EP0847300A1

EP0847300A1 - Filter assembly

Info

Publication number: EP0847300A1
Application number: EP96929738A
Authority: EP
Inventors: Michael B. Whitlock
Original assignee: Pall Corp
Current assignee: Oiltools International BV
Priority date: 1995-08-30
Filing date: 1996-08-30
Publication date: 1998-06-17
Also published as: NO980815D0; WO1997007870A1; NO980815L; AU6901896A

Abstract

A filter for subterranean environments has an inner support member (10; 72), a filter body (12) containing a filter medium disposed around the inner support member, and at least one end cap (14, 16; 76) welded to the inner support member and connected to a lengthwise end of the filter body. The weld (70) is substantially free of martensite. A method for welding a filter includes weld bead which is substantially free of martensite.

Description

FILTER ASSEMBLY

CROSS REFERENCE TO RELATED APPLICATIONS This application claims priority from provisional application Serial No. 60/002987 filed August 30, 1995 and provisional application Serial No. 60/011021 filed February 1, 1996, both of which are herein incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION The present invention relates generally to filters for use in applications such as subterranean wells, including oil and gas wells, water wells, and geothermal wells, and in similar applications in which the filter is exposed to a corrosive and/or high stress environment. More particularly, it relates to filters for the above applications having portions thereof made of different metals fastened to each other by welding. An example of such a filter is one including an inner support member made of a martensitic steel and end caps made of austenitic steel welded to the inner support member.

Filters are frequently used in subterranean wells in order to remove particulates from liquids or gases produced by the wells. Typical particulates which need to be filtered out are sand and clay, and for this reason, well filters for this purpose are often referred to as sand screens.

A well filter typically comprises a filter body mounted on the exterior of a pipe or other tubular support member. The filter is generally installed in series with a plurality of pipes forming a pipe string, and the filter is introduced into the well along with the pipe string. For example, in the case of an oil or gas well, the pipe string is a production string through which oil or gas is transported from a production zone within the well to the exterior of the well. For use in downhole production pipe strings, tubular pipes are required to have quite high yield strength, as this determines the limiting working depth. In addition to high torsion loads, the hanging weight of the string places stresses on the material selected. In environments where additional corrosion resistance is needed, high strength martensitic and duplex stainless steels are most frequently used, having yield strengths ranging from 80 to 100 Ksi.

When a pipe string containing a weld is being inserted into a well, the filter may come into contact with and catch on objects within the well bore. To prevent the filter body from sliding along the inner support member when such contact takes place, it is desirable to provide some restraint to lengthwise movement by the filter body. One method of restraint would be to secure the filter body to annular metal end caps and to weld the end caps to the tubular support member using standard welding techniques.

However, the tubular support member of the filter is frequently a standard API grade of production pipe, which is normally made of medium to high carbon steel, e.g., having a carbon content of about 0.20 weight percent or greater. Production pipe is highly preferred for the support member because it is often desirable to maintain the same strength throughout all portions of the pipe string. On the other hand, it may be desirable from the standpoint of corrosion resistance for the end caps and filter body to be made of a different metal, such as austenitic stainless steel (316 being the most common) and high nickel alloys (for example, Monel).

Such an arrangement can render it difficult to form a weld having the physical properties necessary to withstand harsh downhole conditions. This is due to the composition of the weld, which is usually a combination of the filler material and the metal compositions of the two pieces being joined. In most welding techniques, a filler material, in the form of a separate welding rod or a consumable electrode, is melted via high heat and flows along or into the joint to be welded. The high heat produced by, for example, an arc welder, also melts the adjacent surfaces of the two metal pieces. The melted metal combines with the filler material to produce a weld "puddle" composition which is a mixture of the three. The high heat also affects the adjacent areas of the pipe which are known as the heat affected zone (HAZ). The rapid heating in the HAZ increase the hardness of the pipe, particularly if the pipe has a martensitic phase. This increased hardness can cause failure of the pipe in use due to corrosion or stress cracking, and hence a post weld heat treatment is necessary to temper the pipe, i.e., reduce its hardness. Typically the hardness should be reduce to Rockwell 23 or below.

Heat treatment or "tempering" involves raising the temperature of the metal to a certain level and then allowing the metal to cool under controlled conditions. Usually there are minimum and maximum temperatures for tempering. If the temperature of the heat treatment is below the minimum, tempering will not occur or will be incomplete. If the maximum temperature is exceeded, the physical properties of the metal can be adversely affected. Tempering ranges vary depending on the particular composition of the metal.

