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Mold Materials 12 MoldMaking Technology —— MAY 2017 By James Kaszynski Electroslag remelting (ESR) is a secondary melting operation that can be used to further improve the microstructure of steel. This process takes the initially cast ingot and remelts it under very controlled conditions. This is an actual ESR unit in operation. Defining Mold Steel Toughness T here are many considerations that a designer and mold- maker must balance when selecting a tooling material that will meet or exceed expectations during operation. The part geometry, mold size, polishability requirements and length of the production run all play a role in this decision. An additional concern that is frequently mentioned, but not always clearly defined, is the toughness level of the mold steel. A grade of mold steel with a high toughness level will reduce the likelihood of crack formation and/or propagation should a crack develop. Challenging designs that require thick-to-thin wall transitions, relatively sharp radii or other highly stressed areas will benefit from using an inherently tough mold steel. It essentially provides for an additional level of safety during operation, avoiding unplanned maintenance and downtime. Measuring the Toughness Level The toughness level of a mold steel is determined by a stan- dardized impact test that outlines the sample preparation and test procedure (for example, ASTM A370). The process begins with the tester cutting a sample coupon from the end of a bar or block of mold steel, noting its orientation to the primary rolling or forging direction. Further sectioning and machin- ing of this coupon is carried out to create multiple samples according to specified dimensions. These samples are also heat treated to a hardness level that is representative of the same working hardness range used by the mold during service. 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. The instrument will record the amount of energy absorbed by the sample in terms of foot-pounds (imperial units) or as joules (metric units). The greater the value measured, the greater the toughness level of the steel. It should be noted there may be variations in how the test samples are prepared, however, a Consider grades that have undergone the electroslag remelting process. common method involves grinding a small, V-shaped notch perpendicular to the length direction of the sample and oriented vertically during testing. The pendulum strikes the side opposite the notch, fracturing the sample at this pre-determined loca- tion. This test is referred to as a Charpy V-Notch (CVN) test. There are many variations with regards to the sample prepa- ration, including the use of unnotched test specimens. When reviewing data, make sure you are always comparing values that were collected from samples prepared in a similar manner. Images courtesy of Bohler-Uddeholm Corp.