CN1256458C - A method for preparing powder tool steel by vacuum hot pressing - Google Patents

Abstract

The invention relates to the field of preparing tool steel by powder metallurgy, in particular to a method for preparing powder tool steel by vacuum hot pressing to obtain fully dense powder tool steel without carbide segregation. Put the atomized alloy powder in a graphite mold, put it into a vacuum hot-press chamber, heat it at a vacuum degree of 8×10 ^-1 ~ 8×10 ^-3 Pa to a temperature below the solidus line of 5 ~ 40°C, and then keep it warm. Pressurize at 1-10 MPa, and stop pressurizing when the strain rate reaches below 1×10 ^-3 /min. At the above temperature, the plastic deformation resistance has been significantly reduced, so lower pressure can be used to achieve densification, and a workpiece with a relative density of more than 99.8% can be obtained, in which there is no carbide segregation, the carbide particle size is 3-7μm, and the oxygen content is less than 100ppm. The invention solves the problems of carbide segregation and non-dense structure, etc., and can be used to prepare fully dense powder tool steel without carbide segregation.

Description

A method for preparing powder tool steel by vacuum hot pressing

technical field

The invention relates to the field of preparing powder metallurgy tool steel, in particular to a method for preparing powder tool steel by vacuum hot pressing.

Background technique

Tool steel is one of the most important tool materials. It can be made into various complex shapes of knives and molds, such as milling cutters, gear hobbing cutters, gear shaving cutters, broaching knives, slitting cutters, trimming dies, compression dies and extrusion dies. Compression dies, etc., are used in various forms of mechanical processing. Tool steel contains a large amount of alloying elements to ensure its red hardness, but at the same time it brings the problem of carbide segregation. The use of powder metallurgy can fundamentally overcome the segregation defect, so that Its mechanical properties and service life are improved.

At present, the hot isostatic pressing process is used to form and densify metal powder products at high temperature (≥1100°C) and high pressure (100-150MPa), and its density is close to the theoretical density. Reheating deformation greatly improves the performance of tool steel. But the used cold and hot isostatic pressing equipment of this method is very expensive, and the manufacturing cost is very high. In the literature P.Hellman, et al., Metal Powder Report, 32:79-83, 1977; 35:254-256, 1980;, 47:25-29, 1992 and G.Hoyle's book 'Highspeed steels', 1988 , London, and Butterworths have introduced this method. In addition, powder tool steel is prepared by supersolid liquid phase sintering method, but the sintering window is narrow, that is, the temperature fluctuation must be small, so the process is difficult to stabilize, and network carbides will be formed after sintering. Literature C.S.Wright et al., PowderMetall ., 36: 213-, 1993; 38: 221-, 1995 are detailed.

Contents of the invention

The object of the present invention is to provide a method for preparing powder tool steel by vacuum hot pressing, which can solve the problems of carbide segregation and non-compact structure at the same time.

The technical solution of the present invention is to place 60-325 mesh alloy tool steel powder in a graphite mold, and heat the tool steel alloy powder below the solidus temperature at a vacuum degree of 8×10 ^-1 to 8×10 ^-3 Pa Keep warm at 5-40°C, then pressurize at 1-10MPa, stop pressurizing when the strain rate reaches below 1×10 ^-3 /min, and cool with the furnace after the densification is completed.

The pressure is preferably 2-6MPa; the alloy tool steel can be molybdenum-based high-speed steel, such as M2 (Fe-0.9C-6W-5Mo-4Cr-2V), M4 (Fe-1.3C-6W-5Mo -4Cr-4V), M50 (0.8C-4Mo-4Cr-1V) and M42 (Fe-1.1C-1.5W-10Mo-8Co-4Cr-1V); the alloy tool steel can also be tungsten high-speed steel, Such as T1 (Fe-0.8C-18W-4Cr-1V), T15 (Fe-1.5C-12W-1Mo-5Co-4Cr-5V); the alloy tool steel can also be cold work tool steel, such as Cr12MoV (Fe -12Cr-1Mo-0.5V).

The principle of the present invention is to heat the water-atomized or gas-atomized tool steel powder below the solidus temperature of the material. At this time, the liquid phase has not yet appeared, so it is impossible to appear grain boundary network carbides, but at this time the material The apparent yield stress of the powder is already low, and the plastic flow and viscous flow of the powder material can be produced by using a lower hot pressing pressure, such as 1-10MPa, and thus the void can be eliminated, the densification can be achieved, and the relative density can be greater than 99.8%. The workpiece has no segregation of carbides, and the particle size of carbides is 3-7μm. Since the whole process is carried out in vacuum, the oxygen content of the powder material is reduced to below 100ppm.

