Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum

Definitions

• the present invention relates to a heat-resistant stainless steel.
• the heat-resistant stainless steel is suitable for casting.
• One use of the stainless steel is as a component for a turbocharger, for example the housing of a turbocharger.
• a turbocharger for example the housing of a turbocharger.
• turbochargers In vehicles using turbocharged combustion engines, there is a drive for increasing the operating temperatures of turbochargers to improve engine efficiency. As of the time of writing, operating temperatures often exceed 1050 °C and are projected to rise even further.
• traditional cast stainless steel grades such as 1.4837, 1.4848 and 1.4849 are frequently used to make housings of the hot section of the turbocharger. These steels must withstand oxidation, as well as repeated heated and cooling cycles during which large temperature gradients can develop in the component.
• the present invention provides a steel material including, in mass percent: carbon: 0.35 to 0.6%; silicon: 0.35 to 2.0%; manganese: 6.0 to 21.0%; nickel: 7.0 to 16.0%; chromium: 17.0 to 26.5%; tungsten: 3.5% or less; niobium: 2.2% or less; nitrogen: 0.1 to 0.75%; copper: 4.0% or less; molybdenum: 4.0% or less; sol.
• This steel provides enhanced oxidation resistance compared to prior art steels of this type along with a good balance of microstructural stability at high temperature, strength and cost.
• Such an alloy is suitable for use as a cast austenitic stainless steel for turbocharger applications at temperatures up to 1050 °C and even beyond.
• the steel fulfils the following equation: W Ni + 4W Nb >10. Such a steel has reduced cost for a given microstructural stability.
• the steel fulfils the following equation: 0.065W Cr + 0.047W Cu + 0.008W Fe + 0.017W Mn + 0.327W Mo + 0.484W Nb + 0.12W Ni + 0.013W Si + 0.363W W + 0.325 W V ⁇ 4.6, preferably ⁇ 4.5, more preferably ⁇ 4.0, even more preferably ⁇ 3.5 wherein W Cr , W Mn , W Mo , W Ni , W Si , W W , W Nb , W V and W Cu are the amounts of chromium, manganese, molybdenum, nickel, silicon, tungsten, niobium, vanadium and copper in the steel. Such a steel has lower cost.
• the steel fulfils the following equation: -0.3180 W C - 0.0415 W Si + 0.0111 W Mn - 0.0081 W Ni + 0.0313 W Cr + 0.0252 W W + 0.0222 W Nb - 0.1470 W N -0.0009 W Cu + 0.0230 W Mo – 0.537 ⁇ 0.10, preferably ⁇ 0.05, most preferably ⁇ 0.03
• Such as steel has reduced sigma stability.
• the steel fulfils the following equation: 0.836W C + 0.063W Cr + 0.181W Cu + 0.509W Mn + 0.194W Mo + 20.0W N - 0.606W Nb - 0.143W Ni - 0.437W Si + 0.06W W + 51.429 ⁇ 58.8, preferably 0.836W C + 0.063W Cr + 0.181W Cu + 0.509W Mn + 0.194W Mo + 20.0W N - 0.606W Nb - 0.143W Ni - 0.437WSi + 0.06WW + 51.429 ⁇ 59.0, more preferably 0.836W C + 0.063W Cr + 0.181W Cu + 0.509W Mn + 0.194W Mo + 20.0W N - 0.606W Nb - 0.143W Ni - 0.437W Si + 0.06W W + 51.429 ⁇ 60.0.
• Such as steel is stronger.
• the steel fulfils the following equation: 10 –18 (0.0263 W Cr + 0.1770 W Cu + 0.0399 W Mn + 0.0278 W Mo + 0.1070 W Ni – 0.0817W Si + 0.0342 W W + 0.0051 W Nb ) ⁇ 2.1e-18, preferably ⁇ 2.2e-18, even more preferably ⁇ 3.0e-18
• Such a steel has higher oxidation resistance.
• the steel fulfils the following equation: 10 –4 (-5.35W C + 0.635W Si - 0.231W Mn + 0.583W Ni + 1.5W Cr - 0.452W W – 11.4W N + 1.1W Cu + 12.3) ⁇ 0.0050, preferably ⁇ 0.0055, preferably ⁇ 0.0060.
