WO2001000898A1

WO2001000898A1 - Duplex stainless steel

Info

Publication number: WO2001000898A1
Application number: PCT/SE2000/001235
Authority: WO
Inventors: Pasi Kangas
Original assignee: Sandvik Ab; (Publ)
Priority date: 1999-06-29
Filing date: 2000-06-13
Publication date: 2001-01-04
Also published as: NO20011004D0; DE60018544D1; ZA200101378B; JP3822493B2; CA2342817C; NO333625B1; SE9902472D0; SE513247C2; DE60018544T2; KR100545301B1; ATE290614T1; CN1316019A; EP1117848A1; JP2003503596A; SE9902472L; US6312532B1; EP1117848B1; KR20010072981A; NO20011004L; CA2342817A1

Abstract

A duplex stainless steel alloy has been developed, which contains in weight-%: C maximum 0.05, Si maximum 0.8, Mn 0.3-4, Cr 27-35, Ni 3-10, Mo 0-3, N 0.30-0.55, Cu 0.5-3.0, W 2.0-5.0, S maximum 0.010 balance Fe and normally occurring impurities and additions. The content of Fe is 30-70 volume-%. The steel alloy well suited in those chloride environments, where demands are made on god resistance to crevice corrosion. A relatively high content of W has at the same time given a god effect on both the pitting- and crevice corrosion properties.

Description

DUPLEX STAINLESS STEEL

The present invention relates to a ferritic-austenitic stainless steel with high contents of Cr, N, Cu and W in combination with relatively low contents of Ni and Mo. The material is suitable for applications where high resistance to corrosion is requested, especially in acid or basic environments, where you have high chloride contents at the same time.

Background

Duplex steel is characterized by a ferritic-austenitic structure, where both phases have different compositions. Modern duplex steel will mainly be alloyed with Cr, Mo, Ni and N. The duplex steel grade SAF 2507 (UNS S32750) has mainly been alloyed with high contents of Cr, Mo and N for a high resistance to pitting corrosion. This resistance is often described as a PRE-number (PRE = Pitting Resistance Equivalent =

%Cr+3.3%Mo+16N). The alloy is consequently optimized towards this property and has certainly a god resistance in many acids and bases, but the alloy is above all developed for resistance in chloride environments. During recent yaers have even the elements Cu and W been used as alloying additions. Thus, the steel grade DP3W (UNS S39274) has for example an analogous composition as SAF 2507, but it is alloyed with 2.0%) W as substitute for a share of the Mo-content in the alloy. Likewise, the steel grade Uranus 52N+ (NS S32529) has an analogous composition as SAF 2507, but it is alloyed with 1.5% Cu with the purpose to improve the resistance in acid environments. The steel grade Zeron 100 is a further steel grade which is analogous to SAF 2507, but this is alloyed with both about 0.7% Cu and 0.7% W. The steel grade DTS 25.7NWCu (UNS S39277) is in this composition very similar to SAF 2507, besides that it is alloyed with about 1.7% Cu and 1.0% W. In relation with that it is alloyed with W, a PRE formula was produced, which also includes the element W with a weight corresponding the halve of this for Mo. PRENW=%Cr+3.3(%Mo+0.5%W)+l 6N. All described steel grades have a PRE number irrespective to the calculation method that is over 40.

Another type ferritic-austenitic alloy with high resistance to chloride is the steel grade described in the Swedish Patent 9302139-2 or USA Patent 5,582,656. This type of alloy is characterized by Mn 0.3-4%, Cr 28-35%, Ni 3-10%, Mo 1-3%, Cu maximum 1.0% and W maximum 2.0%>, and has even this high PRE number, generally over 40. The main difference compared with the established superduplex steel SAF 2507 and others is that the contents of Cr and N are higher in this steel grade. This steel grade has been used in environments where the resistance to intergranular corrosion and corrosion in ammonium carbamates is of importance, but the alloy has also a very high resistance to chloride environments.

Description of the invention

The purpose of this invention has been to provide a material with high resistance to chloride environments, at the same time as the material has extraordinary properties in acid and basic environments combined with god mechanical properties and high structural stability. This combination can be very useful in applications for example within the chemical industry, there you have problems with corrosion caused by acids and at the same time have a contamination of the acid with chlorides, which further amplifies the corrosivity. These properties of the alloy in combination with a high strength lead to advantageous design solutions from an economical point of view. There are certainly existing materials with very god properties in acid environments, but these are often steels with high contents of Ni, which makes increases the costs. Another disadvantage with austenitic steels compared with duplex alloys is that the strength in the austenitic steels is usually considerably lower.

