Lubricant behaviour during upsetting at different temperatures

Lubricant behaviour during upsetting at different temperatures S.M. Doraivelu and V. Gopinathan*

Ductile properties of metals are a function of temperature: to utilise the maximum ductility of an alloy and so reduce the number of stages required for forming, use of the optimum lubricant for the given forming temperature and the particular metal is important. To evaluate the behaviour of lubricants under different temperatures a hot upsetting device was designed. Using a 180 t friction screw press, 18/4/i alloy steel and high C - high Cr steel specimens were upset with an initial strain rate of lOs -1 at various forming temperatures up to 400°C using different lubricants

In any metal forming industry the mechanical properties of metals and alloys must be known in order to choose the correct machinery for economic operation. The mechanical properties of metals and alloys are determined by their deformation history. To determine the relevant parameters, compression tests are usually carried out because most metal forming operations, except a few such as wire drawing and stretch forming, involve compressive forces. The compression tests are conducted under plane strain conditions in order to avoid complications in predicting the load. To achieve this condition and to reduce the frictional forces between the tool and the stock, a good lubricant is necessary. At the same time the lubricant should withstand the temperature and eliminate galling or cold-welding of the stock to the tool. All these requirements may not be satisfied by a single lubricant at all temperatures since the properties of lubricants are also temperature dependent. Care must therefore be taken in selecting lubricants with all these requirements. Test materials

High carbon-high chromium steel and 18/4/1 alloy steel are used for making dies, tools and other components. The stress-strain behaviour of these steels at different temperatures and strain rates is not readily available so these steels, whose compositions are shown in Table 1, were selected for study of their stress-strain behaviour. All the specimens were machined from annealed bar stock to a height of 30 -+ 0.01 mm and a diameter of 20 + 0.01 mm. A typical surface profile is shown in Fig 3. A 1.2 mm ¢ hole was drilled to a depth of 10 mm from a cylindrical surface to insert the thermocouple wires.

Fig I The tool.set-up mounted on the press and the instrumentation, I press frame, 2 press ram, 3 top punch, 4 furnace, 5 thermocouple wire, 6 dynamometer assembly, 7 energy selection indicator, 8 hand-operated lever, 9 temperature controller, 10 temperature recorder, 11 sixchannel carrier frequency amplifier, 12 single channel carrier frequency amplifier, 13 modulator, 14 oscillomat recorder

Tooling and experimental procedure

Tooling consisted of a specially designed tubular electric furnace open at top and bottom and mounted on a press bed with a standard electrical resistance strain gauge dynamometer of 80 t capacity placed under the furnace (Fig 1). The back-up plate with the water cooling arrangement is Percentage composition of the test materials Table 1 kept over the strain gauge dynamometer so that it fits exactly into the groove provided in the furnace for alignC Si P Mn S Cr W V ment to the centre of the press bed (Fig 2). Between the back-up plate and strain gauge dynamometer plate a 10 18/4/1 alloy 0.78 0.004 0.40 - 4 . 2 0 17.92 1.2 mm thick asbestos sheet was placed to prevent heat transsteel fer from the bottom die mounted on the back-up plate High carbonin addition to the water cooling provided. A 1mm outer high chromium 1.51 0.44 0.03 0.55 0.015 11.20 -- diameter co-axial chromel-alumel thermocouple bead steel was inserted into the hole provided in the specimen and the wire was bent to 90 ° at two places (Fig 2) to keep *Metal Forming Laboratory, Department of Metallurgy, HT, Madras. it in position in the furnace. 600 036, India i

0-301-679X/79/03 t 2 3 - 0 4 $02.00 © 1979 IPC Business Press

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Each time the die surfaces were cleaned and the lubricant, grease, teflon or glass powder, was applied to both fiat surfaces of the specimen which was kept on the b o t t o m die. The top punch was kept over the specimen as shown in the figure. The punch height was adjusted in such a way as to prevent the ram touching the furnace. A linear variable differential transformer (lvdt) of -+ 50 mm range was used to measure the ram travel. The coil was fixed on the

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press frame and the core was fixed to the ram while upsetting the specimen. The other end of the thermocouple was connected to the temperature recorder and the specimen was heated to the required temperature. Once the required temperature was reached the thermocouple was disconnected from the recorder and connected to the oscillomat recorder through the modulator and the carrier frequency amplifier and the heated specimen was upset inside the furnace itself. At each upsetting, recording of the traces on the photographic paper was started by triggering the start-stop switch of a 36-channel light beam oscillomat recorder just before start of deformation.

