Furnace Atmospheres for Tube Annealing

Rolf Andersson, Torsten Holm, Thomas Mahlo, Sren Wiberg

SPECIAL-EDITION

21. Introduction and basic principles

Metal tubes are annealed to increase their ductility making them easier to bend, machine and form. In the annealing process, the metal is heated above a critical temperature then slowly cooled to room temperature to obtain a softer and less distorted ma-terial structure. At these processing temperatures, unwanted reactions such as oxidation, decarburization and reactions with residual lubricants can occur between the tube surface and the surrounding atmosphere. Proper selection and control of the annealing atmosphere is critical to produce a bright, high qual-ity surface.

A tube annealing furnace must be long to accommodate the length of tubular products. Furnace inlet and exit openings must have heights that let the tubes with the biggest outer diam-eter pass. A roller hearth furnace, see figure 1, is the commonly used furnace type that meets these demands.

To minimize disturbances from air entering the furnace inlet and exit, the height of these openings should be the minimum that allows the tubes to pass through. To avoid oxidation and minimize safety risks, air must not enter the furnace either

long the outside or entering through the inside of the tubing. In addition to keeping the openings as small as possible, flame or mechanical curtains can be used to restrict the air ingress. The atmosphere flow pattern should direct the atmosphere gases through the incoming tubes and purge the air, moisture and residual lubricants from the inner diameter and along the outer surface, away from the furnace.

The main requirement on the furnace atmosphere is that it should be neutral with respect to the metal. Neutrality implies that the atmosphere composition must be controlled so that there are no net reactions between the tube metal surface and the three elements oxygen (O), to eliminate oxidation, carbon (C), to eliminate decarburization of carbon steels, and nitrogen (N), to eliminate nitrogen pick up in stainless steels. (Hydrogen may have to be regarded in special cases). As shown in figure 2, these three elements are present in the furnace atmosphere as the gas molecules oxygen (O2) and nitrogen (N2) but preferen-tially as atoms in other molecules like carbon monoxide (CO) water (H2O) or carbon dioxide (CO2).

Furnace Atmospheres for Tube Annealing

Figure 1: Schematic cross section of a roller hearth furnace with indication of the atmosphere flow pattern.

Heating zone

ExitInlet Cooling zone

Process gas

3Figure 2: Schematic illustration of the atmosphere/metal interaction.

N2CH4

CO H O2

C H3 8

H

O

N

C

H2

O2

CO2NH3

MetalGas/surfaceGas

A furnace atmosphere principally has two major parts, a neutral gas and an active gas. In table 1 the active gas is divided into oxidizing, reducing, carburizing and decarburizing gas compo-nents. It should be noted that for stainless steels nitrogen must be regarded as a nitriding and not a neutral gas. In certain rare cases for steels the risk of hydrogen uptake and consequent hy-drogen embrittlement must be considered.

The neutral or inert part (nitrogen) has the function to purge away oxygen or other undesired gases, to maintain a certain overpressure inside the furnace and to carry the active gas. The active gas has the function to maintain reducing conditions and in the case of steels also to maintain a certain carbon activity. As seen in table 1, the reducing effect from hydrogen (H2) is balanced by the oxidizing effect from water vapor (H2O). The re-ducing and carburizing effect from carbon monoxide (CO) is in the same way balanced by the oxidizing and decarburizing effect from carbon dioxide (CO2). The atmosphere control accordingly requires a certain balance to be kept between oxidizing and re-ducing or/and carburizing and decarburizing species.

The oxygen partial pressure threshold value, above which oxida-tion occurs, and below which the metal stays non-oxidized, is well known for each metal, and is expressed by the equilibrium oxygen partial pressure, PO2(eq), of the atmosphere. The par-tial pressure for each metal oxide is a function of temperature and specific values can be found in chemical handbooks, tables or databases. Quantitative data for the oxygen concentration threshold as a function of temperature are shown in figure 3 for copper (Cu), iron (Fe) and Chromium (Cr). Chromium is cho-sen because it is the alloying element in stainless steel that will oxidize most easily.

