Microstructural Evolution in a 17-4 PH Stainless Steel

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Transcript of Microstructural Evolution in a 17-4 PH Stainless Steel

  • I. INTRODUCTION

    PRECIPITATION-hardened stainless steels arewidely used as structural materials for chemical andpower plants because of their balanced combination ofgood mechanical properties and adequate corrosion re-sistance. 17-4 PH stainless steel is a martensitic stain-less steel containing approximately 3 wt pct Cu and isstrengthened by precipitation of copper in the marten-site matrix [1-8]. After a solution heat-treatment, this al-loy is precipitation hardened by tempering at about580C for about 4 hours. Typical service temperaturesin power plant applications are below 300C, but in-creases in hardness and tensile strength accompaniedby embrittlement was reported at temperatures rangingfrom 300 to 400C after long term aging. Since thesematerials have to serve for a very long period of timeduring the life span of the plants, understanding theembrittlement mechanism at slightly above the servicetemperature is very important.

    The precipitation sequence in 17-4 PH stainless steelbegins with formation of coherent copper precipitates,which occurs during the tempering treatment before ser-vice. These coherent particles were reported to trans-form to incoherent fcc-Cu particles after long term ag-ing at temperatures around 400C [3]. In addition, sincethe Cr concentration in 17-4 PH is within the spinodalline, phase decomposition of the martensite into the Fe-rich a and the Cr-enriched a is expected on aging be-low 450C. Much work has shown that stainless steelsare embrittled when a phase precipitates by spinodaldecomposition [11]. Such a embrittlement is anticipatedin the 17-4 PH stainless steel as well.

    Several studies on the effect of aging 17-4 PH stain-less steel were carried out [3-6]. Early work by Anthony[4] proposed mechanical properties of 17-4 PH are in-fluenced by precipitation of a phase, but no direct evi-dence for a precipitation was presented. Later, Jackand Kalish [3] observed copper precipitation on agingand correlated it to mechanical property changes; how-ever, there was no mention of phase decomposition inthe martensite phase. More recently, Yrieix andGuttmann [10] reported that 17-4 PH stainless steel ex-hibits high susceptibility to aging embrittlement at400C, and they concluded that it was essentially dueto a precipitation. In their study, however, no micro-structural observation results were shown. Employingatom probe field ion microscopy (APFIM) and trans-mission electron microscopy (TEM), Miller and Burke[6] showed direct evidence for a precipitation after ag-ing at 482C. They also reported that significantamounts of iron, nickel and manganese were containedin the e-Cu precipitates even in the overaged condition.However, their aging temperature is rather high com-pared to the service condition of 17-4 PH steel.

    This study aimed to carry out a more complete char-acterization of microstructures in 17-4 PH stainless steelat various stages of heat treatment, i.e., after solutionheat-treatment, tempering at 580C for four hours, andlong term aging at 400C, in order to obtain a betterunderstanding of the embrittlement phenomena on ag-ing.

    II. EXPERIMENTAL PROCEDURES

    The chemical composition of the alloy used in thisstudy was Fe-16.5Cr-4.0Ni-3.4Cu-0.6Si-0.6Mn-0.3Nb-0.06C (wt pct) or Fe-17.5Cr-3.8Ni-2.9Cu-1.2Si-0.6Mn-0.2Nb-0.3C (at. pct). The alloy was solution heat-treatedat 1050C for 1 h and subsequently water quenched.The solution treated samples were then aged at 580Cfor 4 h (tempering). This heat treatment causes precipi-tation of coherent Cu precipitates in the martensite phase

    Microstructural Evolution in a 17-4 PH Stainless Steelafter Aging at 400C

    M. MURAYAMA, Y. KATAYAMA and K. HONO

    The microstructure of 17-4 PH stainless steel at various stages of heat treatment, i.e. after solution heat-treatment, tempering at 580C and long term aging at 400C, have been studied by atom probe field ionmicroscopy (APFIM) and transmission electron microscopy (TEM). The solution treated specimen con-sists largely of martensite with a small fraction of d-ferrite. No precipitates are present in the martensitephase, while spherical fcc-Cu particles are present in the d-ferrite. After tempering for 4 h at 580C, coher-ent Cu particles precipitate in the martensite phase. At this stage, the Cr concentration in the martensitephase is still uniform. After 5000 h aging at 400C, the martensite spinodaly decomposes into Fe-rich aand Cr-enriched a. In addition, fine particles of the G-phase (structure type D8a, space group Fm3m)enriched in Si, Ni and Mn have been found in intimate contact with the Cu precipitates. Following spinodaldecomposition of the martensite phase, G-phase precipitation occurs after long-term aging.

    M. MURAYAMA, Researcher, and K. HONO, Head of 3rd Labo-ratory, are with Materials Physics Division, National Research In-stitute for Metals, Tsukuba 305-0047, Japan. Y. KATAYAMA, iswith Heavy Apparatus Engineering Laboratory, Toshiba Corpora-tion, Yokohama 230-0045, Japan

    Manuscript submitted April 21, 1998.