In the present-day situation there are no duplex stainless steels described that are optimized for this combination of properties, and which then attain those god properties which are described here.

By developing an alloy where high contents of Cr and Ni in combination with the elements Cu and W are used as alloying elements, surprisingly god corrosion properties and mechanical properties have been detected. The alloy contains in weight-%:

C maximum 0.05

Si maximum 0.8

Mn 0.3-4 Cr 27-35

Ni 3-10

Mo 0-3

N 0.30-0.55

Cu 0.5-3.0

W 2.0-5.0

S maximum 0.010 balance Fe and normally occurring impurities and additions. The content of Fe 30-70 volume-%).

Carbide has to be seen as an impurity element in this invention and has a limited solubility in both ferrite and austenite. The limited solubility implies a risk for precipitation of carbonitrides and for that the content should be limited to maximum 0.05%), preferably to maximum 0.03%> and most preferably to maximum 0.02%o.

Silicon is used as a deoxidant under steelmaking and also improves the floability under production and welding. However, high contents of Si favour the precipitation of intermetallic phase, for what reason the content should be limited up to maximum 0.8%>.

Manganese will be added in order to improve the solubility of N in the material.

However, Mn has only a limited effect on the N-solubility in the present type of alloy. Instead there are other elements with higher effect on the solubility. Besides, Mn can in combination with the high sulfur-content cause manganese sulfides, which act as initiating points for pitting corrosion. The content of Mn should for that be limited to between 0.3-4%.

Chromium is a very active element to improve the resistance to the majority of corrosion types. Besides, Chromium improves the strength of the alloy. Furthermore, a high content of Chromium implies that you attain a very god N-solubility in the material. Thus, it is desirable to keep the Cr-content as high as possible to improve the resistance to corrosion. In order to obtain a very god resistance to corrosion the content of Chromium should be at least 27%. However, high contents of Cr increase the risk for intermetallic precipitations, for what reason the content of Chromium should be limited to maximum 35%.

Nickel will be used as austenite stabilizing element and will be added on a suitable level so that the desired content of ferrite will be obtained. In order to obtain contents of ferrites between 30-10% an addition of 3-10%) Nickel is requested.

Molybdenum is a very active element to improve the corrosion resistance in chloride environments and also in reducing acids. A too high content of Mo in combination with that the contents of Cr and W are high, implies the increasing risk for intermetallic precipitations. The Mo content in the present invention should for that be limited to maximum 3.0%>.

Nitrogen is a very active element, which on one hand increases the corrosion resistance and on the other hand increases the structural stability and also the strength of the material. Furthermore, a high N-content improves the rebuilding of the austenite after welding, which gives god properties at welding joints. In order to obtain a god effect of N, at least 0.30%> N should be added. At high contents of N the risk for precipitation of chromium nitrides increases, especially if there is a high chromium-content at the same time. Furthermore, a high N-content implies that the risk for porosity increases because of that the solubility of N in the smelt will be exceeded. For these reasons the N-content should be limited to maximum 0.55 >.

Copper increases the general corrosion resistance in acid environments such as sulfuric acid. It has surprisingly shown that Cu in materials with relatively high contents of Mo and/or W moreover slows down the rapidity of precipitation of intermetallic phase at slow cooling. In purpose to increase the structural stability of the material the content of Cu should exceed 1%> and should preferably exceed 1.5%. Nevertheless, high contents of Cu imply that the solid solubility will be exceeded. By this reason the content of Cu will be limited to maximum 3.0%). Tungsten increases the risk for pitting and crevice corrosion. It has surprisingly shown that the addition of W as substituent for Mo increases the low temperature impact strength. In order to obtain an adequate effect on the impact strength and also the corrosion properties, at least 2% should be added. A simultaneous addition of W and Cu, where W substitutes the element Mo in the alloy with the purpose to improve the pitting corrosion properties, can furthermore be made with the purpose to increase the resistance to intergranular corrosion. However, high contents of W in combination with high contents of Cr and Mo increase the risk for intergranular precipitations. The content of W should therefore be limited to maximum 5%.

Sulfur influences the corrosion resistance negatively by forming easily soluble sulfides. Furthermore the hot workability deteriorates, for what reason the content of S should be limited to maximum 0.010%>.

