EP1529853A2 - Steel for chemistry - Devices - Components - Google Patents

Abstract

Main alloying constituents (wt. %) are carbon (0.22-0.29), chromium (1.1-1.5), molybdenum (0.3-0.6), nickel (3.3-3.7) and optionally vanadium (0.05-0.15). The remainder is iron. Other minority substances which may be present include sulfide and oxide forming elements and impurities.

Description

The invention relates to an iron-based alloy for use as a material for high-pressure components with elevated working temperature, in particular tempered steel for components such as tube heat exchangers in high-pressure polyethylene plants, containing the main alloying elements in% by weight of: Carbon (C) 0.22 to 0.29 Chrome (Cr) 1.1 to 1.5 Molybdenum (Mo) 0.3 to 0.6 Nickel (Ni) 3.3 to 3.7 possibly Vanadin (V) 0.05 to 0.15 Remainder iron (Fe) furthermore sulfide and oxide forming - as well as accompanying and impurity elements. Furthermore, the invention relates to a component with increased working temperature, in particular tube heat exchanger for polyethylene - high pressure systems formed from an iron-based alloy mentioned above.

As materials for components that at elevated temperatures, for example at 300 up to 400 ° C, must withstand high mechanical stresses, such as Tube heat exchanger of chemical plants with an internal pressure of 3000 bar and more, are mostly iron-based alloys according to DIN material no. 1.6604 or material no. 1.6580 or material no. 1.6586 as well as material no. 1.6926 or material no. 1.6944 and material no. 1.6952 are used. to Creating the desired material strength, the parts are austenitized and from the Austenitisierungstemperatur hardened with high cooling rate or quenched and then tempered, this thermal quenching of Material often a relaxation treatment at temperatures up to Tempering temperature follows.

An increase in the tensile strength of the material caused by tempering by means of hardening and tempering also has a significant effect on the other mechanical material properties at room temperature and at elevated working temperatures. An increase of the tensile strength over a value of 1000 N / mm ² to 1100 N / mm ² and above disproportionately increases the 0.2% yield strength of the iron base material, whereby a ratio of 0.2% yield strength (FIG. Rp _0.2 ) to tensile strength (Rm) is adversely affected. In other words, the yield strength approaches tensile strength, significantly lowering the elongation at break and impact strength of the material and significantly reducing crack fracture toughness.

For reasons of operational safety of high pressure components, in particular those of equipment of the chemical industry, become the aforementioned materials only up to that strength thermally tempered, in which the so related elongation and toughness properties of the material as be considered sufficient or comply with regulations. As plant engineering Disadvantage is thus a large wall thickness of the high pressure components required optionally influencing a reaction kinetics of the chemical substances and given a low cost of the reactor or the device. If, for example, high-pressure heat exchangers are sufficient for adjustment high elongation and toughness values of the material with required strength the same designed, the load must be according to the wall thickness large be dimensioned, which is associated with a low specific heat transfer, which requires large thick-walled reactors.

A disadvantage with thick-walled pipes is reaching the so-called leak Rupture criteria, which in the high pressure technique for safety reasons always must be fulfilled. In other words, when in operation of a reactor in the Pipe wall a crack grows, it must first reach the outer surface (= Leak) before an unstable break occurs. As a parameter for unstable break include the critical fracture toughness such as Klc or Jlc or the critical crack length ac Material-specific characteristics are mainly due to the toughness of the material dependent.

The invention seeks to remedy these shortcomings and sets itself the task of a Iron-based alloy of the type mentioned for use for High pressure components with increased strength at high strain and To indicate toughness values of the material.

Another object of the invention is to provide a component, in particular Tube heat exchanger for high-pressure polyethylene plants with improved Use characteristics and / or similar safety criteria formed from an aforementioned iron base material with high strength and at the same time favorable elongation and toughness values.

