T H E R M O M E C H A N I C A L T R E A T M E N T O F

A N D T I N - Z I N C B R O N Z E M I C R O W I R E

Z. A. T i m o f e e v a a n d L. B. Z h e r m u n s k a y a

B E R Y L L I U M

UDC 621.789:62-426:669.725.5

There have been repor t s in the l i te ra ture concerning a substantial increase in the strength of alloys by a cer ta in combination of thermal and mechanical t rea tments [1, 2].

The purpose of this work was to determine the possibi l i ty of improving the physicomechanical p rope r - t ies of microwire made of bronzes B2 and OTs 4-3, which are usedinmanufac tur ing elast ic elements in ins t ruments (tension wires, hangers, springs, etc.).

The investigation was conducted with wire 0.2 mm in d iameter of quenched bronze B2 (TsTU 19-58) ( cr b -< 40 kgf/mm2,6 _> 20% p = 0.1 12.mm2/m) and OTs 4-3 bronze (GOST 5221-50) after additional an-

nealing ~b = 45 k g f / m m 2, 6 = 30%, p = 0.08 ~2.mm2/m).

The wire was subjected to multiple drawing with par t ia l reductions of 8-10%. The total reduction for B2 wire was 75~c and the final d iameter 0.1 ram. The total reduction of OTs 4-3 wire was about 90% and the final d iameter 0.06 ram. The physicomechanical p roper t i es were determined by the method descr ibed in [3].

B2 B r o n z e

As we noted in [31, drawing at room tempera ture leads to a substantial increase in the strength of beryl l ium bronze mic rowi re due to cold working and also the part ia l decomposition of the supersaturated solid solution as the resul t of high local heating.

on~il o b, kgf/mn

180 :o# 1gO ]

1001

p ~. mmZ/m

=-4- o, o9~ : / ~ ' - -"7-

I " I 13' 412 i

~,071 i J/' [

l o i n , E l l 15

-50 0 50 iO0 150 200 250~

Drawing temperature

Fig.1. Variat ion of tensile strength and specific e lec t r ica l res i s tance of B2 microwire with the drawing tempera ture and subsequent aging condtions. 1) After drawing by methods II-V; 2) after subsequent aging 2 h at 320 ~ 3) 40 rain at 270 ~ 4) 2 h at 270 ~

p ~. mm 2/m

r J g,kg?mm 10~:

Ob, Oprop, k

90

~ 70 ~-.T.~ ~ _ _

5O -50 o 50 loo 15o oc

Drawing temperature

Fig.2. Variation of tensile strength, proport ional limit, normal elast ici ty modulus, and specific e lec t r ica l r e s - istance of OTs 4-3 microwire with thermomechanica l t rea tment condi- t i o n s . - ) After drawing by methods II, III, VI, VII; . . . . ) after subsequent aging 1.5 h at 175 ~

Transla ted f rom Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 72-74, March, 1968.

234

TABLE 1

Drawing temp., Bronze Method Nc~ ~

I 20 B2 I I 20 OTs 4-3 Ill -- 40

B2 IV 270 V 150

175 125

VI OTs 4-3 VII

Note: The drawing rate was 10 m/min in method I and 1 m/rain in the rest.

In the investigation of the effect of cold worMng and the de- composit ion of the solid solution the drawing tempera ture and rate were selected so that only one p roces s occur red at a t ime. The drawing conditions are given in the table.

For B2 wire (reduced over 50%) the optimum aging tempera ture is 270 ~ [3], and therefore the wire was aged at this tempera ture for 0.5, 1.5, and 2 h af ter drawing, and also for 2 ha t320~ which is ordinar i ly used for par ts with a large c r o s s section.

After drawing by the usual method at 20 ~ (method I) the strength of the wire was almost 250% higher (cr b = 120 k g f / m m 2) by compar ison with the original condition. According to the x - r a y s t ruc tura l analysis, drawing by this method induces part ia l decom- position of the solid solution, which is indicated by the weak lines of the precipi tated phase and a slight increase in the lattice pa ra - me te r of the c~ solid solution of beryl l ium in copper in comparison with the original quenched condition.

Figure 1 shows the variat ion of the tensile s trength and specific e lec t r ica l res i s tance with the aging tempera tu re and t ime for wire drawn by methods II-V.

