Development of Water-soluble metal working fluids and study of their wear properties

Chidambaram CT, L.A.Kumaraswamidhas, Suman Dutta*

Abstract
Metalworking chemistry evolved from simple oils to water-based technology. The tool life was extended by reducing wear by metal working fluids. This study shows parameters affecting on Wear Scar Diameters (WSD) with different water soluble metal working fluid having different mineral oil and phosphorous EP additive content. The optimum mineral oil and phosphorous EP additive content of this metal working fluid has been estimated using 2-level factorial design. The Mean Ware Scar Diameter (MWSD) is determined experimentally. The test is performed at specific speed, load and temperature adopting four balls tribology test method as per ASTM D4172 test procedure. The results from wear tester indicate the role of mineral oil content and mixture of EP additives. The lubricants with poor wear results possess excellent extreme pressure qualities.
Keywords : Water-soluble metal working fluid, wear scar, four ball tester, 2-level factorial design
Introduction
Metal Working fluid’s principal functions are to accelerate heat dissipation with increased tool service life, lubrication between tool, chip and workpieice with reduced tool wear and improvement of the work piece surface finish. The fluids cool and lubricate the metal – tool interface [1]. Metal working fluid requirements will be removal of heat, transport of metal chips and lubricate between tool and metal. Water soluble metal working fluids and Neat oils are two types of working fluid. The simple aqueous solutions provide excellent cooling but are not adequate for maintaining the good lubrication. Under such circumstances, a compromise is achieved by the use of soluble oils that consist of mineral oil together with EP compounds. These soluble oils function satisfactorily in most of the operations but their EP activities remain to a limited extent [3]. The main advantages of water-based fluids are that they resist the growth of microorganisms and the development of objectionable odors. The functional additives contribute to the total composition which includes emulsification, corrosion inhibition, lubrication, microbial control, lubrication, pH buffering, coupling, defoaming, dispersing and wetting.