The object is achieved according to the invention in that an iron-based alloy is used whose sulfide-forming and oxide-forming as well as accompanying and impurity elements have individual concentrations and / or sum amounts for equivalent-acting element groups values in% by weight: - embedded in solid solution elements: Manganese (Mn) = MIN 0.15 MAX 0.5 Co + Cu + W = MAX 0.31 -Verunreinigungselemente Sulfur (S) = MAX 0.003 Phosphorus (P) = MAX 0.005 P + S = MAX 0.006 - oxygen (O) = MAX 0.0038 - Oxide-forming elements Silicon (Si) = MIN 0.10 MAX 0.25 Aluminum (Al) = MIN 0.008 MAX 0.02 Calcium (Ca) = MIN 0.0001 MAX 0.0008 Magnesium (Mg) = MIN 0.0001 MAX 0.0006 Ca + Mg = MIN 0.00012 MAX 0.0008 Monocarbide-forming elements Ti + Nb + Ta + Zr + Hf = MAX 0.01 Grain boundary occupying elements As + Bi + Sb + Sn + Zn + B = MAX0,015 - gases Nitrogen (N) = MAX 0.001 N + H = MAX 0.01 preferably = MAX 0.008 and the material has a degree of deformation of greater than 4.1 times, wherein the components or components after thermal annealing have largely isotropic mechanical properties and high strength and toughness at a working temperature up to 350 ° C.

The advantages achieved by the invention are essentially to be seen in that by setting or maximizing levels of specific ones Elements and / or element groups in the material a microstructure representation is made possible by thermal quenching, which is both a high material strength as well as a much improved toughness and more favorable elongation values he brings.

It is known to the skilled person and prior art that with sinking Concentration of the impurity elements of an alloy the property values of the material and some can often be improved. High purity alloys, however, tend to be coarse in one Heat treatment, which adversely affects certain material values may have.

In development work it was surprisingly found that alloy technology by lowering or setting the concentrations of some elements or Element groups an advantageous microstructure after thermal quenching of the steel according to the invention can be reached, wherein even at a high Material hardness comparatively much improved strain, - constriction and Toughness values of the material are present. These leaps and bounds Improvements have not yet been scientifically clarified, but it is assumed that these discontinuous property changes with a Avoidance of temper embrittlement phenomena and / or inhibition a grain boundary when relaxing the part at higher Temperatures are justified.

As a result, the mode of action of the alloy according to the invention is intended present elements are set forth, the Main alloying elements related to a thermal remuneration are functionally kinetically matched.

Carbon dissolves when heated in the austenite of the alloy in the solid solution and causes quenching a strain of the crystal lattice and thus hardening of the material. In the alloy according to the invention, C contents of at least 0.22% by weight are required in order to achieve a material hardness of at least 1100 N / mm ² with a coating. If the carbon concentration exceeds 0.29% by weight, more stable carbides and reduced toughness values of the material may be present, so that a content range within narrow limits of 0.22 to 0.29% by weight C is provided.

Depending on the concentrations of the elements, chromium essentially binds Cr ₂₃ C ₆ , Cr ₇ C ₃ and Cr ₃ C ₂ carbides and to a great extent influences the hardening criteria of the material. In order to achieve a desired property profile of the material, at least 1.1, but at most 1.5 wt% Cr are favorable for a desired carbide and Mischkarbidausbildung.

Molybdenum has a deleterious effect on temper embrittlement, is a stronger carbide former as chromium and iron and should be consistent with Cr with a content of at least 0.3 wt .-% in the steel present to a corresponding hardness-increasing effect exercise while tempering the part. Advantageously fine Mo carbides and Mixed carbides are annealed to a Mo content of 0.6% by weight excreted what the ductility of the material at high hardness of the same promotes.

Nickel essentially affects the hardenability of the material and acts toughness promoting. Lower contents than 3.3% by weight are less effective, whereas higher nickel concentrations than 3.7% by weight are too strong Austenite stabilizing effect, whereby a narrow nickel content range of Alloy is justified.

Vanadium with contents of 0.05 to 0.15 wt .-% can be provided in the material be. V as a very strong carbide former acts as a fine micro-alloying element material hardness increase due to extremely fine secondary carbide precipitations when tempering after curing in the temperature range between 450 ° C and 560 ° C. Higher contents than 0.15 wt .-% V, the hardenability undesirable influence and reduce the material toughness.

The iron-based alloy according to the invention has besides the Main alloying elements as the remainder of Iron and Companion as well Impurity elements.

