CA1036912A - Heat treatment of ferrous metals in controlled gas atmospheres - Google Patents

Abstract

HEAT TREATMENT OF FERROUS METALS IN CONTROLLED GAS ATMOSPHERES

ABSTRACT

An oxygen bearing medium comprising oxygen and or a compound constituting oxygen, is mixed in combination with at least one of hydrogen and carbon, a hydrocarbon and a gaseous inert carrier forming the major component (by volume) of the mixture. The mixture is heated to bring about chemical interaction of the components to produce a carbon controlled atmosphere. For safety, the chemical interaction occurs preferably within the furnace itself whereas with conventional generators the gas atmosphere is piped to the furnace.

Description

193~ Z
This invention relates to the heat treatment of ~errous metals and more particularly to controlled atmospheres for the carburising, decarburising, neutral hardening or annealing and carbonitriding of steels.
There are three co~monly used on-site generators for producing protective or controlled atmospheres in heat treatment of metals applications. These are:
(1) Exothermic gas generators, which, depending on the fuel gas/air ratio and the degree of post partial combustion stage ancillary equipment, can produce gas atmospheres suitable as protection in many heat treatment applications for non-ferrous materials and ferrous materials containing low levels of alloying elements; (2) Endothermic gas generators, whose major area of application is in providing a carrier gas for carbon control processing of ferrous components: and (3) Ammonia dissociators, which provide a high fixed composition hydrogen contain-ing gas suitable for annealing/reduction of high alloyed steels and materials or where a high level of reduction is required.
An endothermic generator reqùirs a separate fuel supply for heating purposes and an electrical power supply for example for instrumentation.
It is an expensive machine which requires maintenance and occupies floor space. Further, such generators commonly have a rated output and this output is only ad~ustable within narrow limits. In practice, a bank of generators is used to supply gas atmosphere to a number of furnaces and when the gas atmosphere output exceeds requirements, for example when one furnace is shut down, rather than shut down one generator, which is considered uneconomical, the excess output iB wasted.
One object of this invention i8 to obviate the need for such generators, particularly endothermic generators, by synthesizing control atmospheres from bulk supplied or stored or pipeline gases. This provides a totally flexible system by which it is possible accurately to regulate the supply of high purity gas atmospheres in accordance with variable requirements. In addition, capital, operating and maintenance costs are reduced and non-production time, for example, required for regeneration of endothermic generator catalyst, is minimised.

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What we propose, therefore, is to miX an oxygen bearing medium comprising oxygen and/or a compound constituting oxygen in combination with at least one of hydrogen and carbon, a hydrocarbon and a gaseous inert carrier forming the ma~or component (by volume) of the mixture, and to heat the mixture to bring about chemical interaction of the components in the mixture, thereby to produce a carbon controlled atmosphere. We prefer the chemical interaction to occur within the furnace itself whereas with the conventional generator the gas atmosphere, which is highly inflammable and toxic, is piped to the furnace. There is thereby an improvement in the safety of operation.
According to the present invention there is provided a method of heat treat-ing ferrous metal in a furnace which method comprises the steps of mixing an oxygen bearing medium selected from ~he group; air, carbon dioxide, carbon monoxide;
water vapour and mixtures thereof; hydrocarbon; and an inert gas carrier; to produce a gaseous mixture, each 100 gram moles of which comprises between 2 and 7.4 gram moles oxygen (either as gaseous oxygen or in the form of carbon dioxide, carbon monoxide, water vapour and mixtures thereof); between 60 and 95 gram moles inert gas; and between a trace and 38 moles hydrocarbon containing 7.5 to 38 gram atoms of carbon ( the number of gram atoms of carbon in the hydrocarbon being greater than the number of gram moles of oxygen in the oxygen bearing medium) and deliver-ing said mixture to said furnace chamber which is maintained at or above 690 degrees centigrade and wherein the mixture reacts to form a carbon controlled atmosphere.
The inert gas will normally con~ist of the inert gas carrier except where the oxygen bearing medium is air, in which case the inert gas will consist of the inert gas carrler plus nitrogen from the air.
Where the inert gas carrier is nitrogen, each 100 gram moles of the mixture preferably contains between 70 and 95 gram moles of nitrogen and more preferably between 89 and 95 gram moles of nitrogen.
The hydrocarbon can comprise, for example methane or a higher paraffin. In thi~ connection each 100 gram moles of mixture preferably includes a trace to 19.4 gram moles of hydrocarbon containing 3,42 to 19.4 gram atoms of carbon, and more preferably contains between a trace and 12.7 gram moles of hydrocarbon containing 7.75 to 12.7 gram moles of carbon.

