A study has been made on the microstructure and high temperature mechanical properties after carburization treatment of HP-50 (25Cr-35Ni-0.5C) cast tube which is widely used as cracking tube in petrochemical industries. Pack carburization was used to produce simulated service condition.
The changes in carbide type occurred at carbon content about 2.0% when area fraction of carbide has risen to about 25%.
The blocky carbide in carburized layer was identified by X-ray diffraction as $M_7C_3$ where the other carbide in uncarburized area was $M_{23}C_6$ type.
In tensile test, high temperature ductility is nearly unchanged whereas R.T. tensile ductility was markedly reduced by carburization.
The results of stress-rupture test at 980$^\circ{C}$-35MPa. showed that time to rupture is enhanced by carburizing treatment at 1100$^\circ{C}$ but reduced after carburization at 950$^\circ{C}$. However the strains to rupture increased considerably for both conditions.
Crack nucleation and growth to critical size occurs rather rapidly in the specimen carburized at 950$^\circ{C}$ whereas the process is delayed by carburization at 1100$^\circ{C}$.
The results can be explained mainly by difference in carbide morphology and distribution in matrix. The smooth interface of blocky carbide, which formed in $M_7C_3$ layer, easily accomodates the deformation by boundary sliding and thus cavity & crack formation is reduced.
A boundary zone strengthened by fine secondary carbide precipitates is undesirable from view of creep ductility.
