GB2611082A - A stainless steel - Google Patents

Abstract

An austenitic stainless steel which comprises (by weight): 16-50 % nickel, 10-27 % chromium, 0.1-0.75 % carbon, 2.0-6.5 % soluble aluminium, 0-2.5 % silicon, 0-0.75 % manganese, 0-4.0 % copper, 0-4.0 % molybdenum, 0-3.0 % tungsten, 0-2.0 % niobium, 0-2.5 % vanadium, 0-0.15 % boron, 0-0.04 % calcium, 0-2.0 % zinc, 0-5.0 % cobalt, 0-0.05 % phosphorous, 0-0.1 % sulphur, 0-1.0 % total of lanthanides, hafnium, zirconium, yttrium, cerium and titanium, 0-0.04 % total of selenium, tellurium, antimony, bismuth and lead, with the balance being iron and impurities, and where the following equation is satisfied: 105 x (-0.0689Al + 1.41C – 0.0248Cr – 0.0662Cu – 0.0205Mn + 0.0333Mo – 0.182Nb + 0.0499Ni – 0.219Si – 0.97) ≥-7000. The steel can be used to form turbocharger components, for example a cast turbocharger housing.

Description

A Stainless Steel

Introduction

In vehicles using turbocharged combustion engines, there is a drive for increasing the operating temperatures of turbochargers to improve engine efficiency. Currently, traditional cast stainless steel grades such as 1.4837, 1.4848 and 1.4849 arc frequently used to make housings of the hot section of the turbocharger. These steels must withstand oxidation, as well as repeated heating and cooling cycles during which large temperature gradients can develop in the component. These conditions lead to thennomechanical fatigue where failure is caused not only by cyclic strain as in ordinary fatigue but also by the changing temperature and by surface oxidation, which assists in crack initiation and propagation. Materials for turbocharger housings therefore need to meet several requirements: reasonable yield strength retained to very high temperatures, good resistance to creep and oxidation, as well as a stable microstructure retained over a wide range of temperatures -all at a relatively low cost. Cast austenitic stainless steels reinforced with eutectic carbides are one of the few classes of materials satisfying all these constraints. This invention describes precisely such a type of austenitic stainless steel. The austenitic stainless steel of the invention is suitable for casting and/or for applications in a turbocharger, particularly the turbocharger housing.

The present invention provides a steel which consists of, in mass percent: nickel: 16 to 50%; chromium: 10 to 27%; carbon: 0.1 to 0.75%; sol. Al: 2.0 to 6.5%; silicon: 2.5% or less; manganese: 0.75% or less; copper: 4.0% or less; molybdenum: 4.0% or less; tungsten: 3.0% or less; niobium: 10% or less; vanadium: 2.5% or less; boron: 0.15% or less; calcium 0.04% or less; zinc: 2.0% or less; cobalt: 5,0% or less; phosphorous 0.05% or less; sulphur 0.1% or less; 1,0 wt.% or less in sum of lanthanide elements, hafnium, zirconium, yttrium, cerium and titanium; 0.04% or less in sum of selenium, tellurium, antimony, bismuth and lead; the remainder being iron and incidental impurities; wherein the following equation (1) is fulfilled: 0'(-0.0689Wm + 1.41Wc -0.0248Wc, -0.0662Wc11 -0.0205Wmfi + 0,0333Wm0 -0.182WN1, + 0.0499WN; -0.219W51-0.97)>-7000 (1) wherein WAI, WC, WCr, WN1n. WN10. WNi, Wi, WNb, and WeL are the amounts of aluminium, carbon, chromium, manganese, molybdenum, nickel, silicon, niobium, and copper in the steel.

This alloy provides reasonable yield strength retained to very high temperatures, good resistance to creep and oxidation, as well as a stable microstructure retained over a wide range of temperatures -all at a relatively low cost.

In some embodiments the steel consists of consists of 30 wt% or more nickel, preferably 35 wt% or more nickel, more preferably 37 wt.% or more nickel, even more preferably 40 wt.% or more nickel, most preferably 42 wt.% or more nickel. Such a steel in particular benefits from improved oxidation resistance as well as improved creep resistance.

In some embodiments the steel consists of 45 wt. % or less nickel, preferably 30 wt% or less nickel, preferably 26 wt% or less nickel. Such a steel has a lower cost.

In some embodiments the steel consists of 0.3 wt% or less vanadium, preferably 0,2 wt% or less vanadium. Such a steel has reduced cost.

In son-le embodiments the steel consists of 0.2 wt.% or more silicon, preferably 1.0 wt% or more silicon, more preferably 1.5 wt% or more silicon. Such a steel has increased oxidation resistance and strength.

In some embodiments the steel consists of 1.5 wt.% or less silicon, preferably 1.0 wt. % or less silicon. Such a steel has a increased ductility.

In some embodiments the steel consists of 0.1 wt% or more manganese, preferably 0.2 wt% or more manganese. Such a steel has reduced change of sulphur embrittlement.

In some embodiments the steel consists of 11,0 wt.% or more chromium, preferably including 12.0 wt% or more chromium, more preferably 16 wt% or more chromium and more preferably 22.0 wt% or more chromium. Higher chromium content helps maintain chromia scale at intermediate temperatures In some embodiments the steel consists of 16 wt% or less chromium, preferably 13 wt% or less chromium. Such a steel has improved aluminia scale fon-nation resulting in improved high temperature oxidation resistance, In some embodiments the steel consists of 1.5 wt.% or less tungsten, preferably 1.0 wt.% or less tungsten, more preferably 0.5 wt.% or less tungsten. Such a steel has reduced cost and lower intermetallic content.

In some embodiments the steel consists of 0.1 wt.% or more molybdenum, preferably 1.5 wt% or more molybdenum, more preferably 2.5 wt.% or more molybdenum. Such a steel has increase solid solution strengthening which contributes to creep strength.

In some embodiments the steel consists of 0.5 wt% or less molybdenum preferably 0.3 wt. % or less molybdenum. Such a steel has reduced chance of intermetallic phase formation.

In some embodiments the steel consists of 1.0 wt% or less niobium, preferably 0.2 wt% or less niobium, more preferably 0.01 wt% or less niobium. Such a steel has improved machinability. In some embodiments the steel consists of 0.6 wt% or more niobium, preferably 1.0 wt% or more niobium, more preferably 1.5 wt.% or more niobium, Such a steel has improved high temperature creep.

In some embodiments the steel consists of 0.03 wt% or more boron, preferably 0.07 wit% or more boron. Such a steel has creep resistance.

In some embodiments the steel consists of 0.1 wt% or more vanadium, preferably 0.3 wt% or more vanadium, more preferably 1.0 wt% or more vanadium, even more preferably 1.5 wt% or more vanadium, most preferably 1.9 wt.% or more vanadium. Such a steel has improved high temperature creep resistance.

In some embodiments the steel consists of 0.5 wt% or more tungsten Such a steel has improved creep resistance.

In some embodiments the steel consists of 0.4 wt% or more carbon, preferably 0.5 wt% or more carbon, more preferably 0.475 wt.% or more carbon. Such a steel has improved creep resistance. In some embodiments the steel consists of 0.6 wt.% or less carbon, preferably 0.4 wt% or less carbon, more preferably 0.3 wt% or less carbon, Such a steel has improved ductility.

In some embodiments the steel consists of 3.5 wt.% or more aluminium, preferably 4.0 wt.% or more aluminium, even more preferably 4.3 wt.% or more aluminium most preferably 4.5 wt.% or more aluminium. Such a steel has oxidation resistance.

In some embodiments the steel consists of 5.5 wt.% or less aluminium, preferably 5.0 wt.% or less aluminium, more preferably 3.5 wt.% or less aluminium Such a steel has improved ductility and creep resistance.

In some embodiments the steel consists of 0.2 wt.% or more copper, preferably 0.5 wt.% or more copper, more preferably 1.5 wt.% or more copper, most preferably 2.2 wt.% or more copper. Such a steel has improved castability.

In some embodiments the steel consists of 1.5 wt.% or less copper, preferably 0.5 wt.% or less copper, Such a steel has improved oxidation resistance.

In some embodiments the steel consists of 0.01 wt.% or less boron. Such a steel has improved ductility.

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.

