HK1114815B

HK1114815B - Methods for extending the life of alloy steel welded joints by elimination and reduction of the haz

Info

Publication number: HK1114815B
Application number: HK08104058.3A
Authority: HK
Inventors: Kent K. Coleman; David Wayne Gandy; Ramaswamy Viswanathan
Original assignee: Electric Power Research Institute, Inc.
Priority date: 2004-10-22
Filing date: 2005-09-19
Publication date: 2010-07-16

Description

Method for prolonging service life of alloy steel welded joint by eliminating and reducing heat affected zone

Background

Technical Field

The present invention relates generally to welding. More particularly, the present invention is directed to a method of welding metal components together using buttering (welding) and heat treatment techniques to avoid post-weld heat treatment and eliminate or reduce the heat affected zone.

Description of the related Art

There are two aspects in welding that increase the cost of welding and lead to failure of the post-weld components: the presence of a Heat Affected Zone (HAZ) and a Post Weld Heat Treatment (PWHT) to address the problems caused by the heat affected zone. As is well known in the art, heat from the weld forms a heat affected zone in the metal adjacent the weld. The formation of this haz has adverse metallurgical effects such as notch effects or grain growth, which weaken the metal of the haz. New alloys, such as those containing 2-12 wt% chromium, have been developed to provide higher strength for high temperature pressure applications than previously used alloys and steels. Failures that limit the useful life of components made from such materials tend to occur adjacent welds in the heat affected zone. Furthermore, attempts to develop filler materials that strengthen the weld and reduce the effects of the heat affected zone have not been satisfactory.

Another method for improving the metallurgical properties of the haz is post-weld heat treatment. The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code requires that low alloy steel pipe and Pressure Vessel welding applications be subjected to post-weld heat treatment to achieve toughness, tensile and hardness properties in the weld heat affected zone. However, post-weld heat treatment is generally an expensive process that requires a long time to perform. Generally, post-weld heat treatment requires heating the welded metal piece to a temperature slightly below the first transition temperature of the material. The post-weld heat treatment of a high temperature material containing 2% chromium as specified by the american society of mechanical engineers requires that the material be held at 1350 ° F for one hour per inch of thickness of the material. The ramp rate at which the temperature of the material can be raised to the soak temperature and cooled back to room temperature must be precisely controlled and can take hours to complete. Typical post weld heat treatment operations for 2 "thick pipes made from this material include preparation, time to warm the material, hold time, and cool down time, which may take 24 hours. Multiple welded components may require several post-weld heat treatment operations.

The manufacturer of the components typically performs this post-weld heat treatment in a large furnace that allows several weld joints to be tempered simultaneously. The physical size of the furnace significantly limits the size of the components that can be tempered. Therefore, certain post-weld heat treatment processes must be performed in the field or on-site. In these cases, the parts are welded together and then transported to the site where the post-weld heat treatment is performed. Post-weld heat treatment of such large components is generally performed using a resistance attenuator or an induction heat treatment apparatus. Likewise, the number of weld joints that can be simultaneously subjected to post weld heat treatment is limited by the power and utilization of the post weld heat treatment equipment. In large construction work, the timing of the post-weld heat treatment also becomes an important task.

Furthermore, simply moving the components from the manufacturing plant to the furnace, storage, other manufacturing area or work site can result in failure in the weld and heat affected zone. Assembling the components at the work site creates additional challenges for achieving acceptable post weld heat treatments. Air flow around the components, including external and internal flows, such as from wind and chimney effects, can cause the material to fail to reach a sufficient temperature to develop the desired properties in the post-weld heat treatment. Special care needs to be taken to support the components during the post-weld heat treatment operation, since the high temperature of the post-weld heat treatment operation drastically reduces the strength of the material.

Even with post weld heat treatment, failure can occur near the weld or in the heat affected zone. Accordingly, there is a need for an improved welding process that reduces the effects of heat affected zones and eliminates the need for post-weld heat treatment.

Summary of The Invention

The present invention provides a method of welding two metal pieces together, comprising overlaying a surface of a first metal piece with a first nickel-based filler metal to produce a first overlaid surface having a thickness sufficient to isolate a heat affected zone in the first metal piece from subsequent welding; heat treating at least a heat affected zone of the first metal component after overlaying a surface of the first metal component to produce a heat treated first overlaid surface; overlaying the surface of the second metal component with a second nickel-based filler metal having the same composition as the first nickel-based filler metal to produce a second overlaid surface having a thickness sufficient to isolate the heat affected zone of the second metal component from subsequent welding; heat treating at least the heat-affected zone of the second metallic member after surfacing the surface of the second metallic member to produce a heat-treated second hardfacing; and welding the heat treated first hardfacing surface and the heat treated second hardfacing surface with a third nickel-based filler metal having the same composition as the first and second nickel-based filler metals.

