The invention relates to a screw according to the preamble of claim 1. A screw of this type comprises a shank having a tip, a rear end, which is located opposite the tip, and a longitudi nal axis, which extends through the tip and through the rear end, wherein the shank consists of a first steel material, and further comprises a screw thread helix, which is arranged on the shank and which protrudes from the shank, wherein the screw thread helix consists of a sec ond steel material that is different from the first steel material.

Concrete screws for outdoor applications should often fulfil requirements including:

• high resistance to concrete failure,

• efficient cutting capability of the internal concrete thread, in particular also within rein forcement elements inside the concrete, by the screw,

• high resistance to steel failure during installation and loading of the screw,

• high resistance against brittle failure such as hydrogen assisted cracking,

• high durability and resistance against corrosion,

• efficient manufacturing at low cost.

However, when specifying steel materials for concrete screws, dilemma might arise due to material properties which are contradictory in view of these requirements. For example, the level of hardenability (which can be important for installation and load performance) may be limited by an increase of the risk of hydrogen embrittlement, and corrosion resistance may be limited and contradictory to high hardness.

Several material concepts exist for addressing this issue in concrete screws. W019002044 A1, for example, describes a screw which is, completely, made of martensitic stainless steel of a grade that offers both corrosion resistance and hardenability, in particular due to the special chemical composition of the steel in combination with a nitriding heat treatment.

Other concepts combine different materials within a single screw. US 2010216560A1 discloses concrete screws that comprise a screw body made of (austenit ic) stainless steel, on which hard metal cutting elements, for example welded to the screw body, are provided.

W019170507A1 discloses an example of an axially bimetallic screw, in which the material of a tip section of the screw differs from the material of the remainder of the screw.

US2011/0142569A1 describes screw anchors which are mostly monolithic, with an exception of the foremost part of the screw thread, which is a separate part.

US2010290858 A1 discloses screws comprising a shank-like element and a separate screw thread helix attached thereto, wherein, depending on the intended use, the separate screw thread helix can comprise two separate parts. In the event that a corrosive attack is not to be expected, the shank-like element can consist of common construction steel and the screw thread helix can consist of a hardenable steel comprising a high carbon content. In the event that provision is made for use of the screw anchor in the outside, where corrosive attack is to be expected due to changing environmental influences, the shank-like element can consist of stainless steel, and the screw thread helix can be embodied in two parts, namely comprising a cutting part, which consists of a steel comprising a high carbon content, and comprising a supporting part consisting of a stainless spring steel.

W01 2084385 A1 / DE 102010063682 A1 discloses another screw anchor with a separate cutting helix. The shank is made from malleable steel, which is not heat-treated. The cutting helix can consist of hardened stainless steel, having a surface hardness of about 600 HV. As an alternative, it is proposed to use galvanized carbon steel.

It is an object of the invention to provide a screw that is particular versatile, is particularly easy and/or economical to manufacture and/or can provide particularly good performance and/or reliability.

This object is achieved by a screw according to claim 1. Dependent claims refer to preferred embodiments of the invention.

According to the invention, both the first steel material and the second steel material are stainless steels. Accordingly, the screw of invention comprises a shank and a screw thread helix. These two elements are both made of stainless steels, however of different steel grade. This design can be considered as “radially bimetal”. The distinct design of shank and screw thread helix, re spectively, allows the utilization of different stainless-steel grades for shank and screw thread helix, respectively, which are optimized for individual requirements of these components. Therefore, the requirements mentioned above can be achieved in a particularly equal man ner, i.e. without too much compromise on particular requirements, in particular on load bear ing and cutting function.

The shank is an elongate element. Preferably, it can have, generally, rotational symmetry, in particular with respect to any angle, with respect to the longitudinal axis. For example, the shank can be generally circular cylindrical. In particular when the screw is a concrete screw, the tip can be blunt. However, it could also be pointed. The tip is that end which is intended to be inserted first into the borehole. A head can be attached to the rear end of the shank. The shank could be hollow but preferentially, it is massive.

The screw thread helix constitutes an external screw thread. The screw thread helix is pro vided on the circumferential surface of the shank and winds around the shank. The screw thread helix protrudes radially from the shank so as to engage into an internal screw thread groove, preferably into an internal screw thread groove which is, at least partly, cut by the screw thread helix itself. Preferably, the screw is a concrete screw, in which case the internal screw thread groove in which the screw thread helix is intended to engage can be provided in a concrete substrate, and in which case the screw thread helix can be configured to cut the internal screw thread groove in the concrete substrate, at least partly. Preferably, there are no additional cutting elements provided on the screw thread helix.

The screw thread helix and the shank, respectively, are distinct, as they consist of different materials. In particular, these elements can be manufactured separately and joined thereafter to give the screw. However, the screw thread helix is arranged on the shank so as to transfer tensile load, in particular rearwardly directed tensile load, from the shank into the screw thread helix, which can be achieved by geometric engagement between the shank and the screw thread helix. In particular, the screw thread helix can be connected to the shank.

The screw thread helix and/or the shank can have at least one metallic or non-metallic coat ing and/or at least one metallic or non-metallic inlay. Throughout this document, the terms “axially”, “longitudinally”, “radially” and “circumferential ly” should refer, in particular, to the longitudinal axis of the shank. Rearwards is a direction parallel to the longitudinal axis of the shank, pointing from the tip to the rear end.

