AU621230B2

AU621230B2 - Heat treatment of metals

Info

Publication number: AU621230B2
Application number: AU57592/90A
Authority: AU
Inventors: Robert Franks; Colin John Precious; Paul Francis Stratton
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1989-06-22
Filing date: 1990-06-18
Publication date: 1992-03-05
Anticipated expiration: 2010-06-18
Also published as: JPH03166343A; ZA904677B; CA2019565A1; KR910001073A; AU5759290A; GB8914366D0; EP0404496A1

Description

See reverse side of this form for guidance in completing this part.

Application are as follows: The BOC Group plc are the assignees of BOC Limited whose right to the invention is by virtue of Section 39(1) of the Patents Act 1977 in the United Kingdom.

4. The basic Application() referred to in paragraph 2 of this Declaration was/were the first Application made in a Convention country in respect of the invention, the subject of the Application.

DECLARED Wndleham, Surrey, United Kingd -th i 1s I. d yo '9 0 .4: fiy COMMONWEALTH OF AUSTRALIA FORM

PATEN'

COMPLETE

TS ACT 1952 SPECIFI I N FOR OFFICE USE: Class Int.Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority: Related Art: 09 4 Name of Applicant: Address of Applicant: o Actual Inventor: 4* 4 THE BOC GROUP PLC Chertsey Road, Windlesham, 6HJ, England Surrey Robert Franks, Paul Francis Stratton and Colin John Precious Address for Service: SHELSTON WATERS, 55 Clarence Street, Sydney Complete Specification for the Invention entitled: "HEAT TREATMENT OF METALS" The following statement is a full description of this invention, including the best method of performing it known to us:- 1 1C.v""Z; 1. i a j i i,

;I

t- I- la- HEAT TREATMENT OF METALS This invention relates to the heat treatment of metals. In particular, it relates to the heat treatment of metals which are less readily oxidisable than iron. Such metals include cobalt, nickel, lead, copper, palladium, silver and gold, alloys of such metals, and alloys of mercury.

In manufacturing articles made of metals less oxidisable than iron, it is typically desirable to subject such articles to the step of annealing.

Although the articles are relatively difficult to oxidise, it is still nonetheless necessary to maintain a reducing or non-oxidising atmosphere in the furnace used to perform the annealing operation. It is known that in O° 0) theory nitrogen may be used to form an atmosphere that is inert for the B purposes of annealing. Typically, the nitrogen may be supplied from a o 0o source of nitrogen which has been separated from air by distillation at cryogenic temperatures and need only contain parts per million of reactive o 0 impurities such as oxygen. Such nitrogen can be used in a heat treatment °o0 shop as the atmosphere for a range of different heat treatments. In recent years, it has been found that there are certain economic advantages in producing the nitrogen on the site of its use by non-cryogenic means rather than off-site by a cryogenic distillation process and then transporting the o a nitrogen product to its site of use. There are two main ambient o O920 temperature methods which may be used to separate nitrogen from air. The first is by pressure swing adsorption which entails adsorbing oxygen from 0 the air on an adsorbent to produce a nitrogen product and then periodically 940£ regenerating the adsorbent by subjecting it to a pressure lower than that at which adsorption takes place. The alternative method is to separate air by means of semi-permeable membranes. Known semi-permeable membranes suitable for the separation of air permit oxygen to diffuse through them at a much more rapid rate than nitrogen with the result that the non-.permeare gas becomes enriched in nitrogen.

Such non-cryogenic methods are able to be used to produce a nitrogen product containing in the order of 1% by volume of oxygen more cheaply than cryogenic methods may be used to separate nitrogen from air, provided the cost of transporting the cryogenically produced nitrogen to its site of use is taken into consideration. For many industrial processes the fact that the non-cryogenically produced nitrogen contains in the order of 1% of -2 oxygen as an impurity is not a drawback. However, we have found that in the annealing of metals less readily oxidisable than iron, such an oxygen concentration is indeed a drawback. Although at first sight it may be thought that a suitable annealing atmosphere may be produced by mixing hydrogen with the non-cryogenically produced nitrogen, in for example the annealing of copper, which takes place at temperatures of 400 to 800°C, the reaction between hydrogen and oxygen proceeds sufficiently slowly at temperatures in at least the lower part of this temperature range for there to be a subsLantial risk that oxidation of the copper will still take place notwithstanding the addition of a stoichiometric excess of hydrogen to the atmosphere.