The method of the present invention can be used to weld similar and dissimilar metals. Preferably, the method of the invention is used for welding a martensitic stainless steel with a ferritic stainless steel, an austenitic stainless steel or another martensitic stainless steel.

The method of the present invention allows for sub-component parts to be welded together in a shop or job site without the need for post-weld heat treatment. This provides better utilization of the equipment and improved ergonomics, while reducing the time required for field assembly. The inventive method described herein may be used to substantially reduce the cost and time required to join low alloy pipe and/or pressure vessel materials and may be used in a variety of different welding processes.

These and other features and advantages of the present invention will be apparent from the following description, which, taken in conjunction with the drawings, set forth in detail, illustrates preferred embodiments.

Brief Description of Drawings

FIG. 1 is a process flow diagram according to one embodiment of the present invention; and

fig. 2A-2C illustrate welding of two metal components according to one embodiment of the present invention.

Detailed Description

The present invention provides an improved welding process that reduces or eliminates the effects caused by the formation of heat affected zones during welding and eliminates the need for post-weld heat treatment. Generally, the present invention provides a method of using a weld overlay technique to prepare a first metal piece to be welded with a nickel-based filler material followed by a post weld heat treatment either to temper the heat affected zone or to remove the heat affected zone by normalizing (normalization). The second metal piece is also prepared in a similar or identical manner. The two components are then welded together, either in the shop or in the field, without the need for post-weld heat treatment.

FIG. 1 is a process flow diagram according to one embodiment of the present invention. Process 100 is a method of welding two metal pieces together. It should be appreciated that the metal pieces to be welded together may be of similar or dissimilar metals. For example, low alloy ferritic steels containing less than 12 weight percent chromium may be joined together. Low alloy steels containing less than 12 wt% chromium, such as ferritic steels, may also be joined together with stainless steels containing 12 wt% or more chromium, such as austenitic stainless steels. In a preferred embodiment, the invention is used to join a martensitic stainless steel with a ferritic stainless steel, an austenitic stainless steel, or another martensitic stainless steel. Another preferred embodiment includes joining a 9Cr alloy with another 9Cr alloy, other ferritic alloy, or austenitic alloy, where the 9Cr alloy may include, for example, straight 9Cr, P91, P92, and the like.

It will be appreciated that the above-described,the use of the term "low alloy" steel refers to steels similar to carbon steels, containing less than about 1.65% manganese, 0.60% silicon or 0.60% copper. Unless elements such as chromium, molybdenum, cobalt, niobium, titanium are added to increase the degree of hardening and strength thereof. Examples of low alloy steels includeMo (T11 or P11),cr-1Mo (T22 or P22) and various other ASTM-type steel alloys such as ferritic and martensitic steels. The use of the term "ferrite" refers to a steel that exhibits a predominantly ferritic microstructure at room temperature and cannot be hardened with a heat treatment. The use of the term "stainless" refers to ferrous alloys containing at least 10% by weight aluminum and includes, for example, austenitic stainless steels, ferritic stainless steels, martensitic stainless steels, and precipitation hardened stainless steels. The use of the term "austenitic" stainless steel refers to stainless steels, such as 300-series stainless steels, including, for example, 304, 316, 321, and 347, alloyed with nickel or manganese and nitrogen to provide an austenitic structure at room temperature. The use of the term "martensitic" stainless steel refers to stainless steels that exhibit a martensite dominated microstructure at room temperature with the addition of carbon and that can be hardened by heat treatment, such as 400-series stainless steels, including, for example, 410, 420 and 440. Ferritic stainless steels, such as 430 or 446 steels, contain at least 10 weight percent chromium and have a microstructure of ferrite and carbides at room temperature. Generally, these alloys are not hardened by heat treatment.

In a first step 102, metal pieces to be welded together are prepared for welding. It should be appreciated that "metal piece" refers to any metal piece to be welded. For example, a metal component may be one sub-component part that is ready to be welded to another sub-component part to form the final element or component of the desired device. Thus, the use of the term "metal component" is intended to generally cover all types and shapes of metals to be welded. The preparation made in this step 102 may include various process steps or procedures selected on the metal component to prepare it for welding. For example, a metal member, or a particular surface of a metal member, may be machined into a particular shape by a truck bed. The surface of the metal component may also be ground, typically after arc gouging, or air-arc (air-arc) to remove metal. Electric discharge machining may also be used for precision machining, but this technique is generally slow.

