NÁVRH MATERIÁLU A TEPELNÉHO ZPRACOVÁNÍ PRO VÝROBU VYSOKOTLAKÝCH LAHVÍ URČENÝCH PRO TECHNICKÉ PLYNY A CNG S VYSOKÝMI HODNOTAMI MECHANICKÝCH VLASTNOSTÍ

Abstract

High pressure steel cylinders are an industrial product that reaches to wide range of human being. Those cylinders are being used in field of industrial gases (welding, inertisation), medical (medical treatment, respiratory support, anesthesia), transport (CNG and H2) and many others. Nowadays, the emphasis is put more and more on all their parameters and characteristics like weight, fatigue life, corrosion resistance and many more. Those characteristics are directly connected also with the environmental aspects, e.g. in field of an industrial and medical gases. It is about transporting more gas than more steel and reduction of exhaust gases that is directly connected with it. On the other hand, in the field of personal transportation, the pressure is put on the ecologisation of fuels (development of alternative fuels, e.g. CNG and H2), when the situation in case of tanks serving as an on-board storage of alternative fuels is the same from the point of view that the lighter storage tank ensures higher mileage on eco fuel, therefore also significant reduction of exhaust gases. The improvement of mentioned characteristics, primarily lower weight, mechanical properties have to be increased, mainly Re (yield strenth), Rm (tensile strength), however together with keeping sufficient values of CVN (notch toughness) at - 50 °C, A (elongation) and related microstructural homogeneity. The Ph.D. thesis is focused on analyzing of above mentioned mechanical properties of current 34CrMo4 steel marked NV (micro alloyed with nitrogen and vanadium) on 10 different heats after 23 different heat treatment (HT) modes used for production high pressure seamless steel cylinders and subsequent design of new, adjusted chemical composition of 34CrMo4 steel marked SP (steel without vanadium microalloying) on cylinders made of 2 different heats after 5 different HT modes. Results of experiments (comparison of achieved mechanical properties results, microstructural characteristics, fatigue behavior, SSC resistance and cost effectivity) carried out on material NV were confronted with results of experiments carried out on material SP. The outcome of this Ph.D. thesis is the newly design and cost-effective material 34CrMo4 SP and its optimized HT to achieve high values of Rm (over 1100 MPa) together with favorable CVN values and related microstructural homogeneity. Newly designed material and its HT absolutely fulfil related standards requirements for high pressure seamless steel cylinders and can also be used for a variety of high-strength applications, e.g. crancksahafts, railway axels, drive shafts etc.