THE CARBON CONTENT OF LOW CARBON MARTENSITE

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1 R 406 Philips Res. Repts 15; , 1960 THE CARBON CONTENT OF LOW CARBON MARTENSITE Summary by M. L. VERHEIJKE : : The carbon content of low carbon martensite can be determined by means of dilatometric experiments and resistivity measurements. Both methods are based upon the assumption that decreases in the specimen length and in the resistivity, resulting from the first stage of tempering, are linear functions of the carbon content. Experiments show that this. is an acceptable assumption. Résumé La quantité de carbone prësente dans Ie "low carbon martensite" peut être déterrninée au moyen de mesures dilatométriques et de résistivité. Dans les deux méthodes on suppose que les décroissances de la longueur et de la résistivité de l'échantillon, résultant du premier degré de revenu, sont des fonctions linéaires de la quantité de carbone. Les mesures ont prouvé que la supposition précitée est acceptable. Zusammenfassung Der Kohlenstoffgehalt des Restmartensits (low carbon martensite) kann mittels dilatometrischer Versuche und Widerstandsmessungen bestimmt werden. Die beiden Methoden gründen sich auf die Voraussetzung, dab die mit der ersten AnlaBstufe verknüpften Längen- bzw. Widerstandsabnahmen lineare Funktienen des Kohlenstoffgehalts sind. Die Experimente zeigen, dab diese Voraussetzung zutrifft. 1. Introduetion When, a carbon steel is slowly cooled from a temperature at which the y:modification of iron (austenite, f.c.c.) is stable, two stable phases, a-iron (ferrite, b.c.c.) and cementite (FeaC), are formed by the normal process.of nucleation and growth. When, however, a steel is quenched a metastable nonequilibrium structure arises by a process of shear. This structure is called primary martensite and is essentially a strongly supersatured solid solution. of carbon in a deformed ferrite lattice (b.c.t.). This primary martensite undergoes several changes in structure at higher temperatures. In this paper we shall only deal with the first stage of tempering, occurring with appreciable rate 'at about 150 oe but which is even measurable at room temperature, During that stage -carbide precipitates from the martensite, thus reducing the carbon content of the martensitic matrix: 1 primary martensite-+ low carbon martensite + -carbide (Fe2.4C). (1) b.c.t. b.c.t. h.c.p.

2 438. M.L.VERHEIJKE The carbon content of this low carbon martensite, which appears to be in metastable equilibrium with s-carbide, is reported by a number ofinvestigators (see refs 1-4)) to be about 0 25 % *). However, more recent investigations by Werner et al. 5) indicated that this should be about 0 3 %; by X-ray diffraction analysis they found that for a number of steels this carbon content ranged from 0 25% to 0 33%.. 2. Dilatometric measurements for finding the carbon content Information on the first stage of tempering can be gained from dilatometric measurements, thanks to the fact that the volume of the specimen decreases because of the transformation taking place in this stage. The change in length, álll, which is one-third of the change in the specific volume and is caused by the completed first tempering stage, is assumed by Roberts et al. 1) to be proportional to the amount of precipitated s-carbide, and hence to n-nx, where n is the carbon content of the primary martensite and n«that of the low carbon martensite. This assumption seems reasonable when one takes into consideration the low value of áljl, viz. only -0 2 % in the case of steel with 0 9 % C. Therefore.dljl = a(n-nx). (2) Another assumption which has been used so far is that changes in the specimen length are caused by the tempering reaction alone and not by the relief of internal strain. This assumption, too, is a reasonable one in view of the relatively low temperature for the tempering reaction and the negligible change in the yield point by this transformation. Roberts et al. 1) have determined, from X-ray data, ál]! to be in the case of steel with 0 68 % C. Subsequently they have put n«at 0 25 % and have calculated a. In this way they obtained the necessary data for the calculation of.dljl for every carbon content, however without proving that the above-mentioned assumptions are correct. If, however, ál]! is determined experimentally for steels of different carbon content, then the carbon content of low carbon martensite, ne, can be found with the aid of (2). The results of such experiments are given in this paper. 3. Resistivity measurements for finding the carbon content The electrical resistivity also decreases during the first tempering stage 6)7). This decrease is rather large, viz. almost 30 % for steel with 0 9 % C, measured at room temperature. According to Pitsch 8) the resistivity of a-iron with carbon is *) All percentages refer to weight percentages, PFe-C = PFe (1 + ans + ~np), (3)

