Influence of aluminium and niobium alloying on phase composition, structure and properties of heat- and wear-resistant cast iron of Cr-Mn-Ni-Ti system

Abstract

The paper presents the data on phase composition and structure forming for the alloys and oxide layers, on distribution of elements among the alloy structural components and oxidation surface through the depth of oxide and sub-oxide layers, on variation of wear resistance, scale resistance, growing stability and mechanical properties of cast iron of Cr-Mn-Ni-Ti-Al-Nb system depending on different aluminium and niobium content and thermal accumulating capacity of a casting mould. Complex carbides (Nb, Ti)C are forming in white cast iron during niobium alloying. Quantitative metallographic analysis of (Nb, Ti)C carbides and (Cr, Fe, Mn) 7 C 3 complex carbides was carried out on the samples with examined composition. The tests for scale resistance were conducted, structure and properties of cast iron were investigated. It was determined that chemical composition and structure of oxide layers depend on distribution of alloying elements among the alloy structural components. It was established that the areas of oxide film surface layer, which were formed on eutectics, contain mainly manganese; its concentration is more than 65 %, while Al is 4 % and Cr is 1 %. Manganese leads to increase of defects amount, such as pores, micro-cracks and vacancies, in oxide film during high-temperature gas oxidation; penetration ability of this film also increases, what has a negative effect on metal resistance to further destruction caused by oxidation. The film becomes porous, its thickness enlarges. Aluminium provides favourable influence on forming of the thin protective spinel-type films (dense substance with good metal adhesion) with minimal amount of defects; diffusion through such oxide film is very difficult. The areas of oxide layer, which were formed on austenite dendrites, contain mainly aluminium; its concentration is more than 24 %, while Mn is 16 % and Cr is 12 %. High aluminium content provides small film thickness. Joint alloying by aluminium and niobium leads to simultaneous increase of heat resistance and wear resistance. Wear resistance increased as a result of enlargement of the part of primary carbides (Nb, Ti)C with high hardness in the structure of cast iron. Composition of oxide films includes aluminium which strengthens their protective properties and rises of the alloy scale resistance. Alloying by niobium leads to secondary hardening during cooling in a casting mould. Dispersion particles of М 7 С 3 carbides are forming in solid state, thereby no structure degradation occurs during testing at increased temperatures, and growing stability rises.

The Effect of Cooling Rate on Structure, Basic Mechanical and Special Properties of Complex Alloyed Manganese Cast Iron

“…This is confirmed by the data in Table 2, which indicate the formation of a supersaturated austenitic γ-solid solution containing an increased amount of manganese in the alloy under study. It should be noted that additional saturation of the solid solution with manganese at an increase in the cooling rate during the crystallization of the alloy was established both in the eutectic regions and in austenitic dendritic nuclei [18][19][20][21]. Based on this, the uniform distribution of manganese over the entire volume of austenitic dendrite makes it possible to completely stabilize austenite.…”

Development and Introduction of a New Composition of Wear-Resistant Cast Iron with Improved Performance Properties for Castings of Pump Parts

“…To increase the durability of pump parts castings, their microstructure must be qualitatively different, which can provide cast iron with a new, fundamentally different alloying complex [12][13][14][15]. At the second stage, studies were carried out to establish the relationship between the chemical composition, structure, mechanical properties and wear resistance of white complex-alloyed cast irons.…”

“…With the increase in the microhardness of chromium carbides and their amount, the wear resistance increases. Also, the properties are affected by the inter-carbide distance, with the reduction of which, its wear resistance increases due to the fact that abrasive particles during wear contact carbides more than a soft metal base [19][20].…”

“…But even parts made of the same cast iron composition and working under the same conditions have a different service life. This is due to the fact that changes in the concentrations of alloying elements within the grade composition strongly affect their properties [4][5][6].…”