Industrial Lubrication and TribologyInfluence of tellurium addition on drilling of microalloyed steel (DIN 38MnS6)Eder Costa Nelis Luiz Marcio da Silva Alisson Machado Emmanuel Ezugwu

Article information:To cite this document:Eder Costa Nelis Luiz Marcio da Silva Alisson Machado Emmanuel Ezugwu, (2011),"Influence of tellurium addition on drilling ofmicroalloyed steel (DIN 38MnS6)", Industrial Lubrication and Tribology, Vol. 63 Iss 6 pp. 420 - 426Permanent link to this document:http://dx.doi.org/10.1108/00368791111168990

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Influence of tellurium addition on drillingof microalloyed steel (DIN 38MnS6)

Eder Costa

Federal Centre of Technological Education of Minas Gerais, Divinopolis, Brazil

Nelis Luiz, Marcio da Silva and Alisson MachadoSchool of Mechanical Engineering, Federal University of Uberlandia, Uberlandia, Brazil, and

Emmanuel EzugwuAir Force Institute of Technology, Kaduna, Nigeria

AbstractPurpose – This paper seeks to evaluate the influence of tellurium content on the machinability of the microalloyed pearlitic steel (DIN 38MnS6).Two grades of steels were used, one with high (27 times greater) tellurium content and one with a low tellurium content. Machinability of the steel wasdetermined by the number of holes drilled by the tool before undergoing severe deformation. The drilling test matrix was prepared using a fractionalfactorial design with five input variables studied at two levels (25-1). Other variables investigated include cutting speed (45 and 60 m/min), feed rate(0.15 and 0.25 mm/rev), geometry of the twist drills and use of minimum quantity lubrication (MQL) at the flow rates of 30 and 100 ml/h. Statisticalanalysis of the results revealed that composition of the work material was most influential on tool performance. Addition of tellurium to the steelsignificantly improved machinability, increasing the number of drilled holes by over 100 per cent. The MQL flow rate was the least influential as increasein the flow from 30 to 100 ml/h reduced drill life only by about 9 per cent.Design/methodology/approach – The drilling tests were carried out in the vertical position, up-down, without pre-holes (full drilling). Cuttingspeeds of 45 and 60 m/min and feed rates of 0.15 and 0.25 mm/rev were employed. Drills with two sharpening types were tested. Cutting fluid usedwas vegetable based and applied using the MQL technique at flow rates of 30 and 100 ml/h. The rejection criterion adopted was severe deformation ofthe drills and the number of machined holes was used to measure the machinability of the material.Findings – Of all the variables investigated in this study, the least influential on drill performance is the MQL flow rate. Increase in the flow rate from30 to 100 ml/h reduced drill performance by 9 per cent, contrary to expectation. This is a result of the cooling-lubricant action balance promoted by thecutting fluid applied in low quantities (MQL). The most influential variable on drill performance is addition of Te to the work material which gave overtwofold (103 per cent) improvement in drill performance at the cutting conditions investigated. The Te particles act at the chip-tool interface, reducingthe work necessary to shear the material during chip formation. Increase in both the cutting speed and the feed rate both lowered drill performanceduring machining due to associated increase in cutting temperature which tended to accelerate thermally related wear mechanisms.Originality/value – This work was conducted to evaluate the machinability of a novel alloyed steel employed in the automobile industry. Drilling wasconsidered as most automobile components especially the engine block is designed with many holes which require drilling process.

Keywords Microalloyed steel, Tellurium addition, Minimum quantity lubrication (MQL), Drilling, Steel, Automotive industry

Paper type Research paper

1. Introduction

Microalloyed steels are C-Mn steels, with variable composition

of carbon. They contain small addition (typically less than

0.15 per cent) of alloying elements with great affinity for carbon

and nitrogen. Frequently used alloying elements include

niobium, vanadium, titanium, etc. These steels acquire their

properties of high-mechanical resistance, toughness and

ductility from the combined effects of their chemical

composition and thermo-mechanical treatment without

undergoing subsequent thermal treatments such as tempering

and annealing. They have a wide range of applications in the

automotive industry (Wright, 1990; Aborn, 1977; Pickering

and Garbaz, 1989).

