Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
  • B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
  • B23K35/24—Selection of soldering or welding materials proper
  • B23K35/30—Selection of soldering or welding materials proper with the principal constituent melting at less than 1550 degrees C
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
  • B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
  • B23K35/24—Selection of soldering or welding materials proper
  • B23K35/30—Selection of soldering or welding materials proper with the principal constituent melting at less than 1550 degrees C
  • B23K35/3053—Fe as the principal constituent
  • B23K35/308—Fe as the principal constituent with Cr as next major constituent
  • B23K35/3086—Fe as the principal constituent with Cr as next major constituent containing Ni or Mn

Definitions

• the subject invention is directed toward the art of welding, and more particularly to improvements in welding low carbon stainless steel by controlling or selecting the chromium equivalent to nickel equivalent ratio.
• AISI 316L is a low carbon austenitic stainless steel commonly used in high purity piping systems for the semiconductor, biotechnology/pharmaceutical and nuclear industries.
• 316 stainless steel is an austenitic chromium-nickel-molybdenum stainless and heat resisting steel.
• 316L stainless is a low carbon version of 316 stainless steel with superior resistance to intergranular corrosion following welding.
• European analogs to AISI 316L are DIN X2CrNiMo 17122 and DIN X2CrNiMo 18143.
• weld slag and black spots are used interchangeably because although they differ in their appearance and location at a weld, they consist generally of the same chemical composition.
• Welds are visually inspected and weld slag and black spots are cause for rejection of a weld.
• Weld slag can result in incomplete weld penetration due to interference with heat input to the weld pool.
• Weld slag can also produce corrosion sites, as well as oxygen free sites that promote microbial induced corrosion.
• Weld slag is unacceptable generally for high purity applications wherein welds are expected to be smooth, straight and flat or slightly beaded, and corrosion free.
• the present invention contemplates a new process for controlling the quality of a weld produced when welding weld-grade, on specification austenitic stainless steel parts together, the process comprising forming a weld having a Cr-equivalent/Ni-equivalent ratio, R, of about 1.5 to 2.0, wherein
• the present invention contemplates a new process for improving the autogenous welding of two parts made from the same on-specification, weld-grade austenitic stainless steel comprising determining the above Cr-equivalent/Ni-equivalent ratios of the steels forming the parts to be welded together and rejecting for use in the autogenous welding process all parts made from steels not having a Cr-equivalent/Ni-equivalent ratio of about 1.5 to 2.0.
• the present invention also contemplates a new article of manufacture comprising a welded article comprising first and second parts welded together such that the parts are joined by a weld, the first and second parts being formed from the same or different weld-grade austenitic stainless steels, wherein the weld has the above-noted Cr-equivalent/Ni- equivalent ratio.
• Fig. 1 is a schematic view illustrating one aspect of the invention in which a weld ring is used to match chemistries of two bodies being welded together;
• Fig. 2 is a graph illustrating how ferrite concentration varies as a function of the Cr- eq/Ni-eq ratio in welds produced by autogenous welding of various different 316L stainless steels;
• Fig. 3 is a compilation of photomicrographs showing how weld quality varies as a function of the Cr-eq/Ni-eq ratio of the weld in a number welded articles produced in the working examples herein;
• Figs. 4, 5, 6 and 7 are graphs illustrating how corrosion resistance varies with the Cr- eq/Ni-eq ratio and ferrite content of the weld when certain austenitic stainless steels described in the working examples are autogenously welded.
• welds that is welds having no slag
• acceptable welds are formed from various different types of weld- grade austenitic stainless steels provided that the weld is primarily austenitic but exhibits some ferritic character as reflected by the Cr-eq/Ni-eq ratio R of the weld formed being about 1.5 to 2.0.
• acceptable welds tend to be slightly magnetic, indicating some small amount of retained ferrite in the weld. This has resulted in our further discovery and understanding that the formation of weld slag bears a close relationship to the solidification mode of the weld.
• stainless steel such as type 316L. These are austenitic, austenitic-ferritic, ferritic-austenitic and ferritic.
• austenitic weld will solidify completely to austenite and no further high temperature transformations occur.
• the austenitic-ferritic weld solidifies as austenite and delta ferrite is formed from the melt retained between the austenite dendrites.
• ferrite solidifies first and austenite forms between the ferrite dendrites.
• the austenite phase grows as the ferrite slowly transforms into austenite, resulting in a significant decrease in the volume fraction of ferrite in the final structure.
