EP0154601A2 - Use of an austenitic stainless alloy in weldable high-performance structural elements - Google Patents

Abstract

Yield strengths are the decisive factor for the calculation of mechanically stressed components. Although austenlick steel alloys are corrosion-resistant and weldable, they have the disadvantage of lower 0.2-limits. In chemical apparatus engineering, for example, austenitic steels with high yield strengths are often required.

Through mixed crystal hardening or alloying with nitrogen, the guaranteed minimum values of the 0.2 limits of austenitic steels are raised from around 200 to 300 N / mm ² . In many cases, however, this increase still did not meet all requirements. Another known method of increasing strength is grain refinement. Cold rolling and recrystallizing annealing made it possible to produce nitrogen-alloyed, austenitic steels to be used according to the invention, which had an ultra-fine structural state with average grain sizes of 3.5 μm. Due to the superimposition of nitrogen mixed crystal and ultra-fine hardening, these steels had minimum values of the 0.2 limits of 480 N / mm ² . Manual arc welding with a high-strength, nitrogen-alloyed, corrosion-resistant filler material surprisingly showed that the welded joints created in this way did not break due to coarsening in the seam transition area, but in the ufrefeingra ?ne, high-strength base material.

Description

The invention relates to the use of a corrosion-resistant austenitic iron-chromium-nickel-nitrogen alloy as a material for mechanically highly stressed components with good weldability.

In the chemical industry, e.g. In the construction of apparatus or pressure vessels, and in plants for the generation of energy, steels or alloys are required which, in addition to having sufficient corrosion resistance, are said to be suitable for welding and have high mechanical strengths. Yield strengths such as the 0.2 limits represent the decisive size for the calculation. For this reason, the designer will prefer materials with the highest possible 0.2% proof limits in order to achieve maximum load capacity of the components or because of the raw material and weight savings as well to be able to use thinner workpieces for better workability and weldability. When developing such steels or alloys, there is the difficult problem of maintaining the weldability of the material despite increased strength.

In contrast to ferritic steels, austenitic steels generally have more favorable corrosion properties and are much more weldable, ductile and tough. Since nickel stabilizes the austenitic structure, such steels have at least 7% nickel according to "StahlKey", 13th edition, 1983, Verlag StahlKey Wegst, GmbH, Marbach, pp. 323/324 ff. They also contain more than 16% chromium to achieve sufficient passivity. To avoid intergranular corrosion The carbon content of steels not stabilized with titanium or niobium is limited to a maximum of 0.08%. A further improvement in the corrosion properties is achieved by adding up to 6% Mo, 4% Cu and 3% Si. Increased nickel content of approx. 50% improve the resistance to stress corrosion cracking (see Berg- und Hüttenmännchen Monthly Bulletin 108, 1963, pp. 1/8 and 4 ff.).