The weld must also have reduced hardness for similar reasons. However, a problem arises in tempering the weld due to the weld puddle chemistry. Because the weld puddle is a mixture of the filler material and the adjoining metal pieces, it has constituents which, in combination, can produce "hard spots" in the weld. Hard spots are portions of the weld which cannot be tempered or which have a tempering range lying outside (usually above) that of the pipe. Thus even though the HAZ in the pipe is tempered, the weld will retain hard spots which can cause weld failure. Hard spots often occur when martensite is formed in the weld puddle. This weld martensite differs from that of a martensite pipe because it has additional constituents, such as high levels of molybdenum, which come from either the filler material or the adjoining metal piece, i.e., the filter assembly. In combination with carbon and iron from the pipe, these additional constituents form a martensitic structure which does not temper adequately during the post weld heat treatment. The relationship between austenite and martensite is illustrated in Figure 1 which is a constitution diagram of stainless steel weld metal. As can be seen therefrom, even if a filler material has a composition in the austenite range, the weld puddle may nevertheless become martensitic if the levels of chromium and/or nickel equivalent are diluted sufficiently by mixing with a martensitic production pipe metal. If this occurs, then hard spots can develop as discussed above. Generally, filler material has heretofore constituted austenic or another form of steel having a significant content of iron. In downhole environments exposing the pipe string to corrosive environments such as carbon dioxide, hydrogen sulfide and/or chlorides, these hard spots may readily corrode. Moreover, in some wells, such as geothermal wells, a well filter may be subjected to elevated temperatures during use and may undergo significant thermal expansion with respect to its dimensions when it is assembled. When the tubular support member and the filter body of a well filter have different thermal expansion coefficients, or when the tubular support member and the filter body are heated to different temperatures from each other, the difference in the amount of thermal expansion of the inner support member and the filter body may stress and thereby weaken the weld if it contains hard spots. In extreme cases the filter body may be torn loose from the support member and allow unfiltered fluids to bypass the filter body.

Hence, there is a need in the art for a well filter which can be reliably welded to a metal surface, such as an API production pipe, while maintaining corrosion resistance and physical properties for use in corrosive and/or high stress environments.

Summary of the Invention

According to the present invention, welding between metal parts of a filter may be carried out using a suitable non-ferrous filler metal to create a weld puddle with insufficient iron to form significant amounts of martensite. Because there is little or no martensite in the weld puddle, the resultant weld may exhibit consistent hardness readings below Rockwell C23 after post-weld treatment to temper martensite formed in the heat affected zone (HAZ). In one aspect the invention provides a filter which includes a filter body and at least one weld which is resistant to failure in corrosive environments.

In another aspect the invention provides a filter which is resistant to failure under thermal stress. A filter according to the present invention has an inner support member, a filter body disposed around the inner support member and including a filter medium, and at least one end cap welded to the inner support member with a weld which is substantially free of martensite. In a preferred embodiment, the filter includes two end connectors disposed at opposite lengthwise ends of the filter body, at least one of the end caps being welded to an inner support member. By the term "substantially free of martensite" it is meant that martensite may be present in the weld to the extent it does not adversely affect the in service life of the filter. The weld may also be totally free of martensite. In another aspect the invention provides for a filter for subterranean use which comprises an inner support member capable of transporting a fluid in an axial direction thereof, a filter body disposed around the inner support member and including a filter medium, and an end cap welded to the inner support member and connected to a lengthwise end of the filter body, the weld being substantially free of martensite.

In another aspect, the invention provides for a method for welding two portions of a filter comprising the steps of placing the portions adjacent to each other and forming a weld bead which contacts both portions, the weld bead being substantially free of martensite. In another aspect the invention provides for a method for making a subterranean filter which comprises the steps of welding at least one end cap to an inner support member, wherein the weld is substantially free of martensite, depositing a filter body including a filter medium around the inner support member and subjecting the weld and the inner support member to a post weld heat treatment to temper a heat affected zone on the inner surface member adjacent the weld.

While the invention is particularly suitable for use in connecting a inner support member and end caps of a filter, it can be used to connect together other portions of a filter. For example, it can be used to weld stabilizing fins to the exterior of an inner support member of a filter.

