Review of Peritectic Solidification Mechanisms and Effects in Steel Casting

METALLURGICAL AND MATERIALS TRANSACTIONS 50TH ANNIVERSARY COLLECTION Review of Peritectic Solidification Mechanisms and Effects in Steel Casting GHAVAM AZIZI, BRIAN G. THOMAS, and MOHSEN ASLE ZAEEM Surface quality and castability of steels are controlled greatly by initial solidification. Peritectic steels suffer more from surface quality problems, including deep oscillation marks and depressions, crack formation, and breakouts than other steels. This paper reviews current understanding of the fundamental mechanisms of initial solidification of peritectic steels that lead to these problems. First, different empirical relations to identify peritectic steel grades from their alloy compositions are summarized. Peritectic steels have equivalent carbon content that takes their solidification and cooling path between the point of maximum solubility in d-ferrite and the triple point at the peritectic temperature. Surface defects are related more to the solid-state peritectic transformation (d-ferrite fi c-austenite) which occurs after the peritectic reaction (L + d fi c) during initial solidification. Some researchers believe that the peritectic reaction is controlled by diffusion of solute atoms from c phase, through the liquid, to the d phase while others believe that c growth along the L/d interface involves microscale heat transfer and solute mixing due to local re-melting of d-ferrite. There is also disagreement regarding the peritectic transformation. Some believe that peritectic transformation is diffusion controlled while others believe that massive transformation is responsible for this phenomenon. Alloying elements and cooling rate greatly affect these mechanisms. https://doi.org/10.1007/s11663-020-01942-5  The Minerals, Metals & Materials Society and ASM International 2020 I. INTRODUCTION THE castability of high-quality steel depends on avoiding breakouts, cracks, and surface quality problems. The surface quality of steel products is mainly determined by the early stages of solidification in the meniscus region of the mold.[1–8] Steels which undergo the peritectic transition are the most difficult to cast.[2,9–14] This is attributed to the volume contraction (shrinkage) associated with the peritectic phase transformation.[15–17] This shrinkage leads to the formation of an air gap between the steel shell and the mold during the CC process and decreases the heat flux. This leads to locally thinner and hotter regions of the solidified shell, GHAVAM AZIZI, BRIAN G. THOMAS, and MOHSEN ASLE ZAEEM are with the Department of Mechanical Engineering, Colorado School of Mines, 1610 Illinois Street, Golden, CO, 80401. Contact e-mail: Manuscript submitted April 14, 2020. METALLURGICAL AND MATERIALS TRANSACTIONS B which causes uneven shell growth that leads to surface depressions, deep oscillation marks, cracks, and breakouts.[18–24] Steels within the peritectic composition range include high-strength low-alloy (HSLA) Steels, and recently advanced high-strength steels (AHSS), which are all used extensively in different products due to their excellent mechanical properties, which include high strength and toughness.[25–28] The automotive industry is increasingly utilizing steels with peritectic composition range (AHSS, HSLA, etc.), to reduce vehicle weight. Peritectic steels are widely applied in pipelines, navy vessels, and nuclear power plant components.[28,29] Therefore, there is a great need to produce these steels without defects.[25] Peritectic steel solidification occurs in two distinct stages involving liquid (L), delta-ferrite (d), and austenite (c): the peritectic reaction (L + d fi c) followed by the peritectic transformation (d fi c).[30–33] The peritectic reaction occurs just below the peritectic temperature, when liquid and delta-ferrite are in contact and react to form austenite. This corresponds to the L + d + c three-phase point in the binary phase diagram,[34] or to the 3-phase region on sections through a multicomponent phase diagram, such as shown in Figure 1(a). During the peritectic reaction, an austenite layer grows Fig. 1—(a) Iron-carbon phase diagram showing peritectic steels (shaded) and their phase transformations and (b) schematics of the peritectic reaction and subsequent peritectic transformation. along the L +d interface, as pictured in the upper right frame of Figure 1(b). After the liquid locally runs out, then the remaining d-ferrite transforms to austenite by solid-state transformation. The peritectic transformation starts by thickening of the austenite layer immediately behind the tip of this advancing c platelet, as shown in Figure 1(b),[35] where it proceeds on two fronts, growing into both the liquid on one side of the platelet and the d interior on the other. Later, toward the center of the dendrite, the interior d–c interface has moved far away from the liquid, so further austenite growth into the d ferrite involves only a single moving interface. Note that spatially, these different steps of peritectic solidification occur at different places, and proceed in different directions relative to the thermal gradient, which is oriented downwards in Figure 1(b). In this paper, the proposed mechanisms of peritectic solidification are reviewed in detail and then the effects of peritectic solidification on surface quality, austenite grain size, hot ductility, segregation, and crack formation of cast product are discussed. But first, the different calculations used to identify peritectic steel grades from their compositions are summarized. II. IDENTIFICATION OF PERITECTIC STEELS As casting of peritectic steels is difficult, it is important to identify a peritectic steel from its composition. If a new grade of steel is predicted to be a peritectic, then appropriate actions can be taken at the caster, such as applying appropriate mold powders with high solidification temperature to lower the surface cooling rate,[36–38] and/or casting at a lower speed.[10] Low-alloy commercial steels can be characterized by their carbon content, with corrections according to the other alloying elements. Predicting the occurrence of the problematic peritectic transformation in high-alloy steels is more difficult, and involves the relative amounts of more than one other elements (such as carbon, nickel, and chromium in stainless steels,[39] or in AHSS[40,41]). In the Fe-C phase diagram for low-alloy steels, shown in Figure 1, four different solidification behaviors can be categorized according to their effective carbon content. These include range I, for effective carbon content less than point CA, range II for carbon between CA and CB, range III for carbon between CB and CC, and range IV, for carbon above CC. Steels in range II are known as peritectic steels and are the most difficult to cast because they experience transformation from d to c that coincides with the final stage of solidification. While in range I, d to c transformation starts and ends in the solid state. In range III, the tran (...truncated)