EP0356417A1 - Process for manufacturing stress corrosion crack-resistant tubular bodies, particularly non-magnetizable austenitic steel drill collars, and bodies obtained thereby - Google Patents

Abstract

In a process for the production of tubular bodies that are resistant to stress corrosion cracking, in particular non-magnetizable drill stems and rod sections of austenitic steels, after solution treatment, quenching, and after deformation at a temperature of under 500 DEG C., in order to increase the mechanical properties of the material, and after processing and incorporation of a drilling, the body is heated to a temperature of 220 DEG to 600 DEG C., at least to temperature equalization with a temperature differential of at most 10 DEG C. in the walls of the body. The body is then maintained for at most a time t in minutes at a temperature T in degrees Celsius in accordance with the expression t=10-(T-638)/50 after which it is cooled by the increased withdrawal of thermal energy, at least from the internal surface of the tubular body and the cooled surface exhibits a temperature drop of at least 100 DEG C./min from the starting temperature to the half value between the starting temperature and room temperature.

Description

When sinking a borehole, drill collars and rod parts with high material strength are required to support the weight and stabilize the drill head. In order to be able to control the course of the borehole during drilling and to drill directed boreholes, the inclination and the direction of the borehole must be measured often and at regular intervals, preferably using the earth's magnetic field. In order that such measurements of the earth's magnetic field can be carried out correspondingly precisely or are not influenced, completely non-magnetizable materials are to be used for these drill string parts. To test the non-magnetizable drill collars, it is advantageous to use a method according to EU-PS 14 195.

For non-magnetizable drill collars, only Cu-Ni-Al alloys, so-called Monel-K alloys, were initially used because they are completely non-magnetic, have the required strength properties and are considered to be relatively easy to machine.

However, Monel-K alloys are relatively expensive, so that austenitic steels have been proposed to achieve more economical products for the manufacture of non-magnetizable drill rods and drill string parts.

However, conventional 18/8 CrNi steels have a magnetically unfavorable behavior and have lower strength properties or low yield strengths and poor machinability, so that these materials are not very suitable.

In order to eliminate this unsatisfactory condition, it was proposed according to AT-PS 214 460 to use stable-austenitic steels, in particular manganese-austenites, for non-magnetisable drill collars, the raw parts produced therefrom having to be hardened by cold working in order to achieve high yield strength values of the material . The properties of such collars meet the usual requirements. However, they have the disadvantage that they are not always sufficiently resistant to corrosion attacks, for example aggressive chloride solutions which occur frequently in boreholes, and may tend to show stress corrosion cracking. This can result in breaks that cause the failure of such drill collars.

In order to improve the corrosion behavior with good magnetic material properties and in particular to avoid stress corrosion cracking, according to AT-PS 308 793 alloys with chrome contents of 20 - 25%, nickel contents of 10 - 15% and nitrogen contents of 0.05 were also used for the production of drill collars and rod parts - 0.5% proposed, which are subjected to cold deformation to increase the strength properties.

The use of precipitation hardened alloys with contents of approx. 33% Ni, 18% Cr, 2% Ti, 0.5% Al and 0.004% N should bring about significant improvements in the performance characteristics of drill collars or drill string parts.

However, the high content of expensive alloying elements of these materials can lead to economic disadvantages.

In order to take advantage of the economic advantages of the production of drill collars from non-magnetisable and easily hardenable Cr-Mn steels and their corrosion behavior, in particular the resistance to stress corrosion cracking improve, it was also proposed (AT-PS 364 592) to cause residual compressive stresses in the surface area, in particular of the hollow, of the collars by the action of mechanically triggered impact or pressure energy. Pressurized gas hammer is preferably used, the head part of which carries a firing pin for transmitting the axial impact movement. In terms of their properties, drill collars manufactured in this way largely meet the requirements placed on them in the oil field. However, they have the disadvantage that residual compressive stresses which prevent stress corrosion cracking can only be generated to a small depth below the surface. This is mainly due to the fact that the tools may only have a limited impact energy during surface deformation and multiple strikes should be largely avoided, because otherwise the deformability of the steel is exhausted in the area of action of the firing pin and cracks form. Because the deformation of the zone near the surface has to be area-wide on the one hand, and on the other hand, due to the above reasons, it often has disadvantages, the effect of the method is uncertain and difficult to control. Under a thin surface layer, in which compressive stresses prevail, there are zones with high tensile stresses in particular in the hollow of the tubular part. Surface damage or minor material removal can expose areas with tensile stress, which can lead to increased stress corrosion. Another disadvantage is that high local strain hardening of the material, which is formed in the near-surface zone when mechanical pressure is applied, increases the tendency of the material to pitting. In the event of pitting, the compressive stress layer is undermined and stress corrosion cracking of the part increases. The mechanical application of residual compressive stresses in the surface layer of parts also has the disadvantage that only simple shapes or contours can be treated accordingly last operation must be done without subsequent calibration. It is therefore practically not possible to create residual compressive stresses in the near-surface zone on edges, threaded parts, in corners, holes and back-turns as well as on chamfers and discontinuous surface parts in order to prevent stress corrosion cracking.