0 u i o It is known to purify nitrogen containing about 1% by volume of oxygen by o" o subjecting it to a process in which the oxygen is first catalytically 0o 'o reacted with hydrogen and then the resulting water vapour is adsorbed by j o" means of an adsorbent or getter. The reliance on an adsorption step to B. purify the nitrogen requires the use of a kind of apparatus in which there 00 a ou, are two parallel adsorption stages, one of which is used while the other is regenerated, so as to make possible continuous production of the purified gas. The need for the adsorption stage adds considerably to the capital «B O and running cost of the apparatus and tends to eliminate the economic "00 advantage that would otherwise result from the production of nitrogen °o on-site by non-cryogenic means.

o 0o 0 0 There is thus a need for a method and apparatus of annealing articles of metal less oxidisable than iron which renders the use of non-cryogenically produced nitrogen containing an appreciable quantity of oxygen impurity o attractive from the economic point of view.

0 0i According to the present invention there is provided a method of annealing an article of a metal less oxidisable than iron, comprising the steps of subjecting the article to an annealing temperature in a heat treatment furnace, separating nitrogen from air at the site of the annealing furnace to produce a gas mixture containing at least 95% by volume of nitrogen and a minor proportion of oxygen impurity, catalytically reacting the oxygen impurity with a stoichiometric excess of hydrogen to form water vapour, and passing the resulting gas mixture comprising nitrogen, water vapour and unreacted hydrogen into the furnace to create an annealing atmosphere.

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3- Instead of using a stoichiometric excess of hydrogen the precise stoichiometric amount of hydrogen as required by the reactions: 2H 2 02 2H 2 0 may be employed. Accordingly the resulting gas mixture then contains no oxygen and no hydrogen.

The invention also provides apparatus for annealing an article of a metal less oxidisable than iron comprising an annealing furnace, means for separating a gas mixture comprising at least 95% by volume of nitrogen and oxygen impurity from air, and a catalytic reactor for catalytically o reacting the oxygen impurity with a stoichiometric excess of hydrogen to form a gas mixture comprising nitrogen, water vapour and unreacted 00 a hydogen, wherein said reactor has an outlet in communication with the o o 0oo annealing furnace so as to enable a suitable annealing atmosphere to be 00 a created in the furnace.

0 000 The method and apparatus according to the invention are particularly suited for the bright annealing of the metals less oxidisable than iron.

0 0a Preferably, the nitrogen product contains 0.5-3% by volume of oxygen 00o0 upstream of its catalytic reaction with hydrogen. In general only a small oo if any stoichiometric excess of hydrogen is required. For example, in the bright annealing of copper at a temperature in the order of 600 0 C, suitable oo° annealing conditions can be maintained provided that the ratio of the partial pressures of hydrogen to water vapour in the annealing atmosphere does not fall below 1 x 10 The catalytic reaction preferably takes place over a platinum or palladium 0 o catalyst. Alternatively, copper or nickel catalysts may be used. The catalyst is typically heated by the reaction between hydrogen and oxygen to a temperature of up to 200*C- The method and apparatus according to invention will now be described by way of example with reference to the accompanying drawings. in which Figure 1 is a schematic diagram of apparatus for the bright annealing of -4copper, and Figure 2 is a schematic drawing illustrating apparatus according to the invention including a continuous mesh belt furnace.

