A Corrosion Resistant Alloy (CRA) barrier can provide enhanced corrosion and erosion resistance. Solid Corrosion Resistant Alloys (CRA) such as UNS N06625 are expensive and hence a CRA barrier over carbon steel piping is a cost-effective solution to solid CRA piping for severe service applications. Pipe cladding provide the means for achieving this barrier by a process of bonding two dissimilar metals. 

Application of Barrier

The CRA barrier or pipe cladding can be provided by using one of the following methods:

  • Roll Cladding - is a process of bonding two dissimilar materials by heating and passing them together through a set of hot rollers. 
  • Explosion Bonding - is a process of bonding two dissimilar materials with the help of pressure and heat produced by explosion. The process involves setting of high explosives. The explosive material is spread on the material to be cladded which is kept on top of the base material. On ignition of the explosive, resultant thrust bonds the clad plate on the base plate kept underneath.
  • Weld Deposited Overlay - is a fusion welding technique used to deposit weld metal on the base material to achieve the desired overlay thickness and properties to provide a wear or corrosion resistance surface.
  • Coextrusion - is a process of pushing molten exterior pipe and its liner at the same time through a circular mold. The coextrusion process creates a seamless pipes, however this process cannot be used for large pipe sizes. 

Of the several processes used to manufacture cladded pipes and fittings, the weld overlay process is the most commonly used in the oil and gas industry. The properties of alloy 625 make it an excellent choice for internal corrosion resistant layer in oil and gas applications in sour service to provide local pitting and crevice corrosion resistance; high corrosion-fatigue strength, high tensile strength, and resistance to chloride or sulfide stress-corrosion cracking. This article mainly discusses the application of UNS N06625 weld overlay on carbon steel piping. Weld overlay over carbon steel can be applied in other materials such as stainless steels, super austenitic stainless steels.

Weld Deposited Overlay

Weld Overlay PipesWeld deposited overlay can be applied on both seamless and welded pipes and fittings. Though weld overlay can be performed on pipe sizes as low as 2" NPS, the depth to which the overlay can be performed is limited to between 3m to 4m. This means for smaller diameter pipes you will get shorter pipe spools which have to welded in the field. A good balance is to overlay pipes 6"NPS and above which will allow suppliers to provide weld overlay on lengths up to 12m. An important thing to note when performing overlay is that the base pipe should not get distorted due to excessive heat input of the weld overlay process. To avoid this possibility, a minimum pipe wall thickness should be maintained which can be as low as 7mm depending on the manufacturers overlay technique. The base material thickness should be capable of meeting the design service pressure. The thickness of overlay is not normally taken into consideration for pressure design thickness of piping. Different welding methods can be used to deposit the weld overlay:

  • CO2 welding
  • Shielded Metal Arc Welding (SMAW)
  • Metal Inert Gas (MIG) Welding
  • Plasma Transferred Arc Welding (PTAW)  
  • Submerged Arc Welding (SAW)
  • Tungsten Inert Gas (TIG) Welding also known as Gas Tungsten Arc Welding (GTAW)

The TIG or GTAW weld overlay or cladding process is most popular method as it provides a smooth and consistent weld overlay deposition.

Challenges to providing Weld Overlay

The process of depositing UNS N06625 (Inconel 625) material on the base carbon steel material requires fusion of the deposited Inconel with the base carbon steel pipe. However, good fusion and penetration comes at the cost of dilution of Inconel with Iron (Fe) undermining the cladding process. This limitation inevitably requires the weld metal to be deposited by not less than two layers to achieve a minimum 1.5mm to 2.5mm of undiluted alloy with chemistry matching that of UNS N06625 from the surface. Depending on the severity of application the end user may require undiluted alloy from 1.5mm to 2.5mm thickness. If the end user requires a minimum undiluted thickness of 2.5mm the actual specified overlay thickness may be as high as 4mm to achieve this undiluted thickness with two deposits of weld overlay. In this case, the chemical analysis is carried out at a depth of 2.5mm from the surface to ensure the iron dilution does not exceed 5-10% at this depth. The weld overlay thickness in general can be in the range of 3mm to 5mm depending on the severity of the application. The diluted portion of the weld overlay will exist below the undiluted layer. This layer should generally not have Iron (Fe) dilution of more than 5-10%. Iron dilution of 5% will provided better corrosion resistance. The tolerance on the finished alloy thickness must be kept in the range of +2mm and -0mm.

Undiluted chemistry of Inconel 625

Dilution is related to the speed at which the weld overlay process is carried out. Faster Inconel deposition requires high heat input which creates more iron dilution. Hence a balance has to be maintained in the deposition rate to achieve the required dilution. The GTAW process offers a low deposition rate with controllable heat input and therefore low iron dilution. The filler metal used in the GTAW process is ERNiCrMo-3 for achieving UNS N06625 weld overlay. If the pipe is intended to be used for sour service, it is recommended to limit the sulfur content in the filler wire to less than 0.005 percent. In conventional cold wire TIG cladding, the energy to melt the filler metal comes from the electric arc which limits the melting rates than can be achieved. In the hot wire TIG process the wire is preheated by a separate power source allowing increased melting rates and improved productivity. 

The weld overlay is done in circumferential direction. During the overlay application process the temperature of the base material should not exceed the critical temperature. (The critical temperature of steel defines the phase transition between two phases of steel. As the steel is heated above the critical temperature, about 1335°F (724°C), it undergoes a phase change, recrystallizing as austenite). If the temperature exceeds the critical temperature, then the piping component should be subject to heat treatment to bring the base metal to its original metallurgical state. The base metal material must retain the minimum mechanical property requirements after post-weld heat treatment.

