effect of cyanide bath composition on depth and wear resistance of case

E F F E C T O F C Y A N I D E B A T H C O M P O S I T I O N ON

D E P T H A N D W E A R R E S I S T A N C E O F C A S E

Y a . N. F u n s h t e i n a n d L . S . L y a k h o v i c h UDC 621.785.52.066.6:539.538

Cyanid ing in sa l t ba ths i s wide ly u sed for su r f a c e h a r d e n i n g of s tee l to a shal low depth (0.05-0.40 ram). The toxic NaCN and KCN o r d i n a r i l y u s e d in these baths a re be ing r e p l a c e d with the h a r m l e s s p o t a s s i u m f e r - r ocyan ide K4Fe(CN) G.

Due to the l ack of de ta i l ed da ta on the cyan id ing capac i ty of the ba ths used c o m m e r c i a l l y (Table 1),- with the except ion of compos i t i on [1, 2], we s tudied the p r o c e s s of l iquid cyan id ing in ba ths of d i f f e ren t c o m - posi t ion .

Compos i t i ons 1, 4, and 5 were i nves t iga t ed u n d e r c o m m e r c i a l condi t ions in e l ec t rode baths of the S-100 type without s tee l c r u c i b l e s , and c o m p o s i t i o n s 2 and 3 in an e l e c t r i c c r u c i b l e f u r na c e of the V-20 type. The s a m p l e s w e r e s t ee l s 20 and 25KhGT.

Samples 30 m m in d i a m e t e r and 120-140 m m long were sec t ioned for c h e m i c a l a na l y s i s . The m i c r o - s t r u c t u r e and depth of the ca se w e r e d e t e r m i n e d with s a m p l e s 10 x 10 • 50 m m and s a m p l e s 12 m m in d i a - m e t e r .

Af te r cyan id ing in the v a r i o u s ba ths 2 h at 850~ and quenching in w a t e r the h a r d n e s s of d i sks of s tee l 20 with a d i a m e t e r of 40 m m and a t h i c k n e s s of 10 m m was as fol lows:

Bath No. HRA

1 . . . . . . . . . . . . . . . . . . . . . . 81-85 2 . . . . . . . . . . . . . . . . . . . . . . . 80-84 3 . . . . . . . . . . . . . . . . . . . . . . . 81-84 4 . . . . . . . . . . . . . . . . . . . . . . . 80-82 5 . . . . . . . . . . . . . . . . . . . . . . . 78-82

The samples sectioned for chemical analysis after cyaniding were quenched in oil; the others were cooled in air.

After quenching, the surface zone consists of martensite and the transition zone of troostosorbite.

To determine the effect of bath composition and processing time on the depth of the case and the car-

bon and nitrogen concentrations in it the samples were cyanided at 850~ for 30 rain, 1 h, 2 h, and 3 h. The case is formed most rapidly in the first 30 rain regardless of the bath composition (Fig. 1).

TABLE 1. Compos i t ions of Com- m e r c i a l Baths for Cyanid ing Steel

CaCI2

65

The s a m p l e s we re sec t ioned to d e t e r m i n e the c o n c e n t r a t i o n s of c a r b o n and n i t r o g e n in the case a f t e r cyan id ing for d i f fe ren t t i m e s at 850~ The c o n c e n t r a t i o n of these e l e m e n t s at a depth of 0.025

Composition, % m m a r e g iven in Table 2.

BaCI2 KCI NaC! Na2CO~ K,Pe(CN)6 Bath No. 1 is the m o s t effect ive. The case p roduced in this ba th is f a i r l y wel l s a t u r a t e d with ca rbon and n i t r o g e n and has a high

788782-- ~-~ 32 -- 3,0 h a r d n e s s a f t e r quench ing (HRC 60-64). The d i s advan tages of this ~9 3,0 bath a re that c a l c i u m ch lo r ide is hyg roscop ic and the bath m u s t be

- - 19 15 3,0 48 -- 3,0 ope ra t ed round the c lock .