The invention will be more fully described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows the dependence of oxidation index, chromium activity index and cost index on nickel and chromium levels; Figure 2 shows results of oxidation testing of three examples of the present invention and a comparative example; Figure 3 shows experimental results for Vickers hardness testing for two comparative examples and an example of the present invention; and Figure 4 shows the Larson-Miller parameter values experimentally measured for Example 10 (two points correspond to two separate creep tests) and Comparative example 1 (two points again correspond to two separate creep tests).

The Present Invention Typical heat-resistant steel grades form a continuous, inward-growing chromia scale upon high-temperature oxidation. The chromia scale is adherent and impermeable to oxygen and nitrogen, acting as a protective barrier and limiting the oxidation rate. However, chromia-forming steels are limited to maximum operating temperatures of 1050 °C. Because of its lower density compared to the base metal, the scale exerts stress on the oxide-metal interface. This stress increases with the scale thickness, eventually causing the scale to spall. This exposes the bare, chromium-depleted metal surface to the environment, which now oxidises even more rapidly. Because of the rapid scale thickening above 1050 °C the time needed for a spalling event decreases and the high resulting mass loss rate becomes unacceptably high. Scale spallation and the resulting mass loss is accelerated by rapid heating and cooling conditions typical of turbocharger housings. Such conditions increase the stress on the metal-oxide interface due to the mismatch in the thermal expansion coefficients between the two sides, For components exposed to cyclic oxidation above 1050 °C, chromia-forming materials arc therefore unlikely to provide adequate oxidation protection. A solution to this are alumina-forming austenitic steels (A FA). Alumina scale is much less permeable to oxygen and thickens at a considerably slower rate, offering suitable oxidation resistance at beyond 1050 °C. It is also more resistant to spallation than chromia scale even under cyclic oxidation conditions.

This patent describes a type of austenitic stainless steel forming a partial of fully continuous protective alumina scale with a relatively high yield strength over a range of temperatures as well as a creep resistance comparable to traditional chromia-forming austenitic grades at a comparable cost.

Table 1 illustrates the boundaries of the main alloying elements of the steel of the present invention. Below is an explanation of the effect of each element and several merit indices are described. The merit indices have been developed on the basis of experimental and/or theoretical modelling and have been used to narrow down the composition space of the present invention to define a composition with improved properties for a heat-resistant steel including reasonable yield strength retained to very high temperature, good resistance to creep and oxidation, as well as a stable microstmeture retained over a wide range of temperatures, all at a relatively low cost. A complex trade-off between different properties is made on the basis of the merit indices and results in the elemental limitations in the composition space. Figure I described below shows an example of how the cost, chromium activity and oxidation merit indices are influenced by vaiying nickel and chromium content. The complex interrelationships between merit index and composition are hard to visualise, but Table 2 described below shows the average influence of each element on the different merit indices over the range of the composition.

Bounds /wt% Ni Cr C Si Mn Al Cu Mo Nb W Fe Lower 16 50 35 45 10 0.1 0.75 0,4 0.75 0 2.5 0 1.5 0 2 6.5 3.5 5.5 0 0 4 0 0.5 0 2 0 1.0 0 balance Upper Preferred lower Preferred upper Most preferred 40 27 0.4 0.2 0.75 4.0 4 0,5 2.5 0 0 3 balance lower 45 11 0.6 1.0 0 5.0 1.5 0.3 0.2 0 balance Most preferred 16 0.75 2.5 1.5 balance upper 11 0.1 0 balance 13 0.5 0.5 In order to achieve a high level of creep resistance the steel of the present invention also achieves a certain minimum creep index. This means that the elements present in the steel must be in amounts in order to achieve a minimum creep index. Compositions within the bounds of Table I do not achieve such a creep index. Higher alloyed steels with higher amounts of carbon and nickel and low amounts of aluminium and chromium achieve the desired creep merit index. Additions of molybdenum make achieving the creep merit index more likely whereas additions of copper, manganese, niobium and silicon make achieving the creep merit index less likely.

Merit indices The nickel aluminide index: is a measure of the volume fraction of the nickel aluminide phase stable at high tunperatures. The volume fraction is determined using thermodynamic calculations. A low nickel aluminide index is an indicator of microstructural stability and ductility. An equation proportional to the volume fraction of nickel aluminide is: Nickel aluminide index -(34.487WA1+ 0.209Wcr-0.263Wcu -1.538WI.e + 1.009Wm, + 0.171 WM0 -1.406WN1 ± 16.880W si + 0.949Ww)/100 wherein Wrr, Wmn, W11, WN40, WNI, WSI, WW, WA] and WFc are the amounts of chromium, manganese, copper, molybdenum, nickel, silicon, tungsten, aluminium, and iron in the steel.

Preferably the nickel aluminide index is equal to 0.05983 or less as this results in a low equilibrium volume fraction of nickel aluminide of around 10-15%, more preferably equal to 0.05128 or less or even 0.04274 or less or 0.03419 or less which results in an equilibrium volume fraction of nickel aluminide of below 10%. It is even possible to achieve a nickel aluminide of 0.01282 or less and this is most preferred.

The strength merit index: reflects the yield strength of the alloy at room temperature. A higher value is often, but not always, desired as it indicates that then-nal strains during repeated heating and cooling are less likely to cause plastic deformation, which leads to longer service life of components made of higher strength alloys. The strength merit index is based on two assumptions. First, grain boundary strengthening is constant across the composition space because the as-cast grain size only varies weakly with composition. Second, precipitation strengthening depends only on the volume fraction of precipitates. 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. The equation for the strength merit index is 0.8To Gb Strength index = xiS, + + 300 MPa V2tr Where x is the mole fraction of element i in austenite as predicted from the thermodynamic calculations, :Si is its strengthening coefficient, To is the Taylor factor (value 3.06), G is the shear modulus of steel (74 GPa), b is the length of the Burgers vector (15 mu), I-1, is the radius of precipitates (assumed 1 pm) and co" is their volume fraction. The constant 300 MPa comes from grain boundary and other strengthening contributions.

An equation proportional to the strength based on the composition of the alloy was found: Strength index -0.923 WAi ± 2.752We -0.037We,-+ 2.162W1 + 1.342 Wmia + 2.137 Wm. -0.224 WNb -0.339WNi + 5.684Wsi + 0.667Ww + 128.038 wherein Wcr, Wkin, WCu, WAlo, WNi, W5, WW, WAI, WNh and Wu are the amounts of chromium, manganese, copper, molybdenum, nickel silicon, tungsten, aluminium, niobium and carbon in the steel.

Preferably the strength merit index is 115.385 or more so that a strength of at least about 260-280 MPa is achieved or even 119.658 or more or 121.795 or more where even higher strength up to 285MPa or more is possible. Most preferably the strength merit index is 128.205 or more.

The creep merit index: in high-carbon austenitie stainless steels the creep resistance is governed by two mechanisms -the intrinsic creep resistance of the austenite due to solid solution strengthening and the increase in creep resistance due to the precipitation of secondary carbides at elevated temperatures. Both of these mechanisms are composition-dependent and can be derived with thermodynamic calculations. The combined effect of the two mechanisms has been found by the present invention to be approximated with the following equation: Creep index = 105(-0.0689WAI 1.41We -0.0248We, -0.0662Weu -0.0205W)vl. + 0.0333Wm0 -0.182Wm, + 0.0499WN; -0.219W51-0.97) wherein WAI, WC, WCr, W11n, WAfo, WhC, Wsi, WN1,, and Well are the amounts of aluminium, carbon, chromium, manganese, molybdenum, nickel, silicon, niobium, and copper in the steel.

In the present invention a steel with improved creep resistance over prior art alloys is desired. Therefore, the creep index has a minimum value of -7,000 resulting in a steel material which not only has excellent oxidation characteristics but also good creep resistance. Even greater values of creep merit index are possible and preferably the creep merit index is 0 or greater, more preferably 10,000 or greater, more preferably 30,000 or greater, more preferably 60,000 or greater, more preferably 90,000 or greater and most preferably 1,100,000 or greater.

The chromium activity index: reflects the ability of the alloy to form a protective chromia scale. Chromia forms significantly faster than alumina -the ability to form continuous chromia scale therefore offers protection at the initial stages of oxidation and allows time for the growth of alumina scale at the chromia-metal interface. Unless this condition is met, the formation of a continuous alumina scale may be impeded, A high chromium activity index therefore improves oxidation resistance. In addition, chromia scale is more resistant to hot corrosion and may be desired in applications where sulphur and chlorine ions are present. The activity of chromium is obtained with thermodynamic calculations. The chromium activity index is a function of alloying elements: Chromium activity index -10-'(25.5WAI 23.3Wc, + 13.8Wc" -0.493WFe + 1.11Wm, -1.0Wmo + 22.8 WNb + 4.34WNi + 11.4W51) wherein Wrr, Wmn, W(m, WNIo, Wra and W-Fe are the amounts of chromium, manganese, copper, molybdenum, nickel, silicon, aluminium, niobium and iron in the steel.

