Metal Science and Heat Treatment Vol. 38, Nos. 9 - 1 0 . 1996

UDC 621.78.062.3

PROTECTIVE PROPERTIES OF A NITROGEN ATMOSPHERE WITH AN ADMIXTURE OF NATURAL GAS

Yu. M . B r u n z e r t

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 6 - 10, September, 1996.

The present difficulties with energy supply in Russia require strict control of consumption of natural gas and power and observation of environmental restrictions. This necessitates of the development of economically and environmentally optimum protective atmospheres. For relatively small heat treatment furnaces the use of nitrogen as a carrier gas can be of some interest. In this case environmentally inexpedient exo- and endogen- erators with the corresponding expensive and complex systems for automatic control of the carbon potential become unnecessary. Foreign experience shows that nitrogen can be used by means of a rather simple technology that includes an imported cryostat with liquid nitrogen, a line for feeding natural gas, and the corresponding mixing and proportioning devices. In the present work the possibility of using nitrogen with an admixture of natural gas as a protective atmosphere for annealing is considered.

Heat treatment of steel with the use of a protective nitro- gen atmosphere containing gaseous hydrocarbons is a com- mon practice in some foreign countries [ 1, 2]. Interest in such an atmosphere appeared in the U.S. in the energy crisis of the 1970s. Nitrogen used as a carrier gas arrived from oxygen plants, which made unnecessary the construction of gas gen- erators for the production of exogenous gas or nitrogen-hy- drogen mixture ( - 4% H 2 ) and the corresponding investment. These expenses were especially considerable for metallurgi- cal works, where powerful gas-preparing installations re- quired the construction of large centralized separately located plants. It was very important that nitrogen cartier gas did not require natural gas for its production and was a secondary or side product of the production of oxygen plants.

In the present work 2 we determined the optimum amount of admixture of natural gas in nitrogen. For this purpose, foils of different steels were held at different temperatures in an at- mosphere of pure nitrogen with various proportions of natural gas in a laboratory muffle furnace with a precombustion chamber. Foils 0.1 ram thick were produced from steels fur- ther subjected to a heat treatment under experimental indus- trial conditions, namely, structural steels 25KhGSA (0.25% C, 1.0% Si, 0.9% Mn, i% Cr) and 30KhSNVFA (0.30% C, up to 1.5% Cr, Si, Ni, W, V), tool steel U8 (0.8% C), and vir- tually carbon-free commercial iron with an insignificant con- tent of impurity elements. A foil was placed in a special de- vice in the precombustion chamber, then the chamber and the communicating functional space were blown with pure nitro- gen, and an additive of natural gas was introduced. After this

I I. P. Bardin Central Research Institute of Ferrous Metallurgy, Russia.

370

the foil in the device was placed in the functional space heated to the specified temperature. After a 4-h hold the foil was placed again in the precombustion chamber, where it was cooled rapidly in the same atmosphere.

An analysis of the experimental results (see Table 1) has shown that due to the hold at 800°C in the atmosphere of pure nitrogen (98.98%) all steels were decarburized almost com- pletely. A 8 - 9% addition of natural gas made the atmos- phere virtually neutral at 750°C for carbon-containing steels and commercial iron, although the carbon content in them in the initial state was different (from 0 to 0.8%). A decrease in the amount of natural gas to 4% at this temperature changed the picture, namely, carbon-containing steels were decarbur- ized intensely. At 800°C the addition of 4% natural gas made the atmosphere neutral for structural steels (0.3% C), but the steel with 0.8% C was decarburized markedly. An increase to 8 - 9% natural gas hardly decarburized tool steel but caused some excess carburization of structural steels. At 850°C in the presence of 8 - 9% natural gas the atmosphere was indif- ferent for structural steels but exerted some decarburizing ac- tion on tool steel.