In Figure 1 of the drawings, there is shown a plant 2 for separating nitrogen from air by pressure-swing adsorption. Suitable plants and apparatus for this purpose are for example disclosed in UK Patent Specifications 2073043A and UK 2195097A. The resulting nitrogen typically contains from 0.5 to 3% by volume of oxygen impurity. The nitrogen stream is passed through a catalytic reactor 4 in which its oxygen impurity is reacted with a small stoichiometric excess of hydrogen over a palladium or o o platinum catalyst at a reaction temperature. The resulting gas mixture o "0 comprises nitrogen, hydrogen and water vapour. It is admitted to the heat 0o, treatment furnace 6 which may be of a batch or continuous kind in which an o article made of copper or an alloy of copper such as bronze, copper-nickel, 0 or brass containing up to 15% by weight of zinc, is annealed by immersion 0 in an annealing atmosphere at a temperature of 400 0 -800°C for a period of time typically in the order of 10 minutes to 2 hours. By maintaining the ratio of the partial pressure of hydrogen to the partial pressure of water vapour in the atmosphere and hence in the nitrogen supplied from the 0 catalytic reactor 4 at a value not less than 1 x 10 it is possible to aoo maintain conditions in the atmosphere in which the copper is not 4" deoxidised. Therefore its bright surface is maintained during the o annealing.

Referring to Figure 2 of the drawings, there is shown a conventional continuous mesh belt furnace 10 having an inlet zone 12, 1 metre in length, a hot or heated zone 14, 5.67 metres in length, and a cooling zone 16, 6.86 metres in length. The furnace 10 is provided at its respective ends with baffles 18 or the like to impede the ingress of air from outside the furnace into its interior. The hot zone 14 and the cooling zone 16 have respectively inlets 20 and 22 for gas.

The inlets 20 and 22 are each alternatively able to be placed in communication for source 24 of gas mixture comprising hydrogen, water vapour and nitrogen. The source 24 comprises a plant 26 for separating nitrogen from air by pressure swing adsorption, and a catalytic reactor 28 .1 for reacting oxygen impurity in the nitrogen with hydrogen. A commercially available catalyst comprising palladium supported on alumina is used.

Typically, the catalyst is heated by the reaction between hydrogen and oxygen and the gas mixture leaves the reactor 28 at a temperature in the range of 50 to 150 0

C.

In operation, a gas mixture comprising nitrogen, water vapour and hydrogen from the catalytic reactor 28 is passed into the furnace 10 through either or both the inlets 20 and 22. The hot zone 14 is heated to a chosen temperature typically in the range 500 to 800°C. Copper work to be bright annealed is loaded onto the belt (not shown) of the furnace and the belt is then advanced slowly through the furnace at a speed such that the work typically has a residence time in the furnace of from 30 minutes to 1 hour.

o S As the work passes through the hot zone 14 so it is raised approximately to O B the temperature of the hot zone 14. On leaving the hot zone 14 the work passes into the cooling zone 16 in which it is cooled by contact with the relatively cold atmosphere therein. Typically, the work is at a temperature between ambient and 50°C when it leaves the furnace 10. By employing an atmosphere in accordance with the invention, it can be ensured that bright annealed work leaves the furnace °oe o 0 A number of experimental tests of the apparatus shown in Figure 2 were performed. The results of the tests are set out in Tables 1 to 4 below.

0 0# Table 1 shows the results of comparative experiments in which various different proportions of hydrogen were mixed with nitrogen from a PSA plant oxygen impurity) and the resulting mixture passed into the furnace without a catalytic reaction between hydrogen and oxygen being performed.

It was a feature of the furnace used that entry to the hot zone was by way of a muffle made of an alloy with high nickel and chrome content which had the property of acting as a getter for free oxygen. Accordingly, it was found that substantially all the oxygen entering the furnace with the gas mixture was removed therefrom. It was therefore possible to obtain bright work (at 750 0 C) even when the oxygen level was nominally in excess of the stoichiometric requirement for reaction with hydrogen. Results are given in Table i for the oxygen concentration, hydrogen concentration and dew point in both the hot zone 14 and the cooling zone 16 of the furnace 10 at i, the different hydrogen levels.

