Heat-Affected Zone (HAZ) in Welding Thermal History at HAZ Point

Heat-Affected Zone (HAZ) in Welding Thermal History at HAZ Point
Carbon Steels 300 [amp] 20 [volt] 10 [mm/s] Quick cooling Last session, we showed that welding creates 3 distinctive regions in material which are WP where you have your molten metal, HAZ in a close distance to melt and BM in farther distance. We discussed that WP’s property and microstructure can be controlled by filler metal, but when it comes to HAZ we have the least control on the material aspects of this region. Formation of this region is unavoidable, some process like laser welding gives a very narrow HAZ while other processes like SAW creates a larger HAZ in size. On the other hands, evolution of MS is usually deleterious and makes the HAZ the weakest point of the structures in terms of mechanical and metallurgical aspects When a weld fails, the cause of failure very often roots in HAZ region, for example welded structure’s fracture, rupture, fatigue cracking, corrosion, embrittlement, and so on is dominantly in HAZ. But why, lets look deeply on what is happening during welding to the material We know that welding introduces a high intensity moving heat source with temperature about the melting point that drops to ambient temperature within a few mm distance of weld pool. That means high gradient of temperature in a vicinity of weld. We also agree that heating-up and cooling down have effect on the microstructure. In order to understand the evolution of MS in HAZ, we first need to understand the thermal profile that a point in HAZ is experiencing during welding. For that, lets stick a TC (red spot) in HAZ and close to weld pool. The weld goes by and the TC measures the temperature as shown here. Therefore, when weld pool approaches the TC and pass by, the temperature of that point goes rapidly to high temperature (which is the peak temp here) in a second or two and returns back to below 100 C in less than a minute. Microstructure of materials would definitely evolve due to exposing to such a high temperature. What ever is your material it will be affected and what ever is your MS it will be evolved. Thermal History at HAZ Point Rapid heating

Heat-Affected Zone (HAZ) in Welding Carbon Steels
Evolution of Hypoeutectoid Steels depending on welding thermal history Every material and associated microstructure respond differently to such a thermal profile. And you need to study each material differently, for example evolution of MS in AL is totally different form steels or nickel-based superalloys. The first material that we cover in this course is the common material in welding products that is low carbon steels or mild steel with hypoeutectoid microstructure and carbon content between 0.02 & 0.7. This is actually quit wide range of steels and includes almost all structural steels around you. The study of MS has two major steps: Study the evolution under equilibrium state which is phase-diagram 2) consider the kinetics of evolution to determine that to what extend equilibrium phases are achieved under given thermal cycle. In welding we take the same steps. 1) compare the temperature level to the phase diagram to determine the equilibrium phases that can be formed. and then, because the time is too short in welding, we consider the available time to determine what percentage of the equilibrium evolution can be achieved. So for LCS, lets compare the welding thermal history against the Iron-carbon diagram to determine the equilibrium phases that can form from exposing to this profile of temperature. There are 6 critical points on diagram that are important in HAZ evolution, and these 6 critical points suggests 8 distinct regions of evolution for MS and we will review each one starting from the initial MS which is a mix of Ferrite & Pearlite.

Evolution of Hypoeutectoid Steels depending on welding thermal history The first region. Temperature increases from ambient to A1 (about 715 C)

Heat-Affected Zone (HAZ) in Welding
Carbon Steels The heating part T0 < T < A1 ; The initial microstructure is normally Proeutectoid Ferrite and Pearlite (Lamellar Eutectoid Ferrite and Cementite) Phase diagram suggests that there are some changes in phase fractions of Pearlite (Xp) in this range of temperature that can be calculated by lever rule. The maximum change is about 2 % which is relatively very little. The heating time during welding is also shorter than equilibrium time to complete this small fraction of transformation. Therefore the microstructure does not show much phase change, and experience some tempering only.

Evolution of Hypoeutectoid Steels depending on welding thermal history The next region is the temperature between A1 and A3 in heating trend

Carbon Steels The heating part T1 < T < A3 ; In equilibrium; all α+p  γ that takes time since it is diffusion controlled. Welding time at A1 < T < A3 is in a range of a second or less Therefore we have partially transformed zone Mostly transformation of p  γ that is faster

Evolution of Hypoeutectoid Steels depending on welding thermal history Region 3 starts above A3 and goes up to a critical point called TS or Solution temperature that does not exist in typical Fe-Cementite diagram but plays a critical role in Welding of carbon steels.

