DK1930462T3

DK1930462T3 - Fittings for drinking water installations

Info

Publication number: DK1930462T3
Application number: DK06124248.3T
Authority: DK
Inventors: Peter Arens; Christian Rischen; Frank Kasperkowiak; Patrik Zeiter
Original assignee: Viega Gmbh & Co Kg
Priority date: 2006-11-16
Filing date: 2006-11-16
Publication date: 2014-03-24
Also published as: EP1930462B1; EP1930462A1; ES2455517T3; US20080118388A1; PT1930462E

Description

The invention relates to a method for the production of screw- or push-fit fittings, valves or compression joints for drinking water ducting systems, which are made of a steel alloy, as well as fittings, particularly screw- or push-fit fittings, valves or compression joints, made of a steel by means of laser welding.

Usually nowadays highly cupriferous non-ferrous metal alloys, such as bronze or red brass, are used for the production of fluid conducting components of gas or drinking water ducting systems, for example. The non-ferrous metal alloys used, particularly in the case of systems intended for drinking water supply, are subject to stringent requirements. On the one hand the non-ferrous metal alloys must have particularly high corrosion resistance, since the components in contact with drinking water for example should not corrode even when in use over many years. Apart from a slight tendency of metal ions to migrate into the fluid, the components should also be easy-to-produce and able to be worked on satisfactorily by machine. Due to the rising price of copper, the main constituent of the non-ferrous metal alloys used up till now, alternatives are being increasingly sought which, apart from ensuring satisfactory machining properties, ensure equally good corrosion resistance at low-cost. As stainless steel alloy austenitic steel, for example 1.4301 or 1.4401 steel, has been used hitherto which, although distinguished by good formability, so that fittings, valves or compression joints can be produced at low-cost, has a corrosion resistance, particularly in the case of thick-walled parts, which are in constant contact with drinking water for example, which has room for improvement. Although ferritic steel, which in principle has equivalent corrosion resistance, is well-known, so far it has not been used for the production of fittings, valves and compression joints, since on the one hand these have a tendency to coarse grain formation when welded, which leads to reduced corrosion resistance. On the other hand the production of thick-walled parts, such as valves for example, is problematic due to the reduced formability of the ferritic steel alloys.

The printed publication JP 01180946A discloses, for example, the usage of ferritic steel for the production of pipes conducting highly purified water. A high corrosion resistance, according to the technical teaching of said printed publication, is achieved both from the utilized alloy constitution as well as from an additional coating of the inner surface of the pipe. JP-A 9143630, WO 99/25890 and JP-A 8333656 disclose steel for the production of pipes with improved welding properties.

On this basis the object of the present invention is to propose a method, which permits low-cost production of fittings, valves and compression joints with improved corrosion resistance.

In accordance with a first teaching of the present invention the object indicated above is achieved by a method for the production of screw- or push-fit fittings, valves or compression joints for drinking water ducting systems, which are made of a steel alloy, wherein the steel alloy has the following content of alloying constituents in weight percent: C < 0.05 %, 11 % < Cr < 25 %,

Ni < 5 %,

Mn < 2 %, N <0.25%, optionally: 0.15 % < Ti < 0.8 %, optionally: 1 % < Mo < 2.5 %, the remainder being iron, and wherein a laser welding step is carried out.

Surprisingly, it has been shown that by using the steel alloy with the composition indicated, fittings, valves or compression joints can be produced at low-cost by means of laser welding. Up till now it was assumed that corresponding ferritic or austenitic-ferritic steel alloys were not suitable for the production of corresponding components, since on the one hand these presented problems as regards formability and on the other hand have a tendency to coarse grain formation when welded, so that corrosion resistance is no longer assured in these cases. Due to the advance in laser welding, heat ingress during welding could be reduced to such an extent that any reduction in corrosion resistance virtually no longer arises. Furthermore progress in the field of forming processes, which enable the metal to flow, have led to the fact that fittings, valves or compression joints, despite their wall thickness and the high strains needed for their production, can be made at low-cost. With the use of a steel alloy according to the invention, due to the low carbon content of 0.05 by wt-% maximum, intercrystalline corrosion of the ferritic or ferritic-austenitic steel alloy is substantially reduced. The chrome ratio of 11 - 25 by wt-% likewise leads to an improvement in corrosion resistance. Higher chrome content does in principle increase corrosion resistance. However with increasing chrome content, conversion of the steel alloy into fittings, valves and compression joints becomes more difficult. Nickel is considered to be an austenite former and to this extent determines the amount of the austenitic microstructure in the steel alloy. With a ratio of less than 5 % the austenitic amount is about 50 % of the steel alloy. The lower the nickel content, the higher the ferritic amount of the microstructure.

The formability of the steel alloy can be improved by adding 2 % manganese maximum. A higher manganese amount on the other hand would reduce the corrosion resistance of the steel alloy. With a nitrogen content of N < 0.25 wt-% on the one hand, as the result of nitrogen being present, it is possible to improve resistance of the steel alloy to pitting corrosion. On the other hand as a result of the nitrogen content being limited to the value mentioned, the formability of the steel alloy is not negatively influenced.

In a next embodiment of the inventive use, the steel alloy has a Cr content of 16 % < Cr < 23 % in weight percent. In particular corrosion resistance can be substantially improved by the higher chrome content of the steel alloy.

In order, even with heavier wall thickness, to achieve sufficient resistance of the steel alloy to intercrystalline corrosion, the steel alloy has a titanium content of 0.15 - 0.8 by wt-%.