These very low oxygen partial pressures, of the order 10-6 to 10-30 atm as shown in figure 3, are unrealistic as measurable values in the sense that there are no free oxygen molecules to be measured at a partial pressure of the order 10-30 atm. However, although there is no free oxygen present, there are substantial and measurable amounts of oxygen bonded in the molecules CO, CO2 and H2O. By measuring and balancing the atmosphere ratios of CO2/CO or H2O/H2 it is therefore possible to maintain the atmosphere oxygen control.

2. Specific atmosphere requirements

2.1. For carbon steel tubes

To avoid the formation of iron oxide the oxygen concentration of the furnace atmosphere must be lower than the equilibrium concentration over FeO according to the diagram in figure 3. This equilibrium oxygen pressure and the corresponding ratios for CO2/CO and H2O/H2, expressed as volume concentrations, are given in table 2 for the temperatures 700, 800 and 900C. The compositions given in the table are those that should exist inside the furnace, thus after reaction between all incoming gases such as the supplied atmosphere, air entering via leakages and furnace openings and hydrocarbons from rolling lubricants adhered to the tubes.

Table 1: Atmosphere constituents and their action.

Neutral gas Active gas Reducing Oxidizing Decarburizing CarburizingArgon (Ar) Hydrogen (H2) Water vapor (H2O) Water vapor (H2O) Carbon monoxide (CO)Nitrogen (N2) Carbon monoxide (CO) Carbon dioxide (CO2) Carbon dioxide (CO2) Helium (He) Oxygen (O2) Oxygen (O2) Hydrocarbon (CnHm)

Figure 3: Relation between oxygen partial pressure (atm) and temperature for metal/metal oxide equilibriums for copper (Cu), iron (Fe) and chromium (Cr).

1,0E-33600 700 800 900 1000 1100 1200

Temperature C

Fe/FeO

Cr/Cr2O3

1,0E-30

1,0E-27

1,0E-24

1,0E-21

1,0E-18

1,0E-15

1,0E-12

1,0E-09

1,0E-06Cu/Cu2O

Oxy

gen

par

tial p

ress

ure

(atm

)

4Table 2: Atmosphere equilibrium oxygen partial pressures, PO2 (eq) and corresponding volume concentration ratios of CO2/ CO and H2O/H2, to yield a bright surface on pure iron. The CO2/ CO concentration ratios are given for an atmosphere having 5 volume-% CO and the H2O/H2 concentration ratios are given for an atmosphere having 10 volume-% H2

Annealing wt% C in PO2 (eq) CO2/CO H2O/H2temp., C the steel atm. 700 0 2 1022 0,60 0,33 800 0 1 1019 0,52 0,43 900 0 2 1017 0,50 0,58

If the steel tubes are going to be used as parts subjected to mechanical load, fatigue or wear, which is the case for tube ap-plications in a car, then there is an additional requirement on the atmosphere to be neutral with respect to carbon. Table 3 gives the theoretically calculated oxygen partial pressure of an atmosphere required not only to produce non-oxidizing condi-tions with respect to iron but also to maintain carbon neutrality for steels with different carbon concentrations.

Table 3: Atmosphere equilibrium oxygen partial pressure, PO2 (eq), and corresponding CO2/CO and H2O/H2 volume concen-tration ratios to yield a bright surface and no decarburization. The concentration ratios are given for an atmosphere having 5 volume-% CO and 10 volume-% H2. (The calculations are valid for a steel in the austenitic phase)

Annealing wt% C PO2 (eq) CO2/CO H2O/H2temp., C in steel atm. 800 0,1 3 10-21 0,090 0,075 0,3 2 10-22 0,023 0,019 0,5 7 10-23 0,014 0,011 900 0,1 5 10-20 0,13 0,029 0,3 5 10-21 0,008 0,0092 0,5 1 10-21 0,004 0,0041

When comparing table 3 with table 2, it is seen that the required oxygen equilibrium partial pressures for carbon neutrality is lowered by orders of magnitude with increasing carbon concen-tration in the steel. In other words, to maintain carbon neutral-ity the reducing power of the atmosphere must increase with increasing carbon content of the steel.

2.2. For stainless steel tubes

Avoiding the formation of chromium oxide when annealing stainless steels will require several orders of magnitude lower oxygen partial pressure than avoiding the formation of iron oxide. This was illustrated in figure 3. Hydrogen based atmos-pheres therefore must be used. Dew point analysis is commonly used to monitor and control the reducing power of the atmos-phere.