The low guaranteed 0.2 limits of austenitic steels, which according to DIN 17 440, December 1972 edition, for example for a steel with 18 to 19% Cr and about 9% Ni 185 N / mm ² , can be determined by mixed crystal hardness. or by alloying with up to 0.30% N to 343 N / mm ² (see Japanese Industrial Standard JIS G 4304, 1981, pp. 1301/1304 ff., steel SUS 304 N2). However, such strength increases have not yet met all requirements. In order to achieve a further increase in the 0.2 limits, it was necessary to introduce more nitrogen into the steel up to the solubility limit of approximately 0.55%. Since such quantities of nitrogen bubbles form during solidification or the casting blocks "float" and pores occur during welding, the chromium and manganese contents also had to be increased at the same time. Special steels with 22.5 to 25.5% Cr, 4 to 7% Mn, 2 to 4% Mo and 13 to 17% Ni are known, because of their content of 0.35 to 0.50% N and of low Niobium additives have guaranteed minimum values of the 0.2 limits of 500 to 540 N / mm ² (see ASM Technical Report, 1970, No. C 70-24.2., DEW Technical Reports 13, 1973, p. 94/100 and Proceedings " Molybdenum 1973 ", Noranda Symp. 4, 1973, pp. 43/48) -. Like the nitrogen-alloyed, austenitic steels already listed, they are weldable using the same filler materials. Their pure weld metal has guaranteed 0.2 limits of at least 510 N / mm ² . The disadvantage, however, is that the high chrome and stick The content of these steels makes hot formability more difficult. Furthermore, even at temperatures as high as 1000'C, they still separate intermetallic phases that lead to low strains of approx. 30% and can cause embrittlement after welding, hot straightening or bending. Since chromium promotes ferrite formation in steels, while nickel suppresses it and at the same time delays the elimination of intermetallic phases, the alloys listed have high nickel contents, which make the material more expensive. In chemical apparatus engineering, however, relatively low-alloy steels with only approx. 18% Cr, 12% Ni and 2% Mo are in demand because their corrosion resistance is usually sufficient. In these cases, the low 0.2 limits of these steels of around 200 N / mm ^{2 are accepted} and the addition of nitrogen is dispensed with, which would have only increased the yield strengths to 280 N / mm ² if processing had been somewhat more difficult (compare steel 1.4435 with 1.4406 in DIN 17 440). The usual nitrogen-alloyed, austenitic steels with maximum yield strength values of only 280 to 343 N / mm ² have not yet been widely used. This statement also applies to the higher alloyed, austenitic special steels with nitrogen contents over 0.35% and minimum values of the 0.2 limit of 500 N / mm ² , since their use is naturally more restricted to special cases.

Another method of improving the strength properties of steels is grain refinement. Cold forming followed by recrystallizing annealing on an austenitic steel with approx. 18% Cr and 10% Ni with an ultra-fine microstructure of Size No. granules. 11.5 to 13.5 according to ASTM (6-3 ^p m) generated (see ASTM Special Technical Publication No. 369, 1965, pp 175/179 ). This increased compared to that coarse-grained initial state the 0.2 limit by approx. _{150 N /} mm ² . Since the steel was not alloyed with nitrogen, its 0.2 limit was only 380 N / mm2 overall. The problem of the extent to which such extremely fine-grained, non-convertible steels are suitable for welding has not been addressed.

In the nitrogen-alloyed, austenitic steels discussed, the alloy element niobium is of particular importance. Its effect is based on the precipitation of a complex nitride of the type Nb ₂ Cr ₂ N ₂ , called the Z phase. As a result, grain refinement is achieved even in hot-formed, solution-annealed steels, but this only leads to grain sizes of No. 10 according to ASTM (approx. 10 µm) (see Berg- und Hüttenmännchen Monthly Bulletin 124, 1979, p. 513 ff.). In addition, nitride precipitation hardening is also found, which can assume values of up to 90 N / mm ² (see Thyssenforschung 1, 1969, p. 14 ff.). In order to avoid the precipitation of too much nitride, which would remove nitrogen from the mixed crystal for hardening, these steels have a niobium content which is considerably lower than their sevenfold amount of N, the stoichiometric ratio in the compound NbN.

The hot yield strengths of austenitic steels are also raised by nitrogen mixed crystal hardening and grain refinement. However, the increase in the 0.2 limit due to nitrogen decreases with increasing temperature and is, for example, only about half as large at 400 ° C as at room temperature (see Berg- und Hüttenmännchen Monthly Bulletin 113, 1968, pp. 386/387 ff. ). In contrast, the increase in the 0.2 limit due to grain refinement decreases significantly less with the test temperature (see Metal Science 11, 1977, p. 209 ff.). At even higher temperatures, at which the warm no longer yield point, but the lower creep rupture strength is decisive for the calculation of constructions, this favorable fine grain effect no longer exists. A certain compensation can be achieved by alloying with boron up to a content of 0.015%, since this measure increases the creep resistance of austenitic chromium-nickel-molybdenum steels at temperatures of, for example, 650 ° C (see Rev. Mêtallurgie 59, 1962, p. 651 / 660). This beneficial effect also appears to be seen in such steels which additionally contain nitrogen (see Arch. Eisenhüttenwes. 39, 1968, p. 146 ff. And VDI-Reports 428, 1981, p. 89 ff.). As a result, the area of application in which the hot stretching limit can still be used for the calculation is expanded or shifted to higher temperatures. Due to the susceptibility of austenitic steels to hot cracking when welding with boron, its content is usually limited to 60 to 80 ppm.

