WO2009079972A1 - Austenitic heat-resistant nickel-base alloy - Google Patents

Abstract

Austenitic heat-resistant nickel-base alloy comprising (in % by mass) 0.03-0.1% of C, 28-32% of Cr, 0.01-≤ 0.5% of Mn, 0.01-≤ 0.3% of Si, 0.01-≤ 1.0% of Mo, 2.5-3.2% of Ti, 0.01-≤ 0.5% of Nb, 0.01-≤ 0.5% of Cu, 0.05-≤ 2.0% of Fe, 0.7-1.0% of Al, 0.001-≤ 0.03% of Mg, 0.01-≤ 1.0% of Co, 0.01-0.10% of Hf, 0.01-0.10% of Zr, 0.002-0.02% of B, 0.001-0.01% of N, max. 0.01% of S, max. 0.005% of Pb, max. 0.0005% of Bi, max. 0.01% of Ag, balance Ni and minor components due to the production method, where the sum of Ti + Al is from 3.3 to 4.3%, the sum of C + (10 × B) is from 0.05 to 0.2%, the sum of Hf + Zr is from 0.05 to 0.15% and the Ti/Al ratio is > 3.

Description

 Austenitic heat-resistant nickel-based alloy

The invention relates to an austenitic heat-resistant nickel-based alloy.

The Institute of Marine Engineers, with the "Proceedings" Diesel Engine Combustion Chamber Materials for Heavy Fuel Operation, 1990, provides a summary of the state of the art and the extensive research and development efforts in the field of valve materials made in previous years After that, for this application, it was mainly Alloy 80 A with (in mass%) 0.08% C, 19.5% Cr, 75% Ni, 1.4% Al and 2.4% Ti.

Occasionally, Alloy 81 with (in% by mass) 0.05% C, 30% Cr, 66% Ni, 0.9% Al and 1.8% Ti was used. Occasionally, these alloys are used as valve base materials, wherein the valve seat section is additionally coated with an abrasion-resistant material, as described for example in EP-B 0521821. This document gives the chemical composition (in mass%) of the base material as follows: 0.04 - 0.10% C ₁ ≤ 1, 0% Si, <0.2% Cu, <1, 0% Fe, < 1, 9% Mn, 18-21% Cr, 1, 8-2.7% Ti, 1, 0-1, 8% Al, <2.0% Co, <0.3% Mo, B, Zr, Rest of nickel. Furthermore, a variant of this alloy is also mentioned among other things with 29 - 31% Cr.

At current operating temperatures below 750 ° C, Alloy 80 A has been characterized by a longer life in LCF tests and better abrasion resistance, while Alloy 81 has been tested for its better corrosion resistance under the conditions found in marine diesel engines, for example , Each of these alloys therefore has its own advantages, but none meets all the requirements for mechanical and corrosive properties. The remedy with an additional coating brings with it further undesirable manufacturing and material costs. From a cost point of view unfavorable is also the powder metallurgical manufacturing process. Such costs should be avoided as far as possible. Both US Pat. No. 6,139,660 and US Pat. No. 6,039,919, which describe an alloy of the following composition (in% by mass) for intake and exhaust valves of diesel engines, relate to this: ≦ 0.1% C, ≦ 1.0% Si , ≤ 0.1% Mn, ≥ 25 - <32.2% Cr, ≤ 3% Ti,> 1 - <2% Al, balance Ni. But even this alloy does not provide sufficient hot corrosion resistance. In addition, in the future, more powerful engines, such as marine diesel engines, are operated at temperatures up to about 850 ° C, which also makes higher demands on the valve material, especially since the service life is to be maintained and no additional maintenance is desired.

