ADVANCED MATERIAL JOINING PROCESS UNIT 2 SEMINAR

ADVANCED MATERIAL JOINING PROCESS

1. University of Visvesvaraya College of Engineering (U V C E) State Institute of Eminence, Higher Education Department (Technical), Government of Karnataka. 2023 - 24 / SEMESTER I PRESENTED BY GOPI A (P25UV23T071002) PG - M.Tech (Manufacturing Science and Engineering) COURSE CODE: 18MS1C03 COURSE TITLE: ADVANCED MATERIAL JOINING PROCESSES UNIT - II Heat Flow In Welding: Heat Input, Heat Transfer, Heat Conduction, Convective Heat Transfer.

2. CONTENTS Heat Flow In Welding 3 Modes Of Heat Transfer Conduction Convection Radiation Heat Transfer Rate in Welding Heat Input in Welding Energy Absorption in Welding Peak Temperature and Heat Affected Zone (HAZ) Cooling Rate and Critical Cooling Rate in Welding Preheating in Welding and its Methods Solidification Rate in Welding Driving Forces in Welding References

3. HEAT FLOW IN WELDING Heat Flow/Transfer: It is the science which deals with the rates of energy of heat transfer between two bodies which are initially at different temperature. Importance / Significance of Thermal Effects In Welding : Thermal conditions in and near welds resulting metallurgical structure, mechanical properties, residual stress and distortion. Particular Significance are:- Weld Bead Area Weld Solidification Rate Peak Temperature in Heat Affected Zone (HAZ) Width of HAZ Cooling rates in the weld and HAZ

4. 3 MODES OF HEAT TRANSFER Conduction Convection Radiation

5. CONDUCTION It is the mode of heat transfer from a region of high temperature to a region of low temperature with in a solid, liquid or gas medium or between different medium which are in physical contact with each other. Heat transfer through Conduction Formulae: Q/t = kA(T1 – T2 / d) Q = Amount of Heat transfer t = Time Q/t = Rate of Heat Transfer k = Thermal Conductivity of material A = Cross Section Area T1 – T2 = Temperature difference d = Thickness of material

6. CONVECTION It is the mode of heat transfer by the combined effect of conduction and material transport surrounding fluids Heat transfer through Convection Formulae: Q/t = hA(Ts – Tf) Q = Amount of Heat transfer t = Time Q/t = Rate of Heat Transfer A = Cross Section Area h = Heat Transfer co-efficient Ts = Surface Temperature Tf = Fluid Temperature

7. RADIATION It is the mode of heat transfer from a body at high temperature to a body at low temperature, when the bodies are not in direct physical contact with each other or when bodies are separated from each other in perfect vacuum / space exists. Heat transfer through Radiation Formulae: Q/t = Q = Amount of Heat transfer t = Time Q/t = Rate of Heat Transfer = Stefan Boltzmann Constant = 5.67 X 10 ^-8 W/m^2*K^4 A = Cross Section Area T = Absolute Temperature in Kelvin.

8. Heat Transfer Rate: The rate at which the amount of heat “Q” being transferred per unit time “t”. SI unit of heat is Joule (J) SI unit of time is seconds (s) Heat transfer Rate is Joule / second (J/s) WELDING: Welding is a complex process that involves melting and fusing materials together to create a strong permanent joint and durable bond. One of the most critical parameters that affect the quality and integrity of a weld is the heat input. we will explore what heat input is, how it affects welding and whether it is useful in the welding process.

9. Heat Input in Welding: It is denoted by “q”. Heat input is the amount of heat energy applied to the base metal during welding. It is measured in joules per millimeter (J/mm) or kilojoules per inch (kJ/in). The heat input is calculated by multiplying the welding voltage, welding current, and welding speed. The higher the heat input, the more energy is applied to the weld and vice versa, which can affect the properties of the material and the quality of the weld. Heat input is a critical parameter in welding that affects the quality and integrity of the weld. By controlling the heat input, the welder can achieve the desired penetration, fusion and mechanical properties of the weld. Heat input is essential for welders, as it allows them to optimize their welding parameters and produce high-quality welds that meet the required standards and specifications.

10. Energy Absorption: During welding, the workpiece absorbs only a portion of the total energy supplied by the heat source. The absorbed energy is responsible for the outcome of the welding, including the formation of the liquid pool. the establishment of the time-dependent temperature field throughout the entire weldment and the structure and properties of the weldment. The physical phenomena that influence the energy absorption in the workpiece are unique to each welding process. For a given power source, the extent to which the energy is absorbed by the workpiece depends on the nature of the material, the type of the heat source and the parameters of the welding process. The efficiency of the heat source, η, is defined as the ratio of the energy absorbed by the workpiece to the energy supplied by the heat source, i.e., the fraction of energy transferred from the heat source to the workpiece.