~, -3-10369~Z

Preferably, mixing of the components is effected at or less than ambient temperature and, if desired, the mixture may be preheated before injection into the furnace chamber, to a temperature not exceeding the temperature at which chemical interaction occurs between the components of the mixture.
The mixture is preferably chosen so that the carbon controlled atmosphere within the furnace contains between 3.9 and 10.7 per cent (by volume) carbon monoxide and more preferably between 3.9 and 8.2 per cent carbon monoxide.
The heat treatment concerned may be carburising, decarburising, neutral hardening or annealing or carbonitriding in which latter case, the mixture additionally contains ammonia and preferably between trace and 20 per cent (by volume) thereof.
In all these processes, control of the carbon potential of the control atmosphere is essential if reliable and reproducible results are to be obtained; that is to say, in order to obtain or maintain a desired surface carbon content and desired carbon distribution in the steel.
The term "carbon potential" as used herein indicates the carbon content to which that gas will carburise steel if equilibrium is reached it is customarily measured in percent of carbon in thin strips r~

~103~91Z
or <~-~ims of stf'el Wlli('i~ il<l\i'-' t)eerl br~ lg~lt, ~O sllbstanti~l1 equilibrium wi t,h tne '~a'; ~-tmOSI)i)er'e arlCl tl<lVe .1 SlltJSt,antia1 Iy uniform carbon corlterlt throughout. Ihlls, a yas having a carbon potential of 0.80 percerlt at 1 (. would be in equilibrium with steel containing 0.80 percent o~ carbon at TC., would carburise steel containing 0.70 percen-t of carbon at r c. and would de-carburise steel containing 0.90 percent of carbon at T C. Carbon potential is a func-tion of temperature, however, so that a gas having a carbon potential of 0.80 percent at rc . would have a carbon potential other than 0.80 at either a lower or a higher temperature.
In neutral heat treatment processes the carbon potential must be held equal to the carbon content of the metal surface.
By controlling the hydrocarbon/oxygen bearing medium ratio of the mixture, it is possible to regulate the carbon potential and thereby the migration of carbon as will be described hereinafter. Such control may be to maintain the carbon potential fixed throughout the heat treatment period or to vary the carbon potential during the period. The latter type of control is useful for a technique which we shall designate "layering-in". This involves setting the carbon potential to an initial level to provide a desired case carbon content profile and then changing the level shortly before the end of the run to produce a desired different carbon content, which may be higher or lower than that existing beforehand, at the metal surface.
By this technique, it is possible to achieve almost any desired case carbon content profile.

