Pack nitriding process for low alloy steel

A method of nitriding metal parts is disclosed. A preferred mode requires burying the metal part to be treated in a body of vermiculite or other porous media containing urea or other suitable nitriding agent. Prior to burying, a controlled amount of an aqueous solution of the nitriding agent is absorbed into dry vermiculite; the water of the solution is removed by evaporation leaving a pasty substance coating or impregnating the grains of the vermiculite. The pasty substance coating each vermiculite grain physically forms a thin film on the external particle surface and on the surfaces of its internal porosity; the average film thickness is less than 0.001 inch. The vermiculite bearing the nitriding agent and buried part are heated in a closed container to a temperature at which the agent decomposes at a predetermined slow rate and releases gases bearing nitrogen. Heating is continued for about 4-8 hours at 700.degree.-1300.degree. F (preferably 925.degree.-1050.degree. F for 90% of the applications) to achieve a nitrided case on the metal part of the desired thickness, typically 0.002-0.005 inch thick.

BACKGROUND OF THE INVENTION 
To put a hard, thin, wear and fatigue resistance layer on steel parts which 
have been quenched and tempered, a common processing technique is employed 
called "low temperature nitriding". This is a process which requires 
special heat treating furnace equipment in order to carry out nitriding at 
a temperature in the range of 925.degree.-1050.degree. F. The process 
sequence consists essentially of: (a) roughly shaping the part to be 
treated, often by hot forging, (b) hardening by austenitizing, quenching 
and tempering to a hardness usually in the range of R.sub.c 25-35, (c) 
finish machining, and (d) nitriding at 925.degree.-1050.degree. F. Because 
the nitriding temperature is about the same as or lower than the tempering 
temperature, the hardness of the core material can be maintained in the 
range of R.sub.c 25-35. Such a process is used commercially for making 
some automotive-type gears. 
Other currently used commercial nitriding methods either require the use of 
a molten salt bath (necessitating special control of bath composition) or 
use of a furnace with special equipment to contain an ammonia-bearing 
gaseous atmosphere. These processes are detailed in the eighth edition of 
Metals Handbook, Volume 2. The salt bath technique, as well as the 
technique with a furnace having an ammonia-bearing atmosphere, undesirably 
require considerable care to regulate the nitrogen potential so that 
results are consistent. It is also apparent that with any of the above 
processes, special equipment and considerable manipulation are required to 
obtain good results. This is not always obtainable in certain plant 
situations and certainly it is desirable to reduce the required skills in 
obtaining good results with any procedure. 
Thus in plants or heat treating shops which lack the special equipment for 
low temperature nitriding or the special equipment required for salt bath 
or gas atmosphere nitriding, it would be most advantageous to have a 
method for pack nitriding, as convenient as pack carburizing. However, it 
is important to keep in mind that the science of pack carburizing is 
non-analogous, except as to cost and convenience, because it is carried 
out at temperatures of 1500.degree. F. and above, where low alloy steels 
are austenitic, and is followed by quenching and tempering. 
SUMMARY OF THE INVENTION 
A primary object of this invention is to provide a method of nitriding 
metal parts unrestricted as to heating source and effectively carried out 
without special furnace atmosphere control. 
Still another object of this invention is to provide a method of nitriding 
steel parts by the use of a granular packing having the grains thereof 
thinly coated with a nitriding agent having suitable thermal stability; 
the packing is installed about the part to be nitrided and heated using 
any conventional heating source, such as an electrical resistance heated 
furnace. 
Specific features pursuant to the above objects comprise the use of a 
packing material consisting essentially of dry vermiculite or other porous 
media containing a predetermined quantity of a suitable nitriding agent, 
urea for example, spread on the surface and in the interstices of the 
grains as a thin coating; the packing containing the part to be treated 
therein is heated to a temperature at which the nitriding agent decomposes 
thereby slowly and controllably releasing a nitrogen-bearing gas for 
nitriding the steel part. The impregnated packing is usually a flowable 
material which is placed within a shell or tray during nitriding. After 
decomposition of the nitriding agent, the steel part is exposed to the 
nitrogen-bearing atmosphere for a controlled period of time. The ratio of 
the impregnated packing volume to the surface area of the steel part can 
be relatively low in some applications if the part is packed in an 
airtight, or nearly airtight container. The relation between the depth of 
the nitrided case and the time and temperature of treatment is similar to 
that for other methods of nitriding. 
