Welding lectures 9 10

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Welding lectures 9 10

  1. 1. 10/5/2012Casting, Forming & WeldingCasting Forming & Welding (ME31007)  J u au Jinu Paul Dept. of Mechanical Engineering 1 Course details: Welding Topic Hours1. Introduction to welding science & technology 2-32 Welding Processes 43 Welding Energy sources & characteristics 1-24 Welding fluxes and coatings 15 Physics of Welding Arc 16 Heat flow in welding 27 Design of weld joints 18 Defects, Testing and inspection of weld joints 1-29 Metallurgical characteristics of welded joints, Weldability and welding of various metals and alloys 2 Total 19 2 1
  2. 2. 10/5/2012 Welding Lecture ‐ 9 04 Oct 2012, Thursday, 8.30‐9.30 am y Heat flow in welds 3 3 The welding thermal cycle• Thermal excursion Weld temp. ranges from the ambient temp. of the work environment to above the liquidus temp. (possibly to boiling point and above for some very hi h high-energy-density processes) d it )• The severity of this excursion  in terms of the – temp. reached – time taken to reach them – the time remain at them completely determines the effects on structure (both microstructural for material changes and macrostructural for distortion)• To quantify the thermal cycle mathematically, we need temp. distribution in time and space co- 4 ordinates 2
  3. 3. 10/5/2012 Thermal cycle characterization via thermocouples Temp. p 5 Time Thermal cycle- quasi-steady state• Thermocouples  at various points along weld path• Approach of the heat source  rapid rise in temperature to a peak  a very short hold at that peak  then a rapid drop in temperature once the source has passed by• A short time after the heat from the source begins being deposited,  the peak temperature & rest of the thermal cycle, reaches a quasi-steady state• Quasi-steady state  balance achieved between the rate of energy input and the rate of energy loss or t f i t d th t f l dissipation• Quasi-steady state  temperature isotherms surrounding a moving heat source remain steady and seem to move with the heat source (away from edges) 6 3
  4. 4. 10/5/2012 Thermal cycle- quasi-steady state HAZ S Fusion zoneTemperature isotherms surrounding a moving heat sourceremain steady and seem to move with the heat source 7 Time-Temperature curves Tp Time= ti Temperature Time 8 4
  5. 5. 10/5/2012 Time-Temperature curves• The peak temp. decrease with increasing distance from the source, and more or less abruptly• The maximum temperatures reached ( mA TmB, Tmc) p (T decrease with distance from the weld line and occur at times (tmA, tmB, tmc) that increase. This allows the peak temperature, Tp to be plotted as a function of time• Peak temp. separates the heating portion of the welding thermal cycle from the cooling portion,• At a time when points closest to a weld start cooling, the p points farther away are still undergoing heating. This y g g g phenomenon explains – certain aspects of phase transformations that go on in the heat-affected zone, – differential rates of thermal expansion/contraction that lead to thermally induced stresses and, possibly, distortion 9 Spatial isotherms Tp (Peak temperature) Heating zone Cooling C li zone Cooling zone Heating zone 10 5
  6. 6. 10/5/2012 The generalized heat flow equation• Temp. distribution Controls microstructure, residual stresses and distortions, and chemical reactions (e.g., oxidation)• The influencing parameters – the solidification rate of the weld metal, – the distribution of peak temperature in the HAZ – the cooling rates in the fusion and HAZ g – the distribution of heat between the fusion zone and the heat-affected zone• Requires mathematical formulation to quantify the influence of these parameters 11 The generalized heat flow equation Heat supplied + Heat generated = Heat consumed (for temp rise, melting) + Heat transferred via conduction + Heat loss via convection & radiation 12 6