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Be(allse of'the b-llld up o~' rcsid~ arborl in the furnclce walls, herlting flernerlts arld work stll)r)ort, it is occasionally nececis~ry to "regerler,lte" the l'urnace by burning out the residu~l carbon. The traditional method of doing this in~olves complete shut down f'or an extended period and to enable less l`requent periods Or shut down, the level of residual carbon can be reduced by running the furnace empty but with an input gas mixture containing a controlled amount of oxygen bearing medium which is greater than the amount required for stoichemetry wi-th the hydrocarbon.
This produces an excessively de-carburizing atmosphere;
the oxygen reacting directly or indirectly with the residual carbon. To do this with a conventional endothermic generator system would involve the provision of a supply of oxygen or air not required for normal operation, which is costly.
In an installation operating according to the method of this invention, it is simply necessary to adjust the arnount of oxygen bearing medium in the mixture, as desired.
In all the heat treatment processes mentioned above there are five basic chemical reactions resulting from the introduction of the specified gas mixture in-to the furnace and these reactions are set out below:-I.G.C. + H.C. + ~o~ CO + H2 + I.G.C. (I)where I.G.C. represents the inert gas carrier;~o~
represents the oxygen content of the oxygen bearing medium;
and H.C. represents the hydrocarbon.
In addition to the specified reaction products, there may also be traces of C02 and H20. This reaction is 10~91~
c~rl irr~ versit)~ ),.r ~ ( Om~ t~ r .~ t i (,r, . r ~ i s designated a part~ comt~ustiorl reactior~ hecau!ie of' the Ihw oxygen (2) content rel~tive to ti-e hydrocarbor- (~I.C.) content. In f'act there is a considerable eXC'e!i'i of' hydrocarbon. 'I'he remaining reactions are:-excess H.C. ~ ' C + 112 (2) 2CO ~ ' CF ~~ ('2 (3) where CF is the carbon content of the metal surf`ace, 1-12 + CO ~ 20 + CFe 14) H20 + CO ~ C2 + H2 (5) Traditionally, the carburization process is considered to proceed in accordance with a forward movemen-t (i.e. to the right) in reactions (2), (3) and (4) whereas the opposite is true for decarburization processes.
The reaction (5) indicates the tendency to equilibrium within the furnace chamber.
Generally speaking, to adjust the hydrocarbon/
oxygen bearing medium ratio, is to adjust the carbon potential of the controlled atmosphere with the qualification that the hydrocarbon/oxygen bearing medium raio is never adjusted to a level where the amount of hydrocarbon is less than that required for reaction (1). One exception is, however, the technique of furnace "regeneration" described above which requires an excessively decarburizing atmosphere, i.e. an oxygen rich mixture.
It is preferred to use methane, in one form or another, as the hydrocarbon, but with hydrocarbons of any higher order, decomposition to carbon and methane will occur in addition to reaction (Z). The hydrocarbon may be pure methane, a component of town's gas or any higher order 103~91Z
hydrc~c.~rhor~ Jrlvf-rlier~t l~ nd f~r ecorlornic re~ Jr~
the methar-le is 1ntrodll~ed aS a componfrlt o~ natllral ~as which is preferill)ly preserlt in arl amour~l ol hetweerl a trace and ~0% of volume of the ingoing mixture, depending at least in part upon the he.-l treatmerlt process concerned. Generally, lower hydrocarbor-~~Levels are used in neutral hardening and other neutral heat treatment processes.
Thd inert gas carrier may be any gas which is inert with respect to the five reactions mentioned above and which does not contain elements detrimental to the quality of the metal, for example, it may be helium or argon or any other of the Inert Gases. The cheapes-t and most readily available inert gas carrier is nitrogen. As stated above the nitrogen (I.G.C.) is the predominan-t constituent of the mixture and, further, is preferably present in an amount of 60 - 95% by volume of the mixture. Molecular oxygen which may be introduced as a component of air, preferably constitutes between a trace and 20% by volume of the mixture and, in its combined form, may be introduced as a constituent of water vapour or carbon dioxide. Whereas carbon dioxide (C02) is equivalent to 2' a trace to 40% of wa-ter vapour is required to yield the equivalent oxygen content. It is preferred to use C02 as the oxygen bearing medium since this permits high surface carbon contents to be achieved with high nitrogen dilution, bearing in mind one of the desired objects viz. to improve the safety of operation.
The elevated temperature referred to above depends upon the composition of the ferrous metal to be treated, but, in general, woul,d be above the austenitic transformation temperature vi~. ctbovf~ f~l())( f~r - simple irorl-c.lr~)or alloy. In pra~ti(e, the maximllm temper.lt;~lre att/lirlecl ir-the course of heat tre/ltmerlt wou]d not ~xceed l150(, although it is conceivable that temperatures approactling the upper critical temperature and even the melting point of the metal concerned may be needed.
By way of example, the accompanying f`igure schematically illustrates an embodiment of` apparatus for preparing the mixture of gases required to produce the carbon controlled furnace atmosphere in situ. Each inlet pipeline lOa to d is connected to a separate gas source.
The pipeline lOa is for the inert gas carrier, in -this example nitrogen, pipeline lOb is for the oxygen bearing medium, in this example either agr or carbon dioxide, and -the pipe-line lOc is for the hydrocarbon, in this example natural gas (methane). The pipeline lOd is only used in carbonitriding processes and is connected to a source of ammonia. Each pipeline includes a stop-valve 12a to 12d, a gas pressure control regulator, 14 to d, a flowmeter 16a to d, a flow regulating valve 18a to d and a non-return valve 20a to d. The four pipelines are connected to a common pipe 22, in which mixing of the various components occurs and which supplies the mixture to a conventional furnace. The furnace may be any one of the wide variety of furnaces known in the art utilizing controlled gas atmospheres. In the case of continuous furnaces, however, separate blending or mixing systems such as shown in the figure may be used to introduce into different zones of the furnace mixtures of gas components which will react at the operating temperature of the furnace, to produce the _g_ "

