USE OF NEW HIGH-STRENGTH CORROSION-RESISTANT STEELS ALLOYED WITH NITROGEN IN THE MANUFACTURE OF MULTILAYER BELLOWS

The mechanical properties of the steel were determined by subjecting specimens cut from pipes-layers of multilayer packets to static tensile tests at room temperature. The results obtained (Table i) suggest that the mechanical properties of the steel are functions of the nitrogen concentration (fraction of total means) and grain size. Note that with other conditions equal, an increase in the nitrogen fraction of the total mass exerts a major influence on the ultimate strength, while a reduction in grain size has a significant effect on yield point. For similar grain sizes, therefore, an increase in the nitrogen fraction of the total mass from 0.33 to 0.58% leads to a 90 MPa increase in the ultimate strength type 22-10~2 steel on average, despite a lower carbon concentration, while a grainsize reduction of from 9-10 to 12-13 increases the yield point by 140 MPa for the same nitrogen and carbon fraction in the total mass. As compared with steel 08KhI8NIOT, the ultimate strength of all the steels tested is 1.5 times greater, and the yield point more than two times greater with retention of a sufficiently high level of plasticity. It should be noted that a significant level of strength characteristics, which often exceeds the level attained for liquid-phase alloying even with a coarser metal grain (right up to size 6), is observed in pipes formed from steels with a nitrogen fraction of ~0.6% of the total mass, which are produced by the SDAN method. On the one hand, this enables us to expect an even greater hardening under the same temperature conditions with grain-size reduction; in light of notions concerning the mechanism of creep above the equicohesive temperature (~500~ for this type of steels) [3], on the other hand, the high strength of highnitrogen steel with a rather coarse grain is extremely suitable for the operation of bellows. In the latter case, alloying with nitrogen tends to lessen the contribution of the grain boundaries to creep; in this case, the higher the nitrogen content in the steel,