  7. 7. 10/5/2012 The generalized heat flow equationx = coordinate in the direction of welding (mm)y = coordinate transverse to the welding direction (mm)z = coordinate normal to weldment surface (mm)T = temperature of the weldment, (K)k(T) = thermal conductivity of the material (J/mm s-1K-1) as a function of temperature(T) = density of the material (g/mm3) as a function of temp.C(T) = specific heat of the material (J/g-1 K-1) as a function of ), temperatureVx, Vy, and Vz = components of velocityQ = rate of any internal heat generation, (W/mm3) 13 The generalized heat flow equation• This general equation needs to be solved for one, two, or three dimensions depends on – Weld geometry, – Whether the weld penetrates fully or partially – Parallel sided or tapered, and – Relative plate thickness• 1-D solution  thin plate or sheet with a stationary source or for welding under steady state (at constant speed and in uniform cross sections remote from edges) in very thin weldments• 2 D solution  thi weldments or i thi k weldments 2-D l ti thin ld t in thicker ld t where the weld is full penetration and parallel-sided (as in EBW) to assess both longitudinal and transverse heat flow• 3-D solution  thick weldment in which the weld is partial penetration or non-parallel-sided (as is the case for most single or multipass welds made with an arc source) 14 7
  8. 8. 10/5/2012 Weld geometry and dimensionality of heat flow(a)2-D heat flow for full-penetration welds i thi plates ld in thin l t or sheets;(b)2-D heat flow for full-penetration welds with parallel sides (as in EBW and some LBW)(c)3-D heat flow for partial penetration welds in thick plate(d)3D, condition for near-full penetration welds 15 Rosenthal’s Simplified Approach • Rosenthal’s first critical assumption  Energy input from the heat source was uniform and moved with a constant velocity v along the x-axis of a fixed rectangular coordinate system • The net heat input to the weld under theseconditions is given by Hnet = EI/v (J/m) where  is the the transfer efficiency of the process E and I are the welding voltage (in V) and current (in A), respectively, and v is the velocity of welding or travel speed (in m/s). 16 8
  9. 9. 10/5/2012 Rosenthal’s Simplified ApproachAssumption 2  Heat source is a point source, with all of the energy being deposited into the weld at a single point – This assumption avoided complexities with density distribution of the energy from different sources and restricted heat flow analysis to the heat-affected zone, beyond the fusion zone or weld pool boundary.Assumption 3  The thermal properties (thermal conductivity, k, conductivity k and product of the specific heat and density, Cp) of the material being welded are constantsAssumption 4  Modify the coordinate system from a fixed system to a moving system 17 Rosenthal’s Simplified Approach• The moving coordinate system  replace x with  (Xi) where  is the distance of the point heat (Xi), source from some fixed position along the x axis, depending on the velocity of welding, v =x-vtwhere t is the time 18 9
  10. 10. 10/5/2012 Rosenthal’s Simplified Approach • This equation can be further simplified, in accordance with Rosenthal if a quasi stationary Rosenthal, quasi-stationary temperature distribution exists. • Temperature distribution around a point heat source moving at constant velocity will settle down to a steady form, such that dT/dt = 0, for q/v = a constant The result is constant. 19 Rosenthal’s solution • Rosenthal solved the simplified form of the heat flow equation above for both thin and thick plates in which the heat flow is basically 2-D and 3-D respectively 2D 3 D, respectively. • For thin plates,q =  EI/v = heat input from the welding source (in J/m) EI/vk = thermal conductivity (in J/m s-1 K-1) = thermal diffusivity = k/pC, (in m/s)R = (2 + y2 + z2)1/2, the distance from the heat source to a particularfixed point (in m)K0 = a Bessel function of the first kind, zero order 20 10
  11. 11. 10/5/2012 Rosenthal’s solution • For the thick plate, where d = depth of the weld (which for symmetrical welds i h lf of the weld width, since w = 2d) ld is half f h ld id h i 21 Rosenthal’s solution• Above equations can each be written in a simpler form, giving the time-temperature distribution around a weld time temperature when the position from the weld centerline is defined by a radial distance, r, where r2 = z2 + y2• For the thin plate, the time-temperature distribution is 22 11
  12. 12. 10/5/2012 Dimensionless Weld Depth Vs Dimensionless Operating Parameter • B Based on R d Rosenthal’s solution of th simplified three- th l’ l ti f the i lifi d th dimensional heat flow equation, Christiansen et al. (1965) derived theoretical relationships between a weld bead’s cross-sectional geometry and the welding process operating conditions using dimensionless parameters. • Th theoretical relationship b t The th ti l l ti hi between th di the dimensionless i l weld width, D, and dimensionless operating parameter, n, is shown, where 23 Dimensionless weld depth (D) Vs process operating parameter nChristiansen (1965)• Width of the weld bead canbe determined (w = 2d)• Width of heat-affected zonecan be determined 24 12
