STRENGTH CHARACTERIST ICS

THE NATURE OF BLUE BR ITTLENESS OF STEEL

I .E . Do lzhenkov UDC 620.192.49:669.14

During mechanical tests or deformation of steel at temperatures where a blue oxide film is formed on the surface of the metal the strength increases and the plasticity and ductility decrease [1-3]. This phenomenon is called blue britt leness. It can now be considered proven that heating and holding steel at blue britt leness temperatures do not cause any changes in the mechanical properties at room temperature. Samples tested at room temperature have the same properties as the original samples not subjected to heating. Plastic deformation at room temperature or at a temperature above the blue britt leness temper- ature does not cause any anomalous changes in the properties. Only strengthening occurs, due to the in- crease in dislocation density.

Figure i shows the variation of the mechanical properties of normalized carbon steels in relation to the testing temperature (Fig. la), rolling temperature (Fig. lb), and the tempering temperature after cold rolling with 15% reduction (Fig. lc). It can be seen that blue britt leness develops only at an elevated temperature (Fig. la). Blue britt leness is most evident after roll ing or in mechanical tests at blue britt le- ness temperatures, i.e., as the result of the simultaneous influence of plastic deformation and temper- ature (Fig. 1, a,b). With plastic deformation and an elevated temperature occurring at different times (Fig. lc) the blue britt leness effect is considerably smaller.

One method of studying the nature of blue britt leness is to analyze the elongation diagrams and com- pare them with the temperature curves, and therefore in tensile tests at room and elevated temperatures the complete d iagrams are plotted in coordinates of de format ion force vs elongation (deformation). The intervals between testing temperatures and temper ing temperatures amounted to 25-50~ In the process of rolling at elevated temperatures the total p ressure of the meta l on the rolls was recorded and then the specific p ressure was determined in relation to the rolling temperature by the method general ly used. The temperature interval in rolling was 50 ~ .

The total and specific p ressure of the meta l on the rolls var ies as the rolling temperature increases (Fig. 2) in conformi ty with the change in ult imate strength.

F igure 3 shows elongation d iagrams for steel 40 at different testing temperatures . The curves for steels i0 and U8 are of the same general shape. At room temperature the curve is smooth with a distant yield plateau and "tooth." Wi th increasing testing temperatures the extension of the yield plateau changes (Fig. 4) as well as the elongation d iagram. At 100~ the section of the curve between the yield strength and the ult imate strength becomes sawtoothed; the height of the teeth increases with the force of e longa- tion. At 125~ and higher the sawtoothed section is broader , the curve becoming sawtoothed not only in the plateau area but also in the area of harden ing before failure of the sample , and the height of the teeth increasing. The sawtoothed character is most p ronounced at the max imum extension of the yield plateau. At 175~ the teeth become smal le r and gradual ly disappear; at 200 ~ for steel 40 and 225 ~ for steel i0 saw- teeth occur only in the plateau area, and above 275 ~ the curve becomes smooth again.

Thus , at testing temperatures be low and above blue brittleness temperatures deformat ion is mono- tonic with movement of slip bands (except for a tooth at the point of yield). At blue brittleness temper - atures plastic de format ion results f rom jumpl ike movements of slip bands, first in the section of strength- ening and then also in the section of initial plastic de format ion (in the plateau area).

Dnepropetrovsk Metallurgical Institute. Translated from Metallovedenie i Termicheskaya Obra- botka Metallov, No.3, pp.42-47, March, 1971.

C 1971 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

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Fig. i. Variat ion of mecha- nical propert ies of carbon steels with testing temper - ature (a), rolling temper - ature with 15% reduct ion (b), and temper ing temper - ature after cold rolling (c).