Preferably the chromium activity index is greater than or equal to 0.00455. An even greater chromium activity indexes is achievable and it is preferably that the chromium index is at least 0,0050 or even at least 0.0060 or even at least 0.0070.

The chromium diffusivity index: reflects the ability of the alloy to reform a protective chromia scale or reform it after spallation. Rapid chromia formation facilitates the formation of alumina scale and improves oxidation resistance. Due to the growth of the chromia scale, 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 depend 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. Because of this, chromia scale is able to reform rapidly after a spallation event which decreases the overall oxidation rate.

Chromium diffusivity can be tuned as the interdiffusion of elements in austenite is not constant, but instead depends on the composition of austenite. Nickel and copper in particular are known to increase it. The interdiffusion coefficient of chromium in austenite can be obtained with thermodynamic calculations. An approximation using a linear combination of alloying elements has been found: Chromium diffusivity index = 10-211(171.0WAI + 12.0Wc, + 39.2Wc" + 0.964WF0 -8.56Wm" + 3.09Wme + 6.72WF" + 6.87WF1 -76.9W51+ 4.96Ww) Wherein Wet, WME, Wet, WN1o, WNi, WSi, WW, WAl, WNb and WF" are the amounts of chromium, manganese, copper, molybdenum, nickel, silicon, tungsten, aluminium, niobium and iron in the steel.

Preferably the chromium diffusivity index is greater than or equal to 8.7 E-18. however, even larger values are possible and achieving at least 9.0 E-18, or at least 9.2 E-18 is desirable. It is even possible to achieve 9.5 E-18 or at least 1 E-17 and these values are also desirable.

The cost index: reflects the cost of raw materials (in GBP/kg in 2021) needed to produce an alloy. A simple expression for the cost of alloys can be used, assuming one is starting from pure elements: Cost index = 0.065Wc, + 0.047Wc" + 0.008WFe + 0.017Wm" + 0.327Wme + 0.484W 0.12Wm + 0.013W5i + 0.363Ww+ 0.015Wm wherein Wor, W1Vi11, WNIo, WNi, W51, WW, WM), WA1 and WF, are the amounts of chromium, manganese. copper, molybdenum, nickel, silicon, tungsten. niobium, aluminium. and iron in the steel.

The cheaper an alloy is, the lower the cost index. Desirably the cost index is 7.5 or less. However, even less expensive alloys are possible within the present invention and so a cost index of 6.5 or less or 6.0 or less, 5.0 or less or 4.6 or less are even more desirable.

The oxidation index: following Sato, A., Y-L. Chiu, and R. C. Reed. "Oxidation of nickel-based single-crystal superalloys for industrial gas turbine applications." Acta Materialia 59.1 (2011): 225240.. the requirement to form a continuous alumina scale with limited internal oxidation is a function of the Gibbs free energy of alumina scale fon-nation (AGAI203), which depends on the aluminium activity in the alloy, and the effective valence of the cations in the alumina scale (7,ff.). Both quantities depend heavily on the composition of the alloy. Alloys which form a continuous alumina scale have a high oxidation index value which is a weighted sum of the effective valence in the alumina scale and the Gibbs free energy of alumina formation: Oxidation index = IciAGAbo, + k2Vell, Constants k1 and k2 are phenomenological and have been derived from a large number of oxidation experiments on aluminium-containing austenitic steels from the literature. Thermodynamic calculations are required to obtain accurate values of the oxidation index. A relationship based on the alloy composition was found: Oxidation index = 10-3(4.67WAI + 16.9We -1.45Wer + 5.81Wei, -11.6W111 -4.8Wm. -2.19Wm, + 0.768Wm + 1.23Wsi + 5.14) wherein Wcr, Wivin, Wcu, WNio, WNi, Ws, WA1, Wm, and We are the amounts of chromium, manganese, copper, molybdenum, nickel, silicon, aluminium, niobium and carbon in the steel.

Desirably the oxidation index is greater than or equal to 0.01. Even larger oxidation indexes are achievable and desirably the oxidation index is 0.02 or more or even 0.03 or more. Values of up to 0.045% or more are achievable and these are also desirable.

The oxidation index only indicates the likelihood of formation of continuous alumina scale due to thermodynamic effects. I lowever, because alumina formation is kinetically slow, preferably the alloy can form a protective chromia scale in the first stages of oxidation. This facilitates the formation of alumina scale and suppresses internal oxidation of aluminium in solid solution. To achieve optimal oxidation resistance, alloys should exhibit a sufficiently high Cr activity and Cr diffusivity index values.

The evaluated the approximate effect of the alloying elements on individual merit indices are shown in Table 2 below. The scores in the table were derived as a product of the coefficient of an element in a given merit index multiplied by the average value of the element range in the claimed composition space. These values were then divided by the average of weighted coefficient values to enable comparison of element effects across different merit indices (columns). Large negative values indicate a strong negative effect and large positive values a strong positive effect. The table encapsulates how one element can simultaneously affect several key properties. It also shows that some elements are generally more important than others. For example, the variation in Mn does not affect any merit indices significantly, apart from decreasing the oxidation index. On the other hand, Al has a strong positive effect on nickel aluminide, strength, Cr diffusivity and oxidation indices and a strong negative effect on the creep index.

_Table 2

Merit index Ni Cr C Si Mn Al Cu Mo W Nb Cost 5.07 1.54 0.00 0.02 0.01 0.08 0.12 0.84 0.70 0.62 Nickel -3.27 0.27 0.00 1.49 0.03 10.32 0.04 0.02 0.10 0.00 aluminide Strength -10.98 -0.67 1.15 6.97 0.49 3.85 4.24 4.19 0.98 -0.22 Creep 15.36 -4,28 5.59 -2.55 -0.07 -2.73 -1.23 0,62 0,00 -1.70 Cr activity 1.53 4.60 0.00 0.15 0.00 1.16 0.29 0.02 0.00 0.24 Cr diffusivity 1.74 1,70 0.00 -0.74 -0.02 5,57 0.60 0,05 0,06 0.05 Oxidation 10.11 -10.70 2.86 0.61 -1.73 7.92 4.63 -3.83 0.00 -0.87 With the help of merit indices, complex trade-offs between desired properties were explored and alloys with optimal combinations of these properties were found.

The elements and their ranges in Table 1 were selected for the following reasons: Nickel: stabilises the austenite phase and strongly improves creep resistance due to its slow diffusivity in the austenite matrix phase. In addition, it greatly promotes oxidation resistance in alumina-forming steels by lowering the effective valence of the scale and increasing aluminium activity, thereby promoting the formation of a continuous alumina scale. I lowever, high nickel additions are prohibitively expensive. Nickel can contribute significantly to precipitation strengthening by forming nickel aluminide and gamma prime phases. However, nickel aluminide is an undesirable phase and gamma prime is unlikely to form unless nickel and aluminium contents are both very high. The strength model therefore gives a greater weight to solid solution strengthening. Literature on solid solution strengthening indicates that in Ni-Fe solid solutions Ni has a negligible or even negative effect on yield strength -we therefore consider nickel to have a net negative effect on strengthening.. It was found that the best balance of properties is achieved by a minimum nickel content of 16.0 wt.% and a maximum nickel concentration of 50.0 wt.%. The presence of nickel is beneficial for all merit indices, except for cost and strength. Thus, the minimum amount of nickel is necessary in particular to achieve the desired creep strength, oxidation resistance and desired volume fraction of nickel aluminide and all of these properties increase with increasing nickel concentration. Therefore, it is desirable to increase the nickel concentration to 30.0 wt.% or more, even more preferably 35.0 wt.% or more. Even further increases of nickel concentration to 37.0 wt.% or more or even 40 wt.% or more yet further improve the properties of the alloy at the expense of increased cost and to some extent reduced strength. In an embodiment, nickel is present in an amount of 42.0 wt.% or more and this increases oxidation resistance and creep resistance yet further. If necessary, the reduction in strength, at high concentrations of nickel can be at least partly off-set by using preferred higher silicon and aluminium concentrations and optionally by adding copper and molybdenum alloying elements. For some applications cost is a major factor. Therefore, in some embodiments the amount of nickel is limited to 45,0 wt.% or less or even 30.0 wt.% or less, preferably 26,0 wt.% or less.

