1.Definition of Machinability
The machinability of a part material is usually defined and measured by a material
A property or quality indicating the ease with which it can machine.The machinability of any material may be measured by one or more of the following criteria:
1. Measure Tool Life
The permissible cutting speed for cutting a workpiece material under the condition that the same tool durability is guaranteed;
2. Measu re the quality of the process as Surface finish
3. MEASURE BY UNIT CUTTING FORCE
4. MEASURE BY EXTREME METAL REMOVAL RATE
5. Measure chip braking performance, including CHIP SHAPE
Good machinability should be: tool durability is excellent or in a certain degree of durability of higher cutting speed. The cutting force is small; the cutting temperature is low, easy to obtain better surface quality and chip shape to control or break chips easily.
2.Factors affecting the machinability of workpiece materials:
There are four main factors that affect the machinability of workpiece materials:
• The effect of physical and mechanical properties of materials on machinability
• effect of alloying elements in materials on machinability
• The effect of the material’s metallographic structure on machinability
• The influence of hot working processes on the machinability of materials
1. The influence of material physical and mechanical properties on cutting machinability
(1) the hardness of the workpiece material (including room temperature hardness and high-temperature hardness)
In general, similar materials in the high-temperature hardness of the low workability. Because when the content is hard, the contact length between the chip and the rake face decreases, so the normal stress on the rake face increases and the friction heat is concentrated on the smaller one.
The tool-chip contact surface, to increase the cutting temperature and wear, in the hardness is too high even cause tip .The burning and breaking of the blade.
Similarly, the higher the high-temperature hardness of the workpiece material, the lower the workability. Because in the cutting process, Because the cutting heat makes the cutting temperature rise, under the action of the cutting temperature, the hardness of the tool material and the workpiece material will.
If the hardness of the workpiece material is high at high temperature, the hardness of the tool material and the hardness of the workpiece material at high temperature.
The ratio will decrease, which will increase the wear of the tool. High-temperature Alloy, HEAT-RESISTANT STEEL CUTTING PERFORMANCE IS POOR And for that reason.
The more hardpoints, the more sharp shape and the more widely distributed in the workpiece material, the lower the workability.
The higher the work-hardening property of the workpiece material, the lower the workability. Certain High Manganese steels and AUSTENITIC STEELS
The surface hardness of the cut stainless steel is about two times higher than that of the original Matrix. The material has high hardening performance, first make cutting
The cutting force increases, and the cutting temperature increases. Secondly, the tool scratched by hardened chips, and there is boundary wear behind the pair
Third, when the machine is used to cut the hardened surface, the wear increases.
For Steel, the difficulty of cutting ranges from easy to steep in the following order: ferritic steel something Pearlitic Steel
Carbide section
For high temperature and alloy steels, the machining difficulty ranges from easy to steep in the next order: AUSTENITIC Section Steel
Ni-base hot-stable Alloy fe-base hot-stable Alloy ni-base hot-strength Alloy fe-base hot-strength Alloy–NICKEL-BASE CAST ALLOY.
(2) the strength of the workpiece material (both at room temperature and high temperature)
The higher the strength of the workpiece material, the greater the cutting force, the higher the cutting power, and the cutting temperature.The higher the strength of the workpiece, the worse the workability of the workpiece is.
Alloy Steel and Stainless Steel at room temperature strength and carbon steel similar, but high-temperature resistance is relatively large, so close. The machinability of gold and stainless Steel is worse than that of Carbon Steel.
Materials with a high content of high activation energy alloying elements not only have high room temperature strength, especially at high temperatures.
It’s still solid. For example, the strength of GH37 AlloyAlloy at 600-620C is still 1.4 of 45 # steel
Times. The higher the strength of the material, the higher the resistance to deformation and the higher the deformation power
Cutting force and cutting heat is also very large; the higher the high-temperature strength, the more difficult the processing.
