7

2. LITERATURE SURVEY

2.1 Organization of Literature Survey:

The researchers after carrying out their work publish the same. This published work forms

the base for getting information about the current status of the work carried in that area. It

also helps in providing guidelines for the further work to be under taken. Literature review

helps in knowing the present trend in a particular area of the research and is useful for

planning the future work.

With this objective in mind the available literature on cryogenic treatment and its effect on

properties on various materials in general and tool steel in particular, tool wear, forms of tool

wear, measurement of tool wear, work carried by different researchers on tool wear/ tool life

models, optimization of parameters for yielding maximum tool life etc. is reviewed in this

chapter.

The research carried out by various researchers on effect of cryogenic treatment on the

properties of tool steels, die steels, stainless steel, cast iron, tungsten carbide, copper

electrodes etc. is discussed in section 2.2.

The tool wear, basic forms of tool wear, measurement of tool wear, tool life, research work

carried out by various researchers on different tool work combinations, models proposed for

evaluating tool wear or tool life using different techniques etc. are discussed in section 2.3.

Based on the literature review, the observations made and objectives of the present work are

discussed in section 2.4

2.2 Cryogenic Treatment:

2.2.1. Background on cryogenic processing:

To understand the effects of cryogenic processing it is essential that one should be acquainted

with the heat treating of metals. The primary reason for heat treating steel is to improve its

wear resistance through hardening. Gears, bearings and tooling for example are hardened

because they need excellent wear resistance for extended reliability and performance. The

8

steps in heat-treating are frequently explained in a simplistic manner but it takes significant

skill and experience to execute heat treatments successfully [5].

Steel is normally raised to the austenizing temperature, usually 1200 0C or higher for heat

treating. Austenite is a soft phase of steel and malleable, hence it is very easy to wear the

structure down with repeated use, therefore the need for heat-treating. Gears and other

tooling are often rough machined or formed in the austenitic state. After a predetermined

period of time at the elevated temperature, which is determined by the phase diagram of the

alloy in question, the material will be quenched in a bath that may be oil, water, brine or

polymeric compounds. The rapid cooling (quenching) of the steel in the quenching medium

will cause the atoms in the microstructure to rearrange in the atomic structure that is called

martensite [5]. The maternsite is harder material and hence gives material a good wear

resistance. Hence more is martensite in matrix of material; more is the hard material and thus

possesses higher wear resistance.

2.2.2 How it all started:

Cryogenic processing has been around for many years but is truly in its infancy when

compared to heat-treating. For centuries the Swiss would take advantage of the extremely

low temperatures of the Alps to improve the behavior of their steels. They would allow the

steel to remain in the frigid regions of the Alps for long periods of time to improve its

quality. Essentially, this was a crude aging process accelerated by the very low temperatures.

What we now understand to have happened was the reduction of the retained austenite and

the increase in martensite. By performing this once secret process the Swiss obtained the

reputation for producing a superior grade of steel [5].

The process of experimentation and understanding of the cryogenic treatment of steels really

got under way during World War II at the Watertown Arsenal in Watertown, Mass. It was

under the direction of Clarence Zener who would later go on to develop the Zener diode. At

that time there were no computer controls so the steel tooling would be immersed in liquid

nitrogen for a brief period of time, allowed to warm up, then placed into service. This method

was crude and uncontrolled. Many of the tools would chip and break immediately upon use

because the immersion process would create a very high thermal gradient in the tool and this

would produce micro-cracks in the body. It was also later learned that the cryo-treatment

9

would convert the retained austenite into un-tempered martensite. But the tools that would

not break would experience a greatly enhanced service life [5].

In the 1960’s cryogenic processors used multi-stage mechanical coolers along with insulated

‘cold-boxes’ to gently remove the latent heat from tooling thereby achieving a much slower

cooling rate, concurrent with longer wear lives. Performing a standard wear test (pin-on-disk)

showed that the wear resistance for these steels could be increased by more than 600%. At

this time it was theorized that the increase in wear resistance was a direct result of the

reduction in the amount of retained austenite [5].

Resistance to abrasive wear was investigated in a parametric study. Five tool steels were

tested after conventional heat treatment, after cold treatment at –84 °C (–120 °F), and after

being cryogenically treated at –190 °C (–310 °F). Figure 2.1 shows the results of these

abrasive wear tests. Cold treatment at –84 °C (–120 °F) improved the wear resistance by 18

to 104%, but the cryogenic treatment results show 104 to 560% improvement [6].

0

1

2

3

4

5

6

7

52100 D2 A2 M2 O 1

Wea

r re

sist

ance

(R

wtr

eate

d/R

wun

trea

ted

) -84 C

-190 C

Figure 2.1: Comparison of Wear Resistance Ratios for Five High Carbon Steels Soaked at -840C and -1940C [6]

A study by the IIT Research Institute [7] published in November 1995 for the Instrumented

Factory for Gears sponsored by the US Army ManTech was conducted to study the effects of

the carburizing process and cryogenics treatments in modifying the microstructure of the

material. The results of the tests as presented at the INFAC Industry briefing, June 13, 2000

were that the deep cryogenic treatment gave 50% extra pitting resistance, 5% more load

10

carrying capacity, and a 40 0F to 60 0F higher tempering temperature. Although these

experiments were performed on AISI 9310 material (standard helicopter transmission gear)

the conclusions show promising results, which may be applicable to the general subject of

mechanical and chemical wear resistance.

2.2.3 Cryogenics defined:

Cryogenics is defined as that branch of physics, which deals with the production of very low

temperatures and their effect on matter, a formulation, which addresses both aspects of

attaining low temperatures, which do not naturally occur on Earth and of using them for the

study of nature or the human industry. In a more operational way, it is also defined as the

science and technology of temperatures below 120 K. The reason for this latter definition can

be understood by examining characteristic temperatures of cryogenic fluids given in Table

2.1 the limit temperature of 120 K comprehensively includes the normal boiling points of the

main atmospheric gases, as well as of methane, which constitutes the principal component of

natural gas. Today, liquid natural gas (LNG) represents one of the largest and fast-growing

industrial domains of application of cryogenics, together with the liquefaction and separation

of air gases. The densification by condensation and separation by distillation of gases was

historically – and remains today - the main driving force for the cryogenic industry,

exemplified not only by liquid oxygen and nitrogen used in chemical and metallurgical

processes, but also by the cryogenic liquid propellants of rocket engines and the proposed use

of hydrogen as a “clean” energy vector in transportation [8].

Table 2.1: Characteristic Temperatures of Cryogenic Fluids [K] [8]

Cryogen Triple point Normal boiling point Critical point

Methane 90.7 111.6 190.5

Oxygen 54.4 90.2 154.6

Argon 83.8 87.3 150.9

Nitrogen 63.1 77.3 126.2

Neon 24.6 27.1 44.4

Hydrogen 13.8 20.4 33.2

Helium 2.2 4.2 5.2

11

Cryogenics is basically a derivative of two Greek words - "Kryos" which means cold or

freezing and "genes" meaning born or one that is produced. Technologically, it means the

study and use of materials (or other requirements) at very low temperatures. Deep Sub-zero

treatment of metals and alloys is a deep stress relieving technology. Whenever material is

subjected to any manufacturing operation, it is subjected to stresses. The stress manifests

itself in the nature of defects in the crystal structure of materials. The most commonly

observed defects are in the form of vacancies, dislocations, stacking faults etc. As the level

of stress increases, the density of these defects increases, leading to increase in inter atomic

spacing. When the distance between the atoms exceeds a certain critical distance, cracks

develop and failure takes place [9].

The third law of thermodynamics states that entropy is zero at absolute zero temperature.

Deep subzero treatment uses this principle to relieve stresses in the material. The materials

are subjected to extremely low temperatures for a prolonged period of time leading to

development of equilibrium conditions. This leads to ironing out of the defects in the

material and also attainment of the minimum entropy state. Grain shape and size gets refined

and is made uniform. Defect elimination takes place and inter atomic distance is reduced.

When the material is brought back to room temperature, the defect level reflects an

equilibrium concentration. Compaction of the crystal structure leads to much superior

abrasive, adhesive and erosive wear resistance and enhances corrosion resistance as well as

fatigue strength and resilience [10].

2.2.4 Effects of cryogenic treatment on properties of materials:

Over the past 50 years, large number of reports have been published regarding substantial

benefits that can be realized by treating steel tools at a low temperature, usually near that of

liquid nitrogen, -196 0C.

Despite the fact that several investigations conducted in the 1940’s failed to confirm any

substantial benefits of cold treatments when used as an integral part of the thermal treatment

cycle, reports of large beneficial effects in production operations continue to appear. Today,

the emphasis is on using cryogenic treatment after the conventional heat treatment.

