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Bright Annealing for Stainless Steel Tube

Date: 2014-07-08

When talking about annealing, there are dull and bright annealing systems for a wide variety of parts from engine parts to hand tools to wire and tube.

Bright and dull annealing of stainless steel wire, rod and tube is not a new subject, what is new is the change in technology to achieve the desired result. Most installations in the past used off-line annealing facilities because the technology for a compact installation in-line with the tube mill and welder did not exist. For bright annealing it was a conveyorised furnace using cracked ammonia for the process atmosphere and nitrogen for purging, later bottled or bulk hydrogen proved to be a better alternative as it removed the ever present problem of residual ammonia in the annealing atmosphere.

For dull annealing batch furnaces many gas fired were used and still are, quenching is by batch followed by pickling of the tubes to remove the surface oxide and improve the surface finish.

Market demand for better quality finish has forced both the tube manufacturers and equipment manufacturers to look for a better way of annealing tube. Most of the production is in the austenitic grades although some duplex steels and ferritic steels are annealed but the volume is quite small compared to the austenitic grades in the 300 series.

Of course the other important factor is competition in the stainless steel tube market globally which has brought to many manufacturers the reality their existing plant is inadequate technically and commercially

Induction heating technology is now accepted as the technology of choice for any high speed processing line or lines requiring compact heating combined with rapid response.

Examples of this technology adoption is hardening and tempering of carbon steel rod for spring wire and seam and full body annealing of carbon steel tube, quench and temper lines for pipe, steel bar and wire, copper tube annealing, steel strip coating both metallic and organic and stainless steel tube annealing. All of these examples have one common denominator – they are continuous and it the use of high speed induction heating technology that has made this possible.

METALLURGICAL REQUIREMENTS
Austenitic stainless steels are not hardenable but like most metals suffer from work hardening both during the manufacture of the tube and during later processes. Annealing which is also sometimes referred to as solution heat treatment not only recrystallises the structure but dissolves carbides back into the austenite. Annealing temperatures range from 1050oC to 1150oC although temperature can be altered according to the time at temperature and with induction heating this can be closely controlled. Time at temperature is not a critical issue, it can be too short and where the annealing is in-line this has to be carefully considered as there is a direct correlation between time and machine length.

In the case of dull annealing the time at temperature should be minimised to keep surface oxide formation to a minimum, with induction heating this is rarely a problem as the time at annealing temperature is measured in seconds before direct water quenching.

Bright annealing requires the use of a highly reducing gas atmosphere, hydrogen being the commonly used gas.

A further consideration of in-line tube annealing is the weld zone of the tube which is a continuous seam and becomes ferritic as a result of the welding process. It can be physically detected because of its response to a magnetic field.

As part of any in-line annealing process, it is essential the tube surface is cleaned thoroughly to ensure it is free of any grease, oil or other carbonaceous material before it enters the annealing system. Such residues will lead to surface carburisation and impair the corrosion resistance of the steel.

The annealing process also requires the quenching be rapid, particularly through the upper critical range to minimise carbide precipitation, with dull annealing this is not a problem but with bright annealing it must be carried out under the protective gas atmosphere which means the quench is indirect and slower.

PRACTICAL LIMITATIONS

Heating

For specific markets and applications the use of a quartz tube in the heating coil isolates the hydrogen gas atmosphere from the induction coil and creates a gas tight chamber, but at the same time is transparent to the induced currents from the heating coil. The down side to this approach is the size of the quartz tube relative to the product being heated and the resultant induction heating coil size which lowers the heating efficiency.

The alternative approach is a gas tight induction heating coil but it requires considerable care in manufacture with precision sealing of the enclosure and is generally more costly to produce than the quartz tube approach. It does however deliver a higher heating efficiency.

Quenching
In many if not all installations where the annealing is in-line with the drawing equipment or tube the mill it is not the heating that is the challenge, it is the quench. Where direct quenching using say water as the quenching medium is used for dull annealing there is no problem, however when an indirect quench has to be used such as with bright annealing of stainless steel because of the protective gas atmosphere the rate of heat transfer becomes a critical design element. The impact of indirect quenching is reflected in the overall length of the machine and can in some circumstances where space is limited, prevent the adoption of in-line bright annealing.

Gas System
For bright annealing systems there is one further limitation and that is the protective atmosphere gas required for the bright finish. The tube when annealed passes through a sealed system, so, providing the welding process is sound, mechanical seals at the entry and exit prevent contamination of the hydrogen gas atmosphere. This is important for a number of reasons:

First and foremost is safety, any entry of oxygen into the system and mixing with the hydrogen can create a hazardous situation.
Second, contamination can affect the surface finish of the tube but this is only on the outside of the tube.

For tubulars the inside surface of the tube is not exposed to the hydrogen atmosphere and must rely on a constant stream of argon gas to minimise oxidation not only during welding but through the heating and quenching phases of the annealing process. The practical limitation here is the volume of gas required to fill the tube which is open to the atmosphere at both ends of the mill line.

This limitation sets the upper diameter of tube that is practical to bright anneal in-line with the mill.

BRIGHT ANNEALING SYSTEM
These systems are by far creating the most interest in the market place primarily because they facilitate the production of better finish on the tube surface at much lower cost. Bright annealing of cut lengths of tube has been around for a long time using conveyor type furnaces and continuous in-line systems have been used by some manufacturers for ten years or more because they recognised the benefits would outweigh the costs.

In the last decade developments have taken place in the power supplies bringing to the line higher frequencies with higher operating efficiencies and the quench system technology has improved and will continue to improve with new materials and techniques.

The benefits of a bright in-line system are:

1. Wire, Rod, and Tube leaves the annealer with a bright outside surface requiring no further finishing.

2. Distortion is minimised. The wire, rod and tube is sized in line before it is cut to length thus enhancing the quality of tube produced.

3. Electrical energy is reduced when compared to the use of an electric resistance type furnace. If gas is the energy source for the annealing furnace other considerations must be taken into account:
3.1. Maintenance of refractories.
3.2. Maintenance of muffles or radiant tubes to contain the furnace atmosphere.
3.3. Maintenance of heating elements and connections.
3.4. Cost of furnace start-up time.
3.5. Cost of furnace idle time.

4. Work in progress is reduced.

5. Orders can be progressed faster and small orders filled more economically. (Subject to mill change over).

6. Can be integrated into an existing line. A typical system for tube up to 80mm diameter and subject to line speed has a line foot print of approx 6 metres x 0.75 metre plus the power supply and ancillary equipment which is usually mounted to one side.

7. The existing batch or conveyor type annealing furnace can be scrapped and the floor space released for other uses.

8. Water cooling systems are not contaminated with scale thus maintenance costs are lowered.

9. Labor costs are usually lower as the need to move bundles of tube from one processing cell to another are eliminated.

EQUIPMENT

Heating
The induction heating system for bright annealing is similar to that required for dull annealing in that the major components are the same but there the similarity ends.

The power supply is identical for both bright and dull annealing. Only the induction coil matching changes.

Earlier designs of the induction heating coil were sized to allow for a quartz tube to be inserted that isolated the refractory lined induction coil from the product passing through the centre of the quartz tube. The latest design utilizes an economical solution of a gas tight chamber providing an environment for annealing that is:

1. Impervious gas leakage at elevated temperatures.
2. It is not porous.
3. It can be readily sealed to the entry vestibule and dwell chambers by mechanical seals with expansion compensation.
4. It does not deteriorate with constant use.

From the heating coil the product tube passes through a dwell zone which is unheated but allows the recrystallization to complete and chromium carbides to be taken back into solution.

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