1036~Z
different carbon potentials which may be required at certain stages of the heat treatment process.
The method of treating ferrous metal according to this invention and in particular the way in which the controlled furnace atmosphere is produced and regulated, is much simpler, more versatile and less costly than conventional methods. Furthermore, the results which can be achieved are comparable with known methods of heat treatment as illustrated by way of example in the carburizing test results shown in able I. In these tests, all of the steels used were case hardenable E.N.354, grade steels selected from~E.N. 35B, S.A.E. 8615/8617 and S.A.E. 8620, viz. steels which would respond in a comparable manner to carburization.
It should be noted that, in general, higher alloy steels require a longer time at the carburizing temperature in order to achieve the same depth of penetration and vice versa.
The test results may be divided into three basic groups according to their nitrogen dilution levels; the first runs 1 to 4 being of the order of 60 to 70 percent by volume, the second runs 5 to 7 being of the order of 70 to 80 percent by volume and the third runs 8 to 10 being of the order of 80 to 90 percent by volume. The tests involved, in each run, raising the temperature of the furnace to 925 degrees C while at the same time, passing the three componènt gaseous mixture therethrough.
After introducing the charge of steel components which caused a reduction in temperature to approximately 800 degrees C, the temperature was allowed to recover. Following recovery, the furnace was maintained at 925 degrees centigrade for a period of six hours during which the ingoing mixture 1031i91;~
was sur,pl i~d at the rate s~ in 'I`at)le r. I hf`
temper.l~Urr wa~ t~lrn re~ ed to 8~,0 ~ bef'ore rrm-val .Ind quenching of the st;cel comp~n~nts. Qllellrll:irlg Wcl'i e~'~'ecte~
in oil at a temperaturr- of' 110(. 'Ihe iloclcwell hardness (Rc) and visual etched rase depth were rnrasured be~'ore tempering.
Table I gives the composition ol' the ingoing mixture both in terms of the flow rate setting Or' the valves 18a to c (see figure) and as a percentage by volume of' the mixture. These flow rates and the total rlow rate of the ingoing rnixture are given in standard cubic fee-t/hour (S.C.F.H.). In the Table, "N.G." refers to na-tural gas and "O.B.M." refers to the oxygen bearing medium which for runs 1 to 7 is air and for runs 8 to 10 is carbon dioxide.
For each of the three basic groups of tests, it can be seen that the amount of surface carbon is increased by increasing the N.G./O.B.M. ratio. The carbon content case profiles obtained compare favourably with those which could be achieved using traditional carburizing methods.
Table II is the log of a single test in which a batch of piston pins weighLng 1600 lbs. and having a total surface area of approximately 300 ft. were carburized.
The pins were 2" outside diameter x 1" inside diameter x 6"
long and were made of steel designated A.I.S.I. 8620.
The object of the test was to achieve the following specification:-Surface harness - 56 to 62 Rc Case - 50 Rc min. to a depth of .040"
to .070"
- .070" to .100" total case depth - max 10~ retained austenite - maxm 5~ dispersed carbide lU3~91;Z
((,r~ - ~'5 t~ 4~
Ihe la~oratory test r-slllts perf->rmed on a se(ti()rled part indicated:-(a) Hardness Surface hardness = 59 RcCore hardness = 28 Rc.
(b) Metallographic rrotal case depth = .070"
Retained austenite (by point count) = 10%
No carbides or grain boundry oxides presen-t (c) Microhardness Depth Below Rockwell "C"
S face (inches) Hardness Remarks .006 58 .010 58 .020 56 .030 54 .040 50 Rc 50 min.
.050 46 (to meet .060 38 specified .100 29 requirement) .200 28 In this test the oxygen bearing medium was carbon dioxide and the hydrocarbon was methane. By tracing the process of the test it will be seen that by varying the CH4/C02 ratio of the input mixture, variations in the carbon potential as measured by the well known shim test can be achieved in order to meet a required heat treatment specification.
As examples of the application of the method of this invention to carbonitriding, the following tests were carried out:-Test I: An air motor cylinder of A.I.S.I. 8620steel was treated with the object of obtaining a min carbonitrided case depth of 0.025". The time/temperature/

103~Z
atmcsphere cycle wa~ ~IS ~e~ out hf'lC~W.

