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Application of Direct Resistance Treatment to Localized Areas onAdvanced High Strength Steel Sheet

Jerry E. Gould*, Mark Harris**, Dan Sakkinen**, and Warren Peterson**EWI, Columbus OH

**Johnson Controls, Plymouth, MI

Abstract

The potential for creating local designed variations in microstructure has the potential toradically change how advanced and ultra- high strength steels are used in vehicle components.Such local changes in the microstructure offer the potential for tailored combinations of strengthand ductility, both reducing vehicle weight and improving structural performance. This studyaddresses development of a new technology for creating such local changes in microstructure.The process, termed direct resistance heat treatment (DHRT) is analogous to resistancewelding with in-situ tempering. Current pulses are used to first locally transform themicrostructure (austenite to martensite reaction) then temper the martensitic microstructure toachieve desired levels of performance. Work here was done on a 1-mm thick 10B21 steel.Practices were first developed to achieve fully martensitic microstructures, and thencombinations of temper currents and times were used to achieve differing final hardnesses.Thermal analyses taken from the resistance spot welding literature were used to validate theprocessing times employed for DRHT. These analyses showed that for the material studied,stable temperatures (during heating) could be achieved in about 50-ms, and the workpiececould effectively quenched in about 100-ms. Data was then curve fit to in-situ tempering modelspreviously used for resistance spot welding. Mechanical evaluations showed that the locallyhardened areas were effective for both diverting plastic deformation, and even developingcracks on notched specimens. It was noted that during DRHT, tempered zones were typicallycontained within the initial martensitic zones. These results are inconsistent with previousresistance spot welding work, and are believe related to geometry and differences betweenapplying AC (previous study) and MFDC (current study) current.

Introduction

Newer generations of automotive steels continue to offer improved combinations of strength andformability. Products common in the marketplace today include both advanced high strengthsteels (AHSS) (Refs. 1-2) as well as ultra-high strength steels (UHSS) (Ref. 3). These steelsinclude chemistries higher in carbon and hardenability additions compared to previousgenerations of products. Optimum combinations of strength and ductility are provided by uniquecombinations of phase constituents (ferrite, martensite, tempered martensite, banite, austenite)as well as refinement of the resulting microstructures. The properties of these steels haveallowed designers to simultaneously reduce gauges (and thus structural weight) while improvingoverall vehicle structural performance (crash, NVH, stiffness, etc.).

Current designs of vehicles make use of multiple sheet steels (strength levels, gauges) to inefforts to maximize performance. The approach requires acquisition and blanking of multiplegrades of steels, with subsequent welding and joining to achieve a final structure. An alternativeapproach is to provide focused heat treatment to achieve local properties tailoring on a singlecomponent. The approach is termed localized heat treatment. Resulting components can thenbe designed with arrays of tailored microstructures, with potential for dictating deformation paths(for example during a crash) or improving overall loadbearing capability of the part.

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Recent efforts have been focused on laser heat treatment (Ref. 4) and even segmentedquenching dies (Ref. 5). Laser heat treatment has been shown to allow local tailoring ofproperties, achieving a range of strength levels and achievable ductilities on dual phase steels.The major challenge for laser based heat treatment is penetration of the effected zone. Thermalexcursions during localized laser heat treatment are through conduction, suggestingtemperature gradients both in the through thickness and planar directions. Thicknesses ofsteels that can be successfully heat treated in this way are therefore limited. Alternately, theuse of segmented dies implies that heat treatment must be done as part of the forming process,limiting flexibility of the method.

In this research, a new method is proposed for achieving localized heat treatment of advancedand ultra- high strength steels. This process is termed direct resistance heat treatment (DRHT).DRHT utilizes the inherent resistivity of the steel itself to create heat, and the high thermalconductivity of the copper electrodes to achieve rapid cooling rates. The basic approach is anadaptation of previous work conducting in-situ heat treatment of AHSS steel resistance spotwelds (Refs. 6 – 8). In that work, it was first noted that basic resistance spot welding typicallyresults in martensitic weld nuggets. However, applying a second pulse of current facilitatedtempering of that martensitic microstructure, with a localized decrease in hardness andimprovement in apparent toughness.