Carbon Steels The heating part T3 < T < TS ; Finishing α+p  γ very soon above A3 due to super heating that vastly nucleis γ Increasing temperature push the system toward change in γ state to reduce the total energy of the system thermodynamically by different mechanisms. The grain growth is one mechanism but cannot begin because the boundaries are locked by precipitates of Carbide, Nitride,… (like NbC, VC, Mn3C). This lock remains in effect until TS when the precipitates starts dissolving into γ. Since the grain growth is locked, the system needs an alternative to reduce the thermodynamic energy at elevated temperature. As such, recrystallization become operative. This happens very often in work-hardening materials, and forms new γ grains at high angle boundaries (e.g. > degree misorientation) to reduce high energy of stacking fault accumulated there. This creates a fine grain region if the peak temperature remains between A3 and TS Lets make it more clear what is happening in this region

Evolution of Hypoeutectoid Steels depending on welding thermal history The next region or region 4 combines both heating scheme and cooling scheme one from TS to peak temperature (Tp) and then from peak to A3. Austenite grain growth is the dominant process of evolution in MS.

Carbon Steels • At longer times, average grain size increases. -- Small grains shrink (and ultimately disappear) -- Large grains continue to grow • Callister’s Relation: elapsed time coefficient dependent on material and T. grain diam. at time t. exponent typ. ~ 2 A recall from “Introduction to Material” on grain growth So we need to look into the root of this equation to accommodate the temperature effect as a variable not constant. • This is derived for constant temperature (constant K) when time controls the grain growth.

Carbon Steels The heating part followed by cooling Heating; TS < T < Tp & Cooling Tp > T > A3 ; The grain growth is basically formulated as rate equation that is diffusion controlled and occurs by the migration of grain boundaries as well as correlated with the size of grains. Diffusion term Rate decreases while grain growing The root relation is the general rate equation with diffusion term as shown here. The k is material constant, Q is the activation energy of grain growth which means lower activation energy, easier for GB to migrate , and capital T, the temperature that exponentially increases rate at higher values. The term g or grain size that appears on the denominator indicates that a smaller grain grows faster and the rate decreases while the grain is growing. The integration of such a rate equation turns to this relation including the integration of T(t) from t1 to t2. Now we can accommodate the change of temperature over time. The integration turns: t2 t1

Carbon Steels t2 t1 Callister’s relation for constant temperature In welding, integration required using thermal history T(t) If we assume the temperature is constant over time which is the assumption in Callister’s relation then the integral becomes a constant and you can obtain the Calister’s formula. So this is a general form In welding, however, you need to take the integral one for heating part from TS to Tp and add it to the cooling part from Tp to A3. TS

TS Tp A3 (The grain size is almost doubled)
Here is an example to help you practically use it in an Excel sheet. Lets say you have measured done some sort of thermal analysis for your welding and obtain the thermal history of your location in HAZ. This is the columns T and t here and the interval between TS to A3 is highlighted. The change in time (dt) is characterized every 0.1 second Material constants are given, and the initial grain size is 30 micron or ASTM # 7 You have a column that calculates the values inside the integral for every temperature Then you multiply it by dt, and the integration is the sum of these values by definition. So we add up all highlighted numbers in the last column and this is the value for the integral term. The rest is simple algebraic calculation and it turns that the grain size has changes from 30 micron to 64 micron, almost doubled. You can see that, although the time is about 5 seconds but the high temperature causes the grain size doubled. Metallurgically, and mechanically this means a lot. A3 ASTM Number changed from 7 to 5 at 5 [mm] from weld (The grain size is almost doubled)

Carbon Steels Effect of Grain Size on Mechanical Properties Normally, materials’ microstructure particularly alloys are optimized in terms of grain size, and usually a fine grain structure is desire to give both high strength and high toughness. Effect of grain size on yield strength (Hall-Petch relationship) Very important to know the size of grain growth region and the distance from the welding centerline Normally, materials particularly alloys are optimized in terms of grain size, and usually a fine grain structure is desire to give both high strength and high toughness. Doubling the size of grains in HAZ suggests that the area beside the weld has noticeably lower strength and toughness. In other words, HAZ breaks prior to other region in tensile test, or factures at lower impact force. If you remember we discussed that the HAS is normally the weakest point of the structure, now you can understand it why? The failure is more likely to happen here due to dropped material property; yield strength, ultimate strength, & toughness. plus welding process defect such as undercut is frequently happen in this region that triggers cracking as well. Therefore it is very important to determine the size of region where the grain growth is operative. The integration method that I explained is not very welcomed in industrial practice due to complexity of calculation. The method is very common in computational algorithm for predicting MS in simulation software. In industrial practice, a simplified method can be used. The simplified method look at Tp only to determine the size of this region. This method ignores the profile of T-t that means it does not consider how fast or slow you are approaching the Tp. It simply compares Tp against the phase diagram. The grain growth starts when Tp is above TS. So as it is shown here, the grain growth region is between welding pool solidification line and where the Tp is equal to TS. d = 30  σy ≈ 155 [Mpa]  σy drops 21 [Mpa] in HAZ d = 64  σy ≈ 134 [Mpa] Low carbon steel

