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Thermomechanical treatment of beryllium and tin-zinc bronze microwire

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    Z. A. T imofeeva and L. B. Zhermunskaya


    UDC 621.789:62-426:669.725.5

    There have been reports in the l iterature concerning a substantial increase in the strength of alloys by a certain combination of thermal and mechanical treatments [1, 2].

    The purpose of this work was to determine the possibil ity of improving the physicomechanical proper- ties of microwire made of bronzes B2 and OTs 4-3, which are usedinmanufacturing elastic elements in instruments (tension wires, hangers, springs, etc.).

    The investigation was conducted with wire 0.2 mm in diameter 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 kgf /mm 2, 6 = 30%, p = 0.08 ~2.mm2/m).

    The wire was subjected to multiple drawing with partial reductions of 8-10%. The total reduction for B2 wire was 75~c and the final diameter 0.1 ram. The total reduction of OTs 4-3 wire was about 90% and the final diameter 0.06 ram. The physicomechanical propert ies were determined by the method described in [3].

    B2 Bronze

    As we noted in [31, drawing at room temperature leads to a substantial increase in the strength of beryl l ium bronze microwire due to cold working and also the partial decomposition of the supersaturated solid solution as the result 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/' [

    lo in ,E l l 15

    -50 0 50 iO0 150 200 250~

    Drawing temperature

    Fig.1. Variation of tensile strength and specific electr ical resistance of B2 microwire with the drawing temperature 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, proportional limit, normal elasticity modulus, and specific electrical res- istance of OTs 4-3 microwire with thermomechanical treatment condi- t ions . - ) After drawing by methods II, III, VI, VII; . . . . ) after subsequent aging 1.5 h at 175 ~

    Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 72-74, March, 1968.


  • 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- composition of the solid solution the drawing temperature and rate were selected so that only one process occurred at a time. The drawing conditions are given in the table.

    For B2 wire (reduced over 50%) the optimum aging temperature is 270 ~ [3], and therefore the wire was aged at this temperature for 0.5, 1.5, and 2 h after drawing, and also for 2 hat320~ which is ordinari ly used for parts with a large cross section.

    After drawing by the usual method at 20 ~ (method I) the strength of the wire was almost 250% higher (cr b = 120 kgf /mm 2) by comparison with the original condition. According to the x- ray structural analysis, drawing by this method induces partial decom- position of the solid solution, which is indicated by the weak lines of the precipitated phase and a slight increase in the lattice para- meter of the c~ solid solution of beryll ium in copper in comparison with the original quenched condition.

    Figure 1 shows the variation of the tensile strength and specific electr ical resistance with the aging temperature and time for wire drawn by methods II-V.

    Drawing at room temperature at a slow rate (method II) increases 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 l8kgf /mm 2) even though decomposition of the solid solution is inhibited.

    On the x- ray patterns after drawing by methods II and HI one also observes weak lines of precipit- 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 kg f /mm 2) is attained by drawing at 270 ~ (method IV), since in this case there is intense decomposition of the solid solution, which is confirmed by the reduction of the specific electr ical resistance and the increase of the lattice constant of the solid solution ( a = 3.580 kX).

    In subsequent aging the strength increases in all cases due to the decomposition of the solid solution, and the lattice constant of the solid solution increases correspondingly. Aging 40 rain at 270 ~ increases the strength from 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 time to 1.5 h leads to some increase in strength (by 5-8%), while further aging (2h) has almost no effect. The specific electr ical resistance decreases from 0.095 on aging 40 rain to 0.085 on aging 2 h at 270 ~

    The lattice parameter is the same for all drawing conditions afteraging 2 h at 270 ~ (a = 3.600 kX).

    With an increase of aging temperature to 320 ~ the strength decreases by 10-15% and the specific electr ical resistance decreases 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 microstructure is identical after drawing and after subsequent aging under the dif- ferent conditions.

    OTs 4 -3 Bronze

    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 experimentally that 170-180 ~ is the optimum aging temperature for OTs 4-3 microwire reduced over 50%. Aging at this temperature does not decrease the strength attained in drawing and has a favorable effect on the structure and the stressed state, the elastic after effect being reduced several times.

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


  • The physicomechanical propert ies o fmicrowi re after drawing at low and high temperatures were com- pared with the propert ies obtained after drawing at room temperature (methods I and I I) .

    As can be seen in Fig.2, a high drawing temperature lowers the mechanical propert ies . The highest values of strength, proport ional l imit, and elast ic ity modulus are attained after drawing at -40 ~ and the lowest values at the high drawing temperatures . The specific e lectr ical res istance is minimum (0.078 ~2.mm2/m) after drawing at 175~ which indicates the least deformation of the crysta l lattice and, conse~- quently, the lowest mechanical propert ies .

    The physicomechanical propert ies after a high drawing rate at 20 ~ (method I) are identical to those after drawing at negative temperature (method I I I) . Subsequent aging 1.5 h at 170-180 ~ ra ises the tensile strength, proport ional limit, and elasticity modu lus , and lowers the specific electrical resistance.


    Drawing of B2 and OTs 4-3 bronze microwi re at room temperature at a rate of approx imate ly I0 m/ ra in produces the highest phys icomechan ica l properties.

    1 9

    2. 3.


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