RU2332279C2 - Method of making complex figure thin-walled sintered bars from heavy alloys based on tungsten - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention pertains to powder metallurgy, particularly to making objects from heavy alloys based on tungsten. To make complex figure thin-walled sintered bars from heavy alloys based on tungsten a mixture is prepared, containing 94-96 mass.% of tungsten, and nickel and iron constituting the remaining percentage. Hydrostatic pressing is done, with obtaining of pressing with initial pressure on the support before sintering of not more than 0.03 MPa and liquid phase sintering. The method allows for reducing warping when sintering thin-walled work pieces, reducing machining allowance and prevents breakage of thin-walled work pieces when sintering.

EFFECT: reduced warping when sintering thin-walled work pieces, reduced machining allowance and prevention of breakage of thin-walled work pieces when sintering.

1 dwg, 1 tbl

Description

The claimed invention relates to the field of powder metallurgy, in particular to methods for manufacturing products from heavy alloys based on tungsten.

A known method of manufacturing cermet products from hard alloys, including sintering under pressure of a load mounted above the products on supports of a material that allows the supports to lose their bearing capacity at the end of sintering (AS 442895 (USSR). A method of manufacturing cermet products. Karmazin P. N. and Kalia M.A. Publ. In BI No. 34, 1974).

The disadvantage of this method is the difficulty in eliminating warping of the complex figure.

The closest analogue is the method of pressing hollow articles of metal powders (as.with. 1279748, B22F 3/02, publ. 04/30/1985), according to which the powder is pre-compacted with a density of 30-40% by elastic lower and upper punches with a force of 15- 20 kgf The final compaction up to 70% of the density is carried out by a rigid punch made shaped. The seal is carried out by moving the matrix in the opposite direction to the pressing direction, and the final seal is carried out by rotating the punch.

The disadvantage of this method is the difficulty in eliminating warping of a complex figured thin-walled large-sized product made of a tungsten-nickel-iron alloy.

The objective of the invention is to improve the quality of the workpiece by reducing deformation and the absence of destruction of the material of the workpiece during sintering in the region of the lower end of the complex-shaped thin-walled hollow large-sized products.

The technical result achieved by using the invention is as follows:

- the absence of destruction of the material of sintered thin-walled bulky workpieces;

- a significant reduction in warpage of sintered billets;

- reduction of machining allowances.

The technical result is achieved by the fact that in the method for producing sintered billets from heavy tungsten alloys, comprising mixing powders containing 94-96% by weight of tungsten, the rest is nickel and iron, hydrostatic pressing, liquid-phase sintering of the obtained compacts in a hydrogen atmosphere, sintering is carried out by weighing providing initial pressure on the stand on which the compact is installed, not exceeding 0.03 MPa.

For alloys with a high specific gravity of the composition of tungsten-nickel-iron (tungsten-nickel-copper), the content of the binder phase (solid solution of tungsten in nickel-iron (nickel-copper) equal to 6-8% by mass is the boundary (L. Konyukhova ., Eiduk Yu.A., Vodopyanova LS Strength and deformation of W-Ni-Fe and W-Ni-Cu alloys at elevated temperatures. Non-ferrous metals, 1974, No. 10, pp. 57-59). phases of more than 8% by weight of tungsten grains generally do not contact each other and are surrounded by a binder phase.When the content of the binder phase is reduced from 6% by weight and neither e, in particular, for the VNZh-3.5-1.5 alloy, the contact area of tungsten grains begins to increase more and more, that is, a frame of tungsten grains becomes uniform in volume of the product.The lower limit of the content of the binder phase in tungsten sintered to a compact state alloys is determined experimentally and is 2% by weight.

During sintering, products containing a binder phase of more than 8% by mass are more susceptible to creep (creep), but not destruction, under their own weight and, as a consequence, warpage than products with a binder phase content of less than 6% by mass. The creep of a tungsten-based material with a binder phase content of more than 8% by weight is mainly determined by the creep of the plastic binder phase, because tungsten grains are interspersed in the binder phase and are generally not connected to each other.

Heavy alloys containing less than 6% by weight of the binder phase are characterized by a structure in which tungsten grains are in contact with each other. For them, there is a sharp decrease in strength and deformation properties during sintering and an increase in the probability of material destruction due to a decrease in creep.

