TECHNICAL PAPER

Correlating tool wear, surface roughness and corrosion resistancein the turning process of super duplex stainless steel

Carlos Ancelmo de Oliveira Junior •

Anselmo Eduardo Diniz • Rodnei Bertazzoli

Received: 3 August 2013 / Accepted: 27 November 2013

� The Brazilian Society of Mechanical Sciences and Engineering 2013

Abstract Super duplex stainless steels are extremely

corrosion-resistant alloys designed for very demanding

applications that expose them to corrosive environments,

such as seawater. Due to their chemical composition and

microstructure, which provide high mechanical strength

and thermal resistance as well as high ductility, the

machinability of these alloys is generally poor, resulting in

long production cycles and high tooling costs. Moreover,

machining may be harmful for the corrosion resistance of

the alloy. The goal of this research is to study the turning

operation of UNS 32750 alloy, known commercially as

SAF 2507, and its influence on the alloy’s corrosion

resistance in practical applications. Tests were performed,

using cutting speed and cooling conditions with low and

high fluid pressure as the input variables. The results

indicate that turning with PVD-coated inserts under high-

pressure cooling resulted in long tool lives, good workpiece

roughness and high corrosion resistance of the material

after machining. The most frequent wear mechanism found

during the tests was notch wear, while the main tool wear

mechanism was attrition.

Keywords Machining � Super duplex stainless steels �Pitting corrosion � High-pressure cooling

1 Introduction

1.1 Duplex and super duplex stainless steels:

properties, machinability and corrosion resistance

Stainless steels form a specific group of materials that are

used in a variety of industrial applications. The main fac-

tors that make these materials useful for the production of

components for the chemical, food and aerospace indus-

tries, which require long-life components, are their high

corrosion resistance and mechanical strength. Duplex

stainless steels, the main focus of this research, currently

represent only about 1 % of the total worldwide application

of stainless steels [1]. However, the increasing demand

resulting from the growth of the oil and gas industry has led

to a corresponding trend for the wider use of this material.

Duplex stainless steels are Fe–Cr–Ni–Mo alloys con-

taining up to 0.30 % weight of nitrogen and presenting a

biphase microstructure consisting of roughly 50 % of fer-

rite and 50 % of austenite in volume. To be considered a

duplex stainless steel, an alloy must have a Pitting Resis-

tance Equivalent (PRE) index higher than 20. The so-called

super duplex stainless steels must have a PRE above 40 [2].

The PRE is calculated using Eq. 1 (percentage in weight):

PRE ¼ %Crþ 3:3 %Moþ 0:5 %Wð Þ þ 16 %N:ð1Þ

The combination of elements such as chromium,

molybdenum and nitrogen provides good chemical

stability in saline environments like seawater. Therefore,

duplex stainless steels can safely replace other alloys with

high tin, chromium or nickel content [2].

Due to their biphase microstructure, duplex stainless

steels present a good combination of the properties

Technical Editor: Alexandre Mendes Abrao.

C. A. de Oliveira Junior � A. E. Diniz (&) � R. Bertazzoli

Faculty of Mechanical Engineering, University of Campinas,

Rua Mendeleiev, 200, Campinas, SP 13083-860, Brazil

e-mail: anselmo@fem.unicamp.br

C. A. de Oliveira Junior

e-mail: carlos.ancelmo@sandvik.com

R. Bertazzoli

e-mail: rbertazzoli@fem.unicamp.br

123

J Braz. Soc. Mech. Sci. Eng.

DOI 10.1007/s40430-013-0119-6

characteristic of ferritic and austenitic steels: high

mechanical strength, good toughness and high corrosion

resistance in several environments [3]. These micro-

structures and properties are obtained by increasing the

chromium, molybdenum and nitrogen content when

compared with austenitic steels. These three elements

increase the corrosion resistance, while nitrogen also has

a favorable effect on mechanical strength. These steels

exhibit particularly good thermal resistance when exposed

to temperatures of up to 400 �C, i.e., double that of au-

stenitic steels. However, their resistance to fluency is low

[3].

Due to the high hardening rate and hardness of duplex

stainless steel alloys, the lives of the tools employed to

machine them are shorter or the alloys’ production times

are longer than in the machining of austenitic alloys.

