High-temperature hydrogen resistance of stainless steels

At elevated temperatures, the influence of hydrogen on various mechanical characteristics of martensitic and disperse-hardened austenitic steels is different. The maraging steel has better characteristics of durability and plasticity and the critical values of static and cyclic crack resistance at temperatures of 450–600°K than the austenitic steel with intermetallic hardening. As a result of the intense temperature softening, its ultimate and yield strengths are much lower than for the austenitic steel. The austenitic steel has higher resistance in terms of the threshold value ∆K _th. At room temperature, the low-cycle fatigue limit proves to be most sensitive to the action of hydrogen, whereas at 673°K, the parameter K _fc for the maraging steel decreases.     

Influence of residual stresses on the contact durability of surface-hardened 20KhN3A steel

Conclusions We have established that surface-plastic deformation of improved steel by rolls leads to greater compression residual stresses near the surface of a product than spinning with ball under the same forces. In addition, after surface-plastic deformation by rolls, there appear tangential residual stresses in the axial and circumferential directions, which favors an increase in the contact durability of the specimens. Cemented steel subjected to surface-plastic deformation with rolls has the greatest contact durability. With increase in compression residual stresses due to plastic deformation in oils and surfactants, the contact durability of chisel steel increases. Addition of surfactants to water in the process of surface-plastic deformation even more efficiently increases the contact durability of steel. We recommend to perform surface-plastic deformation of steel with rolls in the process of a contact stress of 7 GPa in the most active aqueous solution of a DS-10 Sintanol spirit, which ensures the optimal level of residual compression stresses, maximum length of hardening, and increase in contact-fatigue strength.Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 4, pp. 117–119, July–August, 2004.     

Antifriction properties of heat-resistant alloys in hydrogen

We investigate the influence of hydrogen on friction properties of heat-resistant KhN55MBYu alloy and establish the main regularities of variation in the sliding friction coefficient in the process of operation of a shaft-bush couple depending on the temperature and pressure of hydrogen as well as on the rate and type of loading. Some recommendations concerning the efficient use of certain plasma and self-lubricating coatings for the operation of valve hydrogen engines are given. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 36, No. 2, pp. 108–112, March-April, 2000.     

Alloying of steel with a superequilibrium amount of nitrogen from a gaseous atmosphere in the process of electroslag remelting

Conclusions We designed the structure of and manufactured an installation for electroslag remelting under elevated pressure in a gaseous atmosphere, which enables one to obtain ingots 10 kg in weight under a pressure of up to 10 MPa. We showed the fundamental possibility of alloying steel with nitrogen from a gaseous phase in the process of electroslag remelting. For the remelting of 20KhN3A steel under a nitrogen pressure of 5 MPa, we obtained a sevenfold increase in the concentration of nitrogen (from 0.0166 to 0.1081 mass %). For alloying with silicon nitride, hardness remains on the level of the hardness of the parent metal (HRC 27-32). For gaseous alloying with nitrogen, hardness increases to HRC 47–50. Gaseous alloying has the following advantages: (a) the absence of a change in the chemical composition of metal due to admixtures contained in the nitrated alloying composition; (b) a considerable reduction in the cost of nitrous metal due to the elimination of nitrated ferroalloys (ferromanganese, ferrochrome, ferrosilicon, etc.) from the technological process. In the case of alloying of 20KhN3A steel by using a superequilibrium content of nitrogen, structural changes manifest themselves in the formation of white interlayers along grain boundaries, increase in the thickness of grain boundaries, and change in the density, morphology, and mutual orientation of rejections of the needle-type phase. We recorded a decrease in the lattice parameter and the appearance of the line of austenite. The addition of activation admixtures to fluxes leads to a sharp increase in the ability of metal to adsorb nitrogen in the process of electroslag remelting. At a pressure of gas-like nitrogen of 0.1 MPa, its concentration in metal increases by a factor of 4.5. This means that, in some cases, nitration is possible even without the use of expensive high-pressure equipment. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 36, No. 3. pp. 115–117, May-June, 2000.