50--7020~15 20--15 30 30 Baths 2 and 3 have good cyan id ing capac i t i e s . 65 10 25 35 ~ 30

Our i nves t i ga t i ons showed that n e u t r a l sa l t s take p a r t in the r e a c t i o n s , a f fec t ing the r a t e of the cyan id ing p r o c e s s . The p r e c i p i - t a t ion r a t e of a tomic c a r b o n is h ighes t in ba ths con ta in ing a lka l i e a r t h m e t a l s . The r e s u l t s ob ta ined a g r e e with those in [3, 4].

B e l o r u s s i a n Po ly techn ica I I n s t i t u t e . T r a n s l a t e d f r o m Meta l loveden ie i T e r m i c h e s k a y a Obrabo tka 10, pp 8-11, October , 1968. Metal lov, No

767

m m

o,,o

0,3

/ ! 2 3h

110 L-

5O

.~ 30

ZO

Cyaniding time

Fig. 1 Fig. 2

Ji Steel 20

? 3 ~ ? 3

Fig. 1. Variation of case depth with process ing time and bath composit ion (bath numbers given on the curves).

Fig. 2. Effect of bath composition on the wear res is tance of steels 20 and 25KhGT. 1) Bath 1; 2) bath 3; 3) bath 5. The c ros s hatched columns show the wear of the steels after 30,000 revolutions.

TABLE 2. Effect of Cyaniding Time at 850~ on the Concen- trat ions of Carbon and Nitrogen in the Case

~ d I Steel

~ C, r~ ~ b 1 3 070

m

0,5 0,78 1 1 0,82

2 0,88

0,5 0,80 3 1 0,70

2 0,75

0,5 0,68 4 1 0,70

2 0,72

5 ~,5 0,55 0,62

2 0,60

20 Steel 25KhGI

N, % C, %IN, %

0,37 0,75 0,33 0,32 0,85 0,34 0,34 0,90 0,36

0,30 0,92 0,30 0,32 0,97 0,30 0,27 0,83 0,32

0,25 -- -- 0,28 - - - -

0 , 2 4 - - - -

0,15 0,29 0;0 051 0,30 0,50 0,27

5.

In the liquid bath K4Fe (CN)6 decomposes with formation of KCN. The presence of CaC12 and BaC12 increases the carbur iz ing capacity of the bath due to the precipitat ion of large amounts of active atomic carbon:

CaC12 + 2 KCN--+Ca (CN)2 + 2 KCI, Ca (CN)~--+ CaCN2 + C.

At the surface of the bath calcium cyanamide interacts with oxygen f rom the air and decomposes with format ion of atomic ni t ro- gen:

CaCN~ -{- ON ~ CaO -~ CO -~ 2 N.

Carbon monoxide decomposes , with precipitat ion of atomic c a r - bon:

2CO~--CQ+C.

Par t of the carbon monoxide escapes f rom the bath into the a tmosphere [1].

The react ions given above also occur in baths with bar ium chloride.

Calcined sodium carbonate, absorbing carbon and nitrogen, reduces the cyaniding rate [3]:

Na~CO~ + 4C + N~---+ 3CO+ 2NaCN.

The cost of bath No. 1 is little more than 40% that of bath No. 3 and about two-thirds that of bath No.

To determine the effect of the bath composition on the wear res is tance the samples were tested after cyaniding 2 h at 850~ and quenching.

Six samples of steels 20 and 25KhGT were cyanided simultaneously in baths 1, 3 and 5 for wear r e s i s - tance tests and also samples for chemical analysis to determine the carbon and nitrogen concentrat ions of the case. The resul ts of the chemical analysis are given in Table 3. The wea r - r e s i s t ance samples of steel 20 were quenched in water , those of 25KhGT in oil. After quenching, par t of the samples were subjected to low-tempera ture tempering (170-190~ for 1 h and others were aged 40 rain at 100~