Brief Description of the Drawings

For a full understanding of the invention, reference should be made to the following detailed description and drawings, wherein

Figure 1 is a constitution diagram of stainless steel weld metal. Figure 2 is one embodiment of a filter of the invention;

Figure 3 is a cross-sectional view of one embodiment of an end cap of the invention;

Figure 4 is a cross-sectional view of one embodiment of a weld of the invention; and Figure 5 is a cross-sectional view of another embodiment of a weld of the invention.

Description of Preferred Embodiments

Figure 2 illustrates an embodiment of a filter according to the present invention. The illustrated filter may be used within an oil or gas well to remove sand and other particulates from a fluid produced by the well, but as stated above, a filter according to the present invention can be used in a wide variety of applications and is not limited to a specific type of well or to wells generally. Rather, the invention is useful in any type of highly corrosive and/or high stress environment where untempered martensite filter welds might fail. The filter includes a tubular inner support member 10, a filter body 12 disposed around the inner support member 10 and including a filter medium, and end caps 14 and 16 disposed at each lengthwise end of the filter body 12 and connecting the filter body 12 to the inner support member 10 in a manner preventing particulates from bypassing the filter body 12.

This embodiment is intended to be connected in series with a tubular pipe string which is inserted into a well. The filter can be installed at any desired location in the string, but usually it will be positioned near the bottom end of the string. The string may include one or more of the filters, either connected directly with one another or separated by a length of pipe or other members. The filter can be deployed vertically, horizontally, or at any other angle within a well.

The inner support member 10 provides rigidity to the filter and serves to axially transport filtrate which has passed through the filter body 12 to an unillustrated conduit (such as a tubular pipe string) connected to the inner support member 10 for transporting the filtrate outside the well. Although, the inner support member 10 is usually a hollow, tubular member and has perforations, pores, or other openings in its peripheral wall which permit fluid to flow into the hollow center of the inner support member 10, it need not be hollow as long as it is capable of transporting filtrate. For example, the inner support member 10 may be a solid, porous member through which filtrate can flow axially, or it may be a solid member having axial channels in its outer surface for the transport of fluid. For reasons of strength, it is usually cylindrical, but other shapes may be employed, such as a shape with a polygonal or oval cross section, and the cross section may vary along its length.

In a preferred embodiment, the inner support member 10 comprises a cylindrical pipe having a uniform cross section over most of its length and having perforations for filtrate formed over a portion of its length in a region on which the filter body 12 is mounted. The inner support member 10 may be equipped with connecting portions at one or both of its ends to enable the inner support member 10 to be connected to other members. In Figure 2, the inner support member 10 has an externally threaded section 20 formed at each lengthwise end which can be screwed into an internally threaded box of a pipe or into a standard pipe connector for joining the threaded sections of two pipes. When the filter is intended to be connected in series with a string of production pipe, a perforated production pipe is particularly suitable as the inner support member 10, since the threaded connectors of the production pipe will have the same strength as that of the connectors of the pipe string to which the filter is to be connected. If the inner support member 10 is expected to be subjected to only low tensile, torsional, or radial compressive forces, light-weight lockseam tubing may be employed for the inner support member 10. If the filter is to be installed at the tail end of a pipe string or other conduit, the lower end of the inner support member 10 may be closed off with a bull plug or similar member.

The inner support member 10 can be made of any weldable metallic material capable of withstanding the conditions to which the inner support member 10 is to be subjected during installation and use. When the inner support member 10 is formed from a length of production pipe (commonly referred to as a pipe joint), it will typically be made of steel.

The length of the inner support member 10 is not critical, and one or more filter bodies 12 can be mounted on a single inner support member 10. Members other than a filter body 12 and end caps 14 and 16 can also be mounted on the inner support member 10, such as collars or conventional centralizers for guiding the filter as it is inserted into a well bore.

The filter body 12 contains a filter medium which filters a well fluid to form a filtrate. The filter body 12 may have any structure capable of performing the intended removal of substances from the fluid being filtered. For example, it may be a pre-packed body, a wire-wrapped body, a sintered metal unitary body, a wire mesh body, or any other type of filter body. For this reason, the filter body 12 is shown only schematically in the drawings.

