Nickel-Free stainless steel for biomedical applications

Details Nickel-Free stainless steel for biomedical applications Nickel-Free stainless steel for biomedical applications

1999-12-03

2001-07-31

Yee, Deborah (Department: 1742)

Alloys or metallic compositions

Ferrous

Nine percent or more chromium containing

C420S059000, C420S079000, C148S327000, C623S924000

Reexamination Certificate

active

06267921

ABSTRACT:

The invention relates to a grade of nickel-free austenitic stainless steels which is corrosion resistant in physiological media, and which can satisfy all of the minimum values for characteristics and properties specified in the International Organization for Standardization (ISO) standard 5832-1 “Wrought stainless steels for surgical implants” and which can be produced in conventional furnaces without any special apparatus for pressurizing the furnace.
The very low residual nickel content in that class of steel can cause allergic reactions to that element on internal and/or external application in or on the human body.
WO91/16469 discloses a non-magnetic austenitic stainless steel having, in particular, both mechanical properties and good corrosion resistant properties under stress. That steel has a composition that is designed to be used in drilling equipment and which consequently is not affected by the problems of nickel leaching into physiological media.
Unfortunately, in the biomaterials field, the trend in legislation in a number of countries is currently towards limiting or even eliminating nickel from alloys which come into contact with skin or which are used to manufacture temporary (bone reconstruction) or permanent (prosthesis) prosthetic material. It has been known for a number of years that nickel ions leach from certain biomaterials into the human body and can lead to localized irritation and even to infection in some cases. Intracutaneous tests have shown that 15% of the population can be susceptible to allergic reactions to nickel.
Austenitic stainless steels alloyed with nickel are particularly important in this regard because they are widely used to produce parts which are implanted or parts which come merely into contact with the human body, such as:
temporary or permanent bone implants;
external fracture reduction fixings;
watches and watch straps.
Such steels alloyed with nickel have until now been used because of their excellent compromise between their various characteristics, such as intrinsic mechanical strength, stability against a variety of different forms of corrosion, and surface resistance, for example frictional resistance.
The implant field, currently the most critical as regards risks of allergy, has formed the basis for studying the steel of the present invention.
In this field, the use of alloys other than such steels has suffered from technical shortcomings or economic problems connected with the cost of such materials or their implementation.
According to the recommendations of the ISO system, austenitic steels can currently be used to produce implants for human use; such steels are defined in ISO standards 5832/1 and 5832/9. Their nickel content must be in the range 9% to 15% and their use can thus result in nickel leaching even in the absence of any corrosion mechanism developing.
The passive film which coats them and endows them with corrosion stability can leach nickel ions as a result of mechanical defects under frictional conditions or as a result of electrochemical equilibria with the surrounding medium, which involves slow simultaneous dissolution and reconstruction of that surface film.
Further, nickel-containing austenitic stainless steels are known to be sensitive to stress corrosion mechanisms or to fatigue corrosion mechanisms when they are subjected to stresses which exceed a certain “threshold” in relation to plastification of micro-domains in contact with the corrosive medium: nickel contributes to the development of these corrosion mechanisms, which are particularly disastrous since they can result in the part breaking.
Nickel-free ferritic steels which are more resistant to stress corrosion cannot be used as human implants because of their ferromagnetic properties.
According to ISO standard 5832-1, in order to provide corrosion stability in physiological media, chromium, which endows steels with stainless properties beyond a minimum content of about 13% and molybdenum which completes and substantially stabilizes the stainless nature of chromium steels, must both be added in amounts which are capable of satisfying the following relationship:
(%Cr)+3.3(%Mo)≧26,
expressed as the concentration by weight of the alloying elements.
In addition to chromium and molybdenum, manganese-containing austenitic stainless steels contain different proportions of manganese and nitrogen which modify their stainless nature: manganese, which is less noble than nickel in the electrochemical series, substantially affects the corrosion stability of stainless steels, while nitrogen is known to improve it.
Recent studies have tended to show that to a first approximation, the corrosion stability of manganese stainless steels in chlorinated media is linked to their composition as regards the principal alloying elements in accordance with the following relationship:
(%Cr)+3.3(%Mo)+30(%N)−(%Mn)
which give a value that characterizes the corrosion stability of such steels.
Thus the higher the value, the better the corrosion stability of the steel.
Preferably, the proportion of alloying elements is such that:
(%Cr)+3.3(%Mo)+30(%N)−(%Mn)≧26
so as to obtain corrosion stability in the media of the envisaged application which is at least equivalent to that of grade D as described in ISO standard 5832-1.
Austenitic stainless steels containing manganese can contain carbon, with the aim of stabilizing the austenitic structure. However, adding carbon even in amounts as low as 0.1% by weight leads to a very substantial degradation in corrosion stability. Carbon forms carbides with the elements chromium and molybdenum in particular, and as a result the austenitic matrix located around the carbides is depleted in the elements chromium and molybdenum which are nevertheless present to endow the steel with stainless properties. Types of corrosion rather like intergranular corrosion can develop. Further, carbon degrades stress corrosion stability.
In the precise case of applications in physiological media, the carbon content of a stainless steel must remain below 0.1% by weight, preferably below 0.06%.
Because of the above technical problems connected with known steels, there currently exists strong demand, which the invention intends to satisfy, for non-magnetic austenitic stainless steels with a very low nickel content and with the following properties:
no ferromagnetism;
a mechanical strength equal to or greater than that of existing steels described in ISO standards 5832/1 and 5832/9, with no brittleness;
a corrosion stability in physiological media which is substantially better than or equal to that of the steel defined by ISO standard 5832/1;
able to take a high polish;
good fatigue behavior;
better resistance to stress corrosion in physiological media up to stresses approaching their 0.2% conventional elastic limit;
very low numbers of inclusions and undesirable residual elements;
manufacturing costs equivalent to or lower than those of traditional austenitic steels, in particular as regards developing a specific alloy composition which can be produced by conventional means without a remelting step carried out under high nitrogen pressure; and
good hardening capabilities by work hardening.
The invention achieves these aims by providing a steel with the following composition as percentages by weight:
15% to 20% of Cr;
2.5% to 4% of Mo;
15% to 24% of Mn;
0.6% to 0.85% of N;
0.1% to 0.5% of V;
0 to 0.25% of Ni;
0 to 0.06% of C;
0 to 0.25% of Si;
0 to 0.02% of B;
0 to 0.5% of Nb+Ta;
0 to 0.5% of Co;
0 to 0.5% of Cu;
0 to 0.002% of S;
0 to 0.02% of Ti;
0 to 0.02% of Al;
the remainder is constituted by iron and impurities, and the conditions are as follows: 5
(%Cr)+2.5% (% Mo)≦27  (I)
(%Cr)+3.3(%Mo)≧26  (II);
and
Log(%N)+0.0605(%N)=−1.3+[125(%V)+80(%Nb)+52(%Cr)+19(%Mn)]×10
−3
−[4.3(%Cr)
2
+0.35(%Mn)
2
]×10
−4
+0.17(%Cr)
3
×10
−5
  (III)
In a preferred composit

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