High pressure metal pipe applications used in the oil and gas industry will often benefit from the use of high strength alloys to resist the stresses caused by the internal pressure of fluid flowing through the pipe. The mechanical or yield strength of an alloy can be increased by various thermal or mechanical processes or a combination thereof. Examples of such processes include: cold-working or drawing for mechanical strengthening; precipitation hardening or quenching and tempering for thermal strengthening; and control-rolling plus accelerated cooling for combined thermo-mechanical strengthening.
The yield strength of an alloy represents the stress below which deformation of the alloy is entirely "elastic"; when the alloy is stressed in the elastic region, the alloy will return to its original shape when the stress is removed, and there will be no permanent deformation. In high pressure pipe applications, it is desirable to increase the yield strength of the alloy and thereby increase the elastic region in which stress resulting from pressure of the fluid flowing in the pipe will not result in permanent deformation of the alloy.
One specific example of a process for increasing the yield strength of an alloy is a plastic deformation process commonly known as "cold-working". Alloys are "plastically deformed" when they are deformed to the extent that they do not return to their original shape when the stress is removed. A number of changes occur in the microstructure of an alloy when it is plastically deformed; during deformation, each individual grain must "give" to produce the anticipated overall deformation. This deformation causes each grain to become stronger and, therefore, more difficult to further deform. With a cold-working process, plastic deformation is performed below the critical temperature of the alloy (i.e., the temperature at which grain recrystallization initiates) and there is a gradual increase in the hardness or yield strength of the alloy and a decrease in its ductility. For example, the yield strength of nickel-based alloys, which are often used in pressure pipe applications, can be increased by about 50 ksi-100 ksi with this cold-working deformation process. Typically, the resulting yield strength will be about 4 times the yield strength of the untreated alloy.
By the nature of their fabrication, pre-strengthened pipe applications will contain a considerable number of welded joints, each of which typically has a tensile capacity lower than that of the base alloy. As described further below, when an alloy is welded by conventional practices, the strength of the base alloy is locally, but permanently, reduced by the welding heat, which results in a welded joint which is substantially weaker than the unwelded, pre-strengthened, base alloy. Accordingly, the benefit of increasing the tensile capacity of the base alloy using thermal or mechanical strengthening methods is substantially decreased because of the weakness of the welded joint resulting from the welding process.
The process of welding will produce a heat-affected zone in the alloy adjacent to the weld metal. The heat-affected zone is that portion of the alloy which has not been melted, but whose mechanical properties or microstructure have been altered by the heat of welding. FIG. 1 is a schematic of a standard welded joint 2 having a weld metal 4, a base alloy 6 which has been thermally or mechanically strengthened, and a heat-affected zone 8. If the chemical composition of the weld metal 4 is essentially the same as that of the base alloy 6 (prior to strengthening of the base alloy 6), then the weld metal 4 of such a welded joint 2 will be the weakest, the heat-affected zone 8 will be stronger than the weld metal 4 but weaker than the base alloy 6, and the base alloy 6 will be the strongest. If the chemical composition of the weld metal 4 and the base metal 6 are not essentially the same, the heat-affected zone 8 will, in some instances, be weaker than the weld metal 4. But, in either instance, the base alloy 6, if thermally or mechanically strengthened prior to welding, will usually be stronger then either the weld metal 4 or the heat-affected zone 8.
With a standard welded joint 2, having a mechanically or thermally strengthened base alloy 6, the welded joint 2 will typically fail along a plane 10 which is located in the weld metal 4. Accordingly, failure of the welded joint 2 will occur in the weld metal 4 and thus at a tensile load below that which could otherwise be sustained by the unwelded, pre-strengthened, base alloy 6. For example, when two nickel-based, cold-worked, alloy pipes, each having a tensile capacity of about 140 ksi (after cold-working), are welded together using a weld metal 4 having a tensile capacity of about 120 ksi, to form a standard welded joint 2, the welded joint 2 will typically fail at the tensile capacity of the weld metal 4. The tensile capacity of the welded joint 2, at the plane of failure 10 located in the weld metal 4, would therefore be about 86% of the tensile capacity of the unwelded, pre-stengthened, base alloy 6.
When welding two pipes, a cap of excess weld metal may be formed which is often considered a by-product of the welding process. With standard welding practices, there is no minimum cap height required; indeed, welding codes and practices often limit the maximum cap height (e.g. to 1/8"). As described further below, although a high-cap welded joint (as depicted in FIG. 2) may have a tensile capacity which is greater than the tensile capacity of a standard welded joint 2, when the base alloy has been strengthened by thermal and/or mechanical treating methods, it is not recommended that a high cap of excess weld metal be relied upon to contribute to the load-carrying capacity of the welded joint.