In industrial tests in the Krasnyi Oktyabr' Plant we used a three-chamber continuous furnace for soft annealing of rolled sheets (Fig. 1). The need for this work was caused by the ab- sence of any atmosphere other than the investigated one in the plant for this furnace. In the course of annealing, stacks of sheets were subjected to a cyclic motion over the roller hearth with stops and holds in each chamber. Metal was heated in the furnace with radiating tubes. The furnace had three cham- bers, namely, one for heating and holding (I), one for holding and regulated cooling (//), and one for jet cooling by a pro-

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TABLE l I 2 3

Steel Regime of annealing of foils lee, s , vol.% C, %

25GSA 8000C, 4 h 0.018

30KhSNVFA 0.017 U8 0.011

25GSA 800°C, 4 h 4 0.26

30KhSNVFA 0,3 I

U8 0.44

25K.hGSA 800*C, 4 h 8 - 9 0.37 30KhSNVFA 0.49 Commercial iron 0.48

U8 0.71 25KhGSA 750"C, 4 h 4 0.017 30KhSNVFA 0.108

U8 0.180

25KhGSA 750°C, 4 h 8 - 9 0.25 30KhSNVFA 0.35 Commercial iron 0.016

U8 0.71 25KhGSA 850°C, 4 h 8 - 9 0.35 30KhSNVFA 0.35 Commercial iron 0.09

U8 0.62

Notation: Vo.s ) volume fraction of natural gas, C) content of carbon in the foil a / ~ annealing.

tective atmosphere (nitrogen) to the temperature of the shop (111). The last chamber was equipped with air-cooled tubes. Thick sheets of alloyed and carbon structural steels hot-rolled and pickled (in order to remove the scale) were annealed. Af- ter rolling (before annealing) the steel was somewhat decar- burized, but the decarburization was within the standard value for annealed steels. In order to cool the metal at a regu- lated rate the second chamber was equipped, like the third one, with air-cooled tubes (in addition to the heating ones). For all heat-treatable steels the standards and specifications envisage that the permissible depth of deearburization after annealing should not exceed 0.20 - 0.25% of the thickness of the sheet on each side. The requisite softening of the steels was controlled by the hardness, the strength parameters, and in some cases (additionally) by the elongation. The carrier gas was nitrogen with a purity of 99.98% that arrived from the units for separation of air in the oxygen plant. In addition, natural gas was fed into the functional space. The flow rate of the gas in the zones, the composition of the gas in the func- tional space, and its moisture content were controlled (the lat- ter by the dew point).

When testing nitrogen atmosphere with natural gas ad- mixtures we studied various temperature modes of moderat- ing annealing: complete, incomplete, and subcritical.

Results of experiments with underannealing are pre- sented in Fig. 2 and can be used for determining the optimum amounts of the natural gas admixture to nitrogen. The an- nealed metal has the form of thick (6-mm) sheets of high-al- loyed structural steel with 0.25% C and up to 1.5% Cr, Si, Ni,

Protective Properties of a Nitrogen Atmosphere with an Admixture of Natural Gas 371

N2 N 2 CH 4 N2 5

Fig. 1. Diagram of a three-chamber continuous furnace with a roller hearth and of the inlets for the components of the protective atmosphere and the cir eulating flows: 1 ) charging gate; 2 ) knuckle; 3 ) heat gate; 4 ) rollers; 5 ) dis- charging gate; L 11,111) numbers of the furnace chambers.

CH4, voi. % vol. % n~

1.

1.

4.0

3.ol& ,.oi/,V o

I.C ~ O.

0f 4 6 8

CO vol. %

/0, / 0.4

l / / J ~ o.2 /

6 8 4 6 Vu.s, vol.%

005\ o

td. p ,oc

-20'

-30

-50 4 6

td.p

~ h

Vn.g, VOI.*/Q

Fig. 2. Composition of the furnace atmosphere and dependence of the dew point ta. p and the decarburization depth hd,xaco in steel 25KhSNVFA on the volume fraction of natural gas added to an atmosphere of pure nitrogen:x) the process temperature is 720°C; O) 750°C; O) 780"C.