3 CARBON CONTENT OF LOW CARBON MARTENSITE where ns = percentage of carbon in solid solution, np = precipitated carbon (as carbide). Although Pitsch only examined this relationship for a-iron with low carbon content, it is quite likely that it also applies for the case of martensite with carbon in solid solution and -carbide, of course with other values for a and f3. This may be deduced from the findings of King and Glover 6), who stated that the resistivity of primary martensite is a linear function of its carbon content up to 1 4% C. Hence Pmart.+ carbide = po(1 + ans + f3np), (4) where po is the fictitious resistivity of martensite with carbon content...:_o. In the case of primary martensite np = 0 and ns = n = the total percentage. of carbon, or Pprim.mart. = po(1 + an). (5) At the end of the first tempering stage lls=nx per cent of carbon will remain in solid solution in the low carbon martensite and np = n-nx per cent of carbon will be present as i-carbide. The following equation then holds: P!.c.m.+ e-carbide = po{1 + anx + f3(n-nx)} (6) for n > nx. It must be possible to determine ne with the aid of the expressions given above when the resistivity of primary and tempered martensite has been measured for different carbon contents. A small number of such experiments will be described in the following section. 4. Experimental procedure The" various data of the steels which were examined are given in tables I and 11. Metallographic inspection showed that the steel structure was homogeneous and that no decarburization had occurred at the edges. The dilatometric experiments were carried out on specimens quenched in a 10% aqueous solution of sodium hydroxide so as to obtain a sufficiently high cooling rate for the steel with a carbon content of 0 2 %. This was also verified metallographically and by means of hardness tests. Due to the short aging time at room temperature (5 sec) the retained-austenite content was negligible, even at 0 9 % C. In the case of the steel with 0 2 % C the sub-zero cooling mayalso be omitted, since MF is above room temperature (MF = martensite finishing temperature). Two differentmethods of tempering were adopted for the dilatometric experiments. With the first method the specimens were kept at 200 C in a drying

4 440 M. L. VERHEIJKE TABLE Chemical composition, specimen dimensions' and hardening conditions of the steels used in the dilatometric experiments C (%) Mn Si Cr Ni Cu I specimens length: 50 0mm; diameter: 3 0mm austenizing 30 min 950 Cin 75%N2 + 25%R2 quenching by dropping into an aqueous solution of 10% NaOR + immersion in liquid nitrogen (aging time at room temperature: about 5 sec). TABLE 11. Chemical composition, specimen dimensions and hardening conditions of the steels used in the resistivity measurements C co Mn Si Cr Ni Cu Al specimens: length (mm) diameter (mm) austenizi~g 30 min 900 C in R2 30 min 850 C in H2 quenching by dropping into an aqueous solution. of NaCI + immersion in liquid nitrogen (aging time at room temperature: about 5 sec).

5 CARBON CONTENT OF LOW CAR1l0N MARTENSITE 441, stove for 15 'minutes; the second method consisted in the specimens being slowly warmed up in a Chévenard dilatometer up to 200 C. In the first case ".the shortening of the specimen due to tempering was determined by means of precision length measurements at room temperature. In the second case the change in length could be found with precision length ~easurements as well as from the dilatometer curve. It may be expected that, due to the heat treatments described above, the first tempering stage will have been completed 1), whereas the third *) tempering stage will not have been initiated yet. This was confirmed by separate dilatometric tests. The second *) tempering stage does not take place because there is no retained austenite. The resistivity measurements were done at room temperature prior to and after a IS-minutes tempering period during which the temperature Was200 C. A small correction (about 0 5 %) was introduced for the decrease in resistivity taking place in the intervál between sub-zero cooling and the first measurement. In this interval the test specimens were mounted in the measuring set-up and temperature equalization took place; it took about 10 minutes. The magnitude of the correction was found by extrapolation of the resistivity changes at room temperature in a separate series of experiments. 5. Results The results of the dilatometric experiments are given in table Ill. The reading accuracy was one part in a hundred thousand; the reproducibility of the measurements was very good, which is to be seen from that table. TABLE III Change in length (Lll/l) during the first stage of tempering method of tempering 0'19-0'20% C % C 0, % C 15 min 200 C (rate of heating about 2000 C/h) *) Chévenard dilatometer up to 200 C (rate of heating o *) oc/h) *) repeated once. " *) 2nd stage of tempering: transformation of retained austenite.. 3rd stage of tempering: low carbon martensite + e-carbide -+ ferrite + cementite.

