Effect of Alloying Elements in Creep Resistant Steels

Effect of Alloying Elements in Creep Resistant Steels

The composition of the alloy has a great bearing on the phases formed and the sequences of precipitation. It is useful to examine the effect of each element on the alloy properties even though the problem became extremely complicated due to element interaction. The elements used to make up the composition of these steels are added for many reasons. Most elements are added to stabilise phases which are beneficial to creep resistance or to suppress phases which are detrimental. Some are added for long-term solid solution strengthening or to improve the corrosion resistance of the alloy. The influence on microstructure of the main alloying elements can be summarised as follows:

Chromium: This is a ferrite stabilising element and a carbide former. Large Cr additions of 9-12% provide the necessary oxidation and corrosion resistance, as well as the strengthening of the material by precipitation of carbides. Additions larger than 11% Cr were found to markedly increase corrosion resistance at 650°C. However, high Cr concentrations promote the formation of delta-ferrite. In addition, several authors have demonstrated that high Cr contents (around 12%) increase the driving force for precipitation of Z-phase, which decreases the creep strength.

Cobalt: This element is used in order to stabilise the austenite field. Co was found to remain in solid solution in 12% Cr steels, even with concentrations up to 10 wt%. Co also raises the Ms and the Curie temperature and it is expected to slow down diffusion processes, reducing the coarsening of the precipitates.

Copper: This is a very effective element to avoid the formation of delta-ferrite, which has a detrimental effect on the mechanical properties of the steel. At concentrations higher than 0.5% Cu prevents a further sharp decrease of the Ac1 temperature. Cu has a low solubility in ferrite and forms Cu-rich precipitates, which may provide nucleation sites for Laves phase formation.

Carbon: C occupies interstitial sites in both austenite and ferrite, with a greater solubility in austenite. C stabilises the austenite relative to ferrite. It is essential for the formation of carbides which causes the secondary hardening of the 9-12% Cr steels.

Nitrogen: N also occupies interstitial sites in the iron lattice and is an austenite stabiliser. Increasing N stabilises MX precipitates, which are fine and generally desirable for creep strength. The addition of N to B containing steels should be restricted due to the formation of BN, which can offset the beneficial effects of both B and N.

Silicon: Si is a ferrite stabilising element and can influence the kinetics of carbide precipitation. Si additions have been found to accelerate the precipitation and coarsening of Laves phase. Si additions can also promote the formation of delta-ferrite phase so that austenite stabilising elements are often added purely to counteract this effect. Si is very important in the formation of protective oxidation layers.

Vanadium: V is a ferrite stabilising element and a strong carbide former. It may also combine with C and N to form fine V-rich carbides and carbonitrides (MX precipitates) which significantly improve the long term creep strength.

Niobium: Nb is a ferrite stabilising element which forms stables MX precipitates with C and N. The effect of Nb depends on the austenisation temperature which governs the amount of MX precipitates which is taken into solution.

Tantalum: Ta, as Nb, is a ferrite stabilising element which forms stable MX precipitates with C and N. The Ta-rich MX precipitates were found to be beneficial to the creep rupture strength. They are extremely stable and show slow coarsening rates during creep.

Titanium: Ti is a strong nitride and carbide former and can improve the creep rupture strength of ferritic steels. Ti precipitates showed a very slow coarsening rate. However, Ti in combination with N may promote the formation of large TiN, which may decrease the creep strength.

Molybdenum: Mo is a ferrite stabilising element and also forms carbides. Additions of Mo can stabilise the M2X phase and the M23C6 phase. Large additions (> 1%) have been found to promote formation of M6C, Laves phase and delta-ferrite in 9% Cr steels.

Tungsten: W is a strong carbide former which promotes the formation of Laves phase and stabilises the ferrite. W is also well known to increase the high temperature strength via solid solution hardening. W additions have been found to generally improve the creep strength of ferritic heat resistant steels. Abe et al. reported that increased W concentrations in 9% Cr alloys reduce the coarsening rate of M23C6 precipitates. This influences the coarsening of martensite laths due to the pinning effect of M23C6, thus delaying recovery of the lath martensitic microstructure.

