213 Materials Science and Engineering, 18 (1975) 213--220 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands Heat Treatment of Cold Extruded Polycarbonate: Some Implications for Design Engineers C.S. LEE, R.M. CADDELL and A.G. ATKINS Department of Mechanical Engineering, The University of Michigan, Ann Arbor, Mich. (U.S.A.) (Received in revised form August 6, 1974) SUMMARY tensile strength as compared with the virgin material; the same cannot be said about yield Polycarbonate was subjected to various stress. A detailed discussion regarding these combinations of mechanical - thermal histories two properties is given in the Appendix since to investigate such effects on subsequent ten- there appears to be confusion in the published sile mechanical properties. This was accom- literature regarding the meaning of these terms. plished by "cold extruding" the material ini- From a design viewpoint, the yield stress tially; nominal "reductions in area" of 18, 40 of a solid is usually of greater concern than is and 64% were used. Cold extruded bars were the tensile strength which merely defines the then heat treated at three temperature levels, maximum load carrying capacity. Thus, studies all being less than T z (150°C) of polycarbonate. which concentrate on tensile strength measure- As compared with the material that was only ments alone may not be of paramount interest cold extruded, it was found that in general, to the design engineer. heat treating tends to raise the yield stress The results reported in this paper suggest while lowering the tensile strength, elastic that the mechanical behavior of cold formed modulus and stress at fracture. The results PC, subjected to subsequent heat treatment, suggest that a desired combination of proper- may be altered to produce a combination of ties may be obtainable by the use of a cold properties most useful to the design engineer. work - heat treating sequence. It appears that the yield stress of the cold ex- truded material (extrudate) may be increased without a noticeable decrease in tensile strength. INTRODUCTION This suggests a relatively unexplored method for altering the mechanical properties of The influence of cold forming upon the polymers. subsequent mechanical behavior of various polymers has received the attention of several investigators. Cold rolling of sheet material EXPERIMENTAL PROCEDURE was used in some studies [1 - 4] as was cold extrusion [ 5,6]. Cold drawing, in its historical All test specimens were produced from a context in metal forming, has also been re- single bar of PC; it was 19.1 mm (0.750 inch) ported [6]. Increases in tensile strength of in diameter and obtained from a commercial cold rolled polymers have been reported [7] source. and biaxial cold rolling improved the deep A tensile specimen of 6.35 mm (0.250 inch) drawability of polymers [8]. Perhaps because diameter and having a 50.8 mm (2 inch) gage of its attractive combination of properties length was machined from the bar so as to (strength and ductility), polycarbonate (PC) determine the properties of interest of the "as- has been used in many of these studies. received" material. Solid cylinders of 17.1 mm When the nominal amount of "cold working" (0.675 inch) diameter and 76.2 mm (3 inches) exceeds 15 to 20%, PC displays an increase in long were machined from the supply bar pre- 214 histories were loaded at a crosshead speed of /_STEEL ROD 8.33/~m/sec (0.05 cm/min) to produce thir- teen sets of load - extension data. During the early portion of each test, an Instron extenso- meter was employed to sense length changes and to drive the recording device. In those tests where a localized neck formed and even- V / / //% !// / / //I tually stabilized, concurrent measurements EXTRUDED "-'-I Ir"~ of load and neck diameter were monitored to POLY- "~ ~--DIE OUTLET rSEE TABLE i ] CARBONATE [FOR BIAMETERSJ provide continuing information. All diameter measurements were obtained with a pair of Fig. 1. Schematic illustration of extruding operation. point micrometers. Each set of load - extension (or diameter) paratory to being cold extruded. By using data was converted to true stress - true strain three dies of varying outlet diameters, three information using the standard definitions "nominal" reductions were available; these that were about 18, 40 and 64% respectively in terms of percent reduction in area. Figure 1 a - ~ and e = In = 2 In , (1) is a schematic version of the extrusion opera- tion. Four extruded specimens were produced where L and A (or D) correspond to instanta- for each reduction, with one specimen per re- neous values