Cutting Of Hardened Steel Biology Essay

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Hardened steel cutting is of great interest for today's industrial production and scientific research. Machine parts and tool made of hardened steel are supposed to be high performance and function to their maximum strength. More and more machine parts are expected to meet very high surface finish which is usually achieved by cutting processes such as grinding. Hardened steels are becoming more and more relevant in modern machining due to their extensive utilization in machine tools and process industry. As presented later in this report that hard cutting is seriously regarded as an alternative for grinding operations under certain circumstances.

Properties of Hardened Steel:

Hardened and case hardened steel acquires various mechanical properties which in turn affects subsequent cutting process. Ferrous work pieces can achieve high hardness by Martensitic Transformation and Carbide Precipitation.

Fig.01: Hardened and microstructure of hardened steel (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

The carbon content influences martensitic hardening and should not be less than approximately 0.25%. Otherwise, the cooling rate necessary for martensitic transformation is difficult to reach.

Fig.02: Martensitic Structure Transformation (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

Optical economical and technical designing of mechanical parts and machines requires comprehensive knowledge of all failures and their probable causes. All machine and mechanical parts are volume and surface loaded. Volume loads are basically classified as mechanical and thermal loads leading to deformation and fracture. On other hand surface load cause corrosion and wear. See Fig.03.

Fig.03: Possible Machine Parts Failure (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

Mechanical properties of work pieces can be improved by hardening and tempering or case hardening of ferrous. Besides the improvement of strength and hardness, the fatigue strength is considerably increased [12]. Heat treatment is aimed to improve the hardness of work piece also result in improvement of wear resistance. Fig.04 shows how rolling contact fatigue is influence by workpiece hardness for different alloyed steels. As hardness increased, rolling contact fatigue increased and improves working life to 10 times of initial state. A higher workpiece subsurface hardness generally increases the resistance against wear. Further factors like structure constituents additionally influence the resistance against wear.

Fig.04: Influence of workpiece hardness on rolling contact fatigue (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

CHIP REMOVAL IN HARD CUTTING:

Applying hard cutting as a finishing process requires the generation of machined surface by pure plastic deformation Therefore, the stress, strain and temperature distribution the cutting zone is of interest. The process evaluation of hard cutting requires a full knowledge and understanding of material removal mechanisms. The chip formation in hardened materials is very necessary in this purpose. Based on various cutting parameters and workpiece mechanical properties determine the formation of chip in hardened steel. Komanduri and Brown have classified chip formation in following manners as shown in fig.05. [13]. The wavy chip is generated by cyclic variation of chip thickness caused by oscillation of the shear angle. The low speed cutting of brittle materials result in discontinuous chip.

Fig.05: Classification of Chips (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

The properties of materials such as elastic deformation, plastic deformation, cracks or localized shearing, also known as adiabatic shearing is mainly influenced by temperature, shear distribution, strain and strain rate. In this report, chip formation mechanism is analyzed based on plastic behavior of material and chip shape. For examples, chip formed during high-speed machining of hard material is in form of saw-tooth chip. Such type of chips is formed due to localized shear, adiabatic shear and catastrophic shear resulting in extensive cracks [1]. Chip formation is a result of the changeable cutting conditions, which mainly depend on: 1

Mechanical, thermal and thermo-chemical characteristics of the work material;

Cutting conditions;

Changes in sliding characteristics at the primary zone;

Changes in tribological circumstances at the tool-chip interfaces;

The possible interactions between the primary and secondary shear zone and the dynamic behaviors of the machine-tool system and its linkage with the cutting process.

It is notices that with increase of cutting speed the chip segmentation frequency increases while chip thickness and magnitude of chip segments decrease [2].

Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566

2.1- PLASTOMECHANICS:

Tension test of hardened steel shows the stress-strain curve is almost linear till fracture. Thus there is practically no plastic deformation. Thus, then how smooth surfaces are produced in hardened machining. There are two theories to explain plastic deformation in hardened steel:

Thermodynamics Theory,

Hydrostatic Theory

According to Thermodynamics theory there is a decisive influence of the heat conductivity of the tool material which causes 'self induced heating' of chip formation zone. It should not be too high to keep the heat in the chip formation zone and to generate a sufficient level of temperature. To examine this theory, Brand made cutting temperature experiments under constant conditions varying only the heat conductivity of the tool material [3]. He could not find a significant influence of the heat conductivity on the cutting force (figure.06). This indicates that thermodynamics theory cannot fully explain plastic deformation of hardened materials. This is underscored by force measurement if speed varies. Here, if self induced heating of the chip formation zone would be the dominant effect, the cutting force should decrease with increasing the cutting speed.