6 442 M. L. VERHEIJKE , , xlo- 3 o Roberts et al. o Present work &=-34-2 X10-5 (n-0.30) In?!; 0.30 %C n;á0.30%c O~-L-~~-~-~-~-~~-L-~~~-~ o --~ Percent carbon (n) 3421 Fig. I. Decrease in length due to the first stage of tempering. Figure 1 shows a plot of LJI/1 against the carbon content, giving nz = 0 30%..The value for LJI/1 mentioned by Roberts et al. almost lies on the straight line drawn through our measuring points in fig. LAs may be expected, no first. stage of tempering can be observed in the case of the 0 2% carbon steel: LJI/1 = 0« ). In a dilatometer curve extended to 500 C (not shown here), one would see that the first and second stages of tempering are absent for such a steel, but that the third stage does take place ( C). Jellinghaus 7) and Lement 9) also stated that no first stage of tempering is observed in the case of steels with less than 0 3 % C; Jellinghaus came to this carbon content (%) TABLE IV Resistivity in I:LQcmat 19 2 C to after quenching + after tempering sub-zero cooling 15 min 200 C mean ~ mean

7 CARBON CONTENT OF LOW CARBON MARTENSITE 443. conclusion on the basis of resistivity measurements, Lement as a result of electron diffraction analysis The results of the resistivity measurements are listed in table IV and fig. 2; the values obtained for primary martensite show good agreement with those found by King and Glover 6). So, in eq. (5): po.= 16 4 (J.Qcmand a = 1'40.(%)-1. The lines obeying expressions (5) and (6) interseet at n = 11a; (fig. 2), thus giving: 11a; = O'30 %. SO'r _.0-".i-> _o- _- _-.-«: o King and Glover o Present work -- Primary marfensite --- Low carbon marfensife + ê-carbide O,~-L--L-~~--~~--~~--L-~~--~~--~~ o M W ~ -----I~Percent carbon (n) 3422 and its change due to the first stage of tem- Fig. 2. EÎectrical resistivity of primary martensite pering (measured at room temperature).

8 444 M. L. VERHEIJKE The values for Po and a may now be used to calculate (3, using eq. (6); this gives (3 = 0 31 (%)-1. According to Pitsch 8), who considers the -carbide particles to be insulators in a well-conducting a-iron matrix, the shape of the -carbide particles can be determined with the aid of (3. One finds that these particles have the shape of platelets with a diameter/thickness ratio of 5. Such -carbide platelets have indeed been found electron-microscopically in martensite and a-iron 10)11). Similar investigations by Pitsch 12) indicated that the - carbide platelets in a-iron with 0 02 % C have a diameter/thickness ratio of Conclusions (1) Itis confirmed that the decreases in length and resistivity ofprimary martensite caused by the complete first stage of tempering are linear functions of the carbon content, and the carbon content of low carbon martensite is found to be 0 30 % by dilatometric experiments as well as by resistivity measurements. The results of our experiments are in conformity with those of the recent X-ray diffraction analysis carried out by Werner et al. 5), although this value of the carbon content of Iow carbon martensite is slightly above the generally mentioned one of 0 25 %. (2) No first stage of tempering could be observed during dilatometric experiments on a steel with less than O 3 % C. This observation supports the current theory on this tempering stage. (3) We have applied the Pitsch theory for the electrical conductivity of a medium consisting of a well-conducting a-iron matrix in which insulating -carbide platelets are dispersed, to the results of our resistivity measurements and found that the -carbide platelets in tempered martensite have a diameter/thickness ratio of about 5. However, it is possible that this theory gives only a rough approximation in the case of martensite. Acknowledgement The author wishes to thank Mr J. J. de Jong for helpful advice and stimulating discussions. Eindhoven, August 1960 REFERENCES 1) C. s. Roberts, B. L. Averbach and M. Cohen, Trans. Amer. Soc. Metals 45, , 2) B. S. Lement, B. L. Averbach and M. Cohen, Trans. Amer. Soc. Metals 46, , 3) E. C. Bain, J. Iron St. Inst. 181, , ) O. Krisement, Arch. Eisenhüttenw. 27, , ) F. E. Werner, B. L. Averbach and M. Cohen, Trans. Amer. Soc. Metals 49, , 6) H. W. King and S. G. Glover, J. Iron St. Inst.188, 61-62, ) W. J ellingha us, Arch. Eisenhüttenw. 27,, , ) W. Pitsch, Acta Metallurg. 3, , ) B. S. Lement, Trans. Metallurg. Soc. A.I.M.E. 215, , ) H. K. Görlich and H. Goossens, Arch. Eisenhüttenw. 27, , ) W. Pitsch and A. Schrader, Arch. Eisenhüttenw. 29, , ) W. Pitsch, Acta Metallurg. 5, , 1957.