METHOD OF PRODUCING A TUBE OF A DUPLEX STAINLESS STEEL

Abstract

The present disclosure relates to a method of producing a tube of a duplex stainless steel, in particular a duplex stainless steel suitable for use in fuel injection systems for injection of fuel into the combustion chamber of a combustion engine.

Claims

1. A method of producing a tube of duplex stainless steel comprising the following composition, in weight %, TABLE-US-00004 C max 0.06; Cr 21-24.5; Ni 2.0-5.5; Si max 1.5; Mo 0.01-1.0 Cu 0.01-1.0; Mn max 2.0; N 0.05-0.3; P max 0.04; S max 0.03; and a balance of Fe and unavoidable impurities, and with a PRE-value of at least 23.0, wherein the method comprises the steps of: a) providing a melt of the duplex stainless steel; b) casting a body of the duplex stainless steel from the melt; c) forming a bar of the body; d) forming a tube of the bar by generating a hole therein; e) reducing the diameter and/or wall thickness of the tube by hot extrusion ; f) further reducing the diameter and/or wall thickness of the tube by cold deformation; and g) annealing the cold deformed tube, wherein after step g), the duplex stainless steel of the tube includes 40-60% austenite and 40-60% ferrite and wherein step g) comprises subjecting said tube to a temperature in the range of from 950 C.-1060 C. for a time period of from 0.3-10 minutes and to an atmosphere consisting of a gas mixture comprising 1-6 vol % nitrogen gas and the remainder is H.sub.2 or an inert gas.

2. The method according to claim 1, wherein said temperature range is of from 970 C.-1040 C.

3. The method according to claim 1, wherein said temperature range is of from 1000 C.-1040 C.

4. The method according to claim 1, wherein said annealing step comprises subjected said tube to said temperature for a time period of from 0.5 to 5 minutes.

5. The method according to claim 1, wherein said inert gas is argon or helium or a mixture thereof.

6. The method according to claim 1, wherein the content of nitrogen gas in said gas mixture is equal to or less than 4 vol %.

7. The method according to claim 1, wherein the content of nitrogen gas in said gas mixture is equal to or above 1.5 vol %.

8. The method according to claim 1, wherein step e) comprises subjecting the tube to said hot extrusion at a temperature in the range of from 1100 C. to 1200 C. and a cross-sectional area reduction in the range of from 92-98%.

9. The method according to claim 1, wherein step f) comprises subjecting, without pre-heating, the tube to cold deformation.

10. The method according to claim 1, wherein step f) comprises subjecting the tube to a cross sectional area reduction in the range of from 50-95%.

11. The method according to claim 1, wherein the cold deformation is pilgering.

12. The method according to claim 11, wherein, in said pilgering step, the relationship between the wall thickness reduction and the outer diameter reduction of the tube is expressed as a Q-value, wherein the
Q-value=(WallhWallt)*(OdhWallh)/Wallh ((OdhWallh)(OdtWallt)), wherein Wallh=hollow wall=the thickness of the wall before pilgering Wallt=tube wall=the thickness of the wall after pilgering Odh=hollow OD=the diameter of the tube before pilgering Odt=tube OD=the diameter of the tube after pilgering, and wherein Q is in the range of from 0.5-2.5.

13. The method according to claim 12, wherein Q is in the range of from 0.9-1.1.

14. The method according to claim 1, wherein said duplex stainless steel comprises, in weight %: TABLE-US-00005 C 0.01-0.025; Si 0.35-0.6; Mn 0.8-1.5; Cr 21-23.5; Ni 3.0-5.5; Mo 0.10-1.0; Cu 0.15-0.70; N 0.090-0.25; P less than or equal to 0.035; S less than or equal to 0.003; and a balance of Fe and unavoidable impurities.

15. The method according to claim 1, wherein the tube is a tube arranged for conduction of a fuel in a fuel injection system for injecting fuel into the combustion chamber of a combustion engine.

Description

EXAMPLES

[0056] Two melts were made having the following compositions: Fe is the balance for both

TABLE-US-00003 No C Si Mn P S Cr Ni Mo Cu N 1 0.02 0.5 1.5 <0.035 <0.010 22.2 3.3 0.25 0.25 0.15 2 0.01 0.53 1.09 0.026 <0.003 22.88 3.15 0.12 0.21 0.25

[0057] The obtained melts were then processed accordingly:

[0058] They were casted to bodies by using continuous casting.

[0059] Round bars were then formed by forging and the tubes were then formed by boring a hole therein. The diameter of the tubes was then reduced by by using hot extrusion at a temperature in the range of from 1120 C.-1150 C., the obtained tubes had a cross-sectional area reduction of 96-98%. The hot extrusion was followed by pickling to remove glass beads.

[0060] The diameter was further reduced by pilgering and subjecting the tubes to a cross sectional area reduction thereof in the range of 80-86%.

[0061] The pilgered tubes were then annealed in an atmosphere consisting of a gas mixture comprising about 2% nitrogen gas and remainder argon gas and subjecting the tubes to a temperature of about 1030 C. for a time period of about 1 minute.

[0062] In the pilgering step Q is about 1.0.

[0063] After annealing, the obtained tubes were subjected to a straightening step. Straightening was performed in a roll straightening machine with a combination of bending and ovalization. The tubes were passed through a series of angled rollers which rotated the tube and applied to it a series of bending movements. During straightening the yield strength is exceeded in order to get a permanent change in shape to obtain a straight tube.

[0064] The obtained tubes had on outer diameter in the of 30 mm and the tubes are to be used as a GDI-rail in a fuel injection system for conducting fuel to be injected into the combustion chamber of a combustion engine.

[0065] One additional tube of melt 1 was also manufactured according to the method disclosed above. This tube had an outer diameter of from 8 mm after the pilgering step. This tube was also used as a fuel line in a fuel injection system for conducting fuel to be injected into the combustion chamber of a combustion engine.