Carbide refinement in M42 high speed steel by rare earth metals and spheroidizing treatment

Zhou Xuefeng Fang Feng Tu Yiyou Jiang Jianqing Zhu Wanglong Yin Songyan

(1Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing 211189, China)(2Jiangsu Engineering Research Center of Tool and Die Steel, Danyang 212312, China)(3Sunnywell New Material Technology Co. Ltd., Changzhou 213200, China)

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Carbide refinement in M42 high speed steel by rare earth metals and spheroidizing treatment

Zhou Xuefeng1,2Fang Feng1Tu Yiyou1Jiang Jianqing1Zhu Wanglong2Yin Songyan3

(1Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing 211189, China)(2Jiangsu Engineering Research Center of Tool and Die Steel, Danyang 212312, China)(3Sunnywell New Material Technology Co. Ltd., Changzhou 213200, China)

The influence of rare earth metals and heat treatment on the microstructure and performance of M42 steel has been investigated by means of an optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), transmission electron microscope (TEM), electron back-scatter diffraction (EBSD) and X-ray diffraction (XRD). The results show that M2C is the prevailing type of eutectic carbides in M42 steel. After modification with rare earth metals, M2C eutectic carbides change from the ordered lamellar structure into a circular structure. Despite different morphologies, the two carbides present the same characteristics of microstructure and growth orientation. Compared with lamellar carbides, M2C carbides with the circular structure are much easier to decompose and spheroidize after heating, which remarkably refines the carbide dimensions. The refined carbides improve the supersaturation of alloying elements in martensite and increase the hardness of M42 steel by 1.5 HRC.

high speed steel; rare earth metals; carbide; dimension; spheroidization

High speed steels are widely used in high-temperature applications. Among them, AISI M42 steel is the most popular. The excellent performance of M42 steel is closely related to a great amount of alloying carbides, the mass percentage of which can reach as high as 20%.

Extensive efforts have been devoted to controlling the dimensions and distributions of carbides in high speed steels, such as forging[1-2], modification[3-5]and heat treatment[6-7]. However, carbides are still too large to satisfy the requirements. This is attributed to the morphologies of eutectic carbides, namely the lamellar morphology, which is unfavorable for carbide spheroidization during heating and deformation.

The present work intends to refine the carbide dimensions in M42 steel by modification and spheroidizing treatment. Misch metal was used to modify eutectic carbides and obtain carbides with the circular structure, which is easier to spheroidize during heating. The microstructure evolution of M42 steel was investigated after modification and heating.

1 Experimental

The material used in this study is AISI M42 steel, the composition of which is listed in Tab.1. The steel was remelted and cast in a sand mould. Before casting, 0.2% misch metal was added.

Tab．1 Compositions of AISI M42 high speed steel %

The as-cast microstructure was observed by the optical microscopy (OM), using the Murakami etchant, which provided selective etching of M2C (black), MC (white) carbides without etching the matrix[8]. The specimens were deeply etched and then observed by the FEI Sirion-400 scanning electron microscope (SEM). The compositions of carbides were measured using Genesis 60S energy dispersive spectroscopy (EDS). The remaining mass percentages of rare earth metals were measured by the MAXx LM15 direct-reading spectrometer (DRS).

The microstructure of the eutectic carbides was investigated by the Tecnai G2 transmission electron microscope (TEM) and electron back-scatter diffraction (EBSD). Samples from ingots were heated at 1 100 ℃ for 1, 2 and 4 h, respectively. Then, they were analyzed by X-ray diffraction (XRD), SEM and TEM. The samples, which were heated at 1 100 ℃ for 4 h, were reheated at 1200 ℃ for 10 min and then immediately oil quenched. EDS was used to analyze the compositions of the matrix after quenching. Then, the specimens were triple tempered for 1 h at 550 ℃, and the Rockwell hardness was measured.