  13. 13. 10/5/2012 Dimensionless Weld Depth Vs Operating Parameter nd = depth of penetration of the weld,U = welding speed (m/s),s = thermal diffusivity (k/C) of the base material (as a solid),Q = rate of heat input to the workpiece (J/s),Tm = melting point of the base material (the workpiece), andTo = temperature of the workpiece at the start of welding.For a symmetrical weld bead, the width of the weld bead w = 2d bead 2d, Cross-sectional area of the weld bead can be determinedCan be applied to the heat-affected zone by simply substitutingTH for Tm where TH is the temperature of some relevant phasetransformation that could take place 25 Assignment -21) Find w and d for symmetrical weld bead as shown in figure.2) Fi d th width of HAZ ( h Find the idth f (phase ttransition t iti temp = 730 C)Material steel with Tm= 1510 CE=20 VI= 200 AWelding speed (v or U) =5 mm/s wT0= 25 CArc efficiency =0.9 dK=40 W/mKC = 0.0044 J/mm3. Ct=5 mm 26 13
  14. 14. 10/5/2012 Welding Lecture ‐ 10 05 Oct 2012, Friday, 11.30‐12.30 am y Heat flow in welds 27 27 Effect of welding parameters on heat distribution• The shape of the melt , size & heat distribution, is a function of 1. Material (thermal conductivity, heat capacity, density) 2. Welding speed, and 3. Welding 3 W ldi power/energy d / density i 4. Weldment plate thickness 28 14
  15. 15. 10/5/2012 Effect of thermal conductivity (and material property) on heat distribution • Increasing thermal conductivity – tends to cause deposited heat to spread – Smaller welds for a given heat input and melting temperature • For a given heat input, the lower the melting point, the larger the weld 29 Material Thermal  diffusivity  =k/C (mm2/s)Aluminium 84Carbon steel 12 Austenitic  4 steelEffect ofthermalth lconductivity(and materialproperty) onheat distribution 30 15
  16. 16. 10/5/2012 Effect of welding speed on Shape of Fusion/HAZ Increasing velocityVelocit 0 Low Medium High Very high y Plan Circle elliptical Elongated Tear Detached view ellipse drop tear drop 3-D 3D Hemi- H i Prolate P l t Elongated El t d 3D 3D t tear view spherical spheroidal prolate tear drop spheroidal drop Effect of welding speed 32 16
  17. 17. 10/5/2012 Tear drop formation at very high velocity Continuous tear drops Detached tear drops at very high velocity Weld direction Weld direction Effect of welding speed• For a stationary (spot) weld, the shape, is round (plan view) and approximately hemispherical in 3 D view), 3-D• Once the source is moved with constant velocity, the weld pool and surrounding HAZ become elongated to an elliptical shape (plan view), and prolate spheroidal in 3-D• With increased velocity, these zones become more y, and more elliptical• At some velocity (for each specific material), a tear drop shape forms, with a tail at the trailing end of the pool. 34 17
  18. 18. 10/5/2012 Effect of welding speed• Increasing velocity  elongates the teardrop more and more,  narrows the fusion and heat- affected zone  overall melted volume constant• Very high welding speeds  the tail of the teardrop weld pool detaches  isolate regions of molten metal  lead to shrinkage-induced cracks along the centerline of the weld 35 Efect of the thickness of a weldment•Thick weldment  Small weld pool and heat-affected zone 36 18
  19. 19. 10/5/2012 Effect of energy density• Increased energy density  increases the efficiency of melting,  increases the amount y g of melting (especially in the depth direction)  decreases the heat-affected zone.• Shape of weld pool & HAZ will be distorted by any asymmetry around the joint.• Asymmetry might be the result of the relative thermal mass (e.g., thickness) of the joint (e g elements as well as their relative thermal properties (Tm, k & C) 37 Simplified Equations for Approximating Welding Conditions1) Peak Temperatures  Predicting metallurgical transformations (melting, austenitization, recrystallization of cold-worked material, etc.) at a point in the solid material near a weld requires some knowledge of the maximum temperature reached at that specific location. For a single-pass, full-penetration butt weld in a sheet or a plate, the distribution of peak temperatures (Tp) in the base material adjacent to the weld is given by 38 19