The presence of free cutting additives, such as Pb, Se, Bi, Te,

MnS and MoS2, in steel can improve their machinability.

These additives form a protective layer on the surface of the

tool, reducing the friction force during machining by acting as

lubricants (Trent, 1988). The first element used to improve the

machinability of steels is sulphur which combine with

manganese to form MnS. This compound has the property of

improving machinability by increasing tool life, reducing the

cutting forces, increasing cutting speed and improving the

surface finish of machined components (Troiani, 2005;

Evangelista Luiz, 2007). Little use was made of steels

containing sulphur up to the First World War. The

production of steels with additives to improve machinability

increased significantly during the Second World War.

The search for improved performance during machining led

to the use of lead (Pb) as a new additive in 1939. Pb is an

element with zero solubility in iron at room temperature, thus

promoting precipitation of isolated metallic or MnS associated

inclusions (Aborn, 1977). However, the use of Pb as additive in

steels has some disadvantages such as:. the density of Pb is higher than that of iron, with

associated strong tendency to segregate; and

The current issue and full text archive of this journal is available at

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Industrial Lubrication and Tribology

63/6 (2011) 420–426

q Emerald Group Publishing Limited [ISSN 0036-8792]

[DOI 10.1108/00368791111168990]

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. Pb is a poisonous element with related health hazards

involved (Barretos et al., 1999).

Bismuth presents properties similar to those of lead. It was first

used to improve machinability in Pray et al. (1941). An

advantage of bismuth over lead is that is not poisonous

(Somekawa et al., 2001) and has lower density, thus reducing

the tendency to segregate with improved dispersion of the

metallic inclusions (Evangelista Luiz, 2007). Between1932 and

1935 four patents were used to describe the use of selenium to

improve themachinability of steels as substitute for sulphur in a

variety of applications from carbon steels to stainless steels

(Palmer, 1932-1935). This was followed by several commercial

developments of steels with selenium.The use of tellurium as an additive to improve machinability

of steels as an alternative to selenium and as a substitute for

sulphur was presented in 1932 (Aborn, 1977). The inventors’

preference to use primarily selenium and tellurium as additive

elements was forgotten until 1962 when tellurium was

incorporated to a resulphurized steel with lead addition. Both

Se and Te have similar mechanisms for improving

machinability. They control the morphology of the MnS,

reducing their hot deformability. TheMnS can be enveloped by

either MnTe or PbTe (in leaded steel), which form eutectics

withMnS (Figure 1). This way, the liquid envelope absorbs the

high stress and restricts deformation of theMnS. However, not

all sulphides are enveloped by tellurides and, regardless of themain action mechanism, the Te will promote spheroidizing of

the sulphides. This action, apart from reducing the anisotropy

of the mechanical properties of the material, improves

machinability (Evangelista Luiz, 2007).Despite the importance of cutting fluids in machining

operations (increased tool life, surface finish improvement,

removal of chips from the cutting area, dimensional control,

etc.), a lot have been done to eliminate them in product

manufacture lately due to the high-operational costs and

associated ecological as well as environmental and health

issues (Tasdelena et al., 2007; Heisel et al., 1998). Dry

machining is employed as an alternative to the use of

conventional cutting fluid in machining operations. When this

is not technically viable, as in the case of the drilling process,

the use of minimum quantity lubrication (MQL) is preferred.

These alternatives are becoming more viable due to the

introduction of new technologies such as the increasing use of

materials with improved machinability as well as advances in

the development of the novel materials, coatings and tool

geometries (Miranda et al., 2001; Kubel, 1998; Teeter, 1999).The results of countless research in recent years indicate

that the MQL technique is an established alternative to

conventional application of cutting fluid in drilling processes

with encouraging results, especially for smaller diameter drills

employed for drilling deep holes (Weinert, 1999; Heinemann,

2004; Heinemann et al., 2006). The MQL system can be

defined as the spraying of a minimum amount of lubricant in

a flow of compressed air (Machado and Diniz, 2000).