• the weld is substantially austenite, with a small amount of retained ferrite.
• the solidification mode of welds can be predicted by the chromium equivalent to nickel equivalent ratio, herein referred to as "R", where
• R Cr eq/Ni eq.
• the solidification mode is austenitic or austenitic-ferritic.
• the solidification mode is ferritic.
• the solidification mode is ferritic-austenitic.
• the above R values are approximate in nature and can vary somewhat such as, for example, about ⁇ 0.03.
• austenitic welds containing about 0.3 to 5 wt.%, preferably 0.5 to 3 wt.%, ferrite consistently and reliably exhibit no black spotting or slag formation even if they contain significant slag forming elements such as Ca, Si, Al, Ti and Zr.
• the preferred mode for reduction or elimination of weld slag in accordance with the present invention dictates that the weld produced have a Cr eq/Ni eq ratio sufficient to insure that the weld produced contains a small but suitable amount of retained ferrite.
• the weld should have a Cr eq/Ni eq ratio of at least about 1.5 (e.g. 1.47 or even as low as 1.45) as this insures the weld will contain at least about 0.3 wt.% or so retained ferrite at room temperature.
• the present invention merely renders these slag-forming elements benign by locking them into the crystal structure of the alloy. This avoids the expensive refining practices used in prior practices yet still provides product alloys exhibiting no black spotting or weld slag formation on a consistent and reliable basis. Additional benefits of using ferrite-austenitic solidification mode is that the presence of a small amount of ferrite is known to reduce hot cracking and micro-cracking. The solid solution of the various slag impurities also can contribute to the hardness of the material, further improving the overall strength of the weld.
• ferrite is considerably more prone to corrosion attack than austenite. Therefore, it is also desirable in accordance with the present invention to limit the ferrite concentration in the weld produced to a suitably low value to insure that welded articles produced in accordance with the present invention exhibit suitable corrosion resistance.
• Fig. 2 illustrates the relationship between percent retained ferrite and Cr eq/Ni eq ratio in some of the welds produced in the following working examples when 316L stainless steel articles were autogenously welded together.
• ferrite concentrations approaching 7-10 wt. % were achieved as the Cr eq/Ni eq ratio, R, approached 2.0.
• the corrosion resistance of a weld containing 7-10 wt. % ferrite might be acceptable.
• the ferrite concentration should be no more than about 5 wt. %, preferably no more than about 3 wt. %.
• the Cr eq/Ni eq ratio, R is limited to a maximum of about 1.67, more preferably to a maximum of about 1.55, as this dictates these lower ferrite concentrations in the welds produced.
• the Cr eq/Ni eq ratio of the welds obtained ranges between about 1.5 to 2.0
• the weld is formed solely from the parts being welded together. See, for example, United States Patent No. 5,223,686, the disclosure of which is incorporated herein by reference, in which an orbital welder is used to join adjacent sections of pipes or tubes.
• Other autogenous welding techniques can be used in connection with the present invention, however, such as manual welding.
• the weld formed in autogenous welding is derived solely from the parts to be welded together, achieving the desired Cr eq/Ni eq ratio in accordance with the present invention in autogenous welding is done by selecting the parts being welded to have necessary chemistries. Where the parts to be joined are made from the same alloy heat, this selection process is easily done by insuring that this alloy heat has the desired Cr eq/Ni eq ratio. However, where the parts to be joined are formed from different heats of the same alloy or different alloys altogether, the parts should be selected to have complementary alloys, that is alloys which when melted and combined together form a molten pool having the desired Cr eq/Ni eq ratio.
• Achieving a desired Cr eq/Ni eq ratio R in a particular weld produced by autogenous welding can be accomplished in accordance with the present invention by a variety of different methods. Preferably, this is done by a selection process in which candidate parts are either selected or rejected for welding based on the Cr eq/Ni eq ratios, R, of the steels forming the parts.
• This selection process can be done, for example, at the steel manufacturing level by rejecting from the mill all steels which will not produce a weld having the desired Cr eq/Ni eq ratio. This can also be done at the supplier level by rejecting for acquisition all parts otherwise on specification but which will not produce a weld having the desired Cr eq/Ni eq ratio.
• Another way of achieving the desired Cr eq/Ni eq ratio R in a particular weld is to control the mill process used to produce the steels of the parts to be welded.