The present invention uses graphite as the mould, so as to withstand high temperature and pressure at the same time, and obtain the shape of the required workpiece. Thermocouples are arranged in the mold close to the workpiece, in order to accurately measure the temperature of the workpiece, and ensure that the heating process is close to but not exceeding the solid state. phase temperature.

The present invention adopts the temperature, stress and strain rate control method described in the Chinese patent application (the application number is 01114076.3, and the application date is June 15, 2001), by setting the strain sensor in the vacuum hot pressing device, the strain rate is controlled, To measure the densification process of the powder body, the specific flow of the hot-pressing measurement and control program is as follows: firstly, set the hot-pressing temperature _T , the hot-pressing pressure P, and the termination hot-pressing strain rate , and set the termination hot-pressing strain rate  _{to be} 1 ×10 ^-3 /min; start heating, read the actual hot-pressing temperature T, and judge whether the actual hot-pressing temperature T is greater than or equal to the _set hot-pressing temperature T (5-40°C lower than the solidus temperature), if greater than If it is equal to this value, perform constant temperature operation, otherwise continue heating; pressurize while controlling the constant temperature, read the hot pressing pressure value, and maintain the pressure P; read the actual strain data, and judge whether the strain rate is less than or equal to the termination of hot pressing strain Speed setting value , if the result is confirmed, the heating will be stopped and the pressure will be relieved automatically, and the hot-pressed sample can be taken out after cooling.

The beneficial effects of the present invention are:

1. The tool material with full density and no segregation can be obtained. The present invention adopts the vacuum hot pressing method to prepare powder tool steel, and at the same time solves the problems of carbide segregation and non-dense structure, and heats the tool steel alloy powder below the solidus temperature Keep warm at 5-40°C, pressurize at 1-10MPa at a vacuum of 8× ^10-1-8 × ^10-3 Pa, stop pressurizing when the strain rate reaches 1× ^10-3 /min, and produce powder materials The plastic flow and viscous flow of the workpiece, and thus eliminate the cavity, achieve densification, and obtain a workpiece with a relative density greater than 99.8%, in which the carbide has no segregation, and the particle size of the carbide is 3 to 7 μm.

2. Low cost. The traditional hot isostatic pressing process forms and densifies metal powder products at high temperature (≥1100°C) and high pressure (100-150MPa). The cold and hot isostatic pressing equipment used is very expensive, resulting in cost Very high, but the present invention adopts the vacuum hot pressing method, the pressure on the workpiece is 1-10 MPa, the pressure can be reduced by 1-2 orders of magnitude, the equipment is simple, and the cost is greatly reduced.

3. The present invention can be used to prepare large complex part blanks, such as holes and flanges, etc., and the material utilization rate is high.

Description of drawings

Fig. 1 is the photo of the powder high-speed steel saw blade prepared by this method.

Fig. 2 is the densification curve when the powder high-speed steel saw blade is prepared by this method.

Fig. 3 is the photo of the powder high-speed steel gear shaving cutter prepared by this method.

Fig. 4 is the densification curve when the powder high-speed steel gear shaving cutter is prepared by this method.

Figure 5 is a photo of powder high speed steel slitting shears prepared by this method.

Fig. 6 is the densification curve when powder high-speed steel slitting shears are prepared by this method.

Fig. 7 is the microstructure of powder high-speed steel slitting shears prepared by this method.

Detailed ways

Embodiment 1 Vacuum hot pressing M2 powder high-speed steel saw blade

The geometric dimensions of the saw blade are _φ262mm , _φ18mm , h3.5mm, as shown in Figure 1. The composition of M2 water atomized alloy powder is: Fe-0.9%C-5.5%W-4.5%Mo-4.4%Cr-2.2%V-0.04%O, and the solidus temperature of the molybdenum-based high-speed steel is about 1242°C . The particle size of the powder is 80-325 mesh, and the particle size distribution is shown in Table 1.

The particle size distribution of table 1M2 water atomized alloy powder granularity -60+80 -80+100 -100+160 -160+200 -200+260 -260+320 -320 percentage 14.9 14.5 35.0 11.8 8.4 6.7 8.7

Put the M2 alloy powder in a graphite mold, put it into a vacuum hot-press chamber, heat it to 1225-1235°C under a vacuum degree of 5×10 ^-2 Pa, then pressurize it to 1.3 MPa, keep it for 80 minutes, and strain rate up to 5 Pressurization was stopped at ×10 ^-4 /min, and after the workpiece was taken out, its relative density was measured to be 99.8%, in which there was no segregation of carbides, the particle size of carbides was 3-7μm, and the oxygen content was 60ppm.

