IL33836A - Non-magnetic barrel steel - Google Patents

Description

J Non-Magnetic Barrel Steel The invention relates to the use of a non-magnetic steel for gun barrels.

As material for gun barrels usually steel types are used oontaining for instance 0.35$ C, 1 .5# Cr, 1 .5$ Ni and Ο.276 Mo which have been hardened to a yield strength (σ 0.2) of about 85 kp/mm and an impact strength (KV) of about 5 kpm. However, in those cases when the barrels are to be used on warships and particularly on mine sweepers barrels made of such steel types are not suitable. Modern mines are so sensitive that they can be released by the magnetic field emanating from the barrel material. Therefore there is a need for a gun barrel material of nonmagnetic type having as good mechanical properties as the above-mentioned hardened and tempered steel. In order that the barrels shall have a long life a very high durability is also required at room temperature as well as at elevated temperatures. This is particularly important for barrels of austenitic type which will attain a high temperature at the inside of the barrel due to the relatively low thermal conductivity of the austenite.

It has proved that the above-mentioned requirements of a yield strength of about 85 kp/mm and an impact strength of about 5 kpm are fulfilled by a non-magnetic, precipitation hardened steel alloy having the following composition: c 0.50 - 0.80$ Si 0.05 - 0.80?S Mn 9.0 - 18.0 Cr 2.0 - 6.01» Ni 7.0 - 1 1 .0 # the balance being iron and the usual impurities. Such a steel has the high yield strength and impact strength required for barrels. It remains effectively non-magnetic during all service conditions. Neither cooling down to -1 90°C nor a very intensive cold working will destroy the non-magnetic properties of the steel. Nor will any magnetism due to shooting appear in the steel.

It has now been found that the durability of a steel of the above-mentioned composition can be considerably increased by the addition of molybdenum and tungsten and an increase of the amount of vanadium without the yield strength and the impact strength being affected unfavourably, at the same time as the stable austenitic non-magnetic structure is retained. It has even proved possible to attain a higher yield strength than that of steel alloys having the above-mentioned composition.

Moreover, it has proved that the toughness at temperatures exceeding 500°C is considerably improved. According to the invention this steel is proposed to be used as material for gun barrels, and it contains: c 0. 35 - 0.807-· Mn 4 - 1 5 i W up to 5 the balance being iron and the usual impurities appearing in steel of this type. The amounts of manganese and nickel should be so chosen that a stable austenitic structure of the steel is sure to be obtained. The amounts of molybdenum and tungsten should be chosen so that the sum of the percentage of tungsten and twice the percentage of molybdenum exceeds 2%. Preferably the steel also contains up to 0.6% niobium (columbium) and up to 0.02 boron.

A preferred composition is the following: c 0.40 - 0.60% Mn 7 - 12 % Gr 3 - 6 % Ni 5 - 9 % V 1.0 - 1.5 % Mo 0.8 - 1.8 % W 1.2 - 2.5 % Nb 0 — 0.3 % B 0 _ 0.02% the amounts of manganese and nickel being so chosen that a stable austenitic structure is obtained after solution heat treatment and ageing.

The composition and properties of some of the steel alloys used for the purpose of this invention will appear from the tables at the end of this specification. Table 1 shows the composition of six different steel alloys which has been tested in connection with the invention, and in tables 2 and 3 some examples of the properties of these steel alloys are given.

The yield strength OQ 2 i-s defined as the stress at which a non-proportional elongation corresponding to 0.2% of the original length of the test specimen appears.

It is given in kp/mm and has been determined in conventional manner. Tensile tests at elevated temperature have also been carried out in conventional manner. The impact teste were carried out with a V-notched bar in a Gharpy impact test apparatus. The impact strength has been determined at a temperature of +20°C and has been given in kpm.

As is well known from the use of tool steels the durability increases with increasing hardness and increasing amount of hard carbides. Investigations by means of electrone microscope and X-ray diffraction investigations have shown that the amount of hard vanadium carbide is considerably, increased by the increase of the amount of vanadium from 0.8 in previously used steel up to about 1.2$ in steel used according to the present invention in spite of a reduction of the carbon content. Moreover, in the steel used in accordance with the invention large amounts of tungsten and molybdenum carbides are present. The increase in durability at high temperatures, however, in the first hand is due to the considerably larger hardness at elevated temperatures (table 2). As will be seen in table 1 the steels Nos. 2 - 6 are within the prescribed composition limits, and as appears from table 2 these have a considerably larger hardness at high temperatures (for instance 700°C) than steel No. 1 the composition of which is within the limits of the previously used steel.

It is also previously known that by a special composition steels can be obtained which are non-magnetic and yet have a good yield strength (ση - 85 kp/mm ) 1 without cold working, but these steels have a too low impact strength ( V < 5 kpm) . As will be seen from table 5 both the steel previously used for gun barrels and the steel used in accordance with the present invention have the high yield strength and impact strength (O-Q kp/mm' KV > 5 kpm) required for gun barrels. However, in comparison with the previously used steel the steel used in accordance with the invention exhibits a higher toughness (better contraction values) at high temperatures, particularly if niobium is present in the steel, as will be seen in table 3. This means a valuable improvement in material for gun barrels. The risk of crack formation on the inside of the barrel due to heavy thermal shocks has thereby been reduced.

Table 1. Composition of tested steel alloys Steel Wo. Percentage of C Si Mn Gr Ni Mo W 1 0.70 0.48 10.2 4.2 8.5 _ — 2 0.44 0.40 9.0 4.3 8.1 1.45 1.6 3 0.47 0.47 9.8 4.3 7.9 1.24 1.4 4 0.63 0.44 9.6 4.2 8.5 0.77 3.8 0.51 0.40 9.9 4.6 8.8 1.5 1.9 6 0.45 0.35 7.5 5.2 5.6 1.3 2.1 Table 2. Vickers hardness (HV 30 at +20°C and HV 5 at +700°C Solution heat treatment 1150°C, 1 h, water + ageing Steel No. HV 30 at +20°C after solution heat treatment + ag 600°C, 6 h 650°C, 6 h 700°0, 1 290 340' 400 2 365 445 3 320 380 455 4 455 320 370 445 6 450 Properties at 20 C and 700 C of some of the steel Solution heat treatment: 1150°C, 1 hour, water +

Claims (3)

What is claimed is :

1. The use as material for gun barrels of a precipitation hardening austenitic steel alloy having large hardness, good durability and good toughness at room temperature as well as at elevated temperature and containing carbon 0.35 - 0.80o manganese 4 15 1o chromium 2 12 1o nickel 2 15 1° vanadium 0.6 1.6 io molybdenum up to 3.0 # tungsten up to 5.0 $> the amounts of manganese and nickel being so chosen that a stable austenitic structure is obtained, and the amounts of molybdenum and tungsten being so chosen that the sum of the percentage of tungsten and twice the percentage of molybdenum exceeds 2$.

2. The use for the purpose stated in claim 1 of a steel alloy containing in addition to the constituents mentioned in claim 1 up to 0.3^ niobium and up to 0.02 boron.

3. The use for the purpose stated in claim 1 of a steel alloy containing carbon 0.40 - 0.60 silicfc¼ maximum 0.5$ manganese 7 - 12 $ chromium 3 - β $ nickel 5 vanadium 1.0 - 1.5$ molybdenum 0.8 - 1.8# tungsten 1.2 - 2.5# niobium 0 - 0.35^ boron 0 - 0.02 COHEN ZEDEK