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.