The overall DRHT process is diagrammed in Figure 1. Essentially, working electrodes arebrought into contact (under force) across the area intended for heat treatment. A current isapplied, with subsequent rapid heating of the workpieces. The area of heat treatment is thendefined by the contact area of the electrodes themselves. DRHT is typically done with one ortwo pulses. The first pulse is generally used to active as quenched microstructures. Thesecond pulse is intended to provide tempering of that microstructure.

This research covers an initial investigation using DRHT to locally modify properties of a 10B21sheet steel. The work described includes examination of initial hardening behavior (during asingle pulse), and the influence of processing conditions during a second pulse to modify theresulting microstructure. This effort also describes the results of some initial mechanical testingassessing the potential for localized areas subject to DRHT for providing tailoring of localproperties and affecting deformation behavior.

Experimental Procedures

Material for study is a nominal 10B21 hot rolled steel, 1-mm thick. This is boron bearing steel,intended for hot stamping applications. The sheet is processed in the non-hardened condition.Nominal material composition, as taken from reference literature (Ref 9), is provided in Table 1.The material is designed to facilitate martensite formation under moderate cooling rates. Tothat end, a continuous cooling transformation/ time-temperature transformation diagram for theproduct is provided in Figure 2. This diagram is based on the nominal composition of theproduct, and work by Bhadeshia (Ref. 10). Superimposed on this diagram are a range ofcooling curves. From these results, it can be seen that the product has a critical cooling rate formartensite transformation of roughly 200oC/sec. Materials were cut into two coupon sizes; 25-mm x 25-mm and 50-mm x 150-mm. The smaller coupons were used for all processdevelopment and metallurgical investigations as described below. The larger coupons wereused for tensile testing, as also described below.

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The resistance heating trials were done on a pedestal type resistance welding frame withnominally 12-kN force capacity. Machine was configured with a Miyachi 500A mediumfrequency direct current (MFDC) power supply, coupled with a 140-kVA transformer. Thesystem was operated in secondary feedback constant current control mode. In addition, heattreating currents were measured with a Miyach 326A Weld Checker. Heat treatment was doneusing 90o included angle truncated cone electrodes with a 7-mm face.

The primary responses measured in these studies were various forms of hardness evaluations.Much of this was done on the heat treated area using an Instron Series 2000 Rockwellhardness tester. Testing was done using a nominal 150- m indenter. Detailed hardness testingwas done on metallurgical cross sections. Cross sections were prepared using standardtechniques, and etched in Nital to reveal underlying microstructures. Samples were preparedboth in the through thickness direction as well as in the plane of the sheet. Microhardnesstesting and mapping were done with a LECO LM247AT automated system.

Limited mechanical testing of specimens with locally heat treated areas was also done as partof this study. Testing was done on an Instron 5585 system with a 220-kN load cell. Sampleswere tested with the heat treated areas nominally centered in the gauge length. Samples werealso tested with notches located on either side of the heat treated area. This was done in aneffort to focus deformation in the area of the heat treatment.

Results

Initial efforts were focused on developing a set of processing conditions capable of locallyproducing martensite between the electrodes. Baseline processing conditions were nominallytaken from standard resistance welding practices (Ref. 11) based on a total stack-up thicknessof 1-mm (matching the thickness of the workpiece). Through an initial series to trials, baselinepractices were developed including an electrode face diameter of 7-mm, 4-kN of force, and aheating time of 28-ms. Heating currents were then progressively increased, and the resultingsurface hardness measured. Surface hardnesses were measured in triplicate, and reported asthe average. The process data from these initial efforts is provided in Table 2, and showngraphically in Figure 3. It is of note that for currents above about 12-kA, a uniform hardnessvalue of roughly 50-Rc (500-VHN) was obtained, suggesting a fully martensitic microstructure.Metallurgical characteristics of a fully transformed sample are provided in Figures 4 – 6. Figure4 provides a cross section of a heat treated spot made at roughly 14.4-kA of current. Note thatthe area between the electrodes is fully transformed, and demonstrates a uniform hardness ofabout 50-Rc (500-VHN). There is then a steep transition between the transformed region andthe base metal. Hardness in the base metal is about 70-Rb (130-VHN). Figures 5 and 6provide a macro/planar view of the heat treated area. The area itself is slightly smaller than theelectrode face, and microstructurally uniform (Figure 5). The corresponding map (Figure 6)demonstrates the uniformity in hardness across the heat treated area, as well as the sharptransition to the base metal.

