EP0205828B1 - Method and use of a steel for manufacturing steel pipes with a high resistance to acid gases - Google Patents

Abstract

For increasing the resistance of steel pipes or tubes against acidic medium, by a combination of a heat treatment and the use of an alloyed steel, a compressure stress is built up on the side of the tubes or pipes facing the acidic medium, of up to 30% of the yield limit.

Description

The invention relates to a method for producing welded steel pipes which can be used for the transport of acidic gases and / or oils and which have compressive stresses on the inside facing the acidic gas and / or oil.

Oils and gases containing hydrogen sulfide (H2S) are often transported in welded pipes and are therefore referred to as “acidic”. The media containing H2S lead to cracks in the pipes, which are referred to as "hydrogen-induced stress corrosion cracking". A distinction is made between so-called HIC errors (hydrogen-induced cracking) and SCC errors (stress corrosion cracking). Damage caused by sour gas and sour oil has already occurred in a wide variety of countries a few weeks after the start of operation, with the formation of cracks, particularly alongside the weld seam in the lower part of the pipe, being observed. Both longitudinally welded and spiral-welded conduits are affected by this damage.

Is known, cf. "Steel and iron" 1984, pp. 1357 to 1360, that a very low sulfur content and a high degree of oxidic purity are required for acid gas pipes, for which a pan-metallurgical treatment, in particular a calcium treatment in a base-walled pan, is known. It is also known to thermomechanically roll a steel to achieve certain mechanical properties, in particular to achieve a well-coordinated combination of strength and toughness properties, cf. "Stahl und Eisen" 1981, pp. 483 to 491 and pp. 593 to 600.

US Pat. No. 3,992,231 discloses a method for producing oilfield pipes with improved acid gas properties. According to this known method, a steel with 0.28 to 0.42% C, 0.8 to 1.2% Cr, 0.6 to 1.0% Mo, 0.025 to 0.05% Nb, 0.4 up to 1.0% Mn, 0.2 to 0.6% Si, remainder iron and usual inevitable impurities firstly seamless tubes are produced, which are quenched after an austenitizing annealing. To create compressive stress on the inside of the tube, the seamless tubes are then placed in an oven at a temperature from 540 ° C to below the transition temperature, i.e. Heated to 690 ° C for several hours and then quickly quenched the inner tube wall with water. The tubes produced by this known method are typical oilfield tubes with a screw connection, as can be seamlessly manufactured up to a diameter of approximately 500 mm. Large pipes for long-distance lines, on the other hand, cannot be seamlessly manufactured due to the process. In addition, a steel of the composition mentioned results in poor field weldability. Furthermore, the long-term heat treatment, for which correspondingly large heating furnaces are required, is technically and economically complex.

Quite apart from this, the intended heat treatment lowers the yield strength of the pipe manufactured, so that higher quality grades can only be achieved through appropriate additional measures, e.g. increased alloy additions of expensive alloy elements can be achieved.

US Pat. No. 4,325,748 shows a method for producing sheet steel for welded pipes for the transport of media containing hydrogen sulfide. The method provides a special thermomechanical treatment with a 10 hour hot aging before finish rolling in combination with a special chemical analysis of the steel, which analysis corresponds to that of the present invention. Warm aging in particular prevents the mid-segregation which is detrimental to the acid gas resistance.

It is also known from DE-OS 3 422 781, a method for the heat treatment of an existing pipeline, in which an induction spout placed around the pipeline and a coolant continuously flowing through the pipe provide the temperature distribution necessary for generating compressive stresses on the inner surface Build up wall thickness. The temperature distribution is controlled by a mechanical change in the induction coil geometry (diameter and pitch), which in turn causes a change in the magnetic flux density. With this method, in a stationary process, in particular the connecting round seams lying in a plane perpendicular to the pipe axis are to be subjected to heat treatment between two pipes. This procedure, which is highly discontinuous due to the particular structure of a defined steady state in the area to be treated, does not permit continuous treatment of a helical or axially parallel seam of a welded individual tube, in particular during the manufacturing process.

In addition, the intended continuous internal flow with the cooling medium in the case of single pipe production can only be implemented in a complex manner in terms of plant technology and is characterized by a high demand for cooling medium and high energy input.

Another disadvantage is that with continuous flow of coolant in the steady state, apart from the magnetic flux density, there is no further control option for optimizing the heat treatment!

A method in which welded high-pressure pipes to increase the corrosion resistance are also inductively heated with internal cooling by means of continuous coolant flow is described in US Pat. No. 4,229,235.

