Magnetichardeninginahot-rolledFe-20Mo-5Ni-0.12Calloy
S.S.M. Tavares J.M. Neto² J.R.Teodosio*
Instinto de Fisio. Unisersidade Federal do Rio de Jaseiro CP 68528. CEP 21945-970. Brazil Departamo de Engenhrig Mecinic Uninersidade Federal Fluminene Brazil* Departmento de Engenharia Metalirgica COPPE(EE Uninersidade Federaf do Rio de Janeiro CP 58510 CEP 21945.970 Rio de Joneiro Brazil
Received 6 September 1996; received in revised form 27 August 1997
Abstract
The magnetic properties in an Fe20Mo5Ni0.12C alloy hot-rolled solution treated at 1230°C and aged at 610Cfordifferent times were measured.These properties were pared with those of the same alloy previously cold-rlled solution treated at the same temperature and aged under the same conditions The results for the same alloy withoutreachinga H value of 401 Oe after 240 min. of aging.The precipitation hardening was acpanied by thermomagnetic carbon obtained by other authors have also been considered. The alloy presented magnetic hardening during aginganalysis (TMA) in the range 25800°C and X-ray diffraction. C 1998 Elsevier Science B.V. All rights reserved.
PACS: 81.40.Rs
Keywordx: Magnetic materials; Magnets; Magnetic hardening
1.Introduction
610°C [5]. However. the addition of 0.12% C de-creases the ductility in cold rolling. On the otherhand this alloy is ductile when hot-rolled.
served in FeMo [1] Fe-Mo-Ni [24] and Fe Magnetic hardening during aging has been ob-20Mo5Ni0.12C [5] alloys. These alloys are pos- sible substitutes for Vicalloy I and other Co-Femagnets. The previous results on these materialsindicate that the 0.12 wt% carbon addition to the Fe20Mo5Ni improves its magnetic propertiesand B = 8000 G are obtained after 240 min at [5]. The best results H = 448 Oe B = 15 400 G
Results presented by Wan et al.[1] on theFe-Mo alloyssuggest that the increase in coercivity can be due to a Mo-rich solidsolution (n’ phase)and 入 (FeMo) precipitation. In the thermomag-observed the first in the range of 600700°C ow- netic analysis two transition temperatures (T ) werehigher temperatures (>700°C) attributed to the ing to the supersaturated ferrite and the second atphase formed during the TMA. The first transitiontemperature decreases with the Mo addition indicating a decrease in the exchange interaction inthe α-FeMo solid solution [1].
In this work the magnetic hardening of a hot-rolled Fe20Mo-5Ni0.12C was investigated bymeans of magnetization measurements (hysteresis loops and thermomagnetic analysis TMA) andX-ray diffraction.
mated Siemens type-F diffractometer with Co( 1.79 A) and Cu (. = 1.54 A) radiations. A graphitemonochromator was used.
3. Results and discussion
2.Materials and methods
samples hot-rolled and aged at 610°C. The coercive Fig. 1 shows the curve of magnetic hardening forforce of the hot-rolled material is not so high as inthan the values reached by the alloy without car- the previously cold rolled one [5] but is still higherbon [2].
An ingot of position Fe20Mo5Ni0.12C(wt%) was obtained by induction melting under vacuum.
rolled to a plate 11 mm thick and quencbed in oil. The 43 mm thick ingot was soaked at 1220°C Then it was soaked at 1230°C hot-rolled to 4 mmthick and water quenched.
It is interesting to note that in the initial condi-tion both the cold-rolled and the hot-rolled
For the magnetic measurements cylinders of3 mm diameter were machined and then sliced in dises of 0.20.5 mm thick. The disc-shaped sampieswere treated at 1230°C for 30 min in vacuum andthen quenched in water. After this they were aged at 610°C for times varying from 15min to 20hwith samples sealed in quartz. (Table I). All these treatments were undertaken
Magnetization measurements (hysteresis loopsand TMA) were made in a vibrating sample mag- netometer (VSM) EGG-PAR model 4500. Theoven model 151H in low-vacuum atmosphere TMA data were obtained with a high-temperature(0.1 mmHg)and an applied magnetic field of 100 Oe. The heating rate was 5°C/min and thecooling rate was 2°C/min.
for 1 h at 670 and 750°C and also in the sample X-ray diffraction was carried out in samples agedaged 4 h at 610°C. It was performed in an auto-
Fig 1. Curves of magnetic hardening at 610°C: (a) hot-rolledFe20Mo-5Ni0.12C (b) cold-rolledFe-20Mo5Ni-0.12C [5] and (c) Fe20Mo-5Ni [2]
Table 1 Identificatioe of samples submitted to magnetization measurements
Sample Condition Measurements1 Hot-rolled and treated at 1230C(not agn) Hysteresis loop TMA hysteresis loop2 3 Hot-rolled treated at 1230°C and aged at 610°C for 1 b Hot-rolled treated at 1230°C and aged at 610°C for 15 min Hysteresis loop TMA bysteresis loop Hysteresis loop TMA hysteresis loop4 Hot-rolled treated at 1230°C and aged at 610°C for 4 b As sampie I affer magmetic measurements Hysteresis loop TMA bystersis loop TMA6 Hot-rolled treated al 1230°C and aged at 610°C for 20h Hot-rolled treated at 1230°C and aged at 610°C for 10 h Hysteresis lop Hysteresis loop
Fe20Mo5Ni0.12C alloys have the same values of H (see Fig. 1). In part this is due to the heattreatment at 1230°C undertaken after cold and hot rolling On the otherhand the cold-rolling processin this case the maximum value of H is reached seems to increase the magnetic hardening rate andafter 40 min in parison with 240 min for thehot-rolled alloy (Fig 1).
