Calorimetric Studies of Polymorphic Iron Transformation

Q4 Materials Science Steel in Translation Pub Date : 2024-09-09 DOI:10.3103/s0967091224700384

S. V. Davydov, L. V. Spivak, N. E. Shchepina

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Abstract

Iron polymorphism, as a basic phase transformation in the industrial technology of iron-carbon alloys, manifests itself in three forms in iron heating and cooling. An analysis of the literature data revealed a lack of information about the mechanism of pure iron polymorphic transformation. In this work, an experimental verification of the second polymorphic transformation α-Fe ↔ γ-Fe is carried out using high-resolution differential scanning calorimetry (DSC). The object of the study are samples made from wire of technically pure iron (TPI–99.88% Fe) and high-purity zone-purified iron (ZPI–99.995% Fe). Heating and cooling are carried out in an argon atmosphere (99.9995% Ar). Based on the analysis of DSC curves, the following results are obtained: the value of hysteresis of the polymorphic transformation during thermal cycling is ~11°C; the iron polymorphic transformation is not reversible upon heating and cooling and occurs through different mechanisms; existing ideas about the reversible polymorphic phase transformation of α-Fe ↔ γ-Fe upon heating as phase recrystallization within the framework of a deformation (distortion) transition are untenable. It is experimentally proven that the pure iron polymorphic transformation under equilibrium conditions upon heating proceeds diffusion-free with a sequential change of three phase transformations of different origins. Jump in atomic volumes of polymorph phases α-Fe ↔ γ-Fe in the region of phase transformation is explained by indirect shear plastic transformation of the α-Fe polymorph crystal lattice into the γ-Fe polymorph crystal lattice, as is customary to date, but by the sequential destructuring of the α-Fe polymorph into a mixture of “paracluster” and amorphous phases. Within the framework of these studies, the following unsolved problems are identified: firstly, the process during which a significant amount of activation energy G is absorbed, is unclear: for TPI 2300 ± 150 kJ/mol, for ZPI 2400 ± 200 kJ/mol; secondly, there is no explanation for the mechanism of the iron crystal lattice transition to the amorphous state when heated; thirdly, the temperature of the polymorphic transformation range in carbon steel 20 does not coincide with the similar temperature range on the Fe–C diagram.

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多晶铁转化的量热研究

摘要铁的多晶型是铁碳合金工业技术中的一种基本相变，在铁的加热和冷却过程中有三种表现形式。对文献资料的分析表明，纯铁多晶体转变的机理缺乏相关信息。在这项工作中，使用高分辨率差示扫描量热仪（DSC）对第二种多晶体转变 α-Fe ↔ γ-Fe 进行了实验验证。研究对象是由技术纯铁（TPI-99.88% Fe）和高纯度区纯化铁（ZPI-99.995% Fe）线材制成的样品。加热和冷却均在氩气（99.9995% Ar）环境中进行。根据对 DSC 曲线的分析，得出了以下结果：热循环过程中多晶体转变的滞后值约为 11°C；铁的多晶体转变在加热和冷却过程中不可逆，并通过不同的机制发生；关于 α-Fe ↔ γ-Fe 在加热过程中的可逆多晶体相变是变形（扭曲）转变框架内的相重结晶的现有观点是站不住脚的。实验证明，在平衡条件下，纯铁的多晶体转变在加热时以无扩散的方式进行，三个不同来源的相变依次发生。在相变区域，多晶体相 α-Fe ↔ γ-Fe 的原子体积发生变化，其原因是 α-Fe 多晶体晶格间接剪切塑性变换为 γ-Fe 多晶体晶格，这是迄今为止的惯例，但 α-Fe 多晶体依次解构为 "paracluster "相和无定形相的混合物。在这些研究的框架内，发现了以下尚未解决的问题：首先，吸收大量活化能 G 的过程尚不清楚：TPI 为 2300 ± 150 kJ/mol，ZPI 为 2400 ± 200 kJ/mol；其次，铁晶格在加热时转变为非晶态的机理尚无解释；第三，碳钢 20 的多晶转变温度范围与铁-碳图上的类似温度范围不一致。

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来源期刊

Steel in Translation Materials Science-Materials Science (all)

CiteScore

0.60

自引率

0.00%

发文量

期刊介绍： Steel in Translation is a journal that represents a selection of translated articles from two Russian metallurgical journals: Stal’ and Izvestiya Vysshikh Uchebnykh Zavedenii. Chernaya Metallurgiya . Steel in Translation covers new developments in blast furnaces, steelmaking, rolled products, tubes, and metal manufacturing as well as unconventional methods of metallurgy and conservation of resources. Papers in materials science and relevant commercial applications make up a considerable portion of the journal’s contents. There is an emphasis on metal quality and cost effectiveness of metal production and treatment.