KR20120131124A - FUNCTIONALLY GRADED COMPOSITIONAL CONTROL METHODS TO ELIMINATE DISSIMILAR METAL WELDS DMWs DURING MANUFACTURE OF INTEGRAL HEADERS - Google Patents

Abstract

PURPOSE: A functionally graded compositional control method for eliminating dissimilar metal welds during manufacture of an integrated header is provided to remove the need for heat treatment after welding. CONSTITUTION: A method for manufacturing an integrated header comprises the steps of: preparing a reverse mold for a head assembly(21), filling a header section of the reverse mold with atomized low-alloy steel powder to form a header section, forming a tube section, and integrally melting the atomized powder in a high-temperature high-pressure atmosphere to form a header assembly. The tube section forming step comprises the steps of: filling a first portion of a tube section of the reverse mold with atomized low-alloy steel powder(22), filing a second portion of the tube section of the reverse mold with atomized steel powder, which gradually proceeds from low-alloy steel to austenitic stainless steel, to form a transition portion, and filling a third portion of the tube section with atomized austenitic stainless steel powder. [Reference numerals] (20) Designing reproduction header assembly; (21) Preparing reverse mold; (22) Filling mold with atomized powder; (23) Evacuating and sealing mold; (24) High-temperature high-pressure processing; (26) Maintaining at sintering temperature; (27) Cooling to room temperature; (28) Thermal processing?; (29) Removing from mold; (30) Washing/Grinding; (31) Boring; (32) Chamfering

Description

FUNCTIONALLY GRADED COMPOSITIONAL CONTROL METHODS TO ELIMINATE DISSIMILAR METAL WELDS (DMWs) DURING MANUFACTURE OF INTEGRAL HEADERS}

This application claims the benefit of Provisional Application No. 61 / 489,507, filed May 24, 2011.

The present application is directed to a method for manufacturing a header assembly that is free of DMWs and eliminates the need for post-weld heat treatment.

Many thermal power plants are built for continuous base-load operation and will now begin to explain important periodic operations. Significant deformations on components such as headers and hot piping are typically associated with periodic practices, which often lead to component degradation, cracking, and resulting component failure. In addition, periodic operation can result in thermal gradients at various locations along the length of the header, which can result in overheating and damage at such locations. If damage occurs, the facilities often face the dilemma of replacing the entire head.

Conventional header design and fabrication requires the manufacture of long, thick pipe sections, the holes being bored to accommodate the header stub tubes. Carbon or low alloy steel and stainless steel headers are typically manufactured using rolled and welded (R & W) plate sections or extruded pipe sections. The penetrations (or holes) are machined into the header in a particular orientation around the diameter and along a certain length, followed by a stub tube to the part. The stub tubes are connected by various welding methods and processes depending on the manufacturer. This typically results in a header assembly made entirely (header and tube) from low alloy steel (eg 2-1 / 4Cr-1Mo) or creep strength hardened ferritic steel (grade 91).

The second weld is required in the field to connect the header / tube assembly to the austenitic stainless steel boiler tube. In this application, dissimilar metal welds (DMWs) are applied between SS boiler tubes and low alloy or CSEF stub tubes. Normally, nickel-based filler metal is used to complete tube-to-stub tube welding. Unfortunately, this is a difficult welding to perform in the field and can lead to failures due to carbon migration and reduced creep strength problems inherent to DMWs over time at usable temperatures. One unique challenge for CSEF steel in this conventional approach is the need to perform on-site post-weld heat treatment (PWHT) on the stub tubes. Due to the proximity of the stub tube to the header, incorrect PWHT can damage the stub tube or the header itself.

These and other disadvantages of the prior art are solved by the present invention, which provides a method of manufacturing a header assembly that is free of DMWs and eliminates the need for post-weld heat treatment.

