Metal Science and Heat Treatment VoL 38, Nos. 9 - 1 0 , 1996

HARDENING FROM THE LIQUID STATE

UDC 669.14.018.253.3:621.78.08

STRUCTURAL TRANSFORMATIONS IN BORON-CONTAINING HIGH-SPEED STEEL HARDENED FROM THE MOLTEN STATE

IL A. Oganyan, t Ya. N. Oganyan, t and A. G. Roshchupkin t

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 12 - 15, September, 1996.

It is known that steels of the RtM5 type containing over 0.1% 13 become brittle due to the formation of a coarse eutectic along grain boundaries. In previous works we have shown that methods of hardening from the liquid state can be used to increase the boron content in this steel to 0.5 - 1% without increasing the brittleness. A special heat treatment involving controlled transformation from an amorphous-crystalline state to a micro- crystalline one made is possible to increase the ductility and curing properties of the steel. However, the mechanism of this transformation was not considered earlier. The present article is devoted to this problem.

It has been shown in [1 - 3 ] that high-speed steels of the R6M5 type that contain over 1% boron can be fabricated in an amorphous state by hardening from the liquid slate. The same steels with a lower boron concentration have an amor- phous-crystalline structure after hardening. The alloys are ductile in the amorphous initial state and are embrittled after high-temperature annealing. On the other hand, the alloys with an amorphous--crystalline initial state are brittle and be- come ductile after heat treatment by a special regime. Here ductility is understood as the capacity of a rapidly quenched ribbon to withstand bending to 180 ° without failure.

In the previous work [3] we established a relation be- tween the carbon and boron content in steel and the tempera- ture of its transformation to a ductile state. It has been shown that the higher the concentration of these elements in the steel the higher the temperature of the transformation. However, the mechanism of the corresponding structural change in the metal has not been considered.

In the present work we investigated structural transfor- mations occurring upon heating in the tested steel (1.13% C, 5.8% W, 4.9% Me, 4.0% Cr, 1.7% V, 0,5% B, the remainder Fe) hardened from the liquid state. The metal was melted in a vacuum induction furnace in an alundum crucible in an argon atmosphere. The ingots were cut into 30-g pieces, placed in quartz ampoules, and melted by a high-frequency inductor. After this the melt was squeezed out of the ampoule under an excess argon pressure through an opening 0.5 mm in diame- ter, and the resulting stream was cooled on the external sur-

Institute of Solid-State Physics of the Russian Academy of Sciences, Russia.

377

face of a massive copper drum 300 mm in diameter rotating

at 3000 rpm. This gave a ribbon about 20 psn thick and 1 mm wide.

The phase composition of the specimens was studied us- ing a DRON-2.0 diffractometer and a Siemens D-500 diffiac- tometer (Germany) in cobalt and copper Ka-radiation. A se- leetive x-ray spectral analysis was conducted on a JXA-5 de- vice. An electron-microscopic study was conducted using JEM-100CX and JEM-1000 transmitting microscopes. Foils were prepared from the ribbon by etching in an electrolyte based on orthophosphoric acid and chromic anhydride. The middle part of the foil cross section was investigated. The process of crystallization of the amorphous-crystalline ribbon was studied by the method of scanning differential cal- orimetry using a DSC 111 device produced by the Setaram firm (France).

In the initial condition (after hardening from the molten state) the tested steel had an amorphous-crystalline structure. In a Debye crystallogram taken from a milled ribbon a well manifested line of bcc iron was clearly seen against the first halo; a diffractogram of the same powder had a peak corre-

sponding to the plane (110) of ct-Fe against a background of the first amorphous halo.

The special features of the microstructure of rapidly hard- ened steel in the initial state were determined using transmis- sion electron microscopy.

The electron diffraction pattern in Fig. la shows point re-

flections corresponding to an ct-Fe-base solid solution and a diffusion halo. The halo is quite smeared and does not contain

individual reflections (except for the six 110 c~-Fe ones),

0026-0673/96/0910-03775 ] 5.00 O 1997 Plenum Publishing Corporation

378 R.A. Oganyan et al.

583.6°C

, I I I I 500 550 600 650 t, °C

Fig. 2. Thermogram of steel with 0.5% B.

which indicates the absence of crystalline particles of a sec-

ond phase exceeding 10 nm in size. The smearing of distant

reflections in the electron diffraction pattern shows that indi-

vidual regions are oriented at angles of at most 10 °. Figure 1 b presents a light-field image of a microstructure

in which we discovered individual dendrite regions 30 - 50 nm

in size under conditions of a near-Bragg orientation. The white network in the dark-field image of the halo re-

gion in Fig. lc represents an amorphous phase. In all prob-

ability, it forms in hardening of a liquid phase of eutectic

composition that solidifies in the form of thin layers between

dendrites ofbcc iron. The width of the layers is 10 - 20 nm.

looo°c i

850°C

I I I I 34 32 30 28

a-Fe

L 26

JL

I I I

24 22 20 Q, deg

Fig. 3. Diffractograms of steel with 0.5% B after annealing at different tem-

peratures (the numbers at the curves): e ) M23(C , B)6; ×) MrC.