• the compositional ranges for certain specified elements are reported by the mill to the customer for each heat of stainless steel delivered.
• a customer may further require that additional residual and/or trace elements be included in the report or "certification" for a variety of reasons. It is not typical, however, for the customer to participate with the mill owner in determining how particular steels will be made. Nor is it typical for the customer to order stainless steels with particular Cr eq/Ni eq ratios or to participate with the mill owner in designing manufacturing runs specifically designed to achieve particular Cr eq/Ni eq ratios.
• the desired Cr eq/Ni eq ratio R of a weld can also be achieved by control with the mill operator of the process used to produce the steels to be welded.
• Standard mill processes can be used to make these steels. Examples are argon oxygen decarburization (AOD), CLU converter process (CLU), vacuum oxygen decarburization (VOD), vacuum induction melting (VIM), vacuum arc remelting (VAR), electroslag remelting (ESR) and electron beam melting (EBM).
• AOD argon oxygen decarburization
• CLU CLU converter process
• VOD vacuum oxygen decarburization
• VAM vacuum induction melting
• VAR vacuum arc remelting
• ESR electroslag remelting
• EBM electron beam melting
• Dilution is typically used when an element cannot be refined out of a melt. It determines selection of scrap for the charge, and addition of other specified alloying elements to bring the residual element into an accepted range.
• various conventional refining techniques can be used to remove these ingredients from the melt. Examples of specific refining techniques are decarburization, deoxidation, desulfurization and dephosphorization. Regardless of what techniques are used, it is important to note that elimination of a trace element from a heat, for example Ti, does not alter the weight assigned to the other trace elements in determining the Cr eq/Ni eq ratio, as described above.
• an effective way of achieving a desired Cr eq/Ni eq ratio in candidate stainless steel parts to be welded together in accordance with the present invention is to control the chemistry of the alloys used to form these parts at the mill, and for this purpose conventional alloy forming and processing techniques can be used.
• the present invention is also applicable to non-autogenous welding in which the weld is formed from an extra material such as a weld rod or electrode (hereinafter “weld piece") in addition to the parts being welded together.
• welding piece an extra material such as a weld rod or electrode
• the parts being joined can be formed from the same alloy heat. More often than not, however, they are formed from different heats of the same alloy or different alloys altogether. In these situations, it is normal practice to match the chemistries of the alloys being welded together, as closely as possible, based on heat certifications obtained from the mill. In addition, it is also normal practice to select the weld piece to have a chemistry intermediate the chemistries of the parts being welded together to achieve a weld matched as closely as possible to both parts.
• black spots and weld slag can also be eliminated in the non-autogenous welding of austenitic stainless steel parts by selecting the welds to have ferrite contents and R valves as described above. Controlling the welds to have the desired chemistry in non-autogenous welding is done in essentially the same way as described above in the case of autogenous welding. However, in the case on non-autogenous welding the composition of the weld piece must also be taken into account in determining the chemistry of the weld ultimately produced.
• weld rings can be used to match chemistries at the weld site. More preferably, a set of weld rings can be provided that each have a different but known chemistry (chemistries can be determined by standard known techniques such as spectrochemical analysis, inert gas fusion, high temperature combustion, or wet analytical chemistry techniques). In addition, the chemistries of the base materials being welded are also determined.
• a weld ring is positioned between the ends of the tubes being welded, with the weld ring having a chemistry selected such that the weld pool formed from the weld ring as well as portions of the tubes which also melt will have the desired Cr eq/Ni eq ratio and therefore solidify to a ferritic-austenitic structure.
• a weld ring is selected that has a Cr eq/Ni eq ratio of about 1.6, and this ring is positioned axially and preferably concentrically between the tube ends being welded.
• a first tube end 10 is to be welded to a second tube end 12. Both tubes have an undesired ratio of about 1.4.
• a concentric weld ring 14 is positioned between the tube ends 10, 12 (in the drawing the relative axial size of the ring 14 is exaggerated somewhat for clarity).
• the weld ring can be formed with similar dimensions as the bodies being welded (for example, in the present example, the weld ring would be formed with similar inside and outside diameters).
• the weld ring material will mix with material from each of the tube ends to produce a weld pool having a Cr eq/Ni eq ratio of about 1.5. When this weld pool solidifies, it will solidify to the ferritic-austenitic mode without the formation of slag or black spots and will further be a smooth well-formed weld.
