An improvement in thermal efficiency of fossil power plant by increasing the operating temperature and pressure of steam requires the use of construction materials of higher creep rupture strength and improved oxidation and corrosion resistance to aggressive atmospheres.
29 laboratory melts of Ni+Mn, Co and Cu bearing 9-12% Cr steels were manufactured and the effects of strong carbide forming elements such as W, Cr, and B and austenite stabilizing elements such as Ni, Mn, Co, and Cu on the RT mechanical properties and creep rupture strength at the range of 600℃ to 700℃ were investigated. Among the alloys studied, alloy compositions showing best creep properties are 2.5-3.5% W, 11.0-12.0% Cr, 1.5-2.0% Ni+Mn (or 1-2% Cu), 0.1% Mo, 0.2% V, 0.07% Nb, 0.10-0.13% C, 0.002-0.003% N, 0.008-0.01% B, ＜ 0.01% Al, ＜ 0.005% O. All the difference between developed alloys and existing 9-12% Cr steels are a removal of Co and Mo, an increase of W, Cr, and B, low Al and oxygen content, Ca deoxidization, and higher austenitizing temperature of 1100℃. The influence of each alloying element on mechanical properties and creep rupture strength is as follows ; Tensile strength at RT increase with W and Cr addition at a rate of 45MPa per %W and 49MPa per %Cr, respectively. But RT Charpy impact energy decrease with an increase of Cr and W. Beneficial elements for the creep rupture strength were B, Cr, W, C, and Cu, while harmful elements were Al, Mn, Ni, and Co. The W and Co addition seem to enhance increase in the amount of Fe and W and as a result the total amount of precipitates after tempering and creep test. The enhancement of short-term CRS with W could be attributed to the formation of Laves phase not solid solution strengthening. W bearing alloys showed two distinct regions of creep behavior at 650℃, which can be described by Norton equation, $ε_m = B \sigma^n$, with two different values for the stress exponent n. At high stresses, the value of n is around 9 - 12 while at lower stresses, n is around 2 - 4. This is the cause of sigmoidal inflections to occur at progressively shorter creep rupture test durations for Co bearing alloys with higher W content. This behavior was shown to be directly associated with microstructural degradation in the form of coarsening of Laves phase.
Developed materials satisfied the minimum creep strain and reduction of area criteria easily, but not always the impact energy requirement demanding particular attention to the tempering treatment. Microstructural examinations revealed that the Cr equivalent criterion by Newhouse is erroneous in high W steels and raised the need for a new criterion.
A new equation for the Cr equivalent is proposed for 9-12% Cr steels as follows : $Cr_eq$ (%) = Cr + 0.8Si + 2Mo + 1W + 4V + 2Nb + 1.7Al + 60B + 2Ti + 1Ta - 2.0Ni - 0.4Mn - 0.6Co - 0.6Cu - 20N - 20C. The equation takes into account the effect of W, B, Ni and Co reasonably well. A conservative rule to be kept to avoid the δ-ferrite formation in this type of steels is proposed as $Cr_eq$ ＜ 10%. For $Cr_eq$ ≥ 10 %, the amount of δ-ferrite (Y) is related to the $Cr_eq$ (X) as follows within the accuracy of ±8%. Y = 340.43 - 71.75*X + 3.77*$X^2$ for 10% ≤ X ≤ 12%. Y = -146.32 + 13.94*X for X ＞ 12%.
Second phases particles extracted from crept specimens include M23C6, Nb(C,N), V(C,N), and Fe2W included M6C for 5.5% W addition and ε-Cu for Cu bearing alloys.