Moreover, the high mechanical strength (high yield stress)

of duplex and super duplex alloys means they require more

energy for chip removal, thus increasing the cutting forces

and heat generated during machining and further shorten-

ing the tool’s life. Increasing the content of elements in the

alloy (particularly nitrogen, which increases the alloy’s

ductility) causes a corresponding decrease in its machin-

ability [4]. Generally, therefore, the higher the PRE, the

lower the alloy’s machinability.

Compared to other stainless steel alloys, the tool life

achieved when machining duplex alloys is about 30 % of

that obtained with ferritic alloys, due to their high yield and

tensile strength. A higher amount of ferrite, i.e., above

60 %, improves the alloy’s machinability. The chips

formed are resistant and abrasive to the tool; this is more

critical in the grades with higher amounts of alloy ele-

ments, such as super duplex, which may generate high

cutting forces [5]. Attempts to increase the PRE index

often cause the material’s machinability to decrease [4].

Therefore, high cutting forces and fast tool wear are typical

factors in the machining of duplex and super duplex

stainless steels [6].

Duplex stainless steels are produced with the lowest

possible sulfur content in order to preserve their corrosion

resistance [6], which, on the other hand, is deleterious for

the machinability, since in other stainless steel alloys,

sulfur combines with manganese to form manganese sul-

fide particles, which contribute to breaking the chip,

decrease the friction coefficient, and thus, improve

machinability.

The biphase structure (austenite–ferrite) of duplex

stainless steels contributes to induce vibrations during

cutting, further increasing the difficulty in attaining good

surface roughness and long tool life. The phases are dis-

tributed randomly and each phase has different properties.

Therefore, each phase contributes differently to chip for-

mation and material removal during machining [2]. This

means that more powerful machine tools and more rigid

fixation of the tools and workpieces are required to over-

come the tendency for vibration in the machining of duplex

stainless steels. In this work, everything (machine, tool,

workpiece and fixations) was very rigid for the finish

turning operations carried out. Therefore, this vibration

tendency was not verified.

High mechanical strength, high ductility and low ther-

mal conductivity favor several tool wear mechanisms, such

as diffusion, abrasion, attrition and built-up edge (BUE),

which shorten tool lives when compared to the machining

of common steels [7]. The most common tool wear

mechanisms in the machining of duplex stainless steels are

abrasion and diffusion, which are favored by high cutting

temperatures, causing flank and crater wear. Notch wear at

the end of the contact between cutting edge and workpiece

(end of the depth of cut) also occurs due to the oxidation

resulting from the difference in temperature between the

tool region in contact with the workpiece and its sur-

roundings outside the cut, and is also caused by the friction

of highly hardened burrs. High ductility causes the com-

pressed chip material to form a burr at the end of the depth

of cut. Because this material also has a high hardening

ratio, this burr is very hard and generates a furrowing effect

in this region of the tool, causing considerable notch wear

[8]. Plastic deformation and edge chipping caused by the

impact against chips are also observed. Small tool entering

angles are preferable to prevent/minimize burr formation

and notch wear, since this type of tool increases the length

of contact between tool and workpiece, improving the heat

and load distribution [5].

Hassiots and Petropoulos [9] stated that pitting corrosion

resistance is related to the roughness of a machined sur-

face. The lower the roughness, the higher the corrosion

resistance. Bramam et al. [10] correlated the stress-corro-

sion resistance of a machined surface with its residual

stress. Corrosion resistance increases as the residual stress

introduced during machining decreases.

Gravalos et al. [11] carried out pitting corrosion tests on

turned super austenitic stainless steel surfaces, evaluating

the influence of surface roughness and residual stress of

these surfaces on pitting formation. They turned several

workpieces using different values of feed, depth of cut and

cutting speed and verified that no variable exerted a sig-

nificant effect on residual stress, while feed exerted strong

influence on surface roughness. After that, they submitted

these turned surfaces with different values of roughness

and residual stress to a pitting corrosion test and concluded

that surface roughness is more important than residual

stress in pitting formation. Low surface roughness con-

tributed in reducing pitting, since the higher peaks of the

surface roughness profile contributed to make the pitting

formation easier.

J Braz. Soc. Mech. Sci. Eng.

