Effect of High Magnetic Fields on theMartensite Transformation

RICHARD FIELDS AND C. D. GRAHAM, Jr.

The effect of high magnetic fields up to 132 kOe on the martensite transformation hasbeen investigated in two alloy steels, 52100 bearing steel and a type 410 stainless steel.In both cases the martensite start temperature is raised by the application of a magneticfield, and the increase in Ms is linear with field. The rate of formation of martensite isnot affected by the field. Numerical values for the entropy of the austenite-martensitereaction can be obtained from the experimental results, and are in reasonable agree­ment with previous results and with theoretical calculations.

IT is well established that the application of a mag­netic field can influence the formation of ferromag­netic martensite from a nonferromagnetic austenite.The magnetic field favors the formation of the ferro­magnetic phase, and hence raises the martensite start(Ms) temperature. The clearest and most extensiveexperiments on martensite formed during coolingappear to be those of Satyanarayan, Eliasz, andMiodownik;' who give references to earlier (mostlyRussian) work. We do not consider the effect ofmagnetic fields on the formation of isothermal mar­tensite, which has also been fairly extensively investi­gated. Satyanarayan et al conducted experiments ontwo alloy steels, whose compositions are given inTable I. They followed the formation of martensite infields of 0 and 16 kOe, using the metallographicGremnger-T'roiano" technique, and found a clear in­crease in the Ms temperature in the presence of themagnetic field.

The Satyana ray an et al analysis of the results isbased on that of Estrin." It is shown that the onlysignificant effect of the magnetic field is to lower thefree energy of the ferromagnetic phase relative to thenonmagnetic phase by an amount IH, where I is thesaturation magnetization of the ferromagnetic phaseand H is the effective magnetic field. If I is in cgsemu/cm" (Gauss/4rr) and H is in Oersteds, the quantityIH is in erg/ern"; this can be converted to cal/rnol forcomparison with thermodynamic quantities as conven­tionally expressed.

If t.T is the increase in martensite start tempera­ture due to a field H, !H/t.T =&(t.G)!aT = ssr>',where t.G is the free energy difference between aus­tensite and martensite at T = Ms, and e:,sY-Ci' is theentropy of the transformation. The quantity !H/t.Tcan also be regarded as an approximate measure oft.G,jTo - Ms, where t.GE is the strain energy as­sociated with the formation of the first martensiteand To is the temperature of thermodynamic equili­brium between austenite and martensite. Somewhatless accurately, IH/t. T is approximately equal tot.Go/To, where t.Go is the free energy difference be-

RICHARD FIELDS is Research Student, Department of Engineer­ing, Cambridge University, England. C. D. GRAHAM, Jr., is Professorof Metallurgy and Materials Science, University of Pennsylvania, Phila­delphia, PA 19174, where Richard Fields was formerly a student.

Manuscript submitted September 12,1975.

METALLURGICAL TRANSACTIONS A

Table I. Alloy Compositions

Alloy C Mn Si Ni Cr Mo

*Comp.2 0.3 0.6 0.2 2.8 0.6 0.6*Camp.1 1.0 0.5 0.2 0.007 1.552100 1.02 0.36 0.16 1.41

410 0.1\ 0.44 0.37 0.2 12.18

*Ref. 1

tween austenite and martensite at absolute zero, andTo is as previously defined. Satyanarayan et al givea neat graphical derivation of these expressions.Values of t.Go, t.GE, and To can only be obtained in­directly for the austenite-martensite transformation,and experiments with magnetic fields are useful toprovide confirmation of the estimates of these quan­tities. The entropy of transformation, however, is adirect experimental value resulting from the experi­ment.

Satyanarayan et al used only magnetic fields of 0and 16 kOe, and assumed that the change in Ms waslinear with field. The present experiments were un­dertaken to test this assumption, using the high-fieldBitter magnets at the Laboratory for Research on theStructure of Matter at the University of Pennsylvania.

EXPERIMENTS

A series of magnetic fields up to 132 kOe were ap­plied during the martensite transformation in two al­loy steels: a 52100 bearing steel similar in composi­tion to Alloy 2 of Satyanarayan et al and a type 410ferritic stainless steel. Neither requires waterquenching to form martensite, which greatly easesthe experimental difficulties; compositions are givenin Table I. The 52100 alloy was annealed for threedays at 1000°C to dissolve the chromium carbide thatis normally present.

The experimental arrangement is shown in Fig. 1.The samples were cylinders, 25 mm long by 12.5 mmdiam, drilled axially for thermocouples. In each run,a sample was austenitized for 1 hat 925°C and thenprecooled to about 200°C above Ms: the 410 stainlesswas precooled in air, but the 52100 had to be rapidlycooled in a hot oil bath to avoid formation of ferriteand carbide. The sample, still austenitic, was then

VOLUME 7A,MAY 1976-719

H

Fig. I-Experimental apparatus. Magnet is a water-cooledcopper solenoid, 28.5 mm bore, capable of producing 130kOe at 9000 A, 400 V.

370

360

350

u 340°w 170ll::::>

/I-et 160ll:wQ.

::;: 150wI-

140 /'130 .120

0 20 40 60 80 100 120 140

I I I I I I IU f- 410 -°<, 14 f-. -wl- f- -lI) -.-- - - - -.--z 1.2 f- • -wl- f- -ll:et 1.0 f- -::;:

Table 11. Summary of Results

Magnetization* !!.T QC IH cal---Alloy 41TM, Gauss I, emu/mol Ms. QC Mf' kOe !!.T'mol-K

Camp. 2 20,600 11,600 321 0.28 1.0Camp. 1 21,300 12,100 101 0.31 0.952100 21,300 12,100 121 0.34 0.8

410 17,700 10,000 342 0.15 1.6

"Martensite magnetization assumed equa! to the corresponding ferrite.

