Технические науки/1. Металлургия

Zhiguts Yu.Yu.¹, Homjak B.Ya.², Bilak I.V.¹

¹Uzhgorod National University, Ukraine

2Mukathevo State University, Ukraine

SYNTHESIS OF SPECIAL THERMITE CAST IRONS AND THEIR PROPERTIES

The experimental studies carried out by the authors were stimulated by the necessity to confirm the development of a complex of theoretical notions. This work was carried out to synthesize the different-class thermite cast irons.

Taking into account the data of the studies, we have developed and realized the production of the different carbon steels as a result of the aluminothermic reduction of the iron cinder with introduction of carbon and ferrous alloys into the thermite [1, 2]. When carrying out the thermite smelting according to the suggested method, one has to take into account the "activity" of the elements that compose the metallothermic burden. Speaking about the inhomogeneity of the element distribution in the casts synthesized by the aluminothermic reduction of oxides, one must indicate the sequence of the above oxides reaction with aluminum. First the most easily reducible elements (Fe, Ni, etc.) are reduced, whereas hardly reducible oxides move to the slag melt. Afterwards the thermite metal that contains the excessive aluminium, while passing the slag layer, reduces the hardly reducible oxides as well.

Continuing this work, we have carried out the experiments in the research-production conditions with the use of the above burden compositions for the cast additions of the alloy steels [2, 3]. The further experimental micromeltings were directed to synthesize the four types of the high alloy cast irons – the nihards, i.e. the analogues of the industrial cast irons (the types I, II, III ones and a special one). The chemical composition of the above alloys shown in table 1 confirms the correctness of the burden calculation results [1, 4]. In this case the synthesis reaction was carried out both in the chill mold-type reactor and in that with the graphite lining to find the influence of the heat removal regime on the nihard mechanical properties (table 2).

Table 1. Chemical composition of the thermite nihards (mass %)

Synthesized nihard	Alloying element content
	С	Si	Mn	Ni	Cr	Mo	S	P
Type І	3.2–3.5	0.4–0.7	0.3–0.5	4–4.5	2–2.5	–	£0.05	£0.05
Type ІІ	2.7–3.2	0.4–0.7	0.3–0.5	4–4.7	2–2.5	–	£0.05	£0.05
Type ІІІ	1–1.5	0.4–0.7	0.3–0.5	4–4.5	1.4–1.6	–	£0.05	£0.05
Alloy 3-2-1	3.2–3.5	0.4–0.7	1.2–2.0	3–3.5	1.5–2.0	0.8–1.0	£0.15	£0.40

Table 2. Mechanical properties of the thermite nihards

Cast iron type	Casting method	sb	НВ	su
		МPа
І	Chill reactor	270–320	570–640	470–890
	Graphite pot	280–320	600–670	490–710
ІІ	Chill reactor	380–450	590–630	560–770
ІІІ	Graphite pot	–	370–410	–
Alloy 3-2-1	Graphite pot	–	490–560	–

The use of the graphite pot leads to a slight nihard strength increase. In general, the micromelting conditions establish such intense melt cooling regime that, in principle, the influence of the reactor lining stops dominating. All the casts made of the nihards had no external signs of shrink holes and cast cutting and macroanalysing confirmed these conclusions. No chemical composition liquation over the cast volume was found. Synthesized thermite wear-proof cast irons, i.e. nihards, relate to the chromium-nickel martensitic cast irons, graphitizing probability of which at the alloy synthesis using the aluminothermic method decreases significantly due to the large temperature gradient and high rates of heat removal. The microstructural analysis has shown that in the thermite cast irons the cementite content is not less than 50% that results in the 1000 – 1050 НV hardness.

It is well known that the cast iron wear-resistance at the abrasive wear depends on the structural components microhardness, shape, location and number. The principal phases in the nihard structure (as the X-ray spectral analysis has shown) that influence most intensively the wear-resistance are cementite and the more wear-proof Cr, Мо etc. carbides. The X-ray spectral analysis has found in these cast iron structures, besides the , and carbides, the carbides that provided the 15 GPа НV microhardness. That of the carbides is 10.0-10.5 GPа, while for and it is 14.5–17.5 GPа. At the same time, the alloyed thermite cast irons at the manganese content increase [1, 4] demonstrate, despite high synthesis temperatures, the castability worsening with the shrink-off conservation within 1.6–2.2 %. To improve the casting properties and the quality of the cast made from the thermite nihard, the metallothermic reactor was heated up to 420–520 К. The synthesized cast irons are badly processed by cutting. The nihards are ready to produce microcracks even at grinding. This results in the necessity to use the low-temperature cast softening with the 4–6 hr exposure or normalization with subsequent softening.

Conclusions. The results of the theoretical and experimental studies presented above are related to the synthesis of materials by combined technologies based on the metallothermy. On the basis of developed methods of calculations the compositions of burdens have been found and a wide spectrum of different-type alloys have been synthesized. The specific features of smelting using combined methods have been found, the mechanical properties and the structure of alloys produced have been studied, the recovery coefficients for the alloy elements in the metallothermic and combined processes have been found.

References:

[1]. Zhiguts, Yu.Yu., & Lazar, V.F. (2014). The technologies and features of alloys synthesized by combined processes. Uzhgorod, Invazor.

[2]. Zhiguts, Yu.Yu. (2004). The method for calculating the metallothermic charge for the supply of cast irons. Engineering Science, 9, 43-46.

[3]. Zhiguts, Yu.Yu. (2003). Gray and white special thermite cast irons. Optimization of production processes and technical control in mechanical engineering and instrument making, 480, 148-153.

[4]. Zhiguts, Yu. Yu. (2008). White special thermite cast irons. Metals and casting of Ukraine, 11/12, 9-11.