In terms of corrosion behavior, in particular the resistance to intergranular corrosion after welding, the austenitic steels listed in DIN 17 440, December 1972 edition, with up to 0.22% N alloyed steels are equated with steels without nitrogen. They are all suitable for welding if the carbon content is limited to <0.07% for wall thicknesses smaller than 6 mm and to <0.03% for thicknesses over 6 mm. According to AD leaflet HP 7/3, April 1975 edition, only parts over 50 mm thick in pressure vessel construction are to be annealed after welding.

The delivery condition of the corrosion-resistant austenitic steels is determined, among other things, by a heat treatment which is referred to as "quenching". It is an annealing at at least 1000 ° C with rapid cooling. This ensures that all chrome carbides, -Nitrides and intermetallic phases are solved. Another purpose of this measure is to largely reduce the dislocations brought about by deformation as a result of recrystallization and recovery, so that a state of optimum corrosion resistance and toughness which is poor in internal stresses is finally obtained. However, one takes into account that in austenitic chromium-nickel steels about 0.2% N and approx. 0.03% C are already dissolved at 900 ° C, so annealing is permissible even at such a low temperature according to the explanations given, if care is taken to ensure that, for example, cold-formed steels are fully charged at such temperatures. can constantly recrystallize and before and after this heat treatment there are no intermetallic phases. Accordingly, in pressure vessel construction according to AD leaflet HP 7/3, April 1975 edition, after cold forming of nitrogen-alloyed austenitic steels, annealing at 900 ° C is permissible instead of "quenching".

The joint welding of austenitic steels is assessed with the help of weld joint samples. These are flat tensile specimens according to DIN 50 120, September 1975 edition, with a continuous transverse seam in the middle. This ensures during the tensile test that the weld metal, seam transition and base material are subjected to the same stress since they are arranged one behind the other in the direction of the tensile force. The sample is suitable for determining the tensile strength and fracture position. It is disadvantageous that it can only be used to determine the yield strengths imprecisely because weld metal, material in the transition area and unaffected base material deform plastically to varying extents within the measuring length or expand permanently. In the case of austenitic steels, the unaffected base material G and the weld seam S come into consideration as the fracture position, while in the transition region U the seam to the base material normally does not break occur. The strength properties would not be found in these transition areas because they are too narrow. If the seam breaks, the strength of the weld metal is decisive. However, since the weld metal is more or less mixed with the base material, the tensile strength of the pure weld metal is determined on longitudinal samples of specially prepared seams, for which reproducibility does not occur, for reasons of reproducibility. Their manufacture is described in DIN 32 525, Part 1, December 1981 edition. The degree of melting or the mixing ratio is mainly dependent on the welding current, which determines the penetration depth, the number of layers and the welding process. Furthermore, all measures to reduce the heat input, such as rapid welding in train tracks, low working temperatures and avoiding preheating, are advantageous. In single-layer welding with the usual amperages, the mix of the weld metal through the base material for TIG (tungsten inert gas), E (electric arc hand), MAGM (metal shielding gas) and UP (submerged powder) welding is approximately 20, 30, 40 and 55%. When multi-layer welding of thick cross-sections, this mixture is significantly reduced. In contrast, it is of course 100% when welding thin workpieces without additives.

In the technical regulations, the suitability of new steels for welding is to be proven as part of so-called process tests. A key example in this context for austenitic steels is AD leaflet HP 2/1, February 1977, entitled "Process test for welded joints". This regulation mainly deals with the production of test pieces from sheet metal by butt welding under manufacturing conditions, so that, among other things, material, welding process, position, additive and auxiliary materials are set. Flat tensile specimens according to DIN 50 120 are to be taken transversely to the seam from the test sheets and tensile strength and fracture position are to be determined. Welding suitability is mainly given when the minimum tensile strength values for the base or filler material, based on its pure weld metal, are achieved in this connection test.