Compar i son of the temperature curves (Fig. i) with the plots of deformat ion force vs deformat ion (Fig. 3) indicates that the change in mechan ica l propert ies at blue brittleness temperatures cor responds to the appearance, deve lopment , and subsequent weaken ing and d isappearance of sawteeth on the elonga- tion curves. The appearance of sawteeth cor responds to the initial increase of the ult imate strength and decrease of the plasticity. The max imum deve lopment of sawteeth on the elongation curves cor responds to the max imum ult imate strength and the min imum plasticity on the temperature curves. This indicates that the processes leading to the appearance of sawteeth on the elongation curves induce the deve lopment of blue brittleness. However , the decline of the sawteeth (decreasing number of teeth, nar rower saw- toothed section, smal le r height of teeth, increasing distance between teeth) still does not lead to a reduction of the ult imate strength and yield strength or an increase of ductility (reduction in cross-sectional area). On ly after complete d i sappearance of sawteeth f rom the elongation curves does the ult imate strength de- c rease sharply, the yield strength decreas ing noticeably and the reduct ion in area and elongation increas- ing on the temperature curves. The temperature at wh ich the deve lopment of blue brittleness is max imum is approx imate ly 50-75~ higher than the temperature of the max imum deve lopment of sawteeth on the elongation curves.

The reasons for the occur rence of a tooth on the curves were d iscussed in [4].

It can be seen in Fig. 3 that with increasing testing temperatures up to the blue brittleness temper - ature there is not one but many teeth. The elongation curves become sawtoothed not only in the yield sec- tion but also beyond that, up to failure of the sample . The sawtoothed section of the elongation curve is evidently due to success ive pinning and breakaway of dislocations, wh ich is indicated by the identical level

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of the breakaway stress (upper yield point) on the extension of the yield plateau and the identical stress level of mov ing dislocations (lower yield point). The sawtoothed section on the plateau must re- sult f rom the following mechan ism, Wi th increasing tensile test temperatures un impeded plastic f low occurs briefly after the appear - ance of the first tooth. The diffusion mobil i ty of impur i ty a toms (carbon and nitrogen), wh ich increases with the temperature , per - mits rapid migrat ion of impur i ty a toms in the stress fields a round dislocations and leads to pinning of dislocations. The steel becomes "stagnant, ~ and plastic deformat ion again changes to elastic defor- mation. Because of the strong interaction of dislocations with at- mospheres and insufficient mobi l i ty of a tmospheres at blue brittle- ness temperatures , plastic deformat ion can occur again only if the dislocations break away f rom the a tmospheres , which requires appli- cation of a force exceeding the pinning force. The deformat ion force thus increases.

After a stress is attained sufficient for the dislocations to break away from atmospheres (upper yield point) the dislocations are again freed from their atmospheres and begin to move under the influence of the applied stress. The deformation force decreases to a level sufficient to maintain the movement of the dislocations, and a new tooth is formed on the elongation curve.

Evidently there is no substantial increase of the dislocation density in the section of the yield plateau during plastic deformation, the sawteeth result pr imari ly from the successive pinning and unpinning of existing dislocations, and the maximum and minimum flow stresses do not change for a given testing tem- perature.

The extent to which dislocations are pinned by atmospheres increases in proportion to the develop- ment of plastic deformation. A substantial percentage of the dislocations pinned in the process of plastic deformation at the blue britt leness temperature remain pinned in the course of further deformation [5, 11]. The critical stress required to renew plastic deformation will increase for this reason.

After the stresses reach a certain magnitude, new Frank--Reid sources emit a quantum of disloca- tions that generates dislocation rings and loops which expand in the slip planes and create slip bands that propagate with relative ease; the deformation force decreases and a tooth is formed on the elongation curve. However, the fresh dislocations remain unpinned only a short time. Cottrell atmospheres are rap- idly formed around them, the steel becomes "stagnant" again, the slip bands cease to expand, and the deformat ion force increases again.

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The rising stresses tr igger the operation of existing and new Frank--Reid sources, which emit another quantum of moving disloca- tions, a new tooth is formed on the elongation curve, etc. The mosaic block boundaries [6], grain boundaries, and aggregates of cementite [7] may act as Frank--Reid sources. Due to the emission of more and more new dislocations by F rank-Re id sources the dislocation density must increase during plastic deformation at blue britt leness temper- atures.

Measurements of the coercive force and mosaic block size of carbon steels in relation to the rolling temperature showed [2] that plastic deformation at blue britt leness temperatures causes a substan- tial increase in the coercive force and a decrease of the block size.

