Evaluation of Metallurgical Quality of Die Steel

In the manufacturing process of the mold, the service life of the mold and the precision, quality and surface properties of the mold are not only related to the design, manufacturing accuracy, machine tool and operation conditions of the mold, but also closely related to the mold steel and its heat treatment process. Then how to evaluate the metallurgical quality of mold steel?

1. Uniformity of chemical composition of die steel

Die steel is usually an alloy steel containing multiple elements. When the steel is solidified from the liquid state in the ingot mold, due to the segregation and crystallization, the various elements in the molten steel are unevenly distributed in the solidified structure and form segregation. This chemical composition The segregation will cause differences in structure and properties, which is one of the important factors affecting the quality of steel.

Reducing the segregation degree of die steel can effectively improve the performance of steel.

In recent years, many metallurgical plants have been working on the production of steel with uniform composition and fine structure.

2. Looseness of die steel

Looseness is a manifestation of the incompactness of steel.

Looseness mostly occurs in the upper and middle parts of the ingot, which is caused by the concentration of more impurities and gases in these places.

Due to the existence of loose defects, the strength and toughness of the steel are reduced, and the roughness of the processed surface is also seriously affected. And plastic molding mold parts have stricter requirements.

For example, forging dies and punches with deep cavities are required to have a porosity of no more than grade 1 or 2, and steel used for plastic molds such as dials or light-transmitting parts requires a porosity of no more than grade 1.

3. Inhomogeneity of die steel carbide

Carbide is an essential component of most die steels. In addition to carbides that are soluble in austenite, there will also be some residual carbides that cannot be dissolved in austenite.

The size, shape and distribution of carbide have a very important influence on the performance of die steel.

The size, shape and distribution of carbides are related to the smelting method of steel, the solidification conditions of the ingot, and the hot working deformation conditions.

The carbides of the hypereutectoid steel may form powder carbides at the grain boundaries, or the carbides may be elongated to form banded carbides during processing deformation, or both. In ledeburite die steel, there is a primary carbide. In the process of thermal deformation, most of the network-like eutectic carbides can be broken, and the carbides first extend along the deformation direction to produce ribbons. With the increase of the degree of deformation, the carbides become uniform, small. The inhomogeneity of carbides has a great influence on quenching deformation, cracking and mechanical properties of steel.

4. Segregation of die steel

Segregation is the performance of the inhomogeneity of the composition and structure of the steel, which is a defect that often exists in the inspection of the low-magnification structure of the die steel. It is formed during the solidification process of the steel ingot and is related to the chemical composition of the steel and the pouring temperature. Generally divided into dendritic segregation, square segregation, point segregation and so on.

Due to the existence of dendritic segregation, the mechanical properties in different directions show obvious differences.

The square segregation is formed due to the accumulation of more impurities and pores at the end of the columnar crystal and the equiaxed region of the ingot during the crystallization of the ingot. Severe square segregation has a significant impact on the quality of steel, especially for parts with a large amount of machining or mold parts with stressed cores. In addition to affecting the isotropy of the mechanical properties of the die steel, the segregation also has a certain influence on the polishing performance of the die.

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