材料期刊网

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。

The process of flake formation in steel was investigated by means of SEM, TEM EPMA and AES as well as acoustic emission technique. The results show whatever size of a flake may be, an "unwelded" smooth micropore was observed on its surface. The comparative experiment was made on the fracture surface of the dehydrogenated specimen, the "unwelded" micropores of identical type were also observed. It may be ascertained that the micropores are interdendritic voids and/ or hydrogen bubbles remaining in the solidified steel ingot, and not internal crack after hot-rolling. These voids and bubbles are unable to weld closely by hotrolling and form the nuclei of flakes in steel. These flakes grow gradually up during air cooling. Their growth will be continued and sometimes stopped till 20—30 days. The acoustic emission detected only signals for flakes growing, no one for sudden cracking, i. e. no incubation time for nucleation occurred. Three principal features of the micromorphology of flake fracture are trasgranular quasicleavage, intergranular wave-like stripes and elongated smooth "unwelded" micropores. The abovementioned mechanism of the flake formation may also illustrate the process of "bright-flakes" formation in Al alloy forgings.