Such a structure is typical for steels after eutectic crystal-

lization at very high cooling rates. In our case the cooling rate

determined from measured distances between axes of den-

drites of second order (20 - 50 nm) exceeded l 0 6 deg/sec.

Structural Transformations in Boron-Containing High-Speed Steel Hardened from the Molten State 379

The rigid skeleton of the eutectic component of the steel

seems to be the cause of intergrain or interparticle fracture in

the ribbon in the initial state.

Data obtained by the method of differential scanning cal-

orimetry show that crystallization in the studied amorphous-

crystalline steel with an atomic fraction of boron and carbon

380 R.A. Oganyan et al.

Fig. 5. Structure of steel with 0.5% B after annealing at 1000°C: a) in a light field; b) in a dark field.

equal to about 7% occurs in the 500-650°C temperature range in one stage with an exothermic peak at 584°C and a thermic effect of 36.7 kJ/kg (Fig. 2). This indicates that the amorphous component of the metal begins to crystallize at 550°C and ceases to crystallize at 650°C.

In order to investigate the phase composition of steel in the temperature region we were interested in, we annealed it in the 5 5 0 - 1000°C range for 2 h. Diffractograms of the an- nealed specimens photographed in cobalt Ks-radiation are presented in Fig. 3. It can be seen that the phase composition of the metal is mainly changed at 650°C. At this temperature the diffractogram contains the principal lines of bee iron, M23(C , B)6 carboboride, and M6C carbide. The same phases are the main ones after annealing at a temperature up to 1000°C. Some lines remained unidentified. Assumably, they correspond to cementite Fe3C.

It should be noted that segregation of an M6C carbide from the solid solution occurs in the amorphous-crystalline steel at about 650°C. In steel fabricated by the conventional technology the process of secondary hardening occurs at 550°C.

After annealing in the entire temperature range, the pre- dominant phase was bcc iron; the total amount of the carbides M23(C , B)6 and M6C was about 20% (10% each). After pro- longed annealing (6 and 8 h) the proportion of the phases changed, bcc iron still remained the base phase, but the amount of M23(C, B)6 exceeded that of M6C by almost a fac- tor of two.

Let us consider what changes occur in the structure of the steel in the annealing process.

In "in situ" experiments conducted directly in the column of a high-voltage JEM-1000 electron microscope changes in the structure of the steel were observed after heating all the wax up to 850°C (see Fig. 4).

It has been established (Fig. 4a, b) that after heating to 750°C the microstructure of the steel does not undergo any substantial change. The eutectic network is retained along grain boundaries, and the dendrite structure is retained inside the grains.

The increased number of point reflections in the electron diffraction pattern (Fig. 4c) indicates crystallization of the amorphous component in annealing.

After the metal is heated to 800°C, the amorphous phase decomposes, forming a chain of highly disperse spherical segregations that repeat the configuration of the initial skele- ton (Fig. 4d, e). Dendritic grains disintegrate in the process of

the ct ~ y transformation. In subsequent heating (to 850°C) the grains and particles

of the excess phase grow in size. The continuous chain of segregations breaks into separate regions (Fig. 4f). Some seg- regations that have been positioned along boundaries of den- drite cells are enclosed by newly formed grains (Fig. 4g). The

size of the austenite grains is about 3 lain and that of the seg-

regations is 0.1 - 0.2 ~tm. It follows from the analysis of the microstructure in heat-

ing that all the processes described occur intensely upon pas-

sage through the temperature of the et ~ y transformation (- 8oooc).

As the annealing temperature is increased, the matrix grains and the particles of the excess phase grow. The number of particles of the excess phase decreases due presumably to dissolution of the finest particles and the resulting growth of the coarser ones and also coagulation of particles.

This restructuring creates favorable conditions for the de- velopment of processes of plastic deformation and provides bending to 180 ° without failure.

A 2-h annealing of specimens at 1000°C was conducted in evacuated quartz ampoules in a KS-400 furnace with Silit heaters. After thinning, the structure of the ribbons was stud- ied under an EM-100CX electron microscope.