The object of the invention is now to raise the low minimum values of the 0.2 limits of the usual nitrogen-alloyed, corrosion-resistant austenitic steels to a level of approximately 500 N / mm ² without reducing their good weldability, with an increase in the alloy contents being excluded. This object is achieved in that a corrosion-resistant austenitic alloy with the known chemical composition specified in claim 1 is used as a material for corrosive and mechanically highly stressed components of good weldability in such a way that after cold forming and recrystallizing annealing high 0, 2 limits can be reached due to the formation of an ultra-fine-grained structure with average grain diameters of less than 10 µm (greater than No. 10 according to ASTM) and the use of fillers made of high-strength, nitrogen-containing, corrosion-resistant steel or nickel alloys, which means that welding is possible, which is a property of the base material, ie the alloy, is justified - despite the very fine grain - after joining by welding not to break in the seam transition area. In further claims, embodiments of the invention are given which relate to the degree of cold rolling, the recrystallization temperature, the guaranteed minimum values of the 0.2 limits and the uses of the steels and alloys to be used according to the invention.

The greatest advantage of the steels to be used according to the invention can be seen in their high 0.2 limits, without the weldability being reduced by the ultrafine grain. To the best of our knowledge of the behavior of metallic materials, it would have been expected that welded joints made from such extremely fine-grained, non-convertible steels would break due to grain coarsening in the seam transition area with relatively low strength. The essential advantage of the invention would be lost. Preliminary examinations according to Table 1 surprisingly showed that the welded joint specimens produced according to DIN 50 120 with transverse seams did not break in the transition but in the base material unaffected by the heat of welding if the solidification by nitrogen mixed crystal hardening and grain refinement did not exceed a certain level. This limit was for steels with approx. 0.2% N with a tensile strength of about 825 N / mm ² .

E (= U x I x 60/v) betrugen für den 2,5-mm-Stab 80 A, rd. 17 cm/min und etwa 6,5 kJ/cm sowie für die 3,25-mm-Elektrode 110 A, ungefähr 19 cm/min und 8 kJ/cm. Die Schweißversuche wurden so ausgeführt, daß Brüche nur im Grundwerkstoff der Flachzug-Verbindungsproben auftreten konnten. Nahtbrüche wären zwar im Sinne der Erfindung auch zulässig gewesen, sie hätten jedoch nicht eine klare Darlegung der erfinderischen Idee gestattet. In der Praxis mag in solchen Fällen im Sachverständigengutachten für die Belastbarkeit von Bauteilen die 0,2-Grenze des reinen Schweißgutes zugrunde gelegt werden, wenn die Zugfestigkeit der in der Naht gerissenen Verbindungsprobe ausreichend hoch war. Um bei den Schweißversuchen die Wahrscheinlichkeit für solche Brüche gering zu halten, wurde ein in seiner Festigkeit den hohen 0,2-Grenzen des ultrafeinkörnigen Grundwerkstoffes angepaßter, niobhaltiger Schweißzusatzwerkstoff mit 0,38 % N, 25 % Cr, 21,5 % Ni, 5 % Mn, 3,6 % Mo u. 0,035 % C verwendet. Es. handelt sich um die vom Hersteller angegebenen Richtwerte für die Stabelektrode Thermanit 20/16/510, die eine Mindest-0,2-Grenze von 510 N/mm² ihres reinen Schweißgutes besitzt (siehe auch S. 2, Zeilen 24/35). Weiter war es zur Vermeidung von Nahtbrüchen notwendig, die Aufmischung des relativ hoch legierten Zusatzwerkstoffes durch die stickstoffärmeren, erfindungsgemäß zu verwendenden Stahllegierungen, deren Ultrafeinkörnigkeit in ihrem Schweißgut natürlich nicht mehr vorhanden ist, niedrig zu halten. Die relativ hohen Schweißgeschwindigkeiten bzw. Ausziehverhältnisse und die niedrige Streckenenergie (Wärmeeinbringen) beim durchgeführten Lichtbogenhandschweißen gestatteten, die Nähte durch möglichst viele Lagen aufzubauen und wenig aufzumischen.The connection samples were taken from test pieces which had been obtained by welding two sheets in the tub position. Your seam preparation can be seen in Figure 1. The 10 mm sheets were provided with a Y-seam (web height 2 mm), the thinner ones with a V-seam (without web). The welds were carried out in multiple layers with the opposite side after the root had been ground beforehand. After each pull caterpillar was laid, the work temperature was below 150 ° C. Elevations in the seams were processed at the sheet metal level. Welding was carried out at the positive pole at a voltage U of 23 V using the rutile-based, coated electrode Thermanit 20/16/510, which is commercially available. The pull-out ratio (bead length / length of molten rod) was 0.7 to 0.8 or 0.8 to 0.9 for the 2.5 or 3.25 mm electrode. The other welding parameters such as direct current I, speed v and the distance energy calculated from this