DETAILED DESCRIPTION 
A preferred mode for carrying out the invention herein, comprises: 
(a) Preparation of a granular packing medium, the medium being capable of 
being wetted by an aqueous solution of urea or other nitriding agent 
meeting the thermal stability of this invention and being capable of being 
dried without suffering significant degradation in its mechanical 
strength. Materials useful as a packing medium herein may be selected from 
the group consisting of vermiculite, charcoal granules, porous clay 
granules, porous ceramic granules, etc. Such materials should be selected 
because they possess all of the following characteristics: (1) are 
chemically inert, (2) have a high absorption capability, (3) are stable at 
high temperatures, (4) have a particle shape which is easily packable and 
possess adequate mechanical strength that is not easily degraded by 
temperatures typical of nitriding. 
(b) Preparation of an aqueous solution of urea, or equivalent 
nitrogen-bearing agent. Suitable nitriding agents includes those compounds 
which are characterized by a relatively slow release of nitrogen-bearing 
gases at typical nitriding temperatures (925.degree.-1050.degree. F.). 
Most nitrogen-bearing organic and inorganic compounds, when heated into 
this temperature range, decompose rapidly releasing ammonia or other 
nitrogen-bearing gases, often leaving behind, in the case of organic 
compounds, a carbonaceous residue. The nitrogen-bearing gases released 
from these compounds can react with steel to form nitrides; however these 
are not suitable nitriding agents because all the nitriding gases are 
released in a matter of a few minutes after reaching the nitriding 
temperature. Penetration of nitrogen into the steel to a significant depth 
(0.002-0.005 inch) requires several hours for nitrogen diffusion. During 
this time, nascent nitrogen must be continually supplied to the surface of 
the metal. Thus, suitable agents for nitriding must either (1) have 
sufficient thermal stability at the nitriding temperature so that they 
decompose slowly, releasing nitrogen-bearing gases over a time period 
measured in hours, or (2) decompose on heating to form another compound 
which has the necessary thermal stability. 
The concentration of the nitrogen-bearing agent is adjusted in the solution 
to provide predetermined amounts of the agent per unit volume of the 
packing medium when dehydrated. The amount of nitriding agent desired in 
the packing medium is primarily a function of (1) the amount of medium per 
unit surface area of parts to be nitrided, and (2) the desired thickness 
of the nitrided case. The preferred nitrogen-bearing agents are urea 
(NH.sub.2 --CO--NH.sub.2), guanidine carbonate [(NH.sub.2).sub.2 
CNH].sub.2 H.sub.2 CO.sub.3, dicyanodiamide [(NHC(NH.sub.2)NHCN)], and 
cyanuric acid (HCNO).sub.3. The agent must be selected on the basis of (1) 
its ability to slowly release a nitrogen-bearing gas capable of nitriding 
steel at typical nitriding temperatures, and (2) its ability to be readily 
and thinly dispersed on the packing medium by means of an aqueous solution 
thereby avoiding direct contact between the nitriding agent and the part. 
Thus, special cleaning operations after nitriding, to remove residues 
produced by thermal decomposition of the nitriding agent, are not needed. 
(c) Absorption of the aqueous solution containing the agent into said 
packing medium; this may be carried out in trays at ambient temperature 
and pressure. The mixture is then dried by heating to a temperature of 
100.degree. F.-200.degree. F. for a period of about 24-48 hours, removing 
excess water. The packing medium will thus be impregnated with a soft 
solid or pasty substance which is urea or other nitriding agent as 
specified above. The resulting dehydrated packing medium is unique in that 
it has the nitriding agent distributed physically in a thin film about 
each of the packing medium granules. Dispersion of the agent as a thin 
coating on the packing medium inhibits agglomeration of the agent if it 
melts on heating (as urea does), and assures a uniform, controlled 
distribution of the agent about parts of complex shape. Some prior art 
methods have employed granular urea as a direct packing medium. This is 
likely to be unsatisfactory because (1) far more urea is consumed than is 
needed to nitride the part, (2) since urea melts at about 273.degree. F., 
the part would become coated with urea. Some of the thermal decomposition 
products, which would adhere to the part, are not water soluble, posing 
subsequent cleaning problems, and (3) it is not possible to nitride in a 
controlled manner, regulating the supply of nitriding agent to assure 
adequate nitriding without forming thick all-nitride surface layers. 
(d) The impregnated packing medium is arranged about the part to be 
nitrided in a container with a loose fitting cover which will allow gas to 
escape, but restrict the entry of air. The packing medium must have been 
thoroughly dried before this step. Any water remaining will lead to 
oxidation of the steel and interfere with the nitriding process. 
