DE2720805A1 - METHOD FOR PRODUCING WORKPIECES FROM ALLOY STEEL - Google Patents

Description

The invention relates to a method for the production of workpieces from alloy steel with high mechanical Properties and resistance to stress corrosion cracking in the presence of hydrogen or of media suitable for the development of hydrogen.

The invention is particularly applicable to steel workpieces with a very high elastic limit of up to 930 MPa (95 kg / nm), which work either in hydrogen or in hydrogenated oases under pressure or in the presence of nascent hydrogen generated by chemical reactions, as is the case, for example, with workpieces of the The case is that the crack formation by stress corrosion in liquid or practically saturated with hydrogen sulfide (HgS) media have to resist under pressure.

310- (76/44) -TBk

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These steels can be used to manufacture forged, rolled or welded workpieces.

The invention also relates to the workpieces obtained by this method.

For several years, attempts have been made to produce steels that retain sufficient mechanical properties, when the workpieces made of these steels are in a hydrated atmosphere or in the presence of nascent Working hydrogen. Indeed, there was one embrittlement of the steel and one, especially in the oil wells Risk of cracking of the workpiece due to stress corrosion mainly found in a liquid medium or in a gaseous medium containing a considerable amount of hydrogen sulphide. Therefore, the FR-PS 1 077 011 describes a steel for the production of deep oil well pipes, in particular have a certain aluminum content.

Although this research is quite old, the steels developed to resist cracking in the presence of hydrogen had rather limited mechanical properties, and most importantly, an elastic limit of no more than 640 MPa (65 kg / mm). Such mechanical properties, which are sufficient for oil wells, are not sufficient for other applications, for example cylinders for the transport of hydrogen under pressure. Namely, in order to obtain improved mechanical properties, such a steel must have a martensitic tempering structure, and it has hitherto been thought that a martensite structure having good resistance to stress corrosion in the presence of hydrogen would be incompatible. So it seemed impossible to find steels with high mechanical properties and still the

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Establish suitability for working in the presence of hydrogen.

Despite this opinion, which was generally widespread among experts, the applicant undertook research into this Problem to be solved, and has found a steel, its composition and its heat treatment in very precise Limits are set that allow the desired properties to be obtained.

The invention therefore relates to a method for producing workpieces from alloy steel with high mechanical properties and resistance to cracking caused by stress corrosion in the presence of hydrogen, with the indication that a steel of the weight composition:

0.15-0.30 % C, 0.1-0.5 % Si,

ο, 5 - 1, 5 % Mn, 1 - % Cr, ο, 25 - 1 % Mo, 0 - 1 % Ni, 0 ο, 1 % V and / or Nb,

Balance Fe and inevitable impurities

and subjecting it to a heat treatment comprising a first austenitizing step at a temperature between 875 and 950 ⁰ C with subsequent cooling at a rate above the critical martensitic speed and a second step consisting of a metallurgical tempering at a temperature between 620 and 725 ⁰ C consists, the tempering temperature by systematic

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Fragility tests on samples is determined and for a certain composition within the specified limits at the same time obtaining an elastic limit

740 MPa «Re« 930 MPa (75 kg / mm ² «. Re <95 kg / mm ² ) and a breaking strength of 88O MPa« R «1080 MPa

(90 kg / mm ² «Rei 110 kg / mm ² ) and a fragility index in the presence of hydrogen P <30 % allows which value

_{x 100}

is where £. nc the constriction of an electrolytic Away with the sample not loaded with hydrogen and ^ -c mean the constriction of a test piece loaded with hydrogen by electrolytic means.

According to a first embodiment of the invention, the steel has the following composition:

ο, 15 - 0.30 % c, ο, 15 - 0.5 % Si, 0, 5 - 1 % Mn, 2 - % Cr, ο, 25 - 0.5 % Mo ο, 5 - 1 % Ni, 0 - 0.07 % V and / or Nb,

Remainder Fe and unavoidable impurities.