E (= U x I x 60 / v) for the 2.5 mm rod was 80 A, approx. 17 cm / min and about 6.5 kJ / cm and for the 3.25 mm electrode 110 A, about 19 cm / min and 8 kJ / cm. The welding tests were carried out in such a way that fractures could only occur in the base material of the flat tensile connection samples. Seam breaks would also have been permissible in the sense of the invention, but they would not have allowed a clear statement of the inventive idea. In practice, the 0.2 limit of the weld metal may be used as a basis in such an expert opinion for the load-bearing capacity of components if the tensile strength of the connection sample torn in the seam was sufficiently high. In order to keep the likelihood of such fractures low during the welding tests, a niobium-containing welding filler material with 0.38% N, 25% Cr, 21.5% Ni, 5 was adapted to the high 0.2 limits of the ultra-fine-grained base material % Mn, 3.6% Mo u. 0.035% C used. It. are the guideline values given by the manufacturer for the Thermanit 20/16/510 stick electrode, which has a minimum 0.2 limit of 510 N / mm ^{2 for} its pure weld metal (see also p. 2, lines 24/35). Furthermore, in order to avoid seam breaks, it was necessary to keep the mixing of the relatively high-alloy filler material low by means of the low-nitrogen steel alloys to be used according to the invention, the ultrafine grain of which is of course no longer present in the weld metal. The relatively high welding speeds or extension ratios and the low path energy (heat input) when performing manual arc welding allowed the seams to be built up by as many layers as possible and not to be mixed up.

Tabelle 2 weist je ein Ausführungsbeispiel von drei erfindungsgemäß zu verwendenden Stahllegierungen aus, die nach dem angegebenen Schweißverfahren gefügt wurden. Die Ermittlung der 0,2-Grenzen erfolgte an Prüfstücken, deren Nähte wie bereits beschrieben und in Bild 1 dargestellt, vorbereitet worden waren. Aus Gründen der Genauigkeit und Reproduzierbarkeit im Sinne der Ausführungen auf S. 6, Zeile 29, wurden die Dehngrenzen jedoch nicht an den Flachzug-Verbindungsproben, sondern an zusätzlich aus dem gleichen Prüfstück entnommenen Rundzugproben nach DIN 50 125, Ausgabe April 1951, ermittelt: Bild 2 zeigt die Lage dieser Proben und deren Aufteilung im Prüfstück. Die Tabelle 2 läßt die Vorzüge der erfindungsgemäß zu verwendenden Stahllegierungen erkennen: Hohe, zwischen 504 und 553 N/mm² liegende 0,2-Grenzen, die hauptsächlich durch Überlagerung von Stickstoff-Mischkristall- und Ultrafeinkornhärtung erzielt wurden, da die Stähle rd. 0,2 % N enthielten sowie Korngrößen zwischen 2,8 und 4,5 pm besaßen. Ferner ist erfindungsgemäß die Schweißeignung gut, da die Schweißverbindungsproben nicht im Nahtübergang, sondern im unbeeinflußten Grundwerkstoff brachen. Für Stähle ohne Molybdän wie beispielsweise solche der lfd. Nr. 1 und 2 sind danach Mindestwerte der 0,2-Grenzen von 450 N/mm² gerechtfertigt, für molybdänlegierte Stähle wie vom Typ der lfd. Nr. 3 erscheinen demgegenüber 0,2-Grenzen von mind. 480 N/mm² angemessen. Diese Mindestwerte dürften Festigkeiten entsprechen, die vom Werkstoff mit an fast 100 %iger Sicherheit grenzender Wahrscheinlichkeit erreicht werden. Gegenüber den üblichen austenitischen Stählen ist danach eine Steigerung der 0,2-Grenzen um rd. 150 % zu verzeichnen, während im Vergleich zu den weniger gebräuchlichen stickstofflegierten, austenitischen Stählen immerhin noch um 60 % höhere Streckgrenzen erzielt werden.

