Basic Research Of Weld Process And Microstructure Modeling .
Basic Research of Weld Processand Microstructure Modeling for aHot-Rolled High Strength SteelFinal ReportSubmitted byKatie Strader, Bin Wang, Prof. Cuixin Chen,Prof. Wei Zhang and Prof. John C. LippoldWelding Engineering ProgramDept. of Materials Science and EngineeringThe Ohio State UniversityTo(Version 1 - 09-21-2015)1
ContentsExecutive Summary . 31. Introduction . 42. Research Objectives . 43. Experimental Approaches . 63.1. Gleeble-based physical simulation of CGHAZ microstructure . 63.2. Samples for Charpy impact testing . 73.3. Charpy impact testing . 73.4. Microstructure characterization . 84. Approaches for Integrated Weld Modeling . 84.1. Equation for grain growth in CGHAZ . 84.2. Equation for CGHAZ hardness . 94.3. Equation for Charpy impact toughness of CGHAZ . 104.4. Welding heat transfer model . 104.5. Integrated weld modeling . 115. Results and Discussion . 125.1. Welding thermal simulation in Gleeble . 125.2. Base metal microstructure. 135.3. Gleeble-simulated CGHAZ microstructure . 155.4. CCT diagram for CGHAZ of BS900D steel . 205.5. Charpy impact toughness . 225.6. Verification of microstructure modeling results . 235.7. Testing applications . 246. Summary and Conclusions . 267. Acknowledgements . 278. References: . 272
Executive SummaryExtra-high strength steels (e.g., BS900D) are increasingly deployed in construction machinery andheavy manufacturing industries to build strong yet light-weight structures. These advanced steelshave a carefully engineered base metal microstructure, which can be significantly altered bywelding thermal cycles. The end users of extra-high strength steels desire incorporating weldinginto their “virtual” manufacturing for product performance optimization to significantly decreasethe time and cost of new product development. However, developing an effective way to modelthe weld mechanical properties of extra-high strength steels remains a major technical challenge.Addressing this technical challenge, the overall goal of this research is to develop the basicknowledge of weld microstructure and mechanical properties of extra-high strength steels.BS900D, a hot-rolled extra-high strength steel developed by Basosteel for the constructionmachinery industry, is chosen for the study. It is focused on the coarse-grained heat affected zone(CGHAZ), the most critical region in a welded joint.Gleebe based physical simulation is used to produce samples with bulk CGHAZ microstructureunder different cooling rates. Microstructure models are established based on the experimentaldata of prior austenite grain size, hardness and Charpy impact toughness. The microstructuremodels are incorporated into a finite-element based weld heat transfer model. This integratedmodel is capable of calculating the weld temperature distribution and mechanical properties forvarious arc welding processes (e.g., gas tungsten arc welding, gas metal arc welding andsubmerged arc welding) over a wide range of heat inputs. The developed framework of integratedweld modeling provides an experimental and analytical foundation for understanding the CGHAZproperties of other extra-high strength steels in the future.3
1. IntroductionComplex, transient physical processes take place during welding due to the interaction betweenthe heat source (e.g., arc) and the workpiece material (e.g., high-strength steel).[1,2] These physicalprocesses include rapid heating and melting of the base metal, vigorous molten metal flow in theweld pool driven primarily by the surface tension stress (or Marangoni stress) and Lorentz force,solidification of the molten pool, and subsequent cooling during which various solid-state phasetransformations take place resulting in the final microstructure. The microstructure changes incurby welding can have detrimental effects on weld properties such as reduction in ductility andfracture toughness and softening of heat-affected zone (HAZ).Although the theories for welding-induced microstructure changes are well established andpublished in a variety of open textbooks,[3,4] the development of practical solutions (e.g.,optimizing welding parameters, pre-heating and post-weld heat treatment or PWHT) areoftentimes obtained through an experimental trial and error approach. As extra-high strength steelsare increasingly deployed in construction machinery and heavy manufacturing industries to buildstrong yet light-weight structures, the trial and error approach can be costly in term of time andresources required to conduct all the experiments. To address this challenge, many computationcodes (both commercial and open source) have been developed to calculate the evolution oftemperature and microstructure during welding. In particular, with the widespread of computedaid engineering (CAE) tools, the end users of extra-high strength steels desire incorporatingwelding into their “virtual” manufacturing for product performance optimization.Extra-high strength steels have a carefully engineered base metal