In a next step 104, each surface of each metal piece to be subsequently welded to another metal piece is overlaid with a nickel-based filler. Specifically, a nickel-based filler is welded to a specific surface of each of the metal pieces that are subsequently welded together. The application of this nickel-based filler to the surface of each metal piece may be referred to as a "weld overlay". Preferably, the nickel-based filler contains at least 10 wt.% or more nickel, and more preferably contains about 40-70 wt.% nickel, and still more preferably 40-60 wt.% nickel. Additionally, the filler material may be any material known in the art that will depend in part on the composition of the metal pieces being welded together. Examples of some preferred filler materials that can be used in the WELD overlay and dissimilar metal Welding of the present invention include INCONEL Welding Electrode (Welding Electrode)182, INCONEL filler material 82, and INCO-WELD A Electrode. It will be appreciated that the nickel based filler preferably has the same composition as the weld overlay on the or each metal piece.

The actual circumstances or techniques for implementing or welding such a weld overlay are well known to those skilled in the art. In particular, any welding step and technique known in the art may be used to apply the weld overlay. However, the weld overlay should be applied at a thickness sufficient to isolate the heat affected zone formed during the weld overlay process from the heat generated during subsequent welding. That is, the weld overlay should be thick enough so that the haz is not affected by the subsequent welding operation, i.e., the process of welding the two metal pieces together.

Further, after the weld overlay is applied, each metal piece may be machined again to the appropriate weld geometry in preparation for the final weld between the two metal pieces in step 104. Thus, if such machining is performed, the thickness of the weld overlay should be sufficient to allow for the reduction in thickness thereof resulting from such machining. The thickness of the overlay may vary depending on the type of welding process, the composition of the filler, and the composition of the alloy used.

In a next step 106, each metal piece is heat treated. The specific heat treatment performed should be sufficient to heat treat at least the heat affected zone in each metal piece. That is, each metal component should be subjected to heating such that at least the heat affected zone of either component is heated to a desired temperature to achieve a desired effect. In one embodiment, this heat treatment comprises heating each metal piece, or at least the HAZ thereof, to a temperature sufficient to normalize the HAZ, e.g., in one embodiment, the heat treatment comprises heating each metal piece to A_C3Above the transition temperature, the heating is different depending on the alloy and the corresponding chemistry. Normalizing the heat affected zone causes the heat affected zone to return to its original or original base metallurgical state. Preferably, this heat treatment is carried out in a furnace to obtain better control than other heating methods or equipment.

It should be noted that such normalizing heat treatments may be used where the metal pieces are all low alloy metals having a chromium content of about 2-12% by weight. Such normalizing heat treatment may be used where each of the metal pieces comprises a low alloy ferritic steel piece. Such normalizing heat treatment may be used in the case where one of the metal members comprises a low alloy metal member and the other metal member comprises a stainless steel member. This normalizing heat treatment may be used where one of the metal pieces comprises a low alloy ferritic steel piece and the other metal piece comprises an austenitic stainless steel piece. Such normalizing heat treatment may also be used in the case where one of the metal pieces comprises a martensitic stainless steel piece and the other metal piece comprises any one of a low alloy ferritic steel, an austenitic stainless steel, or a martensitic stainless steel.

In another embodiment, the heat treatment comprises subjecting each toThe metal component is heated to a temperature sufficient to temper the heat affected zone to achieve sufficient heat affected zone toughness, tensile and hardness properties. For example, in one embodiment, the heat treatment includes heating each metal piece to a_C1Above the transformation temperature, which varies according to the alloy and its corresponding chemical properties, but is lower than A_C3And (3) temperature.

It should be noted that the tempering heat treatment may be used in the case where one of the metal members comprises a low alloy metal member and the other metal member comprises a stainless steel member. The tempering heat treatment may be used in the case of a low alloy ferritic steel member of the metal members. For example, the tempering heat treatment may be used in the case where one of the metal pieces comprises a martensitic stainless steel piece and the other metal piece comprises a low alloy ferritic steel. It should be noted that in some instances it may be beneficial to use a normalizing heat treatment for one metal piece and a tempering heat treatment for another metal piece.

It should be noted that the advantage of heat treating each metal piece before it is welded together avoids the need for post-weld heat treatment. Therefore, in the case of metal components comprising sub-components of particularly large elements, heat treating the individual sub-components may be easier than having to use a post-weld heat treatment on the entire finished element or component of the device. Likewise, expensive field post-weld heat treatments can be avoided. Furthermore, it should be noted that this heat treatment can be carried out in a workshop or in the field.