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The results set out in Table 2 relate to a set of experiments similar to those of Table 1 save that the mixture of nitrogen, oxygen and hydrogen was reacted in the catalytic reactor 28 upstream of being admitted to the hot zone of the furnace through the inlet The Experiments illustrated in Table 3 and 4 are respectively comparable to those set out in Tables 1 and 2 with the exception that the gas mixture is not introduced directly into the hot zone 14 through the inlet 20. Rather, it is introduced directly into the cooling zone 16 through the inlet 22.

Table 3 shows experiment's in which the gas mixture by-passed the catalytic 30 reactor 28, whereas Table 4 relates to experiments in which the catalytic reactor was used. The low dew points obtained in the experiments Sol illustrated by Table 3 indicate that when the gas mixture is supplied directly to the cooling zone, a failure to react the oxygen with at least a stoichiometric quantity of hydrogen in the catalytic reactor 28 will tend a: to result in work that is not bright even if the hot zone is operated at a temperature as high as 7500C.

It must be further borne in mind that if there is used a furnace 10 which does not have a muffle that acts as a getter for oxygen and if the hot zone 14 is operated at a temperature substantially lower than 750°C, there is ?0 a substantial risk that copper work will not be given a bright finish.

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-7 TABLE 1 H 2 /0 2 IN 2 MIXTURE ADMITTED DIRECTLY INTO HOT ZONE H 2

ADDITION

BY VOLUME)

ZONE

(H HOT; C =COOLING) ATMOSPHERE COMPOSITION H2 DEW POINT 0

C)

40 4 o 40 I. 40 4000 4 04 40 4 o 04 04 4 044 4 10 04 4 400 0 44 44 4 404 4 1.45% 2.05 910mW 875mV 880mV 880mW 915mV 93OmV 935iiW 945mV 955mV 955mV 965mV 965mV 975mV 970mV 0% 0% 0.07% 0.1% 0.7 0.65% 1. 25Y.

1.3% -6 -26 b +6 -6 h +6 +9 +9 44,~4 4 4 0440 0 00 44 0 4 40 04 4,~00 4 04 00 0 0 0 040000 0 0 3.15 3.9 4.25 2.2% +10 i 2.1 91h 2.5% 2.6% 3.1% 3.1% +101h 10 h +11 TABLE 2 H2/02/N2 MIXTURE CATALYTICALLY REACTED UPSTREAM OF ENTRY INTO HOT ZONE H 2ADDITION BY VOLUME)

ZONE

(H =HOT; C =COOLING)

ATMOSPHERE

H 2

COMPOSITION

DEW POINT 0

C)

044 0 #40 0.7 1.45 2.05 7 l5nV 4Oppm 865rnV 845rnV 845mV 8 6OmV -21'h -29 845mV 0.15% 860mV 0.2% *090 0 0 4 4 4 :3.15 910mW 920mV 950mV 950mV 965mW 960mV 970mV M7 mW 0 9 5% 1. 2% 1.9% 2.05% 2.5% 2.6% 3.15% 3.1% 0 +12 +14 +11 121h +12 +12 +12 111h 4.25 -9- TABLE 3 H2/02/N2 MIXTURE ADMITTED DIRECTLY INTO COOLING ZONE H 2

ADDITION

BY VOLUME)

ZONE

(H =HOT; C =COOLING) ATMOSPHERE COMPOSITION H 2 DEW POINT 4ppni 4000ppm 0 0o 0 00 0 0 0 a 03 000 000 0 0 0 00 0.7 1.45 2.05 2.6 3.9 4.25 780mW ppm 850mV 56OmV 93OmV 900mV 945mW 93OmV 97 5mV 955mV 980mV 960mV 0% 0% 0% 0.45% 0 .15% 1.05% 0.75% 1.55% 1.4% 2.55% 2.65%- 3.4% 3.3% 4.2% -21 31 h +3 -29 9 V -24 61/2 -16 +7 -12% +7 121h +6 -12% I it It 10 TABLE 4 H2/02/N2 MIXTURE CATALYTICAjLY REACTED UPSTREAM OF ENTRY INTO COOLING ZONE 11ADDITION BY VOLUME)

