EP0577898A1 - Stainless non-magnetic steel with high manganese and chromium content, resistant to stress corrosion and useful for drilling equipment, also the process for manufacturing rods from this steel - Google Patents

Abstract

Steel with austenitic structure, characterised by the corresponding mass proportions of the following elements: Carbon </= 0.025 %, preferably </= 0.020 %, Manganese 15 to 25 %, Chromium 10 to 15 %, Nickel < 0.25 %, Molybdenum 1.0 to 1.3 %, Nitrogen 0.30 to 0.50 %, Silicon < 1.0 %. The process of manufacture comprises the following stages: production of a starting ingot made of a steel as above; first homogeneous heating, to a temperature above the recrystallisation temperature of the steel; blanking the bar by hot drawing of the ingot; first cooling to ambient temperature; second homogeneous heating; forming the bar by forging and dressing in the press; second cooling to ambient temperature; and lastly final machining. In this way it is possible to produce a cold-worked nonmagnetic steel bar of length greater than 2.5 m exhibiting, over its whole length and homogeneously, a yield point, in lengthwise direction, of at least 760 MPa in the case of finished products of diameters of between 79 and 175 mm and of 690 MPa in the case of finished products of diameters of between 178 and 254 mm, and an insensitivity to intergranular corrosion and to stress corrosion.

Description

The present invention relates to non-magnetic stainless steels with high mechanical strength characteristics, as well as the parts made of these steels and their manufacturing methods.

The invention typically applies to the production of parts for the oil drilling industry, in particular for offshore drilling ( off-shore ), such as "drill-collars" , which are parts non-magnetic mechanics placed at the head of the drilling train allowing the entire column to be guided, the latter being made up, for the rest, of elements made of conventional ferromagnetic steel.

These parts must have characteristics of non-magnetism, high mechanical strength and high resistance to corrosion.

First, the magnetic permeability of these parts must be as low as possible, in any case less than 1.01 and generally less than 1.005. This essential physical property makes it possible to guide the drill string in its non-rectilinear progression in the medium to be drilled, by combining the component of the magnetic field created by a magnet housed in the drill collar with that of the terrestrial magnetic field. This property of non-magnetism is also required for other parts than the drill rods, in particular the “stabilizers” or various other measurement or control equipment which is less critical from the mechanical point of view, but which must nevertheless be made of non-magnetic steel.

The second requirement is the ability to withstand the stresses to which these parts are subjected during work, whether these stresses are in bending, in traction or in torsion. In this regard, the minima imposed ( SPEC 7 of May 28, 1984) have been established by the API ( American Petroleum Institute ), providing in particular, in longitudinal direction, a yield strength E _{p 0.2} in traction at less 760 MPa for diameters between 79 mm (3 1/8 ") and 175.0 mm (6 7/8") and at least 690 MPa for diameters between 178 mm (7 ") and 254 mm ( 10 ").

Thirdly, and this is where the most serious difficulty lies, it is essential that the parts produced offer the greatest resistance to the various modes of corrosion, taking into account the aggressive environments in which the parts are brought to work.

In general, the most significant risk is that of stress corrosion cracking caused by the nature of the soil and the additives added to drilling muds to increase drilling speeds, which in particular contain significant proportions of chlorides ( sodium, magnesium, potassium, calcium, in particular), particularly active taking into account in particular the high working temperatures, close to 80 to 100 ° C and sometimes higher, which exacerbate all the mechanisms of corrosion.

By nature, stress corrosion is localized in highly stressed areas from the mechanical point of view, especially in the bores of parts, where the stresses generated by the machining operation on the work hardened metal are added to the work stresses. ; the result of all these stresses can then largely exceed locally the value of the elastic limit of the metal and cause cracking there.

In addition, many wells are typical environments for corrosion delayed by hydrogen in contact with ClNa + H₂S mixtures, the corrosion mechanism possibly being, depending on the structure of the metal, transgranular and / or intergranular.

However, a metal subject to intergranular corrosion (following the precipitation of chromium carbides and carbonitrides in the grain boundaries which, associated with a zone depleted in chromium on either side of the joint, "sensitizes" the metal to corrosion intergranular) is also sensitive to the stress corrosion mechanism, which originates in grain boundaries, and this mechanism can combine very quickly with corrosion in transgranular mode.

Corrosion resistance must therefore be adapted to the various environments in which the tools produced using these steels are likely to work.