• the ring 14 can initially be tack welded as at 20 to either or both of the tube ends 10, 12 prior to performing the orbital welding process.
• a welding kit 30 can be provided that has a number of weld rings 14 of various and known Cr eq/Ni eq ratios for use at the welding station.
• a suitable container 32 can be used to store the weld rings 14. The welder can select a weld ring that most closely will match the known chemistries of the bodies being welded to produce a weld with a ferritic- austenitic solidification mode (based on the weld having a Cr eq/Ni eq ratio in the range of 1.45 to 2.0).
• the weld ring is selected so that the weld formed has a ferrite content of 3 wt. % or less. Even more preferably, the two pieces being welded together will also have ferrite contents of 3 wt. % or less.
• the weld ring chemistry can further be selected to match other elements such as the sulfur content, it being known that when welding heats with different sulfur contents where the spread is more than 0.01 weight % (for example, 0.001% S welded to 0.012% S), the weld arc will tend to deflect along the surface to the low sulfur heat while penetrating more deeply into the higher sulfur heat. This arc wandering can result in uneven and incomplete weld penetration.
• a weld ring can be selected with the appropriate sulfur content to match the two tube materials at the weld.
• the present invention is applicable to a wide variety of different weld-grade austenitic stainless steel alloys.
• certain grades of austenitic stainless steel cannot be welded, as a practical matter.
• the corrosion resistance of the welds produced are unacceptably low.
• the hardness and/or strength of the welds is inadequate.
• the present invention is directed to an improvement in welding those austenitic stainless steels which can be acceptably welded, which are referred to herein as "weld-grade" alloys.
• the present invention is particularly applicable to stainless steel alloys having the following compositions:
• alloys to which the present invention is applicable are the 300 series austenitic stainless steels, such as alloy 316, 317 and 304.
• the present invention finds particular applicability to low carbon stainless steels, i.e. stainless steels containing 0.03 wt.% or less carbon, such as 316L, 317L and 304L stainless steels.
• the present invention finds particular applicability to the alloys described above which also contain more than insignificant amounts of slag forming elements, i.e. Al, Ti, Si, Ca and Zr.
• slag forming elements i.e. Al, Ti, Si, Ca and Zr.
• prior art approaches to eliminating weld slag and black spotting have centered around keeping slag-forming elements below certain maximum tolerable concentration levels as set forth in Table 2. These approaches are very expensive, since severe refining procedures and/or expensive starting materials must be used.
• the alloys being welded can include one or more of these slag-forming elements in concentrations greater than the above maximum tolerable levels. Therefore, the use
• Table 5 shows the concentration levels of slag forming elements which can be exceeded in alloys processed by the present invention without formation of weld slag or black spots.
• the column headed "Prior Art Maximums" in this table shows the levels of slag forming elements regarded as maximums in the prior art for avoiding black spots and weld slag, as set forth in the above Table 2, and indicates that these concentration levels can be exceeded in the steels being welded in accordance with the present invention without forming black spots and weld slag.
• Table 5 shows that alloys containing more than 0.1 wt.% silicon, for example, can be welded without formation of black spots or weld slag in accordance with the present invention, even though 0.1 wt.% is regarded in the prior art as the maximum tolerable concentration of this element for producing welds free of black spots and weld slag.
• Table 5 also shows that more contaminated alloys can be welded without formation of black spots and weld slag even though they contain much higher levels of slag forming elements, for example, more than 0.75 wt.% or even more than 1.5 wt.% silicon.
• the Cr eq/Ni eq ratios of the alloys selected for welding are controlled to within fairly tight ranges, e.g. 1.45-1.55, 1.5-1.67, etc.
• it may be difficult to accomplish this control by adjusting the chemistries of these alloys at the mill using conventional alloy forming and processing techniques.
• the present invention in still another embodiment, provides a new, simplified process for adjusting the chemistries of candidate austenitic stainless steels during manufacture to achieve these narrow Cr eq/Ni eq ratios.
• copper is considered an undesirable trace element for stainless steel chemistries, because there is no practical way of refining copper out of iron-containing alloys. Accordingly, the concentration of copper is minimized in manufacture of most stainless steels, with copper being present typically at a background levels of only about 0.10 wt % or less, and rarely does the copper content exceed 0.50 wt %.
• copper is intentionally added to candidate alloys to reduce the Cr eq/Ni eq ratio to within the desired range of 1.45-1.67.
