Study on primary carbides precipitation in H13 tool steel regarding cooling rate during solidification

Abstract

This study aims to investigate the primary carbides precipitation in H13 steel solidified at relatively high cooling rates, ranging from 300 to 6,000 °C•min -1 , based on in situ observations with a high temperature confocal laser scanning microscope. In the cooling rate range investigated, the solidification microstructure becomes more refined as cooling rate increases and the relationship between the secondary dendrite arm spacing (SDAS), λ 2 , and cooling rate, .T, can be expressed as λ 2 =128.45.. Regardless of cooling rates, two kinds of primary carbides, i.e., the Mo-Cr-rich and V-rich carbides, are precipitated along the interdendritic region and most of them are the Mo-Cr-rich carbides. The morphology of Mo-Cr-rich carbide is not obviously influenced by the cooling rate, but that of V-rich carbide is obviously affected. The increasing cooling rate markedly refines the primary carbides and reduces their volume fractions, but their precipitations cannot be inhibited even when the cooling rate is increased to 6,000 °C•min -1 . Besides, the segregation ratios (SRs) of the carbides forming elements are not obviously affected by the cooling rate. However, compared with the conventionally cast ingot, the SDAS and primary carbides in the steel solidified at the investigated cooling rates are much finer, morphologies of the carbides have changed significantly, and SRs of the carbides forming elements are markedly greater. The variation of primary carbide characteristics with cooling rate is mainly due to the change in SDAS.

Effect of Cooling Rates on the Characteristics of Carbides during Solidification of D2 Cold‐Work Die Steel

“…[1,2] However, the high-carbon and high-chromium contents of D2 cold-work die steel promote the formation of excessive amounts of M 7 C 3 carbides during solidification. [3,4] Large quantities and phases of carbides, such as networks formed by strip and herringbone M 7 C 3 carbides along dendrite boundaries, are difficult to eliminate in subsequent processing. This drawback makes D2 cold-work die steel susceptible to crack formation under external forces, which causes early cracking of the material in service.…”

Mechanism of Tempered Sorbite Formation and Related Enhanced Mechanical Properties for a Typical High Carbon Steel Billet Under Strong Cooling Intensity

“…The linear intercept method was used to measure SDAS in the optical images. [12,24] To ensure the reliability of the measured SDAS results, five micrographs were selected and at least ten lines were drawn in one micrograph. The cooling rates can be estimated from the SDAS of the continuous casting billet specimens on the basis of the following equation: [22]…”

“…With these drawbacks, the continuous casting billet could hardly achieve the best mechanical properties, leading to degrading of the finished product. [12][13][14][15] According to Han's [16] research, in some cases of excessive secondary cooling intensity condition, the temperature around the billet corner was around 500 °C, when it was out of the secondary cooling zone, while the temperature of the center surface was > 900 °C. The temperature difference between the billet corner and center surface after secondary cooling exceeded 400 °C.…”

Influence of Cooling Parameters on the Microstructure and Primary Carbide Precipitation in GCr15 Steel

“…Loria [ 11 ] also found the faster cooling has refined solidification microstructure, but it did not change the type of carbides. With the high‐temperature confocal laser scanning microscope (HT‐CLSM) applied, Zang et al [ 12 ] indicated that the primary carbide is obviously reduced and the microstructure is refined with a higher cooling rate. As the primary carbide cannot be eliminated in the solidification process, Li et al [ 13 ] investigated the carbide resolution in the heat treatment and found the diameter of undissolved carbides increase with the austenitizing time, which indicates the carbide dissolution and coarsening occur simultaneously.…”

“…In the previous study, the alloy addition [ 6–8 ] and solidification rate [ 9–12 ] on the microstructure and carbide precipitation are studied, and the carbide resolution in heat treatment [ 13–15 ] are also investigated. It is commonly known that the cooling rate can affect the solidification behavior, while the temperature interval on the microstructure and primary carbide precipitation was rarely reported.…”