It is certainly known to produce nickel-based "superalloys" having all of the desired properties; the very high nickel content (up to 55%) makes the alloy insensitive to stress corrosion. However, due to their very high price, their use has been very limited, in practice excluding the production of large parts.

The route generally followed consists rather in making austenitic steels based on manganese-chromium containing non-negligible amounts (0.2 to 0.6%) of nitrogen, the hardening element par excellence of austenites and also generator and stabilizer of this phase. Other elements, such as nickel and molybdenum, are added to it, but in smaller quantities.

Such a steel is for example described in EP-A-0 277 065, which describes a steel comprising mass proportions of at most 0.006% of carbon, 19 to 26% of manganese, 7 to 13% of chromium, approximately 0 , 3% nitrogen and 0.6% silicon, less than 0.2% nickel and less than 0.1% molybdenum.

In all cases, whatever the grade of steel produced, this steel is hardened by hot work hardening.

However, this work hardening has the disadvantage - and this for all the grades of steel based on manganese-chromium known up to now― of causing the precipitation of carbides and carbonitrides of chromium in the grain boundaries inducing, as we have explained above, sensitivity to intergranular corrosion and, simultaneously, to stress corrosion, which also arises in grain boundaries.

This drawback does not exist with non-hardened steels (hyperhardened metals), but the latter do not have sufficient mechanical characteristics for the intended uses.

To combat the risk of cracking, and therefore to annihilate the effects of corrosion following work hardening, treatment has hitherto been carried out for all steel grades. surface mechanics in places most exposed to stresses.

This treatment was carried out either by shot peening or by hammering, this operation having the effect of compressing the treated surface to a certain depth (up to 1.5 mm for shot blasting and up to 5 mm for hammering), thus allowing the part to properly resist stress corrosion at the place thus treated.

This mechanical intervention for the surface treatment, however, led to a non-negligible additional cost for the part produced.

In contrast, the present invention provides a steel grade which overcomes the aforementioned drawbacks and which, while offering the required properties of non-magnetism and high mechanical performance, either by nature - and not after surface treatment - resistant to stress corrosion and intergranular corrosion. In this way, the parts produced can be used naked, directly after shaping, without it being necessary to provide a surface and local treatment of the sensitive surfaces.

In other words, the invention provides a grade of non-magnetic steel which, once hardened, possesses the mechanical properties of a hardened steel but resists stress corrosion and intergranular corrosion as well as a hyper-tempered metal , not hardened.

The invention also proposes, as will be seen, a process which lends itself perfectly to the production of any non-magnetic part and with a high elastic limit, in particular non-magnetic drill collars and similar parts (stabilizers, in particular), which are parts typical to have these characteristics and which, moreover, are the most difficult to manufacture given their very large dimensions.

To this end, the steel of the invention is a steel with an austenitic structure characterized by the respective mass proportions of the following elements: carbon ≦ 0.025%, preferably ≦ 0.020%, manganese 15 to 25%, chromium 10 to 15%, nickel <0.25%, molybdenum 1.0 to 1.3%, nitrogen 0.30 to 0.50%, silicon <1.0%.

It will be noted in particular, in this composition, the very low carbon content combined with a low nickel content and a high molybdenum content. The reasons and consequences of these particular choices will be explained below.

In addition, as will also be seen, the choice of a very low carbon content makes it possible to implement a specific, particularly advantageous process for manufacturing a homogeneous bar.

• élaboration d'un lingot de départ en une nuance d'acier du type que l'on vient d'indiquer,
• premier chauffage homogène, à une température supérieure à la température de recristallisation de l'acier, typiquement entre 1240 et 1280°C environ,
• ébauchage de la barre par étirage a chaud du lingot,
• éventuellement, élimination des extrémités du lingot,
• premier refroidissement jusqu'à température ambiante,
• second chauffage homogène, à une température typiquement comprise entre 800 et 1000°C environ (suivant la section de l'ébauche et le diamètre de la pièce à fabriquer),
• mise en forme de la barre par forgeage et dressage à la presse,
• second refroidissement jusqu'à température ambiante, et
• usinage final.

This process includes the following steps:

• development of a starting ingot in a steel grade of the type just indicated,
• first homogeneous heating, at a temperature higher than the recrystallization temperature of the steel, typically between 1240 and 1280 ° C.,
• roughing of the bar by hot drawing of the ingot,
• possibly elimination of the ingot ends,
• first cooling down to room temperature,
• second homogeneous heating, at a temperature typically between approximately 800 and 1000 ° C. (depending on the section of the blank and the diameter of the part to be manufactured),
• shaping of the bar by forging and dressing with the press,
• second cooling to room temperature, and
• final machining.