One group of these companion and impurity elements are the elements Mn, Co, Cu and W incorporated in the solid solution.
Manganese affects the hardenability of the steel, binds off the residual sulfur content and is advantageously provided in a concentration range of 0.15 to 0.5 wt .-% in the steel. Lower levels can cause too low a sulfur activity, which increases the risk of breakage and the property profile are adversely affected. Although Co, Cu and W are elements which can be incorporated in certain concentrations in the mixed crystal, however, they have an extremely disadvantageous effect on the ratio in concentrations above 0.31% by weight rp 0.2 , R m

For a given high tensile strength, the value for the 0.2% proof stress increases of the material in the case of sum quantities (Co, Cu and W) of greater than 0.31, resulting in a ratio with disadvantage of over 0.95.

The impurity elements sulfur and phosphorus cause sinking Held to a mechanical improvement expected by the skilled person Characteristics of the material, however, should be extreme in terms of the required high property profile of the tempered material values of 0.003 wt .-% S and 0.005 wt .-% P at a cumulative concentration of 0.006 wt .-% not exceed.

Dissolved oxygen in the steel is bonded by oxide-forming elements, wherein oxide inclusions are formed, which are the material properties, especially the toughness and elongation deteriorate. Also through Remelting processes can not completely eliminate the oxidation products Alloy are eliminated so that their oxygen content is 0.0038 maximum Wt .-% should be.

In case of intended fusion, processing and remuneration of the Material to get the highest hardness good further property values, it is important to adjust the oxide-forming elements in the intended levels to on the one hand, the complete deoxidation to form favorable mixed oxides in to obtain finely divided form and on the other hand, a grain boundary assignment, the can cause a sudden toughness reduction, with certainty off. Of particular importance is the content of Ca and Mg, which Summengehalt in the range between 0.00012 wt .-% and 0.0008 wt .-% are should.

It was surprisingly found in view of a favorable effect of V, that the other monocarbide-forming elements Ti, Nb, Zr and Hf are consistent adversely affecting the toughness and breaking susceptibility of high strength tempered material, resulting in a maximum sum concentration of this Elements in the alloy of MAX 0.01 wt .-% justified.

If, as provided according to the invention, the grain boundary elements As, Bi, Sb, Sn, Zn and B with a sum content of less than 0.015 wt .-% in the alloy is also at high hardness values of the coated material the ductility of the same given sufficiently. A crossing of this However, cumulative concentration value promotes a deformation-free Brittle fracture tendency.

Although the strong nitride formers in the alloy according to the invention are low Have contents, but is a highest cumulative concentration of N + H of 0.01 wt .-%, with the advantage of 0.008 wt .-% required to a desired Be able to achieve property level of the material.

If the material is hot worked by forging or rolling and a Degree of deformation greater than 4.1 times, can after a thermal Tempering of the part, in particular a rod or a pipe high Strength values and thereby significantly improved toughness properties at a Working temperature of 350 ° C can be achieved.

A further increase in the achievable property level of components can be achieved when using an alloy according to the invention, if one or more of the individual concentrations and total contents of the elements in% by weight of: Mn = MIN 0.15 MAX 0.4 Co + Cu + W = MAX 0.24 S = MAX 0.0008 S + P = MAX 0.005 O = MAX 0.0011 Si = MIN 0.1 MAX 0.20 AL = MIN 0.005 MAX 0.018 Ca + Mg = MIN 0.0001 MAX 0.0006 Ti + NB + Ta + Zr + HF = MIN 0.001 MAX 0.008 As + Bi + Sb + Sn + Zn + B = MAX 0.010 N + H = MAX 0.008 given are.

Advantageously, the alloy is prepared by means of pan metallurgical processes and / or using the ESU process and / or the Vacuum arc furnace process, because this production is also a Segregation in the block minimizes and thus the prerequisite for essentially creates the same material properties in the longitudinal and transverse direction of the part.

The further object of the invention is in a component, in particular

Tube heat exchanger for high-pressure polyethylene plants, formed of an iron-based alloy having a composition as described above, achieved in that the component has a tensile strength Rm of the material of greater than 1100 N / mm ² and a 0.2% yield strength at 320 ° C. of greater than 880 N / mm ² ..

