Crystallographic characteristics of Delta-ferrite transformations in a 304L weld metal at elevated temperatures

Delta-ferrite transformations and their crystallographic characteristics in a Type 304L austenic stainless steel weld metal at elevated temperatures were observed mainly by transmission electron microscopy. Two types of ferrite morphologies, lacy and vermicular, coexist in the Type 304L steel weld metal. The lacy ferrite takes approximately the Kurdjumov-Sachs (K-S) relationship with austenite, but the vermicular one does not. Since the K-S relationship leads to a low-energy δ/γ boundary, the boundary coherency is higher in the lacy ferrite structure than in the vermicular one. These crystallographic characteristics are maintained in each ferrite structure during heat treatments. The two types of ferrite structures behaved quite differently in ferrite shape change and sigma precipitation on exposure to high temperatures. At 1073 K (1472°F), the ferrite shape tends to become rectangular and long-slender with a directionality in the lacy sructure, while roundish in the vermicular one.