• Such a steel has enhanced ability to form a protective chromia scale.
• the steel includes 0.2 wt% or less niobium, preferably 0.1 wt% or less niobium. This reduces cost.
• the steel includes 7.5 wt% or more nickel, preferably 9.0 wt% or more nickel and more preferably 10.0 wt% or more nickel, most preferably 12.0 wt% or more nickel. This improves oxidation resistance.
• the steel includes 14.5 wt% or less nickel, preferably 14.0 wt% or less nickel, more preferably 13.5 wt% or less nickel, even more preferably 13.0 wt% or less nickel, optionally 10.0 wt% or less nickel. This reduces the cost of the steel.
• the steel includes 1.75 wt% or less silicon, preferably 1.6 wt% or less silicon, more preferably 1.0 wt% or less silicon, even more preferably 0.7 wt% or less silicon, even more preferably 0.6 wt% or less silicon and most preferably 0.5 wt% or less silicon.
• the steel includes 6.5 wt% or more manganese, preferably 7.0 wt% or more manganese, more preferably 8.0 wt% or more manganese, preferably 9.0 wt% or more manganese, more preferably 9.5 wt% or more manganese and even more preferably 10.5 wt% or more manganese.
• the steel has improved strength.
• the steel includes 17.5 wt% or less manganese, preferably 15.5 wt% or less manganese, more preferably 15.0 wt% or less manganese, even more preferably 13.0 wt% or less manganese, even more preferably 9.0 wt% or less manganese and most preferably 8.0 wt% or less manganese.
• Such as steel has improved creep resistance and oxidation resistance.
• the steel includes 19.0 wt% or more chromium, preferably 19.5 wt% or more chromium and more preferably 22.0 wt% or more chromium. Such a steel has improved oxidation resistance.
• the steel includes 25.5 wt% or less chromium, preferably 21.0 wt% or less. Such a steel has reduced cost. In an embodiment the steel includes 3.0 or less tungsten, preferably 0.5 wt% or less tungsten, more preferably 0.2 wt% or less tungsten, most preferably 0.1 wt% or less tungsten. Such a steel has reduced cost and fewer brittle phases. In an embodiment the steel includes 0.5 wt% or more molybdenum. Such a steel has high strength and creep resistance. In an embodiment the steel includes 3.0 wt% or less copper, preferably 2.2 wt% or less copper, more preferably 0.5 wt% or less copper.
• Such a steel has increased high temperature strength and ductility.
• the steel includes 3.0 wt% or less molybdenum, preferably 2.0 wt% or less molybdenum, more preferably 1.25 wt% or less molybdenum, even more preferably 1.1 wt% or less molybdenum, even more preferably 0.5 wt% or less molybdenum, most preferably 0.1 wt% or less molybdenum.
• Such a steel has lower cost and fewer brittle phases.
• the steel includes 2.0 wt% or less niobium, preferably 1.5 wt% or less niobium, more preferably 1.0 wt% or less.
• the steel has reduced cost and improved resistance to intermetallics formation.
• the steel includes 0.5 wt% or more niobium, preferably 0.75 wt% or more niobium.
• Such a steel has improved long term microstructural stability.
• the steel includes 0.7 wt% or less nitrogen, preferably 0.65 wt% or less nitrogen, more preferably 0.6 wt% or less nitrogen , even more preferably 0.5 wt% or less nitrogen, even more preferably 0.4 wt% or less nitrogen, more preferably 0.35 wt% or less nitrogen, most preferably 0.3 wt% or less.
• Such a steel substantially avoids the risk of porosity developing and lowers the risk of the formation of brittle chromium and molybdenum nitrides.
• the steel includes 0.15 wt% or more nitrogen, preferably 0.20 wt% or more nitrogen, more preferably 0.25 wt% or more nitrogen, even more preferably 0.30 wt% or more nitrogen.
• Such a steel has higher strength.
• the steel includes 0.4 wt% or more silicon, preferably 0.45 wt% or more silicon, most preferably 0.5 wt% or more silicon. Such a steel has better form filling and deoxidation properties.
• the steel includes 0.25 wt% copper, preferably 0.5 wt% or more copper, more preferably 1.0 wt% or more copper, even more preferably 1.5 wt% or more copper, most preferably 1.75 wt% or more copper.