11. How does Heat Input affect Welding? Heat input plays a critical role in welding, as it affects several key factors, including the penetration, dilution, distortion, and mechanical properties of the weld. The heat input also affects the heat affected zone (HAZ), which is the area surrounding the weld that experiences thermal changes due to the welding process. A high heat input can lead to excessive melting and distortion, while a low heat input can result in poor penetration and insufficient fusion. Peak temperature and Heat Affected Zone(HAZ): The weld thermal cycle of a particular location exhibits peak temperature and cooling rate as function of time apart from other factors. Peak temperature distribution around thee weld-Centre line determines:- Shape of the weld pool Size of heat affected zone (HAZ) Type of metallurgical transformation and so mechanical  properties of weld and HAZ.

12. Variation in heat input and initial plate temperature affects the peak temperature distribution on the plates along the weld line during welding. An increase in heat input by increasing the welding current (for a given welding speed) in general increases the peak temperature of a particular location and makes the temperature distribution equal around the welding arc (almost circular or oval shape weld pool). Increase in welding speed however makes the weld pool (peak temperature distribution) of tear drop shape.

13. Is Heat Input Useful in Welding? Yes, heat input is a useful parameter in welding, as it allows the welder to control the amount of heat energy applied to the base metal and adjust the welding parameters accordingly. By optimizing the heat input, the welder can achieve the desired penetration, fusion, and mechanical properties of the weld. The heat input can be used to qualify weld procedures and ensure that the weld meets the required standards and specifications. Heat flow Theory effects in solid is determined by: Work piece Thickness Edge, End Effects Thermal Conductivity and Specific Heat Heat Source Distribution Convection in the Weld Pool Latent Heat Absorption and Release

14. Cooling Rate in Welding: The cooling rate experienced in the weldment is a function of the rate of energy dissipation. The final metallurgical structure of the weld zone is determined primarily by the cooling rate from the maximum, or peak, temperature achieved during the weld cycle. Cooling rates are particularly important for heat treatable steels. The critical cooling rate for the formation of martensite in these steels is often commensurate with that likely to be encountered in welding. Calculations of Cooling rate: Thickness of the plate to be welded directly affects the cross sectional area available for the heat flow from the weld which in turn governs cooling rate of a specific location. Accordingly, two different empirical equations are used for calculating the cooling rate in HAZ, a) thin plates b) thick plates, depending upon the thickness of plate and welding conditions.

15. Two methods have been proposed to take decision whether to use thick or thin plate equation for calculating the cooling rates and these are based on 1) Number of passes required for completing the weld. 2) Relative plate thickness. The relative plate thickness criteria is more logical as it considers all the relevant factors which can affect the cooling rate such as thickness of the plate (h), heat input (Hnet), initial plate temperature (T₀), temperature of interest at which cooling rate is desired (Ti) physical properties of plate like (specific heat C, density ρ). Relative plate thickness (τ) can be calculated using following equations

16. h{ρC(Ti-T₀)/Hnet}½ Thin plate cooling rate equation is used when relative plate thickness τ< 0.6 and thick plate cooling rate equation is used when τ > 0.9. If value of τ is in range of 0.6 to 0.9 then 0.75 is used as a limit value to decide the cooling rate equation to be used. Cooling rate (R) equation for thin plates: {2ᴫkρC (h/ Hnet)(Ti – T₀)³} ⁰C/sec…..(1) Cooling rate (R) equation for thick plates: {2ᴫk(Ti – T₀)²}/Hnet ⁰C/sec …………..(2) Where: h is the plate thickness (mm), k is thermal conductivity,  ρ is the density(g/cm³),  ⁰C is specific heat (kCal/⁰C.g), Ti is the temperature of interest (⁰C), T₀ is initial plate temperature (⁰C).

17. Critical cooling rate (CCR) under welding conditions The critical cooling rate in the hardening of steels is the lowest cooling rate/minimum continuous cooling rate required to form a 100% martensite structure while minimizing internal distortions and stresses. (Martensite is a metastable interstitial solid solution of carbon in iron. It is formed when austenite is quenched rapidly to room temperature.) This welding speed is identified as critical welding speed (say 10mm/min in this case) above which cooling rate of the weld & HAZ becomes greater than critical cooling rate. This abrupt increase in hardness of the weld and HAZ is attributed to martensitic transformation during welding as cooling rate becomes greater than critical cooling rate owing to the reduction in heat input (Hnet) with increase of welding speed. Using welding conditions corresponding to this critical welding speed for a given steel plate, critical cooling rate can be calculate using appropriate cooling rate equation.

18. Preheating in Welding: Preheating in welding helps ensure weld quality and reduces the occurrence of cracking and other problems. Preheat is often used for this purpose when welding hardenable steels. Operations commonly use preheat before welding steel or steel alloy pipes or plates that are 1 inch thick or more. Ex: Oil & gas, transmission pipelines, power plants, structural construction, mining, shipbuilding and heavy equipment applications often require preheating in the shop and field. Preheating involves heating the area around the weld joint or the entire part to a specified temperature before welding. This reduces the cooling rate of the weld and drives out moisture. This in turn helps prevent hydrogen buildup and the potential for cracking. Operations can use several methods or welding preheat, including induction, open flame, resistance heating and convection ovens. Each one has benefits and drawbacks depending on the application. The best preheating method for a specific application often depends on the material thickness, size of the weldment, project timeline, budget, availability of personnel and expertise.