  20. 20. 10/5/2012 Peak TemperatureTo = initial temperature of the weldment (K)e = base of natural logarithms = 2.718 = density of the base material (g/mm3)C = specific heat of the base material (J/g K- I)h = thickness of the base material (mm)y = 0 at the fusion zone boundary and where Tp = TmTm = melting (or liquidus) temperature of the material being welded (K)Hnet = EI/v (J/m) 39 Width of the Heat-Affected Zone• Peak temperature equation can be used to calculate the width of the HAZ.• Define Tp  Tre or Tau• The width of the HAZ is determined by the value of y that yields a Tp equal to the pertinent transformation temperature (recrystallization temperature, austenitizing temperature, etc.).• Equation cannot be used to estimate the width of the fusion zone, since it becomes unsolvable when Tp =Tm T• (Remember the assumption in Rosenthal’s solution of the generalized equation of heat flow,  Heat was deposited at a point, and there was no melted region, but just a HAZ) 40 20
  21. 21. 10/5/2012 Assignment 3- Part A A single full penetration weld pass is made on steel using the following parameters. Tm= 1510 C, E=20 V I= 200 A Welding speed (v or U) =5 C V, A, mm/s, T0= 25 C, Arc efficiency =0.9, C = 0.0044 J/mm3. C, t=5 mm, Hnet= 720 J/mma) Calculate the peak temperatures at distances of 1.5 and 3.0mm from the weld fusion boundaryb) Calculate the width of HAZ if the recrystallization temperatureis 730 Cc) Find the influence on the width of HAZ if a preheated sampleis used (Assume preheat temp =200 C)d) Find the influence on the width of HAZ if the net energy isincreased by 10% 41 Solidification rate• The rate at which weld metal solidifies can have a profound effect on its microstructure, microstructure properties, and response to post weld heat treatment.• The solidification time, St, in seconds, is given by 42 21
  22. 22. 10/5/2012 Solidification rate • L is the latent heat of fusion (J/mm3), • k is the thermal conductivity of the base material y (J/m s -1 K-1), • , C, Tm and To are as before • St is the time (s) elapsed from the beginning to the end of solidification. • The solidification rate, which is derived from the solidification ti lidifi ti time, h l d t helps determine th nature of i the t f the growth mode (with temperature gradient) and the size of the grains (will be discussed in detail on a separate lecture) 43 Cooling Rates• The rate of cooling influences – the coarseness or fineness of the resulting structure – the homogeneity the distribution and form of the phases and homogeneity, constituents in the microstructure, of both the FZ and the HAZ for diffusion controlled transformations• Cooling rate determines which transformation (phase) will occur and, thus, which phases or constituents will result.• If cooling rates are too high in certain steels  hard, untempered martensite  embrittling the weld  adds susceptibility to embrittlement by hydrogen. p y y y g• By calculating the cooling rate, it is possible to decide whether undesirable microstructures are likely to result.• Preheat  To reduce the cooling rate. Cooling rate is primarily calculated to determine the need for preheat. 44 22
  23. 23. 10/5/2012 Cooling Rates-Thick plates• For a single pass butt weld (equal thickness), For thick plates,R = cooling rate at the weld center line (K/s)k = thermal conductivity of the material (J/mm s-1 K-1 )T0 = initial plate temperature (K)Tc = temperature at which the cooling rate is calculated (K) 45 Cooling Rates-Thin plates For thin plates, h = thickness of the base material (mm) p = density of the base material (g/mm3) C = specific heat of the base material (J/g K-1) f f ( / 1 pC = volumetric specific heat (J/mm3 K-1)Increasing the initial temperature, T0 or applying preheat,decreases the cooling rate 46 23
  24. 24. 10/5/2012Relative plate thickness factor   ≤ 0.75 Thin plate equation is valid   0.75 Thick plate equation is valid 47 Assignment 3- Part BFind the best welding speed to be used for welding g p g6 mm steel plates with an ambient temperature of30 C with the welding transformer set at 25 V andcurrent passing is 300 A. The arc efficiency is 0.9and possible travel speeds ranges from 5-10 mm/s.The limiting cooling rate for satisfactoryperformance is 6 C/ at a temperature of 550 C f i C/s t t t f C. 48 24
  25. 25. 10/5/2012Assignment 3- Part B-Solution 49Assignment 3- Part B-Solution 50 25

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