Apparatus and method for welding steel boiler tubes

Apparatus and method for welding steel boiler tubes in a space restricted area using a pulsed-arc welding process. The process includes the use of a consumable insert ring having a T-shaped profile and flux material to provide deep weld penetration by a GTAW welding machine. Notches in the consumable weld ring allow for visual inspection of tube fit prior to weld.

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gas tungsten arc welding process for welding steel boiler tubes and more particularly, to a flux assisted insert welding that uses a consumable insert ring to substantially increase weld penetration.

2. Brief Description of Prior Art

Steel boiler tube replacement welds are currently done manually and generally characterized by tight tolerances on the radial clearance between the tubes which present special challenges. To allow the welder access to the joint and ensure complete penetration, the tube ends are prepped with a 37 degree bevel or other special end prep geometry. Due to the volume of material removed by the bevel process, multiple weld passes are required to complete the weld. As a result, the process requires a considerable amount of material preparation time and cost. Although manual welding can achieve acceptable weld quality, a highly skilled welder is required, which only exacerbates the complexity and expense of the welding process. Gas Tungsten Arc Welding (GTAW) using a flux for increasing weld penetration is known. U.S. Pat. No. 7,285,747 discloses an insert ring design used in the joining of steel tubes by a GTAW process. The '747 insert ring is used as a filler metal and includes a pair of grooves to retain a penetration enhancing flux while transporting and positioning the insert ring. However it was found that once the flux was applied to the '747 insert ring, the flux would dry before the ring was finally positioned. Once dried the flux had a brittle or “flaky” nature, and would fall off. As a result, the flux retention aspect disclosed in '747 adds more complexity to the process, results in severe welding defects, and causes quality control issues.

To the best knowledge of the applicants, a suitable, commercially practicable method has not been found for welding in a space restricted area, such as a boiler tube replacement application, using GTAW in combination with a consumable insert ring and where the use of a flux is desired in order to provide the penetration capabilities required by the design of the joint and the material properties.

As will be seen from the subsequent description, the preferred embodiments of the present invention overcome shortcomings of the prior art.

SUMMARY OF THE INVENTION

Briefly stated, the present invention is directed to an efficient method of welding thick section piping disposed in a tight space using a single pass. The process relates to a pulsed-arc welding process that includes the use of a consumable insert ring and flux to provide deep weld penetration by a GTAW welding machine.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with the present invention, a gas tungsten arc welding process for welding steel boiler tubes in a space restricted area is disclosed. More particularly, the described method relates to a pulsed-arc welding process for steel tubes that includes the use of a consumable insert ring and flux to provide deep weld penetration by a GTAW welding machine. Specifically, it will be noted in the following description that the present method discloses an efficient method of welding thick section piping disposed in a tight space using a single pass. In the broadest context, the process consists of components and steps configured with respect to each other so as to attain the desired objective.

The present invention is now exemplified by a particular embodiment which is illustrated in the accompanying drawings:

The method designated as numeral10for welding together steel boiler tube ends15and20of steel boiler tubes17and22includes in a preferred embodiment an orbital welder device12, a weld ring14and a flux material16. The orbital welder includes a weld head12A similar to any commercially available low profile weld head on the market. Other welding techniques well known to those of ordinary skill in the art can be used in the art.

In the preferred embodiment, the weld ring14is supplied in the form of a weld ring T-shaped profile. The T-shape insert ring dictates that the tube ends have a flat face to ensure good contact with the ring. This allows for a simple tube preparation when compared to the prior art process of adding a 37 degree bevel or other special end prep geometry when manual tube welding. As such, preparing tubes, both new and old, is a simpler process, which results in cost savings.

As illustrated, the T-shaped structure14defines an outer band14A in perpendicular relationship with an inner band14B. More particularly, the inner band14B extends from an interior surface14D of the outer band14A such that the outer band14A is perpendicular to the inner band14B. As best shown inFIG. 2the outer band14A includes at least one notch14C. As will be further discussed, in application, the at least one notch14C visually assures the tube ends15and20properly abut the inner band14B prior to welding.

Filler metal can be formed into the weld ring using commercially available material conforming to the composition of the base tube such as, but not limited to, INMs2, IN515 and IN521 materials. The weld ring14is appropriately dimensioned to slip onto each end of the tube ends15and20being welded together. The weld rings14shape also aids in joint alignment. Weld ring14can be formed by any convenient process such as machining, sintering, stamping and so forth.