According to a second preferred embodiment of the invention , the steel has the following composition:

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0.15-0.50 % C,

0.1-0.5 % Si,

0.5 - 1.5 % Mn,

1 -2 % Cr,

0.5 - 1 % Mo,

Balance Fe and inevitable impurities.

The invention will now be described on the basis of several non-limiting examples and the single figure which graphically shows the annealing temperature in ⁰ C as the abscissa and the elastic limit Re in MPa and the fragility index F in % as the ordinate and the course of corresponding curves for steels according to the invention .

Steels with different compositions were produced, which were set within very precise limits Form sheets of various thicknesses between 25 and 60 mm. A certain number of Test pieces which have been subjected to the heat treatment defined above, the tempering temperature in the specified limits varied. The test pieces were subjected to a brittleness test by cathodic hydrogen after a subject to the following method:

Tensile test samples are placed as cathodes in an electrolysis system with acidified water (solution with 10 % pure hydrochloric acid); the cathode position protects the sample against corrosion; on the other hand, nascent hydrogen is released on the surface of the sample. Part of this nascent hydrogen penetrates the metal. After 48 hours of such loading, the sample is broken in the tensile test, and its constriction at break is compared with the constriction of a comparative sample, which is none

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was subjected to such loading with hydrogen. When the metal is susceptible to fragility from hydrogen is, the necking of the sample loaded with hydrogen is less than that of the comparison sample (loss of ductility of the metal due to the hydrogen). A fragility index is defined by hydrogen with the formula

■ η ν £ HO - ^ C

where £ - nc is the constriction of the unloaded comparison sample and ^. c denotes the constriction of the loaded sample. The greater F % , the more susceptible the metal is to embrittlement or brittleness due to the hydrogen.

It can be assumed that the fragility index must not exceed 30% in order to achieve satisfactory behavior, which value is apparently variable as a function of the conditions of use.

One of the features of the invention is based primarily on the fact that certain compositions adopted for which the refractive index representing depending on the tempering temperature curve at about 600 ⁰ C showed a large drop in such a way that from a fairly accurate tempering temperature, which can can set to 620 ⁰ C, the fragility index dropped to a satisfactory value in the order of 25 % and remained at this level up to the transition point ACl. The single figure shows the F and Re curves for the steels according to the invention. The curves drawn for each exact composition would have the general shape of the curve types shown in the diagram.

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This is possible thanks to systematic attempts at fragility define a minimum tempering temperature from which the fragility index is satisfactory.

Since the elastic limit Re is a function of the tempering temperature reduced to the transition point AC1, it is possible to adjust the tempering temperature within these limits to choose which allows to obtain the necessary mechanical properties, however, the more one increases strives for mechanical properties while limiting the tempering temperature, the more important the fragility index which can become unacceptable in a very narrow temperature range.

In addition to this specification of the tempering temperature, there is also the need to create a martensitic structure thanks to a extremely rapid cooling to preserve that on each pail above the critical rate of martensitic deterrence must lie. In general, cooling takes place at a rate of the order of 100,000 ° C./h before.

Until now it was thought that it was impossible to use a martensitic structure in the presence of hydrogen, because such steels can break spontaneously under relatively low loads. However, the applicant arrived at this Result through precise adjustment of the contents of the various steel components, which apparently react with one another.

The steel to be used has thus been determined within fairly narrow limits, which are, however, still within

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define two steel families.

In the first case, the steels have a chromium content of 2-3 % and contain vanadium and / or niobium. Their composition is the following:

0.15-0.30 % C,

0.1-0.5 % Si,

0.5-1 % Mn,

2 - 3 % Cr,

0.25 - 0.5 % Mo,

0.5-1 % Ni,

0 - 0.1 % V and / or 0 - 0.07 # Nb,

Remainder Pe and inevitable impurities.

This steel can be used to produce workpieces with a thickness ranging from 25 to 60 ram.

This is how the following steels were made:

Example I.

