Note: Descriptions are shown in the official language in which they were submitted.
~05~
The present invention relates to a process for nitrid-
ing iron and steel in a salt bath, which is free from or has a
low concentration of cyanide and contains substantially potassium
cyanate, sodium cyanate, potassium carbonate and sodium carbonate
: i.e. a process which does not have a signiicant adverse effect
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on the envlronment.
For nitriding iron and steel, mixtures of alkali metal
; cyanides, alkali metal cyanates and alkali metal carbonates have
been used as molten baths heretofore. At an operating temperature
of approximately 570C said molten baths are in the form of fluid -
melts. For this purpose salt melts containing between 20 and 40%
., ~,j .
h j~ of cyanate, expressed as KCNQ, and 30 to 60% of cyanide, expressed
ii~ as NaCN, and the rest being alkali metal carbonate, are usually
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used (German Patent 1 149 035). The workpieces are exposed to
the action of the melt for several hours and a flow of air is
advantageousl~ passed through the melt. By diffusing nitrogen
into the surface of the work piece, layers are formed which cause
an increase in both the abrasion resistance and the alternating
bending stren~th of the workpiece. At the same time the bath
loses cyanide and cyanate and is enriched with carbonate, which
is ineffective for the nitriding process. In order to maintain
the efficiency of these baths, they must be regularly replenished
with pure alkali metal cyanide to regenerate them. Thus, a subs-
tantial portion of the salt bath must be discharged each time in
order to remove the carbonate and to reduce the volume. These
so-called spent salts always contain cyanide and thus are highly
.~.`, .
` toxic.
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Apart from the fact that the handling o~ the highly
; toxic cyanide requires special precautions for the preparation
and operation of the bath, the detoxication of the spent salts
; or their removal With subsequent safe disposal re~uires very
substantial expenditure. Moreover, the waste water obtained
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; upon chilling and rinsing the treated parts is highly toxic due
^ ~ to the cyanide residues still adhering to these parts and must
therefore be detoxicated before discharging it into the sewers.
. .
Attempts have therefore been made to use cyanide-free
` salt melts for nitriding iron and steel having medium and high
carbon contents (see Japanese Application 47-27089). ~Iowever,
~- in this case the melt must be kept free from oxygen and nitrogen
must be passed through the melt to suppress excessive formation
of carbonate in the salt bath which would increase the liquidus
temperature too markedly and decrease the nitriding action.
Apart from this costly rinsing with nitrogen~ this nitriding bath
has the further disadvantage that only workpieces having a
, relatively high carbon content can be nitrided and that spent
`i salts are still obtained during regeneration. In fact the spent
salts are substantially free from cyanide but they contain large
, . ~.
~~ amounts of cyanate which must also be removed.
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~ - The present invention provides a process for nitriding
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iron and steel having low carbon content in a salt bath, which
;~ form good nitriding layers and can be regenerated without producing `
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; 20 spent salts and are fluid in order to reduce losses due to salt -
; adhering to surfaces of the iron and steel and are as free from
cyanide as possible or at least have a low cyanide concentration
in order to avoid the formation of toxic waste water, i.e. a
',' process which has no significant adverse effect on the environment.
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According to the invention the salt bath for said
-` nitriding comprises a salt melt which contains substantially
~; potassium cyanate, sodium cyanate, po'assium carbonate and
'`"'!' sodium carbonate and through which air is passed. The salt
~ bath contains 25 to 57% by weight of cyanate, expressed as cyanate
i?. ' '`'
ion, 0 to 5% of cyanide, expressed as cyanide ion, the rest being
carbonate and alkali`-metal ion, such a bath may be regenerated ~
by the addition of polymeric carbon-nitrogen compounds without ;
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skimming off spent salts. Salt melts containing 33 to 42~ by
weight of cyanate, expressed as cyanate ion, were found to be
particularly suitable for the salt bath.
- According to the present invention therefore there is
- provided in a process for nitriding iron and steel in a salt
,~ bath at elevated temperature the improvement in which the salt
j~ bath consists substantially of a melt of potassium cyanate,
,~ sodium cyanate, potassium carbonate and sodium carbonate through
; which air is continuously passed, said melt containing 25 to
'i~; 10 57% by weight of cyanate, expressed as cyanate ion, 0 to 5% by
i; weight of cyanide, expressed as cyanide ion, the remainder being
~ carbonate and alkali-metal ions.
