MULTICOMPONENT OF

DIFFUSION

LOW-CARBON I.

STEELS

Gogachev

and

G.

SATURATION IN

GASEOUS

MEDIA*

Pushev

UDC 669.14:621.785.53

Diffusion saturation with chromium, aluminum, and silicon was conducted on widely used low-carbon steels, which have relatively high mechanical properties and good workability. Saturation was conducted in gaseous media. This permits simultaneous or sequential saturation with several elements, with changes in the concentration and flow rates of the gases. Saturation was conducted as follows. Gas carriers HCI and H 2 were cleaned and fed into the first chamber, where they react with chromium, silicon, and aluminum to form the saturating phases. The saturating phases were fed into the second chamber, in which favorable conditions were maintained for diffusion saturation of the samples. For operating safety, the second chamber was washed with nitrogen and hydrogen from which water and oxygen were removed by means of ammonia, active copper, H2SO4, and CaCI 2. As a safety measure cleaner and developer.

in case of reverse

flow, a tank should be placed between

the two tanks with the

Saturation was conducted with changes in the parameters of diffusion saturation - temperature, time, and flow of the gas carrier (see Table i). The samples were subjected to electron probe analysis, spectral analysis, x-ray analysis with sectioning, and metallographic analysis. The composition and depth of the diffusion coating were determined. The quantitative data from the electron probe analysis were analyzed in view of the effect of the sample on the saturating elements. The ~ ~/oCr,si results were further analyzed to take into account the mutual effects of the s a t u r a t i n g e l e m e n t s . The s p e c t r a l a n a l y s i s of the diffusion 2C ?,0 coating was conducted on sloping s e c t i o n s , which gave m o r e p r e c i s e data on the quantitative distribution of the d e m e n t s through the depth 78 ,8 of the diffusion coating. "/6

;6

\ 12

,2

o l\\l s

7,e

2

z2

Qualitative a n a l y s e s r e v e a l e d the p r e s e n c e of c h r o m i u m , a l u m i n u m , and silicon in all s a m p l e s . The quantitative distribution of e l e m e n t s through the depth of the diffusion coating is shown in Fig. 1.

,~

\

'>t

Sectioning and p h a s e a n a l y s i s r e v e a I e d a solid solution of c h r o miu_m in iron and FeA1 on the s u r f a c e of ai1 s a m p l e s . The depth of * This p a p e r will be p r e s e n t e d at the All-Union Symposium on New D e v e l o p m e n t s in Metal Science and Heat T r e a t m e n t of Metals and A l loys, S e p t e m b e r 10-12, 1975, in Minsk.

', \ \ -

TABLE

log

200 JOg

#00

1

500

Nc~

--~

I

45

4

curves.

2 3

P e o p l e ' s Republic of B u l g a r i a . No. 8, pp. 20-22, August, 1975.

Composition of saturating medium, ml

Z

Fig. 1. Distribution of elements through depth of diffusion coating. The sample numbers are given'on the

Translated

I40 Fe Cr; 30 Fe SI; 30 AI; I20 AI:Oz; 5 NH,CI

15

from M etallovedenie

1100

4

1150

4

I 1000

3

1150

4

i Termicheskaya

Obrabotka-Metallov,

9 76 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. 10011. No part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15.00.

1 659

this zone was 30-40 # in samples ples 1 and 4.

2 and 3 and i00 # in sam-

A solid solution of chromium in ~ iron and Fe~AI were observed in a zone 50 ~ deep below the surface zone. Only a solid solution of chromium and aluminum in ~ iron was observed at the bottom of the diffusion coating. " Metallographic analysis revealed an uneven surface layer with cracks (Fig. 2), followed by a homogeneous layer with good adhesion to the base metal. The thickness of the uneven l a y e r was 100 ~ on all s a m p l e s except s a m ple 2 (50 ~). It can be a s s u m e d that aluminum penetrates into the base metal in the following m a n n e r . In the saturation t e m p e r a t u r e range of 1000-1150~ the 7 phase is saturated with around 1% A1, F u r t h e r saturation with aluminum c r e a t e s conditions for the f o r m a t i o n of 7 solid solution, and t h e r e f o r e a heterogeneous mixture of solid solutions Fig. 2. Microstructure of samples 1 (~ + ~/) is f o r m e d a f t e r formation of "/ solid solution. The (a) and 2 (b) after diffusion saturation amount of 7 solid solution d e c r e a s e s and the amount of c~ with chromium, aluminum, and silicon. solid solution i n c r e a s e s until the 7 solid solution d i s a p p e a r s (• completely and the o~ solid solution alone r e m a i n s . With cooling of the samples to room temperature the -/ solid solution transforms to a ferrite-pearlite mixture, in which the amount of dissolved aluminum is equal to about 1%. The two-phase mixture ( T + ~) disappears at room temperature, while a sharp increase of the aluminum concentration should be expected, as is confirmed by the electron probe data (Fig. I) and data from [I]. The Fe-Al phase diagram indicates that the T region extends down to approximately 1% AI, followed by a heterogeneous region (7 + ~), which agrees with our results. At room temperature chromium is found in the solid solution, with no jump in concentration as is characteristic of aluminum. It can be assumed that at saturation temperatures of 1000-1150 ~ chromium does not saturate the 7 phase and is precipitated from it. This assumption is confirmed by the Fe -Cr phase diagram, where it can be seen that the 7 phase is saturated with about i0~ Cr. This concentration of chromium