DE-OS-3 004 872 discloses a method for increasing the internal pressure resistance of thick-walled fuel injection pipes in the wall thickness-diameter ratio of 1: 3 to 1: 4 for pressure levels up to 1000 bar, the pipe with subsequent or simultaneous internal cooling by spraying water is moved by an induction heater.

This process, which is quite sensible for stainless steel pipes, is not only due to the completely different process Ren dimensions not applicable to conduits, but would also cancel the advantageous strength state resulting from the usual thermomechanical treatment of the primary material of conduits.

It is also known from DE-PS 2 716 081 to use a controlled rolled steel with a yield strength of at least 40 HB, consisting of 0.01 to 0.13% carbon, 0.1 to 1.0% silicon, 0.7 up to 2.0% manganese, at most 0.1% total aluminum, 0.004 to 0.03% titanium, 0.001 to 0.009% total nitrogen, 0.01 to 0.10% niobium and 0.01 to 0.15% vanadium and / or 0.05 to 0.40% molybdenum with a total niobium and carbon content of at most 0.005% and at least 0.004% titanium nitride with a particle size of at most 0.02 µm, 0 to 0.6% chromium, 0 to 1.0% copper , 0 to 4.0% nickel under the condition [(% Cu) + (% Ni) 1: 5 + (% Cr) + (% Mo) <0.90% balance iron including melting-related impurities, after annealing at a maximum 1,150 ° C and a subsequent hot rolling with a cross-sectional decrease of at least 50% at a temperature of at most 930 ° C and a final temperature of at most 830 ° C as a material for objects such as pipes f for arctic pipelines must have high cold toughness.

Although 5 mm thick and 1 mm ground specimens of this steel were tested for hydrogen cracks after immersion in an H2S solution, the results of these tests do not allow any conclusions to be drawn about either hydrogen-induced cracking or hydrogen induced induced stress corrosion cracking in the weld area of welded pipes, in particular spiral welded large pipes, since these are apparently samples from the strip.

Different standards, e.g. the US standard NACE Standard TM-02-84, was created specifically for the testing of samples taken from welded pipes. For this purpose, a cross section of a welded tube is shown in FIG. 1 a, from which samples 1, 2 are taken. In Fig. 1 b, an enlargement of the sample 1 from Fig. 1 a is shown in cross section, specifically different types of cracks are shown there, with

I cracks along the boundary between base material and HAZ,

11 HIC-like cracks, SCC cracks in the HAZ for steels that are still somewhat HIC sensitive, parallel to the surface and stair-like through the wall,

111 cracks from the geometrical notch of the seam elevation starting from the pipe wall in Q + T treated pipes and

IV cracks - weakening of the grain boundaries by welding heat - along the weld seam in steels with low carbon and niobium content.

“HAZ” is to be understood as the heat-affected zone next to the weld seam (heat affected zone). HIC errors can occur on samples without voltage and SCC errors on samples with voltage.

The HIC errors are defined in accordance with the aforementioned US standard as shown in FIG. 1c (sample according to FIG. 1 as

CSR - “Crack Sensitivity Ratio”, ratio of cracked area to sample area in percent,

CLR - Crack Length Ratio, ratio of crack length to sample length in percent and

CTR - “Crack Transverse Ratio”, ratio of the crack width to the sample width in percent, whereby for the so-called sour gas or oil pipes for these types of error compliance with the following upper limit values on small samples according to the state of the art is required:

If small samples of 100 mm x 20 mm x wall thickness are tested on perfectly manufactured, welded pipes, they meet the above requirements. However, if entire sample tube rings are placed in a corrosion solution according to the US standard NACE TM-01-77 (National Association of Corrosion Engineers), then cracks occur in the area of the weld seam according to FIG. 1b. These cracks are caused by the high tensile stresses from the welding process, especially when pearlite lines are present in the structure. The cracks can be differentiated according to different types I to IV according to FIG. 1 b and are referred to as SCC (Stress Corrosion Cracking).

The invention has for its object to provide a method of the type mentioned, by means of which the disadvantages of the methods according to the prior art are avoided and by means of which welded steel pipes with improved resistance to stress corrosion cracking, i.e. in particular, resistance to attack by acidic gases such as hydrogen sulfide, carbonic acid and acidic oils for long-distance pipelines are easy to manufacture and moreover have good field weldability. In particular, the invention is based on the object of specifying a method by means of which the above-described errors in the finished, welded tubes for the transport of acidic gases and oils are to be avoided without such a method impairing the mechanical properties, in particular the yield strength , ie to humiliate.