W, and V. With increase in the amount of natural gas in the furnace atmosphere the concentration of active reducing components (H 2 , CO) increased. For a large content of natu- ral gas it was close to (but still somewhat lower than) the con- tents of H 2 and CO 2 in an atmosphere of purified and dried lean exothermic gas. An 8% addition of natural gas had an almost full effect on decarburization in the process of heat

372 Yu. M. Brunzel'

90o*c

/sh !05, I 900°C

I I I I

1

b 650°C

4h 1 _ H i l l -~

Fig. 3. Regimes of true annealing: a) annealing with decelerated cooling; b) isothermal mmealing; L 11,111) ambers of the furnace chambers.

treatment. With increase in the amount of natural gas the dew point (tct p ) changed continuously toward decreased moisture contents. In the presence of more than 8% natural gas carbon black appeared in the functional space and on the sheets.

We also studied the parameters of a nitrogen atmosphere with added natural gas in true annealing (Fig. 3), i.e., at tem- peratures higher than those considered above. We annealed thick-sheet alloyed structural steel of grade 30KhSNVFA of martensitic class. The addition of natural gas to the furnace amounted (on the basis of the previous experiments) to 6 - 8% of the fed pure nitrogen.

In true annealing by both studied regimes the rolled stock was not decarburized in the process of heat treatment, and the hardness and mechanical properties of the metal corre- sponded to the requirements of the standard.

It should be noted that the temperature of 900°C at which the steel was annealed is a characteristic hardening tempera- ture for many structural steels. The fact that we obtained posi- tive results in preventing decarburization under conditions of true annealing allows us to conclude that the atmosphere em- ployed can be used successfully for hardening structural steels.

We conducted a spheroidizing soft annealing of high- carbon steel (0.85% C) in a nitrogen atmosphere with 8% natural gas by two regimes, namely, (a) a hold at 760 - 740°C and slow cooling (underannealing) and (b) a hold at 720°C (subcriticai isothermal annealing). Both regimes provided the requisite softening of the steel. It should be noted that the high-carbon steel with 0.85% C annealed at a low (subcritical) temperature was characterized by a hardness of 229 - 170 HB

and a decarburization depth within 0 - 0.25% of the thickness

of the sheet, which met the requirements of the specifications. In addition, the process of spheroidizing annealing was inten- sified at a low (subcritical) temperature. The duration of the subcritical annealing was reduced by 50% compared to the underannealing (10 and 21 h, respectively). Presumably, this can be explained by the effect of the intensification of the dif- fusion mobility of carbon discovered in [3, 4] and hence the intensification of carbide coagulation at a subcritical tem- perature. It is important that annealing at a lower temperature increases the service life of the radiating heating tubes.

An analysis of the results obtained has shown that the op- timum amount of added natural gas is about 8% of the con- sumed nitrogen, which imparts protective properties to the at- mosphere formed in the furnace in a wide range of annealing temperatures and carbon concena'ations in the heat-treated steel. An optimum addition of natural gas creates in the fur- nace an atmosphere providing a low rate of carbon mass transfer in the atmosphere-metal system, i.e., creates a viau- ally indifferent atmosphere. The preparation of such "stabi- lized" nonequilibrium atmospheres with a low rate of carbon mass transfer opens up new possibilities in the development of protective atmospheres.

Such an approach to the preparation of protective atmos- pheres differs from the conventional methods, for example, attainment of an equilibrium carbon potential in the atmos- phere. In the case of a kinetically inactive stabilized atmos- phere its carbon potential does not have technological value, because in slow mass transfer of carbon it can be attained only for hold times exceeding the duration of actual anneal- ing processes.

It can be seen from the data presented that decarburiza- tion can be limited only in the case of a low moisture content in the furnace (t,tp = - 30°C or lower). This requires a high tightness of the furnace space. Since we used a furnace with a gate opened for charging, the condition of airtightness was violated periodically.