Fig.06: Heat Conductivity and Cutting Force (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

According to Hydrostatic theory, material even brittle is deformed plastically if shear flow limit is achieved. In such region chip thickness is extremely small (few micrometers)

resulting in a geometric condition which leads to effective rake angle of -60Ëš to -80Ëš. Such condition is achieved due to high hydrostatic pressure with a little undeformed chip thickness and a highly negative rake angle.

The material behavior in the work zone and so the thermo-mechanical mechanisms strongly depend on cutting parameters and especially on chip thickness 'h' and cutting speed 'Vc' Figure 3.4 shows the influence of the chip thickness on the chip formation process.

Fig.07: Chip Thickness and Chip Formation (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

In 1985, Recht introduced the adiabatic shear theory to characterize the chip segmentation process during hard cutting operations. The thermo-plastic instability is present where flow stress decreases due to thermal softening caused by increase in strain, thus, offsetting strain hardening. Proponents of this theory refer to this to explain the chip segmentation. It is assumed that for some reasons a thermo-plastic instability occurs along a line, extending from the tool tip and curving upwards to the free surface of the workpiece [3].

According to Nakayama, the segmented chip will generate, if the shear strain on the free surface of the workpiece attains an ultimate value ґc. In a simplified model the crack initiates at a point Q of a free workpiece surface. The free surface at Q must be a principle stress direction. Hence the shear plane includes an angle of π γ/4 with the free surface. If Ψc, is the inclination angle and Φ the shear angle, it is Φ = π /4 - Ψ [4].

In cutting experiments of a bearing steel of 760 HV (~62 HRC), the rake angle is varied between γ = -10˚ and γ = -50". In spite of this wide variation of the rake angle, crack inclination Ψc, is almost constant and amounts to Ψc = 30 [4].

Considerable experimental evidence supports the concept that the root source for saw- tooth chips is cyclic cracks that initiate at the free surface of the work and proceed downward along a shear plane toward the tool tip and not adiabatic shear [5].

Fig.07: Crack initiation during saw-tooth chip formation (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

Cutting experiments show that the chip formation in hard turning starts with crack initiation near the free surface. Cracks propagate and end up in a plastically deformed region close to the tip of the cutting edge (fig.08). Fig.08: Micro Structure and Chip Formation (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

According to the hypothesis mentioned above, the crack initiates from the free surface to a point where transition from brittleness to ductility takes place due to higher hydrostatic pressure near the tool tip area. On the basis of this hypothesis Elbestawi concluded that the crack in the free surface will initiate at the critical angle Φcr against the cutting direction when the surface layer energy reaches its maximum value at the minimal applied cutting pressure. Nakayama showed that crack initiated at an angle Φcr = 33˚. Table.01 summarizes the different models for chip formation in hard cutting.

Table.01: Models for Chip formation in Hard Cutting

This various theories and observations are summarized as follows. Material loads are determined as high compressive stresses and high temperatures in the tool tip which lead to deformation of the material. If this condition fails from a distance from tool tip and chip thickness of 20 microns, segmented chips are formed. If workpiece properties such as stress and strain are met, crack initiation occurs with minimal compressive stresses and local thermal softening with high strain rates.

FORCES AND STRESSES:

Matsumoto observed that in cutting hardened steel, resultant forces and stress distributions in the contact area are substantially influenced by the hardness of the workpiece material. The forces occurring in cutting soft steels are relatively high and decrease if hardness increases. With the hardness exceeding 50 HRC the cutting force increases suddenly. The passive force is known to alter the maximum strain value in the chip and the chip type [6].

Fig.09 shows the relationship between components of machining force and the workpiece material hardness.

Fig.09: Relationship between Forces and Material Hardness. (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

Two types of cutting phenomenon are observed based on hardness of the workpiece material. When material hardness range between 30 and 50 HRC, continuous chips are observed. Increase in hardness causes decrease in cutting forces which is explained by tool-chip contact temperature effect. There is a rise of tool-chip contact temperature with increase in hardness of material; shear force on the rake face is reduced. The shear angle increases and chip thickness decreases while tool-chip area is reduced. This all leads to the reduction of cutting forces.

When hardness of workpiece exceeds 50 HRC, cutting forces suddenly increase and chip segmentation appears. With increasing the hardness of the workpiece, two contradicting factors appear, affecting the cutting mechanisms. First factor relates to increase in yield stress due to workpiece hardness; the second factor is reduction of yield stress due to cutting heat generation. Chip becomes brittle when hardness of steel exceeds certain value and deformation energy is small. This way generated heat is reduced and material softening does not occur [8]. Due to high shear force is caused by decrease in temperature at rake face when segmented chip appears. The high friction force results in high cutting force and high strain in the chip [6].