Carbon Steels For a case that close-to-pseudo-equilibrium behaviour is expected; Variation of MS in transverse direction can be approximated by peak temperature ; The simplified method looks at Tp alone to determine the size of this region. This method ignores the profile of T-t that means it does not consider how fast or slow you are approaching the Tp. It simply compares Tp against the phase diagram. The grain growth starts when Tp is above TS. So as it is shown here, the grain growth region is between welding pool solidification line and where the Tp is equal to TS.

Carbon Steels For a case that close-to-pseudo-equilibrium behaviour is expected; Variation of MS in transverse direction can be approximated by peak temperature; The simplified method looks at Tp alone to determine the size of this region. This method ignores the profile of T-t that means it does not consider how fast or slow you are approaching the Tp. It simply compares Tp against the phase diagram. The grain growth starts when Tp is above TS. So as it is shown here, the grain growth region is between welding pool solidification line and where the Tp is equal to TS.

Carbon Steels Assignment 3 – HAZ Microstructure; Consider the welding of 0.01 [m] thick plate of 1018 steel. Using the general thick-plate solution of Rosenthal with the information given in table below, plot temperature vs transverse distance from weld path (x = 0, z = 0, and y increment of [m]), compare it against the pseudo-binary Fe-C equilibrium diagram with critical points given in slide 12, and draw a schematic picture similar to sample in slide 14 for size of different microstructure regions. Quantity Value Description Unit κ 51.9 Thermal Conductivity J/msK ρ 7870 Density kg/m3 Cp 486 Specific Heat J/KgK Q 3200 Heat from Heat Source J.s or watt V 0.0024 Arc Traveling Speed m/s T0 300 Initial Temperature K Deliverables: Your Excel file of calculation including a plot of temperature vs transverse distance from weld path, highlighted critical points on it, and a discussion about size of different microstructure regions. (consult with slide 14) Due date: Friday 22th November 2013 mid-night

Carbon Steels Exam sample

Carbon Steels Step 5; Consider Kinetics of Austenite Decomposition (on Cooling) Cooling A3 > T TS γ  α+p Diffusional decomposition of Austenite to daughter products of Ferrite and Pearlite. A Kirkaldy assumed a single continuous function to describe both nucleation & subsequent growth for each diffusional products (α+p). XA XF A Xp XF BS MS Ce = Cαe = C0 = 0.14

Carbon Steels General rate equation ; Diffusional transformation of one phase to another (α  β) Integration [α] : concentration of element A n= 2 n= 3 Diffusional transformation of one phase to two phases (γ  α + β) The phase change is not immediate and it is a time dependent process with a rate that decreases when concentration or fraction increases.

Carbon Steels General rate equation ; Diffusional transformation of one phase to two phases (γ  α + β) The phase change is not immediate and it is a time dependent process with a rate that decreases when concentration or fraction increases.

Carbon Steels Rate Equation for α ; Kirkaldy, Watt & Henwood shown that the rate is a function of Temperature Super cooling Grain size Volume fraction of α Volume fraction of γ Presence of other alloys , and the power m & p are proportional to volume fraction of the other phase m = 2/3(1-X) p = 2/3 X

Carbon Steels C0 = 0.14 MS BS A A TS Cαe = Ce = XF Xp Ferrite (α) Phase fraction; XA

Carbon Steels Time integration of Ferrite (α) Phase fraction; Integration from tA3 to (tBS or tx=1) tA3 to tBS tA3 to tx=1

Carbon Steels C0 = 0.14 MS BS A A TS Cαe = Ce = XF Xp Rate Equation for pearlite ; Similar to α, Pearlite uses the same form but different constant terms: XA Integration from tA1 to tBS

Carbon Steels Time integration of Ferrite (α) Phase fraction; Integration from tA1 to (tBS or tx=1) tA1 to tBS tA1 to tx=1 If the final fraction of α+p is not 1, the remainder fraction evolves to Bainite or Martensite