is not observed

in the diffusion layer.

The amounts of silicon are minimal; it is found only in the surface layer. From the results obtained one can determine the quantitative distribution of chromium, aluminum, and silicon in the samples. The s u r f a c e of all s a m p l e s w h e r e the gaseous phase was f o r m e d directly in the c h a m b e r consists of a solid solution of c h r o m i u m and silicon in ~ iron, with aluminum in the form of FeA1, which is denoted as phase fi2 on the F e - A 1 d i a g r a m . The depth of this l a y e r is 30-100 #. It is followed by a l a y e r 50 # in depth, consisting of a solid solution of c h r o m i u m in ~ iron, with aluminum in the form of F%A1, which is denoted as phase ~l on the F e - A 1 d i a g r a m . Only a solid solution of c h r o m i u m and aluminum in ~ iron is observed below this l a y e r . Sectioning and x - r a y a n a l y s i s r e v e a l e d no sections of the diffusion coating that consist of both the solid solution of aluminum in ~ iron and F%AI or Fe~AI and FeAlo Electron probe analysis showed that there is no lump in the concentration of aluminum in these zones. This leads to the assumption that the change from one structure to another occurs without a heterogeneous region. The data in the literature also confirm the fact that there is no heterogeneous solution between the ~ solid solution and /3i phase or fi2 phase [i]. If it existed the electron probe would register a concentration jump, which was not observed. Thus, it can be stated that aluminum is in the form of FeAI and Fe~AI in these zones, constituting a hardened c~ solid solution. This assumption is also confirmed by data in [2], where it was noted that fll and fi2 phases have superstructures based on bce lattices. The unit cell of these superstructures consists of eight bcc cells arranged in twos in the direction of each crystallographic axis, with aluminum atoms in the center of four small cells and iron atoms in the center of the remaining smali ceils and at all corners. The constants of this lattice are double those of the ~ phase lattice, and thus the stresses should be far

660

stronger in these sections. Metallographic confirms the presence of these stresses.

analysis showed

cracks in the diffusion layer (Fig. 2), which

The results indicate that the thickness of the diffusion coating varies with the treatment temperature. The coating is thickest (6 = 650 #) after treatment at 1150 ~ Other factors that influence the thickness of the diffusion coating are the processing time and the composition of the saturating mixture and the sample itself. The depth of the diffusion coating reported was reached in samples

processed

for a very long time.

Under identical processing conditions, the amount of chromium in samples of steel 15 is approximately double that in samples of steel 45. Evidently this is dtte to decarburization, which occurs at a higher rate in steel 45 in steel 15. Since the diffusion mobility of aluminum and silicon is high, increasing amounts silicon in the saturating gas increase the depth of the diffusion coating.

of aluminum

and

It was also noted that with increasing amounts of chromium in the diffusion coating the amount of aluminum decreases. This may be due to the lower diffusion coefficient of chromium as compared with aluminum. It can be assumed that the lower the diffusion coefficient of an element, the more the diffusion of that element is slowed down. CONCLUSIONS i. The depth of penetration of aluminum

is considerably larger than that of chromium

and silicon.

2. The aluminum concentration must be reduced in order to avoid formation of a superstructure, which leads to brittleness of the diffusion coating. 3. The depth of penetration of the reagents is substantially larger when the gaseous phase is formed directly in the saturation chamber. LITERATURE I.

2.

CITED

M. Hansen and K. Anderko, Constitution of Binary Alloys, McGraw-Hill, New York (1958). A. D. Le Claire et al., Diffusion in BCC Metals [Russian translation], Metallurgiya, Moscow

(1969).

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