768

TABLE 3. Distribution of Carbon and Nitrogen Through the Case of

Control Samples Cyanided at the Same Time as Wear Resistance Samples

E E 4

0,025 0,05 0,10 0,15 0,20 0,30 0,40

Bath 1 1 Bath 3 Bath 5 Steel 20 Steel 25KhG_Tj St_ee120 . S t e e - - - 125KhG_ ~

c,% N,% c, ~o N,% ~ C,% N,% C,% N,%

Steel 20 3reel 25KhGT

c,% N,% C,9 N,%

0,85 0,78 0,69 0,62 0,55 0,46 0,34

0,29 0,94 0,25 0,90 0,21 0,83 0,16 0,80 0,10 0,72 0,06 0,62 -- 0,47

0,3t 0,90 0,28 0,82 0,24 0,74 0,17 0,65 0,15 0,51 0,08 0,40

- - 0,31

0,23 0,15 0,12 0,07 0,03

0,86 0,29 0,84 0,27 0,80 0,22 0,76 0,18 0,69 0,11 0,52 0,05 0,36 --

0,60 0,58 0,52 0,49 0,42 0,37 0,29

0,30 0,60 0,15 0,60 0,09 0,56 0,07 0,54 0,05 0,46

- - 0,39 0,33

0,27 0,11 0,06 0,03

Laboratory tests of wear resistance were made with rolling friction in the MI machine under conditions of 10% slippage, no lubrication, load of 60 kg, 60,000 rpm.

The samples were paired with high-speed steel heat treated to HRC 62-63. The rollers tested were i0 mm wide with a diameter of 40-~176 The samples were run in (5000 revolutions) before the wear tests. The wear was determined from the weight loss of the samples after 30,000 revolutions. The maximum stress on the contact surface was determined by the Hertz formula. With a load of 60 kg it was 459 kg/ram 2.

P r e l i m i n a r y t e s t s showed that the wear r e s i s t a n c e of the s a m p l e s aged at 100~ af te r quenching is 25-70% higher than that of s a m p l e s t e m p e r e d at 170-180~ a f te r quenching.

The bas i c wear r e s i s t a n c e t e s t s were made with s a m p l e s aged a f te r quenching (Fig.2). The wear r e - s i s t ance of the s a m p l e s cyanided in bath No. 1 was h igher than that of s amples cyanided in the other ba ths . The wear r e s i s t a n c e of s t ee l 20 is more than double that of a l loy s tee l 25KhGT.

The same r e s u l t s were obtained in s l iding f r i c t ion t e s t s of r o l l e r s in a r ing, with the a rc of c i r c u m - f e r ence of the r o l l e r = 22.2 mm. The t e s t s were made under a load of 25 kg for 20,000 revolu t ions without lubr ica t ion . The wea r was m e a s u r e d eve ry 10,000 revo lu t ions . The r ings were made of s tee l ShKhl5 heat t r e a t e d to HRC 61-62.

The h igher wear r e s i s t a n c e of cyanided s a m p l e s of s t ee l 20 as compared with s tee l 25KhGT can be expla ined b y the e leva ted concent ra t ion of r e s i d u a l aus teni te in the case of s t ee l 25KhGT.

X - r a y ana lys i s showed that the amount of r e s i d u a l aus teni te in s a m p l e s of steel 20 a f te r cyaniding and heat t r e a t m e n t did not exceed 7%, while in s a m p l e s of s t ee l 25KhGT (after cyaniding, quenching in oil , and aging) it was 26-32%. It is a l so poss ib l e that the d i s t r ibu t ion of r e s i d u a l s t r e s s e s in s tee l 20 (after the full cyc le of t r ea tmen t ) i s more f avorab le than in s tee l 25K_hGT.

CONCLUSIONS

I. Bath No. I, which is more economical and has the best cyaniding capacity, produces a high-quality case with good wear resistance.

2. When the eyaniding bath is used only a few days a month then baths 2 and 3 are more expedient.

3. For a smooth surface on machine parts after quenching in oil it is expedient to use bath 5.

4. Carbon steel can be used for machine parts not subject to dynamic loading or high specific pres- sures ; after cyaniding and quenching they should be aged at 100~

I.

2, 3. 4.

LITERATURE CITED

Ya. N. Funshtein, Increasing the Wear Resistance of Machine Parts and Tools by Chemieothermal Treatment [in Russian], Belarus' (1965). Ya. N. Funshtein, AvtomobiI'naya Promyshlennost', No. 1 (1965). A. T. Kalinin, Chemieothermal Treatment of Automotive Parts [in Russian], Mashgiz, Moscow (1954). A. N. Minkevich and A. T. Kalinin, Modern Methods of Heat Treating Steel [in Russian], Mashgiz, Moscow (1954).

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