The hardening process investigated above was then used as the basis for a series of quenchand temper trials. In these trials, a best case hardening condition (4-kN force, 14-kA current,28-ms time) was followed up by a 1-s quench and then a temper pulse. The approach wasselected to parallel previous work on tempering spot welds in similar materials (Refs. 6 – 8).Temper times were selected ranging from 10-ms to 32-ms. Samples were again evaluatedthrough surface hardness measurements. For each temper time, samples were made atincreasing current levels, the hardness assessed through triplicate measurements, and thenreported as the average value. The results for the various temper times are provided in Figure

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7. These results show that for each temper time, increases in current result first in softeningthen re-hardening. The only exception to this was the data for the 10-ms temper time. Thesoftening is related to tempering the martensitic microstructure. Here, longer temper times ofcourse result in accelerated effects. Re-hardening appears to result from formation of austenite(and decomposition products on cooling) at the temper increases at higher currents. This issimilar to the results seen previously for tempering of AHSS spot welds (Refs. 6 – 8). Thisprevious work on spot welds developed tools for mapping hardness response during in-situtempering. Details of this mapping are provided in the Discussion section below. To producesuch maps, hardness data generated was first reduced to characteristic combinations of tempercurrents and times that achieved specific levels of hardness. This was done by extrapolatingthe data provided in Figure 7, and is summarized in Table 3. Fitting of that data as described inthe Discussion section below, resulted in the Hardness-Time-Current map shown in Figure 8.This Figure shows iso-hardness lines ranging from 35-Rc (350-VHN) to 50-Rc (500-VHN),generally indicating that higher currents are required at shorter temper times to achieveequivalent levels of hardness. Also shown on this curve is the expulsion (melting) limit ofcurrent and temper times.

Based on this developed temper diagram, combinations of currents and times were selected formetallographic assessment and mechanical testing. Conditions were selected to result inhardness values of less than 35Rc after processing. Selected processing conditions includedtemper times ranging from 18-ms to 30-ms. For each set of conditions, two metallographic andfive tensile specimens were prepared. Preliminary metallographic results for the heattreatments done with 18-ms temper times are presented in Figures 9 – 11. The observationshere differ dramatically from that seen with the as-hardened specimens. The cross section(Figure 9) indicates that tempering occurred only within an interior area of the hardened region.Here the hardness can be seen to drop from 50-Rc to about 35-Rc, while a wider area ringmaintained the original hardness. This radial distribution of microstructures is evident in theplanar section (Figure 10), and the resultant hardness map (Figure 11). Details of themicrostructures at the various radii for this specimen are shown in Figures 12 – 15. Figure 12indicates that the center of the heat treated area is essentially tempered martensite. Figure 13shows the transition from this tempered area and the un-tempered larger radius area. Figure 14provides detail of the microstructure in the un-tempered martensite ring. Finally, Figure 15shows the transition from the edge of the martensitic ring and the base microstructure. Similarresults were seen with each of the temper times examined in this study. Figure 16 provides acomparison of the planar hardness maps for heat treated areas for each of the temper times.All look remarkably similar. Finally, details of the microhardness data taken from cross sectionsat each temper time are provided in Figure 17. Here the differences in hardness from center tomartensitic ring to base metal are evident. The only discernable difference in these traces isassociated with the tempered region at the center.

As mentioned, representative specimens with a heat treated zone in the center of the gaugelength were mechanically tested. These specimens were produced at the various temper timesas described above. Three of the five samples produced at each temper time were tested inthis way. All specimens appeared to fail away from the heat treated area at nominal base metalproperties. A view of a sample failed specimen is provided in Figure 18. From this figure it isclear that the hardened region provided constraint to material deformation, and drove neckinginto adjacent soft material. This is indicative for the potential for deformation path modificationwhen using these localized hardened zones. To better understand the relationship, notchedtensile specimens were also tested. Here, notches were added to each side of the hardenedarea to promote localized deformation. The gauge length of a pre-tested specimen is shown in

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Figure 19. Visual results from three of the notched tensile tests are shown in Figure 20. In allcases, failure initiated at the notch but was deflected around the local DRHT area.