The densification curve of the workpiece is shown in Figure 2.

Example 2 Vacuum hot pressing M4 powder high-speed gear shaving cutter

The difference from Example 1 is:

The geometric dimensions of the shaving cutter are: φ _outside 128mm, φ _inside 70mm, φ _hole 31mm, h22mm, see Figure 3.

Water atomized M4 alloy powder is used, its composition is: Fe-1.25%C-6.5%W-4.5%Mo-4.2%Cr-4.2%V-0.05%O, the solidus temperature of the molybdenum-based high-speed steel is about 1240°C, the particle size of the powder is 80-325 mesh, and the particle size distribution is shown in Table 2.

Table 2 Particle size distribution of M4 water atomized alloy powder granularity +60 -60+80 -80+100 -100+160 -160+200 -200+260 -260+320 -320 percentage 0.1 14.8 14.6 34.9 11.9 8.3 6.8 8.6

Put the M4 alloy powder in a graphite mold, put it into a vacuum hot-press chamber, heat it to 1210-1230°C under a vacuum degree of 8×10 ^-2 Pa, then pressurize at 4.5MPa for 50 minutes, and the strain rate reaches 2.5× Stop pressurization at 10 ^-4 /min. After taking out the workpiece, the measured relative density is 99.9%, in which there is no segregation of carbides, the particle size of carbides is 3-6μm, and the oxygen content is 90ppm.

The densification curve of the workpiece is shown in Figure 4.

Example 3 Vacuum hot pressing M4 powder high-speed steel slitting shears

The difference from Example 1 is:

The geometric dimensions of the longitudinal scissors are 276mm _outside φ, 200mm _inside φ, and h25mm, see Figure 5. Used powder raw material is with embodiment 2.

Put the alloy powder in a graphite mold, heat to 1210-1235°C under a vacuum of 8×10 ^-3 Pa, pressurize at 2.3MPa, hold for 55 minutes, and stop heating when the strain rate reaches 5×10 ^-4 /min After taking out the workpiece, the measured relative density is 99.85%, in which the carbides have no segregation, the carbide particle size is 3-7μm, and the oxygen content is 70ppm. The densification curve of the workpiece is shown in Figure 6, and its microstructure is shown in Figure 7.

Example 4 Vacuum hot-pressed T1 powder high-speed steel sample

The difference from Example 1 is:

The geometric size of the sample is φ50×h20mm, and the water atomized tungsten-based T1 high-speed steel alloy powder is used, and its composition is Fe-0.8%C-18%W-4%Cr-1%V-0.05%O. The phase line temperature is about 1290°C, and the particle size of the powder is 80-325 mesh. Put the T1 high-speed steel alloy powder in a graphite mold, put it into a vacuum hot-press chamber, heat it to 1260-1280°C at a vacuum degree of 6×10 ^-1 Pa, then pressurize at 9 MPa for 30 minutes, and the strain rate reaches 2 Pressurization was stopped at ×10 ^-4 /min, and after the workpiece was taken out, the relative density was measured to be 99.9%, in which the carbides had no segregation, the carbide particle size was 3-6μm, and the oxygen content was 90ppm.

Claims (4)

1, a kind of method with vacuum hotpressing powdered preparation tool steel, it is characterized in that: 60～325 order alloy tool powdered steel are placed graphite jig, are 8 * 10 with the tool steel alloy powder in vacuum tightness ^-1～8 * 10- ³Insulation when being heated to following 5～40 ℃ of solidus temperature under the Pa, the 1～10MPa that pressurizes then is when strain rate reaches 1 * 10 ^-3Stop pressurization when/min is following.

2, according to the described method of claim 1, it is characterized in that: the described 2～6MPa that is pressurised into vacuum hotpressing powdered preparation tool steel.

3, according to the described method with vacuum hotpressing powdered preparation tool steel of claim 1, it is characterized in that: described alloy tool steel is that molybdenum high speed steel, tungsten are rapid steel or cold work tool steel.

4, according to the described method with vacuum hotpressing powdered preparation tool steel of claim 3, it is characterized in that: described molybdenum high speed steel is M2, M4, M50 or M42; Described tungsten is that rapid steel is T1 or T15; Described cold work tool steel is Cr12MoV.

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