Increasing the relaxation resistance of heat resistant Ni−Cr alloys by conditioning

Conclusions The effect of repeated loading (conditioning) in the first period of relaxation on the relaxation resistance of heat resistant Ni–Cr alloys depends on the working temperature. At 750–800°C the effectiveness of this treatment is adequate for practical use (although it is smaller than at 700–725°C). At temperatures up to 850°C conditioning cannot prevent the intensive relaxation weakiening that is characteristic of Ni–Cr alloys at this temperature.Central Scientific-Research Institute of Ferrous Metallurgy. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 53–57, October, 1972.     

Principles of alloying of nickel alloys,hardenable by internal nitriding

Internal nitriding is a process of surface saturation with nitrogen, in which there forms a diffusion layer, consisting of nitride particles that are uniformly distributed in a solid solution but without a surface zone of continuous nitrides, as in conventional nitriding. This is achieved by selection of the alloy chemical composition and the nitriding process regimes. The presence of this diffusion layer opens up broad possibilities for the hardening of various alloys, since a high level of brittleness of the layer is avoided. This is an advantage over conventional nitriding.Moscow Highway Institute (MADI). Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 5–8, March, 1994.     

Oxidation of nickel base alloys strengthened by different hardening effects

Abstract

Three nickel base alloys strengthened by different hardening effects were investigated by thermogravimetry in air under isothermal conditions. The alloys investigated were γ′-Ni₃ (Al, Ti)-hardening alloy 80A (75Ni, 21Cr, 2·5Al, 1·7Ti, DIN No. 2·4952),solid solution hardened alloy C22 (59Ni, 21Cr, 13Mo, 3·5 Fe, 2·8W, DIN No. 2·4602) and a new high nitrogen containing and nitride hardening alloy N (61Ni, 27Cr, 10W, 1·4Ti, 0.2N). Tests were conducted in air between 900 and 1100° C for 48 h. Parabolic oxidationrates were determined and the formation of the oxide layer was investigated by optical microscopy and SEM. Oxidation data showed that the hardening mechanism has almost no influence on the oxidation kinetics. All of the alloys investigated formed chromia layers. After initial transient stateoxidation, the kinetics followed a parabolic law. Alloy 80A had the highest oxidation rate of the investigated alloys, which is attributed first to its lower chromium content and second to the formation of chromium carbides. At grain boundaries, internal oxidation, mainly of aluminium andtitanium, took place. The Al and Ti contents of alloy 80A were too low for the formation of a protective inner oxide layer of one of the two elements to take place. Alloy C22 showed the best resistance to oxidation since its chromium content of 21% is close to that for the minimum in the kineticsof oxide formation that has been found for binary Ni–Cr alloys. Additionally, there were no chromium rich precipitates to shift this chromium content to values that would result in higher oxidation rates. The nitride-containing alloy N contained a higher chromium content of 26%, whichled to a higher oxidation rate than that for alloy C22. A certain amount of inner oxidation took place, especially at coarse Cr₂N precipitates. Conclusions are presented about the optimised chemical composition of chromia laye-forming nickel base alloys for minimised oxidationrate.     

Measurement of the saturated dislocation pinning force in hydrogenated nickel and nickel base alloys

Static strain ageing experiments are used to measure the saturated dislocation pinning force as a function of the hydrogen concentration in hydrogenated nickel and binary nickel—16 wt% chromium. The role of hydrogen transport by mobile dislocations on hydrogen embrittlement is demonstrated.     

Regenerative heat treatment of blades of high-temperature nickel alloys

An analysis of the operation of blades of gas turbine engines (GTE) manufactured from cast nickel alloys has made it possible to determine some of the causes of failure in operation, such as changes in the geometric dimensions or degradation of the microstructure. As a rule, the first cause is considered to be decisive. In order to increase the service life of blades partial regenerative repair of them is envisaged. In this case the dimensions of the blades are usually restored without allowing for possible changes in their structure, which can have a negative effect on operation after the restoration. For this reason, the second cause can become decisive in the reduction of their service life. This work is devoted to investigating the regularities of the degradation of the structure and properties of the material of cast GTE blades made of a high-temperature nickel alloy of the ZhS6U-VI type under operating conditions. A structural criterion determining the operating capacity of the olades and the ultimate level of structural degradation are established. A regenerative heat treatment to be conducted after partial exhaustion of the opet acing resource accompanied by structural changes that occur within the critical range of the structural criterion is suggested.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 16 – 20, May, 1996.     

Friction nitriding of iron alloys

Conclusion Changes in the structure of the surface which occur during heating by friction in ammonia connected with a strongly dispersed grain structure and an increase in the extent of grain boundarires created by the high density of constantly moving and interacting crystal structure defects (vacancies, dislocations, disclinations) leads to intensification of the impregnation process, an increase in nitrogen concentration in the layer, and as a consequence of this to marked hardening and also an increase in the wear and corrosion resistance.For extensive use in practice of friction nitriding it is necessary to develop special equipment which does not require large capital expenditure.Kiev Polytechnic Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 10–12, January, 1991.     

Increasing the heat resistance of nitrided low-carbon martensite steels

Low-carbon martensite steels are characterized, by a high set of mechanical, technological, and operational properties. Alloying of these steels with active nitride-forming elements makes it possible to nitride them in order to obtain wear-resistant surface layers. However, the core of martensite steels loses strength in nitriding due to the development of tempering processes. The present paper is devoted to new low-carbon martensite steels alloyed with V, Ti, and Cu for improving their heat resistance and to the regimes of their nitriding. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 17–20, January, 1998.