The filter body 12 need not have any particular shape. Usually, it will have an inner periphery which is similar in shape to the outer periphery of the inner support member 10, and its outer periphery will usually be rounded (such as cylindrical) to make it easier for the filter body 12 to pass through well casing.

In most applications, fluid will normally flow radially inward through the filter body 12 during filtration. However, in some applications, such as acidizing of a well, air sparging, water injection, and enhanced oil recovery applications, fluid may be directed radially outward through the filter body 12.

Examples of filter bodies which are particularly suitable for use in wells for oil and gas and in other subterranean environments and which can be used in the present invention are disclosed in WO96/ 18022 which is hereby incorporated by reference in its entirety. The filter bodies described in that application include a supported porous medium, which is a filter medium including a foraminate support member, such as a mesh, and particulates sintered to the foraminate support. A supported porous medium provides a filter having excellent damage resistance, meaning that the filter substantially retains its filtering integrity even when significantly deformed.

An example of a supported porous medium for use in the present invention is a sintered supported porous metal sheet material disclosed in U.S. Patent No. 4,613,369 which is herein incoφorated by reference in its entirety. This material, which is available from Pall Corporation under the trademark PMM^®, can be manufactured from a wide variety of metals, such as nickel, iron, chromium, copper, molybdenum, tungsten, zinc, tin, aluminum, cobalt, iron, and magnesium, as well as combinations of metals and metal alloys including boron-containing alloys. For a filter intended for use in a well for oil or gas, nickel/chromium alloys are particularly suitable. Of these, AISI designated stainless steels which contain nickel, chromium and iron are particularly preferred.

In addition to some type of filter medium, the filter body 12 may include a variety of other layers and components, such as drainage layers to assist the flow of fluid into the filter medium and the flow of filtrate into the inner support member, cushioning layers to prevent abrasion of the filter medium, diffusion layers placed between layers of filter medium to permit edgewise flow of fluid, layers for selectively blocking flow through portions of the filter body, and a protective member, such as an outer cage or wrap, for protecting the filter body from erosion and damage.

The end caps 14, 16 serve to connect the filter body 12 to the inner support member 10 in a manner which prevents particulate matter large enough to be removed by the filter body 12 from bypassing the filter body 12 and flowing into the inner support member 10. In applications in which it is expected that axial forces may be applied to the filter body 12 during installation of the filter in a well, at least one of the end caps is preferably capable of resisting these forces to prevent the filter body 12 from sliding along the inner support member 10 under the applied forces. In applications in which the filter body 12 is expected to be subject to thermal strain (either expansion or contraction) different from that of the inner support member 10, at least one of the end connectors is preferably capable of lengthwise movement relative to the inner support member 10 to permit the thermal strain to take place without damage to the filter body 12. In a preferred embodiment in which one or both of the end caps are welded to the inner support member in accordance with the invention, the lack of substantial martensite in the weld may allow sufficient flexing to prevent failure of the weld under thermal stress.

The end caps are not restricted to any particular materials but frequently are chosen to be of the same material as the filter medium so that the filter body and the end caps can be readily joined to one another. Examples of some suitable materials for the end caps are austenitic stainless steels, nickel alloys, and chrome-nickel alloys. Examples of suitable shapes for the end caps are shown in PCT WO96/ 18022.

Figure 3 illustrates one embodiment for an end cap welded to an inner support member. An end cap 40 includes an outer surface 42 facing away from the wrapped layers 44 of a filter medium and an inner surface facing layers 44. The end cap 40 seals the wrapped layers 44. A plurality of annular steps 46 for supporting the various wrapped layers 44 are formed on the inner surface of the end cap 40. In this embodiment, each step 46 supports a single one of the layers 44, but alternatively each step 42 may support more than one layer. The outermost step 46 supports a protective tube 48 and is secured to the tube 48 by a suitable method, such as welding 50. The outer surface of the end cap 42 may be sloped to make it easier for the filter to pass smoothly through a well bore. Instead of having steps, the end cap 40 can have a smooth slope on its inner surface, and the wrapped layers can be welded to the slope.

To assemble the filter of Figure 3, the end caps 40 are first welded to the inner support member 52 with a desired spacing between them, and then the various layers of drainage mesh and filter medium are wrapped one by one around the inner support member 52 and the steps 46 of the end caps 40 and welded to form longitudinal seams. The protective tube 48 is then installed around the wrapped layers and connected to the end caps 40. The assembled filter may be heat treated after any welding step.

