Microstructure and mechanical properties of two-step Cu-alloyed ADI treated by different second step austempering temperatures and times

摘要

Austempering ductile iron (ADI) is an attractive material due to its excellent comprehensivemechanical properties. However, the deficit in elongation and toughness always threatens its security application.Two-step austempering process is an effective way to improve elongation and toughness simultaneously. In thepresent work, the influence of the amount, morphology and distribution of ferrite and austenite on mechanicalproperties of ADI under different second-step austempering parameters has been analyzed. Results show thatthe amount of austenite and its carbon content decrease with increasing of second-step temperature. Carbidebegins to precipitate as second-step austempering temperature reaches 380 °C. These factors together influencethe mechanical properties of two-step Cu-alloyed ADI. Impact energy and fracture toughness are stronglyaffected by second-step austempering temperature, and are dramatically decreased with increase of second-stepaustempering temperature. Elongation remains constant when the second-step temperature is below 360 °C, andthen it is rapidly decreased with further increase of second-step temperature. Strength is slightly influenced bysecond-step temperature. Ferrite morphology is not influenced by second-step austempering duration, while blockyretained austenite size is slightly decreased with the increasing of second-step austempering time. The amountof retained austenite is decreased while the carbon content of retained austenite is increased with the extendingof second-step austempering time. The substructure of austenite is transformed from dislocation to twin whensecond-step austempering time exceeds 60 min. Strength and elongation are improved slightly with extending ofsecond-step time. Impact energy and fracture toughness initially decrease with the extending of second-step time,and then remain constant when the time is longer than 60 min. This is a result of austenite content decreasing andcarbon content of austenite increasing. The second-step austempering time mainly influences austenite contentand its carbon content, which is a result of carbon diffusion behavior variation.