123

1.2 Cooling and lubrication using high-pressure fluid

in machining

The use of high-pressure cooling/lubrication is a technique

which may have good results when machining materials

with low machinability, such as stainless steels and heat-

resistant alloys. When conventional application of the fluid

is used, it spreads over the cutting zone under low pressure

and often does not suffice to ensure long tool life and low

cutting forces. A more efficient application of cutting fluid

using properly directed jets of fluid under high pressure

(70 bars or more) may result in low heat generation and

better heat dissipation, possibly delaying the progression of

tool wear and facilitating chip break-up [5].

Cutting fluid is conventionally applied on the chip

(position A in Fig. 1). In high-pressure cooling/lubrication

systems, the fluid jet is directed either toward the chip–tool

rake face interface (position B) or toward the workpiece-

tool flank face interface (position C in Fig. 1) [12].

When the fluid is directed toward the interface between

the chip and tool rake face, it forms a wedge on the tool

rake face, reducing the contact between chip and rake face

[13] and thereby reducing friction, improving the cooling

effect and increasing chip breakability [14]. The force of

the high-pressure fluid flow on the chip–tool interface may

break the chip efficiently, which is very important in the

machining of very ductile alloys such as the super duplex

stainless steel used in this work [15]. Compared to the

conventional cooling/lubrication system, the high-pressure

injection of cutting fluid slows down the tool’s diffusion

wear mechanism significantly [16].

In this work, several turning experiments were carried

out on SAF 2507 super duplex alloy using a PVD-coated

grade ISO M25 tool, conventional and high-pressure

cooling systems, and different cutting speeds to evaluate

the influence of these input variables on tool life and

workpiece surface roughness. The influence of these

parameters on the corrosion resistance of components

machined with fresh and worn tools was also analyzed.

2 Materials, equipments and experimental procedures

All the turning experiments were performed on a CNC

lathe operating with 22 kW in the main spindle and a

maximum rotation of 6,000 rpm.

Tool flank wear was measured several times during the

experiments, using an optical microscope connected to a

computer equipped with image processing software. This

enabled images of the wear land to be recorded and flank

wear height to be measured. At the end of the experiments,

the worn inserts were analyzed by scanning electron

microscopy (SEM) coupled to energy dispersive spectros-

copy (EDS).

The surface roughness of the turned workpiece was also

measured several times during each experiment, using a

portable roughness meter.

The workpieces used in the experiments consisted of

round bars of super duplex stainless steel UNS S32750,

commercially known as SAF 2507. Table 1 describes the

chemical composition of this material, while Table 2 lists

its main mechanical properties. All the values listed in

Tables 1 and 2 were supplied by the manufacturer.

Cylindrical workpieces with dimensions of

Ø120 9 100 mm (the useful length in the turning opera-

tion) were prepared for the turning experiments, which

were performed in successive cylindrical passes of the tool

on the workpiece. Workpieces were no longer used when

their diameter reached Ø70 mm. Each experiment was

concluded after the tool cut 1,400 m of turning perimeter

Fig. 1 Main directions of the fluid jet (adapted from [12])

Table 1 Chemical composition of SAF 2507 [17]

C max Si max Mn max P max S max Cr Ni Mo N

0.030 0.8 1.2 0.035 0.015 25 7 4 0.3

Table 2 Mechanical properties of SAF 2507 [17] (in metric units)

Proof strength Tensile

strength

Elong. Hardness

HRC max

Rp0.2a

(MPa min)

Rp0.2a

(MPa min)

Rm (MPa) Ab (%

min)Ab

2 (%

min)

550 640 800–1,000 25 15 32

J Braz. Soc. Mech. Sci. Eng.

123

(perimeter of one revolution multiplied per the number of

revolutions of the workpiece) or its notch wear reached

VBN = 0.8 mm, whichever occurred first. In fact, as it will

be seen in Fig. 2, notch wear never reached 0.8 mm and,

consequently, all experiments were finished after 1,400 m

of turning experiments.