The protective tube 48 need not have any particular structure. In this embodiment, it comprises a spiral welded perforated tube of a suitable material, such as stainless steel.

Welding techniques which are useful in the present invention include the various well known welding processes such as gas tungsten-arc welding (GTAW), gas metal-arc welding (GMAW), and shielded metal-arc welding (SMAW).

Gas tungsten-arc welding employs inert-gas shielding with tungsten electrodes. Direct current is used in welding steel, copper and nickel alloys, while alternating cuπent is normally used for welding aluminum and magnesium in order to remove oxide film by reverse polarity. The tungsten electrode is non-consumable and is used in conjunction with a filler rod which supplies the filler material.

In gas metal-arc welding, a consumable, continuous electrode is used with inert-gas shielded metal arc welding and requires no flux. This method is most advantageously used in welding reactive metals, such as titanium, which can undergo atmospheric contamination and/or in welding aluminum and stainless steel which may be subject to increased porosity.

Plasma-arc welding can also be used and employs a plasma which is produced by the heat of an electric-arc-gas mixture. This type of welding is generally used with material having a thickness greater than 3/30 seconds of an inch. (2.4mm). In a prefeπed embodiment, one or more end caps are welded to the inner support member using one of the above welding methods of the invention. For this purpose, a fillet weld between the inner support member and the end caps is generally the most convenient type of weld. However, a butt weld may alternatively be employed. The inner support member is preferably welded to one or both of the end caps using a filler material which is non feπous or has a low iron content which does not form martensite in the weld puddle. Examples of suitable filler materials include nickel, nickel alloys, chrome-nickel alloys, and nickel-copper alloys. In a prefeπed embodiment, an inner support member of L80-13Cr is welded to an end cap formed of 316L stainless steel using Inconel 82 as the filler material. L80-13Cr has the following composition (weight % basis): carbon 0.15-0.22; manganese 0.25-1.0; chromium 12.0-14.0; nickel 0.5 MAX; copper 0.25 MAX; phosphorous 0.02 MAX; sulfur 0.01 MAX; silicon 1.0 MAX. Inconel 82 has the following composition (weight % basis): carbon 0.10 MAX; manganese 2.25-3.5, iron 3.0 MAX, phosphorous 0.03, sulfur 0.015, silicon 0.5, copper 0.5, nickel 67.0 MIN, titanium 0.75, chromium 18.0-22.0, and Columbian 2.0-3.0.

The non-feπous filler material acts as a buffer between the end cap and the inner support member and creates a weld-puddle chemistry with insufficient iron content to form martensite. Because no martensite is formed, the weld can be subsequently tempered by heat treatment, if desired, to reduce its hardness to a desired level.

The welding of an end cap to an inner support member can be performed in one or more weld passes. An example of a method of forming a fillet weld employing two passes is illustrated in Figure 4 in which a cross-sectional view of a finished weld formed with two passes is indicated generally by the number 70. With the end cap spaced from the welding region (not shown), a first welding pass is performed on the inner support member 72 with a filler material to lay a first bead 74 of the filler material on an inner support member 72 around its entire periphery. The bead 74 is essentially a cladding layer or a buttering layer. After completion of the first pass, an end cap 76 is moved to the welding region so as to abut the bead 74 formed in the first pass, and a second pass is formed with the same filler material to form a second bead 78 and thereby fuse the end cap to the bead 74 deposited during the first pass.

The advantage of the two pass method is that the buttering layer reduces the contribution of the inner support member to the overall weld puddle chemistry. Since the second pass bead 78 only contacts the buttering layer and not the high iron content metal, the second pass bead 78 does not incorporate enough iron to form martensite. The only iron in the second bead thus comes from the low concentration in the buttering layer (picked-up from the inner support member) and the iron in the end cap, if any.

Alternatively, an end cap may be welded to an inner support member using a single weld pass as illustrated in Figure 5. The bead 80 contains insufficient iron to form martensite. A single pass is particularly useful if the end cap is itself made of a non-feπous or low-iron material such as nickel alloy (Inconel, for example).

An austenitic stainless steel end cap may also be welded to a martensitic inner support member in a single pass by incorporating a large amount of the filler material into the weld puddle and as little as possible of the metal from the inner support member so that the resultant puddle chemistry has a high nickel and chrome content and a low iron content. As can be seen with reference again to Figure 1, a sufficiently high content of chrome and/or nickel (or their equivalents) will prevent the formation of martensite in the weld puddle.

