The effects of Cu on long term creep rupture strength was investigated for ferritic 12% Cr steels bearing 1-2Cu, 2.5-3.5W, 0.008B after quenching, tempering, and subsequent prolonged (maximum 10000 hours) creep rupture test.
Creep rupture test were performed at 600℃, 650℃, and 700℃ in air atmosphere under constant loads. TEM observations, XRD analysis and electrolytic precipitates extractions were carried out for precipitation kinetics.
Cu was added to avoid δ-ferrite in A~F alloys and Co was added to avoid δ-ferrite in L, M, N alloys. Cu effects on long time creep rupture strength of 12%Cr steels was investigated by comparing creep rupture strength of these two species of alloy.
Combined addition of W, Cr and B was effective to the increase of creep rupture strength. Co added alloys showed higher creep rupture strength than Cu added alloys at 600℃ and 7O0℃. However, the Co alloys' creep rupture strength was decreased rapidly at 6S0℃ with creep time, thus Cu added alloys' creep rupture strength was higher than that of Co added alloys in long term side(> 10000 hours), it can be accounted for the premature precipitation of bulky type Laves phase and depletion of W level in matrix caused by the combined addition of Co and W.
In Cu added alloys, there are two types of Laves phase. One is bulky type. Other is rod type. Rod type Laves phase's nose temperature in time temperature precipitation diagram is about 600℃. Nose temperature of bulky type Laves phase is about 650℃. Composition of two types of Laves phase is almost same.
Rod type Laves phase precipitated from as tempered state is dissolved and disappeared at 650℃ with creep time and wt% of W in electrolytic extracted precipitates of Cu added alloys is lower than that of Co added alloys. This means that the content of W in the matrix of Cu added alloys is higher than that of Co added alloys, in the conditions of same wt% of precipitates and same content of W in two species of alloys. So that the reason for higher creep rupture strength of Cu added alloys than that of Co added alloys at 650℃ in long time side is effective solid solution hardening by higher W content in the matrix of Cu added alloys.
$M_{23}C_6$, $Fe_2W$ type Laves phase, Nb(C,N), $M_2X$, V(C,N), ε-Cu were identified by X-ray diffraction of electrolytic extracted precipitates residues, and Cu did not participate in precipitates.
In case of 700℃ tempering it is considered that many precipitates nucleates on Cu-rich phase or ε-Cu and independent ε-Cu particle could not be observed. But in case of 730℃ tempering independent pure ε-Cu particle was found. From this, it is considered that precipitation behaviour of ε-Cu is associated with the tempering temperature.
Amount of electrolytic extracted precipitates increased rapidly with creep time and was saturated in time of 2000 hours at all creep temperature.
Creep rupture elongation of Cu added alloys is no more than 10% despite of the high elongation more than 20% in RT tensile test. But creep rupture elongation of E alloy tempered at 730℃ is about 10% and it showed the highest creep rupture strength in long time side at 650℃. So, tempering study is required to obtain high creep rupture strength in long time side and creep rupture elongation more than 10%.