microstructure, which issignificantly altered by welding thermal cycles. Computational simulation of microstructureevolution during welding and resulting mechanical properties is still evolving. In particular, as thefinal weld microstructure can strongly depend on the chemistry and the initial microstructure, theexisting microstructure models are typically limited to some particular chemical compositions ofthe steels for which those models are calibrated. Developing an effective way to model the weldmechanical properties of extra-high strength steels remains a major technical challenge.2. Research ObjectivesAddressing this technical challenge, the overall goal of this research is to develop the basicknowledge of weld microstructure and mechanical properties of extra-high strength steels.BS900D, a hot-rolled extra-high strength steel developed by Baoshan Iron and Steel ResearchInstitute (hereafter referred as Baosteel for brevity) for the construction machinery industry, ischosen for the study. It is focused on the coarse-grained heat affected zone (CGHAZ), the mostcritical region in a welded joint.The specific objectives and tasks are: Gleeble physical simulationo Dilatometry for continuous cooling transformation (CCT)o Microstructure characterization and hardness testing4
o Charpy impact testingIntegrated weld modeling:o Weld process modelo Microstructure model of hardness and Charpy toughness (-40 C and roomtemperature)Testing applicationsAn overview of the research tasks is illustrated in Figure 1. In particular, the Gleeble physicalsimulation was used to generate the experimental data under controlled heating and coolingconditions. Such experimental data was used to develop empirical equations for describing weldmicrostructure including prior austenite grain size, hardness and Charpy impact toughness. It isnoted that the individual phase fractions (e.g., those of bainite and martensite) were not calculated.As discussed in details later, this is because that the tested CGHAZ of BS900D steel comprised amixture of bainite and martensite that had similar morphology and hardness and were difficult toquantitatively differentiate from each other. The empirical equations were integrated with a weldheat transfer model based on Abaqus finite element analysis (FEA) code to predict the jointmechanical properties. The integrated weld model was tested with experimental data ofautogenous gas tungsten arc welding (GTAW).Figure 1: Overview of research tasks.Although the model was tested for autogenous GTAW, the integrated weld model is capable ofconsidering other commonly used welding processes such as gas metal arc welding (GMAW) andsubmerged arc welding (SAW) and a wide range of welding heat inputs. Moreover, the frameworkdeveloped can be expanded to other extra-high strength steels in the future.5
3. Experimental ApproachesAs discussed earlier, the base metal studied is BS900D, an extra-high strength steel. Thecomposition and the mechanical properties of base metal are summarized in Table 1.Table 1: Chemical composition and mechanical properties of BS900D base metalComposition (wt%)CSi Mn Cr MoTiNbBV AltPSBS900D 0.16 0.22 1.2 0.28 0.29 0.018 0.016 0.0012 0.04 0.04 0.0114 0.0029Hardness: 325 VickersTensile strength: 965 MPa65 J (longitudinal direction)43 J (transverse direction)Impact toughness:3.1. Gleeble-based physical simulation of CGHAZ microstructureThe samples used for Gleeble physical simulation were machined from a 8.5-mm-thick BS900Dplate provided by Baosteel. As shown in Figure 2, the rectangular bar shaped samples (dimensions11 6 100 mm) were cut perpendicular to the rolling direction of the plate.Figure 2: Samples used for Gleeble physical simulation of CGHAZ microstructure.A Gleeble 3800 thermomechanical tester at OSU was used for the physical simulation ofCGHAZ microstructure. The experimental setup for Gleebe physical simulation is shown inFigure 3. The sample was placed between a pair of water cooled copper grips which, in turn, wereattached to a pair of low-force jaws. A controlled resistive heating was used to heat the sample inwhich the low-force jaws permitted the free expansion and contraction of the sample arisen due tothermal expansion. The thermal cycle to induce a CGHAZ microstructure consisted of a heatingrate of 85 C/sec to a peak temperature of 1350 C, holding at peak for 1 and final “free” coolingdown to room temperature. The cooling rate was varied by changing the free span between thepair of copper grips from 10 to 50 mm; the shorter the free span, the faster the cooling rate. Adilatometer was placed on the rectangular sample to record the volume change on heating cooling.It is noted that the free cooling was used to more accurately determine the phase transformationtemperatures from the dilatometry curve based on the deviation in slope (i.e., coefficient of thermalexpansion).6