ZONE

(H =HOT; C COOL1ING) ATMOSPHERE COMPOSITION H 2 DEW POINT Al 0 09 go 00 40000 0 09 00 00 00 0 4p pm 4500 ppm 815mV 950ppm -24 -36 0.7 1.45 2.05 2.6 825mV 0% 88OmV 0. 27/ 0 00 99 0 0000 3.15 895mW 9 2OmV 9 35mV 945mV 960mV 960mV 9 7 0,,W 970mV 960mW 980mW 0.55%.

0.7% 1.5% 1.65% 2.3% 2.2% 3.1% 3.2% 3.65Y%

MY.%

+6 -32Y2 131h 0 11 t- +8 +81h +8 +8 2 +8 0.

4 4.25 -i i I: 11

NOTE

Oxygen concentrations were measured using an oxygen probe. The temperature of the gas from the hot zone at the sampling point was 750 0 C: the temperature of the gas from the cooling zone at the sampling point was 200°C. Most results are shown in Millivolts The actual oxygen concentration can be calculated using the formula E 0.0215.T. loge (20.95/% 02) where E is the oxygen probe reading in mV T is -he temperature in Kelvin at the oxygen probe location.

%02 is the concentration of oxygen expressed as a percentage by volume.

8o a 0 0 0 8 00 00e 9 0 o0 a o 0 0 004 0 000* *0 4 tt

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Claims

1. A method of annealing an article of a metal less oxidisable than iron, comprising the steps of subjecting the article to an annealing temperature in a heat treatment furnace, separating nitrogen from air at the site of the annealing furnace to produce a gas mixture containing at least 95% by volume of nitrogen and a minor proportion of oxygen impurity, catalytically reacting the oxygen impurity with a stoichiometric excess of hydrogen to form water vapour, and passing the resulting gas mixture comprising nitrogen, water vapour and unreacted hydrogen into the furnace to create an annealing atmosphere.

2. A method of annealing an article of a metal less oxidisable than iron, B comprising the steps of subjecting the article to an annealing 0 temperature in a heat treatment furnace, separating nitrogen from air s 8 at the site of the annealing furnace to produce a gas mixture o« containing at least 95% by volume of nitrogen and a minor proportion of oxygen impurity, catalytically reacting the oxygen impurity with a S"o stoichiometric quantity of hydrogen to form water vapour, and passing the resulting gas mixture comprising nitrogen and water vapour into the furnace to create an annealing atmosphere. o ao

3. A method according to Claim 1 or Claim 2, in which the nitrogen product contains 0.5-3% by volume of oxygen upstream of its catalytic reaction with hydrogen. a6

4. A method according to any one of the preceding claims, in which the catalytic reaclton takes place over a platinum or palladium catalyst. t

5. A method according to any one of the preceding claims, in which the metal is cobalt, nickel, lead, copper, palladium, silver or gold, or an alloy of at least one of such metals, or as alloy of mercury.

6. A method according to any one of the preceding claims, in which the metal is bright annealed.

7. A method according to any one of the preceding claims, in which the resulting gas mixture is introduced directly into the cooling zone of a 13 continuous furnace.

8. Apparatus for annealing an article of a metal less oxidisable than iron comprising an annealing furnace, means for separating a gas mixture comprising at least 95% by volume of nitrogen and a minor proportion of oxygen impurity from air, and a catalytic reactor for catalytically reacting the oxygen impurity with a stoichiometric excess of hydrogen to form a gas mixture comprising nitrogen, water vapour and unreacted hydrogen, wherein said reactor has an outlet in communication with the annealing furnace so as to enable a suitable annealing atmosphere to be created in the furnace. 0 6 00 00 4 06 0

9. Apparatus according to claim 8, in which the catalytic reactor includes a platinum or palladium catalyst. DATED this 18th Day of June, 1990 THE BOC GROUP PLC Ki.L' I v. s 6 4P 41 I 4 i