• Such a steel has enhanced ability to form protective chromia scale.
• the steel includes 0.4 wt% or more carbon, preferably 0.45 wt% or more carbon.
• Such a steel has increased high temperature strength and creep resistance.
• the steel includes 0.55 wt% or less carbon, preferably 0.51 wt% or less carbon, more preferably 0.5wt% or less carbon.
• Such a steel has improved creep resistance and ductility.
• W Ni + 6W Cu ⁇ 10.1 preferably W Ni + 6W Cu ⁇ 10.5.
• Such a steel has further improved oxidation resistance.
• Such as steel has even lower nitride volume fraction.
• the steel includes 0.3 wt% or less vanadium, preferably 0.2 wt% or less vanadium. This improves ductility.
• a cast product is comprised of the above steel. The steel is optimised for casting of components for use in high temperature applications.
• a turbocharger housing is comprised of the above steel. The steel is well suited to such a purpose. The turbocharge may conveniently be cast. The term “consisting of” is used herein to indicate that 100% of the composition is being referred to and the presence of additional components is excluded so that percentages add up to 100% by weight.
• Figure 1 shows improvement in oxidation resistance of the present invention versus two prior art steel compositions
• Figure 2 shows improvement in tensile strength of the present invention versus three prior art steels
• Figure 3 shows improvement in microstructural stability of a steel of the present invention compared to a prior art steel
• Figure 4 shows Vickers hardness values for examples of the invention and comparative examples in the as-cast condition
• Figure 5 shows the microstructures of Examples after casting and stress relief treatment and aging at 800 °C for 140 h.
• Composition space and selection metrics A modelling-based approach used for the isolation of the present alloy with the aim of addressing at least some of the above issues is described here.
• This approach utilises a framework of computational materials models combined with machine learning to estimate design-relevant properties across a very broad compositional space.
• this alloy design tool allows the so- called inverse problem to be solved; identifying optimum alloy compositions that best satisfy a specified set of design constraints.
• the first step in the design process is the definition of an elemental list along with the associated upper and lower compositional limits.
• the compositional limits for each of the elemental additions considered in this invention – referred to as the “alloy design space” - are detailed in Table 1. These limits were selected by the inventor on the basis of the explanations given below.
• Nickel stabilises the austenite phase and improves creep resistance due to its slow diffusivity in the austenite matrix phase. In addition, it promotes oxidation resistance by having a positive effect on chromium activity and diffusivity.
• high nickel additions are prohibitively expensive and severely lower the nitrogen solubility in the melt, severely limiting the achievable yield strength.
• High levels of nickel also stabilise brittle nitrides.
• Nickel is present in at least 7.0% or more in order to achieve a fine balance between oxidation, creep, chromium activity and diffusivity and microstructural stability whilst being limited to 16.0 wt% or less in order to meet the cost, strength and microstructural stability criteria. If the balance of properties desired leans more towards lower cost, the maximum amount of nickel is limited to 14.5 wt% or less, preferably 14.0 wt% or less, more preferably 13.5 wt% or less or even 13.0 wt% or less. Nickel can be present in even lower amounts. However for amounts of nickel below 10.0 wt% (which is a possible upper limit), the inventors have found that copper additions are preferred, as described below, in order to maintain oxidation resistance.
• oxidation resistance is the primary aim for the alloy
• nickel additions are increased accordingly, for example to include nickel in an amount of at least 7.5 wt% or more, or 9.0 wt% or more or even 10.0 wt% or more.
• Such steels have improved oxidation resistance at the expense of increased cost.
• Even higher oxidation resistance is achieved in those embodiments where nickel is present in an amount of 12.0 wt% or more.
• Chromium provides solid solution strengthening and improves creep resistance and is also the main source of oxidation resistance, forming a protective chromia scale which acts as a barrier to further oxidation at high temperatures. To maintain sufficient oxidation resistance, its lower range is limited to 21.5 wt% or more,.
• chromium Even better oxidation resistance is available at higher levels of chromium, so preferred minimum amounts of chromium are 22.0 wt% or more, preferably 22.5 wt% or more, even more preferably 24.0 wt% or more, or even 24.5 wt% or more chromium where high oxidation resistance is important.
