Z-phase in creep resistant steels

Table 1: Chemical composition and heat treatment of 9 to 12 % Cr steels discussed. Wt % bal. Fe.

Figure 1: Steel grade 92. Tempered martensite microstructure. Scale bar: 0.5 µm.

Figure 2: Large (a) and small (b) Z-phase particles co-existing in a 10.5 % Cr steel after 43,000 hours at 600 C. Scale bar: 0.2 µm, 50 nm.

Figure 3: a) Driving force for Z-phase precipitation in 9 to 12 % Cr steels. Z650 is an experimental 12 % Cr steel. b) Very fine Z-phase particles in steel Z650 after heat treatment 650 C/3,000 hours. Scale bar: 50 nm.

Table 2: Cr content of steels compared to experimentally found Z-phase contents. Steels are ranked according to Z-phase content.

Carbide name: No data
Record No.: 943
Carbide formula: No data
Carbide type: No data
Carbide composition in weight %: No data
Image type: TEM
Steel name: Creep resistant steel
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: No data
Steel composition in weight %: No data
Heat treatment/condition: No data
Note: Creep resistant steels
Links: Because of their combination of high creep and fatigue strength and moderate cost, the martensitic 9 to 12 % Cr steels are vital materials for further increases in steam parameters of fossil-fired steam power plants. Recently, new stronger steels like the 9 % CrW steel Grade 92 for thick section boiler components and steam lines, and the 9 % CrCoB steels FB2 and CB2 for large forgings and castings, have been introduced in commercial power plant projects. These steels have enabled the construction of power plants with supercritical live steam parameters up to 600 C/300 bar. The improved efficiency of these new plants compared to earlier sub-critical steam plants (540 C/180 bar) corresponds to a reduction of some 30 % in specific CO2 emission.
In order to further increase the steam parameters of steel-based power plants up to a target value of 650 C/325 bar it is necessary to double the creep strength compared with Grade 92, and at the same time the resistance against steam oxidation must be improved. If the oxidation protection is to be achieved through alloy additions instead of surface coatings, it is necessary to increase the Chromium content in the steels from 9 % to 12 %. However, so far all such attempts to make stronger 12 % Cr steels have failed because the high chromium content introduced severe microstructure instabilities in the tested steels, which led to breakdowns in long-term creep strength.
Since the well-known steel Grade 91, the long-term creep strength of the new generation of martensitic creep resistant steels relies strongly on particle strengthening by fine MN nitrides based on V and Nb. Additions of about 0.15 to 0.25 % V, 0.04 to 0.08 % Nb and 0.02 to 0.07 % N are common to all of the new steels, due to results obtained by Fujita in the late 1970s. However, recent research has demonstrated that the MN nitrides, (V,Nb) N, may be replaced by the thermodynamically more stable Z-phases, Cr(V,Nb)N, which precipitate as coarse particles and dissolve the fine nitrides. This phase transformation has been found to be a main cause for observed long-term microstructure instabilities in the new martensitic creep resistant steels. Thus, the Z-phase transformation is a main cause for the lack of success to develop strong martensitic steels with 12 % Cr for improved oxidation resistance. In the present paper the Z-phase transformation is described in detail, and its influence on the long-term stability of 9 to 12 % Cr steels is discussed. Finally, a possible way to overcome present difficulties to combine high Cr content with long-term microstructure stability is outlined.

Creep and strengthening mechanism: The 9 to 12 % Cr steels have mainly tempered martensite microstructures formed during a final normalising and tempering heat treatment, although for some compositions considerable amounts of Delta-ferrite may form in the microstructure. The tempered martensite consists of prior austenite grains separated by high angle grain boundaries. These grains are subdivided by high angle boundaries into blocks of martensite laths. After tempering the blocks consist of sub-grains with an average width of 0.3 to 0.5 µm separated by low angle boundaries. The dislocation density of the tempered martensite is high (in the order of 10 exp(14) m2), Figure 1.
The technically interesting stress and temperature ranges for creep testing and service exposure of the 9 to 12 % Cr steels are 250 MPa to 20 MPa at 500 C to 700 C. In these ranges the main creep mechanism in the steels is dislocation creep. This leads to glide of free dislocations and migration of sub-grain boundaries during creep, resulting in a reduction of the dislocation density and growth of sub-grains. These deformation processes can be delayed by precipitate particles, which pin dislocations and sub-grain boundaries.
A number of different precipitates can be found in the 9 to 12 % Cr steels. Depending on their particle hardening effect and stability against coarsening or dissolution during longterm exposure the precipitates determine the microstructure stability of the steels. Precipitate hardening should be regarded as the most significant creep strengthening mechanism to obtain high long-term creep strength in the 9 to 12 % Cr steels. The most useful precipitates are the Cr carbides M23C6, the intermetallic Laves phases Fe2(Mo,W) and the MN nitrides (V,Nb)N. The MN nitrides precipitate in high number densities as very fine particles, which are much more stable against coarsening than the other precipitates [14]. As such they contribute significantly to longterm precipitation hardening in the steels. Boron addition to the steels may stabilise the M23C6 carbides, especially near the prior austenite grain boundaries.