The optimum properties of Fe-20Mo-5Nj0.12C hot-rolled and previously treated at 1230°Cwere H =401 Oe B =6387 G B =14813 G and (BH)mx = 0.67 MG Oe obtained after 240 min at610°C (Table 2 and Fig 1).
Fig. 2 shows the TMA curves of samples 14 (seeTable 1)at increasing temperatures.Samples 1 and 2 present the same behaviour: a magnetic phasc-FeMo supersaturated) with transition temper-
Table 2Magnetic properties obtained affer aging at 610°℃
Samplc (BH） (MGOe)H.(Oe) B (G) 3654 (D）g 0.232 1 275 206 4811 17673 18165 0.364 3 375 10 6387 3996 6557 14509 1 0.67 0.636 7 402 392 4197 12652 12558
Fig. 2TMA curves at increasing temperature fr the hot-rolled Fe20Mo-5Ni0.12C alloy: (1) not aged (2) aged 15 min at610 C (3) aged 1 h at 610°C and (4) aged 4 h at 610°C.
ature (7.) 625°C (taken at the inflection point). With the precipitation of a Mo-rich phase theCCC-matrix loses Mo and a second transition istransition belongs to an a-ferrite with lower Mocontent
Sample 3 (previously aged at 610°C by 1 h)shows an intermediate situation: the magnetization increasing may indicate additional precipitationobserved. during the TMA. The transition at 714°C is then
Finally sample 4 (aged at 610°C by 4 h) presentsonly the transition at 714°C. After 4h aging at 610°C the material presents a stable structure ofoccurs during the TMA. α plus precipitates and no additional precipitation
Fig.3 shows the TMA curves of samples 14at decreasing temperatures.Samples 1 and 4 have the same behavior presenting only the transitionat 714°C. Samples 2 and 3 present a similar be- havior with another phase formed at 387C.The net magnetization of all samples are higherat decreasing temperatures (see Fig. 4 for an cxample).
decreasing temperatures for sample 5 which is Fig. 4 shows the TMA curves at increasing andsample 1 annealed by a first TMA (see Table 1) 1tditional precipitation occurs during the TMA and presented a behavior very similar to sampie 3. Ad-
Fig3.TMA cuvesat decreaingtmperaturefo thehot-rolled Fe20Mo5Ni0.12C alloy: (1) not aged (2) aged 15 min at610′C (3) aged 1 h a1 610°C and (4) aged 4 b at 610°C.
after (A) TMA dsta coliectioe of sample 1. Fig. 5. Comparison between hysteresis loops before (B) and
the first TMA) Fig. 4. TMA curve for sample 5 (sample 1 after annealing due to
ing temperature a transition at 387°C is detected. the transition at 714°C is observed. With decreas-
canindicate the precipitation of two phases during 1 h respectively. The results piled in Table 4served in a previous work [6]. It is interesting to aging (FeMo) and a (Fe Mo) C carbide ob-note the change of d value of the (1 0 0) peak offerritedeoaging.Itmcansthatinthe solubilized state the lattice parameter of ferrite is 0.29133 rmand after aging it is 0.28709 m. As expected the degree of supersaturation of ferrite decreases withthe precipitation.
Hysteresis loops were obtained before and afterB are shown in Table 3. This table shows that all the TMA data collection. The results of H B anddue to the TMA annealing The hysteresis loops ofsample 1 before and after TMA data collection are shown in Fig. 5. As shown in this figure the TMAcauses magnetic hardening in this sample withthe H increasing from 275 to 392Oe. Also an increase in the remanence B while a decrease into that obtained in FeMo[1]. In the samples the saturation B is observed.This result is similarpreviously aged by 1 and 4h the TMA causesa kind of overaging which makes the H values to decrease.
4.Conclusions
The Fe20Mo5Ni0.12C alloy hot-rolled andhardening during aging at 610°C. Optimum proper- solution-treated at 1230°C presented a magneticties were B = 6387 G B=14813 Gand (B） 0.67 MG Oe obtained after 240 min at 610°C.
samples aged at 610°C for 4 h 670 and 750°C for Figs. 68 present the X-ray diffractograms of
Table 3Coercive field (H.) remanent inductance (B ) and saturation inductance (B) before and after TMA data collection
Sample Before TMA After TMAH (Oe) B (G) B (G) H (Oe) ()’g B (G)2 275 302 3654 4811 18165 17673 392 328 4708 5169 15182 154593 4 375 401 6557 6387 14509 325 369 6919 6420 15393 15352
Fig. 6. X-ray difractogram of sample aged 4 h at 610°C (KaCu radiation)
Fig. 7. X-ray diffractogram of sample aged 1 h at 670°C (KaCo radiation).
bilized sample showed two transition temperatures Thermomagnetic analysis (TMA) of the solu-(T= 625°C and T=714°C) The precipitationofFeMo and/or (Fe Mo)C at about 660°C is responsible for the second transition temperature since it causes the reduction of the Mo content of
the α-ferrite matrix. The T: belongs to a super- saturated α and T.2 belongs to a low Mo αL.
The sample previously aged at 610°C by 1 hpresented additionalprecipitation during the TMA. On the other hand in the sample aged at610°C by 4 h no additional precipitation occurs. It