The following turning tools were used:

(a) Tool holder with Capto C6 fixture in the machine tool

turret, ISO code PCLNL-45165-12HP. This tool

holder had a built-in cooling system to inject the

fluid under high pressure towards the tool rake face;

(b) Rhombic shaped inserts with point angle of 80�,

negative geometry, nose radius of 0.8 mm, ISO code

CNMG 120408-MM;

(c) The cemented carbide grade was ISO M25 grade PVD

multi-coated with TiAlN and TiN layers, total coating

thickness of 4 lm [18].

All the turning experiments were performed with an

abundant flow of mineral emulsion at a concentration of

7 %. Two conditions of fluid application were used. The

first one was the lathe’s regular cooling system, which

directed the fluid to the tool rake face under a pressure of

15 bar (low-pressure cooling). The second condition

involved applying the fluid through a nozzle inserted into

the tool holder, which also applied the fluid directly on the

tool rake face under a pressure of 70 bar. In this paper, we

refer to this condition as ‘‘high-pressure cooling’’.

The cutting parameters were chosen based in the tool

supplier’s recommendations [18]. The feed and depth of

cut were kept constant at 0.15 mm/rev and 1 mm, respec-

tively, which are typical values of these parameters for

finish turning operations with this kind of tool. Finish

turning operation was used in the tests because it is the last

operation to produce a cylindrical surface and, therefore, it

is the operation that determines the conditions of the sur-

face designed to support corrosion.

Two input variables, each one at two levels, were cho-

sen: cutting speed (110 and 130 m/min) and cooling

pressure (low-pressure and high-pressure fluid). Each

experiment was performed in triplicate.

The workpieces used in the turning experiments were

reused in the corrosion experiments. Because these exper-

iments required ring-shaped workpieces, a 30-mm-diameter

central hole was drilled into the 70-mm-diameter bars used

in the turning experiments, and its face turned using radial

feed. To evaluate the influence of the parameters employed

in the experiments (cutting speed, fluid pressure and tool

condition—fresh and worn) on the workpiece pitting cor-

rosion resistance, the same tool cutting edges (some fresh

and others worn) and conditions as those used in the turning

experiments were employed. To machine the specimen

surfaces used in the corrosion tests, the tool holder was

shifted 90� in the machine turret, in order to use the same

portion of the cutting edges in both kinds of experiments,

the cylindrical turning experiments and the radial turning

used to prepare the corrosion test samples. After facing, the

rings were cut with a cutoff blade, giving them the fol-

lowing dimensions: external diameter 70 mm; internal

diameter 30 mm; thickness 3 mm. These rings were sub-

sequently cut into smaller pieces (10 mm) to render their

dimensions suitable for the corrosion tests.

To accelerate the corrosion process, these samples were

etched with ferric chloride, as recommended by the ASTM

G48-A standard test method [19]. The ferric chloride solu-

tion was prepared with 100 g of FeCl3 dissolved in 900 ml of

distilled water previously filtered to remove impurities. After

etching, the samples were cleaned with magnesium oxide

paste, immersed in water, then dipped in acetone and dried in

the open air. They were then analyzed by SEM under 1009

magnification and their pitted areas were measured.

The results obtained in the experiments were analyzed

using analysis of variance technique and Minitab software

according to Montgomery [22].

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 500 1000 1500

No

tch

Wea

r V B

N (m

m)

Turning Perimeter (m)

PVD Grade ISO M15(GC 1025) Notch Wear VBN x Perimeter

HP 130

HP 110

LP 130

LP 110

Fig. 2 Flank wear in the notch

region vs. machined perimeter

in all the experiments (HP and

LP high and low-pressure

cooling)

J Braz. Soc. Mech. Sci. Eng.

123

3 Results and discussion

Figure 2 illustrates the tool flank wear behavior during one

of the repetitions of the experiments against turning

perimeter (perimeter of one revolution multiplied per the

number of revolutions of the workpiece). The most sig-

nificant wear mechanism in these tools was always notch

wear, which made the wear curves to behave as shown in

this figure, i.e., the wear height increased abruptly due to

micro-chipping in the notch region. This figure also shows

that the experiment with vc = 110 m/min and high-pres-

sure cooling resulted in the lowest notch wear rate.

Figure 3 shows an SEM image of the flank wear land of

the tool used in this experiment. Note that the region at the

end of the depth of cut was the one with the highest wear

height (notch wear) and also that the edge presented micro-

chipping along the wear land.