(e) The packed part is heated to a temperature (at least above 800.degree. 
F.) for a period of time to decompose the impregnated nitriding agent, and 
thus allow a nitrogen bearing gas to be evolved for transferring nitrogen 
to the steel surface. A molecular nitrogen gas (N.sub.2) is not desirable 
for nitriding. A nitriding temperature between 925.degree.-1050.degree. 
F., typical of other nitriding, processes, is satisfactory. 
(f) Hold the part at the nitriding temperature for a period of time to 
produce a predetermined depth of nitrided case in the steel part and to 
obtain a predetermined degree of hardness for the selected surface zone of 
said part. Heating may be provided by any type of heating source, capable 
of producing the temperatures required. The period of time to carry out 
nitriding with this method should be 4 to 24 hours. With longer times the 
atmosphere generated by the nitriding agent may be dissipated and 
oxidation of specimens can occur. 
For any given alloy, the depth of the hardened case increases with an 
increase in the time for nitriding, an increase of the permitted 
temperature of nitriding, and an increase of the concentration of 
nitrogen-bearing agent (urea, for example) with respect to the packing 
medium. The greatest surface hardnesses, however, are produced at lower 
nitriding temperatures with nitriding times of 8 hours or slightly less. 
It is well known that certain alloys (particularly those containing 
chromium, aluminum, and molybdenum) respond more favorably to nitriding 
than do other alloys. The heat treating variables of time, temperature and 
content of nitrogen-bearing agent must be determined by experimentation 
for any particular part and/or alloy. When urea is used, the content may 
be varied from 25-100 gms urea/liter of packing medium. Using a urea 
content of 40 gms/liter of packing medium, a nitriding temperature of 
950.degree. F., and a nitriding time of 6 hours will produce a nitrided 
case of about 0.005 inch thickness on most alloys. 
Comparative test data was generated to corroborate the advantages of the 
method herein, using non-special economical apparatus to achieve results 
equivalent to that with specialized equipment. The following test sequence 
was undertaken. Test pieces of SAE 5140 steel were hardened by heat 
treatment to R.sub.c 38, then the surfaces were ground. Two of these 
pieces were placed in a 2000 cc. pyrex beaker in a manner to be buried or 
packed in about 1800 cc. of dry granular material therein. The granular 
material was two liters of vermiculite (serving as a dry packing medium) 
and was allowed to absorb 600 mm. of a water solution containing 50 grams 
of urea. Typically 1 liter of dry vermiculite will absorb about 300 mm. of 
water. The vermiculite was dried by placing it in a shallow tray and 
heated to a temperature of 120.degree. F. for 24 hours. A thermocouple was 
inserted about 11/2 inches down into the vermiculite to monitor 
temperatures. The beaker was closed with a loose fitting cover and placed 
in an electrically heated furnace preheated to 970.degree. F. A period of 
about one hour transpired before the buried thermocouple reached the 
temperature of the furnace. When the thermocouple reached the furnace 
temperature, the container was held for an additional period of about four 
hours at 970.degree. F. After nitriding, the hot parts were shaken out of 
the vermiculite, removed from the beaker and then air cooled. 
The parts were lightly oxidized; such oxide was removed by soaking in a 50% 
aqueous solution of HCl. The pieces were sectioned, nickel-plated and 
mounted in bakelite for metallographic examination. Microhardness traverse 
readings were made using a Knoop indentor and 1 kilogram load. Results for 
one traverse are shown in Table I. 
Table I 
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Distance from 
Knoop Approximate Equivalent 
Edge, Inches 
Hardness in Rockwell C 
______________________________________ 
.001 736 60 
.002 689 58 
.003 660 56 
.004 579 52 
.005 549 50 
.010 423 42 
.016 392 39 
.022 361 36 
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Upon comparison with the data in FIG. 6, page 153 of volume 2 of the eighth 
edition of "Metals Handbook", it is clear that the hardness gradient shown 
by Table I is typical of nitriding for this type of alloy steel at this 
combination of time and temperature. The surface of the nitrided piece has 
a very thin nitride layer of about 0.0002 inches thickness, also typical 
of parts nitrided by other methods. 