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In order to understand the effects of wear, researchers are able to understand the reaction of two moving surfaces between the metal working fluid and the material [4]. For minimizing the wear, the lubricant plays important role to increase the life time of mechanical components. Extreme Pressure (EP) additives such as sulphur, chloride and phosphorous actually form metal complexes with the metal surface at elevated temperatures. Any of the EP additives or in combination used in formulation of water soluble metal working fluids to impart good lubricity. Wear to the cutting edge is usually accompanied by rounding of the tool’s tip and permissible wear width is an indicator of tool service life [6]. For this paper, 5% of different water soluble metal working fluids employed to Four ball Tribotester, following the ASTM Standard D4172 [7]. The analyses were focused on the wear scar diameter and the results showed that having poor wear results have excellent extreme pressure qualities. The optimum value of mineral oil and phosphate esters were estimated by 2-level factorial design Level Factorial Design.
2-Level Factorial Design
A 2-level factorial design with two factors have been used for this study. The factor considered here are concentration of mineral oil and phosphorous EP additives. The design points are shown Fig. 1 and values of independent variables at two levels are given in Table 1.
Materials and Methods.
The following section illustrates the experimental procedure and the materials and equipment required for this purpose
Materials
Chrome alloy steel standard balls as per AISIE-521000 of 12.7mm diameter, extra polish grade 25 of Rockwell C Hardness 64 – 66 HRC. Every test, new four balls were used. These balls were cleaned by using Acetone and wiped by fresh lint before starting the test.
Water Soluble Metal Working Fluids
The function of Metal working fluids are to aid the cutting and grinding operations with good finish and maintain the work piece quality by extending the life of machine tools. The chemical additives are used to formulate metal working fluid will contribute for stability, lubricity and microbial control. Water soluble metal working fluids with different mineral oil content such as 40%, 43%, 55%, 70% and 90% were defined as test candidates for the study to represent the entire performance bandwidth of the metal working fluids. The above concentrates are mixed with water at 5% concentration are for this test.
Water soluble metal working fluid concentrates with 40% mineral oil content was subjected phosphorous EP Additives dosages with 0.01%, 0.02% and 0.03% for providing extreme pressure lubricity. These concentrates are diluted with water at 5% and conducted wear study as per ASTM D 4712.
Apparatus
Four-ball wear tester used for investigation of understanding the characteristics of lubricants. The above tester uses four balls. Three balls are firmly held in a ball pot which contains the lubricant to be tested and are pressed with one ball on the top. The top ball rotated at 1200 rpm on the three balls at the bottom.
The four-ball tribotester machine TR-30L manufactured by Ducom India. This tester is used to measure the wear scar. The oil cup assembly, collect and the ball bearings are the important components shown in the figure 2. Before conducting the test, these components are cleaned with acetone.
Test Procedures
Steel ball bearings were placed in the ball pot assembly and tightened by using a torque wrench such that bottom steel balls prevented from moving during the testing. In the collector, the spinning ball at the top is locked and tightened onto the spindle. The test metal working fluid fed into the ball pot assembly. The assembly is installed into disc in the four ball machine. The test load of 392N (40 kg) applied slowly to avoid shock loading. The metal working fluid being tested was heated to 75oC by the tribotester’s built-in heater. Set the drive motor to drive the top ball at 1200 rpm about one hour when the temperature reached. Oil cup assembly removed after one hour by switching off the heater. The metal working fluid under test drained from the oil cup. The steel balls were wiped by using lint free industrial wipe. The surface of the bearings was captured and wear scar measured by using microscope.
Result and discussion
With the special microscope, the wear scar diameters of bottom ball bearings were measured and the mean values were calculated. Table 3 shows the Mean Wear Scar. Mean wear scar diameter increases with the load gradually. The mineral oil content plays important role in wear and shown in Table 3 and figure 3 and it has been observed that wear scar value reduced with increase in mineral oil content in the water soluble metal working fluid concentrate. The relation between MWSD and MO content can be represented by the Equation 1 that has R2 = 0.9684
MWSD = 0.00003(MO)2 – 0.0081(MO) + 1.1257 (1)
As our aim is to develop water based metal working fluid using minimum MO content. Further down, 40% mineral oil content concentrate taken for study and imparted with Phosphate additives and mean wear scar diameter found. It was observed by increasing the antiwear additive, the MSWD decreases as shown in Table 2. Data in Table 2 have been used to develop the 2-level factorial design. The surface and contour plot of this design are given in Fig. 4 and Fig. 5 respectively.
The software MINITAB 15 was used for the 2-level factorial design and Table 4 and Table 5 represent the result of the analysis. The regression equation is given by Eqn.2.
MWSD = 0.7075 – 0.0675xMO – 0.0575x PE + 0.0275 x MO x PE(2)
The result of “Analysis of Variance” is given in Table 5. This table gives the value of various errors and the effect of various terms. The response MWSD have been optimized using the MINITAB 15 software. The study shows that 0.03% phosphate EP additive and 90% MO gives the best result of 0.61 within the experimental region. However, we are using 40% MO as discussed earlier. Therefore, the 40% MO and 0.03% phosphate EP additive gives the MWSD of 0.69. The optimum value can be calculated from the Fig. 4 and Fig. 5 based on the maximum allowable MO content.
Conclusion
The tribology behavior of different metal working fluids at load (392N) were evaluated by Four ball tribotester machine and the results were compared. From the results, it can be concluded as
1.From the observation of wear scar condition, all the metal working fluids show abrasive wear.
Severe adhesive wear found only in metal working fluid where there is low mineral oil content
and rest of the samples found good.
2.Phosphate esters containing metal working fluid tend to impart anti-wear property to metal
working fluids and with increase in quantity will leads to instability.
3.There was the general tendency for the wear, as indicated by the increased scar diameter, to
increase the extreme pressure properties were improved.
4.Incorporation of antiwear additives for mineral oil containing 90% water soluble working fluid
leads to instability.
5.A combination of 40% MO and 0.03% phosphate EP additives can be used production of water
based metal working fluid.