Chromium: provides solid solution strengthening, improves creep resistance and is also a source of oxidation resistance, forming a protective chromia scale which acts as a barrier to further oxidation at high temperatures. Fast-growing ehromia scale provides oxidation resistance in the early stage of oxidation and later facilitates the formation of the slower-gowing and more protective alumina scale. Sufficient chromium content is therefore necessary for oxidation resistance. However, high chromium additions stabilise ferrite and various detrimental intermetallics including the sigma phase. Their presence severely reduces alloy ductility and oxidation resistance. Thus, the chromium content should be 10.0 wt.% or more and 27.0 wt.% or less. Increasing chromium content within this range helps maintain the chromium scale at intermediate temperatures. Therefore, preferably the chromium content is at least 11.0 wt.%. In some embodiments where oxidation resistance is paramount chromium content is preferably 12.0 wt.% or more, even more preferably 16.0% or more and most preferably 22.0 wt.% or more. However, higher concentrations of chromium can result in lower oxidation resistance due to poor formation of alumina scale at higher temperatures. Therefore, in some embodiments chromium is limited to 16.0 wt.% or less, preferably 13.0 wt.% or less.

Silicon: provides solid solution strengthening, improves alloy castability and deoxidises the melt. However, other elements in the invention may also serve the above roles so the presence of silicon is optional. In addition, high silicon additions stabilise Ci phase and Laves phase which are intermetallics detrimental to ductility. Therefore silicon is an optional element The maximum allowable amount of silicon is therefore defined as being 2.5 wt.% or less. Nonetheless, a small amount of silicon, for example 0.2 wt.% or more, is beneficial particularly in increasing the strength of the alloy and improving low temperature oxidation resistance. Conversely, decreasing the amount of silicon, for example to 1.5 wt.% or less improves ductility as this results in the precipitation of fewer intermetallic phases. Further improvement in ductility occurs at the desirable level of 1.0 wt.% or less silicon.

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. It improves secondary precipitation which contributes to creep resistance. I lowever, excessive carbon additions result in unfavourably large volume fractions of interdendritic carbides which adversely affects the ductility of the alloys. Therefore, the carbon range is 0.1 wt.% or more and 0.75 wt.% or less. Improved creep resistance can be achieved by increasing the carbon content, for example to 0.4 wt.% or more or even 0.45 wt.% or more or more preferably 0.475 wt.% or more. However, increasing carbon content too far can result in reduced ductility so that in an embodiment the steel contains 0.6 wt.% or less carbon. In some embodiments carbon is reduced even further, for example to 0.4 wt.% or less or even 0.3 wt% or less with consummate improvements in alloy ductility.

Manganese: provides solid solution strengthening, stabilises the austenite phase and neutralises the embrittling effect of sulphur impurities by forming manganese sulphide. However, excessive additions stabilise primary delta ferrite which may precipitate in the as-cast material and decreases its creep resistance and acts as a nucleation site for the detrimental sigma phase. In addition, manganese is also known adversely to affect the oxidation resistance by favouring the fon-nation of manganese-rich oxide phases which are permeable to oxygen and prone to spallation. The inventors have found that the benefit of manganese is in many but not all cases outweighed by the disadvantages and that the necessary physical properties can often be achieved in the absence of manganese. Manganese is therefore an optional element, and manganese is limited to 0.75 wt.% or less, preferably 0.5 wt.% or less or even 0.3 wt.% or less. In an embodiment manganese is added in an amount of 0.1 wt.% or more thereby to lower the risk of sulphur embrittlement. In an embodiment, manganese is added in an amount of 0.2 wt.% or more, thereby further decreasing the chance of sulphur embrittlement Molybdenum: provides solid solution strengthening and creep resistance. However, excessive additions raise the cost of the alloy and tend to stabilise various brittle intermetallics such as sigma and Laves phase. Therefore, molybdenum is an optional element and the amount of molybdenum is limited to 0.4 wt.% or less. Small mandatory additions of molybdenum of 0.1 wt.% or more can provide solid solution strengthening without the risk of appearance of deleterious intermetal lies. Even higher additions of molybdenum of 1,5 wt.% or more or even 2.5 wt.% or more can significantly increase solid solution strengthening and are thereby optional features of the present invention. On the other hand, in an embodiment the steel is limited to 0.5 wt.% or less molybdenum, (or even 0.3 wt.% or less molybdenum) so that the chance of the appearance of sigma and Laves phases is reduced.

Tungsten: similarly to molybdenum tungsten provides solid solution strengthening but with a lower benefit to creep resistance. however, excessive additions raise the cost of the alloy and tend to stabilise various brittle intermetal lies such as sigma and Laves phase. In some embodiments of the invention, tungsten can be present optionally but tungsten is limited to 3.0 wt.% or less. Preferably tungsten is present at an even lower level of 1.5 wt.% or less or even 1.0 wt.% or less or 0.5 wt.% or less. On the other hand, if a better creep resistant alloy is required, tungsten can be present in an amount of 0.5 wt.% or more, particularly for steels with a low molybdenum content, Copper: stabilises the austenite phase and improves oxidation resistance in both chromia forming alloys by increasing Cr activity and diffusivity as well as alumina forming alloys by decreasing the effective valence of the alumina scale. However, high Cu contents may reduce castability by may causing Cu segregation to grain boundaries which embrittles the material and may even cause the presence of Cu-rich liquid films at temperatures near 1000 °C. Liquid films catastrophically reduce high temperature ductility and strength. Cu also stabilises various carbides phases and promotes the precipitation of carbides in the melt. This leads to large primary carbides detrimental to ductility and fatigue life. Therefore, copper additions can be beneficial and copper is optional but in any casc limited to 4.0 wt.% or less. Because of the risks of copper, in some embodiments copper is limited to 2.5 wt.% or less, preferably 2.0 wt.% or less, more preferably 1.5 wt.% or less or even 0.5 wt.% or less. On the other hand, because of the potential benefits of adding copper in an embodiment copper is added in an amount of 0,2 wt.% or more, particularly for improved oxidation resistance. Increasing amounts of copper further improve oxidation resistance further so that amounts of 0.5 wt.% or more, preferably 1.5 wt.% or more and even 2.2 wt.% or more may be beneficial for certain applications.

Niobium: Niobium significantly increases creep resistance and high temperature strength by forming hard and stable interdendritic carbides. However, excessive additions are expensive, stabilise the ferrite phase and promote the formation of brittle intermetallics such as sigma and O phase.

Therefore, niobium is an optional element and in any case is limited to 2.0 wt.% or less. In order to keep the cost of the alloy low and ensure better machinability, preferably niobium is present in an amount of 1.0 wt.% or less, more preferably 0.2 wt.% or less or even 0.01 wt.% or less. On the other hand, small additions of niobium of an amount of 0.6 wt.% or more or even 1.0 wt.% or more result in the formation of a network of very hard MC carbides in the interdendritic regions which can increase the strength of the material at very high temperatures and so these levels of niobium are preferable. In an embodiment, niobium is present in an amount of 1.5 wt.% or more.

Aluminium: Aluminium improves oxidation resistance by forming a protective oxide scale on the surface of the alloy. It can in some instances also significantly improve the yield strength of the alloy up to moderate temperatures by forming gamma prime (Ni3A1) precipitates with nickel. To form a continuous aluminia scale, the activity of aluminium in the alloy needs to be sufficiently high. However, excessive additions stabilise Ni Al precipitates which may reduce ductility and decrease creep resistance. Therefore, aluminium is present in an amount of 2.0 wt.% or more and 6.5 wt.% or less. Increasing aluminium content, for example to 3.5 wt.% or more or even 4.0 wt.% or more results in better oxidation resistance. For this reason, increasing aluminium content up to 4.3 wt.% or even 4,5 wt.% is even more preferred. Reducing the maximum amount of aluminium to 5.5 wt.% or less or even 5.0 wt.% or less aluminium may be beneficial to increase ductility and increase creep resistance. In one ern bodiment aluminium is limited to an even lower amount of 3.5 wt.% or less.