• high chromium additions stabilise ferrite and also encourage the formation of the detrimental sigma phase, making it stable even at very high temperatures where it precipitates more rapidly. The presence of sigma phase severely reduces alloy ductility and oxidation resistance.
• High chromium additions also encourage the formation of chromium nitride precipitates at high temperatures. Their presence also reduces alloy ductility and oxidation resistance.
• chromium is limited to 26.5 wt%, preferably to 25.5 wt% or less and even more preferably to 21.0 wt% or less for applications where alloy ductility is particularly important.
• Silicon provides solid solution strengthening, improves alloy castability and deoxidises the melt. As an element which is able to form a protective oxide scale much like chromium it also improves oxidation resistance via the “third element effect”. The presence of silicon also increases fluidity and in combination with deoxidising properties thereby improves the quality of castings. This latter effect is not given an associated merit index, but contributes to the requirement to have a minimum amount of silicon of 0.35% or more described below.
• silicon content is therefore limited to 2.0 wt%, more preferably to 1.75 wt% or less or 1.6 wt% or less, even more preferably to 1.0 wt% or less, optionally 0.7 wt% or less or 0.6 wt% or less and most preferably to 0.5 wt% or less.
• Silicon is required for better form-filling and deoxidation of the melt so the lower bound is 0.35 wt% or more, preferably 0.4 wt% or more, more preferably 0.45wt% or more or even 0.5 wt% or more silicon.
• Aluminium behaves in a similar way to silicon and deoxidizes the steel and helps with oxidation resistance by forming protective oxides scale for chromium containing alloys. However, high concentrations of aluminium in the presence of nitrogen in the melt result in the formation of hard and coarse aluminium nitrides which adversely affect ductility and fatigue limit. Accordingly an upper limit of the Al content is 0.1%, and more preferably is 0.050%.
• the "Al” content means “acid-soluble Al", that is, the content of "sol. Al”.
• Carbon provides solid solution strengthening, stabilises the austenite phase and forms the characteristic network of interdendritic eutectic carbides which give the alloys in the present invention its characteristic creep resistance and high-temperature strength. Because of the above considerations and in view of the creep merit indices calculated during alloy development, its minimum content is limited to 0.35 wt% or more, preferably to 0.4 wt% or more, more preferably to 0.45 wt% or more. However, excessive carbon additions result in unfavourably large volume fractions of interdendritic carbides which adversely affects the creep resistance and ductility.
• the carbon content is therefore limited to 0.6 wt% or less, preferably to 0.55 wt% or less and even more preferably to 0.51 wt% or less or even more preferably 0.5 wt% or less.
• carbon is 0.45wt% or less or 0.4wt% or less.
• Manganese provides solid solution strengthening, stabilises the austenite phase and increases the nitrogen solubility without excessively stabilising various brittle nitride phases. Manganese also increases the work hardening rate in austenitic steels which has a beneficial effect on increasing fatigue life, especially in the low-cycle regime. Because these steels may experience fatigue because of thermal cycling, relatively higher Mn than sometimes used is specified.
• Molybdenum and tungsten provide solid solution strengthening and creep resistance and increase the solubility of nitrogen. However, excessive additions raise the cost of the alloy and tend to stabilise various brittle phases such as sigma and laves phase. Their content is therefore limited to 4.0 wt% or less in the case of molybdenum and 3.5 wt% in the case of tungsten.
• both molybdenum and tungsten are limited to 3.0 wt% each or less. In one embodiment the sum of these elements is 3.0 wt% or less. In certain embodiments where cost and absence of brittle phases is paramount, tungsten is limited to 0.5 wt% or less or even 0.2 wt% or less or even 0.1 wt% or less. Molybdenum may be limited to 2.0 wt% or less, 1.25 wt% or less, 1.1 wt% or less, or 0.5 wt% or less to reduce cost and reduce the prevalence of brittle phases. A small mandatory amount of molybdenum may be advantageous where high strength and creep resistance is required, for example 0.5 wt% or more.
• Molybdenum is an optional element.
• the alloy contains 0.1 wt% or less molybdenum.
• Tungsten is an optional element.