The theoretical plane of failure for a welded joint having a "high-cap", which is not removed after welding, is illustrated in FIG. 2. The high-cap welded joint 17 depicted in FIG. 2 consists of weld metal 21, high-weld cap 24 and a portion of the pre-strengthened base alloy 16, on either side of the weld metal 21 including at least the heat-affected zone 20. The outer boundary of the heat-affected zone 20 is indicated by dashed lines 18. Assuming that the chemical composition of the weld metal 21 essentially matches the chemical composition of the base alloy 16 (prior to thermal or mechanical strengthening), then the welded joint 17 will, in the best case, fail along the plane 22 located in the heat-affected zone 20. The high weld cap 24 will theoretically push the plane of failure 22 out of the weaker weld metal 21 and into the stronger heat-affected zone 20. However, as with the standard welded joint 2 shown in FIG. 1, failure along plane 22 (plane 10 in FIG. 1) will still occur at a tensile capacity below that which could otherwise be sustained by the unwelded, pre-strengthened, base alloy 16 (base alloy 6 in FIG. 1). For example, When two cold-worked, nickel-based, alloy pipes, each having a tensile capacity of about 140 ksi (after cold-working) are welded together, using a weld metal 21 having a tensile capacity of about 120 ksi, to form a high-cap welded joint 17, the welded joint 17 will theoretically fail at the tensile capacity of the heat-affected zone 20 (about 130 ksi). However, this will only occur if the plane of failure 22 is actually located in the heat-affected zone 20. The tensile capacity of such a welded joint 17 would therefore be about 93% of the unwelded base alloy 16 tensile strength.
As discussed above, the increased tensile capacity of the high-cap welded joint 17 (FIG. 2), in comparison to the standard welded joint 2 (FIG. 1), arises because the high-weld cap 24 will theoretically force the plane of failure 22 into the stronger (with respect to the weld metal 21) heat-affected zone 20. Whereas, the plane of failure 10 of the standard welded joint 2 will typically reside in the weaker (with respect to the heat-affected zone 8) weld metal 4. Notwithstanding this increase in tensile capacity, it would be difficult to achieve reliable and consistent strength through the plane of failure 22 in the heat-affected zone 20 using a high-weld cap 24, because of the high degree of variation in welding parameters which impact the size and extent of the heat-affected zone 20. More specifically, variation in each welder's methods and in welding parameters, such as heat, amperage and voltage, make it impractical and essentially impossible during actual production welding to predict the strength through the plane of failure 22 located in the heat-affected zone 20.
In addition to the foregoing, when a high-cap welded joint 17 is formed with two pipes having a base alloy 16 which has been strengthened by thermal and/or mechanical treating methods, the tensile strength in the heat-affected zone 20 will still be less than the tensile strength of the pre-strengthened base alloy 16. Accordingly, the area of lowest strength will still be in the high-cap welded joint 17, and in a typical weld design it is common practice to attempt to have the welded joint be at least as strong as the base alloy. For the reasons set forth above, a high-weld cap 24 is not recommended to increase the tensile capacity of a welded joint when the base alloy has been strengthened by thermal and/or mechanical treating methods. Furthermore, a high-weld cap 24 is usually not used with unstrengthened alloys (such as unstrengthened carbon steel) because the weld metal used is usually stronger than the unstrengthened base alloy. A high-cap weld is therefore usually not needed to increase the strength of the base alloy.
As discussed above, neither the process of forming the standard welded joint 2 (FIG. 1) nor the process of forming the high-cap welded joint 17 (FIG. 2) will reliably result in a welded joint having a tensile capacity which is at least substantially equal to the tensile capacity of an unwelded, pre-stengthened, base alloy. With both of the above described processes, the benefit of increasing the tensile capacity of the base alloy using thermal or mechanical strengthening methods is substantially decreased because of the weakness of the welded joint resulting from the welding process. As a result, mechanical joining processes (such as connections using screw threads) rather than welding processes, have heretofore been used to join pipes having thermally and/or mechanically strengthened alloys when strength requirements for a particular application exist. Yet, mechanical joining processes are impractical for various pressure pipe applications where the connections are exposed to bending and other load conditions for which they are not designed (e.g. pipes used in subsea pipeline installations).
For the foregoing reasons, the need exists in the industry for a method for welding thermally and/or mechanically strengthened alloy conduits to produce a welded joint having a tensile capacity which is at least substantially equal to the tensile capacity of the unwelded, pre-strengthened, base alloy, thereby allowing the full strength of the base alloy to be developed throughout the welded joint of the conduits.