The present invention also provides a salt bath for
i
~, use in nitriding iron and steel consisting substantially of a
melt of potassium cyanate, sodium cyanate, potassium carbonate
and sodium carbonate and containing 25 to 57~ by weight of
. ~ .
cyanate, expressed as cyanate ion, 0 to 5~ by weight of cyanide,
i` expressed as cyanide ion, the remainder being carbonate and
~'r,`'~` alkali-metal ions.
;i~' 20 It has been surprisingly found that workpieces of iron
and steel having carbon contents, which are normally used in
practice, can be nitrided with good results in a salt melt which
are poor in cyanide or free from cyanide, when said salt melt
consists substantially only of potassium cyanate, sodium cyanate,
! potassium carbonate and sodium carbonate and contain 25 to 57
of cyanate, relative to the cyanate ion.
Salts baths containing a high proportion of potassium
. salts as compared with the sodium salts are very suitable for the
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~;: nitriding process according to the invention and the favourable
~; 30 range of operation for the sodium-potassium weight ratio extends
;: from 0 tol.5~Baths with a sodium-potassium weight ration of 0 to
~ 0.5 were found to be particularly suitable. By using salt melts
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in potassium the melting point of these salt baths is reduced
so that at an operating temperature of approximateIy 570C
flui~ melts are formed and the salt losses upon removing the
treated workpieces are thus kept low. These baths-can still
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cope with high carbonate contents which are formed from the
cyanates during the operation of the bath and usually increase
~, the melting point.
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,~ The special advantage of the baths also lies in that
',;` they can be regenerated by addition of polymeric carbon-nitrogen
compounds, as for example, Melon, polymeric hydrogen cyanide or
polymeric urea, with no significant yield of spent salts. During
'~ the operation of the nitriding baths according to the invention
`; CN and CNO are reacted to C032 by atmospheric oxygen and by
` the nitrogen yielded to the workpieces. Said C032 is ineffective
~i for the nitriding process. By add:ing the regenerating agents
mentioned above excess carbonate can be converted into cyanate
without increase in volume and the yield of spent salt associated
therewith.
~ii; In order to increase the durability of the iron and
'i 20 steel parts, they must be rapidly chilled after the treatment
in the nitriding bath. Chilling of the parts in nitrite~nitrate
salt baths was not possible in the conventional nitriding salt -
' ~aths because the adhe`ring cyanide-containing salt residues
,
reacted explosively with the nitrite-nitrate melt. From the
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baths according to the present invention (which are poor in ~-
cyanide or free from cyanide~ non-hazardous chilling in nitrite-
nitrate melts is possible. Cyanide and cyanate are oxidized to
.. . .
;~ carbonate and nitrogen in a quiescent reaction, so that neither
Xi` cyanide nor cyanate residues get into the waste water. Even
upon chilling from the salt bath according to the present inven-
;~, tion in water, the waste water from the hardening operation is
non-toxic and free from cyanide. Small amounts of cyanide in
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the nitriding salt bath which might get into the water upon
chilling can be destroyed immediately by adding sodium hypo-
chlorite to the waste water. A separate detoxication of the
waste water obtained from th~ hardening operation can thus be
~` dispensed with. This substantially facilitates the treatment
of the waste water.
The nitriding action of the salt baths used in the
process according to the invention increases as the cyanate
content increases. Therefore, the carbonate~cyanate ratio
`;'.
should be advantageously between 0 and 1. However, it is not
~` possible to keep nitriding baths completely free from carbonate
since carbonate is formed by the reaction between cyanate and
,:,
atmospheric oxygen. The addition of cyanide can be dispensed
with completely, but cyanide is a:Lways found in the bath in
small amounts during the nitriding treatment. However, the
~ cyanide content remains below 4% by weight. In the process
k according to the invention the salt melts can also contain up to
30% by weight of alkali metal chloride.
The present invention will be further illustrated by
way of the following Examples.
Example 1
In a cylindrical titanium crucible, having a diameter
of 35 cm and a height of 70 cm, 75 kg of KCNO, 22 kg of Na2CO3
and 3 kg of K2CO3 were melted down and heated to 570C. A
~- fluid melt was thus obtained and 200 litres of air are passed
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through it per hour.
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In this bath samples of Ck 15 steel in the form of
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, small plates were treated for two hours and then chilled in
salt water. The chilling water was subsequently found to contain
no detectable amounts of cyanide.
When testing the repeated alternating flexual strength
it was found that ~BW =12 kp per sq mm prior to the treatment
had increased to~ BW ~42 kp per sq mm after the nitriding
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treatment. A layer of iron-nitride compound having a thickness
~;, of 18 to 22 ~m was detected on the samples by means of metallo-
graphy and X-ray photography. Towards to the interior of the
core said layer changed over into a diffusion zone of 0.55 mm.