Sintering (shrinkage) of pre-pressed billets of the composition W-Ni-Fe begins at a temperature of 950-1000 ° C and ends at 1450-1500 ° C. During sintering, the porosity of the preforms decreases from 40-50% to 0%. At the same time, the strength of the workpieces increases. The process of free sintering is associated with the danger of deformation and destruction of the material in the region of the lower end of the workpiece under the action of gravity and friction force of the end face of the workpiece on the stand during shrinkage. The maximum pressure above which deformation and destruction of the material is observed in the region of the lower end of the preform is determined by the weight of the preform, the area of the support, the chemical composition of the alloy, the temperature and the duration of sintering. To obtain billets from heavy tungsten alloys with a tungsten content of 94-96% by weight, the rest is nickel and iron, with optimal physicomechanical properties, the optimal sintering regime (T, ° C and t, hour) has been studied and worked out. Therefore, to obtain high-quality billets from alloys of a given chemical composition, there must be a certain weight and bearing area of the billet.

The claimed invention differs from the prototype in that sinterings are pressed with a certain chemical composition with a tungsten content of 94-96% by weight, the rest is nickel and iron, with a weight that provides initial pressure on the support on which the pressing is installed, not exceeding 0.03 MPa . As a result of this, the quality of complex-figured thin-walled large-sized blanks is improved due to reduced deformation and the absence of destruction of the workpiece material in the region of the lower end support part.

Pressings, which, before sintering, act on a support with a pressure of more than 0.03 MPa, can be made integral in height. Separately, each component must be sintered and then, for example, welded by diffusion welding.

The starting materials for the manufacture of thin-walled complex-shaped blanks were standard tungsten, nickel, and iron powders. The mixture contained 95 W; 3.5 Ni and 1.5 Fe,% by weight, and prepared by mechanical mixing.

The mixture was pressed in a hydrostat with a pressure of 150 MPa in special molds containing a steel complex-figure model and an elastic cover. A complex-shaped compact was removed from the mold (see drawing).

Sintering of complex-shaped blanks was carried out in an induction furnace in a hydrogen medium according to the regime:

heating to a temperature of 950 ° C in 40 minutes;

exposure for 120 minutes;

heating to a temperature of 1500 ° C in 180 min;

exposure for 120 minutes;

cooling with the oven.

Preliminary experiments on the sintering of control blanks showed that this sintering mode allows to obtain optimal physical and mechanical properties.

After sintering, the presence of defects on complex-shaped blanks was detected, geometric dimensions were measured, and the average density of the blanks was determined by hydrostatic weighing. The results of measurements and analysis of the quality of the workpieces are shown in the table.

Analyzing the quality of sintered blanks, we can conclude that the quality of blanks No. 1-8, 19-20 is good. Defects in the form of cracks, swelling, and delamination in the equatorial part at a height of 50-60 mm from the supporting end were observed in blanks No. 9-18 in fact, the workpiece was destroyed. Due to these defects, it became difficult to measure the linear dimensions of the sintered blanks, which was reflected in the low linear shrinkage (1) and high degrees of warpage (2) of the blanks.

where ΔD, ΔH, Δd, Δh is the difference between the diametric and height dimensions of the compact and the workpiece after sintering;

D, H, d, h - diametric and height dimensions of the compact.

- максимальная линейная усадка по высоте, %;

- maximum linear shrinkage in height,%;

- минимальная линейная усадка по диаметру, %.

- minimum linear shrinkage in diameter,%.

According to the experimental data of the table, it is clear that the limiting pressure of a hemispherical pressing of heavy tungsten alloy VNZh 95 on a support is 0.03 MPa, above which during sintering, the workpiece is destroyed.

The use of the present invention made it possible to obtain, after sintering, a preform of the correct shape, without signs of fracture (cracks, sinks, pores), to reduce machining allowances.

Claims (1)

A method of manufacturing complex-shaped thin-walled sintered blanks from tungsten-based heavy alloys, comprising preparing a mixture containing 94-96 wt.% Tungsten, the rest is nickel and iron, hydrostatic pressing and liquid-phase sintering of the presses on a support in a hydrogen atmosphere, and the pressing is carried out to obtain a press having an initial pressure on the stand before sintering not more than 0.03 MPa.

Images