructure of the nitrided samples subjected to laser melting (original magnification (a) × 100; (b) x 570).

4. C o n c l u s i o n s The conclusions derived from the present work may be listed as follows: (1) C h a n g e s i n t h e m i c r o s t r u c t u r e o c c u r r i n g i n t h e a r e a c l o s e t o t h e surface under the laser action are associated primarily with the compound layer. (2) I n t h e r e g i o n o f t h e l a s e r - b e a m m e l t e d l a y e r t h e p r o d u c t s a r e f i n e r i n structure and become more irregular at high power intensities. (3) I n g e n e r a l , t h e h a r d n e s s v a l u e s v a r y m o r e i n t h e l a s e r - b e a m m e l t e d zone than in the region which was only nitrided. Furthermore, the value of the hardness drops in the laser-melted zone owing to mixing of the surface layer with the material under the surface.

References 1 B. S. Yilbas, J. Chin. Inst. Eng., •0(5) (1987) 543-547. 2 B. S. Yilbas, R. Davies and Z. Yilbas, Int. J. Mach. Tools Manuf., 29(4) (1989) 499 503. 3 B. S. Yilbas, G. Cinar and N. Kahraman, Proc. 8th Int. Conf. on Lasers and Optoelectronics in Engineering, Springer, Berlin, 1987, pp. 415-421. 4 B. S. Yilbas and Z. Yilbas, Pramana J. Phys., 31 (4) (1988) 1-17. 5 B. Edenhofer, Heat Treat. Met., 1 (1974) 23--28. 6 T. Bell, T. Rees and V. Korotchenko, in Proc. Ion Plating and Allied Techniques, Metal Society, Edinburgh, 1977, pp. 230 237. 7 M. B. Karamis and A. M. Staines, Heat Treat. Met. 3 (1989) 79-82. 8 B. S. Yilbas and R. Davies, Proc. 2nd I F H T Int. Seminar, Lisbon, SPIE, Bellingham, 1989, pp. 25 27.