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2.3.4.5.6 .

7.

A. P. Baskakov et at., "Use of subme rged burn ers to obtain a low -te mpe rat ure i2uidized bed," Tsv et-nye Metally, No. 2, 66 (1972).A. P. Bas ka kov et al., "Fu rnac e for tempe ring steel wire in a ftuidized bed heated with subm erg edbu rn er s, " Stal' , No. 12, 1134 (1972).A. t ) . Gulyaev, IIeat Tr ea tm e nt of Steel [in Russi an], Mashgi z, Moscow (1953), pp. 205, 260.I. I. Novikov, Theo ry of II ea tT re at me nt in Metals [in Russian], Metalturgiya , Moscow (1974), p. 274.V. D. Sadovskii , "Transfo rmation of austenite to martensi te," in: Phase Transform ation s in Ir on -Carbo n Alloys [in Russia n], Mashgiz , Sverd lovsk (1950), p. 65.V. Ya. Zubov and A. P. Migachev, "Inst rumen t for testing cutting instr umen ts," in: I mprov ement ofTechno logy and Means of Mechanization [in Russian[, Sel'khozi nstitut, Sverdlovsk (1974), p. 56.

D E T E R M I N I N G T H E R M A L F A T I G U E O FC A S T I N G O F A L U M I N U M A L L O Y S

V. G. Gorb ach , V. G. Aiek h in ,a n d G . L . K ' u r g a n o v a

S T E E L S F O R D I E

UI)C 536.495:621.746.073

One of the main reasons for the failure of molds for pres sur e casti ng is the therm al str ess result ingfrom the tem pera ture gradient on the surface and through the section of die -ca stin g molds [1, 21. Peri odicther mal cycli ng leads to cracks on the surface (crazing) and in the body of the molds. The magnitude of thethermal stres s depends on the mechanical and thermophysical properti es of the material , the heating andcooling rate s, and the maximum and minimum t emp era tur es of the cycle ]3].

Determining the heat resist ance and the suitabili ty of new materials for die-casti ng molds is a majorproblem , and ther efo re attemp ts have been made to calculate the heat resis tan ce of die -ca stin g molds [4, 5].

The r esistan ce of die-cas ting molds depends on the plastic deformation of the surface during heatingand cooling in each castin g cycle . Ttle deformati on has been dete rmine d by calculati on [5].

In this work the deform ation was determ ined e xperi menta lly. The combined influence of mechanic alload and thermal st ress , i .e. , the principal reasons for the failure of die-ca sting molds, was simulated bystretching the samples with simultaneous cyclic changes in temperature . The deformation of the sampleduring thermal cyclinc cha rac ter ize d the servic e life of the molds, since there is an analogy between themechanical and thermal effects on the material .

The quantitative cha racte risti c was the deformation in uniaxial s tre ss relative to one thermal cyclebetween the maximum and minimu m te mpe rat ure s on the surface of the mold.

In conducting tests simulati ng the conditions of opera tion of die -ca stin g molds the res ista nce of thematerial to the influence of cyclically changing temperatu res under constant toad was determined quantita-tively, with determ inatio n of the number of cycl es to failure . We also analyzed the stres s and strain, det er-mined the behav ior of the mate rial quantitatively, and calculat ed the heat resis tan ce of die -ca stin g molds fromthe test results .

The numbe r of cycl es to failure of die -cas ting molds was determi ned by the formula given in [5]:, \ : = - q ~ - , ( 1 )~ - p

where N is the number of pre ssin g cycl es the mold can withstand before therma l fatigue crack s ap pear; ep isthe total defor mat ion (%) of the sample in the rma l fatigu e test s; C is a coeff icien t taking into acco unt the duc -tility of the material and determined by the formula

C=ln / ' 1001~_ / , (2)\ 1 0 0 - ]where ~, is the reduction in section determ ined in therm a[ fatigue tests.

Tran slat ed from Metallovedenie i Ter mic hes kay a Obrabotka Metallov, No. 11, pp. 63-66, November , 1977.