The content of ferrites is important in order to obtain god mechanical properties and corrosion properties and also god weldability. From the corrosion and weldability point of view it is desirable with a ferrite content between 30-70%> in order to obtain god properties. High ferrite contents imply furthermore that the low temperature impact strength and also the resistance to hydrogen embrittleness run the risk of detoriating. The ferrite content is therefore 30-70%, preferably 35-55%).

Example

In the example below the composition of some experimental heats is shown. The compositions of those join not necessarily in the patent claims, but are just included in order to illustrate the influence of different alloying elements on the properties. The optimum composition of the steel grade according to the invention does consequently not necessarily need to occur among the examples.

A number of experimental heats were produced by casting of 170 kg ingot, which was hot forged to round bars. Those were extruded to bars, from where the test material was taken. Table 1 shows the composition of experimental heats with a calculated PRENW- number with the formula PRENW=%Cr+3.3(%Mo+0.5%W)+16%N. Table 1. Composition of experimental heats, weight-%>

Production

Material for all heats was produced by ingotcasting, hot forging and extrusion. Some variations cracked under producing because of high amounts of intermetallic phase. From Table 2 appears how the production was running: Table 2. Results of the production of heats

There is a relation between the content of the alloy and the tendency of cracking under forging. Consequently no heats with a PRENW -number at 45.5 or higher pass the forging without cracking. If the content of Mo is over 2% it is necessary that the content of W is maximum about 1%> in order to avoid high quantities of intermetallic phase. On the other hand, if the content of W is high, it is necessary that the content of Mo is low in order to avoid intermetallic phase and thereby cracking. The relation is illustrated graphically in Figure 1.

Structural Stability

The samples were annealed at 800-1200°C in steps of 50°C. The temperature when the quantity of intermetallic phase became negligible, was determined at this with the help of studies in a lightoptical microscope. The material was then annealed at this temperature with three minutes holding time, thereafter the samples were cooled with a rate of 140°C/min and also 17.5°C/min to room temperature. The quantity of sigma- phase in this material was counted with help of counting the points under a light optical microscope. The results appear from Table 3.

Table 3. Quantity of sigma-phase after cooling with different rates from 1100°C to room temperature.

It appears that material with high content of W has a very god structural stability, especially if the content of Mo is low (heat 605089). Totally unexpected it has shown that even material with high content of Cu and low content of N (heat 605089) under slowly cooling (17.5°C/min) has a better structural stability than material with a low content of Cu and also a high content of N (heat605084). It is known that the addition of the element N increases the structural stability in duplex steels, while the effect of Cu is more uncertain. However, heat 654796 with both a low content of Mo and low Cu- content has inferior structural stability at slowly cooling (17.5°C/min) than heat 605085 with 2% Cu, in spite of the fact that heat 605085 has a content of Mo near 3%. The relation is illustrated graphically in Figure 2. The relation between Mo, W and Cu and the favourable effect of the addition of Cu are illustrated graphically in Figure 3, where the influence of Cr, W and Cu on the cracks under hot working is shown. The cracking under hot working depends in this case mainly on the occurrence of intermetallic phase. Mechanical properties

The strength and the impact strength were measured for some heats. The results appear from Table 4.

Table 4. Mechanical properties (tensile test at room temperature and impact strength at room temperature and at -50°C).

For all materials a high yield point in tension was obtained and also the impact strength at 20°C is high. For the impact strength at -50°C it has surprisingly shown that the heat 605085 has lower impact strength than heat 605084. The reason for this can be either that the heat 605084 has a lower content of Cu or a higher content of W. Because heat 605089 has both a high Cu- and high W-content this shows a god impact strength at - 50°C, it is probable that a high content of W is to prefer to a high content of Mo if a high impact strength at low temperatures is requested.

Corrosion

Pitting- and crevice corrosion properties were tested by testing in FeCl₃ according to ASTM G48C and also MTI-2. A critical pitting corrosion temperature (CPT) and also crevice corrosion temperature (CCT) were hereby determined. The results of all experiments are shown in Table 5.

Table 5. Critical pitting/crevice corrosion temperature for the tested steel grades.

*) The given value is the average of two experiments.