Although single and double pass techniques are prefeπed, it is contemplated that other welding methods can be used. For example, more than one buttering pass and/or more than one second pass may be employed. After welding, the filter may be subjected to heat treatment to temper the HAZ and reduce its hardness to a desired level (such as below Rockwell 23C). An example of a heat treatment condition is 1450°F for 20-30 minutes, although lower temperatures may also be used. The heat tempering conditions will vary depending on the particular metal used and the degree of tempering (i.e., reduction in hardness) desired. One skilled in the art can readily select conditions which will result in a predetermined hardness reduction. For the heat treatment of an L80-13Cr tubular pipe to obtain a post weld hardness below Rockwell 23C, the treatment temperature is from about 1200 to about 1400 °F (649-760 °C) and preferably from about 1420 to about 1460 °F (771-793 °C). The treatment time is from about 15 to about 60 minutes and preferably from about 20 to about 30 minutes.

The following Example illustrates the invention. Example An inner support member constructed of L80-13Cr was welded to two end caps of 316L stainless steel by fillet welding employing two weld passes using the two-pass method described above and as illustrated in Figure 3. The filler material was Inconel 82 having the chemical composition described above. The welded pipe was heat treated at 1450°F for a 30 minute soak time, followed by cooling in still air. The pipe was then sectioned through the welds at 90 degree intervals around the pipe. Microhardness tests were made in the inner support member, in the first pass weld metal, and in the second pass weld metal. Additional tests were made in an attempt to locate the hardest material in and around the welds. No location was found on either weld that exceeded the maximum allowable hardness of Rockwell 23C.

Claims

CLAIMS:

1. A filter for subteπanean use comprising: an inner support member capable of transporting a fluid in an axial direction thereof; a filter body including a filter medium disposed around the inner support member; and an end cap welded to the inner support member and connected to a lengthwise end of the filter body, wherein the weld is substantially free of martensite.

2. A filter as claimed in claim 1, wherein the inner core comprises martensitic steel.

3. A filter as claimed in claim 1, wherein the inner core has a carbon content of at least about 0.20 weight percent.

4. A filter as claimed in claim 1, wherein the inner core comprises L80-13Cr pipe.

5. A filter as claimed in claim 1, wherein the end cap comprises austenitic steel.

6. A filter as claimed in claim 1, wherein the end cap comprises an iron free alloy.

7. A filter as claimed in claim 1, wherein the weld comprises austenitic steel.

8. A filter as claimed in claim 1, wherein the weld comprises a low iron content alloy.

9. A filter as claimed in claim 1, wherein the weld is a butt weld.

10. A filter as claimed in claim 1, wherein the weld is a filler weld.

11. A filter as claimed in claim 1, wherein the weld is formed in at least two passes including a first pass buttering bead and a second pass bead overlaid on the buttering bead.

12. A method for welding together two portions of a filter comprising the steps of placing the portions adjacent to each other and forming a weld bead which contacts both portions, the weld bead being substantially free of martensite.

13. A method as claimed in claim 12, wherein forming a weld comprises forming a buttering bead on a first filter portion, positioning a second filter portion adjacent the buttering bead, and forming at least another weld bead which contacts the buttering bead and the second filter portion, said at least another weld bead being substantially free of martensite.

14. A method as claimed in claim 12 wherein the step of forming a weld bead includes forming an austenitic weld bead.

15. A method as claimed in claim 12 wherein the step of forming a weld bead includes forming a low iron content alloy weld bead.

16. A method as claimed in claim 12 wherein the welding step includes welding an end cap to an inner support member.

17. A method as claimed in claim 16 wherein the end cap comprises austenitic steel and the inner support member comprises martensitic steel.

18. A method for making a subteπanean filter comprising the steps of welding at least one end cap to an inner support member, wherein the weld is substantially free of martensite, depositing a filter body including a filter medium around the inner support member and subjecting the weld and the inner support member to post weld heat treatment to temper a heat affected zone on the inner support member adjacent the weld.

19. A method as claimed in claim 18 wherein the step of welding at least one end cap includes welding an austenitic end cap to a martensitic inner support member.

20. A method as claimed in claim 19, wherein the step of welding at least one end cap includes welding with a low iron content filler.