This object is achieved according to the invention by the features mentioned in the characterizing part of claim 1.

An improvement in the steel structure through globular shaping of the sulfides formed is preferably achieved by adding Ca. Instead of or in addition to the calcium, titanium, zirconium and / or rare earths can be added individually or in groups in customary amounts.

Further advantageous refinements of the inventions The method according to the invention result from the subclaims.

Accordingly, either the pipe and thus the weld seam on the outside is continuously section by section with the aid of a medium-frequency ring inductor - operated with 0.1 to 5.0 MW - to the required temperature of at least 100 ° C higher than the temperature of 300 to 300 ° C Heated to 680 ° C and then cooled with a water or air spray plate or only the weld area with the immediately adjacent zone on the outside with the help of a medium frequency line inductor - operated with 0.1 to 5.0 MW - in comparison to the temperature on the inside heated at least 100 ° C higher temperature from 300 to 680 ° C and then cooled with water or air jets. In special cases, the welded strip edges or the weld seam can be heated autogenously with gas.

It is important to regulate the mutual influence of heat output, treated area and seam feed speed on the one hand and the temperature distribution over the pipe wall, which is dependent on thermal conductivity, heat transfer and heat radiation, and the partial heat dissipation at seam feed speed on the other. According to the invention, this regulation is carried out in such a way that the product of the power density of the heat supply in watts per square meter and the seam feed speed in meters per second does not fall below a limit value of 10,000 W / (mx sec) with internal water or air cooling of 1-20000 liters per meter of pipe length.

The residual compressive stresses in the inner surface of the pipe are built up to at least one third of the pipe wall thickness.

The advantages of the proposal according to the invention can be seen in particular in the fact that welded steel pipes, HF or sub-powder-welded, with the build-up of a compressive stress on the side facing the acidic medium of up to 30% of the yield strength when using the steels claimed significantly improved resistance to stress corrosion cracking, ie resistance to the attack of acidic gases and acid oils for long-distance lines can be produced, which also have good field weldability and good mechanical properties and are technically easy to produce.

• Fig. 1 a bis c Definition und Darstellung der Rißgrö- ßen, wie zum Stand der Technik erläutert,
• Fig. 2a bis bb eine schematische Darstellung der Wärmebehandlungseinrichtung in zwei Varianten,
• Fig. 3 eine schematische Darstellung der erfindungsgemäßen autogenen Wärmebehandlung,
• Fig. 4 Fehlertypen an Rohrproben nach unterschiedlichen Wärmebehandlungen mit Darstellung der Eigenspannungen in der HAZ,
• Fig. 5 Eigenschaft eines erfindungsgemäß behandelten HF-geschweißten Rohres,
• Fig. 6 Eigenschaften eines erfindungsgemäß behandelten UP-geschweißten Rohres,
• Fig. 7 Tabelle Stahl- und Rohrdaten.

The invention is explained in more detail below on the basis of preferred exemplary embodiments. The drawings show in:

• 1 a to c definition and representation of the crack sizes, as explained in relation to the prior art,
• 2a to bb a schematic representation of the heat treatment device in two variants,
• 3 shows a schematic representation of the autogenous heat treatment according to the invention,
• 4 error types on pipe samples after different heat treatments with representation of the internal stresses in the HAZ,
• 5 property of an HF-welded pipe treated according to the invention,
• 6 properties of a UP welded pipe treated according to the invention,
• Fig. 7 Steel and pipe data table.

A steel that is treated in the ladle after tapping with a lime-fluorspar slag and flushed with argon and then stripped is further homogenized with calcium in a ladle to produce raw material with the highest degree of purity. As with steel desulfurization, the steel is tapped free of slag into the basic pan and rinsed for a few minutes after adding a synthetic slag; after adding lumpy CaSi, the rinsing treatment is continued.

Rest Eisen und unvermeidbare Verunreinigungen.After this treatment, the steel has the following melt analysis (which, due to the increased Nb content, does not correspond to the invention):

Balance iron and unavoidable impurities.

The steel is cast into slabs with a dimension of 200 mm thickness and 1,300 mm width in a continuous caster and then the slab reheated to a temperature of 1,170 to 1,250 ° C thermomechanically to a steel strip of 11.9 mm thickness and 1,300 mm width at one Roll temperature of 850 to 910 ° C rolled out.

Rolling takes place in three roughing stands, with one pass in the first and third roughing stands and with 3 to 5 passes reversing in the second roughing stand. In the finishing line, rolling is carried out continuously in seven stands.