gehalten wird, von dieser Temperatur bzw. dieser Ausgangs­temperatur durch gesteigerten Entzug von Wärmeenergie minde­stens von der Innenoberfläche des Rohrkörpers gekühlt wird und die gekühlte Oberfläche von der Ausgangstemperatur bis zum halben Wert zwischen Ausgangstemperatur und Raum­temperatur einen Temperaturabfall von mindestens 100°C/min aufweist. Vorteilhaft ist es, wenn der Körper von einer Ausgangstemperatur von 280 bis 500°C, insbesondere von 300 bis 400°C, mit einem Temperaturunterschied in der Körper­wand von höchstens 6°C, vorzugsweise höchstens 3°C, abgekühlt wird. Besonders vorteilhaft ist, wenn die Innenoberfläche und die Außenoberfläche des rohrförmigen Körpers gekühlt werden, wobei die Innenkühlung zeitlich mindestens 5 s, vorzugsweise mindestens 20 s, früher und/oder mit höherer Intensität als jene der Außenoberflächenkühlung durchgeführt wird.On the basis of this prior art, the invention was based on the objects of avoiding the above disadvantages and of creating a method for producing tubular bodies which are resistant to stress corrosion cracking, in particular non-magnetizable drill rods and rod parts made of austenitic steels. A further object of the invention relates to tubular bodies which are resistant to stress corrosion cracking and are produced by this method, in particular non-magnetizable drill rods and rod parts made of austenitic steel. This object is achieved in the inventive method in that the body after solution annealing, quenching and after deformation at a temperature of below 500 ° C to increase the material strength and after machining or drilling a hole to a temperature of 220 to 600 ° C, at least up to a temperature equalization with a temperature difference of at most 10 ° C in the body wall, at most a time t in minutes at a temperature T in ° C according to the context

is kept, from this temperature or this starting temperature is cooled by increased removal of thermal energy at least from the inner surface of the tubular body and the cooled surface has a temperature drop of at least 100 ° C / min from the starting temperature to half the value between the starting temperature and room temperature. It is advantageous if the body of one Starting temperature of 280 to 500 ° C, in particular from 300 to 400 ° C, with a temperature difference in the body wall of at most 6 ° C, preferably at most 3 ° C, is cooled. It is particularly advantageous if the inner surface and the outer surface of the tubular body are cooled, the inner cooling being carried out at least 5 s, preferably at least 20 s earlier and / or with a higher intensity than that of the outer surface cooling.

Tubular bodies produced in accordance with this process, in particular non-magnetizable drill rods and rod parts made of austenitic steel, according to the invention have local tensile residual stresses of less than 100 N / mm² in the zones near the surface to a depth of at least 8 mm. It is particularly preferred if the near-surface zones have residual compressive stresses to a depth of at least 4 mm, preferably at least 8 mm, and that the tensile residual stresses that may occur in the entire cross-section of the wall are less than 150 N / mm², i.e. below the triggering voltage for stress corrosion cracking , preferably less than 120 N / mm².

Due to a deformation of the blank at temperatures below 500 ° C, which serves to work harden or increase the yield strength of the material, tubular bodies, especially collars, show considerable differences in the local internal stresses in the wall, etc. compressive stresses on the outer surface and high tensile stresses on the surface of the hollow, i.e. the bore, which are well above the limit for triggering stress corrosion cracking. It has surprisingly been found that in a tubular body consisting of solution-annealed, quenched and cold-formed austenitic material, heating to appropriate temperatures while adhering to certain conditions with subsequent intensified cooling can cause stress states. which, due to plastic deformations in the pipe wall, set an internal stress state that largely has no local tensile stresses above the limit at which stress corrosion cracking is triggered. Furthermore, through an appropriate choice of the starting temperature and a time-graded and / or different internal and external cooling in terms of intensity in the wall of the tubular body, such a state of internal stress can be achieved, in which near the surface to a depth of at least 4 mm compressive stress to rule. When the method according to the invention is used, a residual stress redistribution surprisingly takes place in the wall without the high strength or high yield strength of the material caused by cold deformation being adversely affected. It is important that the temperature differences in the pipe wall after heating to the initial temperature are small, because otherwise the stress redistribution during the intensified cooling is adversely affected or can only take place to a small extent and a desired residual stress state cannot be achieved accordingly. Therefore, the temperature difference in the wall should be kept below 10 ° C. Longer holding times at the initial temperature have an unfavorable effect because this causes the solution-annealed, quenched and work-hardened steels, for example austenitic Mn-Cr steel, to be sensitized to inter-crystalline crack corrosion. It has been found that the sensitization depends on diffusion and carbide formation processes and, if appropriate, nitride formation processes, the temperature (T) the holding time (t) until the material is sensitized logarithmically with the relationship
T = -50 log t + 638
influenced. For this reason, the holding time at the initial temperature should be chosen shorter than the value that results from the following relationship:

Furthermore, it is important that the tubular body is cooled from the starting temperature by increasing heat removal at least from the inner wall, because in the area of the inner surface of the wall the highest tensile stresses originating from cold working or work hardening have to be redistributed. Due to low cooling intensities, no sufficient residual stress redistribution is effected, so that the cooled surface of the tube wall must experience a temperature drop from the starting temperature to half the value between the starting temperature and room temperature of at least 100 ° C./min.

It was quite surprising that the method according to the invention brings about a residual stress redistribution and can be used for the production of tubular bodies which are resistant to stress corrosion cracking, in particular non-magnetizable drill rods and rod parts made of austenistic steels. The prejudices of the professional world had to be overcome, e.g. that heating to higher initial temperatures leads to an unacceptable softening or lowering of the yield point of the cold-formed material, and that low initial temperatures can have no effect, because during the subsequent cooling process, only elastic material deformations occur arise. Furthermore, it was assumed by the person skilled in the art that the increased strength and the high tensile stresses on the inner surface of the tube cause cracks, in particular longitudinal cracks, even when heated to the initial temperature. In particular, the corrosion specialist feared that renewed heating of a material quenched from solution temperature and work hardened would cause sensitization, which would make the material in chloride-containing media susceptible to grain breakdown or intergranular crack propagation.

The invention is explained in more detail below with reference to a drawing and an example:
Fig. 1 shows schematically stress states in the wall of a tubular body.

bei Luftbeaufschlagung und einen Teil

für Wasser­beaufschlagung der Außenoberfläche. Die Spannungsumlagerung durch ein intensiviertes Abkühlen der Rohrwand aus Tempe­raturen von beispielsweise 300°C und 400°C bewirkt, daß in der gesamten Rohrwand die Eigenspannungen unter 150 N/mm², nämlich der Auslösespannung der Spannungsrißkorrosion, liegen und somit der Körper vollkommen spannungsrißkorrosions­beständig ist. Dabei werden an der Innenoberfläche Druck­spannungen bis zu einer Tiefe von größer als 4 mm erreicht.After strain hardening by deformation of the raw body at a temperature below 500 ° C, internal stresses prevail in the tubular body, etc. on the outer tube wall A compressive stresses, which change to the inner tube wall B according to curve ① in high tensile stresses. When heating to an initial temperature of 200 ° C with subsequent, intensified cooling of the inner tube wall, the tensile stresses prevailing there are only slightly reduced, as curve nur illustrates. Curves ④ and ⑤ show the residual stress distributions in the pipe wall when cooling from an initial temperature of 300 ° C (4) and 400 ° C (5). In the area of the outer wall A, the voltage curve ⑤ is shown divided, etc. in one part

when exposed to air and a part

for exposure to water on the outer surface. The stress redistribution by intensively cooling the tube wall from temperatures of, for example, 300 ° C. and 400 ° C. has the effect that the internal stresses in the entire tube wall are below 150 N / mm², namely the triggering voltage of the stress corrosion cracking, and the body is therefore completely resistant to stress corrosion cracking. Compressive stresses down to a depth of more than 4 mm are achieved on the inner surface.

Intensified cooling from an initial temperature of 550 ° C, for example, increases the internal compressive stresses and their effective range on the inner surface of the pipe wall (curve ⑥), which can be used for a recalibration by machining. Curve ② shows the stress curve in a pipe wall, which can be set by a method according to AT-PS 364 592 or according to the prior art, high internal compressive stresses prevailing on the inner surface, but these compressive stresses at a small distance from the surface in pass high tensile stresses. The invention is illustrated further below by a practical example:
A block weighing approx. 3 t from Mn-Cr-N steel with a composition of 0.05% C, 19.3% Mn, 13.6% Cr, 2.1% Ni, 0.23% N (figures in% by weight) rest, in particular iron, was primary deformed by hot forging in a long forging machine into a drill collar blank with a dimension of 0̸ 196 x 8800 mm. The solution was annealed from a solution annealing temperature of 1020 ° C in the water basin. The blank was adjusted, cold forged with a degree of deformation of 15%, straightened, turned and drilled. The dimensions of the semi-finished product were:
AD 0̸ 172.3 x ID 0̸ 70.45 x 9250 mm (AD = outer diameter, ID = inner diameter). The residual stresses on the AD were 0̸ -157 N / mm² (residual compressive stress) and on the ID +390 N / mm² (residual tensile stress), the measured values representing the arithmetic mean of 3 measurements using the ring-core method.