According to one aspect of the invention, a method of manufacturing a header assembly having a tube section and a header section for use in the connection of dissimilar metal between a boiler tube and a header assembly comprises the steps of: providing a reverse mold of the header assembly; Forming the header section by filling the header section of the reverse mold with atomized low alloy steel powder, and forming the tube section. Filling the first section of the tube section of the reverse mold with the atomized low alloy steel powder, the tube section progressively filling the reverse mold with a series of atomized steel powder from the low alloy steel to the austenitic stainless steel. Forming a transmission region by filling a second portion of the tube section; Filling the third portion of the tube section of the reverse mold with atomized austenitic stainless steel powder. The transition zone is disposed between the first and third portions such that the low temperature alloy steel powder of the transition zone is placed after the low alloy steel powder of the first portion and the austenitic stainless steel powder of the transition zone is placed after the third portion. do. The method further includes consolidating and melting the atomized powder in a high temperature, high pressure atmosphere to form a header assembly.

According to another aspect of the invention, a header assembly for use in connection between low alloy steel piping and austenitic stainless steel tubes includes a header section formed of low alloy steel and a tube section extending outward from the head section. The tube section comprises a first low alloy steel section, a second transition section, and a third austenitic stainless steel section for connection to an austenitic stainless steel tube, which are connected to a header section, wherein the second transition section is a first low alloy steel section. It is disposed between the alloy steel section and the third austenitic stainless steel section.

The main constructions regarded as the present invention can be best understood by referring to the following detailed description made in conjunction with the accompanying drawings.
1 shows an integral header to tube attachment according to one embodiment of the present invention; And
2 is a flow chart of a method for manufacturing a header assembly.

Referring to the drawings, a header assembly formed in accordance with one embodiment of the present invention is shown in FIG. 1, which is generally indicated by reference numeral 10.

The present invention utilizes an entirely new manufacturing technique to completely eliminate the need for DMWs in the header assembly. The manufacturing method applies functionally graded compositional control generated by powder metallurgy & high temperature isostatic processing (PM / HIP) to create a smooth composition transition from the stub tube alloy to the boiler tube alloy. In this approach the entire header assembly 10 is manufactured using a PM / HIP that includes a stub tube 12. However, the novelty of the approach is that of the steel powder, where the last one to two inches of the tube 12 is produced in a functionally sorted composition and gradually applied from the low alloy steel powder atomized to the atomized austenitic steel powder atomized. The transition section 14 formed by the continuous is used to change from the low alloy steel (or CSEF steel) section 13 to the austenitic stainless steel section 15. This is achieved through the use of control of the PM configuration to gradually transition the alloy from low Cr (2-1 / 4Cr or 9Cr) to 18Cr austenitic stainless steel boiler tube alloy. The use of this process eliminates the DMW normally required to connect the header tube assembly to the boiler tube. By producing a functionally classified monolithic header, all heat treatments can be performed in a controlled shop environment, eliminating the need for post-weld heat treatment in the field.

HIP / PM technology eliminates rolling & welding or extrusion manufacturing steps when header sections are produced as a complete system. More importantly, HIP / PM technology joins the stub tube 12 to the header 11 when the stub tube 12 and the header 11 are integrally formed in one continuous PM / HIP process. Remove it. Most importantly, classify the composition of the stub tube 12 into 18wt% Cr stainless steel, where a dissimilar metal welding (DMW), conventionally performed on site, can be welded from a low-chromium alloy steel or CSEF steel to a stainless steel superheater piping. by graded). With reference to FIG. 2, the process includes the design of an exact duplicate of the header section including the complete tube section (FIG. 1) except for a typical DMW joint that can be obtained from the diagram of the header (block 20). Next, a reverse mold (container) of the header section is created in two halves (or more) from the carbon steel material that establishes the final shape of the header section (block 21). The molds are assembled together and then filled with atomized low alloy steel powder to fill the mold (block 22). The construction of the stub tubes is classified by filling with a continuous layer of chromium content that decreases from 18% wtCr stainless steel to 9% or 2 1/4% Cr ferritic steel. The mold is then emptied using a vacuum to remove any potential air pockets and then sealed by welding (block 23).

The entire assembly is then inserted into a HIP furnace and subjected to high temperature and pressure (typically under an inert argon atmosphere) to integrate and sinter the powder into the final shape of the header (block 24). The assembly is maintained at the sintering temperature for a given period of time (block 26) and then allowed to cool to room temperature (block 27). The additional heat treatment will require the header to be in normalized and tempered state during service (block 28). This final heat treatment can be performed in or out of the can. When allowing the header to return to room temperature, removal of the can is required (block 29).