According to Sahm and Schneider (1996) the flow of the

system usually varies from 10 to 100ml/h and the air pressure

from 4 to 6 kgf/cm2. This mixture (air and oil), with minimum

amounts of fluid, is enough to substantially reduce friction at

the tool to avoid material adherence and to expel the chips

from the cutting area as well as moderately cooling the tool-

workpiece set up (Costa, 2004).The primary objectives of this study is evaluation of the

influence of tellurium content and the flow rate of the cutting

fluid (vegetable based) on the machinability of the

microalloyed steel DIN (38MnS6).

2. Experimental procedure

The drilling tests were carried out in the vertical position,

up-down, without pre-holes (full drilling). Cutting speeds of 45

and 60m/min and feed rates of 0.15 and 0.25mm/rev were

employed.Drillswith twosharpening typeswere tested.Cutting

fluid used was vegetable based and applied using the MQL

technique at flow rates of 30 and 100ml/h. The rejection

criterion adoptedwas severe deformation of the drills (Figure 2)

and the number of machined holes was used to measure the

machinability of the material (NT MECH 038, 1997). The

following strategy was adopted for the drilling tests. All the first

tests as specified in the cutting conditionswere identified as trial

A and all the repeated tests at the same cutting conditions were

identified as trial B. If the difference in the number of holes

produced between trials A and B is greater than 20 per cent, a

third test (trial C) was carried out. Tool life attributed to the test

was calculated as the arithmetic average of the number of holes

for all the drilling trials in each test.The drills used is the M42 (10 per cent Co) HSS with TiN

coating (10HSS-Co.TiNw) with a diameter of 10mm, helix

and point angles of 30 and 1308, respectively. These drills had

Figure 2 Severely deformed drills at the end of tool life

Drill type A Drill type B

Figure 1 A steel micrograph showing an inclusion of sulphidesurrounded by manganese telluride

Source: Costa (2004)

Influence of tellurium addition on drilling of microalloyed steel

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two different types of sharpening, identified as TiN-A and

TiN-B (Figure 3). The geometric detail “gash” observed in

the TiN-B drill helps to improve the self-centering property of

the drills.The machine tool used for the drilling operation is a Vertical

CNC Machining Centre, with main motor power of 9 kWand

maximum spindle speed of 10,000 rpm. Blind holes of 30mm

long, resulting to a length/diameter ratio of 3 (L/D ¼ 3), was

always adopted. Integral biodegradable non-toxic cutting fluid

was used for the drilling tests. The cutting fluid is insoluble in

water, consisting mainly of vegetable oils (Soya, corn and

canola) and anticorrosion additives. This fluid was externally

applied in small amounts (MQL), at the flow rates of 30 and

100ml/h. Table I shows typical characteristics of this oil.The apparatus used to spray the fluidworkswith a continuous

flowof compressed air, set to about 4.3 bar, and an intermittent

spray of oil at the rate of 1 pulse/s. The cutting fluid was carried

by a hose of smaller diameter reaching the nozzle, inside another

hose carrying the compressed air. The mixture of compressed

air-fluid was injected externally on the tool-workpiece interface

using two nozzles. Figure 4 shows the positions of the MQL

nozzles used. Considering that the flow of the cutting fluid in

MQL systems usually varies from 10 to 100ml/h (Sahm and

Schneider, 1996; Costa, 2004), two flow rates were chosen to

study their influence on tool life: one closer to the lower limit

(30ml/h) and the other at the upper limit (100ml/h).The work material is a microalloyed pearlitic steel, grade

DIN 38MnS6, supplied in two different versions. One with

higher (0.0027wt%) tellurium content and the other with

very low tellurium content (0.0001wt%) as shown in the

italicized column of Table II. The average hardness of the

cross section of these steels, designated as “with tellurium”

and “without tellurium”, were 258 and 252HV, respectively.