Further, after heat treating each of the metal pieces at step 106, each metal piece may again be machined to the appropriate weld geometry in preparation for the final weld between the two metal pieces. Thus, if such machining is performed, the thickness of the weld overlay should be sufficient to allow for the reduction in thickness thereof resulting from such machining. The thickness of the overlay may vary depending on the type of welding process, the composition of the filler, and the composition of the alloy used.

In a next step 108, each of the heat treated hardfaced metal members are welded together with a nickel-based filler. As discussed above, the nickel-based filler comprises at least 10 wt% or more nickel, and more preferably comprises about 40-70 wt% nickel, and even more preferably comprises 40-60 wt% nickel. Further, the filler material may be any material known in the art that will depend in part on the composition of the metal pieces being welded together. Examples of some preferred filler materials that can be used in the WELD overlay and dissimilar metal welding of the present invention include INCONEL welding electrode 182, INCONEL filler material 82, and INCO-WELD A electrode. Preferably, the nickel-based filler used to weld the two metal pieces together is the same as the nickel-based filler used to create the weld overlay.

Those skilled in the art will appreciate that any method and technique known in the art may be used to weld the two metal pieces together. Similarly, any device known in the art may be used. It should be noted that this welding operation will not require a subsequent post-weld heat treatment because sufficient performance has been obtained in the respective heat-affected zone of each metal piece. Furthermore, the nickel-based filler provides sufficient metallurgical properties for this weld. Thus, the two welded metal pieces may be placed in service without having to use a post-weld heat treatment.

Fig. 2A-2C illustrate welding of two metal components according to one embodiment of the present invention. Fig. 2A shows two metal pieces 202, 204 to be welded. As described above in connection with fig. 1, each metal component may be prepared according to preparation methods known in the art, such as machining. Fig. 2B illustrates the application of weld overlays 206, 208 to each metal piece. In this particular case, weld overlays 206, 208 are applied to the end surfaces of each metal piece 202, 204. As described above in connection with fig. 1, each of the metal pieces 202, 204 with the weld overlays 206, 208 will then be heat treated either by a normalizing heat treatment or a tempering heat treatment. In addition, each metal member may be further processed by machining to obtain a desired surface shape. Fig. 2C shows the result of the final welding step, wherein filler 210 is used to weld the weld overlays 206, 208 and thereby weld the two metal pieces 202, 204 together. Also as previously described in connection with fig. 1, the final welded component may be placed in service without the need for post-weld heat treatment.

Various embodiments of the present invention have been described. These descriptions are intended as illustrations of the invention. It will be apparent to those skilled in the art that modifications may be made to the invention as described herein without departing from the scope of the claims set out below. For example, it should be understood that the present invention may be used for welding of similar or dissimilar metal components. Moreover, it should be understood that while the present invention is generally described as applied to the welding of two metal components, the present invention may be used in any application where metals are welded together. For example, the invention may be used in power, chemicals, petroleum, steel, transportation, and pulp and paper. More generally, any process in which a low alloy steel weld is used may utilize the present techniques. Further, the present invention may be used to weld a martensitic stainless steel to any one of a ferritic stainless steel, an austenitic stainless steel, or another martensitic stainless steel.

Claims

1. A method of welding two metal members together, comprising:

overlaying a surface of a first metallic component with a first nickel-based filler metal at a thickness sufficient to isolate a heat-affected zone of the first metallic component from subsequent welding to produce a first overlaid surface;

heat treating at least the heat affected zone of the first metal component after overlay welding of the first metal component surface to produce a heat treated first overlay welded surface;

overlaying the surface of the second metal component with a second nickel-based filler metal having the same composition as the first nickel-based filler metal at a thickness sufficient to isolate the heat-affected zone of the second metal component from subsequent welding to produce a second overlaid surface;

heat treating at least the heat affected zone of the second metallic component after surfacing of the second metallic component surface to produce a heat treated second hardfacing; and is

The heat treated first hardfacing surface is welded to the heat treated second hardfacing surface with a third nickel-based filler metal having the same composition as the first and second nickel-based filler metals.

2. The method of claim 1, wherein the first, second, and third nickel-based filler metals each comprise a nickel content of greater than 10 wt.%.

3. The method of claim 2 wherein the nickel content is about 40-60% by weight.

4. The method of claim 1, wherein each of said heat treatments each comprises heat treating at a temperature sufficient to normalize a heat-affected zone of the metal component to be treated.

5. The method of claim 4, wherein each of said heat treatments is respectively comprised above the respective A of the metal piece to be treated_C3The temperature of the heat treatment is set to a predetermined temperature.