Limitation of the molybdenum content to less than or equal to 0.5 by wt-% lowers the cost when using the steel alloy for the production of fittings, valves or compression joints. A substantial improvement in corrosion resistance however can be attained by the alternative addition of molybdenum, so that the Mo-content in the steel alloy is limited to 1.0 % < Mo < 2.5 %, preferably 1.5 % < Mo < 2.5 % in weight percent.

Preferably the nitrogen content of the steel alloy is limited to N < 0.03 %, preferably N < 0.015 % in weight percent, in order to improve the formability of the steel alloy.

In a next refined embodiment of the invention a steel alloy, which has a nickel content of 1 % < Ni < 3 % in weight percent, is used. The ratio between the ferritic and austenitic microstructure is adjusted by means of the Ni-content. Preferably the amount of the austenitic microstructure is 50 % maximum. With corrosion resistance as good as that of ferritic steel having a much lower Ni-content, the austenitic ferritic steel alloy has improved tensile strength values and fatigue limit characteristics in corrosive fluid compared to austenitic steel and improved formability compared to the ferritic steel alloys.

As a result of using the steel alloy and laser welding for the production of screw-or push-fit fittings, valves or compression joints for drinking water supply systems, highly corrosion resistant systems can be provided at low-cost.

In accordance with a second teaching of the present invention the object indicated above is achieved with screw- or push-fit fittings, valves or compression joints, due to the fact that these were produced by the method according to the invention and the steel alloy according to the invention. With regard to the advantages of the fittings, valves and compression joints produced in this way, reference is therefore made to the previous remarks on the method for the production of screw- or push-fit fittings, valves or compression joints according to the invention.

There is now a variety of possibilities for configuring and refining the inventive use as well as the inventive components. In this respect reference is made to the claims subordinate to patent claim 1 as well as to the description of exemplary embodiments in conjunction with the drawing. The drawing shows in

Fig. 1 a]-d] perspective views of four exemplary embodiments of fittings, valves and compression joints according to the invention.

An exemplary embodiment of the fittings according to the invention is the valve housing 1 shown in Fig. la] for example. The valve housing 1, ranked among the fittings, has fairly thick walls in some parts and can be produced from solid metal by machining for example, but also by drop forging the ferritic or ferritic-austenitic steel alloys containing the alloying constituents indicated above.

In particular a titanium content of 0.15 - 0.8 by wt-% ensures good resistance of the valve housing 1 to intercrystalline corrosion, despite the heavy wall thickness. When laser welding the valve housing 1 according to the invention, attention must be paid to ensure that heat ingress is as little as possible, since otherwise the steel alloy has a tendency to heavy grain growth and intercrystalline corrosion. According to the invention, welding is therefore performed with laser methods, which are optimized for as little heat ingress as possible. By adding up to 2.5 by wt-% molybdenum the valve housing 1 can be additionally protected against intercrystalline corrosion.

Although the ferritic or ferritic-austenitic steel alloy according to the invention is limited in its formability, compared to deep drawing steel, the formability permits the production of end caps 2 (Fig. lb)), which were made by deep-drawing from a strip of corresponding steel alloy, provided the metal is able to flow during transformation. In a further cold forming step an O-ring groove 3 can be made in the end cap 2 for example.

Fig. lc) shows a compression joint 4. The compression joint 4 consists of a pipe with an over bend 5 and connection pieces 6 at both ends. The over bend 5 is produced by bending, whereas the connecting pieces 6 are brought into the compression joint 4 by upsetting and widening steps for example, but also by using an internal high pressure forming process. The connecting pieces 6 can be pushed onto a corresponding connecting piece, from a pipe for example, and compressed with this, so that a tight connection is obtained.

The T-piece 7 shown in Fig. Id) has O-ring groves 3 at the two opposite connection ends, which are intended for the connection to compression joints for example. The T-piece 7 for example can be made by internal high pressure forming of a longitudinally welded pipe, the thread 8 being produced by machining for example. However, the thread 8 can also be produced by roller-burnishing. As further forming processes upsetting, crimping, necking out and widening can be mentioned. The use of the ferritic or ferritic-austenitic steel alloy according to the invention not only permits the use of conventional forming techniques, but results in the production of particularly corrosion resistant and low-cost screw- or push-fit fittings, valves or compression joints for drinking water ducting systems.

Claims

1. A process for producing screw or plug fittings, fittings or compression fittings for drinking water installations made of a steel alloy consisting of the following by weight of alloying components: C <0.05%, 11% <Cr <25%, Ni <5 %, Mn <2%, N <0.25%, optionally: 0.15% <Ti <0.8%, optionally: 1% <Mo <2.5%, residue: iron and at which a laser welding step is performed .

Process according to claim 1, characterized in that the steel alloy has a Cr content of 16% <Cr <23% by weight.

Process according to one of Claims 1 or 2, characterized in that the steel alloy has a Mo content of 1.8% <Mo <2.5% by weight.

Process according to one of claims 1 to 3, characterized in that the steel alloy has an N content of N <0.03%, preferably N <0.015% by weight.

Process according to one of claims 1 to 4, characterized in that the steel alloy has a Ni content of 1% <Ni <3% by weight.

6. Screw or plug fitting, fitting or compression fitting for drinking water installations, consisting of a steel alloy with the following alloying components by weight: C <0.05%, 11% <Cr <25%, Ni <5%, Mn <2%, N <0.25%, optionally: 0.15% <Ti <0.8%, optionally: 1% <Mo <2.5%, residue: iron characterized by the fact that the fitting has a laser weld seam.