• Nitrogen provides solid solution strengthening and stabilises the austenite phase. The lower nitrogen content is therefore limited to 0.1 wt% or more, more preferably to 0.15 wt% or more. In some embodiments the amount of nitrogen is at least 0.20 wt% or 0.25 wt% or even 0.3 wt% or 0.4 wt% or more yet further to increase strength.
• excessive additions stabilise the formation of brittle chromium and molybdenum nitrides at high temperatures. They may also cause processing issues whereby the nitrogen solubility in the melt is exceeded.
• the upper nitrogen content is therefore limited to 0.75 wt% or less, more preferably to 0.6 wt% or less, even more preferably to 0.5 wt% or less. An even lower level of 0.4 wt% or less is also possible, or even 0.35 wt% or less or 0.3 wt% or less. Copper: weakly stabilises the austenite phase, provides creep resistance and improves oxidation resistance by increasing Cr activity and diffusivity.
• Cu-rich liquid films at temperatures near 1000 °C where the material is otherwise fully solid. Liquid films catastrophically reduce high temperature ductility and strength. Cu also stabilises various carbide and nitride phases, promoting their formation from the melt which leads to a coarse microstructure adversely affecting mechanical properties.
• the copper content is therefore limited to 4.0 wt% or less, more preferably to 3.0 wt% or less or even 2.5 wt% or less copper or 2.2 wt% or less copper of 0.5 wt% or less copper.
• W Ni + 6W Cu ⁇ 10 W Ni + 6W Cu ⁇ 10.1 or even W Ni + 6W Cu ⁇ 10.5 to ensure high oxidation resistance.
• copper is present in an amount of 0.25 wt% or more, preferably 0.5 wt% or more or even 1.0 wt% or more for improved oxidation resistance. Further improvements in oxidation resistance result from embodiments with 1.5 wt% or more copper and even further improvements result in those embodiments with 1.75 wt% or more copper.
• Niobium significantly increases creep resistance and high temperature strength by forming hard and stable interdendritic carbides and promotes the formation of Z phase at the expense of chromium- molybdenum nitrides.
• Z phase generally has a finely dispersed morphology and coarsens slowly compared to the quickly coarsening chromium-molybdenum nitrides which often precipitate as brittle pearlite-like cellular colonies.
• excessive additions are expensive, stabilise the ferrite phase and promote formation of brittle intermetallics (sigma, G phase).
• niobium content is therefore limited to 2.2 wt% or less, preferably 2.0 wt% or less, more preferably to 1.5 wt% or less or even 1.0 wt% or less.
• niobium in mandatory for instance where long term microstructural stability is desired in which case niobium is present in an amount of 0.5 wt% or more, preferably 0.75 wt% or more.
• niobium is not present at all or only represent in an amount of 0.2 wt% or less or 0.1 wt% or less or even 0.02 wt% or less to keep the cost of the steel low.
• Niobium is an optional element.
• Phosphorus Phosphorous is an impurity.
• Sulphur Sulphur is an impurity. Sulphur segregates at the grain boundaries, decreases the SSC resistance of the steel. Because it lowers the cohesive strength of grain boundaries it can also drastically decrease the ductility and strength of the material. However, sulphur is also known to improve machinability. Sulphur content is 0.3% or less.
• a preferable Sulphur content is 0.05% or less, and more preferably is 0.01% or less.
• a preferable Sulphur content is 0.005% or less, and more preferably is 0.003% or less.
• the Sulphur content is as low as possible.
• a higher content of these elements is desirable as it makes alloys more easily machinable.
• their combined content is preferably more than 0.005% but less or equal to 0.04%.
• Calcium is added to the melt and tends to favour the formation of soft oxide inclusions over hard ones typically formed by Si and Al, both of which may be present in the melt. It also favourably modifies sulphide inclusions. Both mechanisms contribute to better machinability of the alloys.
• the calcium content is preferably more than 0.005% but less or equal to 0.04%.
• the alloys in the invention may contain any combination of minor alloying elements not exceeding 1.0 wt% in total sum.