In general cryogenic treatment using nitrogen as cryogen consists of following stages:

• Slow cooling (i.e., without thermal shock) at a approximate rate of 2.5 0C/min from

ambient temperature to the temperature of liquid nitrogen approximately -196 0C

12

• holding at low temperature up to prescribed period, known as soaking period

• the part is then removed from the liquid nitrogen and allowed to warm at room

temperature in ambient air and

• single-cycle tempering by reheating to a moderately elevated temperature (150 to 315

0C) for about 1 or 2 hours, is usually performed after cryogenic treatment to improve

impact resistance, reduction in brittleness of the part etc. although double or triple

tempering cycles are sometimes used.

Cryoprocessing as discussed earlier is the process of cooling a material to extremely low

temperatures to generate enhanced mechanical and physical properties. In a review carried

out by Wayne Reitz et. al. [11] on cryoprocessing of materials effect of cryogenic treatment

on properties of steel and some copper alloys is described. Research carried out by various

researchers has shown that cryogenic treatment has improved wear resistance, dimensional

stability, electrical and thermal conductivity, and hardness. The metallurgical aspects include

reducing the amount of retained austenite, increasing carbide formation, and enhancing short-

range diffusion. The processing steps in the cryogenic treatment for different materials are

critical.

The effect of cryogenic treatment on properties of different materials with respect to

literature available is discussed in following section.

a) Steels:

In case of conventional heat treatment after hardening, tempering is carried out cooling the

material up to room temperature. There remains certain austenite known as retained

austenite in the microstructure. For eutectoid HSS steel the Mf temperature is approximately

–500C and hence after cooling at room temperature retained austenite is present in the

microstructure [12]. This retained austenite can be transformed into martensite, by reheating

the material which causes distortion on its body. This non tempered martensite may cause

cracks particularly in complex shape HSS tools [13]. Instead if subzero treatment is given to

these materials the retained austenite gets transformed into martensite giving more

dimensional stability in the tool microstructure.

Initially, super cold treatment apparently converts any retained austenite into martensite, and

the martensite is tempered as the steel returns to the room temperature. The martensitic

structure resists plastic deformation much better than the austenitic structure, because the

small carbon atoms in the martensitic lattice ‘lock together’ the iron atoms more effectively

than in the more open centered cubic austenite structure. Tempering the martensite makes it

13

tougher and better impact resistant than un tempered martensite. Martensite is harder and has

more wear resistant structure. Additionally, the cryogenic treatment of high alloy steel, such

as tool steels, results in the formation of very small carbide particles dispersed in the

martensitic structure, between the larger carbide particles present in the steel. These small,

hard carbide particles within the martensite matrix help support the matrix and resist the

penetration by foreign particles in abrasive wear. Also these carbides strengthen the material

without any appreciable change in the hardness [9].

In case of steels it has been observed that wear resistance gets improved from 92 % to 817 %.

Table 2.2 shows the results obtained when the materials were treated at –80 0C and –196 0C

respectively. This increase in wear resistance is basically attributed to conversion of retained

austenite into martensite up to Mf (martensite finish) temperature and thereafter precipitation

of ultra fine carbides [14].

Table 2.2: Percent of Increase in Wear Resistance after Cryogenic Tempering [14]

Material that showed significant improvement

AISI Description At –800C At –1960C

D2 High Carbon /chromium die steel 316 % 817 %

S7 Silicon tool steel 241 % 503 %

52100 Bearing steel 195 % 420 %

O1 Oil hardening cold die steel 221 % 418 %

A10 Graphite tool steel 230 % 264 %

M1d Molybdenum HSS 145 % 225 %

H13 Chromium/molybdenum hot die steel 164 % 209 %

M2 Tungsten/moly HSS 117 % 203 %

T1 Tungsten High speed tool steel 141 % 176 %

CPM 10V Alloy steel 94 % 131 %

P20 Mold steel 123 % 130 %

440 Martensitic stainless 128 % 121 %

Material that did not show significant improvement

430 Ferritic stainless 116 % 119 %

304 Austenitic stainless 105 % 110 %

8620 Carbon steel 97 % 98 %

C1020 Graphite cast iron 96 % 97 %

T2 Tungsten High speed tool steel 72 % 92 %

14

Barron R. F. [15] has attributed the improvement of the wear resistance of these tools to

another mechanism besides the transformation of the retained austenite into martensite. He

verified that the tool steels submitted to conventional heat treatment presented only a small

amount of retained austenite, but those submitted to cryogenic treatment showed better

performance during machining. This new mechanism would be time and temperature

dependent due to the long period (8 hours or more) during which the tools would have to stay

at cryogenic temperatures. He found that the microstructure before the cryogenic treatment

had a relatively large carbides (20µm) dispersed in the matrix. However, after cryogenic

treatment these carbide particles were refined and particles up to 5 µm were observed in the

microstructure. The carbide refinement could in such a way contribute to the improvement of

the wear resistance of the tool. Barron thus attributed this achievement both to austenite

transformation and to the presence of hard and small carbide particles well distributed among

the larger carbide particles within the martensite matrix.

Collins and Dormer [16] studied the effect of deep cryogenic treatment (DCT) on D2 cold

work tool steel. It was found that the DCT samples gave lowest wear rate compared to

conventionally heat treated (CHT) samples. The results were attributed to conversion of

retained austenite into martensite, precipitation of fine carbides and finer distribution of

carbides in the tempered microstructure. This increased toughness and wear resistance both.

V. Leskovek et. al. [17] used vacuum treated M2 HSS for comparing wear resistance due to

cryogenic treatment. The specimens were cryogenically treated for – 1960C, with one hour

soaking. These specimens along with conventional specimen were subjected to sliding wear

test. Also the fracture toughness and hardness was measured. The wear resistance of

cryogenically treated specimens was better than that of conventionally treated specimen.

There was improvement in fracture toughness and hardness in case of cryogenically treated

specimens.

In another research carried out by V. Leskovek et. al. [18] the micro structure of M2 HSS

was found to be modified by combined effect of vacuum treatment and cryogenic treatment

in order to optimize the ratio between hardness and fracture toughness. It was also confirmed

by using a ‘Navy C’ ring test that the dimensional changes of this type of steel can be

controlled by this treatment.

15

Alexandru et. al. [19] carried out research on steels equivalent to M2. The specimens were

subjected to combination of quenching, tempering and cryogenic treatment up to –70 0C.

Tables 2.3 (a) and (b) show these combinations of heat and cryogenic treatment cycles. The

specimens were tested for turning operation and it was found that the life was improved from

22 minutes to 51 minutes. The microstructure was analyzed and the carbide particles were

quantified using SEM, X-ray difractometer, quantitative metallorgaphy and differential

dialometer. The results obtained confirmed that there is an increase in carbide precipitation

(form 6.9 % to 17.4 %) reduction in retained austenite (from 42.6 % to 0.9 %) and increase in

martensite content (from 66 % to 81.7 %).

Table 2.3 (a): Various Heat and Cryo Treatments [19]

A Quenching 1230 ◦C

B Quenching 1230◦C+ double tempering 560 ◦C

C Quenching 1230◦C + sub-zero (−70◦C)

G Quenching 1230 ◦C + sub-zero (−70◦C) + tempering 560 ◦C

H Quenching 1230◦C + tempering 560◦C + sub-zero (−70◦C)

M Quenching 1230◦C + tempering 560 ◦C + sub-zero (−70◦C) + tempering 560◦C

N Quenching 1230◦C + tempering 560 ◦C + sub-zero (−70◦C) + tempering 560◦C

+ sub-zero (−70◦C) + tempering 560 ◦C

Table 2.3 (b): Results of the Heat and Cryo Treatment [19]

A

(%)

M

(%)

C

(%) NC

T

(min)

A 42.6 66.5 6.9 - - B 12.8 74.6 12.5 23410.24 22 C 7.5 84.9 7.6 – 38 G 2.2 85.3 12.5 30928.49 49 H 13.3 71.2 15.5 23788.52 47 M 1.6 19.9 18.5 42869.81 51 N 0.9 81.7 17.4 69646.09 45

A: austenite; M: martensite; C: carbides

NC: amount of carbides smaller than1µm/mm;

T: tool life.

16

R. F. Baron [20] during his research carried out on cryogenic treatment to various materials

including M2 HSS observed significant improvement in the wear resistance. The M2 steel

was cryogenic treated at –84 0C and soaked for 24 hrs. He also found that the wear resistance

further increased when cryogenic temperature was -1960C.

