Metal Science and Heat Treatment Vol. 38, Nos. 9 - 10, 1996

UDC 620.184.4

HEAT TREATMENT FOR EXHIBITING THE OF CAST HYPOEUTECTOID STEEL

DENDRITIC STRUCTURE

A. A. Zhukov, V. A. ll'inskii, and L. V. Kostyleva

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 10- 12, September, 1996.

Different methods exist for etching microscopic and mac- roscopic steel specimens in order to exhibit its primary den- dritic structure [1]. We have developed a method for exhibit- ing this structure in hypoeutectoid steels of the ferrite-pearlite class in a conventional metallographic analysis by etching a microscopic polished specimen with nital or picral (solutions of nitric or picric acid in alcohol). For this purpose the steel is subjected to a special provoking heat treatment, namely, heat- ing to a temperature in the interval Ac 3 - (Ac 3 + 20C), a hold of about 30min, and slow cooling to 600C at a rate

< 300/h. Further cooling to room temperature can be con- ducted at any rate, because after the decomposition of the austenite is completed, the microstructure of the steel does not undergo substantial changes.

As a result of such heat treatment pearlite segregates in the form of very small regions between branches of dendrites of the primary structure of the steel, whereas the branches themselves remain purely ferritic. Figure la presents the structure of steel 35L on a surface separating a zone of co- lumnar crystals and a zone of coarse equiaxial grains with randomly oriented dendrites. The "trunk" of the dendrite (the first-order axis) is quite discernible, as are the "branches," which have some side arms (third-order axes).

Figure l b presents a similar microstructure of the surface layer of the same casting (a zone with more randomly ori- ented dendrites). It can be seen that here the dendrites do not have many branches and the structure consists predominantly of nonbranched "trunks."

Such a dendrite structure in steel after the described heat treatment is caused by the following factors. In carbon steels of type 20L-35L and in low-alloyed and alloyed steels mi- crosegregation of the components usually results in their ac- cumulation between branches of the dendritic structure. These components can either increase the activity of the dis- solved carbon, or decrease it, which happens more often.

Mn, Cr, V, and other carbide-forming elements and, un- der some circumstances, A1 and Cu in austenite belong to ele- ments of type i with e~ < 0 at temperatures below 900C

i = d In ?c/dN~ is the interaction parameter, )'c is the (where ec

coefficient of carbon activity, N. is the concentration of the component in atomic fractions) [2]. Si and Ni belong to ele- ments of typej with e~ > 0. The effect of Ni is weak and am-

biguous (it is possible that gcNi' like e cu, changes sign with de- crease in the austenite temperature). Therefore, only silicon remains in groupj. However, in steels with 0.02-0.35% C silicon segregates in thhe "conventional" way, whereas in

Fig. 1. Microstructure of steel 35L at an internal boundary of a zone of columnar crystals of a casting (a) and in the surface layer of the casting (b), Nital etching, x 100.

374 0026-0673/96/0910-0374515.00 1997 Plenum Publishing Corporatiol

Heat Treatment for Exhibiting the Dendritic Structure of Cast Hypoeutectoid Steel 375

~1 r [ r~'r \Mn IwV 1" ,t ,

1 . !~

20 40 60 80 S, I.tm

Fig. 2. Structure of amorphous-crystalline steel with 0.5% B in the initial state: a) electron diffraction pattern, b ) light-field image, c) dark-field image of a halo region.

steels with a higher carbon content the microsegregation of Si changes from direct (Fig. 2a) to inverse (Fig. 2b). The spectra presented in Fig. 2 were obtained by the method of local x- ray spectral analysis by moving a Cameca MS-46 electron microprobe across second-order dendritic branches in cast steel.

Taking into account that the segregation of manganese is always direct for the concentrations studied, we can infer from the spectra presented that silicon segregates differently depending on the carbon concentration in the metal. In low- carbon steels with 0.15% C (austenite with strongly diluted carbon) silicon concentrates predominantly in microvolumes enriched in manganese, i.e., silicon segregates between branches of primary dendrite crystals. The averaged curves of the characteristic x-ray radiation of silicon and manganese are positioned virtually parallel (Fig. 2a). On the other hand, in steel containing 0.48% C the intensities of the charac- teristic x-ray radiation of silicon and manganese are counter- phase (Fig. 2b), which is a sign of inverse microsegregation of carbon, which enriches, at this carbon concentration, the axial zone of primary dendrite crystals.