Samples were made from a steel that formed a sheet metal with a thickness of 60 mm and (by weight) had the following composition:

0.23 % C, 0.27 % Si, 0.74 % Mn, 0.77 * Ni, 2.73 % Cr, 0.3 ^ % Mo, the balance Fe and inevitable impurities.

The critical martensitic speed of this steel is 17000 ° C / h (between 85O and 600 ⁰ C), which allows obtaining a martensitic quenching in the core of a sheet up to 60 mm thick or a round material up to

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95 mm diameter.

This steel was subjected to the following heat treatment, which gave it the martensite structure.

1st step: Austenitizing at a temperature of 875 ^° C

for 1 hour, then cooling in water (quenching at about 100,000 ° C./h).

2nd step: tempering at graduated temperatures from 600 -

700 ^° C. for 1 h.

The structure of the steel treated in this way consists of an annealing martensite, which is globulated, fine and regular is formed in a matrix of free ferrite distributed carbides. This structure is made by etching with a solution of Picric acid made clearly visible in alcohol.

The steel treated in this way was subjected to the brittleness test subjected by cathodic hydrogen according to the method explained above.

From an annealing temperature of 610 ⁰ C the fragility index is satisfactory, but it is preferable in practice to select a temperature of 620 ⁰ C.

The mechanical properties are then as follows

Elastic limit Re = 765 MPa (78 kg / mm ² ) breaking strength R = 883 MPa (90 kg / mm ² )

Constriction ■ £. % = 67 %

Impact strength KCV + 20 ⁰ C = 154 j / cm ² (15.7 kgra / cm ² ).

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For the same quenching treatment, but with a ^{tempering limited to 600 0} C, the mechanical equipment would

properties have not changed much (R = 785 MPa (80 kg / mm)), but one would have obtained a fragility index F % over 80 Jo, i.e. a very unfavorable fragility index.

On the other hand, it is when a steel of the same composition subjected to a heat treatment which does not allow obtaining a martensitic structure, for example an austenitization at a temperature of 910 ⁰ C for 1 h followed by cooling at a rate in the order of 3000 ° C / h and finally, as above, annealing at 600 - 700 ⁰ C h stepped temperatures during 1 determines that the required annealing temperature is ^{1650 0} C to obtain a satisfactory friability index, but the mechanical properties are much lower then:

Elastic limit re = 640 MPa (65 kg / mm-) Breaking strength R. = 785 MPa (80 kg / mm ² ) Constriction £ % = $ 72 t KCV + 20 ⁰ C = 225 j / cm ² (23 kgm / cm ² )

In the case of this steel, it can be seen that the martensitic structure allows the elastic limit to be increased by 125 MPa (13 kg / mm ² ) while maintaining good resistance to hydrogen fragility thanks to precise adjustment of the composition and the heat treatment.

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Example II

Samples were made from a metal of the following weight composition:

0.185 % C, 0.225 % Si, 0.605 % Mn, 0.800 % Ni, 2.37 % Cr, 0.3 ^ 0 % Mo, 0.050 % V, the balance Fe and inevitable impurities.

The critical speed of martensitic deterrence is 65,000 ° C / h, which is getting a martensitic Structure in the core of a sheet metal up to 20 mm thick or a round material up to 45 mm in diameter.

This steel was subjected to the following heat treatment:

1st step: Austenitizing at a temperature of 910 ^° C

for 1 h, then cooling in the order of 100,000 ° C./h.

Step 2: Graduated occasion of treatments 6OO - 7OO ⁰ C

(1 h).

The structure is an occasional martensite.

It was observed that the lowest annealing temperature, reached a satisfactory friability index of from one, 610 ⁰ C, but it is advantageous to choose a minimum temperature of 620 ⁰ C. The mechanical properties are then the following:

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Elasticity limit Re = 930 MPa (95 kg / mm ² )

Breaking strength R = 1000 MPa (102 kg / mm ² )

Constriction «£. % = 67 %

Impact strength KCV + 20 ⁰ C = 152 J / cm ² (15.5 kgm / cm ² ).