Flux material16improves heat penetration in the weld, thus reducing the weld width which reduces the potential for sagging and other weld profile problems that commonly occur during attempts to do a single pass welding operation on thick walled components. By more efficiently directing the heat inward to reduce the weld pool spread, the welding operation uses lower currents for full penetration compared to the currents which would be required without the use of flux. Reduced current allows for welding with relatively smaller lower power weld heads and power supplies and also further aides in maintaining the high strength and corrosion resistance characteristics in the weld.

Flux is typically available in powder form, but in this case is mixed with a liquid carrier to form a paste that is applied. More particularly, for best results when welding on steel, a mixture of penetration enhancing flux with methyl alcohol is preferred. Other flux materials and liquid carriers known in the art may be used. Preferably, flux16is applied to the outer band14A of the weld ring14and on adjacent tube surfaces15and20. The liquid carrier evaporates and the flux remains loosely adhered to the ring14. Surface application as described facilitates the penetration enhancing characteristics of the flux.

FIG. 3shows the side view of the weld ring14with pipe end15inserted to a proper depth as seen in notch14C and with pipe end20partially inserted into the ring14. Visual inspection at the notches14C will ensure proper tube alignment and fit. If pipe end20were welded as shown inFIG. 3, with the gap “G”, it is likely to lead to an improper fit and weld. Preferably, and as shown inFIG. 2, the depth of the notch14C is approximately one-half (½) of the width of the outer ring portion minus one-half (½) the thickness of the inner ring portion. This relationship allows the user to inspect and verify that the end of a tube is in close contact with the inner ring portion14B of the weld ring14. That is to say the notch is approximately the depth to reach the inner ring14B portion. There can be any number of notches14C and the material lost in the outer ring14A will not adversely affect the resulting weld. The inventors have found that approximately three (3) notches14C in the outer ring14A will allow visual inspection and again, not adversely affect the resulting weld.

Alternatively, as shown inFIG. 5, notch14C′ of weld ring14′ extends the entire width of outer ring14A′. This relationship further allows the user to inspect and verify that the end of a tube is in close contact with inner ring portion14B′ of the weld ring14′. There can be any number of notches14C′ and the material lost in the outer ring14A′ will not adversely affect the resulting weld. The inventors have found that approximately three (3) notches14C′ in the outer ring14A′ will allow visual inspection and again, not adversely affect the resulting weld.

Further, as best shown inFIG. 1, the length “L” of the inner ring portion14B is at least the thickness of the pipe end and preferably a bit wider. In the preferred embodiment, length L is calculated as follows:
L=pipe end diameter on ring−a first value*(wall thickness+a second value)
where testing has determined the preferred value of the first value is approximately 2, and the preferred value of the second tolerance value is approximately 0.03.

For example, for a 2% tube: L=2.255−2*(0.180+0.03)=1.835.

Visual inspection at the notches14C will ensure proper tube alignment and fit.

Once the weld ring14is properly fitted between the tubes17and22such that the tube ends15and20abut the inner band14B of the ring14, each of the at least one notches14C is closed by tack welding. The tubes17and22are now welded together using GTAW welding from the outside of the tubes which uses any commercially available low profile weld head on the market. The welding is done in one pass due to the deep penetration provided by the consumable insert and the flux coated thereon. A second weld pass of lower temperature can be performed if required or desired.

Referring toFIG. 4, the method400of joining tube ends15and20is shown. Once a weld ring is formed402the ring14can be placed404on a first tube end15and then a second tube end20is aligned406and fitted into the ring14. The tube ends15and20of tube section17and22are abutted together with weld ring14therebetween. The operator can inspect408using notches14C that the ends15,20are inserted entirely into the ring14so that the ends15,20rest against the inner band14B. There can be one set of notches14C or several around the perimeter of the ring14. Once the weld ring14is properly fitted between the tubes17and22as discussed, each of the at least one notches14C is closed by tack welding410.

Flux paste16can be applied412to the weld ring14an Orbital welder12is used to form a single pass welding414operation. The weld uses a continuous weld segment with pulsing of current and travel speed to achieve complete penetration. The advantages of pulsing are increased penetration on the high current and the avoidance of puddle sagging on the low current.