Use cycle ultra-fine heat treatment to increase die life

Molds, especially fine punching dies, play an important role in the machinery manufacturing industry. Improving the service life of molds is of great significance for improving production efficiency, reducing production costs, and ensuring product quality. The use of high-grade alloy materials can increase the life of the die, but it will increase the cost. The use of grain refinement to improve the mechanical properties of the material can simultaneously increase the strength and ductility at room temperature, and can also effectively increase the fatigue limit, and fine grains can be obtained with proper heat treatment. The heat treatment grain refinement mainly has the deformation heat treatment and the cycle heat treatment. For the improvement of the service life of the mold, the more appropriate heat treatment method is the cycle ultra-fine heat treatment, and the cycle heat treatment only performs the heating and cooling processes, which is a relatively simple and easy crystal Grain refinement method.

In the past, it has been reported that the Ti-46Al-8.5Nb-0.2W alloy is thin casted by a cyclic heat treatment method so that its coarse lamellar structure is transformed into a uniform and fine equiaxed structure. Recently, Zuo Chuanfu et al. adopted a swinging cycle for carbon tool steel (T8A, whose main chemical composition is 0.86C, 0.30Si, 0.25Mn, 0.02S, 0.023P, 0.10Cr, 0.08Ni, balance Fe). The heating-cooling quenching process also achieves grain refinement. The basic heat treatment process is: using two salt baths with temperatures higher than Ac1 and lower than Ar1, put the workpiece into a high-temperature salt bath above Ac1, heat to normal quenching temperature, and then quickly switch to another salt bath. The furnace is cooled down to 30-50°C below Ar1, so it is repeatedly cycled and finally quenched by Ac1 or more. After quenching, it was tempered at 200°C for 2 hours.

Inspection shows that after the heat treatment of the workpiece after the cycle, the martensite is fine, and the carbides and retained austenite are uniformly distributed in spherical particles. The practical application shows that the T8A steel rotor core punching convex and concave die is subjected to 4 cycles of the swing cycle heat treatment process, heated at 770-780°C, isothermal at 660-670°C, and returned at 200°C after the final quenching. Fire 2 hours, its service life increased from 12,000 to more than 40,000, no cracks in the mold surface and internal, the effect is very obvious.

The use of swing-cycle heat treatment to achieve a uniform, fine structure, is mainly to take full advantage of the rapid heating and recycling process: salt bath heating for rapid heating, can rapidly form a large number of austenite nuclei, and thus get a small Crystal grains; the faster the heating rate, the lower the quenching heating temperature, the better the refinement effect. Repeated heating and cooling cycles promote on the one hand the nucleation of the previously heated non-nucleated sites and, on the other hand, refine the nucleated crystallites. Once heated, the austenite grains are refined once to increase the nucleation rate of the next austenite. At the same time, the fine carbides that are not fully dissolved during rapid heating not only hinder austenite grain growth, but also form a non-spontaneous core forming austenite grains, thereby obtaining ultrafine grains.

Tests have shown that in the first few cycles, the hardness and impact values ​​of the specimens increase steeply, which can be significantly improved by 3-5 cycles of heat treatment. When the number of cycles is increased, the hardness value and impact value of the sample do not change significantly. About six cycles of heat treatment are roughly close to the refinement limit. Excessively increasing the number of cycles is not conducive to improving the heat treatment efficiency, and the number of cycles is best at 4-5 times.

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