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到现在为止一直认为,白点的形成过程是先有一孕育期形核,然后再长大的过程。我们运用扫描电镜、透射电镜、声发射和超声波等多种手段对钢中白点形成过程进行了综合研究。主要结果如下: 一、不论白点大小如何,其上总有表面平滑的显微空隙条带。对比试验表明,去氢的无白点断口上仍有同样类型的显微空隙条带。这证明它不是轧制后形成的,而是凝固过程留下的枝晶间隙,或微小氢气泡未被轧合的充氢空隙,这些空隙就是钢中事先存在的白点核。二、热轧后空冷到一定温度,白点从空隙处开始长大。随时间增加,白点断续地缓慢长大,有的白点约二、三十天后长大才逐渐停止。声发射实验表明,不存在白点孕育-长大开裂的声信号峰,只有白点长大的峰。三、白点断口显微形态主要有三种:(1)穿晶的准解理;(2)沿晶的波状条纹;(3)平滑的显微空隙条带。综上所述,白点的形成过程是一个没有一定形核期只有长大的过程,核是钢中原来未经轧合的充氢空隙。这一机制还可以用来解释铝合金大型锻件中“亮片”(即氢白块)的形成过程。
Until now, it has always been assumed that the formation of white spots is the process of nucleation first and then growth later. We use scanning electron microscopy, transmission electron microscopy, acoustic emission and ultrasound and other means of a comprehensive study of the formation of white point in the steel. The main results are as follows: First, regardless of the size of the white point, there is always a smooth surface of the microscopic gap strip. The comparative tests show that there is still the same type of microscopic gap strip on the hydrogen-free white point fracture. This proves that it is not formed after rolling, but the dendrite gap left by the solidification process or the hydrogen-filled voids that the small hydrogen bubbles are not rolled up and are the pre-existing white point nucleus in the steel. Second, air-cooled to a certain temperature after hot rolling, white spots began to grow from the gap. As time increases, white spots intermittently grow slowly, and some white spots about two or three days after gradual growth. Acoustic emission experiments show that there is no white point gestation - the crack growth of the acoustic signal peak, only the white point of the growth of the peak. Third, the white point fracture microscopic morphology are mainly three kinds: (1) transcrystallization quasi-cleavage; (2) along the crystal wavy stripes; (3) smooth microscopic gap strip. In summary, the formation of white spots is not only a certain nucleation phase of the process of growth, the nucleus is the original uncharged steel filled with hydrogen gap. This mechanism can also be used to explain the formation of “sequin” (ie, hydrogen-white) in large aluminum forgings.