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Therefore, the influence of aluminum on certain properties of high-spe ed steels is simi-

lar to the i nfluence of cobalt.

The results of produc tion and labor atory tests of the cutting properti es of tools of

the steels devel oped showed their high effect iveness, especial ly of IIMbF steel with 1% AI.

The life of tools of the tungsten-free steels is two or three times greater than the life of

tools of R6M~ steel.

SYNCHRONOUS HEAT-TREATMENT TECHNOLOGY FOR

HIGH-SPEED STEEL TOOLS

E. A. Smol'nikov and V. A. Markina UDC 621.78:621.9.02:669.14.018.252

In the mass prod uctio n of high -spee d steel tools great difficulties ere caused by the

difference between the time of holding and of hardening and repeated tempering. For instance,

the total time of hold ing in salt baths ~tot (the total time the tools remain in salt solu-

tion) and the equal time for eac h stage of hard ening (preheating, final heating, and stepwise

cooling) for drills with 6 to 50 mm diameter varies betw een 2 and 4.5 min, and in case of

standa rd tempering at 560°C in furnaces or baths, hol ding in each temperin g alone (not count-

ing the time of heating) amounts to 1 h; this usual ly makes it necessary to use separate

equipment and fixtures for harde ning and tempering. This,l in turn, requires the introdu ction

of additional operations of transporting the tools and stacking them in large tempering cages

where the tools are heate d nonuniformly, in consequenc e their quality is impaired, and in ad-

dition this procedure entails unjustified consumption of electric power and lengthy dwelling

of the tools at the tempering operation. Brief double temperin g of tools at 580 or 600°C,

describ ed in [I], made it possible to reduce holding time to 20-30 and 10-15 min, respec-

tively. However, such temperin g conditions did not solve the problem.

For tools of steels R6M5, R6M5K5, R9M4K8 hardened under standard conditions, holding in

tempering may be reduced to 4 6 min by raising the tempering temperature to 630°C, or to

7-8 min by reducing the tempering temperature to 615°C without impairing the mechanical

proper ties [2, 3]. Tests showed that tool life at the same time increas ed by a factor of

1.2-1.3.*

As a result of this research the holdi ng times in the salt baths in harde ning and temp-

ering of the same tool became so close to each other than it beca me obvio usly possible to

make them equal to each other.

On this basis we worked out a technology of synchronous heat treatment enabling all

stages of the operatio ns of hardenin g and temperin g to be carried out on the same plant or

the same productio n line suitable for a fairly broad ass ortment of high-sp eed steel tools.

To carry out shorter tem pering at 615, 630°C, or even at highe r temperatur es witho ut

risking reduced hardnes s of the tools and for extending their life in operation, it was neces -

sary to increase the degree of allo ying of the solid solution; this was attained by extending

the holding time ~tot in heating prior to quenching or by some increase of the hardening tem-

perature.

In our investigation we increased the holding in heating prior to hardening Ttot, deter-

mined by the well-kn own formula [3] Ttot = Tbp + Tdc , by increasi ng the stand ardized time of

dissolving carbides rde by a factor of 1.5-5 since the time of bulk preheating Tbp did not

change.

The tests were first carried out wit h spec imens 6 x 6 x 60 and i0 x I0 x 55 ram of steel

R6M5 and R6MbK5, and then with tools.

*It may be ass umed that it remains at the p~evious l evel (Editor's note).

All-Union Tool Research Institute. Translated from Metallovedenie i Termicheskaya

Obrabo tka Metallov, No. 8, pp. 26-28, August, 1984.

0026-0673/85/0708-0587509.50 © 1986 Plenum Publishing Corporation 87

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rt0 n~n

12F--

7J ~ 56 d

6g 6JO 5~0 650 t °C 6g ~0 600 6~ ~ °C

b

F i g . 1 . D ia g ra m f o r d e t e r m i n i n g e q u a l h o l d

i n g

times (Tto ) in salt baths for hard ening

and tempering of high-speed steels R6M5 and

R6M5K5 in regimes of synchronous heat treat-

ment: a, b) quenchin g at 1220 and 1235°C,

respectively; I) zone of preferable hardening

and tempering regimes; II) zone of reduced

hardness after tempering; III ) zone of insuf-

ficient tempering.

The specimens were quenched at 1220 or 235°C with stepped cooling to 630°C, and then

they were subje cted to two- or three fold temp ering at 615, 630, 640, 650, 660, and 670°C

with equal ho lding times for both operations~ viz., between 1.5 and 15 min.

Then we plotted the curves for steel R6M5 on the basis of the specifi ed gra in size of

the austenite after hardening (no t larger than No. 9) and also of hardness (~HRC 63), bend-

ing stre ngth (Oh ~ 3300 MPa), impact toughness (a ~ 0.3 MJ/m2), and heat resis tance eval u-

ated according to hardness after additional 4-h heating at 620aC, and on condition of choos-

ing regimes ensuring absence of residual austenite after tempering (Fig. i).

These graphs make it possible to determine equal holding times in heating prior to

hardening and in tempering of the mentione d specimens, ensuring that the specified structure

and properties of the selected steels will be attained.

