Abstract
In this paper, the effects of a super-gravity field with multi-rotational speeds on the grain refinement and tensile properties of as-cast H13 steel were investigated systematically. The experimental results showed that compared to the single-rotational speed (conventional) super-gravity field, the as-cast grains of H13 steel can be significantly refined in a multi-rotational speed (speed increased in stages) super-gravity field. In the conventional super-gravity field, with the decrease in rotational radius, the secondary dendrite arm spacing (SDAS) and the prior austenite grain size (PAGS) increase, and the maximum values of SDAS and PAGS are 90 and 55 µm, respectively, while in multi-speed super-gravity fields, at the range of increasing rotational speeds, SDAS and PAGS decrease as the rotational radius decreases. In the three-rotational speed super-gravity field, the maximum values of SDAS and PAGS are 80 µm and 50 µm. In the five-rotational speed super-gravity field, the maximum values of SDAS and PAGS are reduced to 58 µm and 34 µm. Accordingly, both the tensile strength and the plasticity are enhanced when increasing the number of rotational speeds in the super-gravity field, especially for the inner position of the super-gravity sample. The ultimate tensile strengths at outer, middle, and inner positions of H13 steel solidified in the conventional super-gravity field are 1445 MPa, 1378 MPa, and 1023 MPa, corresponding to elongations of 2%, 1.5%, and 0.5%, respectively, while in the five-rotational speed super-gravity field, they are 1408, 1443, and 1453 MPa, corresponding to elongations of 1.8%, 3.9%, and 2.2%, respectively. The mechanism for the grain refinement is that multi-speed super-gravity can reduce the critical nucleation work of austenite and the tangential force produced by increasing the rotational speed break dendrites at the solidification front, refining the solidified structure.
Highlights
H13 hot work die steel is widely used in extrusion, die casting, hot forging and other industries due to its high strength, high hardness, high toughness and temper softening [1,2]
In order to further explore the influence of the super-gravity field on the refinement of a solidified structure, Figure 6a,b summarize the average secondary dendrite arm spacing (SDAS) and the average grain sizes of the samples obtained in super-gravity fields, respectively
The research result provides a theoretical for the refining of the solidified structure of H13 steel industrial ingot by super-gravity
Summary
H13 hot work die steel is widely used in extrusion, die casting, hot forging and other industries due to its high strength, high hardness, high toughness and temper softening [1,2]. In order to further refine the solidified structure at the upper region in the super-gravity sample, many researchers [12,13,14,15,16] have discussed the refinement mechanisms of different alloys in the super-gravity field, such as the ”heavy crystal rain” [12], the increase in the cooling rate of the melt [15], and the smaller critical nucleation work [16]. If the super-gravity refinement method in the literature is applied to industrial-grade steel ingots above 30 kg, the refining effect at the upper region of the ingot will be even worse. The effect of super-gravity on the solidified structure and tensile properties of H13 tool steel was investigated by industrial experiments under the conventional super-gravity field and the super-gravity field with multiple rotational speeds (speed is increased in stages), defined as “the multi-rotational speed super-gravity field”
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