Figure 3: Experimental setup for Gleeble physical simulation.Three thermocouples (TCs) were mounted on the sample surface along the axial direction: one atthe center and the other two at each side of the center TC. The TC data showed that the gaugesection (i.e., length of sample exposed to 15 C of peak temperature) was about 6 mm long atthe center of sample.3.2. Samples for Charpy impact testingThe second batch of Gleeble physical simulation was used to produce samples with bulk CGHAZmicrostructure for Charpy impact testing. For improve consistency in the results, “controlled”force cooling (not free cooling) was used to maintain the specified constant cooling rates for a freespan fixed at 10 mm. The peak temperature was also 1350 C. In addition to 1 s hold at peak, alonger hold time of 10 s was used to approximate the reheated CGHAZ with large prior austenitegrains.3.3. Charpy impact testingAfter the Gleeble physical simulation, the center portion of a sample was machined into a subsizespecimen for Charpy impact testing, as shown in Figure 4. The geometry of subsize Charpyspecimen (dimension 10 5 55 mm) was prepared in accordance to the ISO 148 and ASTM A370standards. It is noted that the dimensions of the subsize specimen are the same as those used inthe prior Charpy testing of base metal performed by Baosteel. Charpy impact testing wasperformed in accordance to ASTM Standard E23 - 12c, “Standard Test Methods for Notched BarImpact Testing of Metallic Materials” at two testing temperatures: -40 and 20 (roomtemperature).7
Figure 4: Schematics showing the subsize Charpy specimen machined from Gleeblesample.3.4. Microstructure characterizationThe Gleeble heat treated samples were prepared for microstructure characterization by followingthe standard metallographic procedure. The sample was etched in a 5% Nital solution forobserving the bainite and martensite microstructure using optical microscope and scanningelectron microscope (SEM).The prior austenite grain size is a critical microstructure parameter as it can influence the kineticsof austenite decomposition into different ferrite micro-constituents including coarse martensiteaustenite (M-A) constituent, which has been linked to reduced impact toughness. Multiple etchingmethods were tested to reveal the prior austenite grain boundary. The best method for etching theprior austenite grain size in CGHAZ of BS900D was found to be:Step 1: Vilella’s Reagent (1% picric acid 5% HCL) for 90 secStep 2: A light polish (1 micron) with normal pressure for 30 secStep 3: 5% Nital for 10 sImageJ, an open source image processing software, was used for grain size analysis. For eachtesting condition, two to three images were analyzed at 500x magnification. The freehand toolwas used to outline and measure the grain boundary areas of 13-20 grains in the CGHAZ regionto determine the average grain size. Some measurements were further confirmed on SEM images.Finally, the macro-hardness was measured using a Leco indentation machine with 1 kg load. Toevaluate the microstructure-dependent hardness, the micro-hardness measurement was done usinganother LECO Microhardness Tester LM100AT with 300 gram load.4. Approaches for Integrated Weld Modeling4.1. Equation for grain growth in CGHAZThe classic grain growth theory indicates that the driving force for grain growth is the decrease ofinterface energy of grains and the kinetics is controlled by diffusion. Under isothermal condition,the rate of grain growth can be described as:8
Q (1)D1 / n D01 / n k exp a t RT where D is the mean grain size (μm), D0 is the initial grain size (μm), k is a kinetic constant, Qa isthe activation energy, n is the growth exponent, R is the universal gas constant, T is the temperature( C), and t is the time (s).During welding, the grain growth occurs under non-isothermal condition of the rapid weld heatingand cooling. For this case, Eqn. (1) can be rewritten in the following integration form:t Qa (2)D1 / n D01 / n k exp dt0 RT (t ) where T(t) is the temperature profile as a function of time. Eqn. (2) represents the summation ofgrain growth over many small isothermal increments to obtain the final grain size.The austenite grain growth is assumed to occur at a temperature above the austenitizingtemperature Ac3. This start temperature of grain growth is chosen to be 900 C. Moreover, thepeak temperature for grain growth is capped at 1350 C, above which the austenite starts totransform into -ferrite phase.In Eqn. (2), there are three material parameters: n, Q and k. These parameters were determinedusing regression analysis of the experimental data of grain size obtained from Gleeble physicalsimulation. The final equation for grain growth in CGHAZ of BS900D is given as: tP 25920 t r 25920 2.38D 2.38 11.25 2.76 1012 exp (3) exp dtt T (t ) t P T (t ) c Q 25920. The initial austenite grain size D0 is taken the sameRas that of the base metal (11.25 µm). tc is the time reaching the grain growth temperature(900 C) during heating, tp is the time reaching the peak temperature, and tr is the time reachingthe phase transformation temperature (also taken as 900 C) during cooling. The first integrationterm on the right hand side of Eqn. (3) is the grain growth during heating and the second term isthe grain growth during cooling.where n 0.42, k 2.76 1012, and4.2. Equation for CGHAZ hardnessHardness equations developed by R. Blondeau et al.[5] have been widely used for weldmicrostructure modeling. For instance, it was used by Ion et al. for developing diagrams ofmicrostructure and hardness for HAZs in welds.[6] Those equations, taking into account the basemetal composition and weld cooling rate, have the following general form:HV a HV bHV log10 Vr (4)where HV is the hardness of a micro- constituent (i.e., ferrite, bainite or martensite), aHV and bHVare simple, linear functions of steel composition, and Vr is the cooling rate at 700 C (in C/hour).9