At this point, the header should be shaped to approximate the final product. Certain cleaning and grinding may be required to ensure that the cans, molds and any residues are removed to obtain the final surface (block 30). At this point a pair of additional steps are also required: 1) boring the stub tube to create an internal penetration (block 31), and 2) chamfering the inner diameter of the bore zone (block 32). . All of these operations are easily performed using CNC milling / boring operations.

It is once again pointed out that the stub tube is now one integral part of the header, where a weld transition between the header and the stub tube is not required in the prior art. Elimination of welds eliminates wedge fixation, associated with thermal expansion concerns, potential fatigue and creep damage issues, and often associated with weld attachments to stub tubes. As an integral stub tube, only welding for attaching the stub tube to the existing boiler tube is required, so that further damage is significantly reduced. Because the shape is carefully controlled, repeatable and smooth transitions between the stub and the header are made, reducing the potential for stress risers. Most importantly, the end of the stub tube has the same stainless steel composition as the superheater to be joined in situ. By functionally classifying the composition by the powder metallurgy method, the DMW joint is removed.

The foregoing describes a method for manufacturing a header assembly that is free of DMWs and eliminates post-weld heat treatment. While specific embodiments of the invention have been described, it will be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiments of the present invention and the best mode for carrying out the present invention are provided for illustration only and not for the purpose of limitation.

Claims (12)

A method of making a header assembly having a tube section and a header section for use in the connection of dissimilar metal between a boiler tube and a header assembly, the method comprising:
(a) providing a reverse mold of the header assembly;
(b) forming the header section by filling the header section of the reverse mold with atomized low alloy steel powder;
(c) forming the tube section,
(i) filling the first portion of the tube section of the reverse mold with atomized low alloy steel powder;
(ii) gradually forming a transition zone by filling a second portion of the tube section of the reverse mold with a series of atomized steel powder from low alloy steel to austenitic stainless steel;
(iii) filling a third portion of the tube section of the reverse mold with atomized austenitic stainless steel powder, wherein the transition zone is disposed between the first portion and the third portion to provide a low alloy of the transition zone. A steel powder is disposed after the low alloy steel powder of the first portion and an austenitic stainless steel powder of the transition zone is disposed after the third portion;
Forming the tube section by; And
(d) consolidating and melting the atomized powder in a high temperature, high pressure atmosphere to form the header assembly,
A method of making a header assembly.
The method of claim 1,
The integrated melting step is performed in an inert gas atmosphere,
A method of making a header assembly.
The method of claim 1,
Evacuating the mold to remove air pockets,
A method of making a header assembly.
The method of claim 3, wherein
Sealing the mold to maintain a vacuum,
A method of making a header assembly.
The method of claim 1,
Further comprising the step of cooling the mold and the integrated powder to room temperature,
A method of making a header assembly.
The method of claim 1,
Further comprising heat treating the header assembly;
A method of making a header assembly.
The method of claim 1,
Further comprising finishing the header assembly in a final form,
The finishing step is:
(a) grinding the outer side of the header assembly to remove any residue;
(b) boring the tube section of the header assembly to create an inner penetration; And
(c) chamfering the interior of the bored tube section
Made by
A method of making a header assembly.
The method of claim 1,
Inserting the mold and atomized powder into a high temperature isostatic processing furnace to integrate and melt the atomized powder;
A method of making a header assembly.
A header assembly for use in connecting between low alloy steel piping and austenitic stainless steel tubes,
(a) a header section formed of low alloy steel; And
(b) a tube section extending outwardly from said header section,
The tube section is:
(i) a first low alloy steel section connected to said header section;
(ii) a second transition section; And
(iii) a third austenitic stainless steel section for connection to the austenitic stainless steel tube, wherein the second transition section is disposed between the first low alloy steel section and the third austenitic stainless steel section; With 3 austenitic stainless steel sections,
Header assembly.
The method of claim 9,
The tube section is integrally formed with the header section,
Header assembly.
The method of claim 9,
The second transition section starts gradually with low alloy steel for mating with the first alloy steel section and gradually increases the chromium content for the austenitic steel for joining with the third austenitic stainless steel section. Formed by a continuum of steel applied as
Header assembly.
The method of claim 9,
Wherein the first alloy steel section, second transition section, and third austenitic stainless steel section form a seamless tube section,
Header assembly.

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