Use of a non-corrosive, martensitically hardening steel

Abstract

The aim of the invention is to produce mechanically driven rotary tools, preferably boring, milling, grinding and cutting tools which are stable, non-corrosive and biocompatible and also have high mechanical strength properties, in conjunction with good ductility properties. To this end, the invention relates to the use of a precipitation-hardenable, martensitic, stainless chromium-nickel steel having the following composition (in weight %): 10 to 14 of chromium; 7 to 11 nickel; 0.5 to 6 of molybdenum; 0.5 to 4 of copper; 0.05 to 0.55 of aluminium; 0.4 to 1.4 of titanium; up to 0.3 of carbon and nitrogen; less than 0.05 of sulphur; less than 0.05 of phosphorus; up to 0.5 of manganese; up to 0.5 of silicon; respectively up to 0.2 of tantalum, niobium, vanadium and wolfram; and optionally up to 9 of cobalt; optionally 0.0001 to 0.1 of boron; the reminder consisting of iron and common impurities.

Description

TECHNICAL FIELD [0001] The invention relates to a novel use for precipitation-hardenable, martensitic, stainless steels for the manufacture of rotary tools for applications with high requirements as regards a combination of high hardness and ductility as well as corrosion resistance, such as drilling, milling, grinding and cutting tools. BACKGROUND AND TECHNICAL PROBLEM POSED [0002] Precipitation-hardenable, martensitic, stainless steels are known from WO 93 / 07303. In this a composition of a stainless steel is described which has a very high strength with simultaneously good ductility. This steel is described as being particularly suitable for the manufacture of injection cannulae, dental instruments and medical instruments on the basis of wire and strip material produced from the named types of steel. Because of the high hardness of the steel, further working had to be restricted to a minimum. [0003] In WO 01 / 14601 A1 a process is described for the manufacture of parts with complic...

AI Technical Summary

Benefits of technology

[0014] Rotary tools according to the invention are preferably drilling, milling, grinding and cutting tools with or without geometrically defined cutting edges, particularly preferably machine-operated rotary cutting machine tools according to DIN 8580. As a rule, such a tool comprises a shaft, a machining tool head and a mounting part. The tool is moved axially and / or transversely. A surprising effect, covered by the invention, is that precipitation-hardenable, martensitic, stainless steel used in accordance with the invention is advantageous in applications in which the combination of high fracture and bending resistance with hardening and corrosion properties plays a crucial role.

[0016] A further surprising effect, covered by the invention, relates to the advantageous combination of good biological compatibility of the precipitation-hardenable, martensitic, rustless steel used according to the invention with good corrosion properties, high ductility and outstandingly high strength of approximately 2,500 to 3,000 N / mm2. This combination permits the advantageous use of this steel in medical applications in which the material remains in the body of the patient for a shorter or longer period of time.

[0023] It was surprisingly shown that the utilization of the type of steel used according to the invention for the manufacture of machine-operated rotary tools, such as drilling, milling, grinding and cutting tools with and without a defined cutting edge, offers special advantages, in particular in dental and surgical applications, due to the outstanding ultimate breaking elongation behaviour of the steel grade compared with steels used hitherto. In the steels used hitherto, hardness and corrosion resistance in particular, as well as biocompatibility depending on the application, were to the fore. In respect of the tensile strength, a compromise has been accepted in the case of steels known hitherto. Through the use according to the invention of the present steel for the manufacture of machine-operated rotary tools, such as drilling, milling, grinding and cutting tools with and without a defined cutting edge, the disadvantages resulting from the fracture behaviour of the products obtainable on the market hitherto have now also been overcome. The tools produced according to the invention combine hardness, maximum corrosion resistance, good biocompatibility and outstanding breaking strength in the products manufactured. The products also remain fracture-resistant when bent and can be bent repeatedly, such as for example in plastic surgery, without sacrificing their outstanding material properties. In addition, the steel grades used according to the invention have good workability and good milling properties when hardened, which proves advantageous in the manufacture of the products. A further advantage of the use of the steel used according to the invention for the manufacture of rotary tools is the relatively low hardening temperature in the range 425 to 525° C., which results in substantial energy cost savings during manufacture.

Problems solved by technology

Because of the high hardness of the steel, further working had to be restricted to a minimum.

The problem of workability was solved in this case with the help of the special manufacturing process, but this cannot be applied to tools, especially rotary tools with complicated geometry.

Because of this extreme length / diameter ratio and the resulting unfavourable moment distribution, such tools are very sensitive to the bending loads applied in practical use.

A slight bending, barely visible to the naked eye, of the drill can result in its generally running untrue and being unbalanced when next prepared by the operator or next used.

Because of the frequently very high rotation speeds this results, in practice, in the drill bits breaking off during operation.

This means not only that the drills have a short life and must frequently be replaced for reasons of safety, but also that there is a substantial risk of injury for operator, patient and bystanders who may be hit by flying parts of tools, as well as a substantial cost factor.

According to the Medical Products Law, tools with small dimensions in particular are designated disposable products by manufacturers, which represents an additional cost for the user.

After the tools have been used once their use on a second occasion is no longer allowed, and the user must use a new tool, which leads to unreasonably high costs.

In addition there are the corrosive media during use, such as e.g. blood and other bodily fluids.

If such dental and surgical instruments are damaged or attacked by corrosion, there is the danger of patients being contaminated by the corrosion residues and exposed to dangerous post-operative complications.

Each of these known alloys has a series of good material properties, such as corrosion resistance, strength, mouldability and / or ductility, but each alloy also has disadvantages and cannot satisfy specific product requirements.

Complex problems and disadvantages of rotary tools currently available on the market are known from practice.

Austenitic rustless steels, e.g. the AISI 300 range, can offer good corrosion resistance in combination with high strength and ductility acceptable for some applications, but a marked cold reduction is required to achieve the high strength, and this means that the semi-finished product must also have a very high strength, which in turn leads to poor mouldability.

A large quantity is favourable for strength, but reduces ductility and mouldability.

It can also have disadvantages during manufacture of semi-finished products, e.g. this steel is susceptible to cracking in the annealed state.

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