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  • Mechanical Properties and Microstructural Evolutionof Simulated Heat-Affected Zones in Wrought EglinSteel

    BRETT M. LEISTER, JOHN N. DUPONT, MASASHI WATANABE,and RACHEL A. ABRAHAMS

    A comprehensive study was performed to correlate the mechanical properties and microstruc-tural evolution in the heat-affected zone of Eglin steel. A Gleeble 3500 thermo-mechanicalsimulator was used to simulate weld thermal cycles with different peak temperatures at a heatinput of 1500 J/mm. These samples underwent mechanical testing to determine strength andtoughness in the as-welded and post-weld heat-treated conditions. The inter-critical heat-af-fected zone (HAZ) had the lowest strength following thermal simulation, while the fine-grainand coarse-grain heat-affected zone exhibited increased strength when compared to theinter-critical HAZ. The toughness of the heat-affected zone in the as-simulated condition islower than that of the base metal in all regions of the HAZ. Post-weld heat treatments (PWHTs)increased the toughness of the HAZ, but at the expense of strength. In addition, certaincombinations of PWHTs within specific HAZ regions exhibited low toughness caused bytempered martensite embrittlement or intergranular failure. Synchrotron X-ray diffraction datahave shown that Eglin steel has retained austenite in the fine-grain HAZ in the as-simulatedcondition. In addition, alloy carbides (M23C6, M2C, M7C3) have been observed in thediffraction spectra for the fine-grain and coarse-grain HAZ following a PWHT of 973 K(700 C)/4 hours.

    DOI: 10.1007/s11661-015-3131-x The Minerals, Metals & Materials Society and ASM International 2015

    I. INTRODUCTION

    EGLIN steel is an ultra-high strength steel alloy thatwas developed at Eglin Air Force Base in the early 2000sand has since been patented in 2009.[1] The steel hasstrength levels similar to AerMet100, AF1410, andHP9-4-30, but at a reduced cost due to a reduction orelimination of expensive alloying elements such as nickeland cobalt, which can both range from 10 to 14 wt pct inthe previously mentioned alloys.

    Table I shows the alloy composition for Eglin steelused in this work. Silicon is added to enhance toughnessand stabilize austenite. Silicon is well known to makecementite precipitation difficult at tempering tempera-tures used for Eglin steel.[26] This is critical becausesimilar alloys can experience tempered martensiteembrittlement due to cementite formation at tempera-tures as low as 548 K (275 C).[2] In order to ensure thatEglin steel does not form cementite while being tem-pered at 473 K (200 C), the increased silicon content is

    necessary. According to the patent document, chro-mium is added to increase strength and hardenability,while molybdenum is also added to increase harden-ability. Nickel is used to increase toughness, andtungsten is added to increase strength and wearresistance.[1]

    Eglin steel typically has a quenched and temperedmicrostructure consisting of tempered martensite with avariety of carbide sizes and morphologies. Paules et al.[7]

    have reported M3C, M6C, and MC carbides that formafter heat treatment with sizes of 180, 250, and 10 to 20nm, respectively. Two heat treatment schedules wereinvestigated. In the first heat treatment, the samples werenormalized at 1363K (1090 C) for 1 hour, followed by asub-critical anneal at 923 K (650 C) for 1.5 hours. Thesamples were then austenitized at 1173 K, 1223 K, and1283 K (900 C, 950 C, or 1010 C) for 0.5 hours, oilquenched, and then tempered at 533 K (260 C) for1 hour. The second heat treatment was the same as thefirst, but did not contain the normalization and sub-crit-ical anneal. M3C and MC carbides were found in allheat-treated samples, but the M6C carbides were onlypresent following the low-temperature austenitizationtreatment. Increasing the austenitization temperaturefrom 1173 K to 1283 K (900 C to 1010 C) caused thedissolution of the M6C carbides.Eglin steel will undergo a variety of fabrication

    processes such as casting, rolling, forging, fusion weld-ing, and heat treating. Welding of Eglin steel will beespecially necessary, and a comprehensive study of

    BRETT M. LEISTER, Associate, is with Exponent FailureAnalysis Associates, Menlo Park, CA, and also with the Departmentof Materials Science and Engineering, Lehigh University, Bethlehem,PA 18015. Contact e-mail: bleister@exponent.com JOHN N.DUPONT and MASASHI WATANABE, Professors, are with theDepartment of Materials Science and Engineering, Lehigh University.RACHEL ABRAHAMS, Research Scientist, is with Eglin Air ForceBase, Eglin AFB, FL 32542.

    Manuscript submitted October 16, 2014.Article published online September 10, 2015

    METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 46A, DECEMBER 20155727

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  • heat-affected zone (HAZ) microstructure and propertieshas yet to be completed for this alloy. Thus, theobjective of this work is to measure the changes inmechanical properties in the HAZ of Eglin steel andcorrelate those changes to the evolution of microstruc-ture as a result of the weld thermal cycle. In addition, apost-weld heat treatment (PWHT) study has beenundertaken in an attempt to restore properties in theHAZ following weld thermal cycles.

