TECHNICAL ARTICLE:

1. Introduction

When steel tubes are annealed at temperatures in the range 600-900°C there are reactions such as oxidation and decarburisation between the tube surface and the surrounding furnace atmosphere. The selection and control of the furnace atmosphere is crucial for avoiding negative effects of these reactions, and to enable the production of tubes with a high surface quality such as brightness and freedom from decarburisation.

2. The Way That the Atmosphere Influences Surface Quality

2.1. The Furnace Atmosphere Constituents

A furnace atmosphere principally has two major parts, a neutral gas and an active gas. In table 1 the active gas is divided into oxidising, reducing, carburising and decarburising gas components.

The neutral or inert part, normally nitrogen, serves to purge away oxygen or other disturbing 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 concentration.

As seen in table 1, the reducing effect from hydrogen (H2) is balanced by the oxidising effect from water vapour (H2O). In the same way the reducing and carburising effect from carbon monoxide (CO) is balanced by the oxidising and decarburising effect from carbon dioxide (CO2). A certain balance between oxidising and reducing and/or carburising and decarburising types therefore is the principle for a good atmosphere control.

2.2. Oxidation

The condition of iron – depending on temperature and on the atmosphere oxygen partial pressure – may become oxidised alternatively to FeO, Fe3O4 or Fe2O3, as shown in figure 1 (FeO, wustite, does not form below 570°C). With reference to the indications in the figure, the threshold oxygen partial pressures at the temperature 900°C are P1, P2 and P3 for the formation of Fe2O3, Fe3O4 and FeO respectively. In a furnace atmosphere with an oxygen partial pressure greater than P1 all three oxides will form. For obtaining a bright, oxide free surface after annealing, it is necessary that the atmosphere oxygen partial pressure is lower than P3.

Figure 1: Stability regions for the iron oxides as function of temperature and oxygen partial pressure (Logarithmic scale for the oxygen partial pressure)

Direct measurement of this oxygen partial pressure with use of an oxygen probe is the standard practice for atmosphere control for instance in furnaces for carburising and hardening. The atmosphere control practice for annealing is, however, to use an indirect determination of the oxygen partial pressure by analysis of the gas species CO and CO2 or H2 and H2O. The commonly used principle is that the atmosphere oxygen partial pressure, PO2, is proportional to the atmosphere concentration ratio of (Vol%CO2)/(Vol%CO) or (Vol%H2O)/(Vol%H2).

Some alloying elements like chromium (Cr), manganese (Mn) and silicon (Si), form oxides more easily than iron. It may, in some cases, be required to consider the selective oxidation of such elements.

2.3. Decarburising or Carburising

Figure 2 illustrates the effect on the surface microstructure from the use of:

a. A decarburising atmosphere, leading to ferrite formation, seen as the white areas in the upper part of figure 2a.
b. A neutral atmosphere, leading to an even microstructure all out to the surface, seen in figure 2b.
c. A carburising atmosphere, leading to pearlite formation, seen as the dark areas in the upper part of figure 2c.

Figure 2: Surface micrographs showing a) decarburisation, b) no decarburising, c) slight carburising

Normally the goal is to eliminate any decarburising. Decarburising from carbon dioxide, CO2, or from water vapour, H2O, occurs via the following decarburising reactions:

C + CO2-> 2 CO
C + H2O -> H2+CO
(C denotes carbon dissolved in the steel)

Carburising will occur as the opposite reaction of the decarburising reactions, for instance as,

2CO ->C + CO2

The decarburising/carburising power is determined by the carbon activity in the atmosphere, which is proportional to the ratios (Vol%CO)2/(Vol%CO2) or (Vol%H2O)/(Vol%H2)*(Vol%CO). A certain carbon activity can be directly recalculated to a corresponding carbon concentration in the steel. (The atmosphere carbon concentration is often expressed as the atmosphere carbon potential, which is more directly related to the carbon concentration in the steel.)

Atmosphere requirements for maintaining carbon neutrality are much stronger than for those avoiding oxidation. Table 1 illustrates this and shows that the required oxygen equilibrium partial pressure for carbon neutrality is lowered by orders of magnitude with increased carbon concentration in the steel. In other words, to maintain carbon neutrality, the reducing power of the atmosphere must increase drastically with increased carbon content of the steel.

Carbon monoxide, CO, may in the presence of oxygen be oxidised to carbon dioxide, CO2. Thereby the CO concentration will decrease and the CO2 concentration will increase. As a consequence the CO2/CO ratio and the reducing power of the atmosphere will decrease. If the CO concentration is kept high the ability of the atmosphere to successfully minimise the disturbing effect from oxygen is lowered.

Table 3: Required atmosphere CO2 concentrations at 900° for bright and carbon neutral annealing in atmospheres with varying CO concentration

Table 3 shows the effect on the required atmosphere CO2 concentration for maintaining carbon neutrality when the CO concentration of the atmosphere is varied. It can be seen that the required atmosphere CO2 concentration increases when the CO concentration increases. Therefore, the practical implication is that it will be easier to maintain carbon neutrality or non-oxidising conditions when the atmosphere has a high CO concentration. This principle is utilised for improved atmosphere control in the atmosphere system described in the next section.

2.4. Lubricant Reactions

Lubricants applied to the tube surface will vaporise, thermally decompose and react when the tubes reach the hot part of the annealing furnace. These lubricant vapours must be effectively removed out of the furnace. If not, there is a risk that stains or discolouration will occur on the tube surfaces after annealing.

