Abstract
AbstractTo obtain the ultrafine martensite for BR1500HS ultra-high-strength steel, a new preparation process of cyclic heat treatment (CHT) with descending heat temperature and holding time along with cycle steps was developed. First, a series of thermal dilation tests were conducted with the temperature range of 420–730°C and the heat rate of 5 K/s on a Gleeble-3800 thermomechanical physical simulator. According to the experimental data, the temperature ranges and the optimal holding time to completely austenitize BR1500HS were determined. Then, to confirm the optimal parameters of CHT for BR1500HS, several tests with various temperatures and cycle steps were conducted and analyzed by optical microscope and scanning electron microscope. Subsequently, the CHTs with decreasing heating temperature and holding time were studied due to the increasing internal energy of steel along with the CHT process. The lath width was measured as a criterion to evaluate the refinement degree in this article. After several loops of heat treatment, the lath width is reduced to 0.268 µm. Finally, the hardness evolution of the specimens subjected to CHT in this study was analyzed and compared with the lath width test results, which justified the effectiveness of the new developed process.
Highlights
To obtain the ultrafine martensite for BR1500HS ultra-high-strength steel, a new preparation process of cyclic heat treatment (CHT) with descending heat temperature and holding time along with cycle steps was developed
The CHT tests with a fixed holding time of 5 min, a fixed Figure 7: The dilatometric curves of BR500HS
Based on the above analysis, four CHT tests at the solution temperature of 960°C with four different steps of one cycle, two cycles, three cycles and four cycles were conducted to obtain the optimal cycle step
Summary
Abstract: To obtain the ultrafine martensite for BR1500HS ultra-high-strength steel, a new preparation process of cyclic heat treatment (CHT) with descending heat temperature and holding time along with cycle steps was developed. A cyclic heat treatment (CHT) containing several loops of solution and quenching can be an effective approach to achieve the fine-grained or even ultrafine-grained martensite phase by controlling the austenite grain size in the solution process before quenching [3,7,8]. In order to obtain the ultrafine-grained martensite, it is a major issue to determine the two vital parameters of this CHT, i.e. austenitic solution temperature and holding time [11,12]. A new preparation process of CHT with varying temperature and holding time in each loop of solution and quenching was designed to acquire the ultrafine-grained martensite with the grain size smaller than 1 μm. The lath width and hardness of each specimen were measured and compared, which indicated the effectiveness of the newly developed preparation process
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