HAL Id: jpa-00216790
https://hal.archives-ouvertes.fr/jpa-00216790
Submitted on 1 Jan 1976HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
A MÖSSBAUER STUDY OF THE FRACTURE
SURFACE AND THE INFLUENCE OF RETAINED
AUSTENITE ON THE FRACTURE TOUGHNESS OF
FE-9NI-0.1C STEEL
K. Kim, L. Schwartz
To cite this version:
A
MOSSBAUER STUDY
OF THE FRACTURE SURFACE
AND
THE INFLUENCE OF RETAINED AUSTENITE
ON THE FRACTURE TOUGHNESS OF FE-9M-O.1C STEEL
(*)
K. J. KIM and L. H. SCHWARTZ Materials Science and Engineering Department Northwestern University, Evanston, Illinois 60201, U. S. A.
R6sum6.
-
L'influence de l'austenite residuelle sur la resilience de l'acier Fe-9Ni-0,lC a kte Btudiee par effet Mossbauer ainsi qu'a l'aide d'essais au choc et de techniques mt5tallographiques. Les rksultats indiquent que la presence dYaust6nite residuelle augmente la rksilience d'une quantitk proportionnelle B la fraction volumique d'austknite. L'examen par effet Mossbauer d'un Cchantil- lon rompu B 77 K a rkvel6 que l'austknite situee dans une region de 300 ym de part et d'autre de la surface de la fracture avait kt6 transformke en martensite. L'augmentation de la resilience de cet acier peut &re attribuke B l'knergie dissip6e dans la transformation de phase austenite-martensite, induite mecaniquement.
Abstract. - The effect of retained austenite on the fracture toughness of Fi-9Ni-O.1C steel was investigated using Mossbauer effect scattering, instrumented impact tests, and metallographic techniques. The results indicate that the presence of retained austenite increases the fracture toughness by an amount proportional to the volume fraction of austenite. Examination of a sample after fracture at 77 Kusing Mossbauer effect scattering revealed that the austenite within 300 pm o f the fracture surface had been transformed to martensite during the fracture. The improved fracture toughness of this steel may be attributed to the energy dissipated in the mechanically induced austenite-martensite phase transformation.
1. Introduction.
-
The influence of retained auste- nite on the improved fracture toughness of nickel bearing alloy steels has been documented [l-31, but the mechanism of this effect remains an intriguing problem. Studies made by Marshall et al. [l] showed dramatic increases in low-temperature toughness when austenite was retained after tempering in the austenite-ferrite two phase field. Two explanations have been proposed for the working mechanism : migration of carbon from martensite to austenite to increase the intrinsic tough- ness of the martensite formed upon quenching ferrite, and/or mechanically induced transformation of the retained austenite in the plastic zone of the advancing crack. The former is expected because the carbon concentration exceeds the solubility limit in the marten- site, while the solubility of carbon in austenite is much higher. The latter mechanism is based on the observa- tion that a phase transformation induced by plastic deformation is capable of absorbing strain energy resulting in increases in the measured fracture tough- ness 141.The lack of a proper method of examining the frac- ture surface has inhibited the quantitative interpreta- (*) This research was sponsored by NSF through the Mate- rials Research Center at Northwestern University and by the American Iron and Steel Institute.
tion of the contribution of retained austenite to the improved fracture toughness. In order to investigate this problem quantitatively, one needs a technique by which the retained austenite determination can be made on the irregular, plastically deformed fracture surface. The usual X-ray diffraction method will tend to be inaccurate because the roughness of the surface and the mechanical deformation present will distort Bragg peaks. This paper introduces Mossbauer effect scattering for the study of fracture surfaces on the grounds that the observed spectrum is free from the influence of both surface roughness and the defor- mation present.
2. Experimental.
-
The steel, whose nominal com- position was 8.95%
Ni, 0.1%
C, 0.45%
Mn, 0.22%
Si, balance Fe, was received as a plate of4
x 12 x 24 inch3 from the International Nickel Company in the normalized condition. The impact specimen blanks were machined from this platein a standard full size V-notch geometry according to ASTM E23-72151.