  • Published in Metall. Mater. Trans. A. Vol. 30A, pp. 345-353. 1999

    tempering treatment, the specimen was aged for 100and 5000 hours at 400C.

    For atom probe analyses, a locally built reflectron-type energy compensated time-of-flight atom probe(1DAP) and a three-dimensional atom probe (3DAP)equipped with CAMECAs tomographic atom probe(TAP) detection system [12] were used. One disadvan-tage of the 3DAP was its poor mass resolution, becauseit was not equipped with an energy compensator forimproving mass resolution. The mass resolution of the3DAP used in this study was limited to m/D m~200 fullwidth at 10 pct maximum (FW10 pct M), which is sig-nificantly lower than that obtained using an energy com-pensated atom probe (~500 FW10 pct M). Thus, Fe2+and Mn2+ ions, which have similar mass-to-charge ra-tios were not distinguished in the 3DAP analyses. Thus,detailed spatial information provided by 3DAP analy-sis was complemented by 1DAP analysis with a highmass resolution. Field ion microscopy images wereobserved at temperatures of 30 - 60 K with Ne as animaging gas, and atom probe analyses were carried outat a specimen temperature of about 30 K, under a UHV(~1x10-10 torr) condition, with a pulse fraction (Vp/Vdc)of 20 % and a pulse repetition rate of 600 Hz. Micro-structures of the specimens were examined with aPhilips CM200 transmission electron microscope(TEM), operated at 200 kV. High resolution transmis-sion electron microscope (HRTEM) observations werecarried out using a JEOL JEM-2000EX, operated at 200kV. Thin foils for TEM were prepared by grinding theslices to a thickness of about 100 mm, then by twin-jetelectropolishing using a 5 pct perchloric acid-acetic acidsolution at 287 K. For long-term aged specimen, ionbean thinning was employed for thin foil preparation,because it was found that Cu particles are preferentiallydissolved by electropolishing.

    III. RESULTS

    A. Mechanical properties

    Figure 1 shows the influence of aging times on yieldstrength at 350 and 400C. For both temperatures, anincrease in yield strength occurs after 10 hours aging,and the strengthening response is much faster at 400C.Increase in the yield strength is almost saturated after10,000 hours aging, and 80 and 90 pct of strengtheningis achieved after 100 and 1000 hours aging respectively.Values of yield strength, tensile strength, elongation andCharpy V-notch energy absorption measured before andafter 5000 h aging at 400C are summarized in Table I.Increases in yield strength, tensile strength occur afterlong term aging accompanied by decreases in elonga-tion and Charpy V-notch energy absorption. This indi-cates that embrittlement occurs as a result of long termaging.

    Yield strength (Mpa)

    Tensilestrength (Mpa)

    Elonga-Tion (%)

    Charpy V-notch energy

    absorp-tion (J)

    Pre-aged alloy(580C x 4h)prolonged aged alloy(400C x 5000h)

    Charpy V-notch energy absorption was measured at 0C.

    895 1085 23 107

    1362 1434 8.3 3

    Table I Changes in mechanical properties of 17-4 PHby aging

    1600

    1400

    1200

    1000

    800

    Yie

    ld S

    trength

    / M

    Pa

    Tem

    pere

    d 10 100 1000 10000

    Aging time / hr

    400 C

    350 C

    Fig. 10.2% yield stress of 17-4 PH stainless steel as a function ofaging time at 400C.

    NbC

    d -ferrite

    1m m

    Fig. 2 TEM bright field image of the martensite phase in 17-4 PHstainless steel after solution heat-treatment. The predominant phaseis lath martensite. Grains of NbC and d -ferrite are indicated.

    and provides balanced strength and toughness as shownin Table. I. This is the typical condition of 17-4 PH stain-less steel before use as a structural material. After this

  • Published in Metall. Mater. Trans. A. Vol. 30A, pp. 345-353. 1999

    B. Solution treated microstructure

    A solution-treated 17-4 PH stainless steel is com-posed largely of martensite with a minor fraction of d-ferrite as shown in Figure 2. The martensite phase isconsist of a lath structure containing a very high den-sity of dislocations. There is no evidence of precipi-tates which suggests the martensite phase is supersatu-rated with Cu and Cr in the solution-treated condition.

    On the other hand, a high density of fine precipi-tates are observed in the d-ferrite phase as shown inFigure 3 (a). In the bright-field image, the precipitates

    are spherical and each precipitate appears to be associ-ated with dislocation. Absence of strain contrast andpresence Moire fringe indicate that the particles are in-coherent with the bcc matrix. In fact, selected area dif-fraction (SAD) pattern taken slightly inclined from the[111] zone show {020}fcc reflections, indicating that theprecipitates are fcc-Cu. The orientation relationship(OR) between the particle a