WO1997039158A1

WO1997039158A1 - Martensitic-austentitic steel

Info

Publication number: WO1997039158A1
Application number: PCT/CH1997/000123
Authority: WO
Inventors: Peter Ernst; Peter J. Uggowitzer; Markus O. Speidel; Reiner Steins
Original assignee: Abb Research Ltd.
Priority date: 1996-04-12
Filing date: 1997-03-25
Publication date: 1997-10-23
Also published as: NO984756L; JP2000510904A; CA2251805A1; NO984756D0; DE19614407A1; PL329050A1; EP0907758A1; CN1216073A

Abstract

This invention concerns a martensitic-austentitic steel consisting essentially (in wt%) of: 8 to 15 % chromium, 0.5 to 2.5 % molybdenum, up to 2 % tungsten, 4 to 10 % cobalt, 0.5 to 6 % nickel, 2.5 to 8 % manganese, 0.1 to 1.0 % vanadium, 0.05 to 0,25 % nitrogen, up to 0.2 % carbon, the rest being iron with impurities. This martensitic-austentitic steel can be used, in particular, as material for rotors or rotor disks in gas turbines operating at temperatures above 450 °C. The adjustment of the two phase martensitic-austentitic structure is done by heat treatment of the solution annealed and quenched steel structure in a temperature range between 550 °C and 650 °C.

Description

Martensitic-austenitic steel

Technical field

The invention relates to a martensitic-austenitic steel, in particular for use as a material for creep-stressed components of gas turbines.

State of the art

The use of temperable chrome steels as a material for use in thermal power plants, in particular as a material for rotors or rotor disks, is state of the art today. Compared to nickel-based superalloys, temperable chrome steels are characterized by better non-destructive testability. In addition, they have comparatively low thermal expansion coefficients and high heat conduction; this increases resistance to thermal fatigue. However, the tempered steels do not meet the desired requirements with regard to heat resistance, creep resistance and toughness at temperatures above 450 ° C.

A hardenable martensitic steel known as X12CrNiMo12 is known. In addition to iron, this steel contains 0.10-0.14% C, 0.10-0.40% Si, 0.5-0.9% Mn, 11-12% Cr, 2-2.6% Ni, 1.3-1.8% Mo, 0.2-0.35% V and 0.02-0.05 % N as well usual impurities. In the temperature range below 450 ° C, this steel has a relatively high hot stretch and a relatively high creep resistance. At temperatures above 450 ° C, however, the hot stretch limit and creep behavior are insufficient. In addition, the steel has a not inconsiderable tendency to become brittle at higher temperatures.

Presentation of the invention

The invention has the object of providing a martensitic-austenitic steel for use in gas turbine rotors or -rotorscheiben genü¬ with a constricting temperature strength and toughness for a temperature range of 450 ° C to ^β to at least 550 C.

According to the invention, this is achieved by the features of the first claim.

The essence of the invention is the setting of a very fine two-phase microstructure consisting of tempered martensite and thermodynamically stable austenite. The steel consists essentially of 8 to 15% chromium, 4 to 10% cobalt, 2.5 to 8% manganese, 0.5 to 6% nickel, 0.5 to 2.5% molybdenum, up to 2% tungsten, 0.1 to 0.5% vanadium , 0.05 to 0.2% carbon, 0.05 to 0.25% nitrogen, remainder iron and usual melting-related impurities.

The advantages of the invention can be seen above all in the fact that the steel formed in a duplex structure has a high hot stretching limit and a high creep resistance even at temperatures above 450 ° C. and has a low tendency to embrittlement due to its high structural stability.

Further advantageous embodiments result from the subclaims. Way of carrying out the invention

The steel developed for use in accordance with the invention essentially contains (measured in% by weight) 8 to 15% chromium, 4 to 10% cobalt, 2.5 to 8% manganese, 0.5 to 6% nickel, 0.5 to 2.5% molybdenum, bis 2% tungsten, 0.1 to 1.0% vanadium, 0.05 to 0.2% carbon and 0.05 to 0.25% nitrogen and can be produced by casting or by powder metallurgy.