2 Results and Discussion

2.1 Modification of eutectic carbides by rare earth metals

Fig.1 shows the typical as-cast structure of M42 high speed steel, consisting of a matrix and coarse networks of eutectic carbides. Eutectic carbides are divided into two types, M2C and MC, distinguished by the Murakami etchant. It is noted that M2C eutectic carbides change from the ordered lamellar structure into the circular structure after modification with rare earth metals. The three-dimensional morphologies are illustrated in Fig.2, from which different characteristics of the two carbides can be clearly distinguished.

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Tab.2 shows compositions of M2C carbides. The amount of alloying elements, such as Mo, V and Cr, is lower in M2C carbides with the circular structure. Rare earth metals are rarely detected by EDS, either in the matrix or carbides, although the remaining amount is approximately 0.01% in total of modified M42 ingots. This suggests that the majority of rare earth metals have been burned out and the remaining mostly exist in inclusions. It is expected that the trace amount of rare earth metals might be distributed on the interface of M2C/matrix, although the amount is too small to be detected by EDS. This theory was proved in previous studies in which Ce was detected on the surface of eutectic carbides by EPMA or ion bombardment[9-10].

Fig.3 shows the microstructure M2C eutectic carbides.

Tab．2 Chemical compositions of M2C eutectic carbide %

The two carbides are both the M2C type with a hexagonal close-packed structure. This indicates that rare earth metals change only the morphology rather than the crystal structure of eutectic carbides. Crystal defects have been rarely detected in the two carbides. This is quite different from the lamellar M2C in M2 steel where micro-twining and stacking faults were observed[11].

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Fig.4 illustrates the Kikuchi patterns of M2C carbides. The Kikuchi patterns rotate very little in different regions of the eutectic carbides, indicating that the two carbides both have a single crystal orientation. It is inferred that rare earth metals influence the growth rather than the nucleation of M2C. It should be pointed out that the growth of M2C in M42 steel may be different from that of lamellar M2C in M2 steel, which presents a polycrystal orientation[11].

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In high speed steels, M2C eutectic carbides are created by an eutectic reaction: liquid → austenite+M2C. M2C and austenite both nucleate from the liquid and grow together at similar rates, thus M2C forms the lamellar morphology. The three-dimensional networks of eutectic carbides suggest that M2C grows somewhat faster than the austenite, acting as the leading phase during an eutectic reaction. During solidification, rare earth metals are severely segregated at the front of the solid/liquid interface, due to the extremely low solubility of iron. It causes high composition undercooling, increasing the growth rates of both austenite and M2C. As a faceted phase, the increment in the growth rates of M2C is less pronounced than that of austenite with a non-faceted interface. To prevent the overgrowth of austenite, M2C must bend and branch frequently, thus forming the circular structure.

2.2 Microstructure evolution of eutectic carbides during heating

Fig.5 illustrates the morphology evolution of M2C carbides in unmodified ingots after heating. M2C eutectic carbides decompose into M6C and MC after 1 h. As the heating time is prolonged, they spheroidize in local regions. Nevertheless, the carbide dimensions are still very large. In contrast, M2C carbides with the circular structure also decompose after 1 h, but the amount of MC carbides is more, as shown in Fig.6(a). It suggests that the stability of M2C carbides may change after modification, confirmed by XRD (see Fig.7). M2C carbides in modified ingots decompose completely into M6C and MC whereas there are still some M2C carbides remaining in unmodified ingots. It demonstrates that M2C more easily decomposes after modification. With prolonged heating time, circular M2C carbides spheroidize clearly, which greatly refines the carbide dimensions.

The decomposition of M2C can be expressed as M2C+Fe(γ)→M6C+MC where the matrix, represented as Fe(γ), provides elements for the formation of M6C and MC[12]. Compared with lamellar carbides, circular carbides have bended and cylindrical surfaces with large specific surface areas. It suggests that M6C and MC may nucleate at more sites at the interface of M2C/Fe(γ), thus accelerating the decomposition of M2C. In addition, the lower amount of strong carbide forming elements, such as Mo, V and Cr, may also reduce the stability of circular M2C at high temperatures.

