UDC 620.178.35

I M P A C T F A T I G U E S T R E N G T H OF C A S T AND W R O U G H T S T E E L 35 AT ROOM AND S U B Z E R O T E M P E R A T U R E S

S. Ya, Yarema and E. L. Kharish

Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 3, No. 4, pp. 436-439, 1967

The tensile impact fatigue strength of a cast and wrought steel was investigated. It was shown that this property i~ more sensitive to the orientation of internal defects relative to the applied stress than to cyclic

('rbtating bending)or static loads. An increase in the ultimate impact strength at lowered temperatures

was observed.

Most of the investigations concerned with the fatigue of structural steels relate to fatigue in rotating bending, while studies of the impact fat igue strength, especially at low temperatures, are few and far between. This is partly due to as- sertions of certain researchers (MacAdam, Toome, M tiller, e tc . , cited in [1]) that the impact fatigue strength of ma te - rials may bees t imated either from the fatigue limit o.1 (at low impact energies) or from the impact strength a k (at high impact energies). However, as was shown in a number of studies [2, 3, 4], the impact fatigue limits may substantially differ from the static fatigue limits. And so, for instance, it was established in [2] that the dynamic coefficient (the ratio o f impact fatigue limit to static fatigue limit) is 1 .14 -1 .19 for steel normalized and tempered at a high tempera- ture, 0 . 97 -1 .01 for steel tempered at an intermediate temperature, and 0 .80 -0 .86 after low-temperature tempering.

Fig. 1. Microstructure (X150) of longitudinal sections of specimens cut in the direction a) parallel and b) normal to the direction of deformation.

Arrows indicate the direction of tensile impact loads.

Consequently, using data on static fatigue strength in design calculations for parts working under conditions of Cyclic im- pact :loads may l ead to large errors. Since impact fatigue tests occupy an intermediate position between standard impact

a n d static:fatigue tesu, and since lowering the temperature leads to an increase in o.1 and to a sharp reduction (below the Critteai temperature), in a k, it is natural to assume that neither of these properties can serve as a measure of the capaci ty of materials to carry cyclic impact loads at subzero temperatures.

This article describes the results of a study of the conditional impact fatigue limit (corresponding to a base of 105 cycles) of cast and wrought steel 35 (0.33% C, 0.64% Mn, 0.24% Si, 0.04% P, 0.043% S) at room and subzero t em- peratures. The tests were carried out on ground cylindrical specimens (d = 6 ram, 1 = 30 ram) of three kinds: a) cut from wrought Steel in the direction parallel to the direction of deformation; b) cut frPom wrought steel in the direction

no rma l to the direction of deformation; c ) c u t from cast steel.

To prepare type a and b Specimens, an annealed steel ingot was hot forged at 830" C to 37.5% of the initial thick- ness. This produced a clearly defined orientation of the internal defects (inclusions) relative to the direction of the ten- sile loads (Fig, 1). The mechanical properties of riormalized specimens are given in the table.

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The tests were carried out on a type DSVO-t50 ram impact machine (with a frequency of 10 impacts/sec) at + 90. - 2 0 , - 4 0 , - 6 0 and - 8 0 ~ C. The impact force was measured with a piezoelectric quartz crystal and recorded on a oscillograph, k specially constructed refrigeration chamber (Fig. 2) made it possible to mainlain the temperature con- stant at levels down to -120" C.

2 ~ to the chamber i ]

3 4 ~1

to potentiometer ~ 5

through LATR-2 f ~ to grid

Fig. 2. Refrigeration chamber for tensile impact fatigue tests.

4a

35

3 0

"2.5

20

% 4

~ ~ ~ ~ X ~ X \

10 s 10* 10 s N, cycles

~0

35

30

25

20 la a 10 ~ 70 s N, cycles

Fig. 3. Impact (continuous lines) and static (broken lines) fatigue curves of stem 35. O e ) Specimens cut in the di- rection normal to the direction of defor- mation; ZXA) cast steel specimens; [~11) specimens cut in the direction parallel to the direction of deformation. (Open and black symbols relate, respectively,

to static and impact fatigue.)

