Features of heat resistant steels and titanium alloys

Question machinability of high-temperature steel quite successfully resolved, high-temperature Nickel-based alloys, while maintaining their physical and mechanical properties at high temperature and having low thermal conductivity and thermal diffusivity, are ill-equipped to handle their cutting.< / span>

 

Developing high contact temperature on the surfaces of the cutting tool with a high specific pressure contributes to «slidenote» (gripe) coming chip with the front surface of the cutting tool, which greatly restricts the use of an instrument equipped with hard alloy, and completely eliminates its use in interrupted cutting.< / span>

Machining of titanium alloys

 

In these circumstances, the cutting process is carried out only by tools from high speed steel R18 or high speed steel, alloyed with cobalt.

 

But since, high-speed steel can withstand temperatures only up to 600°, then the cutting process (when interrupted cuts and strikes) are limited speed (7-10 m/min). To increase substantially the cutting speed (vs specified) is not possible, so researchers of these processes are on the path of increasing resistance, which can be carried out through:

1) the geometric parameters of the cutting tool;

2) the application of cutting fluids;

3) the method of their supply;

4) surveying method of thermal processing of high-temperature alloy to obtain the structures most responsive the cutting.

 

currently, the research of new tool material for efficient processing of high-temperature alloys cutting.

Titanium alloys have low ductility, which substantially affect their deformation when cutting.

If to characterize the plastic deformation of the shear layer with longitudinal shrinkage of the chip, such may be equal and even less than one. This means that the contact of the shear layer with the front surface of the tool is narrow at the pad, and, taking into account the considerable strength of these alloys, a significant tool wear is obtained in presence of high temperature on the pad.

 

as a result, becomes a natural application of cutting tools, equipped with hard alloy. As hard alloys of TK group is more fragile than the VC group, when machining titanium alloys used alloys of group VK, i.e. as well as the General process all low-plasticity materials. Cutting speed can be up to 100 m/min and more.

A Significant influence on the machinability of titanium alloys have, as mentioned above, various gases, of which the most active are H2, 02, i.e. with the increase of their content in titanium alloys machinability deteriorates.

 

Numerous studies on conventional and doped carbon structural steels showed that the depth of work-hardened layer, the degree of hardening, the magnitude and sign (tensile or compressive) residual stresses are dependent on the plasticity of the processed metal cutting, geometry of tool cutting fluids, the degree of bluntness of the tool and the rigidity of the system detail — the machine — tool.

Studies show that residual stress in the layer under the treated surface appear as a result of heat generated:

1) friction from the rear surfaces of the instrument on the treated surface;

2) plastic deformation of this layer.

 

All of these provisions apply to high-temperature alloys. Studies made to determine the effect of hardening on the fatigue life of the parts, change the limit of the fatigue strength of components of carbon and alloyed structural steels, show that in many cases hardening enhances durability of parts, so that there is and different hardening methods.

features of heat resistant steels and titanium alloys

it is Said in one degree or another relates to superalloys and titanium alloys, but also suggests that the presence of residual tensile stresses negatively affect the strength properties of heat-resistant and titanium alloys. If thin-walled parts, such as turbine blades, when work-hardened after machining, the layer of material may be significant relative to the entire thickness of the part, in these cases it is possible to recommend to make the machining so that the hardening (hardening) would be minimal.

 

During the mechanical treatment of alloys based on titanium (such as VTZ and VT5) is allocated the least amount of heat. On this basis, one would expect that the average integral temperature in the deformed zone of these steels and Nickel alloys should be higher than that of alloys based on titanium. However,the results of temperature studies during cutting titanium alloys held in a wide range of cutting conditions, when compared with temperature data for steels show the opposite. For example, the cutting temperature of the titanium alloy up to 800° C already at a cutting speed v = 40 m/min, feed s = 0,ll mm/Rev and depth of cut V=1.5 mm; when cutting the steel 45 the same temperature develops at much higher mode is: v = 100 m/min; s = 0,29 mm/Rev and t = 2 mm.

 

In the cutting zone there is a complex strain and stress state in the presence of plastic deformation compression, shear and tension, which extend far ahead of the cutter and under the treated surface.

the nature of the change of strain and stress at the length and thickness of the chip formation zone is the same as for high-temperature and titanium alloys, and carbon steels. There is only a quantitative difference.