Since the dislocation density is directly proportional to the square of the coercive force and inversely pro- portional to the square of the block size [6], the increase of the coercive force and the decrease of the block size at blue britt leness temperatures can be explained by the increase of the dislocation density. It was shown by transmission electron microscopy [8] that in low-carbon steel the dislocation density in- creases with the temperature up to 200~ and then rapidly decreases in the region of jumplike plastic de- formation.

Thus, the sawtoothed elongation curves for carbon steels tested at blue britt leness temperatures, and consequently blue britt leness, are due mainly to rapid pinning of moving dislocations during deforma- tion and the consequent high dislocation density. At testing temperatures above 250-275~ the elongation curve is smooth throughout.

Impurity atoms acquire such high mobility that pinning of dislocations by atmospheres no longer controls plastic deformation.

As the sawtoothed character of the curve decreases, the effect of blue brittleness on the temperature curves does not decrease. At testing temperatures 50-75~ higher the ultimate strength continues to in- crease and the reduction in area to decrease. Thus, blue britt leness continues to develop for some time after the pinning of dislocations by atmospheres declines as the result of increasing temperature. The further development of blue britt leness evidently results from the formation of finely dispersed precipi- tates on dislocations. The possibil ity that precipitates result from plastic deformation at blue britt leness temperatures needs further investigation. With a high density of the atmospheres (over 10 atoms) the solute atoms are more strongly bound to dislocations than to ordinary carbides or nitrides. Consequently, the formation of precipitates at blue britt leness temperatures, where the dislocation density increases in- tensely, is not very probable [9]. According to [10], the formation of e carbide during tempering of steel quenched to martensite and then cold worked is thermodynamically disadvantageous, since the interaction of carbon with defects is stronger than with iron atoms in the carbide lattice.

If it is assumed that the precipitation of finely dispersed nitrides on dislocations begins at temper- atures corresponding to the still uudescending section of the temperature curves (see Fig. 1), i.e., at tem- peratures where blue britt leness is fading, then on the temperature curves one would expect a second peak (low point) corresponding to the temperature of the maximum development of the formation of preci - pitates on dislocations and outside of dislocations, which in fact was not observed in our work (Fig. 1) or in the work of other investigators [1].

However, it was shown by transmission electron microscopy in [8] that in tensile tests of low-carbon steel at blue britt leness temperatures, part icularly steel with a high nitrogen content (about 0.024%), FeiGN 2 precipitates are formed on dislocations. This indicates that blue britt leness may be due not only to an in- crease of dislocation density but also to the formation of finely dispersed precipitates on dislocations.

Besides blue britt leness, the temperature curves (Fig. 1) show a reduction of the plasticity in the prerecrystal l izat ion temperature range.

I.

2.

L ITERATURE C ITED

G. I. Pogodin-A lekseev, Dynamic Strength and Brittleness of Meta ls [in Russian], Mashinostroenie, Moscow (1966). I. E. Dolzhenkov, Izv. Akad. Nauk SSSR, Metally, No. 4 (1966); No. 5 (1966).

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3. I .E. Dolzhenkov, Metal. i Term. Obrabotka Metal., No. 6 (1967). 4. A .H . Cottrell, Dislocations and Plastic F low in Crystals, Clarendon Press, Oxford (1953). 5. Ekspress-Informatsiya, Metal. i Term. Obrabotka Metal., No.44, Ref. 211 (1963). 6. S .D. Gertsriken et al., Physical Basis of Strength and Plasticity of Metals [in Russian], Metallur-

gizdat, Moscow (1963). 7. Fizika, No. 9 (1967); Abstract 9E446. 8. B. Brindley and J. Barnby, Aeta Met., 14, No. 12 (1966). 9. J. Baird, Iron and Steel, 36, No. 8 (1963)--~

I0. M.L . Bernshtein and R. I.~ntin, Metal. i Term. Obrsbotka Metal., No. ii (1967). Ii. A .H . Cottrell, in: Structure and Mechanical Properties of Metals [Russian translation], Metallur-

giya, Moscow (1967).

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