Table 2 shows an exemplary embodiment of three steel alloys to be used according to the invention, which were joined by the welding process specified. The 0.2 limits were determined on test pieces, the seams of which had been prepared as already described and shown in Figure 1. For reasons of accuracy and reproducibility in the sense of the explanations on page 6, line 29, the yield strengths were not determined on the flat tensile connection specimens, but on round tensile specimens additionally taken from the same test piece in accordance with DIN 50 125, April 1951 edition: Image 2 shows the position of these samples and their division in the test piece. Table 2 shows the advantages of the steel alloys to be used according to the invention: High 0.2 limits between 504 and 553 N / mm ² , which were mainly achieved by superimposing nitrogen mixed crystal and ultra-fine grain hardening, since the steels approx. Contained 0.2% N and had grain sizes between 2.8 and 4.5 pm. Furthermore, the weldability according to the invention is good, since the weld connection samples did not break in the seam transition, but in the unaffected base material. For steels without molybdenum, such as those of serial numbers 1 and 2, minimum values of the 0.2 limit of 450 N / mm ^{2 are} justified; for molybdenum alloyed steels, of the type number 3, on the other hand, 0.2- Limits of at least 480 N / mm ^{2 are} appropriate. These minimum values should correspond to strengths that the material achieves with a probability that is almost 100% certain. Compared to the usual austenitic steels, the 0.2 limits are increased by approx. 150%, while at least 60% higher yield strengths are achieved compared to the less common nitrogen-alloyed austenitic steels.

The cold-forming of the steels or alloys to be used according to the invention is generally carried out for flat products using the Sendzimir or quarto rolling method, for pipes using cold pilgrims from hot-pressed blanks. This results in further advantages, such as better surface properties, more precise dimensions or, by narrowing the tolerances, material savings of 5 to 10% compared to the usually only thermoformed steels, at least for larger wall thicknesses.

Claims (4)

1. Using a corrosion-resistant austenitic alloy at most 0.08% carbon, 0.065 to 0.35% nitrogen, at most 0.75% niobium, but not more than four times the amount of nitrogen in the alloy, 16.0 to 22.5% chromium, 7.0 to 55.0% nickel, up to 4.75% manganese, up to 6.5% molybdenum, up to 3.0% silicon, up to 4.0% copper, up to 0.0080% boron, Remainder iron and unavoidable impurities, - which has the high 0.2-limits after cold forming and recrystallizing annealing due to the formation of an ultra-fine-grained structure with average grain diameters of less than 10 µm and which uses additional materials made of high-strength, nitrogen-containing, corrosion-resistant steel or nickel alloys is suitable for welding, which is due to the property of the base material, ie the alloy, - despite the very fine grain - does not break in the seam transition area after welding, as a material for corrosive and mechanically highly stressed components with good weldability. 2. Use of an alloy according to claim 1, which is recrystallized by single or repeated cold forming by 30 to 75% and by subsequent annealing in the range between 750 and 975 ° C, for the purpose of claim. 1. 3. Use of an alloy according to claims 1 or 2, which in the ultrafine state with nitrogen contents of approx. 0.2% guaranteed minimum values of the 0.2 limits of 450 or 480 N / mm ² , provided niobium or niobium and molybdenum are contained in the alloy, for the purpose according to claim 1. 4. Use of an alloy according to claims 1, 2 or 3 as a material for easily weldable components which are subjected to high mechanical stresses at elevated temperatures at which the hot stretching limit is the basis for the calculation of constructions.

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