In the initial "charging" period (Fig. 4) the composition of the atmosphere in the furnace worsened due to the intro- duction of oxidizing components together with the charge. The reccurrence of charging was determined by the duration of the entire heat treatment. We could trace the kinetics of the variation of the concentration of the oxidizing components of the atmosphere (H20, CO 2 ) after charging a stack of metal sheets with opening up of the gate. It can be seen that td. p changes from - 40 to - 8°C and the concentration of CO 2 in- creases considerably. Then the protective properties of the at- mosphere are recovered due to the arrival of fresh portions of dry pure nitrogen. In about 2 h the dew point attains the initial low value; here the regeneration with respect to CO 2 is com- pleted much earlier.

A favorable circumstance in the operation of this untight furnace was that the time of the heat treatment exceeded the period of the "conditioning" of the atmosphere after charging and that the period in which the atmosphere was worsened coincided with the period of heating the stack. In the general case, under other heat treatment regimes lasting for a time

Protective Properties of a Nitrogen Atmosphere with an Admixture of Natural Gas 373

comparable with the time of "conditioning," it would be im- possible to eliminate decarburization. It should be noted that the airtightness of the furnace was disrupted additionally due to a defect of its structure, namely, burnout of the radiative tubes. This hampered further industrial experiments. How- ever, when the airtightness was satisfactory, we annealed a considerable amount of metal in this furnace without decar- burization and obtained a product meeting the requirements of the standard.

We can conclude that a nitrogen atmosphere with added natural gas can be used with a reliable result only in furnaces with an appropriate airtightness (for example, a muffle fur- nace with preliminary blowing of the functional space to- gether with the charge by pure nitrogen). In continuous fur- naces the suggested atmosphere can be used under the condi- tion that the charge is sluiced at the inlet into the furnace.

CONCLUSIONS

1. A nitrogen atmosphere with an 8 vol.% addition of natural gas protects structural and tool steels from decarburi- zation hi soft annealing (true, under, suberitical) conducted at 720 - 900°C.

2. The atmosphere formed in the furnace in heat treat- merit is characterized by a low rate of mass exchange of carb- on with the metal and is virtually indifferent from the stand- point of decarburization for steel with a carbon concentration ranging between 0.2 and 0.8%.

3. The aforementioned atmosphere can be used under the condition that the moisture content in the functional space of the furnace is maintained at a low level (the dew point is at most -30°C), which should be provided by the design of the furnace.

4. The use of a nitrogen atmosphere with added natural gas does not require generators of protective gas (for exam-

CO; CO 2 ; CH 4 ; H2, vol.%

1.5 X I

I.

-20 ~ , ~

-30 J ( ~""0.,.~ IL=.. -4~ I I I I

10 15 30 45 60 75 90 105 x, rain

Fig. 4. Measured composition of the furnace atmosphere (volume fractions of the components) after cl~rging steel sheets into the furnace. Before charg ing, the furnace temperature was 760°C.

pie, exothermic gas); herre the nitrogen from oxygen plants is quite suitable.

REFERENCES

1. Yu. M. Brunzel', "Controllable atmospheres," in: Results in Sci- ence and Technology. Metals Science and Heat Treatment [in Russian], Vol. 12, VINITI, Moscow (1978), pp. 143 - 181.

2. W. Hewitt, "Nitrogen-based atmospheres - a user's view," Metals Mater TechnoL, No. 7, 344 -348 (1983).

3. B. G. Sazonov, "Experimental diffusion activity of steel ifi the state of pretransformation," Metalloved. Term. Obrab. Met., No. 7, 13 - 15 (1990).

4. A. P. Gulyaev, "The pretransformation state in iron alloys," in: 7th. lnt. Congr. Heat Treat. Mater. (Moscow, Dec. 11 - 14 ), Vol. 1, Moscow (I 990), pp. 15 - 22.