For the same quenching treatment, but ^{tempering limited to 600 °} C., the fragility index due to hydrogen is 100 % at a value of the elastic limit of 945 MPa (96.5 kg / mm), which is very unfavorable.

Example III

Samples were made from a metal of the following chemical weight composition:

0.260 % C, 0.225 % Si, 0.680 % Mn, 0.820 % Ni, 2.49 % Cr, 0.350 % Mo, 0.060 % V, the balance Fe and inevitable impurities.

The critical martensite quenching speed is 20,500 ° C./h, which means that a martensite structure can be obtained in the Inside a sheet up to 57 mm thick or a round material up to 90 mm in diameter permitted.

This steel was subjected to the following heat treatment:

1st step: Austenitizing at a temperature of 910 ^° C

for 1 hour, then cooling to an order of magnitude of 100,000 ° C./h.

Step 2: From 6OO - 7OO ⁰ C graded occasion treatments

(1 h).

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The structure is an occasional martensite.

The tempering temperature required to obtain a satisfactory fragility index was observed to be 625 ° C is.

The mechanical properties are then as follows:

Elasticity limit Re = 932 MPa (95 kg / mm ² )

Breaking strength R = IO3O MPa (105 kg / mm ² )

Constriction i.% = 64 %

Impact strength KCV = 113 J / cm ² (11.5 kgm / cm ² ).

For the same quenching, however, limited to 600 ⁰ C starting the friability index is hydrogen by 100% (mm 101 kg / ²⁾ at a value of the elastic limit of 99O MPa, which is very unfavorable.

Example IV

Samples were made from a metal of the following chemical weight composition:

0.250 % C, 0.245 % Si, 0.720 % Mn, 0.835 % Ni, 2.46 % Cr, 0.345 % Mo, 0.060 % V, the balance Fe and inevitable impurities.

The critical martensite quenching speed is 21,000 ° C / h, which is the maintenance of a martensitic structure in the core of a sheet metal up to 55 mm thick or a round material up to 85 mm in diameter.

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Al

This steel was subjected to the following heat treatment:

1st step: Austenitizing at a temperature of 910 ^° C

for 1 h, then cooling in the order of 100,000 ° C./h.

2. Sch rode: From 6OO - 700 ⁰ C graded occasion treatments (lh).

The structure is an occasional martensite.

It was observed that the 'required to obtain a satisfactory friability index tempering temperature is 620 ⁰ C.

The mechanical properties are then as follows:

Elasticity limit Re = 932 MPa (95 kg / mm ² )

Breaking strength R = IO6O MPa (108 kg / mm ² )

Constriction £. % - 64 %

Impact strength KCV = II8 J / cm ² (12 kgm / cm ² ).

For the same quenching treatment, but with a ^{tempering limited to 600 °} C., the fragility index due to hydrogen is 80 % with an elastic limit of MPa (102.5 kg / mm ² ), which is unfavorable.

A second steel group was also determined with a chromium content of 1 - 2 % and a molybdenum content of 0.5 - 1 %. Their composition is the following:

0.15-0.30 % c,

0.1-0.5% Si, 0.5-1.5 % Mn, 1-2 % Cv, 0.5-1 % Mo,

Balance Fe and inevitable impurities. 809810/0572

For example, the following steels were produced:

Example V

Samples were made from a metal of the following chemical weight composition:

0.257 % C, 0.235 % Si, 0.665 % Mn, 0.080 % Ni, 1.50 # Cr, 0.750 % Mo, balance Fe and inevitable impurities.

The critical martensite quenching speed of this steel is 93,500 ° C / h, which enables it to be martensitic OefUges in the core of a sheet metal up to 23 mm thick or of a round material up to 40 mm in diameter.

This steel was subjected to the following heat treatment:

1st step: Austenitizing at a temperature of 910 ^° C

for 1 h, then Abkünlung in the order of 100 000 ⁰ C / h.

