The present invention relates to an austenitic heat and creep resistant stainless steel. It also relates to the use of this austenitic stainless steel, especially in oxidizing and carburizing environments. Further, the present invention relates to products made of this austenitic heat and creep resistant stainless steel.
S31008 is the most commonly used high temperature stainless steel for applications in the temperature range of 800-1050° C. It is however outperformed by S30815 both in regards to creep resistance and oxidation resistance in cyclic temperatures. It is however so that S31008 performs better in reducing or carburizing environments.
There is a strong need for a steel which has excellent high temperature oxidation and corrosion resistance in combination with very good mechanical like creep properties. Existing high temperature steels lack this combination of features. An object of the design and development of this alloy, an austenitic heat resistant stainless steel, is to produce a combination of high creep strength and good oxidation and corrosion resistance at high temperatures. Heading for a creep strength as excellent as that of S30815 and exceeding that of S31008 and S31400 and an oxidation resistance that is superior to that of the aforementioned commercial grades. This alloy is aimed at fulfilling the requirement of load bearing applications in oxidizing and carburizing environments.
It is an aim of the present invention to provide an austenitic stainless steel that combines excellent creep resistance and oxidation resistance, in isothermal as well as cyclic conditions, with good resistance, in particular in reducing environments. These are requirements often demanded of materials used in applications such as muffle furnaces.
The present invention relates to an austenitic heat resistant stainless steel, intended to replace the existing heat resistant stainless grades S30815 and S31008 for special high temperature applications like muffle and heat treatment furnaces where both oxidizing and reducing environments exist. By means of the invention an austenitic heat resistance stainless steel is provided having even better high temperature corrosion resistance and creep properties, being cost effective and easy to produce.
Surprisingly, it has been found that the austenitic stainless steel according to embodiments provides high temperature corrosion resistance and creep properties and is particularly suitable for high temperature applications in aggressive environments such as heat treatment equipment e.g. muffle furnaces. The austenitic stainless steel according to embodiments can be economically manufactured in a practical and environmentally sound manner.
According to embodiments an austenitic stainless steel has a composition utilizing the benefits of several alloying elements in order to combine good oxidation resistance through the formation of a tight and adhesive oxide layer and to, at the same time, be alloyed in a way to resist carburizing. Furthermore, it is designed in a way to have excellent creep resistance.
A well-defined and balanced alloying with carbon and nitrogen increases the creep strength through the formation of intra- and to some extent intergranular carbides and nitrides; so-called precipitation strengthening.
Chromium and silicon are added in order to have a high oxidation resistance. The amount is carefully balanced in order to not have a negative influence on the structure stability, since both these elements promote the formation of intermetallic and brittle phases such as sigma phases.
Rare earth metals, e.g. cerium has in earlier micro alloyed (MA) grades shown to have an excellent effect on the cyclic oxidation resistance. Thus, rare earth metals are added in an amount optimized to get the benefits of a more elastic and adhesive oxide layer. The amount, however, is limited since it has been shown that a surplus amount of rare earth metals is no longer beneficial for oxidation resistance and that it might cause clusters of oxide inclusions having a negative effect on mechanical properties and formability.
The nickel content is at a level known from other well-known commercially-available high temperature stainless steels but different from other high temperature grades micro alloyed with rare earth metals. Thus, the combination of the elements is utilized in a novel way. The nickel in combination with silicon promotes resistance to carburization.
Total of 15 test melts have been produced, see Table 1. The melts 1-8 are produced using a Mullite crucible and heated up to melt in an Ar protection atmosphere using a high frequency coil. The melt process takes about 10 to 15 min. Each melt is weighed about 600 grams. The melts are forged by using the hydraulic press Interlaken. An in-house software program has been developed that presses the ingot in short bursts to the desired thickness over a predetermined number of steps. The melt is heated to about 1250° C. between each step. The thickness of the final piece is 8 mm.
