Metal Joining 3 PDF

METAL JOINING 3 CONTENTS Oxyacetylene welding with filler rods Flux? What is its role Compatibility of oxyacetylene welding with different materials Pressure gas welding Welding metallurgy-grain structure and residual stress Weldability/ joinability Oxy acetylene welding with consumable electrode...

METAL JOINING 3 CONTENTS Oxyacetylene welding with filler rods Flux? What is its role Compatibility of oxyacetylene welding with different materials Pressure gas welding Welding metallurgy-grain structure and residual stress Weldability/ joinability Oxy acetylene welding with consumable electrode Welding rods come in standard sizes, with diameters from 1.5 to 9.5mm ( 1/16 to 3/8 in.) and lengths from 0.6 to 0.9m(24 to 36 in.). https://www.youtube.com/watch?v=PbJ6KnV3oFc Filler rod/ wire. They may be bare/ coated with flux and its role Functions of a flux To promote the formation of a better bond, fluxes may be used to clean the surfaces and remove contaminating oxide. In addition, the gaseous shield produced by vaporizing flux can prevent further oxidation during the welding process, and the slag produced by solidifying flux can protect the weld pool as it cools. Flux can be added as a powder, the welding rod can be dipped in a flux paste, or the rods can be precoated. The flux ingredients are usually: calcium, calcium carbonate, calcium fluoride, magnesium, potassium and manganese oxides, sodium, silica, and aluminium oxide. The flux ingredients come together to help stabilize the arc and have more control over the mechanical properties. Advantages oxy fuel welding process Oxyfuel equipment is quite portable, relatively inexpensive, and extremely versatile. A single set of equipment can be used for welding, brazing, and soldering, and as a heat source for bending, forming, straightening, and hardening. With the modifications to be discussed shortly, it can also perform flame cutting. Pressure gas welding Heating effects: welding metallurgy Welding metallurgy helps determine the structure and properties across the joint and the need for additional thermal treatments. The molten pool is actually a complex alloy of all four materials. a small metal casting in a large metal mold a metal casting in a metal mold, coupled with an abnormal and widely varying heat treatment Adequate mechanical properties, therefore, can only be achieved by selecting filler rods or electrodes filler rod compositions tend to produce fine, equiaxed grains. Weld pool suffers from defects such as gas porosity, inclusions, blowholes, cracks, and shrinkage. Heat affected zone experiences sufficient heat to bring about structure and property changes, Phase transformations, recrystallization, grain growth, precipitation or precipitate coarsening, embrittlement, or even cracking. Weakest area in the as-welded joint. The size of the heat-affected zone will also increase with increased starting temperature, decreased welding speed, low rates of heat input, slow cooling rate, large area of HAZ increased thermal conductivity of the base metal, and a decrease in base metal thickness. Weld geometry is also High heat input, faster cooling rate, small area of HAZ important, with fillet welds producing smaller heat-affected zones than butt welds. THERMAL-INDUCED RESIDUAL STRESSES If the welded plates in Figure 30-15a are restrained from horizontal movement, additional stresses will be induced. These residual stresses are known as reaction stresses, and they can cause cracking of the hot weld or heat-affected material or can contribute to failure during subsequent use. EFFECTS OF THERMAL STRESSES -Total heat input to the weld should be minimized - Shot peening/Surface rolling When a multipass weld is being made, cracking tends to occur in the early beads where there is insufficient weld metal to withstand the shrinkage stresses. Crack-prevention efforts can also include maintaining the proper size and shape of the weld bead. cooling more uniform or relaxing the stresses by promoting plasticity in the metals being welded. Hydrogen dissolved in the molten weld metal can also induce cracking  WELDABILITY OR JOINABILITY Within a given process, the quality of results may vary greatly with variations in the process parameters, such as electrode material, shielding gas, welding speed, and cooling rate. References: 1. Kalpakjian S and Schmid S R, Manufacturing Processes for Engineering Materials, 6th Edition, Pearson Education, 2018. 2. J.T Black and Ronald A. Kohser, Degarmo’s Materials and Processes in Manufacturing,11th Edition, John Wiley and Sons,2012