Known and introduced 9-12% chromium steels achieve their warm or creep resistance through the stabilizing effect of the finest precipitations on the dislocation network formed in the tempered martensite. In the case of the steel developed for the use according to the invention, in addition to the fine precipitations, a face-centered cubic phase (austenite) is introduced which, due to its low self-diffusion and the formation of phase boundaries, contributes to increasing the heat and creep resistance, martensitic steels with austenite as creep-improving second phase are known in the art. Compared to the steel according to the invention, however, they differ in that their austenite content is not thermodynamically stable and is therefore highly susceptible to thermal fatigue.

The particularly preferred amounts for each element and the reasons for the selected alloy ranges of the alloy according to the invention are set out below.

Chromium dissolved in the mixed crystal increases the resistance to oxidation. Via the formation of hexagonal chromium nitrides, Cr can also improve the Creep resistance contribute. To achieve this, a minimum Cr content of 8% by weight is necessary. However, the chromium content should not exceed 15% by weight, since otherwise δ-ferrite is formed, which leads to a reduction in toughness and high temperature strength. A suitable range for chromium therefore ranges from about 8 to 15% by weight, preferably from 9 to 12% by weight and in particular from about 10% by weight.

Manganese in the present steel increases the nitrogen solubility in the austenite area particularly effectively. In order to dissolve the stable nitrides of the hexagonal type (Cr, V) 2N and of the cubic type (V, Cr) N during solution annealing, a minimum Mn content of 2.5% by weight is desirable. During the tempering treatment, manganese is concentrated in the austenite formed and to a significant extent lowers its martensite start temperature, i.e. Manganese stabilizes the austenite. For this reason too, the manganese content should be at least 2.5% by weight. However, the Mn content should not exceed 8% by weight so that the alloy does not become completely austenitic. A suitable range for manganese therefore ranges from approximately 2.5 to 8% by weight, preferably from 3.5 to 6.5% by weight, and is in particular approximately 5% by weight.

Nickel increases toughness in martensitic chrome steels because it e.g. effectively reduces the δ ferrite content. Similar to manganese, nickel stabilizes the austenitic phase in the duplex structure. For this reason, the alloy should contain at least 0.5% by weight of Ni. Above a nickel content of 6% by weight, the Ac3 point is lowered too much. A suitable range for nickel therefore ranges from about 0.5 to 6% by weight, preferably from 2 to 5% by weight, and is in particular about 3.7% by weight.

Cobalt increases the nickel equivalent to such an extent that the alloy solidifies austenitically from the melt. This avoids nitrogen effusion and thus pore formation. The Co content should therefore be at least 4% by weight. The co- However, the content should not exceed 10% by weight so that the alloy still has a sufficiently high AQ3 temperature. A suitable range for cobalt therefore ranges from approximately 4 to 10% by weight, preferably from 5 to 8% by weight and is in particular approximately 6% by weight.

Molybdenum promotes tempering resistance and heat resistance, so a minimum content of 0.2% by weight should be observed. The Mo content should not exceed 2.0% by weight, since otherwise massive Laves phase formation can occur. A suitable range for molybdenum therefore ranges from 0.5 to 2.5% by weight, preferably from 1.0 to 2.0% by weight, and is in particular about 1.5% by weight.

Tungsten works in a similar way to molybdenum. Because of the risk of lavage phase formation, a content of 2% by weight should not be exceeded. Therefore, the tungsten content is in the range of 0 to 2% by weight, preferably less than 1% by weight.

Together with nitrogen and small amounts of chromium, vanadium forms a dense dispersion of coherent and partially coherent cubic nitrides, which largely determine the heat resistance and creep resistance. At least 0.1% by weight of V should be added. However, since vanadium promotes the tendency to form δ-ferrite, 1.0% by weight of V should not be exceeded. The vanadium content is preferably in the range from 0.15 to 0.65% by weight.

In the atomically dissolved state, nitrogen promotes the diffusion-free martensitic transformation from the austenitic phase during cooling. Remuneration is thus guaranteed. In addition, it forms the aforementioned nitrides with V and Cr, possibly also with Nb, Ti and Ta. Nitrogen should therefore be alloyed in approximately stoichiometric amounts. An upper limit of 0.25% by weight should not be exceeded due to possible N effusion during solidification. Therefore the

CORRECTED SHEET (RULE 91) ISA / EP N content in the range from about 0.05 to 0.25% by weight, preferably in the range from 0.07 to 0.15% by weight.