One contemporary research issue, related to the quest of enhancing flexibility as well as efficiency, being aggressively pursued by the precision manufacturing industry, is of replacing grinding of hardened steel with turning with its feasibility shown in many publications. In order to adapt turning process for surface finish in lieu of girding cutting parameters such as small depth of cut and low feed are adopted to reduce mechanical and thermal loads. Combination of large corner radius and cutting edge generates a smooth surface comparable with a ground surface [9]. This fact was explained by Nakayama [4] as a consequence of the absence of the built up edge and the minimal plastic flow due to the hardness of the work material. Large negative rake angle increases the cutting force Fc to a minor extent while passive force Fp increases remarkably as shown in fig.10. Schmidt performed experiments to study the effects of high negative rake angle in hard cutting due to large cutting edge roundness rß, a small depth of cut ap and large corner radius rE. Remarkable increase of passive force with smaller rake angle is due to two causes: on one hand with decrease in rake angle; a large portion of thrust force (vectorial addition of feed and passive force) is transmitted in passive direction. On other hand increase in cutting force causes friction to increase which subsequently increase passive forces.

Fig.10: Cutting forces and Cutting Edge Radius rß. (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

It can be summarized that the highest shear stress is found that free end of chip-workpiece interface. These shear stress along with y-direction principal stress (decreasing) and decreasing local temperature causes crack initiation with increase depth of cut as depicted in fig.11. Therefore, a segmented chip is formed.

Fig.11: Stresses and Temperature vs Uncut chip thickness. (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

ENERGY AND TEMPERATURE:

The cutting energy in any machining process is almost completely converted into heat. The heat is generated due to mechanism of material deformation, friction and material removal. The friction between rake face and the new generated workpiece in key mechanism effects surface quality of a workpiece.

Ueda investigated the influence of cutting parameters and workpiece material on the temperature of the cutting edge. It can be clearly stated that the temperature rises with cutting speed and with the hardness of the workpiece [8] as shown in fig.12.

The investigation of the influence of the workpiece material hardness on cutting temperatures confirms the results of the examination of the machining force (fig.09). In a higher range of hardness leads an increase of the material hardness to an increase of the cutting force. With increasing the cutting force cutting energy is becoming higher and results in elevated temperatures.

Higher temperature results in high residual stress and emergence of white layer. The residual stress measure using x-ray diffraction increases with thermal power per unit length (Pa). When Pa increases 150 W/m white layers appear in subsurface of workpiece.

Fig.12: Influence of cutting speed on Temperature. (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

APPLICABLE MATERIALS AND THEIR PROPERTIES:

With high cutting forces and process temperature involve in hard cutting, cutting tool with resilient features are required. In table.02, mechanical and thermal properties of different cutting tool materials are given as an overview.

Cermented Carbide K10

Ceramic

PCBN

PCD

Mechan-ical Properties

Density (g/cm3)

14.0-15.0

3.8-5.0

3.4-4.3

3.5-4.2

Hardness HV30

1500-1700

1800-2500

3000-4500

4000-5000

Young's Modulus (GPa)

590-630

300-400

580-680

680-810

Fracture toughness (MPa)

10.8

2.0-3.0

3.7-6.3

6.8-8.8

Therm-al Propertie

Temp. Stability (ËšC)

800-1200

1300-1800

1500

600

Thermal Conductivity (W/K.m)

100

30-40

40-100

560

Thermal expansion (K-1)

5.4

7.5-8.0

3.6-4.9

4.2-4.9

Table.02: Mechanical and Thermal Properties of Tools (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

Following are key characteristics of hard cutting tools:

High hardness of cutting tool is required. Usually it has to be three times higher than that of workpiece. High resistance of contact area against strong impact and stress is required as well.

A high hardness to Young's modulus ratio is required to minimize the quantity of local plastic deformation.

Geometric accuracy and surface integrity is influenced by thermal conductivity of material. The thermal conductivity of cutting tool material influences the expansion of tool and workpiece.

The high specific forces causes on the contact area between tool and workpiece. Therefore, cutting tool materials must have high resistance against mechanical stress.

High thermal stability of the cutting tool material is highly recommended as energy resulting from cutting is almost completely converted into heat.