D. Yun et. al. [21] carried out research on cryogenically treated M2 steel specimens. Table

2.4 (a) shows the various combinations of heat treatment and cryogenic treatment (–196 0C)

with varied soaking period. The results obtained are depicted in the Table 2.4 (b), which

shows that there is an increase of 11.5 % in bending strength, 43 % in the toughness. There

is also increase in red hardness of the material ultimately resulting in improved life. When

milling cutters made of M2 steel were cryogenically treated (cycle D) and used for

machining of piston rings made of grey cast iron, 440 rings were machined with this cutter as

against 220 rings machined with conventional heat treated cutter (cycle

Table 2.4(a): Different Cycles Applied to M2 HSS [21]

A Quenching from 1250◦C + triple tempering at 560 ◦C

B Quenching from 1250 ◦C + 1 cycle 24 h sub-zero at −196 ◦C + triple tempering at 560◦C

C Quenching from 1250 ◦C+ 1 cycle 48 h sub-zero at−196◦C + triple tempering at 560 ◦C

D Quenching from 1250◦C + 3 cycle totaling 48 h sub-zero at −196 ◦C + triple tempering at

560◦C

E Quenching from 1250 ◦C+ triple tempering at 560◦C + 1 cycle 48 h sub-zero at −196 ◦C

Table 2.4(b): Results of Different Cycles Applied to M2 HSS [21]

Route Hardness

(HRC)

Red hardness (HRC) BS (MPa) IT (J/mm2)

600 0C 625 0C 630 0C

A 63.7 60.6 57.8 55.7 2583 3.5

B 64.8 62.1 57.1 57.5 2880 4.4

C 65.0 63.0 59.3 58.0 2873 4.4

D 65.4 63.1 61.7 59.5 3096 5.0

E 64.3 61.8 58.1 57.3 2611 3.9

In an experimental work Fanju Meng et. al. [22] used alloy tool steel with composition (wt

%), 1.44C, 0.3Si, 0.4Mn, 12.2Cr, 0.84Mo, 0.43V, 0.022P, and 0.008S for wear test. After

heat treatment cold treatment at 223K and cryogenic treatment at 93K were carried out.

17

Figure 2.2 shows a typical heat treatment cycle of experiments. Tempering was carried out at

453K for 600s after cold treatment and cryogenic treatment.

Figure 2.2: Heat Treatment Cycle [22] The variation of the wear rate with sliding speeds was plotted for specimens austenitized at

1293K, quenched and ultra-subzero treated at 93K, which is shown in Figure 2.3. The wear

rate of specimens after cryogenic treatment is smaller than that of as-quenched specimens

(without any subzero treatment) for whole sliding speeds. Furthermore, it decreases

dramatically at high sliding speed. The cryogenic treatment results show 110 to 600%

improvements.

0

4

8

12

16

20

0.5 1 1.5 2 2.5 3.5Sliding speed (m/s)

Wea

r ra

te (

x10

mm

/ N

-m)

Without subzero treatment

Cryogenic treatment

Figure 2.3: Wear Rate Verses Sliding Speed [22]

X-Ray Diffraction Analysis volume fraction of retained austenite against the subzero

treatment temperature was further plotted as shown in Figure 2.4 for specimens austenitized

at 1293K and 1373K. The volume fraction of retained austenite is 12% for as quenched

specimens after austenization at a 1293K and approximately 6% for specimens after cold and

cryogenic treatment. However, it decreases with treating temperature going down for

293

333

1373

223 Cold treatment

93

Tem

pera

ture

(K)

Time

Aging

1293

453

Austenization

Cryogenic treatment

Tempering

18

specimen austenitized as 1373K. Cold treatments reduce the volume fraction of retained

austenite drastically. Nevertheless, cryogenic treatment reduces it slightly relative to cold

treatment.

0

10

20

30

40

50 100 150 200 250 300 350 400Temperature (K)

Vol

ume

frac

tion

(%

)

Austenization at 1373 K

Austenization at 1293 K

Figure 2.4: Volume Fraction of Retained Austenite Verses Subzero Treatment Temperature [22]

It was concluded that cryogenic treatment increases wear resistance dramatically, especially

at high sliding speed. The specimens after cryogenic treatment show a minimum of wear rate.

Unlike cold treatment, cryogenic treatment promotes preferential precipitation of fine η-

carbides.

Flavio J. da Silva et. al. [23] carried out work on M2 tool steels. They found that the hardness

and the microhardness of the M2 HSS samples were not significantly affected by the

cryogenic treatment. The samples cryogenically treated showed a fraction very close to 0%

of retained austenite. Practically the 25% in volume of the retained austenite observed in the

untreated sample were transformed into martensite by the cryogenic treatment. A superior

performance of the cryogenically treated tools compared to the untreated ones was observed

in the Brandsma rapid facing test. This difference reached 44% in some cutting conditions.

The difference on the percentage of retained austenite of the cryogenically treated and

untreated samples did not alter the abrasive wear rate at the conditions used for the sliding

abrasion tests. This is possibly due to the ability of the austenite of the untreated samples to

harden during plastic deformation either by workhardening or by its transformation into

martensite. These phenomena may compensate the gain obtained by precipitation of fine

carbides in the cryogenically treated samples. The cryogenic treatment increased the

performance of the M2 HSS twist drills. The gain observed during drilling steels adopting

catastrophic failure as the end of tool life criterion varied from 65% to 343% depending on

the cutting conditions used. Shop floor tests with cryogenically treated coated HSS milling

19

cutters presented worse performance than untreated tools when shaping the top surface of the

teeth of gear rings. Overall the cryogenic treatment had favorable influences on the

performance of the tools tested.

D. Mohanlal et. al. [24] used T1, M2 and D3 steels for comparative study on wear resistance

improvement due to cryogenic treatment at -180 0C for 24 hours with standard heat treated

samples. There was an improvement of 110.2 %, 86.6 % and 48 % in life of T1, M2 and D3

steels respectively. The samples were subjected to flank wear tests. Cryogenic treatment was

found to be superior to TiN coatings.

Cord Henrik Surberg et. al. [25] carried out work on AISI D2 steel. The samples made of

AISI D2 material were hardened using different austenizing temperatures. It was found that

the percentage of retained austenite was increased as austenizing temperature increased.

After this hardening treatment, tempering was carried out. The samples were then subjected

to various combinations of cryogenic temperatures and soaking time. The results obtained

showed that the cryogenic treatment reduced the retained austenite present in the samples. A

single deep cold treatment followed by single tempering cycle was found to be sufficient to

reduce retained austenite below 1 %. It was also observed that colder the cold treatment

temperature and longer the treatment time, higher the hardness of the samples. The deep cold

treatment did not affect the fracture toughness or distortion.

J. Y. Huang et. al. [26] carried out work for studying effect of cryogenic treatment on M2

HSS. The samples were treated for –196 0C and soaked for one week. It was found that

cryogenic treatment improved wear resistance of the samples. The improvement in wear

resistance was attributed to conversion of retained austenite into martensite, increase in

carbide population and volume fraction in the martensite matrix and also homogenization of

carbide distribution.

A. N. Popandopulo et. al. [27] carried out dilatometry studies and microstructure analysis

during cryogenic treatment. They concluded that shock cooling of quenched steel induces

transformation not only of austenite but also martensite. For this reason, the cold treatment

should be used for parts and tools with no retained austenite in order to intensify the

processes of tempering and precipitation hardening. Tool life increases 20–30% in this case.

Cold treatment of tools with 15–20% retained austenite restores the cutting ability to almost

the same level as for tools without retained austenite. To prevent cracking and embrittlement

of tools and to increase tool life by 50–100%, it is recommended that the cold treatment be

conducted after tempering at 560 0C for 1 hour with final tempering at 400°C for 30–60 min.

20

A. Molinari et. al. [28] carried out research work for studying effect of cryogenic treatment

on AISI H13 and AISI M2 HSS steels. The samples of both steels were cryogenic treated at –

196 0C with soaking for 35 hours. Field tests were carried out on different tools. There was

improvement in wear resistance leading to improved life. Also laboratory tests were carried

out on both types of steels. It was found that wear resistance of both steels got improved due

to cryogenic treatment. The improved wear resistance in case of M2 steel was attributed to

the increased hardness and that for H13 steel was attributed to increased toughness.

M. Pellizaari et. al. [29] applied DCT to two different cold work tool steels X155CrMoV12

and X110CrMoV8. Several combinations of heat treatment cycles were used. The

combination of cycles is shown in Tables 2.5 (a) and (b). The samples were subjected to

block on disk dry sliding wear test. Wear resistance of both the cold work steels got

increased due to cryogenic treatment. In case of X110CrMoV8 the effect was found to be

more as compared to X155CrMoV12. This is due to secondary hardening through

precipitation of carbides along with the decomposition of retained austenite.

