Some aspects for increasing of heat-temperature strength of casting complex alloyed nickel-based alloys due to effect of redistribution of alloying elements between γ-solid solution and hardening γ′-phase

The positive effect of alloying elements on the thermal stability of the γ-matrix and the strengthening γ'-phase of casting nickel alloys, and consequently, on the increase in their heat-temperature strength is established. The alloying elements inhibit diffusion processes, thereby increasing the creep resistance of alloys at high temperatures and loads. It is found that the most indicative parameters of the phase composition of the test alloys are the alloying elements distribution coefficients between the γ- and γ'-phases. The basic principles of balanced alloying are formulated, on which the choice of the optimal chemical composition of heat-temperature nickel alloys is carried out.

Interaction of B and Hf with oxygen in a nickel melt at РAr = 0.1 MPa and analysis of nonmetallic inclusions

The deoxidation of complex alloyed nickel-based alloys during melting in vacuum is carried out in the presence of aluminum with strong deoxidizers B and Hf, which requires additional knowledge about the activity of dissolved oxygen. For this, the activity was investigated in model melts Ni – O – (B/Hf, 0.001 – 0.1 wt. % of each) and Ni – O – Al (5 %) – (B/Hf, 0.001 – 0.1 wt. % of each) at 1560 °C and РAr = 0.1 MPa by the method of instantaneous EMF recording using certified sensors. It was shown that a[O] with the introduction of B into the melt without aluminum is 1.3 times less than with the introduction of Hf, and for melts with aluminum, a[O] decreased 1.6 times as compared with Hf. The study of the morphology and composition of non-metallic inclusions of the Ni – O – (B/Hf) systems showed that the inclusions are located along grain boundaries, have various complex configurations and sizes from 2 to 100 μm. These phases contain Ni, Fe, Zr, S, O, as well as Hf, Pb, Bi, Si, Mg after deoxidation of Hf and an increased content of Zr after deoxidation of B.

Cold resistance of new casting Cr – Mn – Ni – Mo – N steel with 0.5 % of N. Part. 1

The authors have studied cold resistance of thelaboratorymetal of a new austenitic grade of nitrogen-containing casting steel (21 – 22) Cr – 15Mn – 8Ni – 1.5Mo – V (Russian grade 5Kh21АG15N8МFL) with nitrogen content of 0.5 % and yield strength of ~400 MPa. The temperature dependence of impact toughness was constructed in the range +20 ... –160 °C and it was shown that the steel is characterized by a wide temperature range of the viscous-brittle transition with T DBT = –75 °C, at which KCV = 120 ± 10 J/cm2. Comparison material – industrial, centrifugally cast 18Cr – 10Ni steel (grade 12Kh18N10-CC) has such a KCV level at +20 °C. It is not prone to viscous-brittle transition, its impact strength decreases more gently and at temperatures lower than –80 °C and its KCV level is higher than that of nitrous steel. However, in the entire range of climatic temperatures, nitrous casting steel with 0.5 % of N exceeds its impact strength. The studied steels have residual δ-ferrite in the cast structure in an amount of up to ~10 % in Cr– Ni industrial steel and a smaller amount in laboratory nitrous steel. It is enriched by chromium, up to 26 and 34 wt. % respectively, and contains ~14 % of Mn in nitrogen steel. Presence of Mn does not affect the nature of fractures at climatic temperatures. However, δ-ferrite of nitrous steel at –160 °C is beyond the cold brittle threshold. Therefore, its fracture obtained at this temperature contains numerous cracks in δ-ferrite crystals. The critical fragility temperature below which this material is not recommended for use is Тк ≈ –110 °С; it was determined by the criterion method. It corresponds to a level of KCV of 68 – 83 J/cm2, higher than the level of KCU at +20 °C, allowed by the standard of the Russian Federation for castings from austenitic class of steels (up to 59 J/cm2 ). Based on a comparison of literature and our own data, it was concluded that it is impossible to ensure high cold resistance and, at the same time, high strength, due to alloying of economically alloyed nickel (up to 4 %) corrosion-resistant steels by 0.5 – 0.6 % of N.