Technical Data & Welding Guidelines for Ferralium 255SD50 Super Duplex Stainless Steel

Transcription

1 Technical Data & Welding Guidelines for Ferralium 255SD50 Super Duplex Stainless Steel

2 Technical Data & Welding Guidelines FERRALIUM Alloy 255 Ferralium 255SD50 Super Duplex Stainless Steel General FERRALIUM 255-SD50 super duplex stainless steel is the latest refinement of FERRALIUM, possessing higher strength and an improved corrosion resistance when compared to the other commercial super duplex grades. FERRALIUM 255-SD50 is used extensively in the Offshore Oil & Gas Industry and Chemical Industry, where high levels of strength, corrosion and wear resistance are required. The following notes are provided to assist in devising effective fabrication methods for this versatile and practical material. Whilst the welding processes and practices discussed are applicable to all duplex and super duplex grades, any parameters quoted should be regarded as a guide only. It is important to use a skilled welder who is supported with a well thought out WPS that includes attention to details such as material joint preparation, welding sequence, cleanliness, gas coverage, current control and temperature monitoring equipment such as thermocouples and the use of approved, clean fillers. It is a feature of FERRALIUM 255-SD50 that all product forms can be easily welded by all of the usual welding methods with the exception of oxy-acetylene welding. As a result, the designer is provided with full scope to incorporate both castings and wrought forms into a single final assembly for optimum results. Only genuine FERRALIUM alloy electrodes or filler wire should be used. This will ensure sound welds and a satisfactory result in the final assembly with respect to both mechanical properties and corrosion resistance. FERRALIUM 255-SD50 should be welded in the solution-treated condition and, although post-weld heat treatment is desirable to restore the corrosion resistance and mechanical properties of heavy section fabrications, this is not necessary with most welded items. A carefully planned, well-controlled welding process should produce a high integrity joint with properties similar to the parent material. Following the welding procedure is essential to ensure that the metallurgy/ microstructure of the weld are optimised to provide excellent strength and corrosion resistance. The properties of the material depend on maintaining the balance between ferrite and austenite within the microstructure. Appropriate welding processes used with FERRALIUM 255-SD50 and other super duplex grades include: Gas tungsten arc welding (GTAW / TIG) Shielded metal arc welding (SMAW) Submerged arc welding (SAW) Flux cored arc welding (FCAW) Gas metal arc welding (GMAW / MIG) Plasma arc welding (PAW) GTAW (TIG) is generally used for lighter sections up to about 1 (0.512") but for heavier sections, which involve multiple passes SMAW or GMAW are preferred. Flux coated FERRALIUM 255-SD50 electrodes have been developed specifically for use with these latter processes: they provide improved ductility in the weld and eliminate the possibility of cracking in heavily restrained sections as the weld joint is built up. 2

3 Surface Preparation Material preparation is important in determining the final quality of the joint. Super duplex grades require similar procedures to austenitic grades. Surfaces to be welded should be inspected for defects, inclusions, nicks, dents, laminations etc. and, if present, the defects removed. If oxide is present on the surface to be welded e.g. following heat treatment, this must be removed to provide a bright metal finish. This can be achieved by mechanical means such as grinding, machining, emery cloth or wire brushing (using a stainless steel wire brush). Another alternative is to totally immerse the material in an acid solution of the following composition. 15% HNO3 + 2% HF (volume by volume) at 18 C minimum Blasting can also be used but care needs to be exercised with the media employed: it is important to ensure that no media or debris is entrapped or remains on the surface. After appropriate surface preparation it is essential to fully degrease with an approved solvent. Once cleaned, the surfaces should be welded within a short time or suitably protected if welding is delayed. Edge Preparations The edge preparations for welding are similar to those recommended for other stainless steels as shown below: Thickness up to 1.5mm Thickness above 10mm 70 no gap Thickness above 1.5mm to Thickness above to 10mm 70 gap equal to 1/2 t t Or 5mm R 10 Edge preparations may be made by shearing, machining or hand grinding to an accurate form. Cutting processes that impart significant heat into the material should be avoided. Joint Fit-up Accurate fit-up of parts to be welded is critical and they should be held in place by tack welds, clamps or jigs. Where tack welds are used, they should be sufficient in size and number to withstand any stresses which may develop during the welding of the joint. Tack welds should be free from oxide or slag and cleaned carefully prior to making the root pass. For butt-welding thin sheet, the use of a copper backing strip is recommended. 3