In the following order from easy to difficult: pearlitic Steel something ferrite Steel something austenitic steel something something
Fe-based heat-stable Alloy ni-base heat-stable Alloy fe-base heat-strength Alloy ni-base heat-strength Alloy Ni-base Alloy ni-base
IRON BASE CASTING ALLOY
(3) the plasticity and toughness of the workpiece material
The elongation expresses the flexibility of the workpiece material. The higher the elongation, the greater the plasticity. When the strength is the same, The more senior the elongation, the greater the area of plastic deformation, and the higher the work consumed by plastic deformation significant.
The toughness of the workpiece material is expressed in terms of the impact toughness ak Value an article with a considerable ak value indicates that it was destroyed
The more energy you absorb, Similar articles, the strength of the same, plastic material cutting force, cutting temperature is higher, comfortable 3
And the tool bond, so the tool wear, has been machined surface is also rough. So the molding of the workpiece material. The higher the cutting performance, the lower the cutting performance. To improve the machinability of high-plastic materials, the
To reduce plasticity, as by hardening or heat treatment (by performing a plastic work, such as cold drawing).
But when the plasticity is too low, the contact length between the chip and the rake face is shortened too much, which makes the chip load (cutting force)
And cutting heat) is concentrated near the edge of the tool, will promote tool wear.
Too Low will make the cutting machinability decline.
Materials with a high content of nickel and chromium (such as 1Cr18Ni9Ti), although not very strong, are based on the structure has many lattice slip systems, significant elongation, considerable plasticity, and twisted lattice
It’s violent, so it’s still very deformed. The elongation rate is big, then the strengthening coefficient is significant, and the friction coefficient between the tool
And the friction angle is also larger, so the cutting force increases, the cutting heat increases, the elongation of the material processing difficulty is also large
Significant, in the following order from easy to difficult: Pearlitic type steel ferritic type stainless steel something austenitic Type Steel
Ferro-siderite-based Alloy ni-siderite-based Alloy.
When analyzing the influence of mechanical properties of different materials on the cutting difficulty, one of them must be the main one contradictory, but at the same time, it also must consider the interaction of other factors. It is challenging to process hot-strength Steel and AlloyAlloy.
The main factor affecting the degree is high-temperature strength, ultra-high-strength Steel is high hardness and room temperature strength, the thermal stability of Steel. And alloys are of high elongation.
To compare the effects of mechanical properties on machining difficulty, the following order can be followed.
Material with significant elongation, a material with high strength and hardness, a material with super strength and hardness. An element of high strength at high temperatures.
(4) the thermal conductivity of the workpiece material
Thermal conductivity is related to the properties of materials: There are many kinds of alloys with high melting point (low thermal conductivity)
The element enters the reliable solution, causes the crystal lattice distortion to create the dislocation to increase and the internal microscopic hole also has the massive
Hard phases (compounds of nitrogen, boron and carbon and intermetallic compounds) with very low thermal conductivity reduce the thermal conductivity
Low. The materials with austenite structure have low atomic activity, low nuclear amplitude and heat factor at the Lattice lattice of Face Center
The difficulty of conduction is also one of the reasons for the low thermal conductivity. When the material is of fine grain structure, excellent grain boundary summary Table
Large area and extremely low thermal conductivity non-metallic inclusions or compounds of nitrogen, boron, and carbon exist between grain boundaries
These materials have high resistance (low heat transfer), so their thermal conductivity is very low.
The thermal conductivity of Steel is in the following order (from low to high) according to its microstructure: austenitic something.
Quenched and tempered martensite troostite troostite troostite troostite troostite troostite pearlite.
The lower the thermal conductivity of the workpiece material, the less cutting heat transmitted through the workpiece and chip.
This will inevitably increase the amount of heat transmitted through the tool, and the device will wear out more quickly.
In general, the high thermal conductivity of the material, they are higher machinability; Conversely
Materials with low thermal conductivity have low machinability.