Vanadium: Vanadium promotes the formation of harder M7C3 carbides over M73C6 carbides during solidification which results in better high-temperature strength. It also promotes the precipitation of secondary MC carbides over M23C.6 carbides during the operation of a turbocharger housing. Secondary MC carbides are more effective at improving creep resistance than M,3C6 carbides. They also coarsen more slowly, which means that the improvement in mechanical properties from secondary precipitation (yield strength and creep resistance) decrease much more slowly over time. However, excessive additions greatly increase the cost of the alloy. In some embodiments of the invention, vanadium can be present optionally. Therefore, vanadium is an optional element and in ally case is limited to 2.5 wt.% or less. Preferably vanadium is present in an amount of 0.3 wt.% or less or even 0.2 wt.% or less because of the disadvantages of including vanadium. Nonetheless, if an alloy with maximum high-temperature creep resistance is desired, vanadium can be included as a mandatory element in an amount of 0.1 wt.% or more. Increasing vanadium further increases high temperature creep resistance so that minimum amounts of 0.3 wt.%, 1.0 wt.% or even 1.5 wt.% or 1.9 wt% are possible.

Boron: Boron segregates to grain boundaries and strengthens them, improving the high temperature strength and creep resistance of the alloy. however, high boron additions may result in decreased ductility. In some embodiments of the invention, boron can be present optionally. Boron is an optional element but may be present up to an amount of 0.15 wt.% or less. Because of the benefits of adding boron, and in particular if improved creep resistance is desired, boron may be added in an amount of 0.03 wt.% or more or even 0.07 wt.% or more. If boron is present less than 0.01 wt.% this leads to a more ductile alloy.

In addition, the alloys in the invention may contain any combination of minor alloying elements not exceeding 1.0 wt%. Minor alloying elements include lanthanides (atomic elements 57 (lanthanum) to 71 (lutetium)) for a more favourable carbide morphology and improved oxidation resistance, hafnium, zirconium and boron for their grain boundary strengthening effect, yttrium for improved oxidation resistance, as well as titanium for improving the coarsening resistance of MC carbides (where M is primarily Nb partly substituted with Ti, V or Zr). The amount of titanium is preferably limited to 0.15 wt.% or less. The presence of titanium can be beneficial as it can partly substitute for low levels or the absence of niobium. Yttrium is preferably less than 0.01 wt.')/0 and cerium is preferably 0.01 wt."/0 or less. Yttrium and rare earths are known to improve oxide scale adherence, and resistance to hot corrosion. On the other hand, high amounts of yttrium and rare earths can result in segregation and reduced mechanical properties in castings. They also pose processing difficulties due to their reactivity.

Phosphorus: Phosphorous is an impurity. Phosphorous segregates at the grain boundaries and decreases the SSC resistance of the steel. Accordingly, the phosphorous content is 0.05 wt. % or less. A preferable Phosphorous content is 0.02 wt. % or less. Preferably, the phosphorous content is as low as possible.

Sulphur: Sulphur is an impurity which segregates at the grain boundaries. Because it lowers their cohesive strength it can drastically decrease the ductility, strength and creep resistance of the material. Accordingly, the sulphur content is 0.01 wt. % or less. A preferable Sulphur content is 0,005% or less, and more preferably is 0.003 wt. % or less. Preferably, the sulphur content is as low as possible. However, in some instances, a higher sulphur content is desirable as it makes alloys more easily machinable. In this case the sulphur content is preferably more than 0.01 wt.% but less or equal to 0.1 wt, %, Selenium, tellurium, antimony, bismuth, lead: these elements are impurities which segregate at the grain boundaries. Because they lower the cohesive strength of grain boundaries, they can drastically decrease the ductility, strength and creep resistance of the material. Accordingly, their content is 0.005 wt.% or less, preferably 0.001 wt.% or less, and more preferably is 0.0003 wt.% or less. Preferably, their content is as low as possible. I lowever, in some instances, a higher content of these elements is desirable as it makes alloys more easily machinable. In this case their combined content is preferably more than 0.005 wt.% but less or equal to 0.04 wt.% Calcium: calcium is added to the melt and tends to favour the fon-nation 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 wt.% but less or equal to 0.04 wt.%.

Up to 2.0 wt.% zinc may be added and this can result in improved oxidation resistance, In an embodiment at least 0.5 wt.% of zinc is added. Zinc is an optional element.

Cobalt behaves similarly to nickel and so the cobalt content may be up to 5.0 wt.%. Cobalt is a slow diffusing element and improves creep resistance In an embodiment, the sum of cobalt and nickel lies in the range 16 to 50 wt.% or any other range given for nickel alone elsewhere in this document. On the other hand, cobalt is even more expensive than nickel so its content is limited to 0.5 wt.% or less. Cobalt is an optional addition. In an embodiment the alloy is substantially free of cobalt.

In addition, the alloy may contain small amounts of other incidental impurities of any element not listed in the section above.

Figure 1 shows the dependence of the oxidation index (solid black contours), cost index (dashed dark grey contours) and chromium activity index (light grey contours) for the composition space in in Table 1 plotted against Cr and Ni content. As can be seen increasing nickel content raises chromium activity merit index and the oxidation activity merit index, both of which are favourable. However, increasing nickel content also results in an increase in cost which is not favourable. As can be seen from Figure 1, increasing chromium content marginally increases cost. However, most importantly increasing chromium content raises chromium activity, However if chromium content is increased too far, this deleteriously affects oxidation resistance. These trade-offs are indicated in Table 2 in which a low cost merit index and nickel aluminide merit index is desired and high strength, creep, chromium activity, chromium diffusivity and oxidation merit indices are beneficial. On the basis of these kinds of trade-offs, the composition space of the alloy described above in table I was determined.

Examples and comparative examples Experimental testing was carried out on some of the Examples and Comparative examples in Table 3. Comparative examples 3, 4, 5 and 6 are experimental austenitic stainless steels while Comparative examplcs 1 and 2 are well-known austenitie heat-resistant steels corresponding to DIN standard grades 1.4848 and 1.4849, respectively. Both are known for good oxidation resistance but relatively low yield strength and are widely used as heat-resistant materials. However, Comparative examples 1 and 2 arc chromia rather than alumina formers. Comparative examples 7 to 11 arc prior art alloys not falling in the scope of the present invention.

7 ablf:: $ Alloy Fe Ni Cr C Si Mn Al Cu Mo Nb Zn Hf V Y B Comparative example 1 bal. 21.5 25.5 0.45 2.4 1.9 0 0 0.3 0 0 0 0 0 0 Comparative example 2 bal. 38.5 20.5 0.45 2.35 1.9 0 0 0.3 1.7 0 0 0 0 0 Comparative example 3 bal. 35.26 17.4 0.53 0.15 9.99 2.65 3 2.94 0.99 0 0 0 0 0 Comparative example 4 bal. 17.5 12.5 0.5 1.5 0.2 4.4 0 0 0 0 0 0 0 0 Comparative example 5 bal. 17.9 13.3 0.543 1.52 0.26 4.06 0 0 0 0 0 0 0 0 Comparative example 6 bal. 25 12.5 0.1 0.1 0.2 5 1.01 1.01 0 0 0 0 0 0 Comparative example 7 bal. 18.7 16.3 0.11 0.31 1.04 6.14 0 0 0.53 0 0 0 0 0 Comparative example 8 bal. 22.2 17.3 0.03 0.72 0.61 5.38 0 0 0 0 0 0 0.04 0 Comparative example 9 bal. 57.5 19 0 0 0 5.5 0 0 0 0 0 0 0 0 Comparative bal. 45 30 0.45 0.8 1 4 0 0 0.5 0 0 0 0 0