Figures 5 and 6 show the microstructure of the steels without

and with tellurium, at the centre and in an outlying area

(4mm from the border), respectively. It can be noted that the

microstructures are similar, with no significant changes from

the centre to the border and from one to the other steel.The work material used for machining had nearly square

cross sections (100 and 65mm height). This dimensions

ensured drilling on both faces since the length of the holes is

of 30mm. To optimize the number of holes, a CNC-program

was generated to allow 67 holes per face, in alternate rows of

seven and eight holes. The lateral distance and the spacing

between the bottoms of the holes in the workpiece were 2.11

and 5mm, respectively.The parameters that were varied during the tests, with their

respective levels, are given in Table III.The combination of all the variables investigated would be

equivalent to 32 tool life tests (25). In agreement with the

strategy adopted where all the tests would have to be repeated

at least once, the number of experiments would be in excess of

64, demanding extra resources of material, tools and machine

hours. In order to minimize the number of drilling tests

required the use of a fractional factorial design at two levels

(2521) (Werkema and Aguiar, 1996), giving 16 experiments

with resolution to isolate the main effects of the input

variables from interactions of any two factors. Table IV shows

the cutting conditions employed in the tests performed.

3. Results and discussions

TableV shows results of thedrilling tests. It canbe seen that tests

numbers 5, 8, 12, 13 and 14 underwent two repetitions. The

statistical results for the 2521 fractional factorial design are

presented in Table VI. This table shows the average effect of

each variable on tool life when there is variation from the lower

to theupper levels.The average life in this table is the average life

obtained from all the runs showed in Table V for each test.Analyses of data contained in Table VI suggest that:

. The average life for the drilling tests is 206 holes.

. Increase in the MQL flow rate from 30 to 100ml/h

reduced drill performance average by 18 holes

(approximately 9 per cent relative to the average life).

Figure 3 Geometric aspects of the TiN-A and TiN-B drills

TiN-A TiN-B"gash"

Source: Costa (2004)

Figure 4 Machined workpiece and position of the MQL nozzles

Source: Costa (2004)

Table I Typical characteristics of the vegetable oil

Property Value

Density, g/ml (20/238C) 0.900-0.940

Boiling point .1008C

Flash point .3008C

Source: Costa (2004)

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. Increase in cutting speed from 45 to 60m/min reduced

drill performance by 144 holes (70 per cent).. Increase in feed rate from 0.15 to 0.25mm/rev reduced

drill performance by 170 holes (83 per cent).. Changing the drill from TiN-B to TiN-A increased the

tool life by 61 holes (30 per cent).

. Significant increase in the tellurium content to

0.0027 wt% in the work material increased drill

performance by 212 holes (103 per cent).

Figure 7 schematically shows the direction and the increasing

order of influence of the variables on drill performance at the

cutting conditions investigated.From the input variables analysed, variation of the MQL

flow rate is the least influential (reduction of drill life by

around 9 per cent) while addition of significant amount of Te

in the work material is the most influential with 103 per cent

improvement in drill performance.Similar cooling capacities were expected for the two MQL

flow rates evaluated, since the flow rate of the compressed air

and pressure (,4.3 bar) responsible for cooling are similar for

all the tests. In terms of lubrication efficiency of the fluid flow

rates, studies show that the area of contact at the chip-tool

interface is very small, suggesting that the necessary flow rate

Figure 5 Microstructure of the microalloyed steel (DIN 38MnS6) without tellurium

50 µm 50 µm

Notes: (a) Central area; (b) outlying areaSource: Costa (2004)

Figure 6 Microstructure of the microalloyed pearlitic steel (DIN 38MnS6) with tellurium

50 µm 50 µm

Notes: (a) Central area; (b) outlying areaSource: Costa (2004)