It is particular advantageous that the second steel material has a Vickers hardness between 550 HV10 and 800 HV10, preferably between 650 HV10 and 750 HV10, wherein Vickers hardness is in particular according to ISO 6507. Accordingly, the second steel material, which is used for the screw thread helix, has a relatively high hardness, which also entails a relatively high proneness to (hydrogen assisted) brittle failure. However, it was - surprising ly - found, that the “radially bimetallic” design of the inventive screw allows to tolerate this proneness, since it can easily be confined to the screw thread helix, which provides capabil ity for load redistribution after any potential load-path failures. The main load path is provided by the shank, which is not affected by the choice of material for the screw thread helix. In contrast, loads transferred via the screw thread helix from the shank to the surrounding sub strate are distributed along the entire screw thread helix length. If that load path is interrupted due to any potential (but still unlikely) local failure of the screw thread helix, the residual loads will be redistributed to the remaining and intact sections of the screw thread helix, without significant effect to the load path. In other word, potential fracturing of the screw thread helix due to brittle failure will not significantly affect load bearing capacity and can thus be tolerated. Accordingly, the inventive choice of Vickers hardness can provide particularly good cutting performance at low expense, without significant compromise on load bearing capacity and/or reliability.

Preferably, the screw thread helix spans at least 33%, preferably at least 40% or 50%, of the length of the shank. Thus, the longitudinal overlap between the screw thread helix and the shank is at least 33%, 40% or 50%, respectively, of the length of the shank, with the length of the shank measured in the longitudinal direction. Accordingly, the screw thread helix has significant longitudinal extension along the shank, and thus, it can preferably be considered the main thread of the screw. This allows to achieve particularly high embedment depth with particularly good thread reliability, further improving performance.

According to a further preferred embodiment of the invention, the first steel material, i.e. the shank’s steel material, has a Vickers hardness below 440 HV10. As a consequence, the re sistance to (hydrogen assisted) brittle failure of the shank is relatively high, leading to particu larly good reliability in the primary load path provided by the shank. As already mentioned above, the screw is preferably a concrete screw, i.e. the screw, in par ticular the screw thread helix thereof, is able to, at least partly, cut its mating internal screw thread groove in a concrete substrate. In particular, a ratio of the outer thread diameter of the screw thread helix to the pitch of the screw thread helix can be between 1 and 2, in particular between 1,2 and 1,6, at least in some regions of the screw thread helix, more preferably at least in some regions of the screw thread helix located near the tip. These are typical dimen sions for concrete screws.

The invention is explained in greater detail below with reference to preferred exemplary em bodiments, which are depicted schematically in the accompanying drawings. Individual fea tures of the exemplary embodiments presented below can be implemented either individually or in any combination within the scope of the present invention.

Figure 1 is a side view of a screw.

Figure 2 is a sectional view, according to A — A in figure 1 , of the screw of figure 1.

Figure 3 is an isometric view of the screw of figure 1.

The figures show an embodiment of a screw. The screw comprises a shank 10 having a tip 11 at its front end, and, at its opposite other end, a rear end 18. The tip 11 is that end of the shank 10 which is intended to be inserted first into a borehole. The longitudinal axis 99 of the shank 10 extends through the tip 11 and through the rear end 18.

The screw further comprises a drive 19 for transmitting torque to the shank 10 for rotating the shank 10 around the longitudinal axis 99 of the shank 10 for installing the screw. In the pre sent embodiment, the drive 19 is a hex drive head connected to the rear end 18. However, this is an example only, and any type of drive could be used, such as a slotted drive, a cruci form drive, a lobular drive, an internal polygon drive, an external polygon drive or a special drive.

The screw furthermore comprises a screw thread helix 20, which is connected to the shank 10 so as to wind around the shank 10. The screw thread helix 20 and the shank 10, respec tively, are materially distinct elements. Whereas both the screw thread helix 20 and the shank 10 consist of stainless steel, the steel grade used for these respective elements is different. ln the present embodiment, the screw thread helix 20 spans, longitudinally (i.e. in the direc tion parallel to the longitudinal axis 99), approximately 80% of the length l _s of the shank 10. The screw thread helix 20 thus forms a main thread of the screw. In the present embodiment, no additional helical elements are provided on the screw.

The screw thread helix 20 has an outer thread diameter d _tr . At least near the tip 11 of the non-installed screw, a ratio of the outer thread diameter d _tr of the screw thread helix 20 to the pitch p _tr of the screw thread helix 20 is between 1 and 2, in particular between 1 ,2 and 1 ,6.

The first steel material, i.e. the material of the shank 10, has a Vickers hardness below 440 HV10, whereas the second steel material, i.e. the material of the screw thread helix 20, has a Vickers hardness between 550 HV10 and 800 HV10, preferably between 650 HV10 and 750 HV10, in particular both according to ISO 6507.

The first steel material can for example be an austenitic (e.g. 1.4404, 1.4301 , 1.4529, or simi lar), a duplex (e.g. 1.4062, 1.4162, 1.4362, 1.4410, 1.4509, or similar), a ferritic (e.g. 1.4105, 1.4113, 1.4521, or similar) or a PH stainless steel (e.g. 15-5 PH or similar).

The second steel material can for example be an austenitic (e.g. 1.4565/1.4566, 1.3808, or similar), a martensitic (e.g. 1.4108, 1.4109, 1.4116, 1.4122, the steel grade described in W0 12084385 A1) or a PH stainless steel (e.g. 17-7 PH).