Title of Invention

A PROCESS FOR WELDING SUPER DUPLEX STAINLESS STEEL

Abstract

This invention relates to a process for welding super duplex stainless steel (SDSS) to enhance pitting corrosion resistance wherein the process comprises of the steps of conditioning of super duplex stainless steel rods is carried by homogenizing the rods at an elevated temperature, for example, 1070°C for approximately one hour followed by water quenching; fixing of super duplex stainless steel rods obtained in step (a) one each to a motor driven unit and a stationery unit at a burn off distance of 3-10 mm; pplying a forging force to the super duplex stainless steel rods fixed in step (b), demounting the super duplex stainless steel welded rods of step (c) and cooled to 25-35°C and removed from the unit.

Full Text

RELD OF INVENTION This invention relates to a process for welding of super duplex stainless steel (SDSS) to enhance pitting corrosion resistance. PRIOR ART Welding is a common technique that has been used extensively for joining the materials. Corrosion is a major concern for welding of Super Duplex Stainless Steel and welding failures take place due to it. Corrosion can reduce the load carrying capacity of a component either by generally reducing its section size or by pitting. Pitting corrosion in particular is highly dangerous because it not only reduces the effective cross section in the pitted region but also introduces stress raisers that in turn initiates cracks and causing thereby premature failures in service. Conventional fusion weld techniques, well known in the prior art is generally employed for joining the materials without filler metal if the thickness is below 2mm and above this thickness, filler metals are employed. However, it has been reported that fusion welding of super duplex stainless steel results in ferrite content more than about 80%. The main disadvantage of the conventional fusion welding technique is that the high ferrite content reduces the toughness of the weld. Another disadvantage of the above technique is that they are susceptible to hydrogen embritllement. The other known fusion welding technique, known in the prior art, utilises high nickel content electrodes. The use of high nickel content electrodes reduces the ferrite content to about 50%. The disadvantage of the fusion welding technique using high nickel content electrodes is that the welds have defects like porosity, segregation resulting in compositional and microstructural inhomogeneity leading to inferior corrosion resistance that results in corrosion related failures. Another disadvantage of the known fusion welding technique is that the corrosion related failures are not only confined to weld region but also occur in the heat affected zone, due to the chromium depletion in the heat affected region. OBJECTIVES OF INVENTION The main object of the present invention is to provide a process for welding of super duplex stainless steel (SDSS) wherein the welds have enhanced pitting corrosion resistance. Another object of the present invention is to provide a process for welding of SDSS wherein the welds have properties equivalent to parent metal. Yet another object of the present invention is to provide a process for welding of SDSS wherein there is no segregation and porosity in the welds. Still another object of the present invention is to provide a process for welding of SDSS wherein the welds are employed in fatigue critical applications like automobiles and aerospace fields. Further object of the present invention is to provide a process for welding of SDSS wherein it is possible to weld dissimilar material combinations. DETAILED DESCRIPTION OF THE PROCESS. According to this invention there is provided a process for welding super duplex stainless steel (SDSS) to enhance pitting corrosion resistance wherein the process comprises of the steps of: (a) conditioning of super duplex stainless steel rods is carried by homogenizing the rods at an elevated temperature, for example, 1070°C for approximately one hour followed by water quenching; (b) fixing of super duplex stainless steel rods obtained in step (a) one each to a motor driven unit and a stationery unit at a burn off distance of 3-10 mm; (c) applying a forging force to the super duplex stainless steel rods fixed in step (b), (d) demounting the super duplex stainless steel welded rods of step (c) and cooled to 25-35°C and removed from the unit. In accordance with the present invention the process for welding super duplex stainless steel (SDSS) comprises of the following steps:- a) Conditioning of the SDSS rods Super duplex stainless steel rods for example 144 mm diameter with a length of for example 60 mm are homogenized at for example about 1070°C for