Table 2.5 (a): Heat Treatment Variants Investigated [29]

Code Treatment

A Q + T1 + T2

B Q + T1 + C + T2

C Q + C + T1 + T2

D Q + C + T1 + D

E Q + C* + T1 + D

Table 2.5 (b): Heat Treatment Variants Investigated [29]

(Q=quenching, T= tempering C* =controlled DCT for 35h,

C=direct immersion in liquid nitrogen for 14h, D= stress relieving treatment)

Treatment X155CrMoV12 X110CrMoV8

Q 980◦C ×35min 1040 ◦C ×1.5h

T1 500 ◦C ×3h 500 ◦C ×3h

T2 510◦C ×3h 540 ◦C ×3h

D 240 ◦C ×3h 300 ◦C ×3h

21

Firouzdor V. et. al. [30] used M2 HSS drills for studying the influence of deep cryogenic

treatment on wear resistance and tool life in high speed dry drilling configuration of carbon

steels. The experimental results indicated 77% and 126% improvement in cryogenic treated

and cryogenic and temper treated drill lives, respectively. The results of wear rate test were

in agreement with drill life test. Wear resistance improvement was mainly attributed to the

resistance of cryogenically treated drills against diffusion wear mechanism, due to the

formation of fine and homogenous carbide particles during cryogenic treatment.

Additionally, transformation of retained austenite to martensite played an effective role, i.e.

improved hardness values.

It is well known that the durability of tool steel could be improved by deep cryogenic

treatment. It has been assumed that the increase of service life of tool steel caused by

decrease of retained austenite and/or by formation of nano-scale fine η-carbide. But the

principles of deep cryogenic treatment remain unclear yet. In this research, to manifest the

effect of deep cryogenic treatment on wear resistance, the specimen was emerged in liquid

nitrogen for 20 hours for deep cryogenic treatment after austenitizing and the following

tempering temperature was varied. The microstructure of specimens was observed using

TEM and the mechanical properties and wear resistance were examined. As the tempering

temperature increased, the carbides became larger and fine carbides were formed above

certain temperature. In the case of deep cryogenic treated specimen, the number of carbides

increased while the carbides size was decreased, furthermore, the fine carbide forming

temperature was lowered also. It was considered that the deep cryogenic treatment increased

the driving force for the nucleation of carbides. As tempering temperature increased,

hardness decreased, while wear resistance and impact energy increased. The deep cryogenic

treated specimens showed this tendency more clearly. It was considered that the wear

resistance is affected not only due to the hardness but also due to the precipitation of fine

carbides, and this carbide evolution can be optimized through the deep cryogenic treatment

[31].

Moore, K et. al.[32] carried out experiments on three types of heat treated tool steels to

determine the effects of various parameters on the hardness by cryogenically treating these

steels as compared to those only quenched to room temperature. The three tool steels used for

experimentation were H13, D2 and Vanadis 4. It was found that increase in hardness was

very dependent on cryogenic temperature and that stabilization can affect obtainable

22

hardness. It was confirmed that for two of the steels the hardness was independent of

treatment time. The optimum austenitizing temperatures and cryogenic temperatures within

the liquid nitrogen range were identified.

P. Cohen et. al. [33] used M1 and T15 HSS materials for studying effect of cryogenic

treatment. They conducted the drilling tests on four commonly machined materials: 1018

low-carbon steel (165 BHN), 340 alloy steel (390 BHN), A-2 tool steel (175 BHN), and 304-

S stainless steel (126 BHN). M-1 HSS drills were used with 118° point angles. For all

materials, there was significant increase in tool life, which was measured as the number of

holes produced before drill failure. Better hole finish and roundness complemented this

improved tool performance. In addition, turning tests were conducted on AISI 4340. T-15

tool-steel inserts were used at cutting speeds of 200 and 300 sfm, a constant feed of 0.0075

ipr, and a 0.050" depth of cut. The cryogenically treated and untreated inserts had a tool

signature of (0°, 5°, 11°, 11°, 15°, 15°, and 1Ú32"). At both speeds, tool life was

significantly longer, with flank wear being the dominant wear mode. At 200 sfm, the

cryogenically treated inserts lasted 127.0% longer than the untreated inserts (2180 min. vs.

960 min.), and at 300 sfm, the cryogenically treated inserts lasted 91.4% longer than the

untreated inserts (155 min. vs. 81 min.). After complete testing in the laboratory, laboratory

data was compared with field data. Compared to untreated drills, cryogenically treated M-1

tool-steel drills used by tool-and-die, automotive, and job shops lasted 200% to 300% longer

in the machining of 304-S, 100% longer in the machining of 4340, and 425% longer in the

machining of 1018-CR material.

Chia-Hung Sun [34] carried out research for studying effects of deep cryogenic treatment on

properties of AISI D2 tool steel. The result demonstrates that decrease in the cold treatment

temperature increase the hardness of the specimens, which were attributed to the greater

under-cooling to create greater activation energy to enhance the transformation of retained

austenite to martensite. The hardness derived from cryogenic treated specimens was slightly

higher than that of the sub-zero treated. This is attributed to the finer carbides precipitation

rendered from greater under- cooling of cryogenic treatment. The results demonstrate that the

content of retained austenite treated from either sub-zero or cryogenic presented almost the

same extent of reduction after they were single or double low temperature tempered.

However, the retained austenite was significantly reduced when they were tempered at high

temperature following a cryogenic treatment or sub-zero treatment. It is accepted that a major

23

factor contributing to wear resistance improvement through sub-zero or conventional heat

treatment is the removal of retained austenite. In the process of cryogenic treatment at

-196 0C, the retained austenite is transformed into martensite while martensite is decomposed

and ultra-fine epsilon-carbides are diffusively precipitated. The mechanism that cryogenic

treatment contributes to wear resistance is attributed to the precipitation of epsilon-carbide,

which enhances the strength and toughness of martensite matrix, rather than the removal of

the retained austenite only, which exhibited the better wear resistance by a minimum of flank

wear.

V. Soundarajan et. al. [35] used hardened carbon tool steel AISI1095 for research purpose.

In this study as a part I hardening was carried out on AISI1095 carbon tool steel of different

L/D ratio with conventional quenching media like purified water, aqueous solution and hot

mineral oil. As part II, the specimens were hardened in which quenching was carried out as

in part I and then these hardened specimens were quenched in liquid nitrogen at subzero

condition. The specimens were tested for hardness and wear loss. The result showed

improvement in wear resistance of the cryo quenched specimens than the conventional.

M. Kalin et. al. [36] have investigated the effect of four different tempering temperatures of

vacuum and cryogenically treated ESR AISI M2 high-speed steel on the resulting

combinations of microstructure, hardness and toughness and their effect on the wear

mechanisms at different loads. The results showed that at relatively high loads the different

treatments resulted in an order-of-magnitude difference of wear resistance, while at low loads

the selected treatments were efficient enough to keep the wear within the mild wear regime

and small variations between the samples. However, the overall wear transition did not occur

at any load used or any sample treatment, although some small differences in wear

mechanisms can be seen, primarily depending on the fracture toughness of the samples.

Nadig D.S. et. al. [37] had conducted an experiment to study the effect of cryogenically

treated HSS tool made of M42 with composition as Carbon 1.1%, Molybdenum 9.5%,

Vanadium 1%, Cobalt 8%, Chromium 4% and Tungsten 1.5%. The square tool of dimension

3/16″x 3/16″x 4″ length which was cryogenically treated at -80oC was subjected to a standard

sliding wear testing pin on disk apparatus. For comparative study a non-cryogenically treated

HSS tool (M42) was tested for wear resistance and frictional force. From the experiments, it

was observed that the Cryo-treated tool has developed higher wear resistance property. The

24

improvement in the wear resistance was around 33%. This improvement in wear resistance is

caused due to the conversion of soft retained austenite to hard martensite, formation of Eta-

carbide and development of a uniform and refined micro structure.

P Sekhar Babu et. al. [38] studied the improvement in wear resistance of M1, EN19 and H13

tool steels after cryogenic treatment. The materials were tested for improvement in abrasive

wear resistance after cryogenic treatment at different temperatures below 0°C. All the

samples were first heat treated as per standard norms and re tempered after cryogenic

treatment. The samples were treated at 0 0C, -20 0C, - 40 0C, -80 0C and -190 0C. It was

observed that the wear resistance improved by 382% for M1 steel, 335% for H13 steel and

by 319 % for E19 steel samples. The improvement in wear resistance in all these materials

showed incremental increase depending on the cryogenic treatment temperature.

Mahmudi, R. et. al. [39] also carried out research on effect of cryogenic treatment on M2

HSS. They compared the levels of hardness, impact toughness and wear resistance of M2

high-speed steel after conventional heat treatment with those imparted by additional sub-zero

and deep cryogenic processing.

Franjo Cajner et. al. [40] used PM S390 MC high speed steel for studying effect of cryogenic

treatment. The samples were subjected to different austenizing temperatures and thereafter

for triple tempering at higher temperatures, i. e. 540 0C. The samples were then subjected to

the deep cryogenic treatment at -196 0C with a soaking time of 25 hours. The cryo treated

samples were subjected to tempering at a temperature of 540 0C for 2 hours. The results

showed that there is an improvement in wear resistance of the material. There was no

significant improvement in toughness due to cryogenic treatment.

