COPPER PRECIPITATION DURING CONTINUOUS COOLING AND ISOTHERMAL AGING OF A710-TYPE STEELS

Precipitation in copper-containing A710 (also referred to as HSLA-80) and modified-A710 steels was investigated by transmission electron mic roscopy. Isothermal aging of as-quenched specimens at 675 degrees C pr oduced epsilon-copper precipitates located primarily at alpha-iron mat rix dislocations. The precipitates exhibited multiple variants of an o rientation relationship (OR) consistent with that reported by Kurdjumo v and Sachs, fine fault formation, and associated streaking in electro n diffraction patterns. For reaustenitized and continuously cooled spe cimens, the primary precipitation event was associated with interphase precipitation of copper at ferrite/austenite interfaces. Interphase p recipitates frequently displayed ORs other than that reported by Kurdj umov and Sachs, although a unique crystallographic variant was observe d within any one region of interphase precipitation, faults were obser ved infrequently, and streaking was not observed in diffraction patter ns. At high temperatures during cooling, precipitate-free ferrite form ed, whereas at lower temperatures, nucleation of copper precipitates o ccurred at ferrite/austenite interfaces for crystals of polygonal ferr ite and Widmanstatten ferrite. This latter feature precludes the forma tion of Widmanstatten ferrite via a displacive mechanism. Interphase p recipitation was not observed for granular ferrite or acicular ferrite . Less-common precipitation events during continuous cooling included the formation of AIN and CuS.