• the copper content of candidate alloys is increased in accordance with this aspect of the present invention to amounts above the typical background levels of 0.10 wt %, more preferably to levels above 0.25 wt %, even more preferably to levels above 0.35 wt %.
• the copper content of candidate alloys can be as high as 0.50 wt % or even higher, thereby permitting the Cr eq/Ni eq ratio to be lowered very easily.
• Adding copper to candidate alloy heats in accordance with this aspect of the present invention can be done in any conventional manner.
• copper can be added to the alloy heat at the ladle metallurgy station after the heat has been otherwise fully compounded but before it is cast.
• copper can be added to the heat during alloy manufacture.
• copper can be one of the original ingredients in the batch subjected to initial melting in the electric arc furnace, or copper can be added along with other element additions during Argon-Oxygen Decarburization (AOD) or other conventional processing in later stages of the alloy manufacturing operation.
• AOD Argon-Oxygen Decarburization
• the desired tighter range on the Cr eq/Ni eq ratio can be achieved using an AOD/VAR.
• VAR vacuum arc remelting
• a cast steel electrode having the desired chemistry for the final product is drip melted into a water cooled copper mold. This remelt is performed under very low pressure conditions, typically not exceeding 0.1 Torr.
• the VAR process is used to remove dissolved gasses in the heat, typically oxygen and hydrogen.
• the VAR process also removes nitrogen, and thus presents an opportunity to further adjust the Cr eq/Ni eq ratio.
• the VAR process also removes manganese. Lowering the nitrogen and/or manganese content in the chemistry will increase the Cr eq/Ni eq ratio.
• a melt can lose about 50% of the nitrogen and 10- 20% of the manganese in the electrode in a near total vacuum, and about 10-20% of the nitrogen and no manganese in a partial vacuum.
• the amount of nitrogen that is removed during a VAR remelt thus depends in part on how low the vacuum is pulled. Since the chemistry of the heat is known prior to the VAR process, the amount of nitrogen and manganese to be removed can be controlled by controlling the vacuum pulled in the VAR system. Thus, if the Cr eq/Ni eq ratio is low (below 1.45, for example), the VAR process can be used to adjust the ratio up into the desired range.
• a combination of the AOD process and the VAR process can also be used to tightly control the final Cr eq/Ni eq ratio of the heat.
• copper can be added to the heat, such as at the ladle metallurgy station, to lower the Cr eq/Ni eq ratio.
• copper may be added to bring the ratio down to just below 1.45, such as about 1.43.
• the VAR process can then be used to raise the ratio to the desired range , for example, 1.45-1.55 by the removal of nitrogen and manganese.
• nitrogen for example, can be added at the ladle metallurgy station during the AOD process so that after the VAR process the target equivalent ratio is achieved.
• Each tube was subdivided into sections, and two sections of each tube were autogenously welded together using a Hobart CT 150 DC autogenous welder.
• the welds were performed in a glove box with a shielding gas of 96% argon - 4% hydrogen, and a weld current of 47 amps and a weld speed of 6.3 in./min.
• the electrode was a standard 3/32 inch thoriated tungsten electrode. Thereafter, the welds formed were visually inspected for black spots and weld slag.
• photomicrographs were taken of each weld, these photomicrographs being set forth in Fig. 3.
• Each of these stainless steel tubes was subdivided into sections. Two sections of each tube were then autogenously welded together using an orbital welding system. The welds were performed in a glove box with a shielding gas of 100% argon. The weld current ranged from 20 to 47 amps, and the weld speed of ranged from 0.5 to 8.3 inches/minute. The electrode was a standard 3/32 inch thoriated tungsten electrode. For each heat, a portion of an unwelded section of tube plus the weld were subjected to corrosion resistance testing. In one test, the pitting potential of the tested metal was determined by ASTM G-61.
• an external power supply is used to gradually raise the electrical potential of the material being tested in a given solution, while electrical current is measured, until pitting corrosion occurs.
• the potential at which the current rapidly increases due to pitting is defined as the pitting potential.
• a higher pitting potential signifies a higher resistance to pitting corrosion.
• the critical pitting temperature was determined by ASTM G-150.
• ASTM G-150 the temperature at which current density increase rapidly beyond a set limit at a set electrical potential is determined.
• An NaCl solution is used, and the electrical potential is held constant in the passive region. Starting at a temperature of 0 °C, the temperature is raised slowly at a rate of 1 °C per minute until pitting occurs.

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