As discussed in details later, the tested CGHAZ of BS900D steel comprised a mixture of bainiteand martensite with similar morphology and hardness. It was difficult to quantitativelydifferentiate one micro-constituent from another. Since the microstructure is predominantlybainite, the model for CGHAZ of BS900D followed the general form of Blondeau’s hardnessequation for bainite. Two constants, aHV and bHV, were determined by regression analysis ofexperimental hardness data measured on the Gleeble samples. The final equation of CGHAZhardness for BS900D is given as:HV – 547.8 185C 330Si 153Mn 65Ni 144Cr 190Mo (5) 156.9 53C – 55Si – 22Mn – 10Ni – 20Cr – 33Mo log10 Vr where the concentrations of alloying elements such as C, Si and Cr are given in weight percent(wt%).4.3. Equation for Charpy impact toughness of CGHAZAlthough this is no lack of experimental data of Charpy impact toughness for CGHAZ in varioussteel welds,[7] a generally-accepted equation to predict the CGHAZ toughness is yet to beestablished. Two parameters have been recognized for their effects on the Charpy impacttoughness: cooling rate and prior austenite grain size. The effect of cooling rate is expected as itinfluences the microstructure formed in the CGHAZ. For the prior austenite grain size, it has beenshown in the literature that the coarsened austenite grains can form a high fraction of M-Aconstituent. The formation of hard M-A constituent has a detrimental effect on the impacttoughness, especially at low temperatures (below -20 C).In this study, the equation for Charpy toughness of CGHAZ has the following simple form:(6)J T aJ bJ t8 / 5 cJ D where JT is the Charpy toughness at tem
3.3. Charpy impact testing After the Gleeble physical simulation, the center portion of a sample was machined into a subsize specimen for Charpy impact testing, as shown in Figure 4. The geometry of subsize Charpy specimen (dimension 10 5 55 mm) was prepared in accordance to the ISO 148 and ASTM A370 standards.
The optional chain can be adapted to the WELD CRAWLER (ZGN-SCN-WELD CRAWL ER). The chain can be used on a pipe that is up to 48" in diameter. 10048079 ZGN-SCN-WELD CRAWLER- OPT-AXIAL The axial weld solution option allows axial weld inspection using the WELD CR AWLER base (ZGN-SCN-WELD CRAWL
immunity (WFI) resist electromagnetic fields emitted by weld guns up to 100 kA/m. Weld-spatter Electromagnetic Weld Fields PROBLEM Hot welding-spatter (a.k.a. weld debris, weld slag, weld berries) sticks to sensor faces and bodies and causes pre
Weld toe radius is the radius which joining the fillet weld and base metal. Figure 2: Weld toe of cruciform shape fillet weld joint The objectives of this paper are as follows: Analysis and compare the effect of weld toe radius on tensile strength of fillet weld
Part of original welding kit. 6) T20004 Fillet Weld Tip ¼" fillet weld tip used for butt and right angle welds. 7) T20005 Fillet Weld Tips 3/8" fillet weld tip used for butt and right angle welds. 8) T20011 Fillet Weld Tips ½" fillet weld tip used for butt and right angle welds. 9) T20009 Ribbon Weld Tip used for welding thermoplastic
the quality of the weld bead generated by the welding system [10]. Distance sensors are also used to identify and track the weld seam during inspection [11]. Seam tracking systems have been developed to guide a weld gun along a weld seam to improve weld bead quality. For these systems, online inspection of the weld bead is still needed to
Busbar Weld If we consider the weld to the busbar to be a resistor, measuring the impedance is as simple as connecting a current source to the weld and measuring the voltage as shown in Figure 2. HI Current Source LO HI Voltmeter LO Weld Figure 2: Simple weld resistance circuit. The impedance of the weld is then calculated using Ohm's Law.
Graphic symbol that indicates weld required Welding symbol Following eight elements: Reference line (required) Arrow (required) Basic weld symbols Dimensions and other data . Seam Weld Elementary Weld Symbols . Edge Weld Surfacing Elementary Weld Symbols . SUPPLEMENTARY SY
The most basic element of a welding symbol is the “weld symbol” itself, which tells what type of weld is required: fillet weld, groove weld, plug weld, etc. For groove welds, the type of joint preparation is specified by the weld symbol. There are 17 different we