In a spiral pipe mill, not shown, the trimmed steel band is formed into a spiral pipe with a dimension of 609.6 mm x 11.9 mm (API material x 60) and the adjacent edges of the steel band are joined together by tack welding and then the pipe is a length of e.g. 18 m separated. The tack-welded pipe is finished welded on a separate welding stand by double-sided sub-powder welding. Wires and welding powder with a high degree of purity and low hydrogen emission are used for welding.

Small samples were taken from the welded pipes and tested in the NACE test solution using the HIC test procedure described at the beginning. In all cases, with CLR = 6% and CSR = 0.5%, the usual requirements with CLR = 15% and CSR = 1.5% were reliably met.

After the testing of small samples (HIC test method) and that of pipe rings known shows different results, especially because of the inherent stresses caused by welding, 300 mm long pipe rings were placed in a large container with the dimensions 850 mm x 850 mm x 450 mm of a hydrogen sulfide load in NACE solution with a possibility of attacking the pipe from outside and inside subjected. The surfaces of the test areas, about 100 mm on both sides of the weld seam and about 200 mm wide opposite the weld seam, were ground on small samples in accordance with the regulations for the HIC test in order to rule out a temporary protective influence of the scale. To simulate the operating pressure of pipelines, tension was applied to the inside of the pipe using a rod. Tensile stresses of 44% of the minimum yield point were applied in the area of the weld seam and opposite in the base material. After the tube rings had been stored in the NACE solution for 96 hours, they were ultrasonically checked in the ground areas and then examined metallographically.

The investigation revealed cracks in the area of the weld seam towards the inside of the pipe, which can be seen as a combination of HIC errors and SCC errors.

In addition to these interlaboratory tests, in which the NACE solution was able to attack from inside and outside, other tests were carried out in which the solution could only be attacked from the inside of the pipe. The application of stresses to simulate an internal pressure was carried out in the same way as previously described, in each case with 44% of the minimum yield point. Again, in the case of sub-powder-welded pipes, crack systems were found in the weld transition area after 96 hours. In addition, cracks appeared in the weld metal.

In order to reduce the residual stresses which are assumed to be the cause of the cracks occurring in the weld metal and in the adjacent heat-affected zones, the tubes were heat-treated by means of a device shown in FIG. 2a.

Fig. 2a shows a spiral welded tube 1, which rests on guide rollers 2 and is guided by means of further guide rollers 3 past the heat treatment device 4 in a spiral at a speed of 0.4 m to 30 m per minute. The heat treatment device 4 initially consists of a medium-frequency ring inductor 5, which surrounds the tube 1 in a width of 50 mm with a spacing of 50 mm and with approximately 0.1 to 5.0 MW for the ring-shaped heating of the tube 1 to a temperature is operated from 300 to 680 ° C. In the interior of the tube 1, a water or air lance 6 is arranged axially, at the head end of which a spray plate 7 is provided at a distance of 5 to 500 mm from the ring inductor 5, by means of which the circumferential zone of the tube 1 heated directly beforehand with the ring inductor 5 sprayed by water or air in an amount of 1 to 2,000 liters per m pipe and thus cooled.

2aa schematically shows a front view of the medium-frequency ring inductor 5 arranged around the tube 1 and of the spray plate 7 arranged inside the tube 1.

In Fig. 2b, a spiral welded tube 1 is also shown, which rests on guide rollers 2 and spirally by means of further guide rollers 3 to another heat treatment device 8, the weld seam 9 is guided past at a speed of 0.4 to 30 m per minute. In this case, the heat treatment device 8 consists of a medium-frequency line inductor 10 - operated with 0.1 to 5.0 MW - with a width of 400 mm, past which the weld seam 9 passes and heated to a temperature of 300 to 680 ° C. becomes. In the interior of the tube 1, a water or air lance 6 is in turn arranged axially, the end of which is bent in a knee shape toward the inner surface of the tube and at the end with a nozzle head 11 in a width which corresponds approximately to the width of the line inductor 10, for spraying water or air in a quantity from 1 to 2,000 liters per m of pipe is provided on the inside of the pipe.

2bb shows a front view of the tube 1 with a line inductor 10 and a bent water or air lance 6 with a nozzle head 11.

In the same way as with a ring or line inductor, the tube 1, as shown in FIG. 3, can also be heated autogenously with gas burners 12 to the left and right of the weld seam 13 and then, similar to FIG. 2bb, with a water or air shower 14 are cooled. The arrow 15 indicates the direction of advance of the tube 1.