A sample from one end of this semi-finished product was exposed to boiling aqueous solution of saturated magnesium chloride (42%, 154 ° C) for one day, after a short time cracks starting from the ID.

The tubular semi-finished product or rod (approx. 700 mm minimum length for the above sample) was heated in an electric furnace at 415 ° C, with a temperature difference in the tube wall at the end of the heating period of 0.8 ° C. In a spray system, jet cooling was initially carried out on the inside surface with a quantity of 1500 - 2500 l / min and after 10 to 30 s, preferably 20 s, also on the outside surface with a cooling water quantity of approx. 100 l / min and meter length with a temperature drop of the surface of approx. 350 ° C, at least to a temperature below 100 ° C.

As a result of this treatment, the internal stress state of the rod on the ID changed from +390 N / mm² (tensile stress) to -410 N / mm² (compressive stress). Internal compressive stresses of - 120 N / mm² were also determined on the outside diameter. Furthermore, after twisting and drilling, the residual stresses over the wall thickness were determined, the tensile stresses being measured being less than + 110 N / mm². A sample made from this rod, which was tested with magnesium chloride in the SCC test described above, remained completely free of cracks.

A drill string part was made from the semi-finished product and further samples were taken from it at the machined points. It was shown in the SCC test that cut-outs made in the pipe wall by milling, turning and planing as well as the NC-cut threads do not cause any cracks, which results from the non-critical residual stress condition in the entire volume of the part.

The method according to the invention is particularly advantageous for austenitic steels with a directional analysis C: max. 0.2% by weight; Mn: 0-25% by weight; Cr: 12-30% by weight; Mo: 0-5% by weight; Ni: 0-75% by weight; N: 0-1% by weight; Ti: 0-3% by weight; Nb: 0-3% by weight; Cu: 0-3% by weight, rest of iron applicable. Mn-Cr austenites with 17-20% by weight Mn and 12-14% by weight Cr and Cr-Ni austenites with 17-24% by weight Cr and 10-20% by weight are particularly preferred. Ni.

Claims (11)

1. A process for the production of stress-crack corrosion-resistant tubular bodies, in particular non-magnetizable drill rods and rod parts made of austenitic steels, characterized in that the body after solution annealing, quenching and after deformation at a temperature of below 500 ° C to increase the material strength and after a Machining or drilling a hole to a temperature of 220 to 600 ° C, at least heated to a temperature equalization with a temperature difference of at most 10 ° C in the body wall, at most a time t in minutes at a temperature T in ° C according to the context

is kept and from this temperature or this starting temperature is cooled by increased removal of thermal energy at least from the inner surface of the tubular body and the cooled surface has a temperature drop of at least 100 ° C / min from the starting temperature to half the value between the starting temperature and room temperature. 2. The method according to claim 1, characterized in that the body is cooled from an initial temperature of 280 to 500 ° C, in particular from 300 to 400 ° C, with a temperature difference in the body wall of at most 6 ° C, preferably at most 3 ° C becomes. 3. The method according to claim 1 or 2, characterized in that the inner surface and the outer surface of the tubular body are cooled. 4. The method according to any one of claims 1 to 3, characterized in that the cooling of the inner surface of the tubular body is carried out earlier in time and / or with a higher intensity than that of the outer surface. 5. The method according to claim 4, characterized in that the inner surface of the tubular body is cooled at least 5 s, preferably at least 20 s, before the outer surface. 6. The method according to any one of claims 1 to 5, characterized in that gases and / or liquids, in particular compressed air and / or water, are used as coolants. 7. The method according to any one of claims 1 to 6, characterized in that additional compressive stresses are caused by the action of mechanically triggered shock or pressure energy in the surface region of the body. 8. Stress-crack corrosion-resistant tubular body, in particular non-magnetizable drill collar and rod part made of austenitic steel, characterized in that the local tensile residual stresses are less than 100 N / mm² in the near-surface zones to a depth of at least 8 mm. 9. Body according to claim 8, characterized in that at least on the inner surface of the wall of the tubular body to a depth of at least 4 mm, preferably of at least 8 mm, residual compressive stresses prevail. 10. Body according to claim 8 or 9, characterized in that the near-surface zones to a depth of at least 4 mm, preferably 8 mm, have residual compressive stresses. 11. Body according to one of claims 8 to 10, characterized in that in the entire cross section of the wall, the local tensile stresses that may occur are less than 150 N / mm², preferably less than 120 N / mm².

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