C. L. gogte et. al. [41] used M2 and T42 tool steel samples for their research work. The M2

and T42 HSS samples were austenized at 1210 0C and 1240 0C respectively. The austenized

samples were then subjected to triple tempering at a temperature of 560 0C. The samples of

both the materials were then cryo treated at different temperatures and different soaking time.

The samples were subjected to optical, SEM, TEM and x-ray micrography to see the results

and also to quantatively note the changes those are taking place in the steels. It was found

that there is a distinct change in the dispersion of carbide and also in the density of carbide

suggesting a dissolution of carbide into the matrix during the deep subzero treatment.

25

In a research carried out by Fanju Meng et. al. [42] Fe-1.4Cr-1C bearing steel was cryogenic

treated at –500C and –1800C. These cryogenically treated specimens were subjected to wear

test. The wear test was carried out on sample on wheel type. The wear resistance of the

specimens treated at –500C and –1800C showed improvement in wear resistance. In case of –

1800C specimen wear resistance improvement was higher than that of in case –50 0C treated

specimens. Also volume fraction of retained austenite was 66 %, 4.4 % and 4 % in case of

nontreated, –50 0C and –180 0C treated specimens respectively.

A. Bensely et. al. [43] used case carburized steel (En353) specimens for studying effect of

cryogenic treatment on wear resistance. The samples were cryogenically treated for –50 0C

(SCT) and –180 0C (DCT). These samples were soaked for 5 hrs and 24 hrs at –50 0C and –

180 0C respectively. These three types CHT, SCT and DCT samples were subjected to dry

sliding wear test. The wear resistance improvement amounted to 85 % in case of SCT

samples over CHT samples and 372 % in case of DCT samples over CHT samples.

In another research carried out by A. Bensely et. al. [44] case carburized 815 M17 steel was

studied for tensile behavior due to cryogenic treatment. Tensile test specimens were

conventionally heat treated and then Shallow Cryogenic Treatment (SCT) (–50 0C) with

soaking for 5 hours and Deep Cryogenic Treatment (DCT) (–196 0C) with soaking for 24 hrs

was given to samples. The CHT, SCT and DCT samples were subjected to tension test. The

results obtained indicate reduction in tensile strength for SCT and DCT samples over CHT

by a factor of 1.5 % and 9.34 % respectively.

S. Zhirafar et. al. [45] used 4340 steel for studying effect of cryogenic treatment on

mechanical properties. The 4340 steel samples were conventionally heat treated and

thereafter cryogenically treated at –196 0C with soaking for 24 hrs. Mechanical testing

involved hardness and charpy impact tests for specimens. The results obtained showed an

improvement in hardness, slightly detrimental effect (14.3 % decrease) in impact energy

toughness and improvement in fatigue limit. The increase in fatigue limit was attributed to

higher hardness and strength of the material due to cryogenic treatment.

It is well known that the wear resistance of tools can be improved using deep cold treatment

to produce nucleation sites for the nano-sized carbides that form in the martensite during low

temperature tempering following the extended treatment times. However, inconsistencies in

26

the processing techniques used, particularly the soaking time at very low temperature and the

steel’s previous thermal history, have led to different conclusions as to the exact mechanisms

involved. This paper reviews current thinking and attempts to explain the different results

obtained in terms of the processing parameters and to elucidate the implications for the

practical application of the technique to obtain optimum properties. An optimized process

route using liquid nitrogen as the coolant is recommended and a possible technique to make

further improvements in the process is suggested. As part of an optimized heat treatment

cycle, deep cold treatment can dramatically improve measured wear by the precipitation of

nano-sized carbides in the primary martensite. The transformation of retained austenite to

martensite is a minor additional benefit. In many practical uses of tools this increase in

measured wear translates into longer tool life [46].

b) Stainless steel:

Load-carrying cruciform joints with LOP were prepared by P. Johan Singh et. al. [47] using

6-mm thickness AISI 304L austenitic stainless steel cold rolled plate. The initial joint

configuration in the case of the cruciform joint was obtained by securing the long plates and

stem plate in a cruciform position by tack welding in a fixture. Subsequently, fillets were

made between the long plate and stem plate using semiautomatic gas metal-arc welding

process, argon shielding gas and 308L electrode. Great care was taken to obtain the best

possible joint alignment. After welding, the fatigue samples were cut into required sizes. The

cut samples were again machined for better surface finishing. Cryogenic treatments were

given to half of the specimens prepared. In general the basic steps in a cryogenic treatment

were as follows:

1. Ramp down: The ramp down in temperature to -185 0C with ramp-down time of 4 to 10

hours range.

2. Hold: At-185 0C, the material was soaked for 20–30 hours.

3. Ramp up: The chamber was then warmed to +150 0C to temper the primary martensite,

after which it was returned slowly back to room temperature.

Two types of cruciform samples with LOP were tested (i) as-welded condition (non treated)

(ii) after cryogenic treatment (treated). The fatigue properties of cryogenically treated

samples have shown improvement. The strain-induced martensite formed during the

cryogenic treatment and the associated generation of compressive stresses in the weld metal

is considered to be effective in fatigue life extension of welded joints in the high cycle

regime.

27

J. D. Darwin et. al. [48] used Taguchi’s method for optimization of cryogenic treatment to

maximize wear resistance of chrome silicon spring steel. Sample of piston rings made of

chrome silicon spring steel SR10 were cryogenic treated using different factors such as

cooling rate, soaking temperature, soaking period, tempering temperate and tempering

period. L9215 array was used for the first experiment set up and L93

4 for the second set up.

All samples were subjected to sliding wear test. The results obtained indicated that soaking

temperature was the most significant factor for improvement in wear resistance. The

optimum soaking temperature which gave maximum wear resistance was –1840C. The

soaking period also affected wear resistance. 24 hours soaking yielded maximum

improvement. The wear resistance was improved by 48 %.

Also 18 % Cr martensite stainless steel SR34 piston rings were cryogenic treated using

different factors using Taguchi’s method and it was found that wear resistance got improved

by 43.8 % by using optimum conditions [49].

c) Cast iron:

Hao-haui-Liu et. al. [50] carried out investigation on 3Cr13Mo1V1.5 high chromium cast

iron due to cryogenic treatment. The testing high chromium iron was cast from 1450 0C as ф

80 mm balls by chilled mould. The ball was cut into sub-critical treatment samples,

cryogenic treatment samples and X-ray diffraction samples. The test samples were subjected

to sub-critical treatment at serials of test temperatures. The holding time was 1hr for each

temperature. After sub-critical treatment, the samples were air-cooled to room temperature or

put into liquid nitrogen directly and held for 3 hrs. The samples were subjected for abrasion

resistance experiments. The investigation had lead to conclusions as

1. Cryogenic treatment can effectively reduce further the austenite content after the sub-

critical heat treatment, but cryogenic treatment can not make retained austenite transform to

martensite completely. Cryogenic treatment can markedly boost hardness and abrasion

resistance of high chromium.

2. Cryogenic treatment accelerates the precipitation of secondary carbides and makes the

secondary hardening peak advanced at a lower temperature.

3. When there was about 20% retained austenite in matrix, abrasion resistance reaches the

maximal, and decreases sharply, if retained austenite content is less than 20%.

28

d) Tungsten carbide inserts:

Seah et. al. [51] did some study on the effect on cryogenic treatment on tungsten carbide and

found that the treatment improved the wear resistance. The increase in wear resistance was

attributed to increase in number of η-phase particles after cryogenic treatment.

A.Y.L.Yong et. al. [52] used sumitomo (sumitomo SNGG 23) cutting tool inserts for

cryogenic treatment. The inserts were treated at –184 0C for a soaking period of 18 hours.

There after the inserts were tempered. The treated inserts were used for turning operation

and flank wear of the inserts was measured using Mitutoyo toolmaker’s microscope. The

results obtained showed improvement in wear resistance of inserts. He also found that the

state of cryogenically treated inserts is not a permanent state, but a metastable state as at high

temperatures wear resistance is reduced at a faster rate.

Sumitomo milling inserts were cryogenically treated to a temperature of –184 0C for a

soaking period of 24 hrs. These inserts were used for face milling of ASSAB 760 medium

carbon steel with feed, depth of cut as 0.15 mm/tooth and 0.1 mm respectively, cutting

speeds used were 294.5, 343.5, 392.7, 409.9, 540.0 and 589 m/min. Flank wear of the inserts

was measured with a toolmaker’s microscope. It was found that cryogenic treatment to the

inserts improved tool life to certain extent [53].

