Method of producing high-strength rods of austenitic steel and a rod produced by such method

Abstract

Rods with the transverse cross-section surface area of at least 150 mm.sup.2 and tensile strength UTS above 1200 Mpa is produced using a plastic deformation that consisted of one-pass hydrostatic extrusion of the billet 1 made of austenitic steel, with the initial temperature of the billet being below 100 C. The reduction R of the transverse cross-section surface area of the biller (1), which takes place during the extrusion, is at least 2.

Claims

1. A method of producing rods of austenitic steel, having a surface area of a transverse cross-section equal to at least 150 mm.sup.2 and the tensile strength (UTS) above 1200 MPa, using a plastic deformation characterized in that the plastic deformation consists of one-pass hydrostatic extrusion of a billet (1), made of austenitic steel and having a temperature lower than 100 C., with the reduction (R) of the transverse cross-section surface area of the billet (1), which takes place during the extrusion, being at least 2.

2. The method according to claim 1 wherein the temperature of the billet (1) subjected to hydrostatic extrusion is equal to room temperature.

3. The method according to claim 1, wherein the reduction (R) of the transverse cross-section surface area of the billet (1), which takes place during the hydrostatic extrusion, is from 2 to 2.56.

4. The method according to claim 1 , wherein the billet (1) subjected to hydrostatic extrusion is made of steel whose chemical composition, expressed in weight percents, is: below 0.1% of carbon, below 1% of silicon, below 2% of manganese, below 0.05 of phosphorus, below 0.03 of sulphur, from 15% to 20% of chromium, below 3% molybdenum, from 8% to 19% of nickel, below 2% of copper, below 0.8% of titanium, below 0.22% of nitrogen, and iron and unavoidable impuritiesbalance.

5. The method according to claim 1, wherein the hydrostatic extrusion of the billet (1) is conducted with a constant linear speed.

6. The method according to claim 1, wherein the pressure of the pressure transmitting medium (5) which extrudes the billet (1) is not lower than 600 MPa.

7. The method according to claim 1, wherein prior to the beginning of the hydrostatic extrusion process, the billet is covered with a copper-based lubricant.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The invention has been illustrated in the enclosed figures of drawing, in which

(2) FIG. 1 is a schematic representation of the hydrostatic extrusion process and apparatus,

(3) FIG. 2 shows profile the so-called of the variation coefficient of hardness distribution CV(HV10) as a function of the increasing reduction R, measured in austenitic steel after subjecting it to one-pass hydrostatic extrusions, and

(4) FIG. 3 shows profiles of the hardness distribution determined on a cross-section of the rod extruded hydrostatically with various reduction degrees.

MODE FOR CARRYING OUT INVENTION

(5) Below has been described the hydrostatic extrusion of three exemplary rods made of austenitic steel using the technology according to the present invention:

EXAMPLE 1

(6) Austenitic steel of the 316L type whose chemical composition, expressed in weight percents, is: below 0.03% of carbon, below 1% of silicon, below 0.2% manganese, below 0.045% of phosphorus, below 0.015% of sulphur, from 16.5% to 18.5% of chromium, from 2% to 2.5% of molybdenum, from 10% to 13% of nickel, below 0.011% of nitrogen, and iron and unavoidable impurities balance, was subjected to hydrostatic extrusion at room temperature with the reduction R=2.31 Billet 1 made of the above described steel had the form of a cylinder with the diameter D1=38 mm and 300 mm long, ended at one side with a cone with the apex angle 2=45 that was fitted to the angle of the die (2). After covering the billet 1 with a copper-based CS-90 lubricant, it was placed in the high-pressure chamber 3 of the extruding apparatus, with the conical end of the billet 1 being inserted into the hollow of the die 2 with the exit diameter of 25 mm. The high-pressure chamber 3 was closed with the piston 4 and filled with a known pressure transmitting medium 5. The increase of pressure in the chamber 3 was due to the uniform motion of the piston 4 in the direction indicated by the arrow in FIG. 1. Once the pressure in the chamber 3 reached the critical value of 970 MPa, the extrusion process began resulting in a rod with the nominal diameter D2=25 mm being produced during a single extrusion pass. The rod thus obtained had the tensile strength UTS=1280 MPa and the yield stress YS=1100 MPa and was elongated by 15%.

EXAMPLE 2

(7) The steel, as described in Example 1, was subjected to hydrostatic extrusion conducted at room temperature with the reduction R=2.56 in the same apparatus as in Example 1. The billet 1 had the shape of a cylinder with the diameter D1=40 mm and 300 mm long and ended at one side with a cone with the apex angle of 2=90 fitted to the angle of the die 2. Billet 1 was covered with a copper-based CS-90 lubricant and then extruded hydrostatically, during a one-pass operation, to the diameter D2=25 mm. The rod thus obtained had the tensile strength UTS=1310 MPa and the yield stress YS=1200 MPa and was elongated by 14.5%.

EXAMPLE 3

(8) The steel, as described in Example 1, was subjected to hydrostatic extrusion at room temperature with the reduction R=2.23 in the same apparatus as in examples 1 and 2, The billet 1 in the form of a cylinder with the diameter D1=37 mm and the length of 300 mm and ended at one side with a double cone with the apex angle 2 24 and =90 fitted to the shape of the die 2. After the billet was covered with a molybdenum disulphide-based Molipas lubricant, it was extruded hydrostatically to the nominal diameter D2=25 mm during a one-pass operation. The austenitic steel of which the extruded rod was composed had the tensile strength UTS=1210 MPa and the yield stress YS=1140 MPa and was elongated by 18%.

(9) The variation coefficient of hardness distribution CV(HV10), shown in FIG. 2, is defined as the ratio of the standard deviation to the average hardness value measured on a transverse cross-section of the extruded rod. As can be seen, the CVHV10 coefficient decreases with increasing reduction R. The character of the profile of this coefficient, which is a measure of the uniformity of the hardness distribution, plotted as a function of the reduction undergoes qualitative changes at the reduction 2. The considerable decrease of the CVHV10 coefficient (about 0.02) gives evidence of the uniformity of the microhardness distribution. Changes of this coefficient are visible in the measured hardness distribution profiles (FIG. 3) on a transverse cross-section of the extruded rod where we can see a well-marked core effect, characteristic of steel after subjecting it to forging, which vanishes with increasing reduction R. The curve (a) in the diagram represents the initial state of the billet material, the curve (b)the rod hydrostatically extruded with the reduction R=1.44, the curve (c)the rod extruded with R=2.31, and the curve (d)the rod extruded with R=2.56. The one-pass hydrostatic extrusion with the reduction above 2 ensures a uniform deformation on the entire cross-section of the rod and, thus, guarantees that the properties of the product obtained will be homogeneous.

(10) A typical commercial application of the rods according to the present invention is the fabrication of fasteners. For example, a screw M16 fabricated of a rod according to the invention can replace a screw M24 class 50 (UTS=500 MPa), which means that the mass of the screw will be decreased by more than a half while its high strength will be preserved.