Results of a research on influence of chromium, molybdenum and aluminium on structure and selected mechanical properties of Ni-Mn-Cu cast iron in the as-cast and heat-treated conditions are presented. All raw castings showed austenitic matrix with relatively low hardness, making the material machinable. Additions of chromium and molybdenum resulted in higher inclination to hard spots. However, a small addition of aluminium slightly limited this tendency. Heat treatment consisting in soaking the castings at 500 °C for 4 h resulted in partial transformation of austenite to acicular, carbon-supersaturated ferrite, similar to the bainitic ferrite. A degree of this transformation depended not only on the nickel equivalent value (its lower value resulted in higher transformation degree), but also on concentrations of Cr and Mo (transformation degree increased with increasing total concentration of both elements). The castings with the highest hard spots degree showed the highest hardness, while hardness increase, caused by heat treatment, was the largest in the castings with the highest austenite transformation degree. Addition of Cr and Mo resulted in lower thermodynamic stability of austenite, so it appeared a favourable solution. For this reason, the castings containing the highest total amount of Cr and Mo with an addition of 0.4% Al (to reduce hard spots tendency) showed the highest tensile strength.
Determined was quantitative effect of nickel equivalent value on austenite decomposition degree during cooling-down castings of Ni-MnCu cast iron. Chemical composition of the alloy was 1.8 to 5.0 % C, 1.3 to 3.0 % Si, 3.1 to 7.7 % Ni, 0.4 to 6.3 % Mn, 0.1 to 4.9 % Cu, 0.14 to 0.16 % P and 0.03 to 0.04 % S. Analysed were castings with representative wall thickness 10, 15 and 20 mm. Scope of the examination comprised chemical analysis (including WDS), microscopic observations (optical and scanning microscopy, image analyser), as well as Brinell hardness and HV microhardness measurements of structural components.
The paper presents influence of soaking parameters (temperature and time) on structure and mechanical properties of spheroidal graphite nickel-manganese-copper cast iron, containing: 7.2% Ni, 2.6% Mn and 2.4% Cu. Raw castings showed austenitic structure and relatively low hardness (150 HBW) guaranteeing their good machinability. Heat treatment consisted in soaking the castings within 400 to 600°C for 2 to 10 hours followed by air-cooling. In most cases, soaking caused changes in structure and, in consequence, an increase of hardness in comparison to raw castings. The highest hardness and tensile strength was obtained after soaking at 550°C for 6 hours. At the same time, decrease of the parameters related to plasticity of cast iron (elongation and impact strength) was observed. This resulted from the fact that, in these conditions, the largest fraction of fine-acicular ferrite with relatively high hardness (490 HV0.1) was created in the matrix. At lower temperatures and after shorter soaking times, hardness and tensile strength were lower because of smaller degree of austenite transformation. At higher temperatures and after longer soaking times, fine-dispersive ferrite was produced. That resulted in slightly lower material hardness.
In the research, relationships between matrix structure and hardness of high-quality Ni-Mn-Cu cast iron containing nodular graphite and nickel equivalent value were determined. Nickel equivalent values were dependent on chemical composition and differences between them resulted mostly from nickel concentration in individual alloys. Chemical compositions of the alloys were selected to obtain, in raw condition, austenitic and austenitic-martensitic cast iron. Next, stability of matrix of raw castings was determined by dilatometric tests. The results made it possible to determine influence of nickel equivalent on martensite transformation start and finish temperatures.
In the paper, a relationship between chemical composition of Ni-Mn-Cu cast iron and its structure, hardness and corrosion resistance is determined. The examinations showed a decrease of thermodynamic stability of austenite together with decreasing nickel equivalent value, in cast iron solidifying according to both the stable and the metastable systems. As a result of increasing degree of austenite transformation, the created martensite caused a significant hardness increase, accompanied by small decline of corrosion resistance. It was found at the same time that solidification way of the alloy and its matrix structure affect corrosion resista
Within the presented work, the effect of austenite transformation on abrasive wear as well as on rate and nature of corrosive destruction of spheroidal Ni-Mn-Cu cast iron was determined. Cast iron contained: 3.1÷3.4 %C, 2.1÷2.3 %Si, 2.3÷3.3 %Mn, 2.3÷2.5 %Cu and 4.8÷9.3 %Ni. At a higher degree of austenite transformation in the alloys with nickel equivalent below 16.0%, abrasive wear resistance was significantly higher. Examinations of the corrosion resistance were carried out with the use of gravimetric and potentiodynamic method. It was shown that higher degree of austenite transformation results in significantly higher abrasive wear resistance and slightly higher corrosion rate, as determined by the gravimetric method. However, results of potentiodynamic examinations showed creation of a smaller number of deep pinholes, which is a favourable phenomenon from the viewpoint of corrosion resistance.
In this paper, the effect of changes the parameters of heat treatment on the structure and the degree of elements segregation was determined, in the context of corrosion resistance of ductile iron Ni-Mn-Cu, containing 7.2% Ni, 2.6% Mn and 2.4% Cu. In the condition after casting, castings of austenitic matrix and 160HBW hardness were obtained. The achieved castings were soaked at 450, 550 and 650°C for 4, 8 and 12 hours, then cooled down at the ambient air. In most cases, the heat treatment resulted in a change in the castings matrix, had the consequence of increasing their hardness in comparison to raw castings. Increasing the temperature and prolonging soaking time resulted in increasing the degree of transformation of austenite, while reducing the degree of elements segregation. This led to the formation of slightly bigger number of pitting due to corrosion, but not so deep and more evenly distributed in comparison to raw castings. Wherein the results of corrosion tests show that heat treatment of castings did not significantly change their corrosion resistance in comparison to raw castings, in contrast to the significant increase in mechanical properties.
A research of wear resistance of an austenitic cast iron with higher resistance to abrasive-wear and maintained corrosion resistance characteristic for Ni-Resist cast iron is presented. For the examination, structure of raw castings was first formed by proper selection of chemical composition (to make machining possible). Next, a heat treatment was applied (annealing at 550 °C for 4 hours followed by air cooling) in order to increase abrasive-wear resistance. One of the factors deciding intensity of wear appeared to be the chilling degree of castings. However, with respect to unfavourable influence of chilling on machining properties, an important factor increasing abrasivewear resistance is transformation of austenite to acicular ferrite as a result of annealing non-chilled castings. Heat treatment of non-chilled austenitic cast iron (EquNi > 16%) resulted in much higher abrasive-wear resistance in comparison to the alloy having pearlitic matrix at ambient temperature (EquNi 5.4÷6.8%).