Behaviour and thermodynamic modelling of Z-phase in 9 to 12 % Cr steels: Z-phase Cr(V,Nb)N was first found in a martensitic 11 % Cr steel by Schnabel et.al. in 1987. In 1996 Vodarek and Strang realised that Z-phase precipitation could explain observed sigmoidal creep behaviour in older 12CrMoVNb steels, since the Z-phase precipitates as coarse particles and dissolves fine MN nitride particles. Recently, the authors made systematic studies of Z-phase content in a number of 9 to 12 % Cr steels, which rely on MN strengthening. Both commercial and experimental grades were investigated after exposure to creep, and a clear correlation between the Cr content of the steels and the observed Z-phase quantity was found, Table 2. Comparisons with creep testing on a number of the investigated steels show that for Cr contents above 10.5 % the strongly accelerated Z-phase precipitation leads to a breakdown in long-term creep strength. In steels with Cr contents below 9 % Z-phase precipitation is so slow that they are largely unaffected up to very long testing times at 600 to 650 C.
Results from detailed studies of the mentioned steels were used to develop a thermodynamic equilibrium model of the Z-phase. This model has demonstrated that Z-phase Cr(V, Nb)N is thermodynamically the most stable nitride in all 9 to 12 % Cr steels containing V, Nb and N. This means that Z-phase should eventually form in all of the new generation 9 to 12 % Cr steels with potentially detrimental effects on their creep strength. However, as seen from Table 2, large differences in Z-phase precipitation rate has been found, which seem to correlate well with similarly observed large differences in the rate of breakdown in creep strength.

Effect of MN and Z-phase on long-term creep strength: The quantitative effect of MN precipitates on long-term creep strength of modern 9 to 12 % Cr steels can be demonstrated by comparisons of selected steels, which all have fully tempered martensite microstructures free of Delta-ferrite. The chemical composition and heat treatment of the discussed steels are shown in The 9Cr3W-3CoVNb and 9Cr3W3CoVNbB steels have very low Nitrogen content and consequently they contain only M23C6 carbides and intermetallic Fe2W Laves phase, but no MN nitrides. The 9Cr3W3CoVNbB steel is stabilised by Boron addition. As such these two steels represent baseline strength levels of the modern 9 % Cr steels without nitrides. The P92 steel contains fine (V,Nb)N MN nitrides in addition to M23C6 carbides, Fe2W Laves phases and Boron stabilisation. Because of the low Cr content the nitrides in steel P92 are stable against Z-phase precipitation up to long-term creep exposures. In short-term tests up to 2,000 hours the three steels have similar rupture strength. Above 2,000 hours the Boron addition in steels 9Cr3W3CoVNbB and P92 is effective, and above 10,000 hours the MN nitrides in steel P92 are effective to stabilise the microstructure and increase the strength. All three steels have similar heat treatment.

If the MN nitrides disappear from the microstructure during creep it has a clear negative effect on the strength. The P122 steel contains MN nitrides, M23C6 carbides, Fe2W Laves phases and Boron similar to the P92 steel, but due to the high Cr-content of 10.7 %, strong precipitation of Cr(V,Nb)N Z-phase occurs above 10,000 hours and this dissolves the MN nitrides. Consequently, the strength of the P122 steel stays at the level of the 9Cr3W- 3CoVNbB steel, which is without nitrogen, but has a similar Boron content as P122. The creep rupture strength of the 9 to 12 % Cr steels can be increased above the level of steel P92 by optimisation of chemical composition and heat treatment. The creep rupture strength of the 9 to 12 % Cr steels can be increased above the level of steel P92 by optimisation of chemical composition and heat treatment.

The Z-phase precipitation process: Even though the Cr(V,Nb)N Z-phase is thermodynamically the most stable nitride in 9 to 12 % Cr steels alloyed with V, Nb and N it seems quite difficult for it to nucleate. Z-phase has never been observed in any of the steels directly after tempering for a few hours in the normal range 650 to 800 C. Even in the high Cr steels, where Z-phase forms most rapidly, widespread Z-phase formation has only been observed after quite long exposure times of 1000 hours or more at 650 C. Experimental studies of Z-phase TTP diagrams indicate that the fastest precipitation happens at 650 C.
Once a Z-phase has nucleated it can grow fast, and large Z-phase particles can be observed after relatively short exposures. The Cr(V,Nb) N Z-phase grows by dissolution of the MN precipitates, which provide V, Nb and N, and by picking up Cr from the steel matrix. In spite of the fast growth, observations of longterm exposed specimens always reveal both large and small Z-phase particles, Figure 2. This indicates that the nucleation of Z-phase is slow and continuous, and thus the nucleation process should be regarded as rate controlling for the Z-phase transformation. The fact that Z-phase grows fast means that even in early stages of the transformation large Z-phase particles can be observed in the microstructure even though only a very limited part of the MN population has been dissolved. Thus, an observation of a few large Z-phase particles in the steel microstructure is not necessarily alarming for the creep stability. Instead, attention should be focused on the rate of removal of the MN nitrides by measurements of their number density.

As mentioned, calculations of the driving force for Z-phase precipitation allows estimations of the precipitation rate of the Z-phase. The obvious strategy is then to minimise the driving force in order to delay the harmful Z-phase precipitation to very long times (beyond 300,000 hours), where it would have no influence on the long-term stability of the steel in practical use. Unfortunately, as shown this leads to limitations on the Cr content, which prevent attaining the necessary oxidation resistance of the alloy.
Scrutinising the thermodynamic model suggests that Z-phases based on CrNbN or CrTaN could possibly serve as strengthening agents, as the driving force for their formation are significantly higher compared to that of CrVN or Cr(V,Nb)N. Finally, calculations of coarsening rates based on thermodynamic data made as described in, indicate coarsening stabilities of the Z-phases similar to or better than the MN nitrides.
Ongoing work at the TU Denmark to study possibilities to use the Z-phase as a strengthening agent has included design and production of a number of experimental high Cr martensitic steels with high driving force for Z-phase precipitation, Figure 3. So far the experimental steels have demonstrated that it is indeed possible to produce a stable distribution of fine Z-phase particles in 12 % Cr martensitic steels.
Reference: Not shown in this demo version.

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