The Fe-Ni phase diagram along with the heat treatment schedule is shown in figure 1. ,Test specimens were prepared in five sets of heat treatments : as quenched specimens denoted A were prepared by austenitizing at 795 OC (y-field) for 30 minutes followed by water-quench t o room temperature ; temperedC6-406 K. J. KIM AND L. H. SCHWARTZ A (795*C/X)MIN/WQ) - - To (595'WIO MIN/WQ) T, (595'Cl l HR/WQ) Tz l595'C/IOHRS/WO) T. (595°C120HRS/WQ) --
OFe
E,
Ni AUSTENiTlZATON TEMPERINGFe-Ni PHASE DlAGRMn HEAT TREATMENT
FIG. 1. -The Fe-rich portion of the Fe-Ni phase diagram and the heat treatment schedule for introducing retained auste-
nite.
specimens denoted T were prepared by tempering at 595 OC for various times ranging from 10 min. to 20 hours and quenched to room temperature.
The mechanical properties were evaluated by hard- ness test (Rockwell (( C ))) and the instrumented impact
test, a technique useful in identifying the effects of material characteristics on yield stress and stored energy [6]. These impact tests were conducted using a Model 64 Universal Impact Testing Machine (Tinius Olsen Testing Machine Company) and load time and energy time curves were obtained.
The austenite retention on the fracture surface as well as in the bulk were determined using Mossbauer effect scattering, detecting either conversion electrons or scattered X-rays, a technique which has proven effective in austenite-martensite analysis [7-91. Detection of the scattered radiation (radiation source of 115 mCi CoS7
in Rh matrix) was accomplished using a SD-300 Detec- tor (Ranger Electronics) with 96
%
He-4%
CH4 flowgas for detection of electrons and 96
%
Ar-4%
CH4flow gas for 6.4 keV X-ray detection. All measurements
were made with an NSEC AM-1 Mossbauer spectro-
meter equipped with an electro-mechanical constant acceleration velocity transducer coupled to a Nuclear Data 2200 multichannel analyzer.
Test specimens and fracture surfaces were examined by scanning electron microscopy and this fractography was correlated with the foregoing test results.
3. Results and analysis.
-
The austenite retention in the quenched and tempered samples was determined using Mossbauer effect scattering. The spectrum for specimens A contained a six-peak pattern showing the fully martensitic character of the quenched specimens. A typical Mossbauer spectrum for the tempered steels is shown in figure 2 for specimen T3, exhibiting 7 peaks,six for martensite plus a single peak for austenite. A more exact curve fitting would require knowledge of the complex substructure associated with the different atomic environments of iron. As demonstrated by Chow et al. [lo], this 7-peak approximation is satisfac-
tory in the retained austenite determination since the corrections in austenite and martensite spectra arising from taking account of the complex substructure are
.995
VELOCITY
.
MH/5FIG. 2. -The Mossbauer spectrum of specimen T 3 obtained using Mossbauer effect scattering (Fe Ka X-rays detected). The spectrum consists of six-lines for martensite and a single line for austenite at - 0.09 mm/s. The velocity scale is with
respect to iron metal.
comparable. This approximation gives rise to a syste- matic overestimate of retained austenite of less than one percent.
After fitting with Lorentzian peaks, the austenite phase concentration was determined from the ratio of the integrated area of the austenite peak to that of martensite plus austenite. For the spectrum shown in figure 2, the austenite concentration was 15.7
%.
In this analysis, the Mossbauer fraction of iron atoms in austenite and martensite was assumed to be identical. However, the iron atoms in the b. c. c. martensite and in the f. c. c. austenite have a slightly different Moss-bauer fraction as shown by Abe and Schwartz [ll].
