VACUUM DEGASSING OF STEEL
Journal of the Institution of Certificated Mechanical and Electrical Engineers, South Africa by F.I. Waal and B.J. Lodewijks (Archive January 1975)
ABSTRACT
The paper gives the basics of the degassing of steel. It includes the history of vacuum degassing, some principles of vacuum metallurgy, benefits that can be achieved, the present state of the technology and expectations for the near future.
INTRODUCTION
The subject of degassed steel and steel alloys is very much in the process of development, notwithstanding the concept being nearly 80 years old. It is an extremely interesting subject. It can also be highly technical and theoretical. The steelmaker must keep a balance between the ideals of the theoretical metallurgist and the economically orientated businessman. For the consumer, as in all other walks of life degassed steel can be a waste, or it could be something essential. An attempt has been made to present the subject as simply as possible, without withholding essential information or knowledge.
HISTORY
The history of vacuum degassing is, perhaps surprisingly, already a long one.
As early as 1886 Aitken suggested that various plate defects might be avoided by treating liquid steel under vacuum and in this way removing gases, especially oxygen.
It was then already known that the oxygen in liquid steel was responsible for various non-metallic inclusions and that its solubility decreased under vacuum. Although Aitken made proposals for an arrangement, the necessary enthusiasm and knowledge to put the proposals into operation were lacking.
The first one to carry out degassing was Baraduc-Muller, who, in 1914, put a hood over a 12,5-ton ladle and evacuated under it. As the best vacuum obtainable was only 0,5 atmospheres only the upper layers of the steel were treated, and the results were not encouraging.
After the second world war, improvements in the capacity of mechanical vacuum pumps indicated that commercial degassing was a possibility.
Bochumer Verein A.G. was the first with an operational plant and during the 1950s some 200 installations followed.
A big step forward was evacuation by steam ejectors from the early 1960s onwards. This enabled the pressure to be reduced from 5 mm to 0,1 mm Hg, besides having greater capacity and being cheaper than pumps.
EARLY APPLICATIONS
The first application of vacuum degassing was on steel, used for large forging ingots. Those alloy steels were susceptible to hairline cracking, noticed on new but cold forgings, it was known that this was caused by the fact that cold steel is initially oversaturated with hydrogen. This hydrogen recombines at the steels grain boundaries, building up high local pressures. In alloy steels with low ductility, this could lead to small local cracks.
A hydrogen level of under 2,5 parts per million (p.p.m.) was found to be safe. This was, and often still is, reached by leaving the forging for a long time at 650°C during which time hydrogen diffuses out of the steel. For a 100 mm diameter forging this time could be 30 hours and for a 500mm diameter forging as much as 12-day Elimination of this costly heat treatment was thus the prime objective of the first vacuum degassing installations, and that they were successful in this was proven by their fast-growing popularity.
Those first installations were either of the ladle degassing type or the stream degassing type.
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With ladle degassing the teeming ladle is placed inside a chamber which is then rapidly evacuated. To ensure that all the steel is treated it is stirred by either an inert gas or by induction. Depending on ladle size and duration, the steel temperature drops 40 - 70°C. This necessitates superheating the steel in the furnace. After treatment, the steel teems in the normal way.
With stream degassing the ladle is placed 011 the vacuum chamber, which contains another "pony" ladle. The steel flows into the vacuum and immediately breaks up in drop- lets. Hydrogen removal is very good (-0,8 p.p.m. after treatment), but heat losses are large some 100°C.
Figures 3 b and c show variations of stream degassing, namely tap degassing and ladle-to-mould degassing. The latter prevents any hydrogen pick up until the steel is solid, but is practicable only for the larger type ingot, i.e. about 50 tons and more.
MODERN TECHNOLOGY
In the late 1950s, the picture changed an increased emphasis began to be placed on cleaner steels, especially as the demand for structural steels with a higher toughness, with emphasis on transverse properties increased. By then degassing was already established in the production of some rolling steels and had cleared the path for those needs.
Most inclusions in steel are non-metallic ones, except MnS formed by the deoxidation or "killing" of the steel.
They are mainly SiO2, Al2O3, and MnO and do not com-pletely float out to the slag. These inclusions adversely affected toughness and fatigue properties. By reducing the oxygen content of the steel prior to deoxidation with Mn, Si and/or Al, fewer inclusions are formed, and hence a cleaner steel result.
Although the ladle degassing and stream degassing tech-niques were quite capable of removing oxygen as well as hydrogen, it was felt that the much high production of clean steels - compared with low-hydrogen steels - did not tolerate the high-temperature losses of those earlier techni-ques. Therefore, in the early 1960s, two new degassing types became popular, the vacuum lift process and the circulation processes.
The vacuum lift process is usually called the DH-process as it was developed by Dormund-Horder in Germany.
The tube or nozzle is dipped into the ladle of steel to be treated, the chamber is then evacuated, and part of the steel is sucked into the chamber until a ferrostatic head of 1,4 m is reached.
By oscillating either ladle or vessel the steel flows in and out and the treated batches are mixed with the rest of the steel every cycle.