When cutting materials with very low thermal conductivity, a large amount of cutting heat transformed by plastic deformation concentrates on the tip of the tool
The contact area (temperature inversely proportional to the square root of the thermal conductivity) of the part can not be transferred rapidly, resulting in a cutting edge
Burn-out. Low thermal conductivity materials are challenging to process in the following order:
Pearlitic Steel something ferritic stainless Steel something martensitic stainless Steel something austenitic steel something something
Fe-based heat-stable Alloy ni-based heat-stable Alloy fe-based HEAT-RESISTANT STEEL Ni-based HEAT-RESISTANT STEEL
Iron-base casting alloy nickel-base Casting Alloy Titanium Alloy
(5) elastic modulus E of the workpiece material
The material with high elastic modulus has high atomic binding force (such as high-temperature AlloyAlloy, high-strength Steel), which changes during cutting
It is difficult to cut because of the high propagation speed of shape wave, extensive plastic deformation area, massive lattice distortion, and considerable deformation resistance
Some materials (such as titanium alloys) with small elastic modulus, although they have low interatomic adhesion and low deformation resistance,
because of the low propagation speed of deformation wave, the little plastic deformation zone and the small contact area of the chip, the cutting unit is cut
Increased pressure. The smaller the elastic modulus is, the higher the deformation of the material is, and the larger the friction coefficient between tool and chip is
When cutting low thermal conductivity materials (such as titanium alloy), reducing heat concentration, easy to burn out the tip. Especially Cut
If the rigidity is small, the deformation is more significant. Because of the muscular elastic after-effect of the material, the actual back angle of the cutting tool decreases
There are friction and wear between the back of the tool and the machined surface.
The second part parts material, and it’s cutting performance-Effect of alloying elements, metallographic structure and hot working process on cutting
2. Effect of alloy elements in materials on machinability
Effect of Matrix of high melting point alloy elements
The lattice atoms with a high melting point, high activation energy, and small diffusion coefficient are not easy to fall off the equilibrium position
It has high softening resistance and recrystallization temperature and excellent strength at high temperatures. If the Matrix Alloy is of high purity and compact structure,
the interatomic cohesion is greater, IT and other alloy elements formed by the material, mostly low thermal conductivity
Oh, honey. When cutting, the plastic deformation resistance is significant, and the extensive deformation work transformed into a large amount of cutting heat
It is not easy to transfer so that the cutting process of deterioration.
The machining difficulty of the alloy substrate is in the following order:
- MG-AL-CU-FE-TI-NI-CO-MO-FO CR-NB-W
Effect of alloying elements
The addition of alloy elements makes the strength and hardness of the alloy increase and the difficulty of cutting increases
From easy to hard:
- for Alloy Steel Cr-w-v-Mo-Ni-Mn-Si
- for Superalloy Fe-Co-v-Mo-w-Ta-Nb-Ti-Al
- for Titanium Alloy Sn-Mn-Fe-Cr-v-Al-c-on
To improve the properties of steel, some alloy elements such as CR, Ni, V, Mo, W, MN can add into the steel，Si, AL, etc. Among them, CR, Ni, V, Mo, W, Mn, and other elements can improve the strength and hardness of steel
The factors such as SI, AL are easy to form hard particles such as alumina and silicon oxide, which make the tool wear worse.
When the element content is low (generally limited to 0.3%), the effect on the cutting force of steel is small, more than this content of water Flat, on the cutting of steel processing, is detrimental.
The strength of steel can be reduced slightly by adding a small amount of sulfur, selenium, lead, Bismuth, phosphorus and so on
It can also reduce the plasticity of steel, so it is beneficial to the machinability of steel. MNS formed by S and MN and S and Fe The FES, which is very soft, can become the stress source in the plastic deformation zone during cutting and can reduce the cutting
Chip cutting force, so that the chip is easy to break, reduce the formation of accumulated disk, thus reducing the roughness of the machined surface
To minimize tool, wear, and tear. Selenium, Lead, Bismuth, and other elements also have a similar effect.
Phosphorus can reduce the plasticity of Ferrite and make the chip easy to break.
The influence of the chemical composition of cast iron on machinability mainly depends on the graphitization of carbon by these elements
Function. The carbon in cast iron can be dissolved in Ferrite and AUSTENITE and can carburize in combined state
There are two forms of graphite (G) in Free State and body (FE3C).
Graphite has a very low hardness and proper lubrication, so when carbon is present in graphite, the cast iron cut.
The higher the hardness of Fe3C, the more abrasive the cutting tool will be, so the higher the content of Fe3C,
the lower the machinability of cast iron.