Using the high material strength, the wall thickness of the High pressure components are reduced because of the 0.2% yield strength at Room temperature and at an operating temperature of 320 ° C substantially spaced from the strength value and thus a high level of safety of the component is present against separation break. Thinner wall thicknesses, for example one Heat exchanger, also cause a higher specific heat transfer, so that the reactor with substantially reduced dimension the same performance or at the same size, the reactor has a higher performance. From Of particular importance is the "leak before crack" criterion.

According to the invention, it is provided that the following mechanical property values measured in the direction of the longitudinal extent and / transverse to the longitudinal extent of the component are present: Elongation A5 > 16/14% Elongation at break A4 > 18/16% Fracture necking Z > 55/45% Notched Toughness AV (RT) > 80/60 y Notched impact strength AV (-40 ° C) > 50/40 y

If the component, in particular tube heat exchanger for high-pressure polyethylene plants, is tempered to a tensile strength Rm of the material greater than 1170 N / mm ² , this has a 0.2% yield strength of greater than 1060 N / mm ² and a 0.2% Yield point at 320 ° C of greater than 930 N / mm ² , a further reduction of the wall thickness of high-pressure components is possible, which can provide significant plant engineering, but also reaction kinetic advantages.

According to the invention, the mechanical property values of this aforementioned higher-strength material are measured in the direction of the longitudinal extent and transversely to the longitudinal extent of the component: Elongation A5 > 15/14% Elongation at break A4 > 17/16% Fracture necking Z > 55/45% Notched Toughness AV (RT) > 80/60 y Notched impact strength AV (-40 ° C) > 50/35 y

Particularly high security against failure, especially against occurrence A separation fracture is achieved with a ratio of the material of 0.2 % Yield strength broken by tensile strength less than 0.94, preferably from less than 0.92.

According to the invention, the component with a crack fracture toughness J _{1C of} the material of greater than 150 kJ / m ^2, measured according to ASTM -E 813, is preferred.

An essential part of the invention is a choice or setting of the current stress intensity factor to fulfill the "leak before break" criterion.

On the basis of investigation results, the invention is set forth in more detail become.

Table 1 shows the chemical composition of two of the invention Materials. The melts were treated by ladle metallurgy and each poured into electrodes. The block of the batch H 75142 was in the Vacuum arc furnace remelted and in a long forging machine 5.85 times to a rod with a diameter of 200 mm  further deformed, off which rod tubes for a heat exchanger of a polyethylene reactor were manufactured. The thermal treatment of the pipe material was carried out on a Strength Rm of about Rm 1250 MPa.

The block of batch G 53227 was prepared by the ESU method. The Further processing to heat exchanger tubes was carried out the same way as the VLBO block.

Fig. 1 shows the points of the processed rod 1 with a diameter of 190 mm , from which the samples were taken. It means: 2 = tensile tests, 3 = impact tests, 4 = special samples

Table 2 shows the measured mechanical values of the material of the Rod material indicated.

The indication "ZVF" stands for tensile test with fine strain measurement, that for " ZVW "stands for hot tensile test at 320 ° C. The indication" KR "indicates a Notched impact strength test at room temperature, those with "KK" means Notched impact strength values at reduced temperature, in the given case - 23 ° C. In order to meet the high safety requirements, the Notched impact strength of the material tested by means of three samples.

The designation A5 stands for the used sample length and that 5x the Sample diameter.

Table 2 shows the inventive comparison of the measured values Improvement of material properties and in comparison with the state of The technical progress of the technical progress concerning the increase of the Property levels of materials for high pressure components, in particular Pipe heat exchangers for plants of the chemical industry.

The mechanisms of action leading to the improvements according to the invention Characteristics of the highly tempered material have been due to extensive Investigations confirmed.

For this purpose, Fig. 2 shows a dependence of 0.2% strain of the Sum concentration of the elements (Co + Cu + W), Fig.3 elongation at break values of the annealed material as a function of the sum concentration of containing elements (As + Bi + Sb + Sn + Zn + B).

From Fig. 2 is clearly a jump increase in 0.2% elongation values of the material seen when this increased levels of the concentrations of (Co + Cu + W) having.