If an atmosphere containing nitrogen is used it is in some cases necessary to regard the risk of nitrogen pick up in the surface. If the nitrogen concentration is above the solubility limit, meaning that nitrides are precipitated, then a negative effect on corrosion resistance will be obtained. Figure 4 shows that the nitriding effect can be appreciable when annealing is made in cracked ammonia. The degree of nitriding is determined by the atmos-phere nitrogen activity, aN, which is proportional to the square root of the atmosphere nitrogen partial pressure, PN2. Although the nitrogen volume share in cracked ammonia is only 0,25, its nitrogen activity is 0,5, thus twice the nitrogen share.

Figure 4: Analyzed nitrogen concentration profiles in stainless steel strip after annealing in cracked ammonia and hydrogen atmospheres respectively.

2.3. For copper tubes

Pure copper is a fairly noble metal with high resistance against oxidation. It means that annealing to a bright surface can be done in nitrogen containing a low hydrogen concentration (

5Flow Train

Heating zoneCooling zone

ControlCabinet

Nitrogen

Endogas

Analysis

Natural GasAir

Natural GasAir

Endogasgenerator

Nitrogen

3. Atmospheres systems

3.1. For carbon steel tubes

CARBOFLEX is the name of a family of atmosphere systems with the capability of carbon control. The major atmosphere composition constituents are nitrogen (N2), hydrogen (H2) and carbon monoxide (CO). Figure 5 describes the features of a CARBOFLEX atmosphere system based on the combination of nitrogen and an external endogas generator.

One strength of the CARBOFLEX system is the flexibility to mix the incoming gas streams of nitrogen and endogas into widely varying ratios to produce atmospheres specifically tai-lored to the alloy, product mix and location within the furnace. Specifically the concentration of CO+H2 can be kept high in the hot furnace zone, where the reaction rate is high for both decar-burization and carburization, and low at the cold furnace inlet and exit to prevent carbon pickup.

The CARBOFLEX control cabinet contains a PLC for control and a PC with a touch screen as the man/machine interface, seen to the left in figure 6. All controls such as setting the at-mosphere parameters, recipe handling, alarm setting, start up and stop, calibration of analyzers and viewing of actual furnace atmosphere data are made from the PC touch screen on the control unit. The system creates a historical record that can be viewed and evaluated for statistical process control and ISO

compliance. The system can be connected to a central computer supervisory system and can be remotely accessed via modem or via Internet.

The patented CARBOCAT equipment, developed by Linde Gas, is an in-situ generator, illustrated in figure 7, in which the endogas is produced directly inside the furnace chamber. Major benefits are: 1) the elimination of a cooling device for the gas, 2) energy savings, 3) floor space reduction and 4) elimination

Figure 5: The CARBOFLEX gas system with nitrogen and external endogas supply and atmosphere control based on gas analysis.

Figure 6: CARBOFLEX control cabinet (left) and flow train (right)

6Catalytic material

Protective gas

Burner

Off-gas

Air (combustion)

Fuel (combustion)

Natural gas/air(protective gas)

Furnace wall

Protective gas

of piping between generator and furnace. Several CARBOCAT generators can be installed in the same furnace for customized atmospheres in different furnace zones.

Results obtained with the CARBOFLEX atmosphere system with external endogas generator or with the CARBOCAT in-situ generator (see below) are:

Precise carbon control, which has resulted in decarburiza-tion free annealing to meet strong quality requirements.

Drastically reduced total gas consumption compared to ex-ogas systems. This is because the flow rate is minimized at any time, based on the atmosphere composition required to maintain the different set points in each moment. External disturbances such as furnace load variations and air leak-ages are automatically compensated for.

Optimized recipes with different flows are at hand to adapt flow and atmosphere composition to tube dimension and tube qualities.

Special idling programs are used to minimize gas con-sumption when there is no production.

The system eliminates substantial costs and manpower for rework of oxidized, decarburized and discolored tubes

Process visualization, recipe handling, historical data col-lection and alarms are appreciated functions.