Drawing at room tempera ture at a slow rate (method II) i nc reases the strength less cr b = 108kgf/mm 2 ) than drawing by method I, probably because there is less local heating. Drawing at a slow rate at -40 ~ (method HI) apparently allows high strain hardening ( Orb= l l 8 k g f / m m 2) even though decomposit ion of the solid solution is inhibited.

On the x - r a y pat terns after drawing by methods II and HI one also observes weak lines of precipi t - ated phase. After drawing by methods I-HI, a = 3.575-3.572 kX* and p ~ 0.1 ~2-mm2/m.

A very high strength ( O-b= 140 k g f / m m 2) is attained by drawing at 270 ~ (method IV), since in this case there is intense decomposit ion of the solid solution, which is confirmed by the reduction of the specific e lec t r ica l res i s tance and the increase of the lattice constant of the solid solution ( a = 3.580 kX).

In subsequent aging the strength inc reases in all cases due to the decomposit ion of the solid solution, and the lattice constant of the solid solution inc reases correspondingly. Aging 40 rain at 270 ~ inc reases the strength f rom 25% (method IV) to 40~c (method I) . The strength becomes identical in the case of methods I and IV.

An increase of the aging t ime to 1.5 h leads to some increase in strength (by 5-8%), while fur ther aging (2h) has almost no effect. The specific e lec t r ica l res i s tance dec reases f rom 0.095 on aging 40 rain to 0.085 on aging 2 h at 270 ~

The lattice pa r ame te r is the same for all drawing conditions af teraging 2 h at 270 ~ (a = 3.600 kX).

With an increase of aging tempera ture to 320 ~ the strength dec reases by 10-15% and the specific e lec t r i ca l res i s tance dec reases to 0.06 ~.mm2/mo The highest strength is found in wire drawn by method I (or b = 163 kgf/mm2).

The difference in strength after different methods of drawing is 20%, while after aging 40 rain to 270 ~ it is only 5~c. The mic ros t ruc tu re is identical after drawing and after subsequent aging under the dif- ferent conditions.

O T s 4 - 3 B r o n z e

To obtain the maximum strength by cold working, the OTs 4-3 wire was drawn at -40 ~ as well as at 125 and 175 ~ (methods III, VI, VII). It has been established experimental ly that 170-180 ~ is the optimum aging tempera ture for OTs 4-3 mic rowi re reduced over 50%. Aging at this tempera ture does not decrease the strength attained in drawing and has a favorable effect on the s t ruc ture and the s t ressed state, the elast ic after effect being reduced severa l t imes.

* For the quenched wire a = 3.570 * 0.003 kX.

235

The phys icomechanica l p rope r t i e s o f m i c r o w i r e af ter drawing at low and high t e m p e r a t u r e s were c o m - pa red with the p r o p e r t i e s obtained a f t e r drawing at room t e m p e r a t u r e (methods I and I I ) .

As can be seen in Fig.2, a high drawing t e m p e r a t u r e lowers the mechanica l p rope r t i e s . The highest values of s t rength, p ropor t iona l l imit , and e las t ic i ty modulus a re attained a f te r drawing at -40 ~ and the lowest va lues at the high drawing t e m p e r a t u r e s . The specif ic e l ec t r i ca l r e s i s t ance is min imum (0.078 ~2.mm2/m) a f t e r drawing at 175~ which indicates the leas t deformat ion of the c ry s t a l lat t ice and, conse~- quently, the lowest mechanica l p rope r t i e s .

The phys icomechanica l p r o p e r t i e s a f t e r a high drawing ra te at 20 ~ (method I) a r e identical to those a f te r drawing at negat ive t e m p e r a t u r e (method I I I ) . Subsequent aging 1.5 h at 170-180 ~ r a i s e s the tensi le strength, proportional limit, and elasticity modulus, and lowers the specific electrical resistance.

CONCLUSION

Drawing of B2 and OTs 4-3 bronze microwire at room temperature at a rate of approximately I0 m/rain produces the highest physicomechanical properties.

1 �9

2. 3.

LITERATURE CITED

V . S. Ivanova and P. K. Gordienko, New Ways of Increasing the Strength of Metals [in Russian], Nauka, Moscow (1964). V. A. Pavlov, FMM, 16, i (1963). Z. A. Timofeeva and L. B. Zhermunskaya, Metal. i Term. Obrabotka Metal., No. 3 (1965).

236