B. S. Ajaykumar et. al. [54] carried out work on cryogenically treated P-30 tools. A statistical

model is developed to find the correlation between the temperatures induced during

machining process with the life of the tool. The purpose of the research was to determine

how cryogenic treatment responded to P-30 inserts by improving its life during machining

process. The result of the experiments indicated an increase in the tool life by an amount of

20 % when job is made to run at a speed of 71.62 m/min for different feed rates. To compare

the changes in temperature between treated and untreated as well as the tool life, analysis of

the variance was applied and it is found that they are statistically significant and all means

are different from one another. The mean tool life between the treated (22.385) and untreated

(19.79) was compared using independent t- test and it is found that the mean difference

between them is statistically highly significant. This shows that the tool life of treated

specimen is much higher compared to the untreated group. It was found that correlation

coefficient between tool life and temperature was negative. Relation between the two through

statistical modeling indicates that an increase in temperature affects the tool life.

29

e) Nickel-Titanium instruments:

Nickel titanium k-files were cryogenically treated at a temperature of –196 0C with soaking

period of 10 min. These files were subjected to a test for evaluating cutting efficiency.

Cryogenic treatment resulted in increased microhardness, but this increase was not detected

clinically. There was no measurable change in elemental or crystalline phase composition

[55].

f) Cr-Zr-Cu alloy spot welding electrodes:

W u Zhisheng et. al. [56] in their investigation used Cr-Zr-Cu alloy spot welding electrodes

for studying effect of cryogenic treatment. The spot welding electrodes were treated at

–150 0C and –170 0C for a soaking period of 2 and 4 hours. The investigation concluded that

electrode life was improved from 550 to 2234 welds by deep cryogenic treatment. The

capacity of heat transmission and the electrical conductivity of electrodes were improved by

deep cryogenic treatment. The improvement in life was attributed to finer grains in case of

deep cryogenic treated electrodes.

g) Cryogenic cooling effects in turning:

Machining of steel inherently generates high cutting temperature, which not only reduces

tool life but also impairs the product quality. Conventional cutting fluids are ineffective in

controlling the high cutting temperature and rapid tool wear. Further, they also deteriorate the

working environment and lead to general environmental pollution. Cryogenic cooling is an

environment friendly clean technology for desirable control of cutting temperature.

Experimental investigation of the role of cryogenic cooling by liquid nitrogen jet on cutting

temperature, tool wear, surface finish and dimensional deviation in turning of AISI-4037

steel at industrial speed feed combination by coated carbide insert was studied by N. R. Dhar

et. al. [57]. The results when compared with dry machining and machining with soluble oil

as coolant indicated substantial benefit of cryogenic cooling on tool life, surface finish and

dimensional deviation. This was attributed mainly to the reduction in cutting zone

temperature and favorable change in the chip–tool interaction.

A tool was modified by Ahsan Ali Khan et. al. [58] to apply liquid nitrogen as coolant

through a hole made in the tool so that liquid nitrogen can be directly applied to the

30

machining zone during machining of stainless steel SUS 304 with carbide tools coated with

titanium carbonitride. It was found that the tool life increases more than four times by

applying liquid nitrogen using the modified tool. Application of this cryogenic cooling was

found to be more effective at higher cutting speeds. It was also observed that cryogenic

cooling is efficient at a higher feed rate rather than a higher depth of cut.

Titanium alloys are being increasingly sought in a wide variety of engineering and

biomedical applications. Machining and grinding of these alloys imposes lot of constraints.

Rapid tool wear encountered in machining of titanium alloys is a challenge that needs to be

overcome. Researchers in order to reduce the cutting zone temperatures and enhance the tool

life investigated a process of cryogenic machining with liquid nitrogen as a coolant. The

effects of cryogenic cooling were studied on growth and nature of tool wear while turning Ti-

6Al-4V alloy bars with microcrystalline uncoated carbide inserts under dry, wet and

cryogenic cooling environments in the cutting velocity range of 70–100 m/min. Cryogenic

cooling by liquid nitrogen jets enabled substantial improvement in tool life through reduction

in adhesion–dissolution–diffusion tool wear through control of machining temperature

desirably at the cutting zone. It was concluded that proper application of cryogenic cooling in

machining Ti-6Al-4V alloy provides reduction in tool wear, which can lead to enhancement

of productivity along with environmental friendliness [59].

The work carried out by various researchers on effect of cryogenic treatment on properties of

different materials as discussed above indicates that some materials give good response to the

cryogenic process and their properties are altered. Some materials show less or no response

to the cryogenic treatment. Tool steels come under the category of good response to the

cryogenic treatment and the basic property improved in case of tool steels is wear resistance.

The improvement in wear resistance of one such newly developed tool steel is the work

undertaken in the present research.

In next section tool wear, types of tool wear, measurement of tool wear, tool life and a brief

review of the literature available on tool wear models, tool life models, optimization of

parameters for maximizing tool life etc. are discussed.

31

2.3 Tool Wear:

2.3.1 Introduction:

As discussed earlier, this section contributes towards literature review related to tool wear.

Tool life is the most important criterion for assessing the performance of a tool material,

machinability of work material and for determining cutting conditions. Flank wear is

generally considered as the decisive factor of the tool life. However, at higher cutting speeds

and metal removal rates, the tool failure may also be caused by cratering [2]. Different types

of tool wear are discussed in the following section.

2.3.2 Geometry of tool wear:

The progressive wear of the cutting tools can take three forms as shown in Figure 2.5:

(i) Wear on the tool flank characterized by the formation of a wear land as a result of the

newly cut surface rubbing against the tool flank, known as flank wear.

(ii) Tool wear on the rake face characterized by the formation of a crater or a depression, as a

result of chip flowing over the tool rake face known as face wear; and

(iii) Wear on the nose of the tool as a result of the work surface rubbing against the tool nose

known as nose wear.

Figure 2.5: Forms of Tool Wear

2.3.3 Flank wear:

As a rule, the extent of flank wear is considered a dependable criterion for judging the life of

the cutting edge. The flank wear is obtained on the flank of the tool. The flank wear can be

more easily observed and measured than other types of wear. It is relatively easy to predict

when a given amount of wear will be reached once the wear rate has been established. The

development of flank wear initially assumes a high rate followed by a more or less linear

increase and finally rises rapidly when the amount of wear crosses beyond the critical value

[2].

32

A typical case of flank wear development with respective to time is shown in Figure 2.6. It

can be seen from this figure that the graph can be divided into three definite regions A, B and

C. In the region A, the wear grows rapidly within a short period of time because during the

initial contact of the sharp cutting edge with the work piece, the peaks of the micro

unevenness at the cutting edge are rapidly broken away. In the region B, the wear progresses

at a uniform rate. In the region C, the wear rate is rapid and may lead to a catastrophic

failure of the tool. In general, it has been found that the most economical wear land at which

the tool needs to be removed and re-sharpened is just before the start of the rapidly increasing

portion of the curve. In the case of high-speed steels, the linear rate of wear section of the

curve is quite flat and the final failure occurs suddenly [2].

0

0.01

0.02

0.03

0.04

0.05

0 20 40 60 80

Tool life, min

Too

l wea

r, in

Figure 2.6: Wear verses Cutting Time [4]

The wear land on the flank will not be generally uniform along the entire cutting edge length.

Depending upon the machining conditions, the following types of wear lands or

combinations of them are generally observed [2].

1. Excessive wear at the nose end of the flank is brought about by deformation of the

tool material which reduces the relief in the area, thus increasing the rate of wear.

This can also be brought about if the crater on the rake face breaks through the nose

area.

2. Irregularities in the wear along the whole cutting edge length due to minute chipping

or attrition of the cutting edge

3. Excessive wear at the line of depth of cut. This can be either due to the work

hardened surface caused by the previous cut or heat treat scales or by other abrasive

materials on the work piece [2].

C A B

C

33

2.3.4 Face wear:

As the cutting speed increases, the tendency of the cutting tool to fail by cratering increases.

The tool-chip interface temperature increases with cutting speed and at the higher

temperatures the rate of material removed from the tool increases. The pattern of crater wear

indicates that the wear in this region is primarily due to the diffusion or chemical reaction

between the tool and chip material. At low cutting speeds a crater may be formed owing to

the action of the chip flowing over the tool rake face. The narrow region close to the tool

cutting edge is protected from the action of the chip by the presence of stable built-up edge.

This cavity of crater has its origin not along the cutting edge, but at some distance away from

it and within the chip contact area. It is to be noted that the maximum tool-chip interface

temperature occurs at a distance from the cutting edge, and in this region the crater is

initiated. As the crater wear progresses with time, it becomes wider, larger and deeper, and

approaches the edge of the tool. If the crater wear is allowed to proceed too far, the cutting

edge becomes too weak and breaks down suddenly. The depth of the crater and the distance

of the center line of the crater from the cutting edge are measured for the quantitative

assessment of the crater wear [2].