3.2. For stainless steel and copper tubes

HYDROFLEX is the name of a family of atmosphere systems with hydrogen (H2) and nitrogen (N2) as major constituents. A

typical HYDROFLEX system set-up for bright annealing of stainless steels is shown in figure 8. If nitrogen pick up is to be avoided a 100 % hydrogen system or an argon/hydrogen system should be selected.

A HYDROFLEX nitrogen/hydrogen atmosphere control sys-tem for copper annealing has the same principle layout as the one shown in figure 8 but with only 1-2 volume-% hydrogen in the gas mixture and with oxygen control in the cooling zone. The control cabinet for a copper annealing system is shown in figure 9.

Figure 8: HYDROFLEX nitrogen/hydrogen system for bright annealing

Figure 7: Principal drawing of the CARBOCAT catalyst

Hydrogen

Flow Train

Heating zoneCooling zone

ControlCabinet

Nitrogen

AnalysisNitrogen

7Results obtained with the HYDROFLEX atmosphere system are: The flow rate is minimized at any time based on the at-

mosphere composition required to maintain the different set points in each moment. External disturbances such as furnace load variations and air leakages are automatically compensated for.

The atmosphere buffering capability (the ratio H2/H2O) in-creases with increased percentage of hydrogen. Increased buffering capacity has the consequence that less gas is needed to maintain the atmosphere quality.

Elimination of ammonia handling and storage. Nitrogen pick up in stainless steels can be completely elimi-

nated by the use of hydrogen or argon/hydrogen mixtures. Optimized recipes with different flows are at hand to adapt

flow and atmosphere composition to tube dimension and tube qualities.

Special idling programs are used to minimize gas con-sumption when there is no production.

The system has eliminated substantial costs associated with scrapping oxidized tubes and manpower costs for rework.

Process visualization, recipe handling, historical data col-lection and alarms are appreciated functions.

4. Atmosphere generationThree common methods of producing furnace atmospheres for tube annealing are:

1) Outside the furnace in separate external atmosphere generators

2) In-situ in the furnace by the mixing and reaction of separate gases or liquids

3) A combination of the 1)- and 2)-methods.

Table 4 shows an overview of the applications for the different at-mosphere production methods. By the notation A=Applicable, used in the table, is meant practically applicable from both the technical and the economical point of view. Pure endogas gives

Figure 9: Copper tubes exiting an annealing furnace, left, and the atmosphere control panel, right.

the possibility of carbon control but endogas is unnecessary rich in carbon and also has high concentrations of the flammable gas species which makes safety precautions required at inlet and exit. It is therfore put within parenthesis in the table. Combin-ing endogas with nitrogen eliminates these drawbacks.

In situ atmosphere generation means that the gases and media are supplied from separate sources and in separate piping to the furnace, where the gases are mixed, reacted and synthesized in-side the furnace. This method can be combined with the use of endogas for carbon neutral steel annealing or with cracked am-monia for bright annealing of stainless steel or copper. An alter-native route to produce a CO + H2 atmosphere is by cracking of methanol according to the reaction: CH3OH = CO + 2 H2. This is also being done in situ in the furnace. By combining nitrogen with endogas or cracked methanol it is possible to produce al-most any N2/CO/H2 composition, thus also those obtained by endo-, exo- or monogas generators.

So CARBOFLEX and CARBOCAT offer a flexible atmosphere systems for tube annealing that accentuates the advantages of in-situ generated atmospheres without the inherent drawbacks.

Table 4: The use of different atmosphere generation methods. A = Applicable, - = Not Applicable

Principle of Method Carbon steels Stainless steels Copper alloys atmosphere Bright Carbon Bright No Pure Brass andgeneration control N-pickup copper bronzeExternal Exogas A - - - A - generators Endogas A A - - - - Monogas A A - - - - Cracked ammonia - - A - - AIn-situ Hydrogen - - A A - A Nitrogen/Hydrogen A - A - A A Nitrogen/endogas A A - - - - Nitrogen/methanol A A - - - - Nitrogen/Natural gas (A) (A) - - - - Argon/Hydrogen - - A A - -Combination Nitrogen/endogas A A - - - - Nitrogen/cracked ammonia A - A A A

4359

0980

03

04 -

1.1

ku

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