~as ~lc~w in S(,i~il ~H4 2 Step N2 (~l~ (~ N~13 Ratio __ __ .
1. Heat up to 900 C. 540

2. First 60 minutes at 900C. 460 67 13 40 5.1

3. Following 180 minutes at 900 C. 510 71 19 40 3.6

4. Last 36 minutes at 900C. 540 45 15 20 3.0 After quenching in oil. The resulting visual etched case depth was 0.032" and the surface hardness was 59Rc. The case profile was found to be:-Depth R Hardness .006" 57 .010" 58 .020" 54 .030" 51 Test II: A ball socket b~dy of A.I.S.I. 12L14 steel was treated with the object of obtaining a carbonitrided case depth of .003" to .005" and a surface which was file hard to Rc60.

Gas Flow in SCFH
CH4:C02 Step N2 CH4 C02 NH3 Ratio 1. Heat up to and first 20 minutes at 871C. 540 2. Following 12 minuOes at 871 C. 480 103 17 40 6.1 3. Last 8 minutes at 871C. 540 48 12 20 4.0 After quenching in oil, the visual etched case lO;~
depth wi~; c~ rl t~ )()5 ~o .()()6" itrl~l t~ rfil~ w.
f`ile h.lrcl to ~c60 ~IS re~J~Iired.
Ihe c.~se proi ile WilS f ourld to be: -Depth Rc llilrdness . 002 " 60 . 004 " 5 .006" 37

Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of heat treating ferrous metal in a furnace chamber which method comprises the steps of mixing an oxygen bearing medium selected from the group oxygen, air, carbon dioxide, carbon monoxide, water vapour and mixtures thereof; hydrocarbon; and an inert gas carrier; to produce a gaseous mixture;
each 100 gram moles of which comprises between 2 and 7.4 gram moles oxygen;
between 60 and 95 gram moles inert gas; and between a trace and 38 gram moles hydrocarbon containing 7.5 to 38 gram atoms of carbon, the said oxygen ranging from gaseous oxygen to a form comprising carbon dioxide, carbon monoxide,water vapour and mixtures thereof, the number of gram atoms of carbon in the hydrocarbon being greater than the number of gram moles of oxygen in the oxygen bearing medium, and delivering said mixture to said furnace chamber which is maintained at or above 690 degrees centigrade and wherein the mixture reacts to form a carbon controlled atmosphere.

2. A method according to claim 1 wherein the carbon potential is controlled by adjusting the volume ratio of hydrocarbon to oxygen bearing medium of the mixture.

3. A method according to claim 1 where the inert gas carrier is nitrogen.

4. A method according to claim 3 wherein each 100 gram moles of mixture contains between 70 and 95 gram moles of nitrogen.

5. A method according to claim 1 where the hydrocarbon is a paraffin.

6. A method as claimed in claim 1,3, or 5 wherein each 100 grams moles of mixture contains between a trace and 19.4 gram moles of hydrocarbon containing from 3.42 to 19.4 gram atoms of carbon.

7. A method according to claim 1 wherein the heat treatment is a carbonitriding process and the mixture is supplemented by a volume of ammonia so that the ratio of ammonia to (ammonia plus mixture) is less than or equal to 20 percent by volume.

8. A method as claimed in claim 1,3 or 7 wherein the furnace is heated to a temperature between 690 degrees centigrade and 1150 degrees centigrade.

9. A method according to claim 1,3 or 7 wherein the oxygen bearing medium is carbon dioxide.

10. A method according to claim 1,3,or 5 wherein the mixing is effected at or less than ambient temperature.

11. A method according to claim 1,2 or 3 and comprising the step of preheating the mixture to a temperature less than the temperature at which chamical interaction occurs between the components of the mixture.

12. A method according to claim 1, 3 or 5 wherein the carbon controlled atmosphere within the furnace contains between 3.9 and 10.7 percent,by volume, carbon monoxide.

13. A method according to claim 1,3 or 5 where the oxygen bearing medium is oxygen.

14. A method according to claims 2 or 3 wherein the carbon controlled atmosphere within the furnace contains between 3.9 and 8.2 percent , by volume, carbon monoxide.