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Fig 5 True stress~true strain values at various test temperatures when grease and Teflon were used as lubricants for 18/4/1 alloy steels

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taken and processed using a simple program to calculate the flow stress/true strain for various points.

Results and discussion Determination of true stress and true strain

True stress was obtained by dividing the load by the instantaneous area, as derived from the specimen's current height, assuming incompressible and homogeneous deformation. True strain was determined directly from the lvdt output using the formula ln(ho/hi) where ho = initial height of the specimen and h i = instantaneous height. Typical force, ram travel and temperature traces on the time base are shown in Fig 4. From this recording the data were

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When the specimens were upset dry (With no lubrication) tbey fractured near the hole provided for inserting the thermocouple. This was due to the large frictional force between the tool and stock. Figs 7 and 8 show the fractured specimens which were upset at various temperatures, The materials not able to withstand the deformation behave in a brittle manner. The no-lubricant condition is therefore not recommended for upsetting of these steels.

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Values of true stress/true strain for both the tested steels at various temperatures are shown in Figs 5 and 6 where grease and Teflon were used as lubricants. For these steels, the flow stress continued to increase from the initial value, reached a maximum and started decreasing with further true strain. This decrease in flow stress at the end of the stroke was largely due to internal heat generation and decrease in strain rate at the end. Effect of lubricants on true stress and ductility

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True stress/true strain behaviour of the steels up to 400°C

From Figs 5 and 6 it can be seen that the flow stress at all temperatures is lower when the lubricant was Teflon rather than grease. In addition, the total deformation obtained when Teflon was used is greater than that obtained wheff grease was used at all except low temperatures where the total deformation is the same in both cases. This clearly

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Fig 7 Fractured specimens o f 18/4/1 alloy steel at various test temperatures

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shows that the ductility increased when Teflon was used as lubricant. When Teflon is used barrelling is almost absent at test temperatures of 300°C, 350°C and 400°C (Fig 9).

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Effect of temperature and lubricants on true stress/true strain behaviour of the steels

True stress/true strain curves obtained at various test temperatures are shown in Figs 5 and 6 for both steels when grease and Teflon were used as lubricants. Since the energy supplied by the grease to compress the specimens was altered in such a way as to get constant initial strain rate at all test temperatures, the total deformation for each test temperature varies as shown in these figures.

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The flow stress values are plotted as a function of temperature for different strains in Fig 10 (a) and (b) for both the steels. It can be seen that a brittle range exists for high C High Cr steel between 150°C - 200°C and for high speed steel between 100°C and 200°C. Though the stress values were lower when Teflon was used at test temperatures, the rate of increase of stress due to the brittle range is greater than the rate of stress increase with grease. The curves in the brittle range show constant rather than increased stress with grease as lubricant.

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Glass lubricants

Glass lubricants were tried at temperatures above 400°C. Flow stress values using glass were lower than obtained when Teflon was used above 400°C since Teflon loses its lubricating properties above 400°C. Galling was eliminated when glass was used and the amount of barrelling was also found to be minimal compared to grease and Teflon.

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Up to 400°C Teflon was found to be a better lubricant than grease from 18/4/1 alloy steel and high C high Cr steel. The effect of Teflon lubricant is found to be minimal. Above 400°C Teflon loses its lubricating properties and glass is found to be a better lubricant for these steels.

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Fig 10 True stress variation as function of test temperature for different true strains for (a) 18/4/1 alloy steel and (b) high carbon high chromium steel

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