Investigations on the influence of internal nitridation on creep crack growth in alloy 800 H
- PDF / 680,152 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 108 Downloads / 283 Views
I.
INTRODUCTION
I N an earlier paper, Ill it was observed that massive internal nitridation occurred around growing creep cracks in Alloy 800 H at 1000 °C in air. Since a marked change of the microstructure is caused by this nitridation, it is imaginable that there may also be an effect on creep crack growth. The effect of internal nitridation on creep strength had already become obvious in investigations discussed by Griinling and Btirgel./21 At 1000 °C, internal nitridation around creep cracks in creep rupture test specimens had led to a decrease of the creep rate by a factor of about 1/5 for the example shown in their paper. The reason for this behavior was an increase of creep strength by particle strengthening due to the precipitation of internal nitrides. The state of knowledge about the role of particles for creep crack growth is discussed to some extent by Riedel. p) Creep crack growth occurs by cavity nucleation and coalescence. Nucleation of cavities results from the rupturing of atomic bonds at stress concentrations, at weakly bonded particles or interfaces, or by vacancy condensation. Oxides and sulfides are quoted as "weak" particles in the above sense. Carbides are not regarded as "weak" particles, and nucleation of cavities will occur by vacancy condensation (in the interface). The effect of nitride particles has not yet been discussed in this respect. After cavity nucleation, subsequent crack growth will again be facilitated by the presence of particles and interfaces of low cohesive strength. The extension of the creep zone at the crack tip is small in such a case, and the macroscopic description of creep crack growth may be made on a linear-elastic basis. As the area covered by "weak" particles increases, the resistance to crack growth
M. W E L K E R , formerly with D E C H E M A , is with Leonische Drahtwerke AG, Roth, Federal Republic of Germany. A. R A H M E L and M. SCHI]TZE are with DECHEMA, Postfach 97 01 46, Frankfurt, Federal Republic of Germany. Manuscript submitted July 8, 1988. METALLURGICAL TRANSACTIONS A
decreases. In the case of strongly bonded particles, theoretical considerations lead to the conclusion that diffusive (Coble) creep is inhibited by the particles and, thus, also diffusive cavity growth. However, the respective analysis suggests that inhibition should only become effective at stresses below those usually applied under creep rupture test conditions. The effect of particles on crack growth may not be very marked in this case. The zone in which creep occurs around the crack tip is large, and the macroscopic description of creep crack growth must take time dependence of the stressstrain field around the crack tip into account. Since the role of nitride particles from internal nitridation for creep crack growth was not clear, creep crack growth tests were performed in air (with massive internal nitridation, i.e., nitride particles at grain boundaries and also in the grains) and in Ar-5 pct H2 atmosphere (with no nitridation). While in a previous paper tq the mechanisms a
Data Loading...