    II. EXPERIMENTAL PROCEDURE

    The chemistry for wrought Eglin steel can be seen inTable I. The Eglin steel was received in the quenchedand tempered condition according to the following heattreatment: normalize at 1339 K (1066 C) for 1 hour (air

    cool), austenitize at 1283 K (1010 C) for 1 hour (waterquench), and temper at 477 K (204 C) for 4 hours (aircool).A Gleeble 3500 thermo-mechanical simulator was

    used to perform the HAZ simulations. Thermal cyclesthat were used for the HAZ simulations were calculatedby Sandias SmartWeld (SOAR) program[8] using a heatinput of 1500 J/mm. Four peak temperatures of 948 K,1098 K, 1198 K, and 1623 K (675 C, 825 C, 925 C,and 1350 C) were used to be representative ofthe sub-critical HAZ (SCHAZ), inter-critical HAZ(ICHAZ), fine-grain HAZ (FGHAZ), and coarse-grainHAZ (CGHAZ), respectively. The peak temperatureswere chosen based upon dilatometry data collected forEglin steel at a range of heating rates. The Ac1temperature ranged from 1052 K to 1086 K (779 C to813 C), and the Ac3 temperature ranged from 1168 K

    Table I. Composition (Weight Percent) of Eglin Steel

    Alloy C Co Cr Fe Mn Mo Ni Si W

    Eglin 0.26 2.70 Bal. 0.65 0.42 1.00 1.00 1.00

    Fig. 1Mechanical properties of as-simulated Eglin steel showing the effect of thermal cycle peak temperature and sample geometry on (a) yieldstrength, (b) tensile strength, (c) percent reduction in area, (d) charpy impact toughness.

    5728VOLUME 46A, DECEMBER 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A

  • Fig. 2Light optical micrographs, scanning electron micrographs, and scanning electron fractographs of Eglin steel base metal and heat-affectedzones in the as-simulated condition. Arrows indicate as-quenched martensite at the prior austenite grain boundary in the ICHAZ sample. Thefracture surfaces shown are from the broken charpy specimens.

    METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 46A, DECEMBER 20155729

  • to 1184 K (895 C to 911 C). Samples used for HAZsimulations were 11 9 11 9 70 mm3 rectangular bars.

    Post-weld heat treatments (PWHT) were conducted at473 K (200 C/392 F) and 973 K (700 C/1292 F) for1 and 4 hours. These temperatures were selected in orderto stay outside the temper embrittlement trough forEglin steel. Recent results by Abrahams et al. haveshown that Eglin steel exhibits a loss in impact tough-ness due to tempered martensite embrittlement betweentempering temperatures of 260 C and approximately650 C.[9] The PWHT temperatures of 473 K and 973 K(200 C and 700 C) were based on the results of thisstudy. The samples used for PWHT had previouslyundergone thermal cycling on the Gleeble 3500 using a1500 J/mm heat input. The samples were heated inambient atmosphere and allowed to air cool followingheat treatment.

    Tensile testingwasperformed inaccordancewithASTME8[10] using a crosshead speed of 0.5 mm/min until yieldfollowed by 5 mm/min until fracture, using samples with agage diameter of 2.87 mm (0.113 in.) and a gage length of11.48 mm (0.452 in.). Samples were also tested with adouble-reduced gage section (DRS) and a gage length of3 mmand a gage diameter of 2.87 mm.TheDRSwas used

    to ensure failure occurred within the uniform temperatureregion of the Gleeble samples. Charpy testing was per-formed at room temperature in accordance with ASTME23[11] using a standard size sample.Synchrotron X-ray diffraction characterization was

    performed at the 12-BM-B beamline at the AdvancedPhoton Source of Argonne National Laboratory. Sam-ples used in the X-ray diffraction experiments were cutfrom the thermally simulated Gleeble samples with an11 9 11 mm2[2] cross section and were polished to a0.05 lm surface finish. The wavelength of the X-radia-tion used was 0.827 nm. When being examined forretained austenite, the samples were scanned from 25 to35 deg in 2h with a step size of 0.01 deg and a count timeof 3 seconds per step. The (200)a and (200)c peaks arecontained within this 2h range. When being examinedfor the presence of carbides, the samples were scannedfrom 19 to 23 deg in 2h with a step size of 0.01 deg and acount time of 10 second per step.Microstructural characterization was performed on a

    Reichert-Jung MeF3 inverted light optical microscope.Samples were prepared using standard metallographictechniques with a 0.05 lm colloidal silica final polish.The samples were etched first in 4 pct picral, cleaned

    Fig. 3Mechanical properties of Eglin Steel showing the effect of post-wel