The circulation or RH-process, developed by Ruhrstahl and Heraeus, is basically the same with the exception that it has two nozzles and neither ladle nor vessel oscillates. Re-placement of the treated steel is here achieved by bubbling argon continuously through one of the nozzles, taking the steel up, which then flows down through the other nozzle.
Both processes need about the same time for a complete treatment, between 10 and 20 minutes. However, both have their vacuum chamber permanently at a high temperature, between 1400?C and 1500?C This is maintained by an elec-tric resistance rod in the upper part of the chamber. In this way temperature losses in the steel are minimized and vary between 10?C and 30?C when no additions are made.
This brings us to another feature of this process. Since they treat small amounts at a time and mix these treated amounts thoroughly with the batch, accurate addi?tions of alloys can make during degassing and their homogeneity in the finished steel is assured. For this purpose, each installation includes an elaborate alloy addition system, with bunkers, vibrators, a weigh hopper, a vacuum lock and timers, all controlled automatically. At present, most steel degassing in the world is done using these processes.
PRACTICE
The general practice is that after refining, that is removing most of the unwanted elements from the steel bath, the furnace taps and at the same time additions to the ladle are made, but not deoxidizing elements such as silicon and aluminum. The aim is to get all other elements close to specification, though on the low side. Before degassing starts; two or more samples are taken from the ladle and it is then hoped that all additions have sufficiently mixed to get a representative analysis of the steel.
After tills, the vacuum treatment starts. Initially, this is done without any additions, to keep the oxygen as free as possible and let it react with carbon.
The chemical reaction and its equilibrium equation are:
Only when enough oxygen has been removed (this can be seen on the pressure recorders) are additions made to trim the steel to specification.
BENEFITS
As already mentioned, the above technique has two big advantages:
- Cleaner steel, as nearly no oxides ate formed when making additions;
- Better accuracy in attaining the right specifications because trimming addition can be made.
Emphasis is laid on one or other advantage depending on the grade of steel being handled. Generally, with the. lower carbon steels only accuracy is important, while with the higher carbon and alloyed steels cleanliness is also important and they are more thoroughly degassed.
Vacuum degassing results in some other benefits, and among them:
- Savings on deoxidation elements.
- The possibility of making additions in a specific sequ-ence. This is important with certain grades to prevent interaction of elements.
- Thorough mixing, and thus homogeneity in the teem-ing ladle, both of analysis and temperature. This, to-gether with cleanliness, has resulted in a big reduction in the occurrence of cracks, both internal and surface.
Referring again to accuracy, this is especially important where silicon is concerned in the case of semi-killed steels. Here, the amount of oxygen, and hence the amount of sili-con, controls the steel between a broken surface condition on the one hand, and piping and thus lamination on the other.
To sum up; the benefits of degassed steel, from the view- point of the consumer, are as follows.
- No hairline cracking, even after a long time. This is important with high carbon and manganese steel (rail- steel) and certain high alloy steels.
- Clean steel, resulting in increased toughness and resis-tance against fatigue.
- Little or no cracks of any kind.
- Little or no lamination.
- Consistent quality. The first ingot of heat has the same analysis and properties as the last.
AN EYE TO THE FUTURE
Although vacuum technology has achieved a lot, this is not the end. New processes are already taking a share of the market. The general trend is not to improve vacuum degassing itself as such, but to reduce work in the furnaces and m this way increase production. Fortunately, this will also be accompanied by the improvement in quality.
The basic idea is to remove the refining stage from the furnace and do tills part of the process at the degassing plant, thus freeing the furnace for the next heat. This process is especially suitable for arc furnaces, where refining takes from 30 minutes to one hour. On special steels, this could be up to 6 hours.
The first of these vacuum-refining processes was the Swedish ASEA-SKF process.
Here refining is split up into degassing and heating, ac-complished by putting different lids onto the ladle. The ladle is stirred by induction, and therefore the analysis accuracy is much improved. A fresh slag gives excellent results in the removal of sulphur, and cleanliness can be brought to values only previously dreamed of commercially.
The newest processes combine degassing and heating and refining in one operation. Stirring is accomplished by blow-ing argon through the ladle. These processes give an even higher degree of cleanliness, as there is no pick up of oxygen during refining.
Whereas the VAD (Vacuum Arc Degassing) process is intended mainly for medium and high carbon steels, the VOD (Vacuum Oxygen Decarburization) process is intended for low carbon stainless steels, where the CO reaction is used to remove the last bit of carbon.
SUMMARY
In this paper, an attempt has been made to give users of steel an insight into degassed steel.
In the basic steel industry, it is often found that from a quality point of view consumers insist on steels, which ex-ceed their actual requirements. They pay for these steels. The same applies to degassed steels. On the other hand, a higher quality - and you now know some benefits obtained by degassing - in conjunction with proper design, could mean substantial savings in many applications.
Because the needs and applications of these special steels, as well as the effects of degassing, are so varied, it is of utmost importance that they are discussed extensively between steelmaker and consumer so that the correct steel is supplied to meet the consumer’s needs. It is hoped that this paper is a contribution to this goal.