Table II Chemical composition of high-strength low-alloy steel grades (DIN 38MnS6)

Composition (wt%) C Mn P S Si Ni Cr Mo V Al

Without tellurium 0.3960 1.4400 0.0180 0.0650 0.5900 0.0500 0.1300 0.0200 0.0040 0.0040

With tellurium 0.3800 1.5000 0.0240 0.0610 0.5400 0.0600 0.1800 0.0300 0.0040 0.0060

Composition (wt%) Cu Pb Ti Nb B Sn Ca H2 N2 Te

Without tellurium 0.1000 0.0030 0.0021 0.0050 0.0008 0.0050 0.0006 0.0002 0.0171 0.0001With tellurium 0.1500 0.0020 0.0017 0.0050 0.0007 0.0070 0.0005 0.0002 0.0159 0.0027

Table III Variables used in the machining tests

Level/value

Parameters Lower Upper

Material Without tellurium – WO/T With tellurium – T

MQL flow rate (ml/h) 30 100

vc (m/min) 45 60

f (mm/rev) 0.15 0.25

Tool TiN-B TiN-A

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to promote lubrication can also be very small. Machado and

Wallbank (1997) made theoretical calculations for the

necessary flow rate to guarantee lubrication by at least a

molecular layer of oil at the chip-tool contact surface,

considering an efficiency of only 1 per cent and arrived at the

value of only 0.1ml/h. From this, the two oil flow rates

employed (30 and 100ml/h) would be adequate to lubricate

the contact area, provided that there is sufficient access.

These facts justify the minimum influence of the MQL flow

rate on drill performance.Variation of the drill geometry from TiN-B to TiN-A

increased drill performance by 30 per cent. Drill type TiN-A

has large main lip relief surface area than drill type TiN-B (by

about 22 per cent). This suggests that distribution of the heat

generated at the cutting area can be different for the two drill

types. Since drill type TiN-B has smaller surface area, it tends

to absorb more heat per unit area leading to an increase in

temperature at the cutting interfaces. The higher

temperatures generated will not only accelerate thermally

related wear mechanisms, but also reduce the shear resistance

limit of these tools (Machado and Da Silva, 2004).Increase in cutting conditions means that larger amount of

material passes through the flow area, consequently increasing

temperature developed in the cutting tool, assuming the

inevitable adherence conditions at the chip-tool interface

(Trent, 1988, 2000). Variation of the cutting speed from 45 to

60m/min (33.3 per cent increase) reduced the average drill

performance by 144 holes. This represents a reduction of 4.3

holes per percentile point increase in the cutting speed.

Variation in the feed produced a reduction of 2.6 holes in drill

performance per percentile point increase in feed. These

figures demonstrate the greater influence of the cutting speed,

relative to the feed, on the performance of the HSS drills

investigated.Of the five variables investigated, chemical composition of

work material was the most influential when drilling the

microalloyed steel. The tellurium rich microalloyed steel gave

more than twofold increase in drill performance. Lower

hardness is a desirable property of work materials as this leads

to lower cutting temperatures and consequently lower wear

rate and improve machinability. However, lower hardness is

usually associated with higher toughness, which can

deteriorate surface finish (Machado and Da Silva, 2004).

The hardness of the work materials, measured along the cross

section, are 252 and 258HV for steels without tellurium and

with tellurium, respectively. Results obtained from the drilling

tests suggest that the higher hardness of the Te rich

microalloyed steel did not affect drill performance, contrary

to expectation.Analysis of the chemical composition of the steels investigated

(Table II) shows that composition of some chemical elements

that increase the material resistance are within the same range

for the twoworkmaterial grades. By increasing the resistance of

the work material, a greater amount of work will be required to

shear the material, generating higher temperature at the chip-

tool interface and setting off the thermally activated wear

mechanisms that will accelerate wear of the cutting tools

(Machado and Wallbank, 1997). Tellurium is responsible for

optimization of the microalloyed steel. Therefore, substantial

increase inTe contentwill lead to significant improvement in the

machinability of the microalloyed steel as illustrated in the

drilling test results. The tellurium tends to modify the shape of

Table V Data obtained from the drilling tests at various conditions

Run Run C Average life

Test A B Deviation (%) (deviation > 20%) (holes)

