11-03-19 Mould Steel properties.
As written previously, of course the steel composition for mould manufacturing plays a key role on the mould as an whole but also in part quality;
Nowadays there are processes (such as ESR) that allow us a better control on the expected Steel behaviour under certain conditions.
The toughness level (tenacidade) of a mold steel is determined by a standardized impact test that outlines the sample preparation and test procedure (for example, ASTM A370). (...) Once prepared, the sample is positioned within a fixture and a pendulum is released from a specific height. As the pendulum strikes and then fractures the sample, it continues to travel on its upward swing. The height it reaches depends on how much energy the sample absorbed. A steel grade that exhibits a relatively high toughness level will absorb significant amounts of energy, thereby reducing the height to which the pendulum climbs.
Factors Affecting Toughness
The homogeneity of the steel, which is the level of banding or segregation of the alloying elements and/or alloy carbides. The other is micro-cleanliness, which is the amount of undesirable, non-metallic inclusions (NMI).
Once the primary melting and ladle metallurgy processes have been completed, the molten steel is cast into ingots. The high melt temperature and the large volume of molten steel poured into each ingot mold results in a relatively slow solidification rate. As the ingot solidifies from the surface towards the core, the nature of the process results in alloy segregation. Essentially, this is a variation in the steel’s chemical composition throughout the cross section of the ingot.
To improve homogeneity and the microstructure of the steel, and to minimize the level of non-metallic inclusions, an additional step in the manufacturing process of the mold steel known as electroslag remelting (ESR), or a more advanced variation known as protective/pressurized electroslag remelting (PESR), may be implemented.
(...) During this process, the portion of the remelted ingot that is in the liquid state is relatively small. The result is faster solidification rates and, thereby, a lower amount of alloy segregation and banding. The chemical composition will be more consistent throughout the cross section of the ESR ingot. In addition, the alloy carbides that form are likely to be relatively small and uniformly distributed.
An added benefit of the ESR process is that the slag bath will extract tramp elements such as sulfur from the melt as each molten droplet of steel passes through it. The process can reduce sulfur to levels of 0.003 percent or lower, reducing the amount of non-metallic inclusions formed upon solidification. The ESR process helps ensure a high level of micro-cleanliness. The lack of brittle sulfide particles within the matrix of the steel greatly improves toughness properties as well as the ability to polish the mold steel to a high surface finish.
Consider mold steel grades that have undergone the additional refinement provided by the ESR process. Increased toughness levels and crack resistance; a consistent, uniform response to etching/texturing processes; and the ability to achieve a lens-quality surface finish during polishing are major benefits.
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