The transformations applied to the bar work harden it, preferably, with a wrinkling factor of at least 1.5.

One can also provide, after the roughing step or, where appropriate, the step of removing the ends of the ingot, an additional step of cutting the ingot or, respectively, of the remaining part of the ingot, in one plurality of distinct lengths, each length then being treated individually by the subsequent steps of the process.

We will also see that, thanks to the steel grade proposed and the process which we have just exposed, the invention allows the production of long non-magnetic hardened steel bars (typically, greater than 2 or 3 m) having over their entire length homogeneous mechanical characteristics, in particular characteristics that meet the values imposed by the API recommendations mentioned above.

This property is to be opposed in particular to the case of bars obtained with the steel grade and the method described in the aforementioned EP-A-0 277 065, which allow only to achieve bars having locally the desired characteristics, two ends and not over their entire length.

The present invention therefore also relates, as a new industrial product, to a hardened non-magnetic steel bar of length greater than 2.5 m, in particular for drilling equipment, having, over its entire length and in a homogeneous manner, a mechanical resistance, in transverse direction, to breaking in tension at least equal to 830 MPa and an insensitivity to intergranular corrosion and to corrosion under stress.

It is thus possible to obtain either a very long end product with homogeneous properties, or, after cutting the bar, sections with homogeneous characteristics, whatever their length and their position in the starting bar.

Example

We will now describe an example of implementation of the invention.

A steel grade with the following composition is produced, using techniques which are in themselves conventional: carbon ≦ 0.020%, manganese 15 to 25%, chromium 10 to 15%, nickel <0.25%, molybdenum 1.0 to 1.3%, nitrogen 0.30 to 0.50%, silicon <1.0%, sulfur ≦ 0.010%, phosphorus ≦ 0.025%.

It will be noted in particular, in this composition, the low nickel content (which is sought as low as possible) because this element, if added in increasing amounts in the steel, would deteriorate the resistance to stress corrosion in the environments that cause it.

On the other hand, the molybdenum content is chosen to be high because, although, as with nickel, a high molybdenum content reduces the resistance to corrosion under stress, the presence of this element is a factor of resistance to pitting and to cavernous corrosion. The properties of the grade according to the invention indeed make it possible to obtain a resistance to corrosion under stress that is so high that the negative impact of molybdenum on this resistance becomes negligible. The presence of molybdenum, previously considered harmful, then becomes advantageous because it provides additional resistance to other forms of corrosion.

Last but not least, with regard to the very low carbon content, this is a very important characteristic because it is necessary to guarantee a microstructure free from intergranular precipitation, in particular from continuous intergranular precipitation. A higher carbon content would reduce the corrosion resistance by causing excessive precipitation of carbides and carbonitrides dehomogenizing the areas adjacent to the grain boundaries.

In addition, this low carbon content makes it possible to implement the process which will now be described.

We will take the example of the realization of a drill rod of 9.300 m in length and 203.2 mm in outside diameter (generally the diameters of these parts vary from 120.6 to 279.4 mm; the example chosen corresponds to a typical drill, located in the middle of the range).

These drill rods are tubular parts, threaded at their two ends and axially bored, over the entire length, to a diameter between 50.8 and 76.2 mm (this value depending on the outside diameter).

The starting metal consists of an octagonal ingot of 10 t, with an average section of around 60 dm² (the extreme ingot weights corresponding to the smallest and the largest diameters are approximately 3.5 and 10 t respectively).

This ingot is firstly heated in a gas oven at a temperature between 1240 and 1280 ° C for a period between 16 and 24 hours.

The ingot thus made homogeneous in temperature is then unwound and subjected to a first thermomechanical transformation consisting of a roughing operation by stretching with a press (4500 t press), making it possible to obtain an octagonal blank of 290 mm on dishes.

This operation, which lasts about an hour, must necessarily result in an octagonal bar whose length is less than 6 m in order to subsequently apply a minimum wrought of 1.5 on the finished product.

After roughing, the two ends of the ingot are removed (about 15% of the total weight) and the rest of the bar is cut in two lengths.

The two sections thus cut are placed on the ground and cooled to room temperature. Two rods are then made from these two 290 mm octagonal blanks, each with a length of around 5.8 m.