_{Tempering treatments graded} from 6oo _ γ ₀₀ c> c (1 h)

The structure is an occasional martensite.

It was observed that the 'required to obtain a satisfactory BrUchigkeitsindex tempering temperature is 620 ⁰ C.

The mechanical properties are then as follows:

Elastic limit Re = 920 MPa (94 kg / mm ² ) breaking strength R = 1020 MPa (104 kg / mm ² )

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Constriction £ % = 65 %

Impact strength KCV + 20 ^ = 137 J / cm ² (14 kgm / cm ² ).

However, there is PUr the same quenching with a limited on 6OO ⁰ C tempering, when the mechanical properties are little changed (Re = 990 MPa, or 101 kg / mm), the friability index in the order of 45%, which is not very favorable.

Example VI

Samples were made from a steel of the following chemical weight composition:

0.255 % C, 0.270 % Si, 1.24 % Mn, 0.10 % Ni, 1.49 % Cr, 0.765 % Mo, balance Fe and inevitable impurities.

The critical martensite ^{quenching speed is 24 000 0} CAi * which allows a martensitic structure to be obtained in the core of a sheet metal up to 50 mm thick or a round material up to 80 mm in diameter.

This steel was subjected to the following heat treatment:

1st step: Austenitizing at a temperature of 910 ^° C

for 1 h, then cooling in the order of 100,000 ° C./h.

2nd step: tempering treatments graded from 600 - 700 ⁰ C (1 h),

The structure is an occasional martensite.

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Ä0

It was observed that the 'required to obtain a satisfactory friability index tempering temperature is 620 ⁰ C.

The mechanical properties are then as follows:

Elasticity limit Re = 920 MPA (94 kg / mm ² ) Breaking strength R = 1020 MPa (104 kg / mm ² )

Constriction ^. % = 65 %

Impact strength KCV + 20 ⁰ C = 142 J / cm ² (14.5 kgm / cm ² )

For the same quenching treatment, but with a ^{tempering limited to 600 °} C., an elastic limit of 100 MPa (102 kg / mm) results in a less favorable fragility index of 50 %.

In all examples, the minimum tempering temperature which allows the best mechanical properties to be obtained was used specified. It is obvious that if one wishes to obtain less high properties, one will end up with a higher one Tempering temperature limited only by the transition temperature ACl can choose, which has the advantage that it does not have to be adhered to with such great accuracy as one is free in this area from brittleness due to hydrogen due to a too low tempering temperature.

Thus, thanks to a careful and precise choice of composition, a quenching steel was made that can withstand high temperature tempering while maintaining high mechanical properties thermodynamically stable structure assumes that for the resistance against stress corrosion in the presence of hydrogen is required.

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The choice of chemical composition, temperature and time of the specified within the exact Limits to the starting up must take into account:

- The quenching possibilities of the workpiece with special consideration of the shape and the Thickness of the workpiece that allow a theoretical thickness to be determined from which the critical quenching speed depends; it is actually essential that the cooling is fast enough to quench at a speed above the critical martensite speed to secure the steel used;

- the desired mechanical properties (elastic limit, breaking strength), but the elastic limit is limited to a maximum of 930 MPa (95 kg / mm ² ) in order to maintain good resistance to embrittlement by hydrogen.

It should also be noted that the workpieces made from the stems so defined can be subjected to welding operations. Here, it is essential to subject them to e ne of relaxation treatment at a very high temperature, which is moving the ch or at most 20 ⁰ C below the temperature of the metallurgical annealing, but never less than 625 ^0C.

Of course, the invention is not restricted to the examples given, which are only intended to provide a precise explanation special execution cases were used. These chemical compositions and the treatment temperatures and times that

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are given in the examples, on the other hand, in particular as a function of the shapes of the workpiece and the properties sought, but simply within the vary narrow limits given above; likewise, the quenching speed must always maintain a martensitic one Enable structure.