example 10

Comparative bal. 30 12 0.1 0 0 5 0 2 1 0 0 0 0 0

example 11

Example 1 bal. 31.2 15.8 0.581 1.23 0.25 2.53 2.97 1.37 0.77 0 0 0 0 0 Example 2 bal. 25.9 16.6 0.553 1.17 0.3 2.77 0 2.85 0.76 0 0 0 0 0 Example 3 bal. 41.4 20.7 0.362 0.97 0.22 2.42 0 3.56 1.03 0 0.083 0 0 0 Example 4 bal. 40.3 21.3 0.562 0.36 0.047 3.39 2.04 0 0.89 0 0 0 0.012 0 Example 5 bal. 25.4 12.9 0.537 0.31 0.21 4.1 1.96 0 0 0 0 0 0 0 Example 6 bal. 43.1 12.4 0.505 0.29 0.22 4.79 1.98 0 0 0 0 0 0.012 0 Example 7 bal. 25 12.5 0.5 0.1 0.2 4.4 2 0 0 0 0 0 0 0 Example 8 bal. 42 12.5 0.5 0.1 0.2 4.95 2 0 0 0 0 0 0 0 Example 9 bal. 40 20 0.5 0.1 0.1 3.5 2 0 1 0 0 0 0 0 Example 10 bal. 32.84 19.72 0.46 0.157 0.21 3.36 1.99 2.88 0.91 0 0 0 0 0 Example 11 bal. 30 17.5 0.5 0.5 0.2 3.5 3 1.5 0.75 0 0 0 0 0 Example 12 bal. 25 17.5 0.5 0.5 0.2 4 0 3 0.75 0 0 0 0 0 Example 13 bal. 40 20 0.3 0.1 0.1 3.5 0 3.5 1 0 0.15 0 0 0 Example 14 bal. 40 20 0.3 0.1 0.1 3.5 0 3.5 1 0 0 0 0 0 Example 15 bal. 32 12.5 0.55 0.1 0.2 4.5 1.01 1.01 0 0 0 0 0 0.05 Example 16 bal. 36 12.5 0.55 0.1 0.2 4.5 2.51 0.51 0 0 0 0 0 0.05 Example 17 bal. 32 12.5 0.55 0.1 0.2 4.5 1.01 1.01 0 0 0 0.5 0 0 Example 18 bal. 36 12.5 0.55 0.1 0.2 4.5 2.51 0.51 0 0 0 0.3 0 0 Example 19 bal. 25.4 12.9 0.537 0.31 0.21 4.1 1.96 0.3 0 0 0 0.5 0 0 Example 20 bal. 43.1 12.4 0.505 0.29 0.22 4.79 0 0.3 0 0 0 1 0 0 Example 21 bal. 25.4 12.9 0.537 0.31 0.21 4.1 1.96 0.3 0 0 0 2 0 0 Example 22 bal. 43.1 12.4 0.505 0.29 0.22 4.79 0 0.3 0 0 0 2 0 0 Example 23 bal. 32 12.5 0.55 0.1 0.2 4.5 1.01 1.01 0 0 0 0.5 0 0.05 Example 24 bal. 36 12.5 0.55 0.1 0.2 4.5 2.51 0.51 0 0 0 1 0 0.05 Example 25 bal. 32 12.5 0.55 0.1 0.2 4.5 1.01 1.01 0 0 0 0 0 0.08 Example 26 bal. 36 12.5 0.55 0.1 0.2 4.5 2.51 0.51 0 0 0 0 0 0.09 Example 27 bal. 32 12.1 0.49 0.43 0.24 4.98 0.99 0.65 0 0 0 0 0 0.039 Example 28 bal. 36.6 11.5 0.508 0.38 0.23 4.9 2.44 0.47 0 0 0 0 0 0.033 The values of merit indices for the alloys in Table 3 are given in Table 4 for comparison, Jib ? 4 Alloy Nickel Strength Creep Cr activity index Cr. Oxidation index alutninide index index,," ! _! Cost index COSL inuex index /le-2 umusmtV index Comparative example 1 -5.62 138 -4.50E+04 0.0069 3.00E-18 4.79 -0.033 Comparative example 2 -6.10 132 2.24E+04 0.0070 3.59E-18 7.21 -0.017 Comparative example 3 1.77 146 4.04E+04 0.0069 9.67E-18 7.37 -0.091 Comparative example 4 5.77 136 -3.38E+04 0.0047 9.67E-18 3.51 0.024 Comparative example 5 4.75 136 -2.59E+04 0.0048 9.18E-18 3.60 0.021 Comparative example 6 5.75 129 -2.95E+04 0.0052 1.26E-17 4.71 0.025 Comparative example 7 10.77 130 -8.95E+04 0.0061 1.40E-17 4.13 0.008 Comparative example 8 8.83 142 -7.90E+04 0.0062 1.27E-17 4.32 0.011 Comparative example 9 8.51 113 1.05E+05 0.0082 1.58E-17 8.36 0.042 Comparative example 10 6.74 122 6.04E+04 0.0101 1.30E-17 7.83 0.006 Comparative example 11 5.63 126 -8.95E+03 0.0054 1.27E-17 6.00 0.019 Example 1 0.21 137 2.74E+04 0.0062 9.07E-18 6.14 0.030 Example 2 0.76 136 1.94E+04 0.0058 8.20E-18 5.94 0.001 Example 3 0.18 130 6.40E+04 0.0075 9.16E-18 8.26 -0.002 Example 4 2.35 128 6.95E+04 0.0079 1.20E-17 7.05 0.035 Example 5 3.04 130 2.50E+04 0.0052 1.13E-17 4.49 0.038 Example 6 5.63 129 1.06E+05 0.0061 1.35E-17 6.45 0.055 Example 7 3.66 130 2.11E+04 0.0051 1.20E-17 4.42 0.039 Example 8 5.86 124 1.02E-H05 0.0061 1.39E-17 6.33 0.055 Example 9 2.05 123 6.56H-F04 0.0076 1.22E-17 7.00 0.035 Example 10 2.01 132 3.57H-F04 0.0072 1.15E-17 7.06 0.014 Example 11 2.64 134 1.58E-H04 0.0067 1.14E-17 6.14 0.029 Example 12 4.09 133 1.23 E-H04 0.0062 1.09E-17 5.94 0.003 Example 13 2.28 126 6.22H-H04 0.0074 1.15E-17 8.04 0.003 Example 14 2.26 126 6.22E-H04 0.0074 1.15E-17 8.04 0.003 Example 15 4.12 128 7.23E-H04 0.0054 1.22E-17 5.49 0.036 Example 16 4.36 128 8.07E-H04 0.0058 1.30E-17 5.84 0.050 Example 17 4.19 128 7.23E-f-04 0.0054 1.22E-17 5.49 0.036 Example 18 4.39 128 8.07H-H04 0.0058 1.30E-17 5.83 0.050 Example 19 3.17 131 2.60E-H04 0.0052 1.13E-17 4.58 0.037 Example 20 5.48 121 1.20E-H05 0.0058 1.28E-17 6.46 0.042 Example 21 3.40 131 2.60E-H04 0.0052 1.13E-17 4.57 0.037 Example 22 5.64 121 1.20E-H05 0.0058 1.28E-17 6.45 0.042 Example 23 4.19 128 7.23E-H04 0.0054 1.22E-17 5.49 0.036 Example 24 4.51 128 8.07E-H04 0.0058 1.30E-17 5.83 0.050 Example 25 4.12 128 7.23 L-F04 0.0054 1.22E-17 5.49 0.036 Example 26 4.36 128 8.07H-F04 0.0058 1.30E-17 5.84 0.050 Example 27 6.32 129 5.3211.-F04 0.0054 1.27E-17 5.36 0.039 Example 28 6.12 130 7.16E-H04 0.0057 1.33 E-17 5.84 0.053 As can be seen from Table 4 all of the examples of the invention achieve most of the desirable merit indices mentioned above. Only example 2 does not quite achieve the desired minimum chromium diffusivity index of 8.7 E-18 or an oxidation index of >0.01. Example 3 also does not quite achieve the desired minimum oxidation index of 0.01 (and is quite expensive). Examples 27 and 28 do not quite achieve the desired nickel aluminide index of 0.0598 or less. Nonetheless, the improvement in properties of the examples of the invention over well-known comparative examples 1 and 2 is obvious both in the poor chromium diffusivity index and oxidation index achieved by those alloys. Comparative example 3 has a low oxidation index due to too high a level of manganese and comparative examples 3-8 and 11 all have low creep merit index. Comparative examples 7 to 10 show too high a proportion of nickel aluminide. Comparative examples 9 and 10 additionally have excessive cost and example 10 has poor oxidation resistance. Comparative examples 9 and 10 achieve good creep resistance but at the expense of excessive cost and too high a volume fraction of nickel aluminide. Comparative example 9 also is relatively weak with a low strength index.

Figure 2 shows a comparison in the mass gain during cyclic oxidation testing of samples of alloys from the invention and comparative example 2. Oxidation testing was conducted in a box furnace at 1100 °C for various durations (indicated in the figure). Samples were put into open alumina boats which were then inserted into a preheated furnace. After exposure, the boats were removed from the furnace, air cooled for 1 h and weighed. Examples 4 and 10 showed stable and moderate mass gain, indicating the formation of protective alumina scale. Comparative example 2 is a chromia-forming alloy which showed rapid and considerable mass loss likely due to scale evaporation and spallation. Thus experimental results indicate indeed that the low chromium diffusivity index and oxidation index of the composition of comparative example 2 result in poor oxidation resistance.