2.3.5 Measurement of flank wear:

Tool wear is determined by observing and measuring the wear as it develops, specifying the

effective cutting time elapsed before a stipulated degree of wear is reached. The flank wear

can be measured readily on a toolmaker’s microscope or a calibrated magnifying glass

having about 10 x magnifications. The flank wear is generally measured from the original

cutting edge. If the flank wear is uniform along the three zones as shown in Figure 2.7 the

mean value VB is determined. If more wear develops in any one part of the cutting edge, the

maximum value VBmax is recorded and for the calculation of the mean value of wear, the

zone containing the maximum value is not considered [2].

Figure 2.7: Regions of Flank Wear [2]

Zone A

Zone B

Zone C

VB

L/4 L/2 L/4

L

34

2.3.6 Tool life:

Tool life may be defined as the effective cutting time between two consecutive re-

sharpening. When the wear reaches a certain value the tool is not capable of further cutting

unless it is re-sharpened. Tool life is the most important criterion for assessing the

performance of a tool material, machinability of work material and for determining cutting

conditions. Flank wear is generally considered as the decisive factor of the tool life [4].

Tool life is an important practical consideration in selecting cutting tools and cutting

conditions. Tool wear and fracture rates directly influence tooling costs and product quality.

Due to these reasons tool life is the most common criterion used to rate cutting tool

performance and the machinability of materials.

In 1977 the International Standards Organization (ISO) introduced the ISO: 3685 [60],

aiming at unifying tool life testing procedures and measured parameters in turning. The

standard defines tool life as the time elapsed until a definite amount of wear has occurred in

the rake face or flank of the cutting tool. This parameter is less useful in practical terms, as it

does not provide information about has many good pieces can be produced, but it provides

definition of tool life, which is independent of workpiece geometry and quality requirements.

More standards followed in 1989, covering tool life testing procedures in face milling and

end milling and a revised edition of ISO: 3685 appeared in 1993 [61]. The aim of these

standards is to unify testing procedures in order to increase reliability and comparability of

test results when making comparisons of cutting tools, work materials, cutting parameters,

cutting fluids, treatments given to cutting tool materials etc.

Tool life of a cutting tool is dependent on various factors such as tool material, process

parameters, tool geometry, work material, cutting fluid and treatments given to cutting tool

materials.

2.3.7 Literature review on tool wear and tool life models:

Various researchers have made efforts to optimize tool life of tools using different

optimization techniques. Taguchi’s orthogonal array technique also has been made use of for

optimizing process parameters to yield higher tool life or lower tool wear. Also efforts have

been made to optimize process parameters used during turning operation to have lowest

values of surface roughness. The response obtained and the input parameters were used to

find out relationship between them using regression analysis.

35

S.K. Choudhary et. al.[1] developed an empirical mathematical model for measurement of

tool wear using cutting parameters such as cutting speed, feed, depth of cut and the force

ratio in case of turning operation. A series of experiments were conducted using C45

workpieces and HSS tool. The mathematical model developed is

w = 0.324 (Ft / Fc) 0.601 + 0.000003N 1.623 f 0.912 d 1.162 D 1.01

where w is flank wear in mm, Ft trust force in N, Fc is cutting force in N, N spindle

revolutions, f is feed in mm/rev, d is depth of cut in mm and D is diameter of work in mm.

This model clearly depicts that flank wear of the tool is a function of cutting speed (as V is

dependent on N and D), feed and depth of cut parameters used in turning operation. Wear

from this experimental model developed was compared with experimental wear values,

which showed a good agreement.

Dragos A. Axinte et. al. [62] proposed a method to allow an experimental determination of

the extended Taylor’s equation with a limited set of experiments and to provide a basis for

the quantification of tool life measurement uncertainty. The model proposed by them is

TL = f (Cp, f, V, VB) = G apa f b V c VB d

where TL is tool life in min, ap is depth of cut in mm, f is feed in mm/rev, V cutting speed in

m/min, VB is flank wear in mm. G, a, b, c and d are constants, which were found out by

regression analysis for different cutting fluids. The method was applied for evaluation of

efficiency of cutting fluid efficiency in turning. Experiments were run in a range of cutting

parameters, according to a 2 3–1 factorial design, machining AISI316L stainless steel with

coated carbide tools. An extended Taylor’s equation for each fluid was found from regression

analysis.

In another research, S. K. Choudhary et. al. [63] used an optimization technique for

machining EN 24 and HSC materials with HSS tool. The cutting speed and feed values were

optimized for a given metal removal rate. The optimized values of cutting parameters showed

sustained improvement in tool life for both work materials. The models proposed for tool

wear of HSS tool with HSC and EN 24 materials respectively are as follows:

hf = 6.64.10-3 V 1.69 f 1.77 t 0.96 and

hf = 5.45.10-4 V 1.86 f 1.36 t 0..87

where hf is height of flank wear of tool, V is cutting speed in m/min, f is feed in mm/rev and t

is machining time in min. This indicates that tool wear is a function of process parameters

and machining time.

36

A.M.A. Al-Ahmari [64] has established empirical models for tool life, cutting force and

surface roughness based on turning experiments. The developed machinability models can be

utilized to formulate an optimization model for the machining economic problem to

determine the optimal values of process parameters for the selected material. Data of 28

experiments when turning austenitic AISI 302 have been used to generate, compare and

evaluate the proposed models of tool life, cutting force and surface roughness for the

considered material.

Mozher [65] conducted 28 experiments on austenitic AISI 302 material having 50 mm

diameter and 500 mm length. He considered four machining parameters (cutting speed (v,

m/min), feed rate (f, mm/rev), depth of cut (d, mm) and tool nose radius (r, mm)). The

considered outputs were tool life (T, min), cutting force (Fc, N) and surface finish (Ra, µm).

The experiments have been done on 16K20 lathe machine using throw away carbide inserts

IN.2, IN.3 and IN.4 based on the design levels of tool nose radius with tool holder T2. He

constructed three models of tool life, cutting force and surface finish using RA technique and

the logarithmic transformation of their first order models and evaluated these models using

the analysis of lack of fit. The model obtained for tool life is as follows:

T = 406.423 r0.038/ v1.051 f 0.289 d 0.219

Guey Jiuh Tzou et. al. [66] used Nickel base super alloy Inconel 718 as a work material for

optimizing cutting parameters for turning operation. Using Taguchi’s L18 Orthogonal Array

cutting speed, feed, different cutting tool inserts, working temperature and ultrasonic power

parameters were optimized. Flank wear of the tools was the response variable. S/N analysis

and ANOVA were employed for optimizing the parameters, which were confirmed by

carrying out confirmatory test.

Non-linear regression analysis techniques were used by S.E. Oraby et. al. [67] for

establishing models for wear and tool life determination in terms of the variation of a ratio of

force components acting at the tool tip. The ratio of the thrust component of force to the

power, or vertical, force component has been used to develop models for (i) its initial value

as a function of feed, (ii) wear and (iii) tool lifetimes. Predictions of the latter model have

been compared with the results of experiments, and with predictions of an extended Taylor

model. In all cases, good predictive capability of the model has been demonstrated. It is

argued that the models are suitable for use in adaptive control strategies for centre lathe

turning.

37

In work carried out by Yahya Isik [68] a series of tests were conducted in order to determine

the machinability of tool steels. In these tests the effects of tool material, type of coating on

the insert (for coated tools) and the cutting parameters that affect the machinability were

taken into considerations. The cutting force data used in the analysis were gathered by a tool

breakage detection system that detects the variations of the cutting forces measured by a

three-dimensional force dynamometer. The workpiece materials used in the experiments

were cold work tool steel AISI O2, hot work tool steel AISI H10 and mould steel AISI

improved 420. The cutting tools used were HSS tools, uncoated WC and coated TiAlN and

TiC + TiCN + TiN inserts (ISO P25). No cutting fluid was used during the turning

operations. During the experiments cutting forces, flank wear and surface roughness values

were measured throughout the tool life and the machining performance of tool steels were

compared. As a result of the experiments conducted. The conclusions concerning the tool life

are as follows:

• Cutting speed (V) is the most influential parameter on tool life, feed rate (f) is the

second most one, and cutting depth (d) is the least influential parameter. The

influence of cutting depth is negligible compared with those of the other cutting

parameters

• At the end of the tool life, considerable increase in cutting forces (Fs) is observed, but

the increase rate varies according to the cutting tool and the workpiece

• The amount of flank wear and the cutting force are appropriate parameters to

determine the tool life

• Prediction of tool wear becomes possible on condition that the cutting speed range,

recommended for the tools, is employed.

Theoretical and experimental studies were carried out by X. Luo et. al. [69] for investigating

the intrinsic relationship between tool flank wear and operational conditions in metal cutting

processes using carbide cutting inserts. A new flank wear rate model, which combines

cutting mechanics simulation and an empirical model, is developed to predict tool flank wear

land width. A set of tool wear cutting tests using hard metal coated carbide cutting inserts

were performed under different operational conditions. The wear of the cutting inset was

evaluated. The results of the experimental studies indicate that cutting speed has a more

dramatic effect on tool life than feed rate. The wear constants in the proposed wear rate

model are determined based on the machining data and simulation results. A good agreement

38

between the predicted and measured tool flank wear land width show that the developed tool

wear model can accurately predict tool flank wear to some extent.