Figure 4 shows a magnified image of the edge shown

in Fig. 3, as well as the results of the EDS analysis of the

tool’s wear land. This figure shows a large amount of

material adhered on the tool wear land. Moreover, the

EDS analysis of this adhered material indicated the pre-

sence of elements from the workpiece, such as Si, Cr, Mn,

Fe and Mo. Therefore, one of the wear mechanisms of

this tool was attrition, which causes a metallic layer to

form when two surfaces are in contact under a moderate

load, at low temperature and low cutting speed [20]. The

resistance of this layer was such that, when the flow of

material forced the surfaces to separate, the rupture did

not occur at the interface between the surfaces but in one

of the metals, causing particles from one material to

migrate to the surface of the other. In machining, adher-

ence of workpiece/chip material on the tool causes the

removal of particles from the tool. Therefore, we created

a hypothesis to explain the wear mechanisms of the tool

used in this experiment. In all the experiments, a burr was

formed at the end of the contact between the tool and the

workpiece (end of the depth of cut), due to the high

ductility of the material. This burr, which was very hard

due to the high work-hardening rate of the material,

produced a furrowing effect in the region of the tool in

contact with it, as described by Biermann and Heilmann

[8]. As the cutting operation proceeded, this furrowing

effect gradually removed the layer of coating at that point

of contact. This uncoated region, with its higher friction

coefficient, enabled the chip/workpiece material to adhere

at that point, thus further favoring the tool’s wear caused

by attrition. The resulting increase in wear in this region

when compared with the other regions of tool/workpiece

contact is illustrated in Fig. 4.

Figure 5 shows the tool wear land of one of the tools

used in cutting under low-pressure cooling and cutting

speed of vc = 130 m/min. In this figure, note the notch

wear and the adhered workpiece material in the notch

region. This leads to the conclusion that the wear mecha-

nisms were the same as those in the first experiment

described (vc = 110 m/min and high-pressure cooling),

i.e., the onset of notch wear was caused by the hard burr,

and after removal of the tool coating in the notch region,

attrition wear gradually favored notch growth.

The other experiments presented the same wear mech-

anism, i.e., notch wear was the highest wear along the

cutting edge. The EDS images and analysis of the wear

land revealed a large amount of workpiece material

adhering to the notch region, indicating that all the tools

underwent the same wear mechanisms, regardless of the

cutting speed and coolant pressure. Therefore, to save

space, the tools used in the other experiments are not

shown here. However, it should be pointed out that the

notch wear sometimes caused chipping of the cutting edge

in that region, as depicted in Fig. 6 (tool used in the

experiment with vc = 110 m/min and cooling pressure of

15 bar).

For this reason, it was not possible to finish all the

experiments with the same value of flank wear (VB).

Therefore, the ratio of ‘‘flank wear at the end of the

experiment vs. machined perimeter’’ (this ratio was called

relative wear index) was used to compare the several

experimental conditions from the standpoint of wear

resistance. The lower this index, the slower the tool’s wear

progression and the higher its wear resistance in that

condition.

Figure 7 shows the values of this index in all the

experiments, while Fig. 8 depicts the main effects of this

index in graphic form. These figures indicate that cooling

pressure is the most influential variable in wear resistance,

i.e., machining with high-pressure cooling (HP) generates a

much slower wear rate than with low pressure (LP). In

Fig. 3 Tool cutting edge in the experiment with vc = 110 m/min and

high-pressure cooling

J Braz. Soc. Mech. Sci. Eng.

123

contrast, cutting speed had a much lower influence, with

higher cutting speeds generating only slightly higher wear

rates. Nevertheless, it should be kept in mind that the

increase in cutting speed was low (\20 %), while the

increase in fluid pressure was very high (from 15 to

70 bar). The experiments with high-pressure fluid

presented lower wear rate due to the more efficient cooling

effect, and hence, lower tool temperature.

Figure 9 shows the behavior of surface roughness (Ra

and Rz) throughout the experiments. An important point

this figure reveals is that the roughness values in most of

the experiments remained stable during cutting, i.e.,

Fig. 4 EDS analysis of the tool

wear land—vc = 110 m/min

and high-pressure cooling

Fig. 5 Tool cutting edge in the experiment with vc = 130 m/min and

low-pressure coolingFig. 6 Tool cutting edge in the experiment with vc = 110 m/min and

low-pressure cooling

J Braz. Soc. Mech. Sci. Eng.