Boron: B stabilises the lath martensitic microstructure, due to the fact that B reduces the Ostwald ripening rate of fine M23C6 carbides by the enrichment of B in the vicinity of prior austenite boundaries.

M23C6: M23C6 is a Cr-rich carbide which may also contain W, Mo, V, Fe and B. The M23C6 has a cubic crystal structure (fcc space group Fm3m) with the lattice parameter varying between 1.057 and 1.068 nm. M23C6 carbides are found in the early stage of tempering, because they nucleate easily on the prior austenite grain boundaries and martensite laths or block boundaries. After tempering the average size of the carbides is about 100 nm, but the coarsening rate is comparatively high, decreasing their influence on creep strength with time. In B containing steels, B will dissolve in M23C6 carbides and substitute for carbon, although only in very small quantities. The enrichment of B in M23C6 carbide promotes the formation of intergranular M23(C,B)6 which may decrease the coarsening rate of the precipitate.

MX: The formation of MX precipitates occurs when strong carbides and/or nitrides formers are added to the alloy (e.g. V, Nb, Ta, Ti). MX carbonitrides often have a cubic NaCl-type structure. Often the lattice parameters have intermediate values, indicating the existence of a solid solution between the different carbonitrides. MX particles usually form during tempering on dislocations within the matrix or on sub-grain boundaries. They increase creep strength by pinning free dislocations and sub-grain boundaries.

Laves Phase: This is an intermetallic phase of the type (Fe,Cr)2(W,Mo) which may precipitate in particular in Mo or W containing steels. Laves phase also contains minor amounts of Si. A Laves phase with a hexagonal crystal structure (space group P63) with lattice parameters a=0.473 and c=9.772 nm is usually found in 9-12% Cr steels. Often the Laves phase does not nucleate during tempering as it is not stable at high temperatures. The nucleation and growth rate is slow. It precipitates intergranularly during service exposure. During the growth step it becomes larger than most other particles, but the coarsening rate is slower than M23C6. W-containing Laves phase usually nucleates faster, thus becoming smaller and more finely distributed as compared to the Mo-containing Laves phase. The Laves phase has in the past been blamed for the breakdowns in creep strength of several 9-12% Cr steels, arguing that it removes W from the matrix, which would imply a loss of solid solution strengthening by W. However, this explanation seems unlikely, as the precipitation strengthening contribution of Laves phase should largely compensate the W depletion.

Z-phase: Z-phase is probably the most stable nitride in 9-12% Cr steels during long-term exposure in the temperature range 600-700°C. It has an empirical formula of CrXN, were X can be Nb, V or Ta. Jack et al. first identified it as a tetragonal CrNbN nitride. Danielsen et al. identified a V containing modified Z-phase with a cubic NaCl-type structure. In a recent work Danielsen reported different Z-phase crystal structures from several authors. According to Danielsen the cubic structure was found to coexist whit the tetragonal structure in the Z-phase. Further investigation showed that the cubic structure of Z-phase was predominant in samples which had been exposed for relatively short times, while the tetragonal diffraction patterns became clearer with longer exposure times.
The Cr content in 9-12% Cr steels has a strong influence on the precipitation kinetics of Z-phase. 11-12% Cr steels have a much higher rate of Z-phase precipitation than 9% Cr steels. Z-phase precipitation causes the dissolution of MX carbonitrides, which are beneficial to creep strength. Hence progressive Z-phase precipitation causes breakdown in creep strength. High Cr steels show creep strength reductions concurrent with Z-phase precipitation, whereas the steels with ~ 9% Cr and limited Zphase precipitation do not show effects. Therefore Z-phase directly contributes to a reduction in the creep rupture strength on 11-12% Cr steels.

Reference: David Rojas Jara, 9-12% Cr heat resistant steels: Alloy design, TEM characterisation of microstructure evolution and creep response at 650°C, PhD Thesis, der Fakultät für Maschinenbau der Ruhr-Universität Bochum, 2011, pp. 11-16.