METHOD OF USING POWDER METALLURGY FABRICATION FOR MANUFACTURING INTEGRAL HEADER AND TUBE REPLACEMENT SECTIONS

Abstract

A method of manufacturing integral head and tube replacement sections includes the steps of providing a reverse mold of a head and tube replacement section, providing an atomized steel powder, and filling the reverse mold with the atomized steel powder. The method further includes the step of inserting the mold into a hot isostatic processing (HIP) furnace to consolidate and sinter the powder into the shape of the head and tube replacement section.

Description

BACKGROUND OF THE INVENTION

This application claims the benefit of Provisional Application No. 61/489,505 filed on May 24, 2011.

This application relates to a method of manufacturing integral header and tube replacement sections.

Many fossil power plants were built for continuous base-load operation and are now beginning to see meaningful cyclic operation. Significant strains on components such as headers and high temperature piping are commonly associated with the cyclic practices often resulting in component degradation, cracking, and eventual failure of the component. Additionally, cyclic operation can result in thermal gradients at various locations along the length of headers which can lead to overheating and damage at these locations. When damage is encountered, utilities are often faced with the dilemma of replacing the entire header or removal of a short section (usually on the order of 3-8 feet in length) of the header.

Carbon or low alloy steel and stainless steel headers are commonly fabricated using either rolled & welded (R&W) plate sections or extruded pipe sections. Penetrations (or holes) are machined into the header at specified orientations around the header diameter and along specific lengths wherein stub tubes are then joined to the component. The stub tubes are joined to the header via various welding methods and processes, depending on the manufacturer. Due to the geometry of the stub to header weld, the welding is often performed manually and inspection is very difficult. Thus, failures in the weld or ligament cracking between penetrations are common failure mechanisms.

Acquisitions of replacement sections often require long lead-times, as much as 12 months or more, with the manufacturer resulting in de-rating of the plant and lost revenues until the replacement section or a new header can be obtained.

BRIEF SUMMARY OF THE INVENTION

These and other shortcomings of the prior art are addressed by the present invention, which provides a method of manufacturing integral header and tube replacement sections that provides a cost-effective alternative to the conventional manufacturing processes used for producing a header replacement, that provides for shorter lead-times, and that improves the overall quality of the header replacement section as a number of attachment welds (tubes to header) can be eliminated.

According to one aspect of the present invention, a method of manufacturing integral head and tube replacement sections includes the steps of providing a reverse mold of a head and tube replacement section, providing an atomized steel powder, filling the reverse mold with the atomized steel powder, and inserting the mold into a hot isostatic processing (HIP) furnace to consolidate and sinter the powder into the shape of the head and tube replacement section.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter that is regarded as the invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:

FIG. 1 shows an integral header to tube attachment according to an embodiment of the invention; and

FIG. 2 is a flow diagram of the method for manufacturing an integral header to tube attachment.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, an integral header to tube attachment section formed in accordance with an embodiment of the invention is illustrated in FIG. 1 and shown generally at reference numeral 10.

The present invention provides an alternative to the conventional manufacturing process which results in shorter lead-times and improved overall quality of a replacement section. It uses a combination of powder metallurgy (PM) and hot isostatic processing (HIP) to generate the replacement section that is near-net shaped with stub tubes produced as part of the overall PM/HIP effort. No welds are required to join the stub tubes to the header. With this process, the problematic weld joints are eliminated entirely and result in a much longer overall life of the integral header assembly. The technology is applicable to headers used in fossil plants, HRSG plants, and any other applications (chemical, petro-chem, pulp & paper) involving headers and header replacement.

The HIP/PM technology eliminates the rolled & welded or extrusion manufacturing steps as a header section can be produced as one completed system. More importantly, it eliminates the joining of the stub tube to the header as the stub tube and header are integrally manufactured in one continuous PM/HIP process. Referring to FIG. 2, the process involves the design of an exact duplicate, Block 11, of the damaged header section including the short tube sections, FIG. 1, which can be obtained from drawings of the header. Next, a reverse mold (container) of the header section is generated, Block 12, in two halves (or more) from a carbon steel material that establishes the final shape of the header section. The mold is assembled together and then filled with an atomized low alloy steel powder to fill the mold, Block 13. Next, the mold is evacuated using a vacuum to eliminate any potential air pockets and then sealed via welding, Block 14.

The entire assembly is then inserted into an HIP furnace and brought to a high temperature and pressure (usually under an inert argon atmosphere) to consolidate and sinter the powder into the final shape of the header, Block 16. The assembly is maintained at the sintering temperature for a given period of time and then allowed to cool to room temperature, Blocks 17 and 18. Additional heat treatment will likely be required to bring the header to a normalized and tempered condition for service, Block 19. This final heat treatment can be performed in or out of the mold. Removal of the mold is required once the header has been allowed to return to room temperature, Block 20.

At this point, the header should be in a near-net shape (near final shape) condition. Some clean-up and grinding may be required to assure that the can, mold and any residuals are removed to obtain a final surface, Block 21. A couple of additional steps are also required at this point: 1) boring of the stub tubes to produce an inner penetration, Block 22, and 2) chamfering of the inside diameter of the bore regions, Block 23. Both of these operations are easily accomplished using CNC milling/boring operations.

It should be pointed out once again that the stub tubes are now an integral part of the header that requires no weld transition between the header and stub tube, a region of considerable problems in the past. Elimination of the weldment, removes thermal expansion concerns, potential fatigue and creep damage issues, and wedging that is often associated with the weld attachment of a stub tube. As an integral stub tube, only welds to attach the stub tube to the existing boiler tubes are required, significantly reducing future damage. Because the shape can be carefully controlled, repeatable smooth transitions between the stub and header are achieved reducing the potential for stress risers.

The foregoing has described a method for manufacturing integral header and tube replacement sections. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.

Claims

1. A method of manufacturing integral head and tube replacement sections, comprising the steps of:

(a) providing a reverse mold of a head and tube replacement section;

(b) providing an atomized steel powder;

(c) filling the reverse mold with the atomized steel powder; and

(d) inserting the mold into a hot isostatic processing (HIP) furnace to consolidate and sinter the powder into the shape of the head and tube replacement section.

2. The method according to claim 1, further including the step of subjecting the mold to a vacuum to eliminate air pockets.

3. The method according to claim 2, further including the step of sealing the mold to maintain vacuum.

4. The method according to claim 1, further including the step of bringing the HIP furnace up to a high pressure and a high temperature to consolidate and sinter the powder.

5. The method according to claim 4, wherein the HIP furnace is brought up to a high temperature and high pressure in an inert gas atmosphere.

6. The method according to claim 4, further including the step of maintaining the HIP furnace at a high temperature and high pressure for a pre-determined amount of time.

7. The method according to claim 1, further including the step of cooling the mold and sintered powder to room temperature.

8. The method according to claim 1, further including the step of heat treating the head and tube replacement section.

9. The method according to claim 1, further including the step of finishing the head and tube replacement section into final form.

10. The method according to claim 9, wherein the step of finishing includes the step of grinding the outer surface of the head and tube replacement section to remove any residuals and obtain a final surface.

11. The method according to claim 9, wherein the step of finishing includes the step of boring a stub tube of the head and tube replacement section to produce an inner penetration.

12. The method according to claim 9, wherein the step of finishing includes the step of chamfering an inside of bore regions of the head and tube replacement section.