The difference in the Debye-temperature of austenite and martensite was estimated from the reported elastic modulus 1121 to be two percent. Neglect of this diffe-
rence corresponds to a systematic overestimate of only 0.1
%
in the austenite concentration. In addition to the above, a further correction of the data obtained for Mossbauer X-ray scattering should be made in order to account for non-linear absorption effects ; however, this correction is negligible in the austenite concen- tration range studied [13].For all specimens, Mossbauer effect measurements were made and the resultant retained austenite analysis is summarized in figure 3a. Notice that the austenite retention ranges from 5
% for
1 hour tempering to 15.7%
for 20 hours tempering showing a linear relationship with tempering time at 595-OC. It was further observed that for tempering times up to15 hours, the austenite concentration remained the same after holding at liquid-nitrogen temperature for 2 hours. A few percent reduction in austenite
concentration was observed for the specimen tempered for 20 hours after holding in liquid-nitrogen. This
TEMPERING TIME AT 595"C, HOURS
FIG. 3. - a) Retained austenite and b) Rockwell hardness (Re) as a function of tempering time at 595 "C.
ture for this phase is below 77 K for samples with less than about 13
%
austenite, and M , is about 77 Kfor the sample tempered for 20 hours. The equilibrium concentration of austenite in the steel at 595 OC is 25
%,
so this thermal stability limit lies far below the equili- brium concentration. The observed results parallel the observation of Marshall et al. [I].The Rockwell hardness (R,) vs. tempering time is shown in figure 3b. The results indicate that tempering
softens the steel within 10 minutes to a level which is then independent of austenite concentration.
A determination was made of the retained austenite
on the fracture surface with reference to the impact energy and the original austenite retention in the bulk sample. The results of the instrumented impact tests for specimens A and T2 are shown in figure 4 along with
the corresponding Mossbauer spectra. Notice that a 9
%
retention of austenite improved the overall frac- ture toughness drastically, e. g. from 7 ft/lb to 73 ft/lbT E V TEMPERATURE, K
FIG. 4. -Results of the instrumented impact test for specimen A and T 2. Impact energy is plotted as a function of test temperature. Also shown is the corresponding Mossbauer
spectrum for each specimen.
when tested at 77 K, and from 47 ft/lb to 143 ft/lb when tested at 295 K. Furthermore, the ductile-brittle temperature, T,, (50
%
energy point between upper and lower limits), was lowered by 40 K. The test results for specimens which contained varying amounts of retained austenite exhibited a linear relationship between the impact energy and the retained austenite concentration. That is,where E is the total energy absorbed, Eo is the energy
absorbed by mechanisms other than transformation of
y
-
a, and F? is the austenite concentration in the test specimen in volume percentage. It is interesting to note, and not yet understood, that the contribution to the impact energy due to retained austenite is independent of test temperature.The fracture surface was examined by Mossbauer effect spectroscopy and the resultant spectrum after fracture for specimen T 3 is shown in figure 5. By
J I
-8.000 6 . Z O . 4.900 -2.Z50 .O€Q 2 . W Y.W 6.750 8.003
VELOCITY
.
MH/SFIG. 5.
-
The Mossbauer spectrum of specimen T 3 obtained using Mossbauer effect scattering on the fracture surface (Fe Ka X-rays detected). The spectrum contains only six-lines for martensite, and shows no retained austenite on the fracture surface. The velocity scale is with respect to iron metal.comparing the Mossbauer spectra for the specimen T 3
before (Fig. 2) and after (Fig. 5) fracturing operation, it is immediately obvious that the retained austenite on the fracture surface has transformed to martensite as a consequence of fracture. This observation, combined with the improved impact energy, indicates that the retained austenite plays an important role in increasing the toughness of this steel and demonstrates that the localized mechanically induced phase transformation is operative in the Fe-9 Ni-0.1 C steel studied. Similar,
non-quantitative, observations have been made by Sarno (unpublished research) using X-ray diffraction.
C6-408 K. J. KIM AND L. H. SCHWARTZ
vation of the plastic zone and by sequential Mossbauer scattering spectra made after electropolishing the fracture surface. It was found that the depth of trans- formation extended to about 300 pm for the specimen T 2 tested at 77 K. The significance of this part of the study lies in the fact that little attention has previously been given to the fracture surface, and that depth scanning had not been applied to this problem by any technique.
Scanning electron microscope studies of the micro- structure and fracture surface were made. A martensite packet size of about 10 p was observed for all speci- mens. The retained austenite appeared as a plate-like structure within and along the martensite lath boun- daries. The fractography indicated that the fracture behavior of quenched specimens could be characterized as quasi-cleavage even at room temperature, while those of tempered specimens could be described as dimpled rupture or transgranular rupture even at 77 K.
The latter observation confirms the presence of heavy plastic deformation for the tempered specimens.
