Que3.1. Explain in brief about weld thermal cycle. What are the factors affecting weld thermal cycle? Answer Weld Thermal Cycle 1. Weld thermal cycle shows variation in temperature of a particular position( in and around the weld) during the welding as a function of welding time. 2. As the heat source( welding bow or honey) approaches close to the position of interest first temperature increases hotting
governance followed by gradational drop in temperature cooling governance. 3. A typical weld thermal cycle as shown inFig.3.1.1 the rate of heating peak temperature, and time needed for attaining the peak temperature, cooling rate. Temperature Weld thermal cycle of position A Weld thermal cycle of position B Welding time 3.1.1. Schematic of weld thermal cycle of two different locales down from the weld centre line. 4. Since distance of the point of interest down from the weld centre line directly affects all the below parameters hotting
and cooling rate, peak temperature of weld thermal cycle thus each position N point offers different and unique weld thermal cycle as shown inFig.3.1.2. Advance Welding 3 – 3 H( ME- Sem- 5) 5. In general, an increase in distance of point of interest down from the weld centre line Decreases the peak temperature. ii. Decreases the rate of heating and cooling. iii. Increases time to attain peak temperature. iv. diminishments rate of cooling with increase in time. position of heat source B A Point of interest 3.1.2. Schematic of welding showing position of two points A andB. Factors Affecting Weld Thermal Cycle 1. Weld thermal cycle varies with distance from the weld centre line but it is also told by heat input rate, quantum of heat supplied for welding, weldment figure, thermal parcels of base essence and original plate temperature. 2. Rate of heat input is primarily governed by the energy viscosity of heat input source which to a great extent depends upon the welding process being used for development of weld joints besides the welding parameters. 3. High energy viscosity processes like tube bow welding and ray ray welding offer advanced rate of heating, peak temperature and cooling rates than low energy viscosity processes similar as gas welding, shielded essence bow welding. Que3.2. Draw a neat graph and figure to illustrate the temperature distribution around a metallic bow butt weld. Answer 1.Fig.3.2.1 shows the temperature distribution around a metallic bow butt weld. Electrode or bow is moving from right to left wing. Heat Flow Welding & Welding Metallurgy 3 – 4 H( ME- Sem- 5) Position along weld Temperature °C 538 1650 1093 1093815538400 °C Melting temperature 3.2.1. 2. The commanding edge of the temperature pattern is compressed, because the bow is moving towards cold essence and the running edge becomes extended due to the bow which leaves preheated essence in its wake. Que3.3. bandy in detail the computation of peak temperature of weld essence. Answer 2. Eq.(3.3.1) is also occasionally known as the Rosenthal equation. 3. Samples of the temperature distribution as represented by a family of isotherms drawn around the immediate heat source position XY aeroplane
) are shown inFig.3.3.1 shows the effect of thermal conductivity by comparing the isotherms for a fairly low thermal conductivity material( say sword) and a fairly high thermal conductivity material( say aluminum) when other processing conditions are the same. 4. The point on any isotherm that’s farthest from theX-axis( or line of stir of the heat source) is at its peak temperature at that moment. 5. Using eq.(3.3.1) and considering temperatures in terms of distance from the emulsion zone boundary, it can be shown that the peak temperature for a thin plate( line source) is given as, Y = Distance from the emulsion boundary at the workpiece face, and Tp = Peak or maximum temperature at a distance Y from the emulsion boundary. 7. Eq.(3.3.2) and eq.(3.3.3) are applicable to single pass processes and have to be applied to each pass by itself. 8. They’re useful for estimating the heat affected zone size and also for showing the effect of preheat on the HAZ size. 9. It’s apparent from the equations that all parameters being constant, preheating increases the size of the HAZ. Also, the size of the HAZ is commensurable to the net energy input. 10. therefore, high intensity processes similar as ray welding generally have a lower HAZ. 11. A high intensity energy source results in a lower total heat input because the energy used in melting the essence is concentrated in a small region. Que3.4. What do you mean by heat affected zone( HAZ) in welding? Why weld generally fails in HAZ? Answer Heat Affected Zone 1. Heat affected zone is the portion near the weld essence zone which is composed of parent essence and didn’t melt but toast to a enough high temperature for a sufficient period. Due to this heating, mechanical parcels and microstructure of this zone have been changed. 2. The HAZ in low carbon sword of normal structure welded in one run with carpeted electrodes or by submerged bow welding process comprises of three metallurgical different regions Grain Growth Region 1. This region is incontinently conterminous to the weld essence zone. 2. In this zone, base essence is hotted
to a temperature well above the upper critical temperature. This results in grain growth and coarsening of the structure. ii. Grain Refined Region 1. This region is conterminous to the grain growth region. 2. In this region, base essence is hotted
just above the upper critical temperature where grain meliorated is completed and finest grain structure exists. iii. Transition Zone 1. This region exists in a temperature range between the upper and lower critical metamorphosis temperatures where partial allotropic recrystallization takes place. Weld generally fails in HAZ due to 1. The failure of weld is due to weld decay. 2. The weld decay is caused by following reasons i. The time and the temperature of exposure. ii. The composition and previous treatment of the weld. Que3.6. How the cooling rate affects the property of the welded joint? Answer 1. The different types of cooling rate give the conformation of different types of response products as shown inFig.3.6.1. 2. Cooling wind( a) shows veritably slow cooling rate( conventional annealing) and it forms a course pearlite with low hardness. 3. Cooling wind( b) involves a briskly cooling rate than wind( a) homogenizing) and it forms the fine pearlite. 4. Cooling wind( c) involves a slow oil painting quench cooling; this results in a admixture of medium and fine pearlite. 5. Cooling wind( d) is typical of an intermediate cooling rate and results into the martensite and fine pearlite structure. 6. Cooling wind( e) is a drastic quench and results into the conformation of martensite structure. 7. Cooling wind( f) obtains a bainite structure by cooling fleetly enough to miss the nose of wind and also holding in the temperature range at which bainite is formed until metamorphosis is completed. 8. Cooling wind( g) is digression to the nose of TTT wind. 9. The wind( g) is critical cooling rate( CCR) for the sword. 10. If cooling rate> CCR Martensite structure will form. 11. If cooling rate< CCR Some softer metamorphosis products similar as pearlite or bainite will form. Que3.7. What do you mean by hotting
rate and cooling rate? How it affects the parcels of weld? AKTU 2018- 19, Marks 10 Answer Heating Rate 1. The heating rate of a work piece depends on how hot the heat source is and how efficiently that heat is transferred to the work. 2. A advanced temperature at the source means a steeper temperature grade between it and the work, and so the heating rate will be briskly. Cooling Rate 1. It’s defined as the difference in the austenitizing temperature and quench temperature divided by the time to cool within some value of computation of cooling rate on the centre line of the weld. Effect of Heating Rate 1. The welding heat input has a great influence on the weldments parcels. 2. Mechanical parcels and durability of weldment depend on microstructure of weld essence. 3. The cross sectional area of a weld is generally commensurable to the quantum of heat input. 4. As further energy is supplied to the bow, further padding material and base essence will be melted per unit length, performing in a larger weld blob. 5. The most important specific of heat input is that it governs the cooling rates in welds and thereby affects the microstructure of the weld essence. 6. A change in microstructure directly affects the mechanical parcels of weld. thus, the control of heat input is veritably important in bow welding in terms of quality control.