DE-C 101 23 566 discloses an austenitic heat-resistant nickel-based alloy which has the following composition (in% by mass): 0.03-0.1% C, max. 0.005% S, max. 0.05% N, 25-35% Cr, max. 0.2% Mn, max. 0.1% Si, max. 0.2% Mo, 2 - 3% Ti, 0.02 - 1, 1% Nb, max. 0.1% Cu, max. 1% Fe, max. 0.08% P, 0.9-1.3% AI, max. 0.01% Mg, 0.02 - 0.1% Zr, max. 0.2% Co, the sum of Al + Ti + Nb being> 3.5%, the remainder being Ni and production-related conditions. The alloy is characterized by additions of (in% by mass) 0.001-0.005% B, 0.01-0.04% Hf, and 0.01-0.04% Y.

The invention has for its object, up to a temperature of 850 ° C hot corrosion resistant material with mechanical properties which are not inferior to those of Alloy 80 A.

This object is achieved by an austenitic heat-resistant nickel-based alloy with (in% by mass)

0.03 - 0.1% C 28 - 32% Cr 0.01 - <0.5% Mn 0.01 - <0.3% Si 0.01 - <1.0% Mo 2.5 - 3, 2% Ti 0.01 - <0.5% Nb 0.01 - <0.5% Cu 0.05 - <2.0% Fe

0.7 - 1, 0% AI

0.001 - <0.03% Mg

0.01 - <1.0% Co

0.01 - 0.10% Hf

0.01 - 0.10% Zr

0.002-0.02% B

0.001 - 0.01% N max. 0.01% S max. 0.005 Pb max. 0.0005% Bi max. 0.01% Ag

Remaining Ni and production-related admixtures, where the sum of Ti + Al is between 3.3 and 4.3%, the sum of C + (10 x B) is between 0.05 and 0.2%, the sum of Hf + Zr is between 0.05 and 0.15% and the ratio Ti / Al> 3.

Advantageous developments of the inventive up to 850 ° C hot corrosion resistant nickel-based alloy can be found in the associated dependent claims.

Such hot corrosion resistant materials achieve mechanical properties that are not inferior to those of Alloy 80 A. In this respect, the material of the invention as a valve material is generally applicable and can be used in particular for future generations of marine diesel engines in the temperature range up to 850 ° C.

Table 1 shows an example of the chemical composition of two inventive examples E1 and E2. For a better comparison, two typical analyzes of the commercial alloys Alloy 80 A and Alloy 81 are listed. The analyzes of the alloys E1 and E2 were obtained from a series of laboratory melts, which were melted in 10 kg blocks in the vacuum induction furnace, then hot rolled and solution heat treated at 1180 ° C for two hours in air with subsequent water quenching. The hardening of the alloys took place by two further annealing:

6 hours at 850 ° C with air cooling followed by 4 hours at 700 ° C with air cooling

The alloys differed in the content of the elements discussed below, so that the evaluation of their mechanical properties and their behavior in the corrosive medium led to the analysis according to the invention.

Table 1

Chemical composition of the alloys E1 and E2 according to the invention in

Compared to Alloy 80 A and Alloy 81

(Dimensions %) Since an Alloy 80 A target according to the present invention was comparable in heat resistance at the use temperature, tensile strength and yield strength were measured at 600 ° C and 800 ° C. Table 2 shows that at 600 ⁰ C Alloy 80 A is comparable and even tighter. At 800 ° C, the alloys are comparable.

Table 2

Tensile strength and yield strength of E1 and E2 in comparison with Alloy 80 A at 600 °

C and 800 ° C

For the investigation of the corrosion behavior first samples in the laboratory were carried out in synthetic oil ash of the following composition:

40% V ₂ O ₃ + 10% NaVO ₃ + 20% Na ₂ SO ₄ + 15% CaSO ₄ + 15% NiSO ₄ .

The atmosphere was air with an SO ₂ content of 0.5%. The samples were swapped out at both 750 ° C and 850 ° C for 20 hours, 100 hours and 400 hours, respectively. In the 400 hours aging, the ash was renewed after 100 hours, 200 hours and 300 hours to maintain the corrosiveness. In the laboratory experiments, the depth of the internal corrosion could be reliably measured.

