GB2229951A - Use of welding inserts - Google Patents

Abstract

A method of welding a water tube to a supporting tube plate in a feed heater employ the internal bore arc welding technique. The method comprises the step of locating between the tube plate 10 and an end of the water tube 11, prior to the arc welding operation, an annular insert 14, preferably a washer, of silicon iron. <IMAGE>

Description

HEAT EXCHANGERS This invention relates to heat exchangers, especially but not exclusively high pressure feed water heaters which are employed to pre-warm charge water before it enters boilers to raise steam.

For convenience and simplicity we will refer hereinafter simply to ~feed heaters.

A feed heater, as is well known to those skilled in the art, includes within its construction tube sheets supporting water tubes, the support being effected by welding the water tubes to the tube sheets.

The weld between a water tube and a supporting tube sheet is effected by a known special internal bore arc welding technique in which a rotatable welding head is inserted through the tube sheet, which may be, for example, between 12 and 24 inches thick, into the interior of the water tube, the weld between the latter and the tube sheet being made without the addition of filler metal since the welding technique employed does not facilitate such addition.

In older feed heaters, say those manufactureed 20 to 30 years ago for example, problems have been and are encountered with solidification cracking of the welds between the water tubes and the supporting tube sheets. The reason for this was the very high sulphur levels, of the order of 0.03 to 0.04%, used in steel making at that time.

Modern steel manufacturing techniques, in order to obviate or mitigate these problems, controlled the composition of steels with respect to the content of other elements employed in the steel making process, for example carbon, was kept low while manganese was maintained at comparatively high levels. Silicon, although only having a minor effect, was also kept low.

Due to advances in radiography, examination of the welds in feed heaters employing radioactive isotopes has shown them to be full of gas porosity resulting in a substantial number of unacceptable and rejected welds.

This, it will be manifest, is a major problem to manufacturers and/or repairers of feed heaters resulting in welding work having to be re-done with consequent increased costs.

The aforesaid gas porosity is the result of the chemical breakdown of oxides and silicates to produce free oxygen as a result of the arc during the welding process. This free oxygen, in the absence of sufficient de-oxidising agents, combines with steel in the weld pool to form carbon monoxide which nucleates as a gas pore.

It is known in conventional welding techniques that oxygen solubility can be reduced with consequent porosity prevention or reduction by adding to the weld pool via the welding comsumable, inter alia, silicon.

The Applicants employ this porosity prevention factor with the special internal bore arc welding technique to provide a novel solution to the major problem of gas porosity in the welds between water tubes and supporting tube plates in feed heaters.

According to the present invention there is provided a method of welding a water tube to a supporting tube plate in a feed heater employing the internal bore arc welding technique, the method comprising the step of locating between the tube plate and an end of the water tube, prior to the arc welding operation, an annular insert of silicon iron.

Preferably, the thickness of the annular insert is calculated to give a weld pool silicon content of 0.30% minimum, taking into account the base level silicon content of the steel to be welded.

Preferably, the annular insert is a washer having- a thickness of between 0.010 and 0.040 of an inch.

Preferably the washer has the same internal and external dimensions (diameters) as the water tube.

Preferably, the washer is made of silicon iron having a composition range of: Mn wt% Si wt% Balance 0.50 to 1.50% 1.50 to 4.50% Fe The effect of the silicon iron washer is to dilute the weld pool with respect to the carbon (reducing the tendency to weld metal tearing in high sulphur steels, while maintaining manganese levels (an element beneficial in mitigating the effect of sulphur) and, at the same time, increasing the silicon level to act as a deoxidant.

Also according to the present invention there is provided a feed heater incorporating a tube plate and water tubes welded together in accordance with the method defined in the preceding five paragraphs.

An embodiment of the present invention will now be described by way of example, with reference to the accompanying drawing which is a sectional view of a supporting tube plate and water tube of a feed heater illustrating the improved welding method.

Referring to the drawing, the supporting tube plate of the feed heater is indicated at 10, and a water tube at 11.

The improved welding method comprises four stages A to D.

The tube plate 10, as is usual, is formed with water passages 12, and, at each passage, a seating 13 (welding location) for the end of a water tube 11.

The improvement in the internal bore arc welding technique, according to the present invention, comprises the use of a metalurgical correction annular insert in the form of a silicon iron washer 14 within the composition range and thickness range defined above.

Stage A shows the relative dispositions of the tube plate 10, the washer 14 and the water tube 11 before assembly.

Stage B shows their assembled dispositions immediately prior to welding. The washer 14, which has the same internal and external diameters as the water tube 11, is disposed within the seating 13, and the end of the water tube 11 butts the washer 14 and extends into the seating 13.

Stage C shows a rotating arc welding head 15 extending through the tube plate passage 12 into the interior of the water tube 11, the location where the arc is struck being indicated at 16.

Stage D shows the completed weld 17 between the tube plate 10 and the water tube 11 from which gas porosity has been obviated or mitigated as a result of using the silicon iron washer 14.

As modern steelmaking techniques consistently produce sulphur levels of 0.010% or less the requirements mentioned above to prevent solidification cracking are no longer relevant.

However the silicon content of these modern steels remain marginally low with respect to weldability (porosity in autogenous welds).

Consequently the technique of using an insert according to this invention can be used with such steels to obviate or mitigate the effects of air disturbances of the argon gas shield and other ambient factors that inadvertently introduce 02 into the weld pool.

The welding method according to the present invention may be employed in other constructional applications similar to that described above.

As a result of the present invention the number of rejected welds is significantly and substantially reduced.

The metalurgical correction washer may be made from materials other than silicon iron and corrections similar to those specified above can be to the welding of other materials.

Claims (7)

1. A method of welding a water tube to a supporting tube plate in a feed heater employing the internal bore arc welding technique, the method comprising the step of locating between the tube plate and an end of the water tube, prior to the arc welding operation, an annular insert of silicon iron.

2. A method as claimed in claim 1, in which the thickness of the annular insert is calculated to give a weld pool silicon content of 0.30% minimum, taking into account the base level silicon content of the steel to be welded.

3. A method as claimed in claim 1 or 2, in which the annular insert is a washer having a thickness of between 0.010 and 0.040 of an inch.

4. A method as claimed in claim 3, in which the washer has the same internal and external dimensions (diameters) as the water tube.

5. A method as claimed in claim 3 or 4, in which the washer is made of silicon iron having a composition range of: Mn wt% Si wt% Balance 0.50 to 1.50% 1.50 to 4.50% Fe

6. A method of welding a water tube to a supporting tube plate in a feed heater employing the internal bore arc welding technique, substantially as hereinbefore described with reference to the accompanying drawing.

7. A feed heater incorporating a tube plate and water tubes welded thereto by the method claimed in any one of claims 1 to 6.