 

The maximum value when cutting heat-resistant alloys and carbon steel reaches a strain of compression, and when cutting titanium alloys—shear deformation.

 

At high temperatures encountered in the cutting process titanium alloys is manifested the activity property of titanium to oxygen and nitrogen from the air. This leads to changes in structure and physico-mehancheskoe properties of the surface layer of machined parts, which in all likelihood may be the cause of reducing its fatigue strength.

 

high-temperature alloys prone to the formation of Nalimov on the front surface of the cutter, thus necessitating the use of cooling lubricant fluids having high lubricity.

In the processing of high-temperature alloys a large amount of heat which increases the temperature of the part and cause a change in its size and shape. To avoid this requires a generous supply of coolant.

 

the greater tendency of high-temperature alloys to shot peening. So, in many companies, high-temperature alloy after receiving the shot peening is not amenable to machining; recommends that before machining the material is first subjected to heat treatment.

Large cutting forces of 3-4 times more force when cutting normal structural steel, and a high coefficient of friction require the use of tools with high cleanliness of working surfaces and sharp cutting edge.

Most superalloys due to the nature of the crystallographic structure of the phase components are very abrasive, therefore, used for processing instrumental material needs to resist impact is either inherently or as a result of the appropriate special processing and created environments.

high-temperature alloys retain considerable hardness and strength at short-term increase in temperature during cutting. The sudden temperature rise and subsequent Bystrovskaya deformation of the tensile strength of the alloy is higher and the viscosity lower.

 

titanium Alloys are treated worse than some stainless steels, but better high-temperature alloys. Relatively rapid wear of cutting edges of the tool during the machining of titanium alloys depends on high chemical activity of titanium, easily coming into connection with all contact metals. This feature of the titanium in its low thermal conductivity and a small contact surface between the cutter. And chip leads to the development of high temperatures in the cutting zone. Titanium alloys often contain inclusions in the form of oxides nitrides and carbides, which have high abrasive properties and contribute to the accelerated wear of cutting tools. Hardening has no significant effect on the wear of the cutting tool.

 

Machining of titanium alloys

Drill it is advisable to sharpen with dual cone intake: 2F = 90 and 118°; the angle of the screw grooves = 28-35°; rear angle of 12°. In the formation of deep holes in the drilling process, you should periodically remove the drill from the hole to clear chips.

For cooling is used sulfatirovnie or chlorinated oil.

For threading use taps with spiral flutes; taps for threading to b mm increments of less than 1.25 mm are dvuhkonusnyj; for larger threads trehchasovye. Cutting gauge teeth and the tap is recommended satyavati. The thread should be cut is not complete; reducing the height of the thread from 75 to 65% improves life tap 2-3 times.

for threading in unalloyed commercially pure titanium is used, the cutting speed v = 12 m/min When cutting threads in titanium alloys v=7.6 m/min. For cooling of taps used sulfatirovnie and chlorinated oil.

When pulling technically pure non-alloy titanium cutting speed permitted by broaches HSS v = 7.6 m/min.

When pulling titanium alloys v = 4.6 m/min.

the Feed per tooth of the broach for roughing teeth of 0.075 — 0.15 mm; finishing of 0.038-0.15 mm.

Processing splavov with a hardness of HRC>37 associated with considerable difficulties due to rapid wear of the broaches.

In the processing of titanium splavov should monitor the status of the drive and to prevent the buildup of titanium on the teeth.

Cooling: abundant stream sulfatirovannah or chlorinated oil. The teeth of broaches are performed with a rake angle of 8°; hind angles of 3° for roughing broaches and the 2° for finish.

Cutting titanium

Cutting bar material with a diameter of 50-90 mm has been successfully produced nosaukumi HSS. The pitch of the canvases depends on the hardness of the material being cut. When NV 275-350 tooth pitch of 4.2—6.2 mm; HB 350— 6.2 mm; NV >350-8,4 mm.

the tension of the webs should be constant and sufficient.

titanium Alloys are cut at between 45 and 70 double strokes per minute handsaw with the flow 0,15—0,23 mm per double stroke. For cooling used sulfatirovanne or chlorinated oil.

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