The parameters of microsegregation of manganese and silicon in these two steels expressed in terms of the effective coefficients of distribution are as follows: K~ M~ = 0.75 (for

Fig. 3. Structure of a decarburized dendrite branch of the second order in heat treated steel 45FL. Picral etching, x 600.

both steels); Ke Si= 0.93 (for the steel containing 0.15% C),

and K si = 1.11 (for the steel containing 0.48% C). Consequently, silicon cannot counteract the effect of

manganese and other elements of type i on the difference in the activities of carbon in different regions of the dendrite structure of steel (at least at a carbon content exceeding 0.25%).

It turns out that only i-type components accumulate be- tween dendrite branches. Since carbon has a high diffusion mobility in austenite, this phase is characterized by partial thermodynamic equilibrium with respect to this component, i.e., only the chemical potential of carbon ta c becomes uni-

form. Sincetac=[ta~z+RTlnNcl+[RTlunyc I (where Nc is

id x ~tc P'c

the carbon concentration in atomic fractions, ~ is the chemi-

cal potential of carbon in its standard state, ~ is the chemical

potential in an ideal solution, ta~ x is the excess chemical po-

tential), the condition ~c = const corresponds to the condition In N c + ln~, = const. Consequently, in the regions where 7c is reduced (due to the presence of an elevated amount of com- ponents of type i), the concentration N should be above the average value. For this reason austenite is inhomogeneous with respect to carbon even when the latter is characterized by an equilibrium distribution.

The amount of carbon is reduced along the axes of den- drites; at the temperature Ar 3 ferrite begins to segregate in these regions. Subsequent portions ferrite are deposited on crystals of it already formed, and therefore, at Ar t , pearlite segregates mainly between dendrite branches, making the dendritic pattern of the metal more pronounced. In steels characterized by inverse segregation of silicon (for C > 0.35%) this phenomenon can be observed metallographically due to polarization of the primary structure with respect to manga- nese and silicon. For example, in the cross section of a den- drite branch in Fig. 3 we can see darker ferrite enriched in silicon and depleted of manganese and coated by a thin bor- der of lighter "deposited" ferrite that undergoes a y--~ ct transformation at a lower temperature.

376 A.A. Zhukov et al.

In the case of austenitization at an elevated temperature the dendritic pattern in the microstructure weakens or is not observed at all, because the centers of ferrite crystallization in austenite lose their activity when superheated. Austenite be- comes more stable under conditions of supercooling and its decomposition occurs at a lower temperature, at which ferrite originates predominantly along grain boundaries rather than in carbon-depleted zones; no dendritic pattern appears in the resulting ferrite-pearlite structure.

The cooling rate of austenite after austenitization and holding affects structure formation just like superheating of austenite. The higher this rate the more supercooled is the austenite and the lower the susceptibility of ferrite to seg- regation in zones with Yc > ~ and, consequently, with

Nc < N~ "n.

It is of interest to note that if the steel is heat treated in order to obtain a uniform femte-pearlite structure (without traces of the dendritic pattern), it can still acquire such a pat- tern in a repeated provoking heat treatment by the regime rec- ommended above. The metal can be transferred repeatedly from one state to the other as long as it retains a dendritic mi- crosegregation nonuniformity with respect to manganese and other i-type components. Moreover, under such "thermal cy-

cles" the steel "polarizes" with respect to manganese and sili- con (in the simplest case of unalloyed steels), i.e., manganese and carbon concentrated between dendritic branches force silicon into the dendrite axial zone [3], which becomes more susceptible to ferrite segregation when austenite is cooled be- low Ar 3 .

Consequently, only true homogenizing of east steel with respect to all its components can eliminate completely the possibility of formation of a dendrite pattern in a ferrite-pear- lite transformation that occurs in the range Ar 3 -Ar I. Con- versely, selective homogenizing annealing can increase the capacity of the steel to form such a pattern.

REFERENCES

1. K. J. Smithies, Metals: A Reference Book [Russian translation], Metallurgiya, Moscow (1980), pp. 200 - 201.

2. i. 1. Dobrovol'skii and V. A. Kupryashin, "Pearlitization of the s~ucmre of cast iron under the effect of copper and aluminum," in: Problems of the Quality and Effective Use of Metal in Me- chanical Engineering [in Russian], lzd. TPI, Tula (1982), pp. 28 - 34.

3. A. A. Zhukov, "On upward diffusion of components of carbon and alloyed steel in homogenizing," Metalloved. Term. Obrab. Met., No. 12, 56 (1976).