FIELD. KOeFig. 3-Effect of magnetic field on the martensite start (Ms)temperature of two alloy steels.

RECORDER

-uT

M

dM/dt

MAGNET

THERMOCOUPLE

SAMPLE

BUCKING COIL

PICKUP COIL

placed in a pickup coil in the center of a high-fieldmagnet, and the magnetic field was set to a prede­termined value. The integrated output of the pickupcoil, which is a direct measure of the magnetizationof the sample and hence the amount of martensiteformed, was plotted directly against the sample tem­perature during the cooling. The magnetization axiswas calibrated using a sample of pure iron. A setof experimental curves for 410 stainless is shown inFig. 2. The Ms temperature for each run was deter­mined by extrapolating the linear portion of thecurve to 0 pet martensite. Fig. 3 shows the Ms tem­peratures, determined in this way, as a function ofthe applied field for both alloys. It is clear from thefigure that the effect of magnetic field on the Ms tem­perature is linear up to at least 130 kOe. (The de­magnetizing field correction is about 1 kOe maximumand has been neglected. Eq. [4] of Satyanarayan et ai,which apparently attempts to account for the demag­netizing field, is not correct when the applied fieldis large compared to the demagnitizing field). It isinherent in the measuring method used here that the

FURNACE

/-- ....................

I

I I I I I I I

OL--'--__-'--__L-__L-_---.JL-_~~::..___'__---'

-

-

-

·410

I I //

//Ref6/

,/

Ref. I /;'cemp.2 v

·/Ref.1;' /' comp.l

I

;';'

;';'

;';'

/;'

,/

1.0 I-

2.0 f-

zoi=et:.ll: uo •LL "­W wI- I-00 00z zw WI- I­a: a:« «:. :.~ f!.w~a:

o 20 40 60 80 100 120 140

FIELD, KOe

Fig. 4-Rate of formation of martensite in 410 stainless steelvs magnetic field.

I I I

0.5 1.0 1.5 2.0

ENTROPY OF TRANSFORMATION s«: col/mol-K

Fig. 5-Rate of formation of martensite vs entropy of trans­formation. Unlabelled point is from Ref. 6, for a steel con­taining 0.6 C, 4 Cr, 8 Ni, 3 Si, and 1 Mo.

400350300250200150100

100.---------------------------,

TEMPERATURE (OC)

Fig. 2-Percent martensite vs temperature for 410 stainlesssteel in various fields. Curves traced directly from x-y re­corder plots.

75wl-ll)

Zwl-ll: 50et::;:

I-ZwUll: 25wQ.

720-VOLUME 7A,MAY 1976 METALLURGICAL TRANSACTIONS A

zero-field value of M, must be obtained by extrapola­tion, since the martensite will not be magnetized andwill give no signal at zero field.

The experimental results, expressed both as in­crease in Ms per kOe of field and as !HIf:J.T, aregiven in Table 11, along with other relevant data. Theequivalent data from Satyanarayan et al is includedin the table; the agreement is good. For comparisonof these results with other magnetic experiments,and with various estimates of f:J.G E , To, and f:J.Go, seethe discussion of Satyanarayan et al.

The data below Ms for the 410 stainless were ade­quate to determine the rate of martensite formation.The amount of martensite increased linearly with de­creasing temperature from about 10 to 50 pet mar­tensite. The rate of formation of martensite was in­dependent of magnetic field, as shown in Fig. 4, andin agreement with a theoretical treatment as outlinedby Magee4 which equates the rate of nucleation ofmartensite to the free energy difference betweenaustenite and martensite. Satyanarayan et al give aplot of martensite formation rate vs f:J.S y -a' for threealloy steels, which confirms the theoretical predic­tion by Brooks, Entwistle, and Ibrahim" of a linearrelationship. Their graph is reproduced as Fig. 5,with an added point for the new data on 410 stainlesssteel. The new point clearly fails to lie on the line.

METALLURGICAL TRANSACTIONS A

CONCLUSIONSWe have found that the martensite start tempera­

ture increases linearly with field up to 130 kOe in twoalloy steels. The rate of formation of martensite isnot influenced by magnetic field. Numerical values forthe entropy of transformation have been obtained, andare in reasonable agreement with other determinationsand estimates.

ACKNOWLEDGMENTSP. J. Flanders helped us greatly with the experi­

mental arrangements, and the Laboratory for Researchon the Structure of Matter, University of Pennsylvania,provided access to the high-field equipment. TheLaboratory is supported by the National ScienceFoundation. SKF Inc. of Philadelphia kindly providedsamples of 52100 steel.

REFERENCESI. K. R. Satyanarayan, W. Eliasz, and A. P. Miodownik: Acta Met., 1968, vol. 16,

p.877.2. A. B. Greninger and A. R. Troiano: Trans. ASM, 1940, vol. 28, p. 537.3. E. I. Estrin: Phys.Metals Metallogr., 1965, vol. 19, no. 6, p. 117.4. C. L. Magee: in Phase Transformations, p. 118, ASM,Metals Park, Ohio, 1970.5. R. Brook, A. R. Entwistle, and E. F. Ibrahim: J. Iron Steel Inst., 1960,

vol. 195, p. 292.6. L. V. Voronchikhin and I. G. Fakidov: Phys.Metals Metallogr., 1966, vol. 21,

no.3,p.119.

VOLUME 7A, MAY 1976-721