The possible regions of equal holding in salt baths in hardening and tempering, presented

in Fig. i, are limited from above by the fact that, firstly, e xcess ively long hold ing or ex-

cessively high quenching temperatures cause intense grain growth and in consequence reduced

strength and impact toughness after tempering~ and secondly, that longer holding times in

temperin g t han in Fig. i lead to reduc ed hardness.

The low er graphs in Fig. i boun d the region III of insuff icien t temperin g.

For steel R6MSK5 an analogous graph has the same shape, in principle (see Fig. i), kut

in plott ing it we used the following charact eristic s: HRC ~ 64; ~b ~ 3000 MPa; a = 0.25

MJ/m ~ .

Compa rativ e test s of tool life showe d that the life of cutter bits 7 × 8 × 2.5 mm in

size~ of steel RI4F4, aft er synchronous heat treatment was 1.3-1.8 times longer than after

heat treatment by the conventional technology.

Wit h the ai d of the graphs in Fig. 1 and the data of [2] we find that for drills with 6,

i0, 20, and 30 mm diameter in hardening at 1220°C and tempering at 630°C the holding times

are 4, 5, 6, and 7 min, r especti vely, and in hard enin g at 1235°C and temp ering at 640~C 3,

4, 5, and 6 min, respectiv ely.

The synchronous heat treatment devised by us and characterized by equal holding times

in hardening and tempering is applicable to all high-speed steels and may he used on semi-

automatic plant and production lines, including those that provide, in addition to harden-

ing and tempering, for operations of washing the tools and of re mo vi ng sa lt s from them.

Here, the time of final cooling in hardening and each tempering may be taken as a multiple

of the holding time in heating, or it may be reduced to the same length by cooling with com-

pres sed air or by water--air mixtures.

588

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The application of synchronous heat treatment makes it possible to attain a higher level

of mechaniz ation in hardening and tempering cutting tools, to shorten the cycle of heat

t r e t m e n t and to ensure large savings by reducing laboriousnessj saving materials and elec-

tric power, an d by improv ing the qual ity of the tools.

Application of the new technology is efficient both in series and in individual produc-

tion of tools.

The temperature of shortened tempering ttem ( °C) in synchronous heat treatment of all

other high-speed steels can be approximately determined by our empirical for mula

~em=600+ 8. . .12) V-ff 1 . . .~ Co,

wher e V and Co are the contents of vana dium and cobalt, respectiv ely, in %.

LITERATURE CITED

I. A . P . Gulyaev, K. A. Malinin a~ and S. M. Saverina, Tool Steels (Handbook) [in Russian],

Mashgiz, Moscow (1961).

2. E. A. Smol'nikov and V. V. Umanets, "Shortened tempering of high-speed steels R6M5,

R6M5K5, an d R9M4K 8," Metalloved. Term. Obrab. Met ., No. 2, 11-15 (1980).

3. Technologica l Instructions "Heat Treatment of Metal Cutting Tools for Conditions of

Automated Production" [in Russian], VNIIinstrument (1982).

EFFECT OF THERMAL CYCLING ON THE MECHANICAL

PROPERTIES OF STEEL 20Kh

Yu. A. Bashnin, L. A. Lisitskaya,

L. M. Semenova, and A. V. Pozhars kii

UDC 621.78:620.18:669.14.018.29

Steel 20 is used as material of many important tractor components sub jected to nitriding

with subsequent hardening and low tempering. In recent years the process of nitriding was

greatly improved: a technology of preliminary heat treatment was worked out as well as con-

trolled atmosphere on the basis of endogas, mak ing it possible to improve the quality and ex-

tend the life of components. Howeve r, the time requi red for the proces s remain ed as before.

In addition, a substantial shortcoming of this process was the increased grain size of the

core of the components; this impaired their mechanical properties and often caused breakage

of gear wheels in operation.

To obtain fine r grain size of the core, regim es of preli minar y heat treatm ent [i] were

devised but they are effective only with high alloy steels. For the same purpose thermal

cycling is used either before or after thermochemical treatment (TCT ) [2].

By thermal cycling, austenite grain is refined to No. II-12 within five to seven cycles.

A characteristic feature of the known methods of thermal cycling is the application of ac-

celerated heating (at the rate of approximately i00 deg/min); this makes it difficult to use

thermal cycling in the process of TCT, e.g., in nitriding. We devised a method of thermal

cycling that causes refinement of the austenite grains both in the surface layer and in the

core of components in the process of TCT itself~ and when the required rate of diffusion

saturation is attained, the method makes it possible to shorten the duration of the process

by~elimina ting the necessity of repeated heating prior to quenching and to improve the re-

quired complex of properties. It was shown [3] that nitriding using thermal cycling (heat-

ing rate 3-4 deg/min to 880°C, cooling at the same rate to the temperature ensur ing full

phase recrystal ization) mak es it possible within four or five cycles to reduce the grain

size of the austenite consi derabl y (to No. ii), to incre ase fatig ue strength, and more than

double the impact toughness of components made of steel 20Kh compared with the conventional

method of TCT.

Moscow Highe r Institute of Metallurgy (MVMI). All-Union Research Institute of Engineer-

ing (VNIITMASh). Translated from Metalloved enie i Termicheskaya Obrabotka Metal ov, No. 8,

pp. 28-30, August, 1985.

0026-0673/85 /0708-0589509 .50 © 1986 Plenum Publishing Corporation 589