A reduction in elongation at break is associated with increased contents of (As + Bi + Sb + Sn + Zn + B). Chemical elements H75142 G53227 C: 0.25 0.23 Cr: 1.27 1.37 Not a word: 0.43 0.43 Ni: 3.43 3.42 V: 0.10 0.093 Mn: 0.31 0.32 Co: 0.05 0.02 Cu: 0.02 0.02 W: 0.02 0.05 Co + Cu + W: 0.09 0.09 S: 0.0005 0.0006 P: 0,003 0,003 S + p: 0.0035 0.0036 O: 0.0009 0.0011 Si: 0.19 0.18 al: 0,014 0.011 Ca: 0.0002 0.0002 mg: 0.0002 0.0002 Ca + Mg: 0.0004 0.0004 Ti: 0.001 0.001 Nb: 0.001 0.001 Ta: 0,002 0,002 Zr: 0,002 0,002 Hf: - - Ti + Nb + Ta + Zr + Hf: 0006 0. 006 As: 0.0032 0.0029 BI: 0.0005 0.0005 sb: 0.0005 0.0007 Sn: 0,004 0.0036 Zn: 0.0005 0.0017 B: 0.0005 0.0005 As + Bi + Sb + Sn + Zn + B: 0.0092 0.0099 N: 0.0045 0.0081 H: 0.00005 0.00008 N + H: 0.00455 0.00818 ESU G53227 VLBO H75142 ZVF- outside / longitudinal Rp0.2 [MPa] 1157 1158 Rm [MPa] 1258 1267 Rp0.2 / Rm 0.920 0.914 A5 [%] 16 17 Z [%] 63 66 ZVF- inside / longitudinal Rp0.2 [MPa] 1159 1190 Rm [MPa] 1259 1284 Rp0.2 / Rm 0.921 0.927 A5 [%] 16 16 Z [%] 66 68 ZVF- outside / across Rp0.2 [MPa] 1170 1163 Rm [MPa] 1270 1275 Rp0.2 / Rm 0.921 0.912 A5 [%] 15 16 Z [%] 53 63 ZVF inside / crosswise Rp0.2 [MPa] 1134 1144 Rm [MPa] 1245 1246 Rp0.2 / Rm 0.911 0.918 A5 [%] 14 15 Z [%] 57 59 ZVW 320 ° C - outside / longitudinal Rp0.2 [MPa] 987 995 Rm [MPa] 1126 1144 A5 [%] 18 19 Z [%] 70 69 ZVW 320 ° C - inside / longitudinal Rp0.2 [MPa] 1028 1025 Rm [MPa] 1154 1162 A5 [%] 17 20 Z [%] 71 69 KR-RT [J] outside / longitudinal 89/100/97 97/105/109 Inside / longitudinal 91/92/90 95/93/96 outside / across 86/83/83 99/88/92 Inside / across 82/85/82 95/93/85 KK - 23 ° C [Y] outside / longitudinal 64/70/68 69/72/79 Inside / longitudinal 60/65/57 79/78/81 outside / across 56/55/54 76/75/75 inside / across 55/51/55 69/74/77

Claims (10)