Discussion

As this work demonstrates, resistance heating can be a very effective method of creating localchanges in steel sheet microstructures. An understanding of how processing variables affectthe characteristics of DRHT can be taken directly from thermal cycle modeling done forresistance spot welding. Heating during resistance spot welding has been previouslycharacterized by a relatively simple expression (Ref. 12):

=+ 1

+ Equation 1

Where I is the heating current, is the electrical resistivity, ks is the thermal conductivity of thesteel, A is the electrode contact area, is the thermal diffusivity, t is the heating time, and x is½ the stack-up thickness. In this equation, T and To are the actual and room temperaturerespectively. This equation is plotted for the 1-mm thick 10B21 steel (using 7-mm electrodes) inFigure 21. The results show that at a given current (12-kA) The temperature initially risesrapidly, and stabilizes at longer times. The break-over point here is about 50-ms. Thisaccounts for the 30-ms to 50-ms range. This accounts for the short heating times used in thisstudy. Referring to Equation 1, the break-over in temperature-time response, is strictly afunction of the x2 t term in the denominator. This term relates to the ability of theelectrodes to conduct heat away from the workpiece. At short times, thermal conduction islimited, and heating in the workpiece is nearly adiabatic. At longer times, the electrodes can bebecome heat sinks, reducing temperature rise rates for a given current. An artifact of this effectis that short times tend to achieve flatter temperature profiles through the workpiece, whilelonger times see progressively wider thermal gradients. Collectively, shorter heating andtempering times were found beneficial for DRHT. Shorter times reduce microstructuralgradients away from the heat treated area, and assure uniform microstructures in the throughthickness direction of the workpiece.

Cooling once the current has been terminated has also been characterized for resistance spotwelding through relatively simple equations (Ref. 6). From that reference, cooling profile can beestimated from the equation:

=1 + 2 2

1 + 2+ Equation 2

Where Tp is the peak temperature (before cooling), and ke and xe are the thermal conductivityand face thickness of the electrodes, respectively. The term “x” in this equation refers to thethrough thickness direction in the stack-up. A value of “0” indicates the geometric center of thestack-up, and the location of peak temperature. A plot of this equation representing theapplication here (assuming a peak heat treatment temperature of 1000oC) is provided in Figure22. The plot indicates that the workpiece under the influence of the electrodes should cool toroom temperature in less than 100-ms.

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The above analysis is provided to demonstrate the suitability of resistance heating for localizedmicrostructure modification of steels. The process (both heating and cooling) has implicit cycletimes on the order of 10’s to 100’s of milliseconds, and is compatible with cycle times necessaryin the automotive industry. While the work done here primarily is focused on full austenization,quenching, and tempering, the analysis presented (and the experimental observations as well)suggest the potential for heating only to inter-critical temperatures (ferrite + austenite) allowingfor a broader range of final microstructures to be produced.

As noted, most of the work conducted in this study was focused on tempering the microstructureafter local full transformation to martensite. As suggested above, impetus for this effort wasprevious work conducting in-situ tempering of resistance spot welds on AHSS (6 – 8). Acomponent of that work addressed developing constitutive equations that defined the shapes ofthese temper curves. That was done by combining the previously defined Holloman-Jaffee(Ref. 13) equation relating times and temperatures to achieve specific tempering responses withEquation 1 above. This produced an equation that related DRHT tempering parameters(currents, times, electrode sizes) to specific tempering responses. Essentially, DRHT temperingto a specific hardness level is defined by a loci of currents and times adhering to the followingequation:

=+ 1

( )Equation 3

Where A*, B*, and C* are empirical constants specific to the steel and hardness desired. For afixed geometry of heat treated area (such as studied here), that equation can be simplified tothe form:

=+ 1

( )Equation 4

Where A**, B**, and C** are modified constants incorporating the electrode area and steelthickness values. This is the form of the equation used to fit the temper diagram as described inthe Results section above.