Girth Welding of CRA Components

Pipes, fittings and flanges that are required to be clad are procured with plain ends without bevelling. Fittings and flanges are procured with 10-15mm additional lengths. Additional weld deposits are made on the bevel ends of pipes, fittings and flanges to allow machining or grinding and matching of the ends after overlay. The out of roundness of the end must meet the required dimension and within the acceptable tolerance limit. The minimum overlay thickness shall be maintained after grinding of mismatch. Bevelling of the ends is done after completing the internal weld overlay.

Inconel Overlay End Preparation

When performing overlay on flanges, the following must be taken into consideration. The final dimensions of the flange after overlay should match the dimensional standard such as ASME B16.5, ASME B16.47 or API 6A flanges. Since overlay will add to the flange thickness, it would be necessary to machine the flanges to such an extent that after accomplishing the weld overlay and final machining, the dimension of flange should match that of the applicable standard. The machining concept does not apply to the inside diameter of the flange. Overlay on the inside diameter of flange will reduce the bore of flange and will match the connecting pipe which also has a weld overlay on the inside diameter. Example drawing below shows carbon steel flange with 3mm UNS N06625 weld overlay on the face of the flange. In this example the inside diameter of the flange is not provided with weld overlay.

Weld Overlay on Flange Face

Material traceability by Cladding Supplier

Weld overlay of piping components involves interfaces with the Cladding supplier. All piping components and spools to be cladded are free issued to the cladding supplier with the inspection release notes and material test certificates. The cladding supplier must have his own material traceability procedure which should include as a minimum the following:

  • Identification number for each item
  • Material heat number
  • Welding consumable heat/lot number
  • Consumable gas lot number
  • Weld overlay status
  • Non destructive examination (NDE) status
  • Dimensional control status

Surface Preparation of Components

All piping components should be internally cleaned to remove scale, rust or grease and make it free from any contamination. All possible sources of contamination such as use of carbon steel wire brushes should be avoided.

Ultrasonic Testing of Weld Overlay Components

Ultrasonic examination or testing (UT) can be employed to establish the overlay thickness and the presence of lack of fusion between the base material and weld overlay or between two individual layers of overlay. UT is carried out in accordance with ASTM A578 - "Standard Specification for Straight-Beam Ultrasonic Examination of Rolled Steel Plates for Special Applications". Ultrasonic examination is carried out from the outside wall. For a pipe, suggested measurement of wall thickness is before and after weld overlay over the entire pipe length, at a pitch of 150mm at four locations around the circumference. The minimum overlay thickness should not be less than the specified thickness. Additionally, UT should be carried out to establish the integrity of the fusion zone.

Corrosion Testing of Weld Overlay Components

In order to assess the corrosion resistance of Inconel 625, Intergranular Corrosion (IGC) tests as per ASTM G28 - "Standard Test Methods for Detecting Susceptibility to Intergranular Corrosion in Wrought, Nickel-Rich, Chromium-Bearing Alloys" are required. The acceptance criteria for UNS N06625 is 36mpy or 0.914mm/year.

Testing to ensure thickness of Weld Overlay

There are a few methods to measure the weld overlay or clad thickness. One of the methods involves measuring the pre-weld thickness at predefined locations at four radial positions at 90° intervals. Post weld overlay thickness measurements are taken after the cladding is complete at the same locations using the same method. The difference between the pre-weld and post-weld overlay will give the actual overlay thickness of the pipe at the predefined locations.

Another method involves using magnetic thickness gauges which measure the reluctance of a magnetic flux path passing through the weld overlay and the base material. The gauges are designed to measure the thickness of the nonmagnetic material on a magnetic base material.

Visual Inspection

Each finished pipe and fitting must be 100% examined both internally and externally in accordance with ASME Section V Article 9 and must be free of defects. Unacceptable surface imperfections must be removed by machining or grinding and the repair area blended into the contour of the pipe or fitting as long as the remaining pipe wall thickness remains above the minimum wall thickness as per the applicable code.

Internal visual examination of the cladded piping must be carried out as per API 5LD section 12.2. The internal overlay clad surface must be visibly free from dirt, grease, oil or other detrimental contaminants. The weld overlay surface must be relatively smooth and continuous and free of cracks and arc burns. The average peak to trough height between adjacent beads must be less than 0.8mm. No gaps between adjacent weld beads must be permitted.

Hardness Testing

If the cladded component is intended to be used for sour service, hardness testing should be performed. Hardness of the weld overlay should meet the applicable requirements specified in ISO 15156 / NACE MR0175 Clause 7.3.3 Figure-6. Hardness testing is performed using Vickers test in accordance with ASTM E92 - Standard Test Methods for Vickers Hardness and Knoop Hardness of Metallic Materials" using 5kg or 10kg load. A minimum of 5 indentations are recommended in each layer of weld overlay up to a maximum of two layers, over the heat affected zone and away from the unmixed zone of carbon steel.

Positive Material Identification

The weld overlay should be subject to PMI chemical analysis.

Pickling after Overlay

The cladded components should be subjected to pickling in accordance with ASTM A380. While performing the pickling process adequate care shall be taken to prevent the carbon steel base material coming in contact with the pickling chemical.