19. When to use Preheat in Welding? Determining if a welding application requires preheat depends on several factors, including the type and thickness of the base material. The welding code typically indicates the use of preheat. To meet the requirements of the code, the welding procedure specification (WPS) for the job will outline the minimum and maximum preheat temperatures as well as the necessary duration of preheating. Often, a part must stay within a specific temperature range for a certain amount of time such as between 250 degrees and 400 degrees Fahrenheit for 30 minutes before welding can start. Welders typically must monitor the base metal’s temperature between weld passes to ensure the material remains within the required range. Common temperature verification tools include crayons, thermocouples, infrared thermometers and thermal imaging cameras, etc.,

20. Methods of Preheating in Welding: Induction: Induction creates a magnetic field that generates eddy currents within the base metal, heating it internally from within. Open Flame: Operators use a fuel gas and compressed air torch (sometimes called rosebuds) to apply flame directly to the metal part. Resistance Heating: Resistance heating uses electrically heated ceramic pads placed on the base metal. The heated tiles transfer heat to the part through radiant heat and conductive heat where the pads are in contact with the part. Oven: Ovens use convection heating. Operators place the entire part inside the oven for preheating.

21. Solidification Rate: The time required for solidification of weld metal depends up on the cooling rate. Solidification time is the time interval between start to end of solidification. Solidification time is also of great importance as it affects the structure, properties and response to the heat treatment of weld metal. The solidification of weld metal takes place in three stages; reduction in temperature of liquid metal, liquid to solid state transformation, finally reduction in temperature of solid metal up to room temperature. It can be calculated using equation:        Solidification time of weld (St) = LHnet/2πkρC(tm-to)² in sec Where L is heat of fusion (for steel 2 J/mm³)

22. Above equation indicates that solidification time is the function of net heat input, initial plate temperature and material properties such as latent heat of fusion (L), thermal conductivity (k), volumetric specific heat (ρC), melting point (tm), initial temperature(to) Long solidification time allows each phase to grow to a large extent which in turn results in coarse-grained structure of weld metal. An increase in net heat input (with increase in welding current / arc voltage / reduction in welding speed) increases the solidification time. An increase in solidification time coarsens the grain structure which in turn adversely affects the mechanical properties. Non-uniformity in solidification rates in different regions of molten weld pool also brings variation in grain structure and so mechanical properties. Generally, centerline of the weld joint shows finer grain structure and better mechanical properties than those at fusion boundary primarily because of difference in solidification times. Micrographs indicate the coarser structure near the fusion boundary than the weld center.

23. Driving Forces in Welding: Several driving forces for fluid flow are present in the weld pool are:- Maragoni, Buoyancy, Electromagnetic and Arc-shear Forces. Marangoni Force: The spatial gradient of surface tension is a stress known as Marangoni stress. This stress may arise as result of variations of both temperature and composition of the weld metal. Frequently, convection in the weld pool results mainly from Marangoni stress, which is determined by the temperature gradient at the surface of the weld pool. The spatial gradient of surface tension is the product of the spatial gradient of temperature and the slope of the surface tension versus temperature plot. Marangoni stress can be expressed as follows: τ = dγ/dT * dT/dy Where: τ = Shear stress due to the temperature gradient lb/(in. min2) (g/[cm s2]), dγ/dT = Temperature coefficient of surface tension lb/(°F)

24. (g/[s2 °C]) and dT/dy = Spatial gradient of temperature, °F/in. (°C/cm) Buoyancy, Electromagnetic Forces and Arc Shear Forces: Buoyancy/up-thrust, tendency of an object to float or to rise in a fluid when submerged. This fluid can be either a liquid or a gas. When the surface tension gradient is not the main driving force, the maximum velocities can be much lower. The electromagnetic force in the weld pool increases as the arc becomes more constricted and produces a change in the motion of the weld pool. when an electric current flows through the workpiece, it will interact with a magnetic field to generate an electromagnetic force. Arc Shear Force, sometimes called "Dig" or "Arc Control" is a similar feature to Hot Start, except Arc Force operates during welding, not just at ignition. When the welding machine senses a short circuit it will deliver a peak of current.

25. REFERENCE Heat Transfer - YVC Rao. Basic Thermodynamics – PB Nagaraj & D Venkatesh. https://arcraftplasma.blogspot.com/2016/09/method-of-calculating-cooling-rate-in.html https://www.millerwelds.com/resources/article-library/preheat-in-welding-what-is-it-and-when-should-you-use-it#:~:text=Often%2C%20a%20part%20must%20stay,minutes%20%E2%80%94%20before%20welding%20can%20start . https://www.linkedin.com/pulse/understanding-heat-input-welding-useful-weldflow-engineers?utm_source=share&utm_medium=member_android&utm_campaign=share_via https://www.hilarispublisher.com/open-access/experimental-determination-of-cooling-rate-and-its-effect-on-microhardness-in-submerged-arc-welding-of-mild-steel-plate-grade-c-25-as-per-is-1570-2169-0022.1000138.pdf

26. THANK YOU