METALLOVEDENIE I TERM. OBRABOTKA METALLOV, April 1960, No. 4, pages 54-56

HB No. 5389

H E A T T R E A T M E N T OF ROLLER C O N V E Y O R FRAMES

Eng. M. YA. BROVMAN South Urals Engineering Works

Our plant builds roller conveyor tables with group drives for transferring stock on blooming and continuous b i t - tet mil ls . To reduce internal stresses, the conveyor frames are heat treated after casting. However, undesirable warp-

age was observed after machining; this led to poor dimensional accuracy and required addit ional fabrication. We thus had to study the effect of thermal and mechanica l treatments on warpage.

~oo :1 _

lJgt~

t,O{l

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0 2 :~ . " . " o 8 10 ~'~ t# I5 t8 ]g Z2 ~.4 2~ Z~ 32 aL~ 36 38 ~ 02 40 Ft~:

Pig. 1. Heat t reatment of toiler conveyor frames: 1) old; 2) new heat t reatment .

Y/ / / / / / / / / / / / / / / / / / / / / 2

Fig. 2. Cross-section of convey- or frame.

Figure 1 shows the heat-treatment cycle used for the frames. From relaxation curves [1], one can determine the expected de- crease of residual stresses after heat t reatment .

Drastic cooting from 900 to 370 ~ in 1 hr or fast cooling

from 670~ is a source of new residual stresses. A cross section of a conveyor frame isgiven in Fig. 2. Most conspicuous after

machining are the lack of paral let i ty of planes A and change of dimensions of a.

Let us consider the shetf a as a canti lever beam of thick- ness h , length l and width b . Let the temperature in the outer

layers of th icknessv be by At less than in the interior. The out-

er layers tend to acquire a length l (1 - na t ) where ct - the co-

efficient of linear expansion. A tensile strain A / i = o l l / E arises

in the outer layers. In the inside layer, the compressive strain

is A.I 2 = ozl /E,where o z and o~ are the stresses in the outer and inner tayers, respectively. It follows that

~ll + M2 = [~At,

From the equil ibrium condition

2~,6v = :~e (h - - 2v),

2~v .

h - - 2v

Solving the system of two equations we get

~K,Zt

~ + h - - 2 - - - - - - T

Let us determine a temperature gradient At m at which the stresses are equal to the yield stress o m

( 2v ) :rn 1 + h - - -2v

~E (1)

After machining, when the lower layer of thickness v is removed, the beam will bend by moment

M : "~ m 6 v ( ' h - -t- ~-) . .

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The angle over which the extreme layer rotates is B = MI / E1 .

The deflection at the beam end f = MI 2/2gi; for carbon steel at room temperature, calculations give At m =

90 + 120~ When the temperature difference between the outside and inside exceeds At m, plastic deformation and

significant distortion after machining arise. In the author's experiments, a beam was rapidly cooled from 900 ~ atAthigher than~ t m. Resistance strain gages

were fixed to the lower surface,while the upper was removed by mi l l ing . The strain was recorded after each pass. For this material , the computed strain in the lower fibers was e = 0.89 x 10--4(v = 1 mm),whereas the actual

measurements yielded e = 0.56 x 10-4; for v less than 1 mm, the actual strains were 1.3-1.5 times less than the calcu-

lated ones. The value ZXt m from the formula is too high and for ZXt<At m plastic deformation will not arise. The frames,

therefore, must be cooled so that the maximum temperature gradient would not exceed At m. This requires knowledge of temperature changes with t ime . The cooling rate of the bearing supports is computed according to well-known

methods. Calculations applied to the most important parts of the frame showed that they can be cooled in air only from 400 ~ Curve 2 in Fig. 1 gives the heat-treatment procedure to be used if the condition ZXt-<Z~tm is to be fulfilled.

An acceptance of the suggested heat- treatment cycle (Fig. 1, curve 2) and copious cooling during grinding con- siderably reduce the warpage of conveyor frames after machining.

REFERENCES

1. N.I. Klyuchnev and G.S. Strizhov, Residual Stresses in High Strength Ductile Iron Castings (Vnutren. Napryazhen- iya v Otlivk. i . Vysokopr. Chug. s Sharov. Graf.) [Book] Akad. Nauk SSSR Press, 1957.

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