Iron-based alloy for use as a material for high-pressure components with elevated working temperature, in particular tempered steel for components such as tube heat exchangers in high-pressure polyethylene plants, containing the main alloying elements in% by weight of: Carbon (C) 0.22 to 0.29 Chrome (Cr) 1.1 to 1.5 Molybdenum (Mo) 0.3 to 0.6 Nickel (Ni) 3.3 to 3.7 possibly Vanadin (V) 0.05 to 0.15 Remainder iron (Fe), furthermore sulfide and oxide-forming as well as accompanying and impurity elements, wherein their individual concentrations and total amounts for similarly acting element groups have values in% by weight: - embedded in mixed crystals: Manganese (Mn) = MIN 0.15 MAX 0.5 Cobalt (Co) Copper (Cu) Co + Cu + W = MAX 0.31 Tungsten (W) - Pollution elements: Sulfur (S) = MAX 0.003 Phosphorus (P) = MAX 0.005 Sulfur (S) + phosphorus (P) S + P = MAX 0.006 - oxygen (O) = MAX 0.0038 - Oxide-forming elements Silicon (Si) = MIN 0.10 MAX 0.25 Aluminum (Al) = MIN 0.008 MAX 0.02 Calcium (Ca) = MIN 0.0001 MAX 0.0008 Magnesium (Mg) = MIN 0.0001 MAX 0.0006 Magnesium (Mg) + Calcium (Ca) Ca + Mg = MIN 0.00012 MAX 0.0008 - Mono carbide-forming elements Titanium (Ti) Niobium (Nb) Tantalum (Ta) Ti + Nb + Ta + Zr + Hf = MAX 0.01 Zircon (Zr) Hafnium (Hf) - boundaries - occupancy elements Arsenic (As) Bismuth (bi) Antimony (Sb) As + Bi + Sb + Sn + Zn + B = MAX 0.015 Tin (Sn) Zinc (Zn) Boron (B) - gases Nitrogen (N) Hydrogen (H) N + H = MAX 0.01 preferably = MAX 0.008 and the material has a degree of deformation of greater than 4.1 times, wherein the components or components after thermal annealing have largely isotropic, mechanical properties and high strength and toughness at a working temperature up to 350 ° C. An iron-base alloy according to claim 1 having one or more of the individual concentrations and sum amounts of the elements in% by weight of: Mn = MIN 0.15 MAX 0.4 Co + Cu + W = MAX 0.24 S = MAX 0.0008 S + P = MAX 0.005 O = MAX 0.0011 Si = MIN 0.1 MAX 0.20 al = MIN 0.005 MAX 0.018 Ca + Mg = MIN 0.0001 MAX 0.0006 Ti + Nb + Ta + Zr + Hf = MIN 0.001 MAX 0.008 As + Bi + Sb + Sn + Zn + B = MAX 0.010 N + H = MAX 0.008 An iron-base alloy according to any one of claims 1 or 2, characterized in that the alloy is produced by means of ladle metallurgy processes and / or using the ESU process and / or the vacuum arc furnace process. Component, in particular tube heat exchanger for high-pressure polyethylene plants, formed from an iron-based alloy according to one of the preceding claims, which component is annealed to a tensile strength Rm of the material greater than 1100 N / mm ² , this a 0.2% mating limit of greater than 1000 N. / mm ² and has a 0.2% yield strength at 320 ° C greater than 880 N / mm ² Rm (RT) > 1100 N / mm ² Rp 0.2 (RT) > 1000 N / mm ² Rp 0.2 (320 ° C) > 880 N / mm ² Component, in particular tube heat exchanger for high-pressure polyethylene plants, formed from an iron-based alloy according to one of the preceding claims, which component is annealed to a tensile strength Rm of the material greater than 1170 N / mm ² , this 0.2% yield strength greater than 1060 N. / mm ² and has a 0.2% yield strength at 320 ° C greater than 920 N / mm ² Rm (RT) > 1170 N / mm ² Rp 0.2 (RT) > 1060 N / mm ² Rp 0.2 (320 ° C) > 920 N / mm ² Component according to Claim 4, with the mechanical property values of the material measured in the direction of the longitudinal extent and transversely to the longitudinal extent of the component of: Elongation at break A 5 > 16/14% Elongation at break A 4 > 18/16% Fracture necking Z > 55/45% Notched Toughness AV (RT) > 80/60 y Notched impact strength AV (- 40 ° C) > 50/40 y Component according to claim 5 with the mechanical properties of the material measured in the direction of longitudinal extension and transverse to the longitudinal extension of the component of Elongation at break A 5 > 15/14% Elongation at break A 4 > 17/16% Fracture necking Z > 55/45% Notched Toughness AV (RT) > 80/60 y Notched impact strength AV (-40 ° C) > 50/35 y Component according to one of claims 4 to 7 with a ratio of the material of 0.2% yield strength broken by tensile strength of less than 0.94, preferably less than 0.92 rp 0.2 rm <0.94, preferably <0.92 Component according to one of claims 4 to 8 with a crack fracture toughness J _{1C of} the material of greater than 150 kJ / m ² measured according to ASTM - E 813 J 1C > 170 kJ / m 2 Pipe component with high internal pressure, in which the "leak before break" - Criterion is met, that is, that the current stress intensity factor is smaller is the critical stress intensity factor of the pipe wall material.

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