In Fig. 4, the initial state and the values of the internal stresses inside the pipe obtained by various methods are absolute and related to the yield strength of the treated and tested spiral welded pipes of dimensions 609.6 x 11.9 mm made of material quality X 60 in a bar chart, whereby Below this bar chart for the initial state (A) and the processes (B), (D), (E), (H) and (I) the samples with the occurring crack types are schematically assigned. Sections of tubes that were shown or treated as described above were tested. The tube sections were kept in H2S-saturated solution for 96 hours at room temperature. A tensile stress of 44% of the measured yield strength (Rp) of the pipe was applied to the inside of the pipe by ovalizing the pipe section. This initial state is designated by A in FIG. 4, it being evident from the associated sample representation that numerous cracks were found both in the weld seam and in the heat-affected zone.

In the diagram, the beam heights, the longitudinal stress and the transverse stress values indicate measured using the disassembly method.

Below the bar chart, important parameters for the initial state A and for the different methods B to N as well as the labeling and the test results for the respective pipe sections are listed.

The pipes according to D and E were heated to 600 and 700 ° C and then cooled from the outside with water.

Although the internal stresses are reduced in this process, cracks continue to occur because compressive stresses develop on the cooling side (here outside) and tensile stresses on the inside of the pipe.

The pipes according to F and G, which were heated to 600 ° C and then cooled in air, are already crack-free and have a reduced internal stress. Only the pipes according to H and 1, which were heated to 640 ° C and 700 ° C and cooled in air, still show cracks.

A method called Q + T (quench and temper), in which the pipe is heated to 940 or 950 ° C, quenched with water from the outside and subsequently tempered at 600 or 640 ° C, also leads to a crack-free sample and an extensive reduction of the internal stresses.

A build-up of residual compressive stresses of approx. 20% of the yield strength in the HAZ on the inside of the pipe facing the acidic medium only takes place with processes M and N, which involve water cooling from the inside with 1 to 2,000 liters per meter of pipe length and with a seam feed rate of 0.45 meters per minute while maintaining a minimum value of 10,000 W / (mx sec) for the product of power density and seam feed speed, a temperature of 600 ° C is reached on the outside of the tube, which is at least 100 ° C higher is the temperature on the inside of the pipe. The tests were carried out with processed and unworked seam elevation; in both cases there are no longer any SCC cracks.

The chemical composition of the steel strip, the associated dimensions of the pipe produced therefrom, the measured mechanical properties in the initial state and after annealing and cooling are shown together with the corresponding micrographs in FIG. 5, in this case for a longitudinal seam hf resistance press-welded pipe.

In the same way as the steel described in FIG. 5 and in the preceding text with its production and treatment, the steel explained in more detail in FIG. 6 with the determined properties is suitable for sub-powder-welded sour gas and oil pipes; in the same way this applies to the steel described in the table in FIG. 7 and to the acid gas-resistant pipes made from it.

Claims (6)

1. Method for the manufacture of welded steel tube, utilisable for the transportation of acid gases and/or oils, from a steel having the following composition:

wherein the ratio Ca:S is greater than 2.25 and the product Ca x S is equal to or less than 0.001,

with the rest iron and unavoidable impurities, wherein the steel is rolled thermomechanically to a strip with pearlitic-ferritic and/or bainitic grain structure,

whereafter the strip is formed into a tube having a ratio of wall thickness to diameter of 1 to 25 up to 1 to 160, the edges of the strip are welded to each other, and the tube finally is moved relative to a heat treatment device to build up compressive stresses on the inside of the tube, wherein at least the welding seam zone on the outside of the welded tube over a maximum width of 400 mm is heated to a temperature of 300 to 680 ° C which is at least 100 ° C higher than the temperature within the tube, and then is cooled from the inside by spraying with water or blowing with air, while maintaining the product of heat input in W/m² and feed speed in m/sec equal to at least 10000 W/m x sec.

2. Method according to claim 1, characterised in that a steel is used with composition

with the rest iron and unavoidable impurities.

3. Method according to claim 1 or 2, characterised in that the tube and the welding seam zone is continuously heated inductively in sectors.

4. Method according to claim 1 or 2, characterised in that the tube and the welding seam zone is heated continuously in sectors autogenously by gas.

5. Method according to one or more of claims 1 to

4, wherein at least the welding seam zone on the outside of the welded tube over a maximum width of 400 mm is heated to a temperature of 550 to 650°C which is at least 100°C higher than the temperature within the tube, and then is cooled from the inside by spraying with water or blowing with air, while maintaining the product of heat input in W/m² and feed speed in m/sec equal to at least 10000 W/m x sec.

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