AD welds slides
Welding of Dissimilar Metals Most of the welds are between dissimilar metals Same General Type with two different sources Same General Type but at two extreme ends of specification Joining of two completely different materials Successful weld between dissimilar metals At-least as strong as the weaker of two metals being joined Often considered weaker than both Properties vary across the weld – variation is much more than conventional welds Lack of overall ductility occurs when one is significantly weaker than the other Prequalification – A necessary requirement Welding of Dissimilar Metals Possible Type of JointsMost of the welds are between dissimilar metals Non-Fusion Joints – Simplest Case Adhesives / Bolting Brazing and Soldering – Generally considered non-fusion Joints Brazing – some metallurgical interaction, conductivity, expansion and corrosion Fusion Joints – A Range of controlling Factors 1. Melting Temperatures 4. Pre & Post Heating 2. Expansion 5. Choice of Welding Processes 3. Thermal Conductivity 6. Weld Pool Properties Welding of Dissimilar Metals Fusion Joints – Controlling Factors 1. Melting Temperatures i. When subject to same heat source, one will be melted before the other ii. When significant difference, one will form larger part of weld pool iii. Probability of hot tears in low melting point alloy iv. Solution – When there is a wide difference, it may become necessary to include a material with intermediate melting temperature (usually a brazing alloy) i.e. buttering 2. Expansion (Coefficient of thermal expansion) 1. When widely different, thermal stresses will be setup which may lead to Cracking during cooling or during service. Distortion Fatigue failure in components subject to thermal cycling (e.g. CrMo Steels and SS304/316 in steam service) See Example Welding of Dissimilar Metals Fusion Joints – Controlling Factors 3. Thermal Conductivity i. When widely different one can become highly heated, leading to distortion ii. Thermal conductivity changes with temperature. It rises for some, decreases for others. iii. Solution 1: Direct the heat source iv. Solution 2: Where distortion is critical, it may require heat input or extraction. 4. Pre and Post Heating i. Pre-heating is usually not much of a problem ii. Post-heating (heat treatment) can cause problems (CS-SS304 Joints) iii. Two different age hardening alloys can have different ageing temperatures Welding of Dissimilar Metals Fusion Joints – Controlling Factors 5. Choice of Welding Process i. Precise location of arc to alloy differential Heat Transfer 6. Weld Pool Properties i. Metal Mixing ii. Dilution Calculations iii. Microstructure Determination iv. Microstructure Stability v. Corrosion vi. Magnetic Effects Welding of Dissimilar Metals Joint Design 1. Selection of a correct filler – eventual joint serviceability 2. Many Dissimilar metals are associated with Stainless Steels 3. Use of Schaeffler Diagram QUESTION A low alloy creep resisting steel (C: 0.2%: Mn:0.6%;Cr:0.25%; Mo: 0.8%;Si:0.2%) has been welded to a 304L type stainless steel (C:0.03%; Cr:17.2%;Mn:1.65%; Ni:11.9; Si:0.6) using a 309 type consumable (C:0.05%; Mn:1.50%; Cr:23.2%; Ni: 13.48%; Si: 0.48%). The proportion of each material in the weld pool is estimated as:Creep Resisting Steel: 25% 304L type stainless steel: 25% 309 consumable: 50%. Calculate the probable composition of the weld metal, and determine from the Schaeffler diagram shown what microstructural constituents are predicted in the weld. Welding of Dissimilar Metals Welding of Dissimilar Metals