The results above definitively showed the benefit of microstructural variations for managingdeformation paths. This was obvious in single tensile specimens (with a hard zone in the gaugelength), as well as notched specimens intended to specifically focus deformation in the region ofthe hardened zone. Microstructural characteristics desirable for such deflecting zones includehigh local yield strengths as well as high fracture toughness. Hard zones, of course act asdeflectors, forcing plastic deformation into softer surrounding metal. This has been well knownin the welding community for decades. However, microstructures must be managed such thatfracture strengths remain well above any local yield strengths. Reduced fracture strengths (e.g.fully martensitic microstructures) run the risk of fully brittle failures, as has been evidenced byresistance spot weld issues with high carbon steels going back decades (Refs. 10, 11). In thisstudy, the focus has been on using specific tempering approaches to achieve this combinationof strength and toughness. Development similar methods using inter-critical annealing (creatingferrite + austenite microstructures at peak temperatures) also offer considerable potential.

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One major challenge seen using quench and temper type DRHT schedules has been thetendency for “coring” or transforming a large spot during austenization/quenching, and thentempering only a smaller area as the second current pulse is applied. The in-situ tempering ofspot welds, on which this work was based, differed from the current study in two major aspects.First, actual spot welds were being tempered. Spot welds by their nature single pointconnections between overlapped sheets. As a result, during tempering the bonded area of thespot weld is the only possible conduction path for current flow. Secondly (and probably moreimportantly) previous work was done with AC rather than MFDC current. Inductive effects withthe AC waveform tend to current densities to the outer periphery of the electrodes.Redistributing the current in this way may be beneficial to achieving more uniform temperingduring the second current pulse. Modifications to the MFDC waveform may be beneficial in thisregard.

Conclusions

The research presented here represents a first effort of locally modifying the microstructure ofsheet steels though the use of direct resistance heat treatment (DHRT). Concepts for DHRTwere taken directly from previous efforts on in-situ tempering of resistance spot welds inadvanced high strength steels. DHRT can be done in one or multiple steps, depending on thetype of heat treatment desired. This work has focused on single and dual pulse heat treatment.The former is for partial or complete austenite transition, and the second for modification of theinitial pulse decomposition microstructure. Work was done on a 1-mm thick 10B21 steel. Thefollowing conclusions can be drawn from this work.

(1) Single pulse heat treatment was capable of fully martensitic microstructures: Increasingcurrents resulted in progressively harder microstructures, stabilizing when a fullymartensitic martensitic microstructure was achieved.

(2) Martensitic microstructures could be effectively modified with short temper pulses: Shorttemper pulses (ranging up to about 32-ms) could be used to consistently temper thedeveloped martensitic microstructure in hardness ranges from 50-Rc down to about 30-Rc.

(3) Simple thermal modeling validated necessary heating and cooling times for effectivequenching and tempering: Simple thermal models developed for spot welding (heatingand cooling) suggested that hating temperatures began to stabilize at around 50-ms,and that cooling to room temperature could be done in less than 100-ms for the steelthickness used.

(4) A preliminary temper map was developed for the 1-mm thick 10B21 steel. Mapping wasdone through generating a range of temper current/time pulse combinations, and fittingthe results to previously developed models for tempering resistance spot welds.

(5) Spatial temper fields did not match those for initial martensite transformed region:Tempered microstructures typically constituted a “core” within a ring of untemperedmartensite. This was inconsistent with previous work on resistance spot welds. Causeswere related to geometric effects (compared to spot welds) and the use of DC current.

(6) DRHT transformed regions acted as effective crack deflectors during destructive testing:Tensile specimens containing DRHT transformed regions in the gauge length effectivelydiverted deformation and cracking to lower strength materials at the periphery. This was

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true even for notched specimens specifically attempting to promote failure through thetransformation region.

References

(1) Davies, R. G. and Magee, C. L. 1979. Physical metallurgy of automotive high strengthsteels. Structure and Properties of Dual Phase Steels, eds. R. A. Kot and J. W. Morris,AIME.

(2) Bleck, W. 1996. Cold rolled, high-strength sheet steels for auto applications. Journal ofMetals 48 (7):26-30.

(3) Hein, P. and Wilsius, J. 2009. Status and Innovation Trends in Hot Stamping ofUSIBOR 1500 P. Steel Research International, 79(2):85-91.

(4) Asadi, M., Frommeyer, G., Aghajani, A., Timokhina, I., and Palkowski, H. 2012. Locallaser heat treatment in dual phase steels. Metallurgical and Material Transactions A.43A(4):1244-1258.