• Minor alloying elements include rare earth elements (REM), particularly lanthanides (element numbers 57 to 71 including lanthanum) for a more favourable carbide morphology, hafnium, zirconium and boron for their grain boundary strengthening effect, yttrium for improved oxidation resistance, as well as titanium (preferably less than 0.1 wt%) and vanadium (0.3 wt% or less, preferably 0.2 wt% or less) for improving the coarsening resistance of MC carbides (where M is primarily Nb partly substituted with Ti, V or Zr), boron may be helpful in strengthening grain boundaries and may optionally be present in an amount of 0.1 wt% or less, and trace amounts of calcium and magnesium may be present as a by-product of slag treatment or deoxidation (up to 0.1 wt% each).
• REM rare earth elements
• lanthanides element numbers 57 to 71 including lanthanum
• hafnium, zirconium and boron for their grain boundary strengthening
• the alloy may contain small amounts of incidental impurities of any element not listed in the section above.
• the alloys in the invention may have a lower cost and therefore the nickel content needs to be reduced. Because nickel has a crucial role in improving oxidation resistance, it needs to be substituted by other elements with a similar role. The most effective of these elements is copper – according to the Cr diffusivity index even small amounts are improve oxidation resistance appreciably. Therefore preferably: W Ni + 6W Cu > 10 wt%. More preferably W Ni + 6W Cu > 12 wt% and most preferably W Ni + 6W Cu > 15 wt%. In some instances, the alloys in the invention may have a lower cost and therefore the nickel content needs to be reduced.
• the microstructure of the steel is mainly austenite reinforced with eutectic carbides.
• the second step relies upon thermodynamic calculations used to calculate the phase diagram and thermodynamic properties for a specific alloy composition. Often this is referred to as the CALPHAD method (CALculation of PHAse Diagrams).
• a third stage involves isolating alloy compositions which have the desired properties as calculated in the second step. The candidate alloys in the investigated composition space were selected based on the various merit indices as detailed below.
• Alloys of the invention in particular were designed to achieve a good balance of the following merit indices: strength, Cr diffusivity, nitride fraction, Cr activity, cost and sigma fraction, which are all described below. Some alloys in the space will achieve a slightly different balance to others, with some favouring very high oxidation resistance, for example, at the expense of higher than optimal cost. As can be seen from the examples in table 1, steels falling within the present invention have good strength, Cr diffusivity, nitride fraction, Cr activity, cost and sigma fraction merit indices compared the comparative example steels which do not meet the compositional requirement. Improved fatigue resistance is also expected because of the relatively large amounts of manganese and improved castability because of the relatively large amount of silicon.
• the strength merit index reflects the yield strength of the alloy at room temperature. A higher value is better as it indicates that thermal strains during repeated heating and cooling are less likely to cause plastic deformation, which leads to longer service life of components made of the materials in with higher strength. It is based on two assumptions. First, grain boundary strengthening is constant across the composition space because the as-cast grain size varies weakly with composition. Second, precipitation strengthening depends only on the volume fraction of carbides and nitrides. The variation in size distribution is neglected due to complex solidification conditions and is assumed to be constant. The variation in yield strength is therefore dominated by solid solution strengthening.
• Equation for the strength merit index is Where x i is the mole fraction of element i in austenite, S i is its strengthening coefficient (taken from various publications, e.g. Ghosh, G., & Olson, G. B. (1994). Kinetics of FCC ⁇ BCC heterogeneous martensitic nucleation—I. The critical driving force for athermal nucleation.
• ⁇ 0 is the Taylor factor (value 3.06)
• G is the shear modulus of steel (74 GPa)
• b is the length of the Burgers vector (2.5 nm)
• r p is the radius of precipitates (assumed 1 ⁇ m)
• ⁇ p is their volume fraction.
• the constant 300 MPa comes from grain boundary and other strengthening contributions.
• Strength index 0.836W C + 0.063W Cr + 0.181W Cu + 0.509W Mn + 0.194W Mo + 20.0W N - 0.606W Nb - 0.143W Ni - 0.437W Si + 0.06W W + 51.429 (1)
• a composition which gives a strength merit index of 58.8 or above is preferred as this ensures that the alloy has sufficient strength for the designed task. More preferably the alloy has a strength merit index of 59.0 or above or even 60.0 or above as this indicates even higher strength.
• the chromium diffusivity index reflects the ability of the alloy to form a protective chromia scale or reform it after spallation.
• chromium concentration directly underneath it is somewhat depleted relative the bulk of the metal.