Figure 3 shows Vickers hardness values (5 kg load, 10 s dwell time) for an example of the invention. Relative to comparative examples, its hardness value is equivalent indicating equivalent yield strength.

Figure 4 shows Larson-Miller parameter values (with a constant term of 20) for creep tests conducted at 950 °C and 60 MPa for Example 10 of the invention and Comparative example 1. Two samples of each alloy were tested -hence four creep tests were conducted in total. Relative to the Comparative example, Example 10 from the invention show larger Larson-Miller parameter values due to their longer creep rupture life.

Claims (40)

1. CLAIMSA steel material including, in mass percent: nickel: 16 to 50%; chromium: 10 to 27%; carbon: 0.1 to 0.75%; so!. 2.0 to 6.5%; silicon: 2.5% or less; manganese: 0.75% or less; copper: 4.0% or less; molybdenum: 4.0% or less; tungsten: 3.0% or less; niobium: 2.0% or less; vanadium: 2.5% or less; boron: 0.15% or less; calcium 0.04% or less; zinc: 2.0% or less; cobalt: 5.0% or less; phosphorous 0.05% or less; sulphur 0.1% or less; 1.0 wt.% or less in sum of lanthanide elements, hafnium, zirconium, yttrium, cerium and titanium; 0.04% or less in sum of selenium, tellurium, antimony, bismuth and lead; the remainder being iron and incidental impurities; wherein the following equation (1) is fulfilled: 10(-0.0689 WA! ± 1.41 Wu -0.0248Wur -0.0662Wo11 -0.0205 Wmn + 0.0333 Wmo -0.182 WNb 0.0499WN; -0.219Ws; -0.97)>-7000 (1) wherein WA], WC, WCr, WNIn, WMo, WM, WS I, WM', and Well are the amounts of aluminium, carbon, chromium, manganese, molybdenum, nickel, silicon, niobium, and copper in the steel.
2. The steel of claim 1. wherein the following equation is fulfilled: 0.065W0 + 0.047Wcu, + 0.008WFG + 0.017Wvh; + 0.327W/ma + 0.484W(th + 0.12WN; + 0.013W51+ 0.363Ww+ 0.015WA; <7.5 (2) preferably 0.065W1 + 0.047Wcu, + 0.008W/Fe + 0.017Wvill + 0.327W/ma + 0.484W(th + 0.12W/NI + 0.013W51+ 0.363Ww+ 0.015Wm<6.5 more preferably 0.065Wc1 + 0.047Wc,, + 0.008W/Fe + 0.017Wm, + 0,327Wmo + 0.484Wmb + 0.12Wmi + 0.013Wsi+ 0.363Ww+ 0.015WA, <6.0 even rnore preferably 0.065W/c, + 0.047Wc,, + 0.008W0 + 0.017Wm, + 0,327Wmo + 0.484WNb + 0.12WNi + 0.013W51+ 0.363Ww+ 0.015WA, <5.0 most preferably 0.065Wc1 + 0.047Wc,, + 0.008W/Fe + 0.017Wm, + 0,327Wmo + 0.484Wmb + 0.12Wmi + 0.013W51+ 0.363Ww+ 0.015WAI <4.6 wherein Wer, W1Vin, W(Tu, WNIo, WN, Ws, WW, W10, WA, and W-F, are the amounts of chromium, manganese. copper, molybdenum, nickel, silicon, tungsten. niobium, aluminium. and iron in the steel.
3. 3. The steel of claim 1 or 2, wherein the following equation is fulfilled: (34.487W2v + 0.209W -0.263W cu -1.538%b + 1.009 Wm, + 0.171 Wmo -1.406 W Ni + 16,880Wsi + 0.949%0/100 <0.05983 (3) preferably (34.487%v + 0.209W/c, -0.263W,A, -1.538Wre + 1.009Wm. + 0.171W/AL, -1.406WNi + 16.880W5i + 0.949Ww)/100 <0.05128 more preferably (34.487%v 0.209W/c, -0.263Wc,, -1.538Wn + 1.009Wm, + 0.171Wmo -1.406WNi + 16.880%1+ 0.949%0/100 <0.04274 even more preferably (34.487Wm + 0.209 wc, -0.263 Wcu -1 538 + 1.009 Wm, + 0.171 WM0 -1.406 W Ni + 16,880W51+ 0.949%0/100 <0.03419 most preferably (34.487W2v + 0.209Wc, -0.263W cu -1 538 WE, + 1.009 Wm, + 0.171 Wmo -1.406 W Ni + 16,880Wsi + 0.949Ww)/100 <0.01282 wherein %Tr, WM11, Wcu, WR40, WNi, WS I, WW, WA] and W-Fc are the amounts of chromium, manganese, copper, molybdenum, nickel, silicon, tungsten, aluminium, and iron in the steel.
4. The steel of claim 1. 2 or 3, where in the following equation is fulfilled: I 0-3(4.67WA: + 16.9Wc -1,45Wc, + 5.81W/cu -11.6Wm1 -4.8Wmo -2.19WNb + 0.768WN 1.23Ws, + 5.14)>0.01 (4) preferably I 0-3(4.67WA: + 16.9Wc -1,45Wc, + 5.81W/c, -11.6Wm, -4.8Wmo -2.19WNb + 0.768WN 1.23W51 + 5.14)>0.02 more preferably I 0--1(4.67WA: + 16.9Wc -1.45Wc, + 5.81Wc, -11.6Wm, -4.8Wmo -2.19Wmb + 0.768WN1 + 1.23 Ws + 5.14)>0.03 most preferably I 0-5(4.67WA: + 16.9Wc -1.45Wc, + 5.81Wc, -11.6Wm, -4.8Wmo -2.19W1m, + 0.768Wm + 1.23W5; + 5.14)>0.045 wherein Weis Wm., Wee, WM0, WNi, \A/Si, WA] Wm and Wc are the amounts of chromium, manganese, copper, molybdenum, nickel, silicon, aluminium, niobium and carbon in the steel.
5. 5. The steel of any of claims 1 to 4, wherein the following equation is fulfilled: 0.923 WAI + 2,752W -0.037Wm-+ 2.162 Wc" + 1.342 Wmn ± 2.137 Wm. -0.224WNb - 0.339 WN; + 5.684W51 + 0.667W + 128.038 > 115.385(5) preferably 0.923WA; + 2.752Wc -0.037Wcb + 2.162Wc" + 1.342Wm, + 2.137Wm" -0.224Wob - 0.339Wm + 5.684W51 + 0.667 Ww + 128.038 > 119-658 more preferably 0.923Wm + 2.752We -0.037Wes + 2.162We. + 1.342Wm. + 2.137Wm. -0.224Wm, - 0.339Wm + 5.684W51+ 0.667Ww + 128.038 > 121.795 even more preferably 0.923Wm + 2.752We -0.037Wer + 2.162Weo + 1.342Wm. + 2.137Wm0 -0.224WNo - 0.339WN; + 5.684W51+ 0.667Ww + 128,038> 128.205 wherein Wm,WM11, W(M, WM0, WM, WSil WW1 WA], WM, and We are the amounts of chromium, manganese. copper, molybdenum, nickel, silicon, tungsten, aluminium, niobium and carbon in the steel.
6. 6. The steel of any of claims 1 to 5, wherein the following equation is fulfilled: 10-5(25.5 WA] ± 23.3 Wc" + 13.8 W co -0.493Wye + 1.11 Wur -1.0Wmo + 22.8Wm5 + 4.34 WNi + 11.4W51)? 0.00455 (6) preferably 10-5(25.5Wm + 23.3Wer + 13.8We. -0.493WF. + 1.11Wm11 -1.0Wm. + 22.8Wm, + 4.34%; + 11.4W51) > 0.0050 more preferably 10-5(25.5 W Al ± 23.3 We, + 13.8 Wee -0.493 WF, ± 1.11 Wlyir -1.0 Wlyin 22.8WNb 4.34Wm + 11.4W50 > 0.0060 even more preferably 10-5(25.5W.m + 23.3Wc, + 13.8Wco -0.493Wm + 1.11WNIn -1.0W.V10 + 22.8WNb 4.34WN; + 11.4W5) >0.0060 wherein Wc", Wm., Wco, WM0, WNi, W51, WA], Wm, and WFe are the amounts of chromium, manganese, copper, molybdenum, nickel, silicon, aluminium, niobium and iron in the steel.