4. Discussion.
-
It has been noted in several studies that the Mossbauer effect is competitive with X-ray diffraction for determining the retained austenite concentration in steels [lo, 11, 131. However, when a rough surface such as a fracture surface becomes the object of investigation, the X-ray diffraction method suffers a loss of low-angle scattering intensity due to the surface roughness, and the mechanical deformation which extends farther than the depth of X-ray penetra- tion broadens the Bragg scattering peaks making quantitative analysis uncertain. On the other hand, the Mossbauer effect scattering technique is still applicable because this method is free from the deformation present and represents the average scattered intensity from the phases present irregardless of the surface roughness.As pointed out in the introduction, it has been suggested that improved fracture toughness might be attributable to the movement of carbon from mar-
[I] MARSHALL, C. W., HEHEMAN, R. F. and TROIANO, A. R.,
Trans. ASM 55 (1962) 803.
[2] Jm, S., HWANG, S. K. and MORRIS, J. W., Jr., Metall. Trans. A 6A (1975) 1721.
[3] .ANTOLOVICH, STEPHEN, D. and SINGH BRINDAR, Metall.
Trans. 2 (1971) 2135.
141 GERBERICH, W. W., ZIEMMINGS, P. L., ZACKAY, V. F. and PARKER, E. R., Fracture 1969 (Chapman and Half Ltd., London) 1969, 843.
[5] ASTM Standard Designation E 23-72.
tensite. This possibility may be ruled out by the follow- ing argument. The diffusion of carbon at 595 O C is so
rapid (r. m. s. jumping distance
-
216 pm in 10 min.) that most of the carbon redistribution would be completed in 30 min. of tempering [14]. Furthermore, the hardness of the martensite matrix shown in figure 3b reached a value which was independent of the austenite concentration after 10 min. of tempering, indicating no further compositional change in the martensite matrix. By taking into consideration that the retained austenite was transformed to martensite to increase the fracture toughness by an amount proportional to the austenite concentration as describ- ed in eq. (I), and that the plastic deformation observed in the fractography of tempered specimens provided the necessary condition for the deformation induced phase transformation, it may be concluded that the observed improvement of fracture toughness is attributable to the retained austenite itself, i. e. mechanically induced transformation is operative in improving the toughness of the steel studied.5. Conclusions.
1) Mossbauer effect scattering provides a unique tool for the investigation of the fracture surface.
2) By suitable heat treatment of Fe-9 Ni-0.1 C
retained austenite up to 13
%
is thermally stable at 77 K.3) The retention of austenite improves the fracture toughness tenfold when tested at 77 K and fourfold when tested at 295 K,
4) Upon fracturing, the retained austenite on all fracture surfaces transforms to martensite.
5) The depth of this transformation extends to 300 pm from the fracture surface for specimen T 2 tested at 77 K.
6) The results indicate that a mechanically induced phase transformation is operative in improving the fracture toughness of Fe-9 Ni-0.1 C steel.
[8] SWARTZENDRUBER, L. J. and BENNETT, L. H., Scr. Metall. 6
(1972) 737.
[9] KIM, K. J. and SCHWARTZ, L. H., Metall. Trans. 7 A 11976) 1567.
1101 CHOW, H'. K., WEISE, R. F. and FLINN, P., NSEC-4023-1, 1969.
[Ill ABE, N. and SCHWARTZ, L. H., Mat. Sci. Eng. 14 (1974) 239. [12] SCHEIL, ERICH and REINACHER, GERHARD, 2. Mefall. 3b
(1944) 63.
[13] SWARTZENDRUBER, L. J., BENNETT, L. H., SCHOEFER, E. A., DELONG. W. T. and CAMPBELL. H. C.. J. Weldinn 53
-
[6] WULLAERT, R. A., Impact Testingof Metals, ASTM STP466, (1974) 1-S.Am. Soc. for Testing and Materials (1970) 148. 1141 ZEMSKIY, S. V., GRIGORKIN, V. I., KUKUSHKMA, V. N. and [7] SWANSON, K. R. and SPLIRERMAN, J. J., J. Appl. Phys. 41 ZAKHARENKOVA, V. I., Fiz. Met. Metalloved 33 (1972)