After annealing at 1000°C the grain size in the matrix

was 5 ~tm (Fig. 5). Particles of the excess phase were posi- tioned along grain boundaries and inside grains. The elec- tron-microscopic examination allowed us to identify particles

of M23(C, B)6 carboboride 0.3 - 0 . 5 ~tm in size and particles

of M6C carbide 0.15 -0 .3 ~tm in size.

Structural Transformations in Boron-Containing High-Speed Steel Hardened from the Molten State 381

A local x-ray analysis did not show any inhomogeneity in the distribution of elements in the metal annealed at tempera- tures below 1000°C, which means that the structure was ho- mogeneous. In steel annealed at 1080°C some inhomogeneity in the distribution of tungsten, molybdenum, and chromium was caused by growth of the grain size.

Analyzing the results obtained and comparing them with published data we can conclude the following. It is known [4] that the main shortcoming of cast high-speed steel is the car- bide inhomogeneity caused by the coarseness of the eutectic structure that appears in massive castings due to the low cool- ing rates. The presence of a eutectic network along grain boundaries causes embrittlement in cast steel. Only with the help of hot forging (rolling) with a high degree of deforma- tion (up to 90°,6) can the rigid eutectic skeleton be broken up and the ductility of the steel increased.

In recent years methods of powder technology, including gas spraying of the melt, have been tested with the purpose of breaking up the structure of the ingot. The use of high cooling rates gave powders with a fine-grained structure and highly disperse carbides. However, the formation of the eutectic net- work along grain boundaries has not been eliminated com- pletely. For example, in powder particles 200-315 pm in size with austenite grains 3 - 5 ~un in size the thickness of the eu-

tectic layer is 1 - 1.5 pm [4]. In our experiments on hardening molten steel of the

R6M5 type with 0.5% B we could attain a cooling rate ex- ceeding 106 deg/sec and thus reduce the size of dendrite cells to 30 - 50 um and the thickness of the eutectic layer to about 10 nm. This allowed us to transform the rigid eutectic skele- ton rather easily into a set of spherical carbides by heat treat- ment of a rapidly hardened ribbon, which improved its ductil- ity. The fact that heating to 750°C does not cause a substan- tial change in the microstructure of the metal compared to the initial state shows that high heat resistance can be obtained in a compact specimen.

Comparing our results with data obtained in [5] in hard- ening molten steel R6M5 at a cooling rate of about l0 s deg/sec, we can state the following. In [5] the matrix grains reached 1 vm in size on the contact side of the ribbon and 3 - 4 ~m on the free surface, whereas in our experiments the grain size was an order of magnitude smaller. It seems

that the introduction of boron into the steel compositions is one of the causes of break up of the structural components.

The method used for breaking up the structural compo- nents of the alloy, in particular, the thinning of its eutectic and its transformation into fine spherical formations, can be useful for other classes of metallic alloys, for example, beat- resistant ones.

CONCLUSIONS

1. The brittleness of amorphous-crystalline ribbons in the initial state is caused by the rigid skeleton of the amor- phous component of the structure.

2. The increase in the ductility of amorphous-crystalline ribbons with increase in the annealing temperature can be ex- plained by the following successively occurring processes: crystallization of the amorphous component of the alloy, dis- integration of the rigid eutectic skeleton and formation of a continuous chain of spherical segregations in its place, break- ing of the continuous chain into separate regions due to disso- lution of fine particles and growth of coarser ones, and growth of matrix grains proceeding especially intensely above the temperature of the (x --) y transformation. As a m- suit of these processes some of the segregations formerly lo- cated on grain boundaries move inside the grains, which pro- motes plastic deformation of the ribbon without its failure.

REFERENCES

1. Patent 2116207 A. G. B. (Great Britain), Publ. 21.09.83. 2. O. V. Abramov, R. A. Oganyan, Ya. N. Oganyan, et al., "Special

features of the structure and properties of microcrystalline tool steel fabricated by hardening from the liquid state," Poroshle Metallurg., No. 10, 89-93 (1988).

3. Ya. N. Oganyan. R. A. Oganyan, and O. V. Abramov, "Effect of the composition of microcrystalline tool steel on its ductility," Metalloved. Term. Obrab. Met., No. 1, 45 -48 (1991).

4. Yu. A. Geller, Tool Steels [in Russian], Metallurgiya, Moscow (1983).

5. V. N. Fedorov and V. V. Smimov, "Crystallization kinetics and structure of high-speed steel R6M5 obtained by hardening from the melt," Izv. Ros. Akad. Nauk, Metally, No. 6, 120-125 (I 993).