9a2 0026-0 673/7 7/111 2-09S 2 S07.50 1978 Plenum Publishi ng Cor por ati on

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TA BLE 1I

T , . uSteel H R C ~ ;>-,.~

50 313 27 52

48 311 32 884Kh4V:,~FS -~- -~ -~- ~-5 0 8 8 7 2 7 82

Note. Numerators refer to steels quenchedfrom 1050" in oil (steels 3Kh2V8 and4Kh4VMFS) and from 900 in water(N21M2T2B,, tempered at 600-630 for 1 h(3Kh2V8 and 4Kh4VMFS), and aged at g60 for I h (N21M2T2B); denominators refer tosteels quenched from the same temperatures,tempered at 6~0 for 1 h (3Kh2V8 and4Kh4VMFS), and aged at ~2~ for 1 h(N21M2T2B;,.

The to t a l de f o r m a t ion wa s de te r m ine d by the f o r m ula~p = ~t ~- ." ( 3 )

W i t h a g i v e n t e m p e r a t u r e d i s t r ib u t i o n i n t h e s a m p l e t h e t h e r m a l e x p a n s i o n i s d e t e r m i n e d b y d e f o r m a t i o n ,a n d t h e r e f o r e t h e r e s i s t a n c e o f t h e m a t e r i a l t o t h e r m a l f a t i g u e i s d i r e c t l y r e l a t e d t o t h e c h a n g e i n d e f o r m a t i o na t o c c u r r i n g t w i c e i n e a c h h e a t i n g a n d c o o l i n g c y c l e a n d i s a fu n c t i o n o f t h e c h a n g e i n t e m p e r a t u r e A T a n d t h ec o e f f i ci e n t o f t h e r m a l e x p a n s i o n a :

' t = I - ~ ' ( 4 )

whe r e # i s P o i s son ' s r a t io .The defor mat i on ~n occu r r in g under the inf luence of constant load was de te rmin ed f ro m the resul ts of

the r m a l f a t igue t e s t s :~ n ~ - ,

n ( 5 )w h e r e 6 i s r e l a t ive e longa t ion ; n i s cyc les to fa i lure .

The n umber of cyc l es to fa i lu re in uniaxia l tens ion was de te rmin ed by means of the IMASh-9-66 appa ra -tus ; the s t reng th of the mate r ia l was c a lcula ted f rom the va lues of the mic roha rdn ess dur ing hea t ing and e lon-ga t ion of the sam ple in vacuum.

The appara tus was equipped wi th two t ime re l ays connec t ed to the hea t ing control , which made i t poss ib leto regul a te the hea t ing and cool ing t imes . The tes ts w ere made under constant load - 20 kgf / mm 2.

We de te rmin ed the number of cyc le s to fa i lure , the re la t iv e e longa t ion and reduc t ion in sec t ion, and ca l -cula ted the defo rmat i on of the sample pe r cyc le .

Up unt i l r ecent l y the formi ng par ts of d ie -cas t in g mol ds were g enera l ly made of s tee l 3Kh2V8 (GOST5950-73) wi th a re la t iv e ly low duc t i l i ty , f rac tur e toughnes s , and res is t anc e to c raz i ng [6]. At the prese nttim e ste el 3Kh2V8 has been r epl ace d with new die stee ls: 4Kh5MFS (TU ChM3-17 6-70) and 4Kh,tVMFS (GOST5950- 73) ; a t t e m pt s ha ve a l so be en m a de to u se m a r a g ing s t e e l s f o r d i e - c a s t ing m olds .

T A B L E 2ct . 1 0 6 , [Steel 1/deg at, c [

29 029 0tKh4VMFS 10,2N21M2T2B 11,4 0,30,3

!),q:~

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TABLE 3

3K}I2VE4Kh4VMFSN21M2T2B

S t e e l

0,495

0 , 4 9 S

0,406

I i ~ l

0,086

0,103

0,030

t p

0,581

059___~80,8910 , 4 3 6

0 , 7 3 2

2,12

1,716

C a l c . c y c l e st o f a i l u r e o fd i e - c a s t i n g m o l d sf r o m t h e r m a 1 a t i g u e

21 60038 20059 500630009@70082 300

, ,N o t e. S e e n o t e i n T a b l e 1 .

