Effect of cerium and lanthanum on the microstructure and mechanical properties of AISI D2 tool steel

Abstract

AISI D2 tool steel has excellent wear resistance with high dimensional stability. This type of steel is suitable for making molds. This paper describes investigations into the effect of adding Ce/La on microstructure of AISI D2 type cold work tool steels obtained by means of optical microscopy, scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectrometry (EDS) and image analyzer. The results showed that after modification with Ce/La, the morphology, size and distribution of M₇C₃ carbides change greatly. The carbide network tends to break, and all carbides are refined and distributed homogeneously in the matrix, and also reduce the size of chromium carbides and increase the dissolution of carbides during heat treatment. The results of mechanical tests show that the toughness of the alloy increased about 75% without reducing the hardness of the alloy.

Introduction

AISI D2 steel is in group of cold work tool steels. There are high chromium carbides in the microstructure of these alloys due to the presence of high chromium (∼12 wt%) and high carbon (1.5–2.35 wt%) contents that cause excellent wear resistance and high strength properties. Due to their high abrasion resistance properties, they are used in the molds [1], [2], [3], [4].

Alloy elements such as chromium and carbon segregate seriously during solidification of this alloy. So, eutectic carbides often form a network along dendrite boundaries in as-cast microstructure, which reduce fracture toughness greatly. These continuous networks of chromium carbide cause deep cracks in large ingots during hot deformation, that sometimes lead to the collapse of large ingots [5], [6]. To solve this problem in the production process of high-alloy tool steels, today's modern manufacturing techniques such as powder metallurgy and spray forming are alternative methods to replace conventional ingot casting. In these methods, due to rapid solidification of molten steel, an alloy is produced in which very fine and uniform dispersion of carbides occurs in comparison to conventional ingot casting [7], [8]. But, conventional ingot casting is still used for lower production costs, due to the high cost of production equipments and their high technology. Thus, ingot casting modification is a basic option in producing high-alloy tool steels. Because of high volume ingots and technical restrictions in ingot casting of these alloys, there is no possibility of increasing the cooling rate in order to reduce cell size, dendrite arm or fragmentize of secondary phases. In other words, for technological reasons, thermal modification cannot be effectively used to improve the casting properties. In such cases, it is necessary to use chemical modification by adding small quantities of some chemical elements.

Recently, adding niobium and titanium has been investigated for refining the eutectic chromium carbides in high chromium cast iron (HCCI) and high alloy tool steels. Presence of these elements can lead to the formation of “TiC” and “NbC” carbides at a higher temperature than the eutectic carbides, reducing the carbon content in the melt and thereby the volume fraction of the eutectic carbides. Continuity of net-like eutectic carbides greatly decreases by adding titanium and niobium, and toughness of ingots cast increases. Replacing eutectic carbides with titanium and niobium carbides (with high hardness) causes an increase in abrasion resistance of the alloy [8], [9], [10], [11], [12], [13]. But conditions of heat treatment of these alloys changed because heat stability of austenite and ferrite phases areas change. So, heat treatments of these alloys need more investigations after adding elements of niobium and titanium. On the other hand, by adding Ti and Nb into D2 tool steel, chemical composition and heat treatment of the new steel is not according to standard of D2 tool steel. Formation of TiC or NbC carbides decreases the amount of chromium eutectic carbides. The dissolved chromium content increases in the matrix and amount of carbon reduces in the matrix. The hardenability of new alloy changes due to quantities of dissolved carbon and chromium as well as, “martensite start” (MS) and “martensite finish” (MF) temperatures change.

Recently, in many investigations rare elements have been used in order to improve high alloy tool steels. These elements change the morphology of eutectic carbides and are better distributed in the matrix of alloy. For example, Wang et al. [14] found that addition of a desirable amount of boron could alter the carbide morphology of high chromium ledeburite steel from strip eutectic and rosette eutectic into divorced eutectic. Moreover, they studied the effect of trace additions of RE–Te–B multiple modifier in the microstructure of CD-2 steel and concluded that with combined addition of trace amounts of these elements, the carbide morphology changed from net-like to granular form [15]. It was also pointed out that CD-2 steel with granular carbide possesses higher toughness and resistance to crack propagation than conventional CD-2 steel with net-like carbides as well.

It is reported that by adding small amounts of some modifying agents to high carbon, high alloy steels, the eutectic solidification progress of austenite/carbides can be modified, and therefore, the grain size, shape and distribution of carbides in the matrix can be improved [14], [15], [16]. Pan et al. [16] have shown that by adding K/Na modifier to high speed steels, the continuous carbide network along dendrite boundaries could be broken up and that carbides are distributed homogeneously in the microstructure.

Previous studies used very small molds (e.g, using impact sample size, “10×10×55 mm³”) in their experiments and rapid solidification of molten alloy happened [14], [15], [16], [17]. Due to differences in experimental conditions (rapid freezing) and in ingot casting of tool steel (slow freezing), the authors can not rely on the results of their researches. In this study, the addition of trace elements was studied for improving the structure of D2 tool steel. But the extent of possible experimental conditions was similar to ingot casting, and the cooling rate of molten alloy was slow.