(5) George, R., Bardelcik, A., Worswick, M.J. 2012. Hot forming of boron steels usingheated and cooled tooling for tailored properties. Journal of Materials ProcessingTechnology, 212(11): 2386-2399.

(6) Chuko, W. and Gould, J.E. 2002. Development of appropriate resistance spot weldingpractice for transformation-hardened steels. AISI/DOE report TRP 9934. American Ironand Steel Institute Technology Roadmap Program Office, Pittsburgh, PA.

(7) Chuko, W. and Gould, J. E. 2002. Development of appropriate resistance spot weldingpractice for transformation hardened steels. Welding Journal Research Supplement,81(1):1s-7s.

(8) Peterson, W. and Gould, J. 2004. Development of spike temper diagrams for a range ofadvanced high strength steels. Sheet Metal Welding Conference XI, AWS DetroitSection, Detroit, MI.

(9) 2011. Standard Specification for Quality Assurance Requirements for Carbon and AlloySteel Wire, Rods, and Bars for Mechanical Fasteners. ASTM Standard F2282-03.Annual Book of ASTM Standards, Vol. 01.08, pp. 592-606. ASTM, Conshohocken, PA.

(10) Bhadeshia, H.K.D.H. and Svensson, L.-E. 1993. Mathematical Modeling of WeldPhenomena eds. H. Cerjack and K. E. Easterling, Institute of Metals, London, pp. 109-180.

(11) 2000. Recommended Practices for Resistance Welding. AWS C1.1-2000. AmericanWelding Society, Miami, FL.

(12) Gould, J. E. 1987. An examination of nugget development during spot welding using bothexperimental and analytical techniques. Welding Journal Research Supplement, 67(1):1s-10s.

(13) Holloman, J. H. and Jaffee, L. D. 1945. Time-temperature relations in the tempering ofsteel. Trans. AIME, pp. 162-223.

(14) Han, Z., Indacochea, J. E., Chen, C. H., and Bhat, S. 1993. Weld nugget developmentand integrity in resistance spot welding of high strength cold rolled sheet steels. WeldingJournal Research Supplement, 72(5):209s-216s.

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(15) Gould, J. E. and Workman, D. 1998. Fracture morphologies of resistance spotwelds exhibiting hold time sensitivity behavior. Proceedings, Sheet Metal WeldingConference VII, AWS Detroit Section, Detroit, MI.

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List of Tables

Table 1 – Representative Chemistry for the 10B21 Steel used in this Study. The chemistrywas taken from standard reference sources (Ref. 9).

Min Max AvgCarbon 0.10% 0.23% 0.17%Boron 0.001% 0.003% 0.002%Manganese 0.80% 1.10% 0.95%Phosphorous 0.00% 0.02% 0.01%Sulfur 0.00% 0.02% 0.01%Silicon 0.15% 0.30% 0.23%Aluminum 0.02% 0.06% 0.04%

Table 2 – Current Range Data used in defining the First Pulse Conditions for LocallyCreating Martensite in the 10B21 Steel.

Sample#

HeatingTime (ms)

ControllerCurrentSetting

(kA)

MeasuredHeating

Current (kA)

MeasuredHardness

(Rc)

1 28 14 16.35 49.52 28 13 15.23 51.03 28 12 14.45 49.04 28 11 13.36 50.25 28 10 12.39 48.06 28 9 11.64 40.07 28 8 10.44 24.08 28 15 17.02 48.9

Table 3 – Combinations of Temper Currents and Times Identified to Achieve SpecificLevels of Hardness. Data was extroplated from the experimental results shown in Figure 7.

Time(ms)

Hardness50-Rc 47-Rc 44-Rc 40-Rc 35-Rc

10 16-kA 19-kA 25-kA 26-kA -16 13-kA 15-kA 16-kA 17-kA 22-kA20 12.5-kA 14-kA 15-kA 15-kA 20-kA24 12-kA 13.5-kA 14-kA 15.5-kA 18-kA28 11.5-kA 11.5-kA 12-kA 13-kA 17-kA32 10-kA 10.5-kA 11-kA 12.8-kA 16-kA

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List of Figures

Figure 1 – Schematic Representation of theDRHT Process. The diagram includes pulsesfor both initial martensite formation andsubsequent tempering.