Carbon Steels C0 = 0.14 MS BS A A TS Cαe = Ce = XF Xp Rate Equation for Bainite ; XA

Carbon Steels Time integration of Ferrite (α) Phase fraction; Integration from tBS to (tMS or tx=1) tBS to tMS tBS to tx=1 If the final fraction of α+p+b is not 1, the remainder fraction evolves to Martensite

Carbon Steels C0 = 0.14 MS BS A A TS Cαe = Ce = XF Xp Rate Equation for Martensite ; Given by K-M relation (Koistenen & Marburger) XA

Carbon Steels Time integration of Ferrite (α) Phase fraction; Integration from tMS to tx=1 At the end; Each phase gets a number between 0-1 with sum equal to 1

Carbon Steels Integration of rate equations; α formation in 1018 steel welding example: We know that C % = 0.14 Mn % = 0.7 A3 = 1088 K G = 5 R = 8.314 tA3 = 12.5 tBS = 14.5 ≤1

Carbon Steels Integration of rate equations; Example: α formation in 1018 steel welding example: x

Kirkaldy Model & extend of Watt & Henwood;

Carbon Steels Study Example; Consider a welding of 1018 steel containing C [wt%] = 0.14 & Mn [wt%] = 0.7, Rosenthal thick plate model generated the thermal history of a point at 5 [mm] distance of weld path as shown below

Carbon Steels Step 1; Critical points (Celicious) (Carbon wt%) TS here is the Mn3C precipitation dissolution temperature; Constants therefore are A=5.9 B=7375, a=1, & b=1 Cm & Cc are concentration of metal (here Mn) & non metal (here C) in precipitate respectively. TL = [K] = [C] Ts = [K] = [C] TS = [K] = [C] A3 = [K] = [C] A1 = [K] = [C] BS = [K] = [C] MS = [K] = [C] Tp = 1561 [K] = 1288 [C] Compositions in wt%

Carbon Steels Step 1; Critical points

Carbon Steels Step 2; Pseudo-binary Fe-C equilibrium diagram (Hypoeutectoid steel) TS Critical points are added to the line that represents the alloy (here C0 = 0.14) A A BS MS C0 = 0.14

Carbon Steels Step 2; Pseudo-binary Fe-C equilibrium diagram (Hypoeutectoid steel) TS A A BS MS Ce = C0 = 0.14

Carbon Steels Step 2; Pseudo-binary Fe-C equilibrium diagram (Hypoeutectoid steel) TS 715 < T[in C] < 912 (T in [C]) A A BS MS (T in [C]) Cα ≈ 3.25x10^-5 (T-20) < T[in C] < 715 Ce = Cαe = C0 = 0.14

Carbon Steels Step 2; Pseudo-binary Fe-C equilibrium diagram (Hypoeutectoid steel) TS 715 < T[in C] < 912 (T in [C]) A A BS Cγ = ((889 – T)/203)^ < T[in C] < 912 MS (T in [C]) Cα ≈ 3.25x10^-5 (T-20) < T[in C] < 715 Ce = Cαe = C0 = 0.14

Carbon Steels Step 3; Determine pseudo-equilibrium phase fraction (Hypoeutectoid steel) The volume fraction is given by the lever law TS A XA XF A Xp XF BS MS Ce = Cαe = C0 = 0.14

Carbon Steels

Carbon Steels Step 4; Consider the Kinetics of transformation 4-1; Analyze the heating part A1 < T < A3 ; A1 A3 Molten Super heated into γ Partially transformed Tempered At y = 0.005 The transformation α+p  γ is completed above A3 due to super heating At y = The transformation α+p  γ is partially completed and reversed to relatively similar MS due to comparable profile of heating and cooling. Re-distribution of C, N, H, … occurs. At y = 0.01 Only tempering and release of work hardening or residual stresses. The current code treats this region by following the psuedo-binary Fe-C equilibrium diagram. It ignores synthetic microstructure model that models nucleation and growth of grains including segregation and solute diffusion.

Carbon Steels Step 4; Consider the Kinetics of transformation 4-2; Austenite Grain Growth Heating; TS < T < Tp & Cooling Tp > T > A3 ; TS A XA XF A Xp XF BS MS Ce = ~ 81 % α + 19 % p Cαe = C0 = 0.14

Heat-Affected Zone (HAZ) in Welding Thermal History at HAZ Point

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Heat-Affected Zone (HAZ) in Welding Thermal History at HAZ Point

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