Very surprising it has shown that W at very high contents, in combination with low contents of Mo (heat 605084) obtains very god pitting corrosion properties. Heat

605085 has a PRENW number, which is higher than this for heat 605084, but in spite of this heat 605084 obtains a considerably higher CPT value at testing according to ASTM G48C. The same is valid for heat 605089, which in spite of that the material has lower PRENW value that heat 605085 obtains a higher CPT value. The resistance to pitting corrosion measured as CCT value shows unexpectedly high values for heat 605084 and heat 605085. For instance the material of type 2507 with a PRE over 40 has a CCT value of approximately 40°C. However, the crevice corrosion properties in heat 605089 are inferior for heat 605085. The differences between those heats are, that 605089 has a higher W-content, but at the same time a lower content of N. In order to obtain a god corrosion resistance with regard to both pitting corrosion and crevice corrosion it is consequently requested, that one has partly a high W-content and partly a high N- content. It also seems to be clear that there is an optimum PRENW value, so that if one has higher or lower PRENW values inferior properties will be obtained. The relation will be illustrated graphically in Figure 4-5.

The mixture in the ferrite phase and the austenite phase was determined with help of microprobe analysis. The results appear from Table 6.

Table 6. Mixture in the ferrite and austenite phase for tested heats

It appears that PRENW in the austenite phase and also in the ferrite phase in all cases except for heat 605088 are superior to 40. Furthermore, for heat 605088 an unacceptable low CCT value was obtained, which thus can depend on that PRENW for the austenite phase is relatively low. For heat 605084 and 605085 PRENW is highest. An observation is that in spite of the PRENW in both the austenite phase and the ferrite phase for heat 605085 is higher than for 605084, heat 605085 has thus a lower CPT according to ASTM G48C testing compared with 605084. The higher content of W in combination with a high content of N, which is retrieved in heat 605084, can explain this effect. Probably, this is the reason for that heat 605085 has an inferior structural stability than 605084 is the higher content of Mo in heat 605085, which increases the risk that the material contains precipitations, which reduce the resistance to pitting corrosion. An optimum PRENW value is in the range of 41-44. For an optimum corrosion resistance PRENW should be in the range of 43-44.

The resistance to intergranular corrosion was measured by carried out the Streicher-test according to ASTM A262 Practice B. This test specifies how the material withstands oxidizing acid environments and also the resistance of the material to intergranular corrosion. The results appear from Table 7.

Table 7. Results of corrosion testing according to ASTM A262 Practice B. The results are average values of two tests for every heat.

It appears that the materials have very low corrosion rates in these tests. The differences are relatively little, but material with simultaneously high Mo-content and high Cu- content shows the highest corrosion rate (heat 605085). If the Cu-content is high, but the Mo-content low a low corrosion rate is obtained (heat 605793, 605088, 605089). The combination of high contents of the elements Cr, Mo, W and N is requested for a god resistance to pitting corrosion. In relation with high Cu-contents it is consequently optimal to foremost use Cr, W and N with purpose to increase the resistance to pitting corrosion if one wants to have a god resistance to intergranular corrosion at the same time. Consequently heat 605089 obtains with 2.0% Cu, 0.98% Mo and 3.8% W very low corrosion rates at Streicher-testing.

The resistance to caustic solution environments was tested in cooking 60%) NaOH (160°C) for some heats. The testing was carried out during 1+3 days. The results appear from Table 8. Table 8. Results of corrosion testing in cooking 60%o NaOH (160°C). Average values of double tests.

There is a relation between the god corrosion properties in NaOH and the content of Cr in the austenite phase, so that the material with high contents of Cr in the austenite phase obtains low corrosion rates at exposure in NaOH. The relation will be illustrated graphically in Figure 6.