Together with chromium, carbon promotes the formation of carbides of the form Cr23C6. Because of its stoichiometry, this cabid extracts more chromium from the matrix than the Cr2N formed in the steel according to the invention. For this reason, the carbon content should not exceed 0.2% by weight, preferably 0.1% by weight.

Niobium, titanium, zirconium and tantalum are alloying elements which together with vanadium can form MX type probe nitrides. Their effect is based primarily on the fact that they increase the stability of V (N, C) excretions even in small additions. If the contents are too high, however, the stability of the nitrides is so high that they cannot be dissolved in the solution treatment. For this reason, the total content of these elements should be limited to 0.5%.

Boron increases the coarsening resistance of excretions. Since it tends to segregate, the content must be limited to 0.02%.

In the case of the impurities which usually occur during production, the elements such as phosphorus, sulfur, antimony, tin and arsenic should not exceed the contents given in Table 2 below. This is necessary to avoid embrittlement.

The steel according to the invention has a martensitic-austenitic structure which is produced by a tempering process. The tempering process consists of solution annealing, hardening and subsequent tempering. Solution annealing should be carried out in the temperature range 1050 ° C ≤ T ≤ 1250 ° C, preferably at 1100 ° C ≤ T ≤ 1230 ° C and especially at 1200 ° C to dissolve all nitrides of the VN form. The austenite content and the degree of hardening of the martensitic phase are set via the tempering treatment. In order to set a desired austenite content of 15 to 45%, a tempering temperature in the range of 550 ° C T T 650 650 ° C is to be selected, preferably 580 ° C T T 630 630 ° C.

Embodiment

An example of a particularly preferred embodiment of the alloys described above, which is referred to as alloy 817, is described in more detail below. The composition of alloy 817 can be found in Table 1 and Table 2. Table 2 shows some maximum levels of possible impurities.

After solution annealing and quench treatment, the alloy was annealed at 600 ° for 4 hours. After this heat treatment, the structure was two-phase martensitic-austenitic with an austenite phase fraction of approximately 37% with a phase range size of 200 to 300 nm.

The properties of the alloy 817 according to the invention are compared in Table 3 with the alloy X12CrNiMo12 mentioned earlier.

According to Table 3, alloy 817 is characterized by consistently better properties compared to alloy X12CrNiMo12. The high heat and creep resistance allow use as a rotor or rotor disk material for gas turbines at elevated temperatures up to 550 ° C.

Table 1

Table 2

Table 3

Claims

claims

1. Martensitic-austenitic steel, consisting essentially of: (measured in% by weight) from 8 to 15% chromium, 0.5 to 2.5% molybdenum, up to 2% tungsten, 4 to 10% cobalt, 0.5 to 6% Nickel, 2.5 to 8% manganese, 0.1 to 1.0% vanadium, 0.05 to 0.25% nitrogen, to 0.2% carbon, the rest of iron with impurities.

2. Martensitic-austenitic steel according to claim 1, characterized in that chromium in the range from 9 to 12% and / or molybdenum in the range from 1.0 to 2.0% and / or cobalt in the range from 5 to 8% and / or nickel in the range from 2 to 5% and / or manganese in the range from 3.5 to 6.5% and / or tungsten 1.0% and / or vanadium in the range from 0.15 to 0.65% and / or nitrogen in the range from 0.07 to 0.15% and / or carbon up to 0.1%.

3. Martensitic-austenitic steel according to claim 1 or 2, characterized in that a maximum total content of 0.5% of the elements niobium, titanium, zirconium and tantalum is contained.

4. Martensitic-austenitic steel according to claim 1, 2 or 3, characterized in that a maximum of 0.02% boron is contained.

5. Use of the martensitic-austenitic steel according to one of claims 1 to 4, as a heat and creep-resistant material in thermal power plants.

6. Heat treatment process for the martensitic-austenitic steel according to one of claims 1 to 4, characterized in that the steel alloy is annealed in the solution-annealed and quenched state in the temperature range from 550 ° C to 650 ° C so that a structure with a volume fraction of 20 % to 50% austenite is formed