The most often applied cutting tool materials for hard turning and face milling operations are AI2O3/TiC-ceramics and PCBN. Their high hardness combined with high temperature stability enables these materials to resist the thermal and mechanical loads in the hard cutting process. The application of cemented carbide tools is also limited by their comparatively low temperature stability. A most important difference between ceramics and PCBN is the value of fracture toughness. A high thermal conductivity and a low thermal expansion coefficient are of importance. Both characteristics favor PCBN as the more adapted tool material for hard cutting processes. The hardness of PCBN is surpassed by PCD. However at low temperatures, the diffusion ability of carbon in ferrous materials is too high so that PCD cannot be applied in hard cutting of steel.

WEAR MECHANISMS:

Hard cutting processes are marked by very high mechanical and thermal loads. Hard cutting tools require having hard and ultra-hard cutting properties.

Figure 4.2 shows tool life in turning of hardened steel, using different ceramic and PCBN inserts.

Fig.13: Tool life for different hard cutting tool. (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

PCBN is the cutting tool material with the longest possible tool life. But the composition of the tool material exerts an influence on the wear mechanisms. The material properties of PCBN can be influenced by the PCBN content, the grain size and distribution as well as the composition of the binder phase which can be ceramic or metallic. For hard turning operations usually ceramic binders are preferred.

The interaction of tool wear with different cutting edge radii is shown in fig.14. Tools with lower cutting edge radius show higher wear resistance. These tools allow a 1.4 times higher cutting time until reaching the width of flank wear of VBc = 100 micron, compared to the tool with the higher cutting edge radius. Both tools show crater wear. Tool with higher cutting edge radius has crater wear is localized completely in cutting edge radius. In comparison the tool with smaller cutting edge radius shows crater wear in rake face which increases the chipping possibility.

Fig.13: Tool Wear. (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

GEOMETRIC ACCURACY:

Surface integrity is a main requirement of workpiece quality for finishing processes. In many applications, conventional lathes deliver sufficient surface qualities in the range of R, = 2-4 µm. Higher requirements on surface quality can be fulfilled by the use of high-precision lathes. Figure 5.3 shows that a surface roughness in the range of Rz = 0.5-1 µm can be achieved. Fine grinding or honing qualities are defined up to a surface roughness of R, < 1 µm. Feed rates of f = 0.1 mm deliver surface qualities in hard turning comparable to fine grinding or honing [11].

An example for the application of high-precision lathes the production of an inner ring of a roller bearing with very close tolerances for the profile contours.

Fig.14: Surface Finish in Hard Turning. (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

One of the main benefits of hard turning is the possibility to avoid the use of cooling lubricants. On the other hand, in the case of dry machining, form errors gain more importance. The form errors of the workpiece occur due to the thermal expansion of the workpiece and cutting material and lead to a reduction of workpiece diameter in feed direction as shown in fig.15. Additionally, the thermal effects lead to deviation of parallelism on the surface lines. A more common way to increase workpiece quality preventing form errors due to thermal effects is the use of cooling lubricants.

Fig.15: Form errors in dry machining. (Source: Cutting of Hardened Steel, CIRP Annals, Volume 48, issue 2, 2000, pages 547-566)

ECONOMICS:

Material Removal Rate (MRR) by far the most important criteria determining the economical aspect of productivity of any cutting process. Usually very high surface finish is achieved by using grinding after rough cutting process but by using hard turning as discussed earlier. Hard-turning is used for both internal and external grinding using same tool thus reducing machining time and cost. Final determination of finishing process is dependent on specific situation. An additional bonus of hard turning is avoiding cutting fluids. The possibility of dry machining means saving considerable costs otherwise caused by buying, monitoring, treatment and disposal of cutting fluids.

ENVIRONMENTAL ASPECTS:

Assumed that 5000 parts of a gear component have to be produced per year, approximately 50 kg chips will result from the over measure machining. This is independent from the chosen manufacturing process. However, in grinding a consumption of coolant has to be considered additionally which reaches up to 8 tons per year. Furthermore, dependent on grinding wheel composition, about 20 cm of abrasive and bond particles are generated during machining and dressing. These particles are mixed with coolant, chips and filter material. In industrial application, it is nearly impossible to separate these materials. Thus, the waste consists of a variety of different, usually detrimental to health and environmentally harmful waste materials; the whole amount has to be disposed under special security conditions.

Cutting leads to more favorable conditions. Due to the possibility of dry machining, there are only chips consisting of not contaminated workpiece material that can easily be recycled. Very small amounts of tool material are to be neglected because they are dissolving easily in the steel matrix. Tool can be disposed (ceramics) or reused after sharpening, but there is no mixture of different materials. Thus, cutting of hard materials can be considered as a very efficient possibility for protection of environment.

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