Figure 2 – Continuous CoolingTransformation/ Time TemperatureTransformation Diagram for a 10B21 Steel.Various cooling rates are superimposed onthe diagram, showing a critical value formartensite formation of roughly 200oC/s.

Figure 3 – Variations in Hardness WhenApplying a Single DRHT Pulse. Data isdeveloped for a heating time of 28-ms, andshows stable peak hardnesses (martensitic finalstructure) above 12-kA.

Figure 4 – Metallographic Section throughan Area Subject to Single PulseHardening. Hardening was done at 14.4-kAcurrent and 28-ms heating time.

0

10

20

30

40

50

60

0 5 10 15 20

Surf

ace

Har

dnes

s(H

Rc)

Measured "Hardening" Current (KA)

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Figure 5 – Microstructural Variations of aSingle Pulse DRHT Hardened Area acrossthe Sheet Surface. Note the narrowtransition from the hardened area the basemetal.

Figure 6 – Planar View Hardness Map of aSingle Pulse DRHT Specimen. Thishardness map parallels the microstructurevariations shown in Figure 5.

Figure 7 – Hardness Variations as aFunction of Temper Current for SeveralDifferent Heating Times.

Figure 8 – DRHT Temper Map for the 1-mmThick 10B21 Steel. Map is based onhardening conditions of 14.4-kA current, 28-ms heating time, and 1-s quenching timebefore tempering.

25

30

35

40

45

50

55

0 5 10 15 20 25 30

Hard

ness

(HRc

)

Temper Current (KA)

10-ms

16-ms

32-ms

24-ms

20-ms

28-ms

5

10

15

20

25

30

10 15 20 25 30 35

Tem

perC

urre

nt(K

A)

Temper Time (ms)

35-Rc40-Rc44-Rc47-Rc50-Rc

Surface Expulsion

26.030.2

31.433.9

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Figure 9 – Macrosection of a DRHTSpecimen Subject to Hardening andSubsequent Tempering. Note that thetempered region is completely contained by aretained martensite ring.

Figure 10 – Planar View of MicrostructuralVariations on a DRHT Sample Subject toboth Hardening and SubsequentTempering.

Figure 11 – Planar View Hardness Map of aDouble Pulse DRHT Specimen. Thishardness map parallels the microstructurevariations shown in Figure 10.

Figure 12 – Microstructural Details of theCentral DRHT Tempered Area. Observedmicrostructures suggest tempered martensite.

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Figure 13 – Microstructural Details of theTransition between the Central Area andMartensitic Ring in a Dual Pulse DRHTSpecimen.

Figure 14 – Microstructural Details of theMartensitic Ring Surrounding theTempered Region.

Figure 15 – Microstructural Details of theTransition between the Martensitic Ringand the Base Metal in a Dual Pulse DRHTSpecimen.

Figure 16 – Comparison of Planar HardnessMaps for Each of the Temper Heat TimesEvaluated in this Study.

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Figure 17 – Hardness Traces Taken fromSpecimens Subject to Dual Pulse DRHTRepresenting the Range of Temper Timesused in this Study. Only variations noted arein the central temper region itself.

Figure 18 – Overview of a Failed TensileSpecimen with a DRHT Zone in the GaugeLength. Note necking and failure away fromthe hardened zone.

Figure 19 – Gauge Length of a NotchedTensile Specimen used to FocusDeformation at the DRHT Zone.

Figure 20 – Overview of Three FailedTensile Specimen with Notches used toFocus Stresses at the DRHT Zone. TheDRHT zone effectively deflected deformationand failure away from the heated area.

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Figure 21 – Estimates of HeatingCharacteristics during DRHT. The plot istaken from modeling used to predicttemperature variations in spot welds, andincorporates conditions used in hardening the1-mm thick 10B21 steel.

Figure 22 – Estimates of CoolingCharacteristics during DRHT. The plot istaken from modeling used to predict coolingcharacteristics of resistance spot welds,assumes a peak temperature of 1000oC, andincorporates conditions used in hardening the1-mm thick 10B21 steel.

0

200

400

600

800

1000

1200

1400

0 50 100 150 200 250

Tem

pera

ture

o C

Weld Time (ms)

=+ 1

0

200

400

600

800

1000

1200

0 20 40 60 80 100

Tem

p(d

eg.C

)

Time (ms)

=1 + 2

1 + 222