It has been determined that various blends of packing medium containing the 
desired nitrogen-bearing agent can be utilized to obtain varying case 
depths. Furthermore, the packing medium may contain a mixture of different 
nitrogen-bearing agents each of which have thermal stability at nitriding 
temperatures and are not of a toxic nature according to this invention; 
but such mixture should preferably be of the compounds suggested below 
which include polymers of cyanic acid and cyanamide. In fact, it has been 
determined that thin steel parts can be effectively through-hardened by 
this process. Furthermore, after nitriding, the spent impregnated packing 
medium can be recycled to be used again in this process. To this end, the 
spent impregnated medium is again saturated with an aqueous solution of a 
suitable nitriding agent, dried, and is ready for reuse. 
The thermal stability of a variety of nitrogen-bearing compounds was 
determined in a series of simple experiments. A measured amount of 
compound (2-10 grams) was placed in a 150 ml. pyrex beaker, the beaker was 
covered with aluminum foil and placed in a furnace at typical nitriding 
temperatures. After a certain time period, the beaker is removed from the 
furnace, and the residue is weighed. A piece of steel can be inserted into 
the beaker with the residue, and reheated to confirm that the residue is 
capable of nitriding steel. Following are examples of this kind of 
experiment.

EXAMPLE 1 
3.5 gms. of ammonium carbonate, (NH.sub.4).sub.2 CO.sub.3 H.sub.2 O, were 
placed in a beaker, the beaker was covered with aluminum foil, and placed 
in a furnace at 975.degree. F. After 10 minutes, all of the compound had 
disappeared. An ammonia smell was detected. This compound does not have 
the thermal stability needed for pack nitriding of steel. 
EXAMPLE 2 
Five grams of Hexamethylenetetramine (C.sub.6 H.sub.12 N.sub.4) were placed 
in a beaker, covered, and heated at 975.degree. F. After ten minutes, 0.27 
gms of a carbonaceous residue remained. A piece of steel heated with this 
residue was not nitrided. Similar results have been obtained with ethylene 
diamine (NH.sub.2 (CH.sub.2).sub.2 NH.sub.2) and formamide (HCONH.sub.2). 
None of these compounds has the necessary thermal stability. 
EXAMPLE 3 
Ten grams of guanidine carbonate were placed in a beaker, covered, and 
heated at 975.degree. F. After 30 minutes, 2.08 grams of residue remained. 
The residue (primarily melamine, (NH.sub.2 CN).sub.3, the trimer of 
cyanamide, according to "The Chemistry of Carbon Compounds", E. H. Rodd, 
editor, Elsevier Publ. Co., New York) is quite stable and does nitride 
steel. After 7 hours of heating, 1.97 grams of residue remained. If a 
piece of steel is placed in the container with the residue, the residue 
disappears more rapidly than without the steel. It is characteristic of 
nitriding with guanidine carbonate (or melamine) that almost no oxidation 
of the steel surface occurs unless there is substantial air leakage into 
the container. 
EXAMPLE 4 
Ten grams of Urea (NH.sub.2 CONH.sub.2) were placed in a beaker, covered, 
and heated at 975.degree. F. After 30 minutes, 0.49 grams of residue 
remained; after 4.25 hours, 0.35 grams remained. The residue is primarily 
cyanuric acid (HCNO).sub.3, the trimer of cyanic acid, and cyamelide, 
isomeric with cyanuric acid, according to "The Chemistry of Carbon 
Compounds", E. H. Rodd, editor. The residue disappears more rapidly when 
it is heated in the presence of steel, which it nitrides. When steel is 
heated in the presence of urea, or its decomposition products, a loose 
reddish oxide (identified as Fe.sub.3 O.sub.4) forms on the steel surface; 
this oxide probably forms as a consequence of the breakdown of HCNO at the 
steel surface. 
The compounds which have been found to have suitable thermal stability for 
this process are (1) those which form polymers of cyanic acid, 
(HCNO).sub.n, when heated, e.g., urea and cyanuric acid, and (2) those 
which form polymers of cyanamide, (NH.sub.2 CN).sub.n, when heated, e.g., 
guanidine carbonate and dicyanodiamide. There are many organic compounds 
derived from urea, guanidine, or cyanamide which behave in this manner. 
Besides the appropriate thermal stability, suitable nitriding agents 
should not be highly toxic or potentially explosive, and should be soluble 
in water and relatively inexpensive. These requirements are met by urea, 
guanidine carbonate, dicyanodiamide and cyanuric acid. Compounds such as 
melamine and cyamelide, which are both insoluble in water, would be 
suitable nitriding agents if they are dispersed on the packing medium by 
some means other than a water solution.