By establishing an oxidising atmosphere, with respect to the lubricant vapours, in the first cold entrance part of the furnace it is possible to remove the lubricants by burning them off. However, the atmosphere must still be in a reducing state with respect to the annealed metal. This is a serious conflict between the requirement of an oxidising atmosphere at the inlet and a highly reducing atmosphere further into the hot zone of the furnace. It is solved in the first place by adjusting the gas inlets, especially for nitrogen, to ensure that the oxidising gas is flowing in a direction out of the furnace. Secondly, the atmosphere in the hot zone of the furnace is adjusted to a fairly high concentration of CO+H2 that yields an ability of the atmosphere to buffer the disturbance from the oxidising combustion atmosphere.

3. Atmosphere System

3.1. General Flow Considerations

A roller hearth furnace, schematically shown in figure 3, is the commonly used furnace type for tube annealing.

Figure 3: Schematic cross section of a roller hearth furnace with indication of the atmosphere flow pattern

Disturbances from air entering into the furnace at inlet and exit openings should be minimised. One precaution is to restrict the height of these openings to a minimum, so tubes can only just pass through. To avoid oxidation and to eliminate safety risks it must also be assured that air is not entering into the furnace through the inside of the tubes. Therefore gas must flow inside the tube in the direction going out of the furnace towards the furnace inlet. Correspondingly the gas flow should be in a direction towards the exit.

These flows are illustrated in figure 3. These flow directions will also assure that lubricants adhered to the tubes are purged as vapour out of the furnace, thus maintaining safety. In practice it can sometimes be successfully observed and controlled when lubricant vapour is exiting out of the tubes (see figure 4).

Figure 4: Oil vapours exiting from the inside of the tubes at the inlet of a furnace

The furnace inlet and exit are cold parts with a temperature below the safety temperature. It is a requirement to ensure that there is no risk of creating explosive gas mixtures at these points. This is achieved by letting inert nitrogen gas into these parts of the furnace in order to lower the concentration of the flammable gas constituents.

The total flow rate required is determined by the total furnace area open to air entry. Naturally the tube inner diameters make up the major part of this area, but there are additional open areas such as below the furnace rollers and openings and leakages at different parts and seals around the furnace. All these openings should be minimised in order to reduce the required amount of gas and to assure both safety and annealing quality.

3.2. Atmosphere Generation

There are three ways used to produce a furnace atmosphere:

1) Outside of the furnace in separate external atmosphere generators
2) In-situ in the furnace by the mixing of nitrogen and in-situ generated endogas or cracked methanol
3) A combination of methods 1) and 2)

By the use of separate gas generators for endo-, exo-, mono-gas and cracked ammonia, a fixed atmosphere composition is produced. The output flow rate can be varied only within certain limits.

Figure 5: Principal drawing of the CARBOCAT® catalyst

With the in-situ method it is possible to create widely varying atmosphere compositions and flows with respect to the position inside the furnace and to the alloy and product being treated.

The patented CARBOCAT® equipment, developed by Linde Gas, is an in-situ generator, illustrated in figure 5, in which 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 of piping between generator and furnace. Several CARBOCAT® generators can be installed in the same furnace.

The third method is a combination of nitrogen and an external endogas generator. Nitrogen based atmosphere systems, supplied by the industrial gas companies, and atmosphere generators, supplied by the furnace producers, were earlier seen as separate alternatives each with individual pros and cons.

The development is now moving in a direction of combining the two systems, taking the best out of each system, and bringing them together for optimal results.

Table 4 shows an overview of the applications for the different atmosphere production methods. Only atmospheres that have low CO2/CO concentration ratios, and that include the use of endogas or cracked methanol, are capable of accurate carbon neutrality control.

Exogas and nitrogen/hydrogen system can yield bright annealing results but cannot avoid decarburisation. In exceptional cases with very tight furnaces with almost zero level air ingress into the furnace, it may be possible to make carbon neutral annealing also with a low buffering atmosphere such as nitrogen/natural gas.

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

3.2. An Atmosphere System

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 6 describes the features of a CARBOFLEX® atmosphere system based on the combination of nitrogen and an external endogas generator.

Figure 6: The CARBOFLEX® gas system with nitrogen and endogas supply and
atmosphere control based on gas analysis

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 tailored to the alloy, product mix and location within the furnace.

The principles for carbon control, lubricant removal and safety – described in the previous sections – can be utilised. Specifically, the concentration of CO+H2 can be kept high in the hot furnace zone, where the reaction rate is high for both decarburisation and carburisation, and low at the cold furnace inlet and exit to ensure safety. At any time the total gas flow into the furnace is minimised by a closed loop atmosphere control, based on the parameters in the process recipe. External disturbances such as furnace load variations and air leakages are automatically compensated for.

The closed loop atmosphere control assures annealing quality results with respect to carbon control and freedom of oxidation or surface discolouration. There is a drastically reduced total gas consumption compared to exogas systems. Special idling programs are used to minimise gas consumption when there is no production.

The CARBOFLEX® control cabinet contains a PLC for control and a PC with a touch screen as the man/machine interface, seen in figure 7.

Figure 7: CARBOFLEX® control cabinet (left) and flow train (right) for an installation including the CARBOCAT® in-situ endogas generator

All controls such as setting the atmosphere parameters, recipe handling, alarm setting, start up and stop, calibration of analysers and viewing of actual furnace atmosphere data are made from the PC touch screen on the control unit.

With the system, historical logged data can be stored, viewed and evaluated for statistical process control. The system can be connected to a central computer supervision system and can be made remotely accessible via modem or via Internet.

4. Concluding Example

The Swiss company Rothrist, a high quality tube supplier, has after a few years operation using the CARBOFLEX® atmosphere system (with external endogas generator), experienced the following comprehensive benefits:

• Overall good experiences with respect to system operation, productivity and quality
• No surface decarburisation
• Incoming decarburised tube material can be decarburised
• Active carburising is possible for special requirements