Therefore, in the chemical composition of cast iron, all can promote graphitization of the elements, such as SI, Al, Ni,
Cu, Ti can improve the machinability of cast iron, and the factors that can prevent graphitization, such as CR, V, MN
, Mo, Co, P, s, and so on will reduce the cutting performance.
Influence of the hard phase
The interaction of alloying elements results in hard aspects, which exist in the material and make the cutting difficult
Increased because:
Alloy elements and nitrogen, boron, carbon or between the formation of extremely high hardness of nitride, Boride, carbonization
Substances or intermetallic compounds are having a certain toughness. The more dense phase, the more hardness, and strength of the material High.
The precipitation dispersion distribution of hard phase from a robust solution is active during heat treatment, quenching and aging treatment
The grain boundary can be changed to prevent the crystal lattice sliding so that the microstructure is stable at high temperature, the high-temperature strength increased, and the cutting deformation resistance increased
The cutting force is large, the cutting heat rose, and the hard phase particles and the tool edge surface cause severe friction. If the grain is coarse
Significant (1 ~ 3 grades) with highly stable sediment aggregates and uniform refinement of dispersed sediment hardening
The processing difficulty increases with the change of phase.
In a word, the more kinds and contents of the solid phase in the material, the more difficult it is to cut.
From easy to hard, in the following order.
- For carbide-reinforced steels and AUSTENITIC HEAT-RESISTANT STEELS AND ALLOYS:
Fe-Cr-Mo-w-v-Ti-Nb-Ta-Zr-Hf
- For intermetallics-reinforced steels and AUSTENITIC HEAT-RESISTANT STEELS AND ALLOYS:
Mn-Cr-Co-w-Mo-Ta-Nb-Ti-Al
3. The influence of the metallographic structure of the material on the machinability
At high temperature, the strength in the coarse grain is more significant than that in fine grain, and the failure stress is not easy to penetrate through the large grain
Under the condition of high cutting heat, the atom diffuses slowly, and it is difficult to cut.
The austenite alloys strengthened by high activation energy elements have large grains of inhomogeneous phase with a lot of dislocations
When the structure cut, the plastic deformation resistance is high, and the deformation heat is high.
The face-centered lattice austenite which is denser than body-centered lattice and not easy to phase change has the characteristics of high plasticity, poor thermal conductivity and knife sticking
Serious.
The cutting of these structures in a body-centered lattice phase with many slip surfaces and a hardened period, such as a titanium alloy
It’s hard to cut and process.
In general, the plasticity of Ferrite is higher, and that of pearlite is lower. Steel contains most of the iron
The cutting speed and tool durability are higher when the blank and a few pearlites. Pure iron (deficient carbon)
Is a complete ferrite, due to high plasticity, its machinability is very low, the chip is not easy to break, it is cutting Low machinability, the disk is not easy to break, the chip is easy to bond in front of the knife surface, has been processed surface roughness Superlative.
When the pearlite distributed in sheets, the cutting tool must keep in contact with the pearlite with a hardness of 800HBW
FE3C contact, so the tool wear. The structure of Lamellar pearlite after spheroidizing is continuous distribution
The Ferrite + dispersed carbide particles, the tool wear is smaller, and the durability is higher.
Therefore, when processing high carbon steel, we expect it to have a spherical pearlite structure. Hard, such as martensite and Sorbate, machined
The higher the degree of organization, the tool wear, durability is very low, should choose shallow cutting speed.
4. The influence of the material hot working process on the machinability
Heat treatment can often change the metallographic structure and physical and mechanical properties of the workpiece material so that it can improve its cutting Machinability.
The hardness of High Carbon Steel and tool steel is on the high side, and there are more reticulate and lamellar cementite structure, so it is difficult to cut
Whittled. By spheroidizing annealing, its hardness can be reduced, and the spheroidized cementite structure can be obtained, so improved For machinability.
MARTENSITIC stainless steel is usually quenched and tempered to reduce plasticity and improve machinability.
Generally, iron castings annealed before cutting to reduce surface hardness and eliminate internal stress to improve its machinability.