of load and area (diameter) and duction used to provide a tensile specimen as Ao is the original area prior to loading. Every described earlier. The remaining three specimens test was carried to fracture. With specimens per reduction were heated for two hours at that displayed a stable neck, fracture always temperatures of 100 °, 117 ° and 140°C respec- occurred when the neck had fully propagated tively; they were then air cooled. Since the to the shoulders. Those that showed no ten- glass transition temperature (Tg) of PC is dency towards localized necking usually about 150°C, the highest temperature used fractured away from the shoulders. had to be less than Tg to prevent all effects of the prior cold extrusion from being erased. This led to an arbitrary choice of 140°C; the EXPERIMENTAL RESULTS AND DISCUSSION other two temperatures were chosen so as to give a spread of possible heat treatment effects. 1. Dimensional changes Following the heat treatment, each of these Table 1A contains information about the nine specimens was machined to produce a dimensions of specimens at various stages tensile specimen similar to that described prior to being machined for tensile tests. The above. Using an Instron machine, the tensile changes are expressed both in terms of dia- specimens of different mechanical - thermal meter and percent reduction in area from the *TABLE IB ~rcentrecovery ofdeformation ~rdifferent m~hanical-~ermM ~eatments Diam. of % Recovery at % Recovery after heat treatment extrudate room temp. (inches) 100°C 117°C 140°C 3min 48 hou~ 0.610 23.8 26.8 + 38.9 41.9 57.3 0.524 12.9 15.4 22.7 25,9 41.9 0.407 20.4 21.2 25.0 27.0 40.9 * A starting of 0.675 inch (area of 0.3578 in 2) was used in all calculations. + Sample calculation (subscripts o,e,r refer to original, extruded and recovered sizes) D o = 0.675 inch, A o = 0.3578 in 2 Area reduced = 0.3578 -- 0.2922 = 0.0656 in.2 D e = 0.610 inch, A e = 0.2922 in.2 Area of recovery = 0.3177 -- 0.2922 = 0.0255 in. 2 D r = 0.636 inch, A r = 0.3177 in 2 % Recovery = 0.0255/0.0656 (100) = 38.9. 215 • o • .~1C0 • o ° ~D u~ cO r~ C~1 w, • • ...... ° o , ° L~. i ,-I o~ o~ O ~J N -d • o o ~D Cq 00 CO ~D 00 OOO 0 C~ @ = 000~D C o~ ..= oC "0 ca o,..~ ~.~ 0 o O r.D t~. ~J o~ , ~ o~ ~J o~ ~D~D~ o. e~ odd 0 .,-, • ~ o ~ ..4 "0 ~c~c~ 0 ~0 o 0 o0 L~- r.D oo o~ c~ N O o o o o o o ~ ~ O I> tl- o 0 + od~ O ~e4d ~+ + tl. ,It + 216 pre-extruded diameter. As may be noted, the specimens show significant relaxation imme- .g 5O diately after being extruded. Although a ~-o ;0 greater recovery in the absolute diameter ac- t[+ companied the higher degree of initial cold ~0 o • oea work no consistent trend pertained in regard 0o to the percent recovery of deformation. Ad- 0• 8o # ocw eee • Bo~ @ ditional recovery may be seen for the heat co I0 o3 treated specimens, with the degree of recovery 50 ~ I- ~°ee°° • i : NOTREATMENT HEAT I-- correlating directly with temperature of heat 100"C--2 HR8 treatment. Table 1B contains the information o 117 'C-2HRS in terms of percent recovery of deformation. 140"0 --2 HRS, % o.; o.'2 or3 o.'4 a'5 0/6 o:7 2 2. Tensile true stress - true strain behavior of TRUE STRAIN non-heat treated specimens Fig. 3. Tensile true stress - true strain curves of PC Figure 2 shows the influence of the degree cold worked 18% by extrusion then subjected to va- of cold extrusion on the tensile behavior of rious thermal treatments. PC for the reductions used in this study. It may be noted that the observed behavior is creases the elastic modulus and decreases the quite similar to that for cold rolled PC where fracture strain in a consistent manner whereas similar levels of "cold working" are used. the true stress at fracture first increases but Values of tensile strength, yield stress based then begins to decrease with ever increasing upon a 1% offset, elastic modulus and the amounts of cold work. Tensile strength increases true stress and strain at fracture are listed in with cold work Table 2. Both English and SI units are included where applicable. It may be seen that small 3. Tensile true stress - true strain behavior of amounts of cold working (here 18%) lead to heat treated specimens a substantial decrease in the 1% offset yield Figures 3 - 5 show the influence of heat stress as compared with the material in the treating the extrudates that had experienced "as-received" condition (i.e. no cold work). 18, 40 and 64% cold work respectively. In all With increasing levels of induced cold working, cases, heat treating raises the yield stress however, the yield stress increases but never while lowering the tensile strength.