The test melts 9-15 are produced using a Leybold-Heraeus vacuum induction furnace having minimum pressure of 4×10-4 bar. The melts are tapped to metal mound in vacuum for producing 65 kg ingots. Heating up to 1250° C., the Fröhling rolling mill with furnaces on both sides is used to hot roll 38 mm thick slab to 10 and 6 mm thick plates, respectively. The rolling speed is 45 m/min. The rolling passes are 7 and 9 for 10 mm thick plate and for 6 mm thick plate, respectively.
Annealing temperature and holding time have been chosen to bring about a fully recrystallized austenite, proper hardness and grain size. Annealing temperature and holding time cover from 1100° C. to 1200° C. and from 0 min to 30 min, respectively.
Not all melts listed in Table 1 fulfill the basic idea behind this austenitic stainless steel to chemically combine main elements like chromium, nickel, silicon, nitrogen and REM of S31008 and S30815. Therefore, the chemical compositions obtained in above test melts result in a target and preferred chemical composition as described below in Table 2. The microstructure investigation, oxidation and carburisation tests, as well as creep test are performed in the most cases using the melts 7, 8, 14 and 15.
Production Process and Products
The austenitic stainless heat resistant steel as defined hereinabove and hereinafter is intended to be used for manufacturing of objects such as semis, plate, sheet, coil, strip, par, pipe, tube and/or wire. The methods used for manufacturing these products include conventional manufacturing processes such as, but not limited to, melting, refining, casting, hot rolling, cold rolling, forging, extrusion and drawing.
Microstructure
Environmental Testing
Mechanical Testing
Microstructure
Environmental Testing
The carbon activity ac is calculated according to:
ac=(K×pCH4)/p2H2 (1)
where pCH4 is the CH4 partial pressure, in the present case content of CH4 in the gas mixture. p2H2 is assumed to be very low, i.e. 0.00001, since the running gas flow and constant supply of CH4 will minimize H2 in the reaction. K is the equilibrium constant and is calculated using standard free energy of formation for the reaction ΔG at temperature T (K) of 1273K (1000° C.).
Mechanical Testing
Testing procedure as described in
Stress in MPa as a function of rupture time in h at 900° C.
One reference point is also given to S31008.
Rupture time increases with decreasing stress.
The rupture time of the austenitic stainless steel is similar to that of S30815.
The rupture strength for the austenitic stainless steel indicates a considerably higher level than that for S31008 at the same given rupture time.
Summary of Findings
According to embodiments the austenitic stainless steel is provided with improved heat resistance and corrosion resistance. According to an embodiment the austenitic stainless steel has finer grain size which improves oxidation and corrosion resistance as well as ductiliy. In a preferred embodiment the austenitic stainless steel has superior cyclic oxidation resistance. In a particular embodiment the steel has superior isothermal oxidation resistance. In a suitable embodiment the steel has superior carburization resistance. In a particularly preferred embodiment the steel has a creep resistance comparable with commercial grades.
In an embodiment the steel contains in weight % carbon <0.20, chromium 20.00-26.00, nickel 10.00-22.00, silicon 0.50-2.50, manganese <2.00, nitrogen 0.10—sulphur <0.015, phosphorus <0.040, rare earth metals 0.00-0.10, and the rest being iron (Fe) and inevitable impurities.
For the stainless steel, carbon is a strong austenite former that also significantly increases the mechanical strength by the formation of carbides. On the other hand, carbon also reduces the resistance to intergranular corrosion just due to the carbide formation, indicating the low carbon content. In embodiments described herein, the austenitic stainless steel contains <0.20 carbon in weight %. Keeping the carbon content <0.20%, preferably at least 0.05% but not more than 0.10% provides an optimization between austenite, mechanical strength and intergranullar corrosion resistance.
Chromium is the most important alloying element for the stainless steels. Chromium gives stainless steels their fundamental oxidation and corrosion resistance. All stainless steels have a Cr-content of at least 10.5% and the oxidation and corrosion resistance increases with increasing chromium content. In addition, chromium carbide and nitride improve mechanical strength. On the other hand, chromium promotes a ferritic microstructure. High chromium also contributes to intermetallic sigma phase formation. In a preferred embodiment the chromium content is at least 24.0 but not more than 26.0% for the austenitic stainless steel.