In a research work carried out by E. Usui et. al. [70] an analytical method is presented, which

enables the crater and flank wear of tungsten carbide tools to be predicted for a wide variety

of tool shapes and cutting conditions in practical turning operations based only on orthogonal

cutting data from machining and two wear characteristic constants. A wear characteristic

equation is first derived theoretically and verified experimentally. An energy method is

developed to predict chip formation and cutting forces in turning with a single-point tool

from the orthogonal cutting data. Using these predicted results, stress and temperature on the

wear faces can be calculated. Computer simulation of the development of wear is then

carried out by using the characteristic equation and the predicted stresses and temperatures

upon the wear faces. The predicted wear progress and tool life are in good agreement with

experimental results.

Hong-Tsu Young [71] carried out work on workpieces made of annealed carbon steel AISI

1045. The wokpieces were machined on a lathe. The cutting speed and depth of cut used in

turning operation were 100 m/min and 1.3 mm respectively. Tool wear was measured using

tool maker’s microscope. The graph plotted between tool verses chip back maximum

temperature indicted that as chip back maximum temperature increased, flank wear of tool

increased. Thus, it was concluded that tool wear is strongly temperature dependent.

F. Boud [72] performed experiments in single point turning. Factors such as cutting speed,

feed rate, tool material, etc. are well known to have an effect on tool wear in metal turning.

He performed experiments on solid carbon steel, by using uncoated HSS insert for studying

effect of bar diameters on tool wear. All experiments were performed under identical cutting

conditions, i.e. cutting speeds were constant as well as the feed rate and depth of cut, also the

same bar was used except for varying the bar diameter. The findings are analyzed in three

separate ways. The conclusion of the research work is that bar diameter has an influence on

tool temperature and, by implication, on tool wear. Thus, a factor not previously considered

in wear theories in turning is shown to be significant.

Lee et. al. [73] carried out work for obtaining the optimal tool life with the ratio of cutting

speed to flank wear value. He claimed that the results have shown a good correlation between

the dynamic tangential force and flank wear.

39

In a research work P. Vamsi Krishna et. al. [74] conducted cutting tests under dry, wet and

solid lubricant conditions to study the process performance in terms of cutting forces, tool

wear, tool temperature and surface roughness in turning. For this purpose, cemented carbide

tool and EN8 steel workpiece were selected. The cutting tests were performed on PSG-124

lathe. Process parameters selected for turning were cutting velocity 110 m/min, feed rate 0.25

mm/rev and depth of cut 1.0 mm. One of the conclusions of the work is that cutting forces

and tool flank wear are reduced because of the formation of a boric acid film on the surfaces.

Thus tool wear is affected by cutting fluid used in turning operation.

Yong Huang et. al. [75] evaluated tool performance as a function of cutting conditions based

on the flank wear criterion using the calibrated tool wear model in finish turning of hardened

52100 bearing steel. A satisfactory match has been reached when comparing model

predictions and experimental measurements. Further, an analysis of variance was carried out

to investigate statistical significance among cutting conditions, and it shows that cutting

speed plays a dominant role in determining the tool performance in terms of tool life,

followed by feed and depth of cut, and overall tendencies agree with predictions from the

general Taylor tool life equation as well as experimental observations.

Kaye et. al. [76] developed a technique for on line prediction of the tool flank wear in turning

using the spindle speed change. A series of cutting tests were performed by them using

various combinations of cutting speed, feed rate, depth of cut and material hardness. It was

concluded that the predictions from the developed model are in well agreement with actual

flank wear measurements.

Alakesh Manna et. al. [77] worked on E0300 alloy steel for optimizing the cutting

parameters such as cutting speed, feed and depth of cut. Three levels of each parameter were

used to find out the surface roughness of the materials and using Taguchi’s approach the

cutting parameters were optimized to yield higher surface finish quality. An empirical

mathematical model for surface roughness due to cutting parameters was evolved and

confirmed.

Sumit Kanti Sikdar et. al. [78] carried out turning operation using CNMG120412N-UJ

inserts for cutting low alloy steel AISI 4340. Flank wear surface area was measured with

help of surface texture instrument. Cutting forces were measured with help of kislter piezo

40

electric dynamometer. Mathematical model between tool flank wear area and components of

cutting forces was developed.

In a research carried out by C. Y. Nian et. al. [79] proved the optimization of multiple

performance characteristics in turning S45C steel bars. Cutting parameters i.e. cutting speed,

feed and depth of cut values selected each at three levels were optimized to give better

surface finish, lower cutting force and higher tool life i.e. lesser tool wear by using Taguchi’s

L27 array.

E Daniel Karby [80] carried out turning operations on a CNC lathe using VNE versa Turn

CCGT 432 –AF inserts for turning steel bars using Taguchi’s L8 OA. The selected

parameters were feed at four levels, speed at two levels and depth of cut at two levels. The

S/N analysis and ANOVA techniques were used to optimize these parameters to yield lower

value of surface finish. Confirmation test for optimum and one of the non optimum

conditions were conducted and robustness of parameters optimization was experimentally

verified.

The brief review as discussed above, of the literature available on tool wear / tool life models

developed for the cutting tools by researchers indicate that tool wear is a criterion used for

judging the life of the tool. Tool wear is a function of the temperature developed in

machining zone, except the initial wear due to plastic deformation. The wear of the tool

increases as temperature developed in machining zone increases. In uniform wear rate zone

the temperature of machining zone is basically function of cutting process parameters i. e.

cutting speed, feed ,depth of cut , cutting fluid used, tool geometry, tool and work material

etc. In case of process parameters cutting speed is the predominant factor for the increase in

machining zone temperature followed by feed and depth of cut. The properties of tool

material such as hardness, red hardness, wear resistance etc. also play an important role in

variation of tool wear.

From the observations of the literature referred above on the cryogenic treatment effect on

material properties, especially tool steel and tool wear as a function of process parameters

and treatment provided to the tool materials, objectives of the present research are finalized.

These observations and objectives of proposed work are discussed in the next section.

41

2.4 Observations and Objectives:

The literature available on effect of cryogenic treatment on properties of materials indicates

that no systematic procedure has been followed by the researchers for studying effect of

cryogenic treatment on different materials. The temperature levels selected, soaking time

used during treatment even for the same material by various researchers varies a lot. Due to

this, the results obtained by various researchers match on gross level but minute details of the

effect are not worked out by the researchers.

Majority of the research carried out on tool steels for studying the effect of cryogenic

treatment is mainly concentrated on either M2 tool steel or die steel i.e. D2. There is very

little research carried out on T series tool steels for effects of cryogenic treatment.

The approach used by all researchers is basically oriented in metallurgical manner i. e. to

trace out the reasons for improvement/reduction in properties of material due to cryogenic

treatment. No efforts are observed to have a mathematical approach in evaluation of the

improvement / reduction in properties of the material due to cryogenic treatment. Therefore,

an attempt is made to develop empirical mathematical models for the first time for T42 tool

steel.

In order to meet out the gap in the available literature; in this present experimentation the

study is undertaken in following manner. These steps are already identified as objectives and

are mentioned in chapter 1.

• Pilot experiments shall be carried out with T42 HSS samples for studying effect

of cryogenic treatment on wear of the tools.

• A mathematical model for wear of the tool with respect to process parameters

such as cutting speed, feed and depth of cut shall be proposed.

• A mathematical model for wear of the tool with respect to cryogenic treatment

temperature, cutting speed, feed and depth of cut shall be proposed.

• To find out the effect of cryogenic treatment on wear properties of the tool by

comparing the two models proposed above.

• Experimentation as per Taguchi’s Orthogonal Array shall be carried out for

conventionally treated T42 HSS tools.

• Experimentation as per Taguchi’s Orthogonal Array shall be carried out for

cryogenically treated T42 HSS tools.

42

• An empirical mathematical model shall be evolved using regression analysis for

evaluating wear of conventionally treated tools using the data obtained from non

cryo tools experiments.

• An empirical mathematical model shall be evolved using regression analysis for

evaluating wear of cryogenically treated tools using the data obtained from cryo

treated tools experiments.

• The effect of cryogenic treatment on wear properties of the tool will be found out

by comparing the two models developed as above (point 7 and 8 above).

• Nomogarphs shall be developed for tool wear of T42 tool steel with respect to

cryogenic treatment temperature, cutting speed, feed and depth of cut, which

can be used as a ready reckoner for predicting the tool wear.

Thus, after identifying the need, objectives of present work and methodology, the

mathematical model development is discussed in the chapter to follow, i. e. Process

Modeling.