1 596 729 18.2 – 6632 77 73 5.2 – 753 171 156 8.8 – 1644 128 150 14.7 – 1395 31 23 25.8 32 296 310 373 16.9 – 3427 153 153 0.0 – 1538 61 107 43.0 102 909 122 141 14.1 – 132

10 306 368 16.8 – 33711 185 160 13.5 – 17312 597 461 22.8 640 56613 133 68 48.9 98 10014 85 62 27.1 65 7115 250 209 16.4 – 23016 29 35 17.1 – 32

Table IV Cutting conditions employed in each drilling test

Test

Flow rate MQL

(ml/h)

Cutting speed

(m/min)

Feed rates

(mm/rev) Drill Material

1 30 45 0.15 TiN-A T

2 100 60 0.15 TiN-A WO/T

3 100 45 0.25 TiN-B T

4 100 45 0.15 TiN-B WO/T

5 30 60 0.25 TiN-B WO/T

6 30 60 0.15 TiN-B T

7 30 45 0.25 TiN-A WO/T

8 100 60 0.25 TiN-A T

9 30 45 0.15 TiN-B WO/T

10 100 60 0.15 TiN-B T

11 100 45 0.25 TiN-A WO/T

12 100 45 0.15 TiN-A T

13 30 60 0.25 TiN-A T

14 30 60 0.15 TiN-A WO/T

15 30 45 0.25 TiN-B T

16 100 60 0.25 TiN-B WO/T

Table VI Effect of each input variable on the average life of the drills

Number of holes

Average life

(holes)

Flow rate MQL

30-100 (ml/h)

Cutting speed

45-60 (m/min)

Feed 0, 15-0, 25

(mm/rev)

Tool

TiN-B-TiN-A

Material

WO/T-T

Effect 206 218 2144 2170 þ61 þ212

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the sulphides, changing them from oblong to smaller globules

which considerably improves the machinability of the material

(Evangelista Luiz and Machado, 2008). The slight addition of

tellurium (0.0027wt%) ensured improved machinability of the

microalloyed steel without jeopardizing its mechanical

properties.

4. Conclusions

1 Of all the variables investigated in this study, the least

influential on drill performance is the MQL flow rate.

Increase in the flow rate from 30 to 100ml/h reduced drill

performance by 9 per cent, contrary to expectation. This

is a result of the cooling-lubricant action balance

promoted by the cutting fluid applied in low quantities

(MQL).2 The most influential variable on drill performance is

addition of Te to the work material which gave over

twofold (103 per cent) improvement in drill performance

at the cutting conditions investigated. The Te particles act

at the chip-tool interface, reducing the work necessary to

shear the material during chip formation.3 Increase in both the cutting speed and the feed rate both

lowered drill performance during machining due to

associated increase in cutting temperature which tend to

accelerate thermally related wear mechanisms.

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Figure 7 Order of influence of the variables on drill performance

ORDER OF INFLUENCE OF THE VARIABLES

MQL flow rate30 → 100

(ml/h)

9%

Cutting Speed45 → 60(m/min)

70%

Feed0.15 → 0.25

(mm/rev)

83%

ToolTiN-B → TiN-A

30%

WorkpieceMaterial

WO/T → T

103%

Influence of tellurium addition on drilling of microalloyed steel

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Industrial Lubrication and Tribology

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Corresponding author

Emmanuel Ezugwu can be contacted at: eoezugwu@gmail.

com

Influence of tellurium addition on drilling of microalloyed steel

Eder Costa et al.

Industrial Lubrication and Tribology

Volume 63 · Number 6 · 2011 · 420–426

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