Nickel is present in all of the austenitic stainless steels since nickel promotes an austenitic microstructure. When added to a mix of iron and chromium, nickel increases ductility, high temperature strength, and resistance to both carburization and nitriding because nickel decreases the solubility of both carbon and nitrogen in austenite. On the other hand, high nickel is bad for sulphidation resistance. In a preferred embodiment the chromium content is at least 19.0 but not more than 22.0 w-% for the austenitic stainless steel.
Silicon improves both carburization and oxidation resistance, as well as resistance to absorbing nitrogen at high temperature. On the other hand, silicon tends to make the alloy ferritic, and promotes to intermetallic sigma phase formation. In a preferred embodiment the amount of silicon in the austenitic stainless steel is further controlled so that the silicon content is at least 1.20 but not more than 2.50 w-%.
Manganese is usually considered an austenitizing element and can also replace some of the nickel in the stainless steel. Manganese improves hot workability, weldability, and increases solubility for nitrogen to permit a substantial nitrogen addition. On the other hand, manganese is mildly detrimental to oxidation resistance, so it is limited to 2 w-% maximum in most heat resistant alloys. In a preferred embodiment the amount of manganese in the austenitic stainless steel is at least 0.50 but not more than 2.00 w-%.
Nitrogen is a very strong austenite former that also significantly increases the mechanical strength. Nitrogen tends to retard or prevent ferrite and sigma formation. On the other hand, high content nitrigen impairs toughness and causes embrittlement. In a preferred embodiment the amount of nitrogen in the austenitic stainless steel is at least 0.12 but not more than 0.20 w-%.
Sulphur and phosphorus are normally regarded as impurities. Sulphur is commonly below 0.010 w-%, while phosphorus is usually not specified. In a preferred embodiment the sulphur and phosphorus content in the austenitic stainless steel is not more than 0.010 w-% and 0.040 w-%, respectively.
Small amount of the rare earth elements (REM) are used singly or in combination to increase oxidation resistance by forming a thinner, tighter and more protective oxide scale in austenitic stainless alloys. Residual REM oxides in the metal may also contribute to creep-rupture strength. On the other hand, a surplus amount of rare earth metals might cause clusters of oxide inclusions having a negative effect on mechanical properties and formability. In a preferred embodiment the REM content in the austenitic stainless steel, mainly cerium and lanthanum, is at least 0.03 w-% but not more than w-%. In a particularly preferred embodiment the REM is cerium and is present in the range of 0.03% to 0.08 w-%
In a particular embodiment the N, C and rare earth metal (REM) contents in the austenitic stainless steel satisfy the relationship:
0.40% N+3×C+3×REM≤0.60% (2)
As described above the stainless steel comprises inevitable impurities. In an embodiment the austenitic stainless steel comprises one or more of the inevitable impurities contains in weight %:
Further embodiments relate to objects formed from the stainless steel according to embodiments of the present invention. In one embodiment is provided an object comprising the stainless steel according to any of the embodiments described herein.
The stainless steel according to embodiments of the present invention has a diverse range of uses. In one embodiment is provided a use of the stainless steel according to any of the embodiments described herein in the formation of an object. In a further embodiment the object formed and/or used according to embodiments is selected from the group consisting of plate, sheet, strip, tube, pipe, bar and wire. Further embodiments relates to uses of objects formed in heat treatment applications. Such object are apt for use in difficult environments. Thus, in an embodiment the object may be used in aggressive high temperature environments, which have oxidizing and reducing carburizing atmospheres, like in muffle furnace and in metal manufacturing process applications.
Number | Date | Country | Kind |
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20206232.9 | Nov 2020 | EP | regional |
This application is the United States national phase of International Application No. PCT/EP2021/080791 filed Nov. 5, 2021, and claims priority to European Patent Application No. 20206232.9 filed Nov. 6, 2020, the disclosures of which are hereby incorporated by reference in their entireties.
Filing Document | Filing Date | Country | Kind |
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PCT/EP2021/080791 | 11/5/2021 | WO |