• the degree of depletion is particularly severe when chromium diffusivity is low. After spallation, the oxidation kinetics depends on the composition of this depleted layer. If the degree of depletion is high, chromia scale may not be able to form and various porous and non-adherent oxides may form instead. It is only when oxygen has penetrated through this depleted layer that the local chromium concentration is high enough to start forming chromia again. Conversely, when chromium diffusivity is high, the degree of chromium depletion is low.
• Chromium diffusivity can be tuned as it depends on the composition of austenite. Nickel and copper in particular are known to increase it. The diffusivity of chromium in austenite can be obtained with thermodynamic calculations.
• Chromium diffusivity index 10 –18 (0.0263 WCr + 0.1770 WCu + 0.0399 WMn + 0.0278 WMo + 0.1070 W Ni – 0.0817W Si + 0.0342 W W + 0.0051 W Nb ) (2)
• a preferred steel achieves a chromium diffusivity index value of at least 2.1e-18 or more, preferably 2.2e-18 or more and even more preferably 3.0e-18 or more.
• the nitride fraction index is a measure of the volume fraction of M2N nitrides stable at high temperature. The volume fraction is determined using thermodynamic calculations.
• a low nitride fraction index is an indicator of microstructural stability and ductility.
• a steel according to the present invention preferably achieves a nitride volume fraction index volume fraction of 0.058 or less, more preferably 0.055 or less or even 0.050 or less and most preferably 0.040 or less.
• the cost index reflects the cost of raw materials (in GBP/kg) needed to produce an alloy. It is assumed that a master alloy of iron and a given alloying element is used in each case, where the fraction of alloying element varies. The cost is then a weighted sum of the master alloys and the remaining pure iron, accounting for the iron contained in the master alloys.
• the index i stands for alloying element
• n stands for the number of alloying elements
• p i stands for the weight fraction of alloying element i in the master alloy (master alloy ‘purity’ – the remainder is assumed to be iron)
• w i is the weight fraction of alloying element i in the alloy
• c i is the cost of the master alloy of element i.
• Index Fe stands for iron.
• the steel has a cost of 4.6 GBP kg -1 or less , preferably 4.5 GBP kg -1 or less, more desirably 4.2 GBP kg -1 or less, more desirably 4.1 GBP kg -1 or less, more desirably 4.0 GBP kg -1 or less, even 3.5 GBP kg -1 or less.
• the sigma fraction index is a measure of the volume fraction of the sigma phase at 800 °C in the alloy when the alloy is in thermodynamic equilibrium. To prevent excessive loss of ductility the sigma fraction index should be low.
• the equation estimates sigma fraction as a function of composition and is derived from regression analysis of thermodynamic calculations (and therefore is indicative of trends, rather than reliable at predicting absolute volume fraction) is:
• Sigma phase fraction index -0.3180 W C - 0.0415 W Si + 0.0111 W Mn - 0.0081 W Ni + 0.0313 W Cr + 0.0252 W W + 0.0222 W Nb -0.1470 W N -0.0009 W Cu + 0.0230 W Mo – 0.537
• a lower number (even negative) for sigma phase fraction is indicative of lower sigma stability at 800 °C.
• the sigma phase fraction index is 0.1 or less, preferably 0.05 or less, more preferably ⁇ 0.04 and most preferably 0.03 or less.
• the creep merit index reflects the hypothesis that much like in nickel superalloys, the microstructure is assumed to be constant during creep and creep-type deformation is confined to the matrix (austenite) phase. Dislocation segments rapidly become pinned at the carbide interfaces. The rate-controlling step is then the escape of trapped configurations of dislocations from austenite- carbide interfaces, and it is the dependence of this on local chemistry – in this case composition of the austenite phase – which gives rise to a significant influence of alloy composition on creep properties.
• the governing equation is Where x i is the molar fraction of alloying element i in the austenite phase and D i is its diffusivity. Because diffusivity changes with composition this leads to a complex relationship. However the results were used to help determine the elemental ranges of the alloy.
• the chromium activity index reflects the ability of the alloy to maintain a protective chromia scale and resist oxidation at high temperature. According to Wagner’s theory of oxidation, the oxidation rate of chromia-forming alloys is inversely proportional to the activity of chromium in the alloy under the given conditions. Therefore, the higher the chromium activity, the slower the oxidation.