Finally, the subject matter of the invention is also those produced from the steels with the properties described Work pieces. Such a steel can be used in various applications such as:

- Devices for the transport or storage, even under high pressures, of hydrogen or hydrogen-containing Gases (e.g. natural gas) containing or not containing hydrogen sulfide, the constructions Can be fixed or moving containers, valves or the moving parts of turbo compressors (Elements of the fan wheels; wheel hubs, flanges, blades).

- Elements of the pillars for the exploitation of oil or gas deposits at great depths, such as the Pipes or the pipe connectors.

It is clear that other applications in the area of the invention.

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Claims (6)

Claims 1. Process for the production of workpieces from alloy steel with high mechanical properties and durability against crack formation due to stress corrosion in the presence of hydrogen, characterized in that that a steel of the weight composition: 0.15-0.30 % C, ο, 1 - ο, 5 % Si, ο, 5 - ι, 5 % Mn, 1 - 3 % Cr, ο, 25 - 1 % Mo, 0 - 1 % Ni, 0 - ο, 1 # V and / or Nb, Balance Fe and inevitable impurities used and subjecting it to a heat treatment comprising a first austenitizing ^{step at a temperature between 875 and 950 0} C with subsequent cooling at a rate above the critical martensitic speed and a second step consisting of a metallurgical tempering at a temperature of 620 to 725 ⁰ C, the tempering temperature being determined by systematic brittleness tests on samples and, for a certain composition within the specified limits, at the same time obtaining an elastic limit 7 ^ 0 MPa $ Re ^ 930 MPa (75 kg / fam ² ^ Re ^ 95 kg / nm ² ) and a breaking strength of 88Ο MPa ^. R ^ 1080 MPa (90 kg / mm ² ^ R ^ 110 kg / mm) and a fragility index ¹ in the presence of hydrogen P <30 % allows which value 809610/0572 BATH ORIGINAL ρ 0 ₌ χ ^ - nc is where ^ lnc is the constriction of an electrolytic Away with the sample not loaded with hydrogen and £ _c the Contraction of a specimen loaded electrolytically with hydrogen mean. 2. The method according to claim 1, characterized in that a steel of the composition: 0.15-0, ^ 0 % C, 0.1 - 0, 5 % Si, 0.5 - 1 % Mn, 2 3 % Cr, 0.25 - 0, 5 % Mo, 0.5 - 1 # Ni, 0 - 0.1 % Ί and / or 0 - 0.07 % Nb, the remainder Fe and unavoidable impurities and produces workpieces with a thickness of 25 to 60 mm. 3. The method according to claim 1, characterized in that one a steel of the composition: 0.15-0.30 % C, 0.1-0.5 % Si, 0.5 - 1.5 % Mn, 1 - 2 % Cy, 0.5 - 1 % Mo, Remainder Pe and inevitable impurities uses and manufactures workpieces with a thickness of 20 to 50 mm, 809810/0572 4. The method according to any one of claims l to 3, characterized in that that the cooling takes place at a speed of 100,000 ° C / h. 5. The method according to any one of claims 1 to 4, characterized in that the heat treatment after welding the workpiece ^{comprises a third step for relaxation at a temperature equal to or at most 20 0} C lower than the metallurgical tempering temperature. 6. A workpiece made of alloyed steel obtained according to the method according to any one of claims 1 to 5, characterized in that that it is made of a steel of the composition: 0.15-0.30 % C, ο, 1 - 0, 5 % Si, ο, 5 - ι, 5 % Mn, 1 - 3 % Cr, ο, 25th - 1 % Mo, 0 - 1 # Ni, 0 - 0.1 % V and / or Nb, Balance Fe and inevitable impurities and after the heat treatment a martensitic tempering structure and, as mechanical properties, an elastic limit 740 MPa <Re ^ 930 4 MPa (75 kg / mm ² ^ Re £ 95 kg / mm ² ), a breaking strength 88Ο MPa ^ R ^ 1080 MPa (90 kg / mm ² £ R $ 110 kg / mm ² ) and has a fragility index in the presence of hydrogen F ^ 30 % . 809810/0572