approximately one hour, and are then water quenched to obtain elongated grains of austerity in ferrite matrix. b\ Fixing of SDSS rods to be welded Two conditioned rods from step (a) are taken and one of the rods is fixed to a motor driven unit and another rod is fixed to a stationery unit. The burn off (length loss during heating stage) between the two rods fixed has a distance of 3-10 mm.After a burn off length of for example 5mm the rotation carried at 1200-1800 rpm preferably at a speed of 1450-1550 rpm for about 1-5 minutes is stopped and an axial force of 1-5 KN is applied to SDSS rod while rotating. Subsequently, a forging force of for example 6-11 KN was applied for consolidation of weld. The weld was allowed to cool at 25-35°C for about 10 minutes and removed from the unit. Thereafter, samples of 6mm height and 10 mm diameter were machined from the weld region to measure the pitting and repassivation potentials. Micro structural comparison of conventional and welds of present invention show that fusion welds contain coarse grains and multiphase microstructure while welds of present invention consist of fine equiaxed grains. In addition, fracto-graphic examination of some of the fusion welds revealed the presence of gas related porosity and hydrogen related cracking, while these are absent in the welds of present invention. The hardness of welds of present invention is found to decrease from the centre of weld, both in axial 85 radial direction. This indicates that the weld centre is harder than parent material and weld periphery. Notch tensile strength of welds of present invention is better when compared to parent material. From the impact studies it is found that the welds of present invention exhibit greater crack propagation resistance compared to conventional fusion welds. The evaluated pitting and repassivation potentials are governed by the principle, higher the pitting potential better is the resistance to pitting corrosion. The difference between the pitting and repassivation potentials is an indication for crevice corrosion resistance. In general, lesser the difference better is the crevice corrosion resistance. For obtaining excellent pitting corrosion resistance, the pitting potential should be very high. For welds of present invention the pitting potential is clearly indicated by its superior pitting and crevice corrosion resistance. It is to be understood that the process of the present invention is susceptible to modifications, adaptations, changes by those skilled in the art. Such modifications, changes, adaptations are intended to be within the scope of the present invention, which is set forth under the following claims. I CLAIM; 1. A process for welding super duplex stainless steel (SDSS) to enhance pitting corrosion resistance wherein the process comprises of the steps of: (a) conditioning of super duplex stainless steel rods is carried by homogenizing the rods at an elevated temperature, for example, 1070°C for approximately one hour followed by water quenching; (b) fixing of super duplex stainless steel rods obtained in step (a) one each to a motor driven unit and a stationery unit at a burn off distance of 3-10 mm; (c) applying a forging force to the super duplex stainless steel rods fixed in step (b), (d) demounting the super duplex stainless steel welded rods of step (c) and cooled to 25-35°C and removed from the unit. 2. A process for welding super duplex stainless steel (SDSS) as claimed in claim 1, wherein the SDSS rod fixed to a motor driven unit in step (b) is rotated at a speed of 1200-1800 rpm preferably at a speed of 1450-1550 rpm for about 1-5 minutes. 3. A process for welding super duplex stainless steel (SDSS) as claimed in claim 1, wherein an axial force of 1-5 KN is applied to SDSS rod while rotating. 4. A process for welding super duplex stainless steel (SDSS) as claimed in claim 1, wherein to consolidate the weld the axial force is increased to 6-11 KN. 5. A process for welding super duplex stainless steel (SDSS) substantially as described and illustrated herein.

Documents:

1109-DELNP-2004-Correspondence-Others (14-01-2010).pdf

1373-del-2003-abstract.pdf

1373-del-2003-claims.pdf

1373-del-2003-correspondence-others.pdf

1373-del-2003-correspondence-po.pdf

1373-del-2003-description (complete)-06-08-2008.pdf

1373-del-2003-description (complete).pdf

1373-del-2003-form-1.pdf

1373-del-2003-form-18.pdf

1373-del-2003-form-2.pdf

1376-DEL-2003-Abstract-(06-08-2008).pdf

1376-DEL-2003-Claims-(06-08-2008).pdf

1376-DEL-2003-Correspondence-Others (14-01-2010).pdf

1376-DEL-2003-Correspondence-Others-(06-08-2008).pdf

1376-DEL-2003-Description (Complete) (14-01-2010).pdf

1376-del-2003-form-13-(06-08-2008).pdf

1376-DEL-2003-GPA (14-01-2010).pdf