• Chromia scale typically spalls once it reaches a specific thickness as stresses at the metal-oxide interface grow with the oxide thickness and typically do not vary with alloy composition. This means that slower- growing scale will last longer before spalling, which minimises mass loss.
• Chromium activity typically scales with chromium concentration in austenite, but is also quite sensitive to the presence of other elements. For example, nickel and copper are known to appreciably increase chromium activity. Chromium activity in austenite can be obtained with thermodynamic calculations.
• Chromium activity index 10 –4 (-5.35W C + 0.635W Si - 0.231W Mn + 0.583W Ni + 1.5W Cr - 0.452W W – 11.4W N + 1.1W Cu + 12.3)
• a chromium activity index of 0.0050 or greater indicates an alloy with superior abilities to form a protective chromia scale and is therefore particularly desirable. More desirable is a chromium activity index of 0.0055 or greater and most preferably 0.0060 or greater. Examples and comparative examples Experimental testing was carried out on the alloys in the table below.
• Comparative example 1 is an experimental austenitic stainless steel while Comparative examples 3,4 and 28 are from a well- known austenitic heat-resistant DIN standard grade 1.4848 known for its good oxidation resistance but relatively low yield strength.
• Example 4.58 69.1 32
• Example 4.17 66.5 26
• Example 4.54 70.3 35
• Example 4.34 68.9 26
• Example 4.47 68.6 38 Example 4.28 69.2 12.5
• Example 4.49 68.0 14
• Example 4.43 68.7 12.8 Example 3.57 63.7 19.5
• Example 3.77 59.3 24.5 Example 30 0.50 0.5 7.0 10.0 25.0 0.0 0.0 0.30 1.0 1.0 0.0508 0.040 2.2E-18
• Figure 1 shows a comparison in oxidation resistance between examples of the invention and comparative examples.
• the data are from a cyclic oxidation test during which semicircular coupons of the materials were exposed to the atmosphere at 1000 °C in a box furnace. After an exposure step, they were air-cooled to room temperature and their weight was measured and recorded.
• Examples 27 and 33 and Comparative example 4 show stable mass gain indicating the formation of a protective chromia scale.
• Comparative example 1 shows rapid mass loss indicating the lack of formation of chromia scale, making this alloy unsuitable for applications at 1000 °C.
• Example 27 and 331 oxidise more rapidly than Comparative example 4, they are sufficiently oxidation resistant for applications at 1000 °C.
• Example 34 also shows a parabolic curve but the mass gain is higher than desired for this temperature.
• Figure 2 shows a comparison in yield strength over a range of temperatures between an example of the invention and three comparative examples. Room temperature tensile tests conformed to ASTM E8-16a while elevated temperature tests conformed to ASTM E21-17. Example 1 significantly outperforms all comparative examples. The example of the invention has the highest yield strength.
• Figure 3 shows a comparison in microstructure between Example 27 (left) and Comparative example 1 in the as-cast condition and after 25.5 h at 750 °C.
• Comparative example 1 In the as-cast state both alloys show lamellar nitrides. In Comparative example 1 these are mostly isolated regions with sparse large nitride colonies ( ⁇ 200 ⁇ m in diameter). In Example 27 the nitride colonies are interconnected, more evenly distributed and no large colonies are present. After 25.5 h at 750 °C, Comparative example 1 colonies dramatically increase in size and volume fraction, which adversely impacts ductility and creep resistance. In Example 27 they grow only somewhat, indicating better microstructural stability.
• Figure 4 shows Vickers hardness values (5 kg load, 10 s dwell time) for examples of the invention and comparative examples in the as-cast condition. Examples significantly outperform Comparative examples 1 and 4.
• Comparative example 1 Hardness of Comparative example 1 is comparable, but its oxidation resistance (Figure 1) is poor, making it unsuitable for high-temperature applications despite its high hardness.
• Figure 5 shows the microstructures of Examples 33 and 34 after casting and stress relief treatment (first column) and aging at 800 °C for 140 h. The microstructure of both Examples stays relatively unaffected, without signs of detrimental phases (cellular nitrides, coarse intermetallic phases), indicating good thermal stability.

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