As a reliable - because they are on the one hand better evaluable and on the other also take into account erosive effects - the corrosion studies in the marine diesel valve itself can be estimated. From each laboratory melt and for comparison also from the material Alloy 81 and 80 A samples were used in a marine diesel valve. This marine diesel valve ran over 3000 hours in the main engine of a high-speed oceangoing ship. Subsequently the samples were taken from the valve and the corrosion attack was examined metallographically. Here, material loss, layer thickness and internal corrosion attack were distinguished in detail from each other.

The following dependencies of the corrosion behavior on the content of the individual alloying elements resulted from the investigations.

Cr: The Cr content must be as high as possible from the corrosion point of view. Metallurgically, however, 32% is a sensible upper limit. This shows the clear difference between the alloy variants with about 30% Cr and those with 20% Cr. The corrosion attack in the first mentioned alloys is at best only half as large. The samples tested in the valve with a Cr content of 30% show a cobblestone-like appearance on macro photographs, which is reflected in the micrographs as a wavy sample surface, which is indicative of only moderate corrosion erosion. In contrast, the poorer samples already show strong even flaking.

Ti, Al: A TkAI ratio of> 3 results in better corrosion resistance than lower Ti: Al ratios. This is attributed to the formation of a Ti-rich seam between the outer oxide layer and the region of internal sulfidation at high Ti contents. Aluminum and titanium have a positive effect on the heat resistance due to the formation of γ'-phase. The sum of the elements Al + Ti should advantageously be between 3.5 and 4.3%. Too high a total content of these elements makes the thermoforming of the material difficult.

Si: Silicon has been found to have no positive effect on corrosion properties and should be no more than 0.5%, better less than 0.1%.

Nb: The niobium-alloyed samples basically have the thinnest corrosion layer, but this has no effect on the material loss itself Protective corrosion layer acts against the progression of the corrosion attack, the Nb content should be limited to a maximum of 0.5%. Furthermore, the Nb influences the material strength due to its high solubility in the γ'-phase. At lower Nb levels below 0.5%, the Ti and Al content need not be adjusted.

B, C: The addition of boron at levels of 0.002 - 0.01% improves corrosion resistance by reducing the internal sulfidation, which preferably proceeds along the grain boundaries, and thus reducing overall corrosion attack. Carbon preferably forms Cr carbides at the grain boundaries. Boron forms borides, which contribute to the stabilization of the grain boundaries and thus to long-term stability. In particular, the forming Cr carbides lead to a Cr depletion in the vicinity of the grain boundaries, which is why at a high C content, the corrosion accelerated progresses. In addition, carbides and borides must not overburden the grain boundaries, as they then hard precipitates greatly reduce the ductility of the material. As a compromise, it has been found that the sum of C + (10 x B) should not exceed 0.1%. Advantageously, said sum is about 0.08%.

Hf: Hafnium is often added to improve the high temperature oxidation resistance and obviously also influences the durability of the samples in vanadium ash and SO ₂ atmosphere positively. Furthermore, Hf also changes the grain boundary properties under carbide or carbosulfide formation. Too high an HF content should be avoided, as otherwise the hot forming is no longer guaranteed. This results in a favorable concentration range between 0.02 and 0.08%, preferably 0.05%. The effect of Hf on the grain boundaries is comparable to the effect of Zr, which is why the empirical formula Hf + Zr <0.10% advantageously results.

Zr: Zirconium forms carbosulfides, which have a positive effect on the long-term strength and also contribute to the hot corrosion resistance by the binding of sulfur. It turned out that a Zr content between 0.01 and 0.05%. The aim is to have a Zr content in the range of 0.02%.

Co: Co is an element that in principle increases the resistance to sulfur-containing media. On the other hand, it is also very expensive, which is why the co-alloying of Co is dispensed with. Due to admixtures in the feedstocks, however, the Co content can reach up to 2% without incurring increased costs.

Fe: The element iron occurs i.a. as an accompaniment element. Reducing the iron content to well below 1% increases the costs, since higher-quality starting materials would have to be selected. With a Fe content limited to 3%, you do not have to expect a significant deterioration of the corrosion resistance and not too high costs of the starting materials. However, an Fe content below 1% should be sought.