123

roughness did not vary with increasing tool wear. More-

over, the behavior along the experiment of the Ra and Rz

curves is very similar. The reason for this is that, as

mentioned earlier, the main type of wear was notch wear,

which did not change the original shape of the tool nose in

contact with the machined surface. The roughness profile

obtained on a turned surface depends directly on the tool

nose shape [21].

Figure 10 shows the main effect of surface roughness at

the beginning of tool life. This effect was calculated using

‘‘Analysis of Variance’’ [22]. Note that under high-pressure

fluid, surface roughness was lower than under low coolant

pressure. An explanation for this is that the material in the

cutting zone is cooler due to the higher cooling efficiency

of high-pressure fluid injection [5], thus causing less plastic

deformation on the surface of the workpiece in the vicinity

of the formed chip. Plastic deformation is one of the factors

that contribute to surface roughness. It can also be seen that

increasing the cutting speed led to a corresponding increase

in surface roughness. A hypothesis to explain this behavior

is that increasing the cutting speed increases the tempera-

ture of the material in the cutting region [20]; hence, the

lower cutting speed (110 m/min) resulted in lower tem-

perature and less plastic deformation on the surface of the

workpiece in the vicinity of the chip. In other words, the

same mechanism that decreased the surface roughness

when using high-pressure fluid also reduced it when using a

low cutting speed. Figure 9 shows that the lowest surface

roughnesses were obtained in the experiment with the

lowest temperature (vc = 110 m/min and high-pressure

fluid).

As described in Sect. 2, the workpieces were turned

under the same cutting conditions as those used in the

regular experiments, using both fresh and worn tools, to

evaluate the influence of the turning conditions on the

corrosion resistance of machined surfaces.

After the pitting corrosion test, SEM images of the

corroded samples were recorded and processed with image

processing software that recognizes the percentage of white

and dark areas on the surface. Figure 11 shows an SEM

image of the sample (Fig. 11a) and the same image pro-

cessed by the imaging software (Fig. 11b). The dark points

in these images correspond to pitting corrosion. Figure 12

illustrates the results of the pitting corrosion tests (pitting

percentage).

In Fig. 12, note that the percentage of corroded areas

produced with high-pressure fluid (machining condi-

tions 5–8) was consistently smaller than with low-pressure

fluid (machining conditions 1–4), regardless of the cutting

speed or tool wear (fresh or worn tools). The cutting pro-

cess may influence the corrosion resistance of a machined

surface because it causes plastic deformation of the

Fig. 7 Relative wear index as a

function of the experimental

conditions of the tests

Fig. 8 Main effects of the factors influencing the relative wear index

J Braz. Soc. Mech. Sci. Eng.

123

machined surface. Moreover, the greater cooling efficiency

of high fluid pressure, which lowers the temperature of the

material in the cutting region [5], and therefore the

temperature of the machined surface, is another factor that

influences corrosion resistance. This result indicates that

lower temperatures in the cutting region are beneficial for

surface corrosion resistance. Figure 13 (built also using

analysis of variance technique [22]) indicates that fluid

pressure was the only significant factor affecting the cor-

rosion resistance of turned surfaces and that, albeit not

statistically significant, cutting speed and tool wear level

were the second and third factors that influenced corrosion

resistance.

Figure 14 shows the main effect of the factors that influ-

ence the corrosion resistance of turned surfaces. This figure

indicates, as mentioned earlier, that the use of high-pressure

fluid generated lower corrosion. When the cutting speed

increased from 110 to 130 m/min, the corrosion rate also

decreased, albeit not as significantly as when the fluid pres-

sure increased (as shown in Fig. 13, not statistically signif-

icant). Increasing the cutting speed caused the cutting

temperature to increase [20], and was expected to impair the

Fig. 9 Workpiece surface

roughness during the

experiments: a Ra, b Rz

Fig. 10 Main effects of the factors that determine workpiece

roughness

J Braz. Soc. Mech. Sci. Eng.