Over 24 hours of use the cyanate content of the melt
decreased from approximately 39% by weight to approximately 37%
by weight and the carbonate content increased from approximately
14% by weight to approximately 16~i by weight~ By adding 1.2
kg of Melon the initial values can be restored without having
to remove spent salts.
Example 2
In the same bath as in Example 1 small sample plates
of different materials were treated for 90 minutes at 560C and
examined. The following iron-nitride layer thicknesses were
obtained:
20 Mn C'r 4 12 - 14 ~m
15 Cr Ni 6 10 - 12 ~m
14 Ni 6 11 - 14 ~m
spheroidal graphite iron 8 - 14 ~m
gray cast iron 10 - 20 ~m
; These layer thicknesses correspond approximately to the
layer thickness of a Ck 15 sample treated in a similar manner.
Example 3
In each of the following salt baths (which were operat-
ed for 24 hours) the initial cyanate and carbonate contents were t
restored without producing spent salts by adding to each bath
; 1.2 kg of Melon or 2 kg of polymeric urea:
' a) 75 kg of KCNO, 20 kg of Na2CO3, 3 kg of K2CO3 and 2 kg of
NaCN. Cyanite content approximately 39% by weight as CNO .
b) 68 kg of KCNO, 10 kg of NaCNO, 8 gk of K2CO3 and 14 kg of
Na2C -
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Cyanate content approximately 42% by weight as CNO .
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~ C) 35 kg of KCNO, 35 kg of NaCNO, 10 kg of K2CO3 and 20 kg of
,.......... Na2C03 ::
Cyanate content approximately 41% by weight as CNO .
Example 4
In a crucible according to Example 1, 100 kg of KCNO
- were melted down and heated to 570C. 200 litres of air were
passed through the fluid meIt per hour. In this bath several
samples of Ck 15 steel in the form of small plates were treated
for 90 minutes and then chilled in salt water. No cyanides could
;~ 10 be detected. The samples had an iron-nitride-compound layer
having a thickness of 8 to 10 ~m.
,,:,,~
~; Over 24 hours the cyanate content of the melt decreased
.. . .. .
, from approximately 52% by weight to 50% by weight. The carbonate
content increased from 0% by weight to approximately 2% by
~` weight. By adding 1.2 kg of Melon the initial values were restor-
~i ed.
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;i Example 5 -
; In a titanium crucible according to Example 1, 58 kg
. of K2CO3 and 42 kg of NaCNO were melted down and heated to 570C.
After 90 minutes it was found that samples of Ck 15 steel had
.; j, . ..
n iron-nitride-compoun~layer having a thickness of 6 to 10 ~m.
~; ;. ..
The cyanide content of the melt which had decreased from approxi-
;,.... . .
~`j mately 27% by weight to 25% by weight in 24 hours, was restored -
~ to its initial value by adding 1.2 kg of Melon. The carbonate
t'.;.'' content, which had increased from 25% by weight to 27% by weight,
was restored to its initial value in the same manner.
;s Example 6
;~ In a titanium crucible according to Example 1, 35 kg
~; of NaCNO, 35 kg of KCNO and 30 kg of Na2CO3 were melted down
` 30 and 200 litres of air were passed through the salt melt per
hour at 570C. On a Ck 15 samiple, which had been treated for
90 minutes, 16 to 18 ~m of an iron-nitride-compound layer was
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detected. The cyanate content of the melt which had decreased
from 41% by weight to 39% by weight in 24 hours, was xestored
j to its initial value by adding 2 kg of polymeric urea.
,, Example 7
~'i In a titanium crucible 64 kg of KCNO, 16 kg of K2CO3,
11 kg of NaCNO, 4 kg of NaCN and 5 kg of NaCl were melted down
~` and heated to 570C. On a Ck 15 sample in the form of small
~; plate, which had been treated for 90 minutes, an iron-nitride~
compound layer having a thickness of 10 to 14 ~m was detected.
.
Over 24 hours the cyanate content of the melt decreased from
~ approximately 40% by weight to 38% by weight and the carbonate ~-
;~ content increased from approximately 7% by weight to 9% by weight.
The initial value was restored by adding 1.2 kg of Melon.
Apart from the materials mentioned hereinbefore any
other steel-alloyed or unalloyed - can be nitrided by the process
of the present invention.
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