6. The method of claim 4, wherein the first and second metal pieces each comprise a low alloy ferritic steel piece.

7. The method of claim 4, wherein the first metal piece comprises a low alloy metal piece and the second metal piece comprises a stainless steel piece.

8. The method of claim 7, wherein the first metallic component comprises a low alloy ferritic steel component and the second metallic component comprises an austenitic stainless steel component.

9. The method of claim 4, wherein the first metal piece comprises a martensitic stainless steel piece and the second metal piece is selected from the group consisting of low alloy ferritic steel, austenitic stainless steel, and martensitic stainless steel.

10. The method of claim 1, wherein each of said heat treatments comprises heat treating at a temperature sufficient to temper the heat-affected zone of the metal component to be treated.

11. The method of claim 10, wherein each of said heat treatments is respectively included at a respective a of the metal component to be treated_C1Above the temperature and corresponding A of the metal component to be treated_C3Heat treatment is carried out at a temperature lower than the above temperature.

12. The method of claim 11, wherein the first metal piece comprises a low alloy metal piece and the second metal piece comprises a stainless steel piece.

13. The method of claim 12, wherein the first metallic component comprises a low alloy ferritic steel component and the second metallic component comprises an austenitic stainless steel component.

14. The method of claim 11, wherein the first metal piece comprises a martensitic stainless steel piece and the second metal piece is selected from the group consisting of low alloy ferritic steel, austenitic stainless steel, and martensitic stainless steel.

15. The method of claim 1, wherein the first and second metal pieces each comprise a low alloy ferritic steel piece.

16. The method of claim 1, wherein the first metal piece comprises a low alloy metal piece and the second metal piece comprises a stainless steel piece.

17. The method of claim 16, wherein the first metallic component comprises a low alloy ferritic steel component and the second metallic component comprises an austenitic stainless steel component.

18. The method of claim 1, wherein the first metal piece comprises a martensitic stainless steel piece and the second metal piece is selected from the group consisting of low alloy ferritic steel, austenitic stainless steel, and martensitic stainless steel.

19. A method of welding two dissimilar metal members together, comprising:

heat treating the first metal component after overlay welding of the first metal component surface to produce a heat treated first overlay welded surface;

overlaying a surface of a second metal component having a different composition than the first metal component with a second nickel-based filler metal having the same composition as the first nickel-based filler metal at a thickness sufficient to isolate the heat affected zone of the second metal component from subsequent welding to produce a second overlaid surface;

heat treating the second metal component after surfacing of the second metal component surface to produce a heat treated second hardfaced surface; and is

20. The method of claim 19, wherein the first metal piece comprises a low alloy metal piece and the second metal piece comprises a stainless steel piece.

21. The method of claim 20, wherein the first metallic component comprises a low alloy ferritic steel component and the second metallic component comprises an austenitic stainless steel component.

22. The method of claim 19, wherein the first metal piece comprises a martensitic stainless steel piece and the second metal piece is selected from the group consisting of low alloy ferritic steel, austenitic stainless steel, and martensitic stainless steel.

23. The method of claim 19, wherein each of said heat treatments comprises heat treating at a temperature sufficient to normalize the heat-affected zone of the metal component to be treated.

24. The method of claim 19, wherein each of said heat treatments comprises heat treating at a temperature sufficient to temper the heat-affected zone of the metal component to be treated.

25. A method of welding two dissimilar metals, consisting essentially of:

preparing a surface of a first metal member and a surface of a second metal member having a different composition from the first metal member for welding;

heat treating at least a heat affected zone of a first metal component to produce a heat treated first hardfacing;

machining the first build-up surface;

heat treating at least the heat-affected zone of the second metallic component to produce a heat-treated second weld overlay surface;

machining the second surfacing surface; and is

26. The method of claim 25, wherein the first metal piece comprises a low alloy metal piece and the second metal piece comprises a stainless steel piece.

27. The method of claim 26, wherein the first metallic component comprises a low alloy ferritic steel component and the second metallic component comprises an austenitic stainless steel component.

28. The method of claim 25, wherein the first metal piece comprises a martensitic stainless steel and the second metal piece is selected from the group consisting of low alloy ferritic steels, austenitic stainless steels, and martensitic stainless steels.

29. The method of claim 25, wherein each of said heat treatments comprises heat treating at a temperature sufficient to normalize the heat-affected zone of the metal component to be treated.

30. The method of claim 25, wherein each of said heat treatments comprises heat treating at a temperature sufficient to temper the heat-affected zone of the metal component to be treated.