4 Welding Process and Welding Material All of the welding processes mentioned above are capable of satisfactorily welding duplex and super duplex stainless steels. The process selected will depend on availability, required quality, type, size, location and orientation of weld, speed and commercial considerations. The method should be optimised for the specific grade of material and type of joint being welded. The technique and weld procedure must be capable of producing the joint with the microstructure and properties required. In GTMA (TIG) welding the non-consumable tungsten electrode is connected to the negative side and the use of a superimposed high frequency current is recommended for ease of start-up. With GMAW (MIG) welding the consumable electrode is connected to the positive terminal. Welding Material To maintain the ferrite austenite ratio and optimise the microstructure / properties of the joint selection of the filler metal is crucial. The filler metal may have an excess of nickel over that in the parent metal in order to counter the production of excessive ferrite within the weld metal due to the rapid cooling. Nitrogen additions of % to the filler metal also stabilise austenite and help suppress ferrite formation. FERRALIUM 255-SD50 filler wires of matching composition to the parent metal are available for the GTAW welding process with a standard length of 1000mm and diameters 1.6mm (1/16"), 2.4mm (3/32") and 3.2mm (1/8"). For SMAW (MMA) processes, which are generally adopted for welding sections greater than 1 (0.512"), FERRALIUM 255-SD50 flux-coated electrodes are available. These are produced in diameters of 2.5mm (3/32"), 3.2mm (1/8") and 4.00mm (5/32") and are supplied in sealed containers. FERRALIUM 255-SD50 flux-coated electrodes are over-alloyed in nickel to ensure that the correct ferrite/austenite balance can be maintained in the as-welded material. Electrodes are supplied in sealed tins and should be dried in an oven at 150 C to remove moisture before use. Welding Procedures Pre-heat is not required and should be avoided. However, care should be taken to ensure that the areas to be welded are free from moisture. The maximum interpass temperature is 150 C for FERRALIUM 255-SD50 and other super duplex grades; temperatures in excess of this can lead to embrittlement of the weld and heat affected zone. When multipass welding is necessary, heat input should be kept as low as possible (consistent with achieving sound welds), particularly for sections greater than 1 (0.512"). GTAW Voltage volts Current amp Gas flow Torch 0.42m 3 /hour, under bead m 3 /hour Normally pure argon (>99.95%) is used as the shielding gas with GTAW. However, with super duplex grades, such as FERRALIUM 255-SD50, it has been found advantageous to add up to 2% nitrogen (as a 98% argon-2% nitrogen mixture) in order to restore nitrogen lost from the weld pool during the welding process. Backing gas of argon with up to 10% nitrogen should be employed. SMAW (MMA) Voltage Current volts amp, 2.5mm diameter amp, 3.2mm diameter amp, 4.0mm diameter 4

5 Post-Weld Heat-Treatment Fabrications of wrought FERRALIUM 255-SD50 welded in accordance with good welding procedures will generally provide very satisfactory corrosion resistance under most conditions. Post weld cleaning to remove any heat tint and blending of any surface discontinuities will also help maximise corrosion resistance. The application of a post-weld heat treatment is only necessary if large amounts of detrimental intermetallic phases are produced through excessively high heat input or slow cooling. In certain situations, due to the severity of corrosion conditions or the complexity of the welded fabrication e.g. when extensive welding of particularly thick sections is involved, it may well be beneficial to carry out post-weld heat-treatment. In such cases it is recommended that full details are discussed with Langley Alloys Technical Department. Welding to Dissimilar Metals FERRALIUM 255-SD50 can be satisfactorily welded to carbon steels, others stainless steels or nickel based alloys. Generally, FERRALIUM 255-SD50 welding consumables should be used under the same conditions described above. For the welding of FERRALIUM 255-SD50 to super-austenitic steels or nickel alloys, it is suggested that nickel alloys filler materials with high chromium and molybdenum contents but without niobium (e.g. E Ni Cr Mo 10 or SG Ni Cr 23 Mo 16) are employed. The Wharf, Lowfield Drive, Newcastle, Staffordshire ST5 0UU Tel: +44 (0) Fax: +44 (0) sales@langleyalloys.com Web: 5