We tested steels 3Kh2V8, 4Kh4MVFS, and maraging steel N21M2T2B (0.03~c C, 0.04% Mn, 0.16% Si,0.012% Cr, 21% Ni, 1.7c,~ Ti, 2.0% Mo, and 0.44% Nb).

The test results are given in Table I.With cycling in the range of 360-650 corresponding to the change in the temperature of molds used for

casting aluminum alloys, the number of cycles to failure was highest for steel N21M2T2B.From the thermal fatigue tests we determined the deformation per cycle, the ductile characteristics,

and coefficient C from formula (I).The deformation t occurring with changes in temperature was determined by formula (4). The data

given in Table 2 were used for the calculations.The results of the thermal fatigue tests are given in Table 3, where it can be seen that steel N21M2T2B

has a higher resistance to thermal fatigue.The results of microhardness tests under a load of 200 gfinthe temperature range of 20-650 are given

in Table 4. It can be seen that with heating to 650 the hardness is highest for steel 3Kh2V8; the hardness ofsteels 4Kh4MVFS and N21M2T2B is lower by a factor of 1.3 and 108, respectively.

Thus, the high resistance to thermal fatigue of steeiN21M2T2Bis evidently due to its higher ductilecharacteristics. This is in agreement with the data from [7], where it was reported that the resistance tothermal fatigue in tension depends to a greater extent on the ductility than the strength.

Operating tests of die-casting molds used in the production of radio and electrical parts from aluminumalloys showed that molds manufactured from steels 3Kh2X'8 and 4_Kh4VMFS have a service life of 30,000 to60,000 pressings, which agrees with the calculations.

The calculated number of cycles to failure for molds of steel N21M2T2B was larger than for molds ofsteels 3Kh2V8 and 4Kh4VMFS by a factor of 2.5 and 1.5, respectively.

Thus, the basic operating conditions of die-casting molds are simulated by the simultaneous influenceof tensile st ress and cyclically changing temperature. However, this does not take into account the conditionsof heat transfer, the effect of molten metal, and the presence of stress concentrators.

T A B L E 4

3 K h 2 V f i4 K h 4 V M F SN 2 1 M 2 T 2 B

S t e e l

7O8

802580

Hardness F - Izwat ( C ;1 0 0 9 A ) 0 3 0 0 4 0 O

6 9 6 6 7 5 6 7 8 6 2 4

501 494 4~4 454

564 569 456 4 3 6

8oo 60o6 0 7 4 9 . 0

as--}435 388

4 2 8 : 2 34

650350286

28116019@

Note . See i to te to Tab le 1 ,

9,ql

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CONCLUSIONSI. The total deformation of a sample under the influence of thermal cycling and uniaxial stress charac-

terizes the resistance to thermal fatigue of die-casting molds.2. Experimental determination of the deformation makes it possible to calculate the number of cycles

to failure of die-casting molds due to thermal fatigue.3. Calculating the thermal fatigue by the method proposed makes it possible to compare different

steels. It is recommended that these calculations be made in selecting materials for dic-castingmolds.4. Maraging steel N21M2T2B is a promising material for die-casting molds used for aluminum alloys.

Io

2.3.4.5.6.7.

L ITERATURE CITEDI. I. Goryunov, "Operation of die-casting molds in pressure casting," in: Interaction of Casting Moldsand Castings [in Russian], Izd. Akad. l~'auk SSSR, Moscow (1962), p. 147.M. N. Kleiner, "Means of increasing the resistance of die-casting molds used for casting copperalloys," in: New Developments in Casting Production [in Russian], Mashgiz, Moscow (1962), p. 225.S. Noesen, S. Williams, and N. Schenectady, Modern Casting, 51. No. 6, 119 (1967).I. I. Goryunov, Die-Casting Molds for Pressure Casting. llandbook [in Russian], Mashinostroenie,Leningrad (1973).L. F. Koffin, "Cyclic deformation and fatigue of metals," in: Fatigue and Endurance of Metals [-Russiantranslation], IL, Moscow (1963), p. 257.Yu. A. Geller and E. S. Golubeva, MProperties and applications of modern die steels for die casting,"Kuznechno-Shtampovochnoe Proizvod., No. 8, 12 (1964).P. Forrest, Fatigue of Metals [in Russian], Mashinostroenie, Moscow (1968).

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