To this end, a second thermomechanical transformation is carried out, starting with a second heating up to a temperature (in the case of this example) of 980 to 1000 ° C., the temperature being a function of the section of the blank and of the diameter. of the part to be manufactured. This heating is carried out in a gas oven for approximately 8 hours, in order to obtain a uniform temperature over the entire length of the bar and over the entire extent of the section.

The two blanks are then subjected, in a single operation, to rapid forging in stamps with simultaneous dressing on the 4500-ton press, this making it possible to obtain a bar of approximately 230 mm in diameter and 10 m in length. The bar is turned over at the end of the operation to forge the end part previously held in the jaws. During this operation, the temperature drops in skin up to 750 ° C.

The bar is then placed on the ground, cooled as quickly as possible, then the dressing is completed.

The thermomechanical forming operations are then completed.

We can therefore proceed to the final machining operations comprising, in itself conventional, the lengthening, the straightening of the faces and the drilling of the bore to 71.4 mm in diameter, these different stages being matched intermediate and final checks.

The drill rods thus produced have been found to comply perfectly with the requirements for resistance to corrosion as well as those for mechanical strength.

To evaluate the resistance to corrosion under stress, samples were taken from industrially manufactured drill collars as indicated above.

With regard to intergranular corrosion, the corrosion samples taken and placed in a boiling sulfocupric medium (test according to ASTM A 262) did not show any beginning of intergranular corrosion.

As regards stress corrosion, a ring test piece ( C-Ring test piece according to ASTM G 38) was stressed either in constant deformation or in constant stress and placed in test environments suitable for causing stress corrosion, the most classic of which is a CL₂Mg, 6H₂O 44% medium boiling at 154 ° C (test according to ASTM G 36), or else in a test medium for delayed corrosion by hydrogen, the most common of which is a ClNa medium with 50 g / l buffered with glacial acetic acid at pH 3 and saturated with H₂S at room temperature (test according to NACE TM 01-77).

The test specimens, constrained to stresses equal to 80% of the elastic limit E _{p 0.2,} showed no rupture after 720 hours of testing, these tests having even been extended without further consequences up to 1000 hours.

As far as the mechanical characteristics are concerned, these comply with the minimum requirements set out in API SPEC 7 , namely: for future test pieces in the longitudinal direction an elastic limit E _{p 0.2} at traction greater than 760 MPa for diameters between 79 and 175 mm and at 690 MPa for diameters between 178 and 254 mm. The resistance is always greater than 830 MPa, the elongation greater than 13%.

Claims (9)

A non-magnetic stainless steel with an austenitic structure based on manganese-chromium, characterized by the respective mass proportions of the following elements: carbon ≦ 0.025%, manganese 15 to 25%, chromium 10 to 15%, nickel <0.25%, molybdenum 1.0 to 1.3%, nitrogen 0.30 to 0.50%, silicon <1.0%. The steel of claim 1, wherein the proportion of carbon is at most 0.020%. A method of manufacturing a bar, characterized by the following steps: - production of a starting ingot of steel according to claim 1, - first homogeneous heating, at a temperature higher than the recrystallization temperature of the steel, - roughing of the bar by hot drawing of the ingot, - first cooling down to room temperature, - second homogeneous heating, - shaping of the bar by forging and dressing with the press, - second cooling to room temperature, and - final machining. The method of claim 3, wherein the first heating is at a temperature between about 1240 and 1280 ° C. The method of claim 3, wherein the second heating is at a temperature between about 800 and 1000 ° C. The method of claim 3, wherein the transformations applied to the bar work harden it with a wrinkling factor of at least 1.5. The method of claim 3, further comprising, after the roughing step, an additional step of removing the ends of the ingot. The method of one of claims 3 or 7, wherein, after the roughing step or, where appropriate, the step of removing the ends of the ingot, there is provided an additional step of cutting the ingot or , respectively, of the remaining part of the ingot, in a plurality of distinct lengths, each length then being treated individually by the subsequent stages of the process. A non-magnetic hardened steel bar with a length greater than 2.5 m, in particular for drilling equipment, characterized in that it has, over its entire length and in a homogeneous manner, an elastic limit E _{p 0.2} , in longitudinal direction, at least equal to 760 MPa for finished products with diameters between 79 and 175 mm and 690 MPa for finished products with diameters between 178 and 254 mm, and insensitivity to intergranular corrosion and to corrosion under constraint.