Fig. 4. Impact fatigue curves of steel 35 at - 6 0 ~ C. 0) Specimens cut normal to the direction of deformation; O) cast steet specimens; O) specimens cut parallel to the direction of deformation. (Broken lines relate to impact fatigue of steel 35.

at 20* C.)

By varying the voltage applied to a spiral 5 placed in a Dewar vessel 4 it is possible to vary the intensity of evapor- ation of liquid nitrogen. Gaseous nitrogen under pressure passes through a flexible tube and a brass coil with radial aper- tures to the chamber 2 and cools the specimen 1. The chamber 2 is insulated with a foam plastic. The temperature is measured with a copper-constantan thermocouple 3. For comparison, specimens of the same diameter were subjected to static fatigue tests on type IMA-5 machines. Test results are reproduced in Figs. 3, 4, and 5.

Figure 3 shows how the strength of stem 35 is affected by preliminary plastic deformation producing a specific orientation of internal defects (inclusions). And so, the conditional impact fatigue limit of specimens cut normal to the direction of deformation is, on the average, 1 .4 times larger than that of specimens cut normal to the direction of de- formation, although the corresponding increase in the UTS and impact strength a k is 1.12 and 1.15 times.

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Type of specimen

UTS, kg/mm 2 lak, d e c a j o u l e / c m 2

�9 at temperature ," C

- - 6 0 +20

47.8 53.0 48.7

-6o1+2o 50.5 l 45.1 58.0 52.0 51.7 47.1

9.60 11.2 10,3

Comparison of the condit ional impact and static fatigue

l imits (the dynamic coefficient Oimpact/O q is 1 . 0 3 - 1 . 0 8 for specimens cut normal to the direction of deformation, 0 .98-1.01 for cast steel specimens, and 0 . 8 4 - 0 . 8 8 for spec i - mens cut paral lel to the direction of deformation) leads to a conclusion that standard tests are less sensitive to the or- ientat ion of internal defects (inclusions) than impact fatigue tests. One should, however, bear in mind that the stresses appl ied in these tests are different ( impact tension and ro-

tat ing bending). Test results (Figs. 4, 5) show that lowering the temperature produces an increase in the condit ional i m - pact fatigue l imit of steel 35 even below the cr i t ica l t e m - perature at which there is a sharp reduction in the impact

strength of this s teeI .

If the process of fracture is divided into two stages,

namely the stage of nucleation of microcracks associated

. . . . . i �84 , ! . . . . . . f i

+20 -20 -40 -60 t , ~

Fig. 5. Temperature dependence of the number of cycles to fracture N of steel 35

fatigued under an impact tensile stress of 3a kg /mm ~. A) Specimens cut normal to the direction of deformation; O) cast steel specimens: e ) specimens cut paral le l to

the direction of deformation.

with microplast ic strains (Nt) and the stage of crack propagation (N2) leading eventual ly to fracture [1], the increase in the condit ional impact fatigue l imi t at lowered temperatures may be attributed tO a lengthening of the stage N~ due to the inhibition of microplast ic deformation. This view is supported by the fact that as the test temperature is lowered, the area of the fatigue crack with a characteris t ic matt appearance on the fractured surface becomes reduced, i . e . , the

stage N 2 becomes shorter although the over-a l l endurance is increased.

Summary

1. Tensile impact fatigue tests are more sensitive to the orientation of internal defects (inclusions) in steel than

fatigue tests in rotating bending or tests under static loads.

2. Lowering the temperature leads to an increase in the condit ional tensile impact fatigue l imit of steel 35.

REF ERENCES

1. E. A. Silkin and A. F. Zasova, ZL, no. 12, 1961. 2. N. N. Davidenkov and E. I. Balyaeva, MiTOM, no. 11, 1956.

3. S . V . [olkachnik, Izv. AN SSSR. OTN, no. 5, 1958. 4. M. U. Katsnel 'son, A. Ya. Malolemev, and I. M. Vysotskaya, Vesmik mashinostroeniya, no. 4, 1962.

10 March 1967 Institute of Physics and Mechanics, AS UkrSSR, L'vov

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