Welding Metallurgy • Study of Welding Metallurgy is important because the overall-mechanical properties of a weldments are determined by the properties of individual microstructure present in the weld deposit and the weld heat affected zone. Major problem associated with welding fabrication is the inability to obtain uniform mechanical properties through out the weldments. Welding Metallurgy is concerned with 1. Melting of electrode and base metal 2. Solidification of weld metal 3. Gas absorption 4. Slag metal reactions
Metallurgical Effects of Welding 1. Weld Metal Weld Metal behaves like a casting with defects and characteristics of a casting. Solidification of metals is usually considered to be a Nucleation and Growth process. Nucleation involves creation of critical sized particles (nuclei) and from which Growth will proceed.
Metallurgical Effects of Welding • Absorption of Gases by Weld Gases such as Hydrogen gets dissolved in the molten metal and may get trapped in the solid metal if the cooling is rapid. These gases may get retained in the microstructure or may form bubbles to form porosity in the meld metal. The gases may also react with the liquid metal or with one another. The gases which often cause trouble are Hydrogen, Nitrogen, Oxygen.
Metallurgical Effects of Welding 3. Slag Inclusions : Slag inclusions are frequently trapped in fusion welds and there is difficulty of removing it. 4. Hot Cracking of Welds : Hot Cracking of welds occurs at high temperatures. This may be due to the low ductility of the base metal. 5. Heat Affected Zone (HAZ): 6. Corrosion of Welds : Depends on the composition of base metal, process employed etc.
Welding Zones If welded joint closely examined, the joint consists of three distinct zones namely 1. Weld Metal Zone 2. Heat Affected Zone 3. Unaffected base metal or parent metal.
Weld Metal Zone • Weld Metal is formed as well as the weld metal solidifies from molten state. This is a mixture of parent metal and electrode (or filler metal). The ratio depends on welding process used, type of joint and plate thickness. • Weld Zone resembles a cast metal.
Heat Affected Zone (HAZ) • Adjacent to the Weld Metal Zone is the HAZ. HAZ is composed of the parent metal that did not melt but was heated to a high temperature for a sufficient period so that grain growth occurred. • Heat Affected Zone is that portion of base metal whose mechanical properties and microstructure have been altered by the heat of welding. • The width of HAZ varies according the welding process and technique. • HAZ consists of three metallurgicallydistinguished regions.
1. The Grain Growth Region : • Grain Growth Region is immediately adjacent to Weld Metal Zone. • In this zone, the metal his heated to a temperature well above the Upper Critical Temperature(A3). • This result in grain growth. 2. The Grain Refined Region : • Adjacent to the Grain Growth Region is the Grain Refined Region. • In this zone, the metal his heated to a temperature just above the Upper Critical Temperature(A3). • In this region finest grain structure exists. 3. The Transition Region : • Adjacent to the Grain Refined Region is the Transition Region. • In this zone, the metal his heated to a temperature between Upper Critical Temperature(A3) and A1.
Unaffected base metal or parent metal. • Outside the HAZ is the parent metal is the parent metal that was not heated sufficiently to change its microstructure.
Welding Stresses • Residual stresses are stresses existing within the weldment in the absence of external forces. • They are in static equilibrium within the body and are internally balanced. Residual Stresses may be classified into two categories. 1. Macro stresses : They are large scale internal stresses and may be developed by inhomogeneous plastic deformation from external loading, by non-uniform heating or by varied chemical diffusion. 2. Micro stresses : They are small scale internal stresses and may be developed by heterogeneities due to difference in elastic modulus, co-efficient of expansion etc.
Causes of Internal Stresses 1. Mechanical Residual Stresses Expansion and contraction of heated metal due to the piece being welded itself 2. Metallurgical Residual Stresses. Phase transformation which takes place while cooling 3. Reaction Stresses. Expansion and contraction of heated metal due to the other parts of structure to which the piece being welded is attached
Effects of Thermal Stresses • Residual Stresses can cause 1. Distortion of work-piece when welded. 2. Distortion of welded objects while machining. 3. Result in weld cracking. 4. Result in brittle fracture. 5. Result in lowered ductility. 6. Affect fatigue strength adversely. 7. Lower creep strength.
Methods for controlling Welding Stresses • Welding stresses may be reduced (before the weld is made) by the following methods. • Structure should be designed so that joints will have slowest residual stresses. For example a double V groove butt joint instead of single V. • Since residual stresses are caused by thermal strains due to welding, a reduction in amount of weld metal usually results in reduction of residual stresses. For example use of a U-groove instead of V-grove should result in reduction of amount of weld metal. • Peening will reduce residual stresses. 4. Cracks are great stress raisers. Crack tack welds should be chipped or melted out before welding work.
Stress Relief Heat Treatment of Weldments 1. Peening : In Peening, outer fibres of the weld are elongated with the help of hammer blows. When properly applied Peening reduces residual stresses to a great extent. But Peening reduces internal stresses of a low intensity. Peening also reduces distortion. Peening should not be employed to the first and last layers of weld. Excessive Peening will result in cracking of weld. 2. Vibratory Stress Relief : In this method, weld structures are subjected to vibrations to relive residual stresses. The weld structure is placed on a platform that vibrates. Up to 25%of residual stresses may be relived by Vibratory Stresses Relief Treatment.
Thermal Heat Treatment : Thermal treatment of weld consists of heating the weld structure uniformly to a suitable temperature for a predetermined period of time followed by uniform cooling. Thermal Treatment is a better substitute than Vibratory Stress relief. • Thermo-mechanical Treatment : In this method, the weld is heated to set up another residual stresses so as to counteract and thereby cancel the original residual stresses due to welding. This is a low temperature treatment when compared to Thermal Heat Treatment. Reductions up to 60% of residual stresses have been reported by this method. • Overstressing Treatment : In this method, the weld is loaded above the yield stress of the metal. When such pressure are removed, the residual stresses are found to be largely disappeared.