7. 7. The steel of any of claims 1 to 6, wherein the following equation is fulfilled: I I 71.0WA, + 12.0We, + 39.2Wet, + 0.964WFo -8.56Wm. + 3.09Wmo + 6.72Wiso + 6.87WN, -76.9W5, + 4.96Ww) >8.7 E-18 preferably 10-20(1 71.0WAi 12-0Wc, + 39.2 W11 + 0.964W -8.56WNin + 3.09WMo (i.72WNb 6.87WNi -76.9Wsi + 4.96Ww) >9.0 E-18 more preferably I 0-2a(171.0WAI + 12.0Wer + 39.2We" + 0.964W. -8.56Wm" + 3.09WN," + 6.72WN, + 6.87WNj -76.9 W5 + 4.96Ww) >9.2 E-13 even more preferably I 0-2°(171.0Wm + 12.0We, + 39.2Wet, + 0.964WNo -8.56Wmll + 3.09Wm0 + 6.72WNN + 6.87WN1 -76.9Ws, + 4.96Ww) >9.5 E-18 10'071.0WA, + 12.0Wer + 39.2We" + 0.964Wco -8.56Wmo + 3.09Wmo + 6.72WNo + 6.87W5, -76.9Ws, + 4.96W w) > 1E-17 wherein Weis Wmo, We., WMo, WNi, Ws, Ww, WAl, WNb and WFe are the amounts of chromium, manganese, copper, molybdenum, nickel, silicon, tungsten, aluminium, niobium and iron in the steel.
8. 8. The steel of any of claims Ito 7, wherein the following equation is fulfilled: 105(-0.0689WAI + 1.41W -0.0248Wer -0.0662Well -0.0205WM" 0.0333Wm0 -0.182WNI, + 0.0499WN, -0.219Wsi -0.97)>-0 preferably 105(-0.0689WA1 + 1.41 We -0.0248Wer -0.0662We. -0.0205 + 0.0333Wmo -O. 82WNh + 0.0499Wm -0.219Wsi -0.97)>-I 0000 more preferably 10'(-0.0689WA1 1.4IWe -0.0248We, -0,0662We. -0.0205Wm. + 0.0333Wm0 -0.182WN1 + 0.0499WN1 -0.219W -0.97)>-30000 even more preferably 105(-0.0689WAi + 1.41Wc -0.0243Wc, -0.0662We1 -0.0205Wmo + 0.0333 WM0 -0.132 + 0.0499 WNj -0.219W si -0.97)>-60000 even more preferably 105(-0.0689WA, + I.41We -0.0248We, -0.0662We. -0.0205Wm. + 0.0333Wmo -0.182W5H + 0.0499WNi -0.219W51-0.97)>-90000 most preferably 105(-0.0689WA1 + 1.41 Wu -0.0248Wc, -0.0662We. -0.0205 WAin + 0.0333Wma -0.182Wm, + 0.0499WN; -0.219Wsi -0.97)>-110000 wherein WAb WC, Wen WNin, WM°, WNI, Ws, WM), and Well are the amounts of aluminium, carbon, chromium, manganese, molybdenum, nickel, silicon, niobium, and copper in the steel.
9. 9. The steel of any of claims 1 to 8, including 30 wt% or more nickel, preferably 35 wt% or more nickel, more preferably 37 wt.% or more nickel, even more preferably 40 wt.% or more nickel, most preferably 42 wt.% or more nickel.
10. 10, The steel of any of claims Ito 9, including 45 wt. % or less nickel, preferably 30 wt% or less nickel, more preferably 26 wt% or less nickel.
11. 11, The steel of any of claims Ito 10, including 0.3 wt% or less vanadium, preferably 0,2 wt% or less vanadium.
12. 12, The steel of any of claims Ito 11, including 0.2 wt.% or more silicon, preferably 1.0 wt% or more silicon, more preferably 1.5 wt% or more silicon.
13. 13, The steel of any of claims Ito 12, including 1.5 wt.% or less silicon, preferably 1.0 wt. % or less silicon.
14. 14. The steel of any of claims 1 to 13, including 0.1 wt% or more manganese, preferably 0.2 wt% or more manganese.
15. 15. The steel of any of claims 1 to 14, including 11.0 wt. % or more chromium, preferably 12.0 wt% or more chromium, more preferably 16 wt% or more chromium and most preferably 22.0 wt% or more chromium.
16. 16. The steel of any of claims 1 to 15, including 16 wt% or less chromium, preferably 13 wt% or less chromium.
17. 17, The steel of any of claims Ito 16, including 1.5 wt.% or less tungsten, preferably 1.0 wt.% or less tungsten, more preferably 0.5 wt.% or less tungsten
18. 18. The steel of any of claims 1 to 17, including 0.1 wt.% or more molybdenum, preferably 1.5 wt% or more molybdenum, more preferably 15 wt.% or more molybdenum.
19. 19. The steel of any of claims 1 to 18, including 0.5 wt% or less molybdenum, preferably 0.3 wt. % or less molybdenum.
20. 20. The steel of any of claims 1 to 18, including 0.15 wt% or less titanium.
21. 21. The steel of any of claims 1 to 20, including 1.0 wt% or less niobium, preferably 0.2 wt% or less niobium, more preferably 0.01 wt% or less niobium,
22. 22. The steel of any of claims 1 to 21, including 0.6 wt% or more niobium, preferably 1.0 wt% or more niobium, more preferably 1.5 wt.% or more niobium.
23. 23, The steel of any of claims Ito 22, including 0.03 wt% or more boron, preferably 0.07 wt% or more boron.
24. 24, The steel of any of claims Ito 23, including 0.1 wt% or more vanadium, preferably 0,3 wt% or more vanadium, more preferably 1.0 wt% or more vanadium, even more preferably 1.5 wt% or more vanadium, most preferably 1.9 wt.% or more vanadium.
25. 25, The steel of any of claims 1 to 24, including 0.5 wt% or more zinc.
26. 26. The steel of any of claims 1 to 25, including 0.5 wt% or more tungsten.
27. 27, The steel of any of claims Ito 26, including 0.4 wt% or more carbon, preferably 0.45 wt% or more carbon, more preferably 0.475 wt.% or more carbon.
28. 28, The steel of any of claims Ito 27, including 0.6 wt.% or less carbon, preferably 0.4 wt% or less carbon, more preferably 0.3 wt% or less carbon.
29. 29. The steel of any of claim 1 to 28, including 3.5 wt.% or more aluminium, preferably 4.0 wt.% or more aluminium, even more preferably 4.3 wt.% or more aluminium most preferably 4.5 wt.% or more aluminium.
30. 30. The steel of any of claims 1 to 29, including 5.5 wt.% or less aluminium, preferably 5.0 wt.% or less aluminium, more preferably 3.5 wt.% or less aluminium.
31. 31. The steel of any of claims 1 to 30, including 0.2 wt.% or more copper, preferably 0.5 wt.% or more copper. more preferably 1.5 wt.% or more copper, most preferably 2.2 wt.% or more copper.
32. 32. The steel of any of claims Ito 31, including 2.5 wt.% or less, preferably 2.2 wt.% or less, more preferably 1.5 wt.% or less copper, most preferably 0.5 wt.% or less copper.
33. 33. The steel of any of claims 1 to 31, including 0.01 wt.% or less yttrium.
34. 34. The steel of any of claims 1 to 33, including 0.01 wt.% or less cerium.
35. 35. The steel of any of claims Ito 34, including 0.5 wt.% or less manganese, preferably 0,3 wt.% or less manganese.
36. 36. The steel of any of claims Ito 35, including 0.01 wt,% or less boron.
37. 37. A steel comprising about 43wt% nickel. about 12 wt% chromium, about 0.5 wt% carbon, about 0.3wt% silicon, about 0.22wt% manganese, about 4.8wt% aluminium, about 2.0 wt% copper with the remainder being iron and incidental impurities.
38. 38. A cast product comprised of the steel of any of claims Ito 37.
39. 39. A turbocharger housing comprised of the steel of any of claim 1 to 37.
40. 40. The turbocharger of claim 38 wherein the turbocharger housing is a cast product.