123

material’s corrosion resistance. However, an increase in

cutting speed, particularly in a low range of speeds such as

those applied here, strongly reduces the specific cutting force

[21], resulting in lower plastic deformation of the machined

surface and thus increasing the corrosion resistance.

Regarding the third variable of the experiments, the use

of fresh and worn tools, Figs. 12, 13 and 14 show that

using worn tools contributes to increase the formation of

corrosion points in super duplex stainless steels. However,

the influence of this variable is even lower than that of

cutting speed. As illustrated in Figs. 3, 4, 5 and 6, the

geometry of the cutting edge of the worn tool differs sig-

nificantly from that of the new tool, having lost much of its

sharpness. The hypothesis to explain the influence of the

cutting edge condition on the surface corrosion resistance

is that this dull edge facilitates plastic deformation, which

in turn introduces crystal defects and causes sliding of

atomic planes. Consequently, the deformed regions present

higher internal stresses that behave anodically and become

subject to pitting corrosion. The influence of this parameter

is not as significant as that of the other two because the

main wear mechanism is notch wear, which does not occur

Fig. 11 Images of the surface machined with high-pressure cooling, vc = 110 m/min and worn tool: a SEM image, b processed image

Fig. 12 Pitting percentage in different machining conditions

J Braz. Soc. Mech. Sci. Eng.

123

at the interface between tool and machined surface, and is

therefore expected to cause less damage.

Surface finish is another factor to be considered when

analyzing the influence of the machining operation on the

corrosion resistance of the workpiece. As mentioned in the

‘‘Introduction’’ of this paper, Gravalos et al. [11] affirmed

that low surface roughness contributes to reduced pitting

corrosion. Therefore, we attempted to find a correlation

between the influence of the experimental input parameters

on corrosion resistance (Fig. 14) and surface roughness

(Fig. 10). At this point, we discovered a similarity between

the two figures: the increase in fluid pressure caused the

roughness to decrease and corrosion resistance to increase

(decrease in pitting percentage). However, any similarity

also ended at this point. Worn tools produced surfaces with

lower corrosion resistance, but not always higher surface

roughness. In addition, increasing the cutting speed led to

more corrosion-resistant surfaces, and in general, higher

surface roughness, which contradicts the findings reported

in the literature [11].

Based on these results, the hypothesis that we propose is

that, in the machining process, the temperature of the

material in the cutting region and the specific cutting force

(calculated force per chip area) are the factors that influ-

ence pitting corrosion resistance. Therefore, to ensure that

workpieces machined in conditions like those used in our

work attain their maximum corrosion resistance, the turn-

ing operation should be performed using high-pressure

Fig. 13 Significant factors that

influence pitting formation after

machining

Fig. 14 Main factors that

influence pitting corrosion

J Braz. Soc. Mech. Sci. Eng.

123

fluid and high cutting speeds. Moreover, tool wear should

be controlled to prevent it from reaching high values.

4 Conclusions

The following conclusions can be drawn based on the

results obtained in SAF 2507 super duplex stainless steel

turning experiments, with regard to cooling pressure and

cutting speed and their influence on tool life, workpiece

surface roughness and pitting corrosion resistance:

• The main type of wear found was notch wear caused by

the furrowing effect produced by the burr, followed by

adhesion/attrition in that wear region.

• The use of the high-pressure cooling system provided

benefits, such as extended tool life and reduction of

workpiece roughness.

• It is important to use high-pressure cooling to attain

long tool lives in this type of machining operation.

Moreover, low cutting speeds are also advisable.

• The input variable that most influenced the corrosion

resistance of turned workpieces was coolant pressure.

Turning with high fluid pressure yielded workpieces

with higher corrosion resistance. Increasing the cutting

speed increased the corrosion resistance, although this

factor was not statistically significant. The condition of

the tool (worn or new) was also not statistically

significant on this parameter, but it can be stated that

worn tools produced less corrosion-resistant surfaces.

• High-pressure cooling and high cutting speed should be

used to ensure the maximum corrosion resistance of

components produced in the conditions tested here. In

addition, it is important to avoid excessive tool wear.

Acknowledgments The authors wish to thank Sandvik Coromant

(Brazil) for the tools supplied for the experiments and also for support

during execution of the tests and analysis of the results.

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