The process of carbide spheroidization is closely related to element diffusion at the carbide/matrix interface, depending on the concentration gradient in the matrix[13]. The element concentration is higher in the matrix adjoining the carbides with a larger surface curvature. Compared with lamellar carbides, M2C carbides with the circular structure have quite a different surface curvature from one part to another. Thus, the concentration gradient is higher in the matrix adjoining circular carbides, which accelerates the spheroidization of circular carbides.

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2.3 Hardness after quenching and tempering

Tab.3 shows compositions of the matrix after quenching. The amount of strong carbide forming elements is higher in modified M42 ingots, particularly for Mo. It is attributed to those much finer carbides in specimens, which are easier to dissolve into the matrix at high temperatures. As M2C carbides are enriched in Mo, the amount of Mo increases more remarkably than other elements. The higher supersaturation promotes the precipitation of secondary carbides from the martensite during tempering. It is found that the hardness of M42 steel increases from 61.0 to 62.5 HRC after modification.

Tab．3 Compositions of the matrix after quenching at 1 200 ℃ for 10 min

3 Conclusions

1) After modification with rare earth metals, M2C eutectic carbides changes from the ordered lamellar structure into the circular structure. Despite different morphologies, the two carbides present the same characteristics of microstructure and growth orientation.

2) Compared with lamellar carbides, M2C carbides with the circular structure decompose more easily and spheroidize during heating, which remarkably refines carbide dimensions and increases the hardness of M42 steel. Therefore, the method of modification with rare earth metals and spheroidizing treatment is an effective method to improve both the microstructural homogeneity and performance of high speed steels.

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稀土和球化熱處理對M42高速鋼碳化物尺寸的細化作用

周雪峰1,2方 峰1涂益友1蔣建清1朱旺龍2尹松艷3

(1東南大學江蘇省先進金屬材料高技術研究重點實驗室, 南京 211189)(2江蘇省工模具鋼工程技術研究中心, 丹陽 212312)(3盛利維爾(中國)新材料技術有限公司, 常州 213200)

采用OM，SEM，EDS，TEM，EBSD，XRD等分析手段,研究了稀土和熱處理對M42高速鋼組織和性能的影響規(guī)律．結果表明,M2C是M42高速鋼主要的共晶碳化物類型．稀土處理后,M2C共晶碳化物形貌由規(guī)則層片狀變?yōu)椴灰?guī)則環(huán)狀．盡管二者形貌差異明顯,但具有相同的微觀結構和晶體取向特征．與層片狀碳化物相比,環(huán)狀M2C共晶碳化物熱穩(wěn)定性較差,高溫加熱時更易發(fā)生分解和球化,使碳化物尺寸明顯細化．小尺寸碳化物在淬火加熱時易于溶解,提高了淬火后基體中合金元素的固溶度,使M42高速鋼硬度增加了1.5 HRC．

高速鋼;稀土;碳化物;尺寸;球化

TG142.7

Received 2014-04-24．

Biography：Zhou Xuefeng (1982—), male, doctor, lecturer, xuefengzhou@seu.edu.cn.

s：The National Natural Science Foundation of China (No.51301038, 51201031, 51371050), the Industry-Academia-Research Cooperative Innovation Fund of Jiangsu Province (No.BY2014127-03), the Natural Science Foundation of Jiangsu Province (No.BK20141306), the Scientific and Technological Innovation Fund of Danyang (No.SY201305).

：Zhou Xuefeng, Fang Feng, Tu Yiyou, et al．Carbide refinement in M42 high speed steel by rare earth metals and spheroidizing treatment[J]．Journal of Southeast University (English Edition),2014,30(4):445-448．

10.3969/j.issn.1003-7985.2014.04.008

10.3969/j.issn.1003-7985.2014.04.008