Optimum composition of alloy according to the invention

It has surprisingly shown that in a duplex steel with a chromium content exceeding 27% very god properties will be obtained if one at the same time adds high Cu- and W- contents to the material and also a high N-content. Accordingly, it has surprisingly shown that addition of high contents of the element W performs god impact strength at low temperatures. A high content of W in combination with a high content of N performs furthermore an outstanding resistance to crevice corrosion in chloride environments; the effect of W on the pitting- and crevice corrosion properties is also surprisingly great. In order to obtain an adequate effect an addition of at least 2% W is requested. Simultaneously high contents of the elements Mo and W have to be avoided, however, up to 4%> W can be added if Mo is limited to below 2%, preferably below 1%>. In order to obtain god corrosion- and impact strength properites and, but at the same time avoiding precipitation of intermetallic phase, the relation %>Mo+0.5%>W<3.52 should be fulfilled, preferably it should be %Mo+0.5%W<3. The addition of the element Cu has also surprisingly in this material shown to slow down the precipitation of intermetallic phase at slowly cooling. This also implies that necessary hot working such as forging can be performed easier without risk for cracking caused by high contents of intermetallic phase in the material. In order to obtain this effect, an addition of at least 0.5% Cu is requested, preferably at least 1.5%. If %Mo+0.5%W> it is requested that %>Cu>l .5 in order to obtain the best hot workability in the material. In order to obtain god corrosion properties the relation %>Cr+3.3(%>Mo+0.5%>W)+16%>N should exceed 40 in the weakest phase. For simultaneously god pitting- and crevice corrosion resistance at the same time the elements W should exceed 2% and N should exceed 0.30%). An optimum resistance to pitting corrosion will be obtained if the PRENW number is in the range of 41-44. Furthermore, for optimum resistance to crevice corrosion PRENW should preferably be in the range of 43-44. With the purpose to obtain a god structural stability at the same time, copper will be added to the material. However, copper an unfavourable effect on the intergranular corrosion in combination with a high content of Mo. In order to optimize the material regarding the intergranular corrosion a high content of Cu should therefore be combined with a low content of Mo. In order to ensure god pitting corrosion properties one should of this reason add high contents of W. In order to obtain god resistance in basic environments the Cr-content in the austenite phase should be at least 28%>.

Claims

1. Ferrite-austenite steel alloy with a content of ferrite of 30-70%>, balance austenite with god warm workability, high resistance to crevice corrosion and god structural stability, characterized in that it contains in weight-%o C maximum 0.05%, Si maximum 0.8%, Mn 0.30-4.0%, Cr 27.0-35.0%, Ni 3.0-10.0%, Mo 0-3.0%, N 0.30-0.55%, Cu 0.5-3.0%, W 2.0-5.0%, S maximum 0.010%, balance Fe and normally occurring steelmaking additions for deoxidization and hot ductility.

2. Steel alloy according to claim 1, characterized in that the content of ferrite is between 35-55%), balance austenite.

3. Steel alloy according to claim 1, characterized in that the content of Mo is 0-2.0%, preferably Mo 0-1.0%.

4. Steel alloy according to any of the preceding claims, characterized in that the content of W is 2.0-4.0%, preferably 3.0-4.0%.

5. Steel alloy according to claim 1, characterized in that the relation %Mo+0.5%W<3.52 is fulfilled.

6. Steel alloy according to claim 1, characterized in that the relation %Mo+0.5%W<3 is fulfilled.

7. Steel alloy according to claim 1, characterized in that the content of Cu is 1.5-3.0%.

8. Steel alloy according to claim 1, characterized in that the relation %>Mo+0.5%>W<3.52 is fulfilled at the same time as the content of Cu not exceeds 1.5%.

9. Steel alloy according to claim 1, characterized in that the relation %Cr+3.3(%Mo+0.5%W)+16N exceeds 40.

10. Steel alloy according to claim 1, characterized in that the relation %Cr+3.3(%>Mo+0.5%W)+16N exceeds 40 both in the ferrite- and the austenite phase.

11. Steel alloy according to claim 10, characterized in that the relation 41<%Cr+3.3(%Mo+0.5%W)+16N<44 is fulfilled.

12. Steel alloy according to claim 2, characterized in that it contains in weight- % C maximum 0.05%, Si maximum 0.8%, Mn 0.30-4.0%, Cr 27.0-35.0%, Ni 3.0- 10.0%, Mo 0-2.0%, N 0.30-0.40, Cu 0.5-3.0, W 3.0-4.0%, balance Fe and normally occurring steelmaking additions for deoxidization and hot ductility and the relations %Mo+0.5%W<3.52 and 41<%Cr+3.3(%Mo+0.5%W)+16N<44 are fulfilled.

13. Steel alloy according to claim 4, characterized in that the relation 41<%Cr+3.3(%Mo+0.5%W)+16N<44 is fulfilled.

14. Steel alloy according to claim 1, characterized in that the content of Cr in the austenite phase is at least 28%>, preferably at least 29%>.

15. Steel alloy according to claim 13, characterized in that the relation 43<%Cr+3.3(%Mo+0.5%W)+16N<44 is fulfilled.