GB2046301A

GB2046301A - The heat treatment of alloys

Info

Publication number: GB2046301A
Application number: GB7940013A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1978-12-15
Filing date: 1979-11-20
Publication date: 1980-11-12
Also published as: GB2046301B

Abstract

An alpha-beta titanium alloy, preferably a Ti-Al-Mo-Sn-Si alloy, is treated by heating the alloy at a temperature in its alpha-beta phase field, eg 900 DEG C, followed by directly quenching the alloy in an inert liquid bath, eg a mixture of sodium and potassium nitrates, or molten tin at a lower temperature, eg 500 DEG C, for a period, eg 24 hours.

Description

SPECIFICATION The heat treatment of alloys The present invention relates to the heat treatment of alloys.

Alloys in which titanium is the major constituent element, which will be referred to herein after as 'titanium alloys', are being increasingly proposed for use in aerospace applications, eg airframe structures and gas turbines. Of particular interest are those titanium alloys known as 'alpha-beta' alloys in which known alpha and beta phases co-exist, these two phases being in discrete regions within the alloy microstructure and having their own distinctive crystal structure.

Such alloys contain, for example, small percentages of aluminium, molybdenum, tin and silicon in addition to titanium.

Commercially available alpha-beta titanium alloys may be supplied as forgings, bar and sheet.

The alloys may be supplied in the heat treated condition or can be heat treated by the user. The conventional heat treatment for Ti-Al-Mo-Sn-Si alloys involves heating the alloy to a temperature in the alpha-beta phase field, eg 900"C, and maintaining it at this temperature for a period of time depending upon component size, typically one hour followed by air-cooling to room temperature and then ageing at an intermediate temperature, eg 500"C, for a number of hours, eg 24 hours.

According to the present invention a method of treatment of an alpha-beta titanium alloy, preferably a Ti-Al-Mo-Sn-Si alloy, includes the steps of heating the alloy at a temperature in its alpha-beta phase field followed by directly quenching the alloy in an inert liquid bath at a lower temperature and then maintaining the alloy in the bath for a period of time.

Preferably the bath is a salt bath, although it might alternatively be a bath of liquid tin.

Preferably the bath temperature is about 500"C.

Preferably the quenched alloy is maintained in the bath for a number of hours, eg 24 hours.

It has been discovered that when Ti-Al-Mo-Sn-Si alloys are treated by the method according to the invention, instead of by the conventional method described above, their resultant tensile properties are improved.

An additional advantage is that residual stresses are minimised, so the invention is particularly suited to the heat treatment of engineering components having relatively complex shapes, for example, bolts for securing aircraft structures.

Embodiments of the invention will now be described by way of example only.

EXAMPLE 1 12.7 mm diameter rolled bar of IMI 550 alloy was supplied by Imperial Metal Industries Limited. This alloy has a weight composition of 4% aluminium, 4% molybdenum, 2% tin, 0.5% silicon, the remainder being essentially titanium. The bar was cut into two samples A and B.

Both samples were placed in an electrically heated furnace through which a gentle flow of inert gas, eg argon was passed continously. The temperature of the furnace was 900"C. Both samples were maintained at this temperature for one hour.

After this period both samples were removed from the furnace. Sample A was allowed to cool in air to room temperature. It was then placed in an electrically heated salt bath containing a mixture of sodium and potassium nitrates. The temperature of the bath was 500on and the sample remained in the bath for 24 hours, after which it was removed and allowed to cool to room temperature again.

Sample B was transferred directly to the salt bath at 500"C. It was maintained in the bath at this temperature for 24 hours, after which it was removed from the bath and allowed to cool in air to room temperature (20"C).

After undergoing the above treatments Samples A and B were tested as follows: 4 in BSF tensile test pieces (two from each of A and B) were machined from the bars and these were subjected to tensile testing at room temperature, according to BS 18, to determine proof stresses, tensile strength and elongation.

The results of the tests are given in the following Table 1.

Material 0.2% Tensile 12.7 mm Heat Treatment Proof Stress Strength Elong % dia bar MPa (tsi) MPa (tsi) on 5.65we IMI 550 1 h 900 C, AC + 24h 1045 1055 1183 1202 13.5 12.5 (Sample A) 500on, AC (67.7) (68.3) (76.6) (77.8) (Conventional) IMI 550 1h 900=C, quench 1223 1206 1394 1394 8.5 9.5 (Sample B) to 500"C, hold for 24h, AC (79.2) (78.1) (90.3) (90.3) h = hour AC = air cooling to room temperature A = cross-sectional area Table 1 shows that the tensile properties of the alloy IMI 550 are clearly improved by the use of the treatment applied to Sample B (ie treatment embodying the invention) instead of the conventional treatment applied to Sample A.

EXAMPLE 2 A bar of IMI 551 with a 9 mm square cross section was used. This alloy has a weight composition of 4% aluminium, 4% molybdenum, 4% tin, 0.5% silicon, the remainder being essentially titanium. The bar was cut into two Samples C and D. Sample C was treated in the same way as Sample A in Example 7 (ie by the conventional treatment) whilst Sample D was treated in the same way as Sample B (ie by the method embodying the invention) but with the primary treatment at 950"C.

Duplicate specimens for Samples C and D were then subjected to the same tests as the specimens from Samples A and B above. The results are given in the following Table 2.

Material 0.2% Tensile 9 mm Heat Treatment Proof Stress Strength Elong % sq bar MPa (tsi) MPa (tsi) on 5.65vex IMI 551 1h900'C,AC+24h 1369 1349 14651438 9 7.5 (Sample C) 500"C, QC (88.7) (87.4) (94.9) (93.1) (Conventional) IMI 551 1h 950"C, quench 1528 1524 1677 1689 4 5 (Sample D) to 500"C, hold for 24h, AC (99.0) 98.7) (108.6) (109.4) Table 2 shows that the tensile properties of the alloy 1MI 551 are clearly improved by the use of the treatment applied to Sample D (ie the treatment embodying the invention) instead of the conventional treatment applied to Sample C.

Claims

1. A method of treatment of an alpha-beta titanium alloy which includes the steps of heating the alloy at a temperature in its alpha-beta phase field, followed by directly quenching the alloy in an inert liquid both at a lower temperature and then maintaining the alloy in the bath for a period of time.

2. A method as claimed in claim 1 and wherein the bath is a salt bath.

3. A method as claimed in claim 2 and wherein the salt of the salt bath is a mixture of sodium and potassium nitrates.

4. A method as claimed in claim 1 and wherein the bath is a bath of liquid tin.

5. A method as claimed in any one preceding claim and wherein the bath temperature is about 500"C.

6. A method as claimed in any one preceding claim and wherein the quenched alloy is maintained in the bath for up to 24 hours.

7. A method as claimed in any one preceding claim and wherein the alloy is a Ti-Al-Mo-Sn Si alloy.

8. A method as claimed in claim 1 substantially as herebefore described and with particular reference to the treatment of Sample B in Example 1 or of Sample D in Example 2.

9. An alpha-beta titanium alloy which has been subjected to the method claimed in any one preceding claim.

10. An alloy as claimed in claim 8 and which has the atomic composition of 4% aluminium, 4% molybdenum, 2%-4% tin, 0.5% silicon, the remainder being essentially titanium.

11. An alloy as claimed in claim 8 or claim 9 and which has been formed into the shape of an engineering component.