Mn: The conditions mentioned for Fe also apply to Mn, whereby the Mn content can be reduced to less than 1% without much effort.

Although the influence of various elements on corrosion resistance and heat resistance is often in opposite directions, could with the alloys E1 and E2 compositions are found that the requirements for the high-temperature corrosion behavior detected and the heat resistance at the same time at temperatures ranging between 600 ⁰ C and 850 ° C fulfill. The good corrosion resistance can be explained by the addition of the reactive elements, such as hafnium and zirconium, without exceeding the selected optimum (0.05-0.10%). Higher levels increase the corrosion attack directed into the material. The limitations of the carbon content <0.1% and that of manganese <1% additionally contribute to the corrosion resistance. For the heat resistance, it has proved to be particularly favorable when aluminum and titanium are added, with their Summenge - as already stated - should be in the range between 3.5 and 4.3%. These warmth strengths make one Coating the seat portion of the valve superfluous, thereby manufacturing costs can be saved.

The alloy can be prepared by the usual methods of a melt operation, advantageously a melting in a vacuum with subsequent remelting in the electroslag process is useful. The formability for the production of rods for further processing to valves, such as marine diesel valves, is given.

The alloy according to the invention is also particularly suitable for the production of valves for large diesel engines in general, that is, for example, for such large diesel engines that are used in stationary facilities for power generation.

Claims

claims 1. Austenitic heat-resistant nickel-based alloy containing (in mass%) 0.03-0.1% C 28 - 32% Cr 0.01 - <0.5% Mn 0.01 - <0.3% Si 0.01-≤ 1.0% Mo 2.5-3.2% Ti 0.01 - <0.5% Nb 0.01 - <0.5% Cu 0.05 - <2.0% Fe 0.7 -1.0% AI 0.001-.0.03% Mg 0.01-≤ 1.0% Co 0.01 -0.10% Hf 0.01 -0.10% Zr 0.002-0.02% B 0.001 - 0.01% N max.0.01% S max.0.005 Pb max.0.0005% Bi max.0.01% Ag Remaining Ni and production-related admixtures, where the sum of Ti + Al is between 3.3 and 4.3%, the sum of C + (10 x B) is between 0.05 and 0.2%, the sum of Hf + Zr is between 0.05 and 0.15% and the ratio Ti / Al> 3. 2. An alloy according to claim 1 containing (in mass%) 28-31% Cr. 3. The alloy of claim 1 or 2, which contains (in mass%) 29 - 31% Cr. 4. An alloy according to any one of claims 1 to 3 containing (in mass%) 2.8-3.2% Ti. 5. An alloy according to any one of claims 1 to 4 containing (in mass%) 2.8-3.0% Ti. 6. An alloy according to any one of claims 1 to 5, which contains as an addition of boron (in% by mass) 0.002 - 0.01%, in particular 0.002 - 0.005%. 7. An alloy according to any one of claims 1 to 6, wherein the sum of C + (10 x B) between 0.05 and 0.1%, in particular between 0.05 - 0.08%. 8. An alloy according to any one of claims 1 to 7, wherein the Zr content is adjusted between 0.01 and 0.05%. 9. An alloy according to any one of claims 1 to 8, wherein the Hf content is set between 0.01 and 0.08%. 10. An alloy according to any one of claims 1 to 9, wherein the following proportionality is given: Zr / Hf = 0.1-0.5%. 11. An alloy according to any one of claims 1 to 9, characterized in that the ratio Ti / Al is between 3.3 and 4.2. 12. Use of the alloy according to one of claims 1 to 11, as a valve material, in particular for usable in diesel engines valves. 13. Use of the alloy according to one of claims 1 to 11, as a valve material for usable in the temperature range up to 850 ⁰ C valves in marine diesel engines. 14. Valve, in particular valve for a large diesel engine, which consists at least partially of an alloy according to one of claims 1 to 11.