Bainite morphologies in sintered steels: influence of materials and process conditions

ABSTRACT

With the introduction of diffusion-bonded (d.b.) powders, metallographers found localised microstructure changes in sintered steels, and frequent presence of bainite. With the sinter-hardening process, the bainite fraction became a tool for rating the method. In this study, depending on different parameters, differences between bainitic structures of homogenous or not-homogeneous sintered steels have been analysed. When using pre-alloys, upper bainite is present with the same morphology. When using d.b. powders, variations of local composition lead to upper bainite of different morphology, each corresponding to a composition range. Quite high Ni local content gives rise to lower bainite, always mixed with martensite. Higher Ni contents lead to incoercible austenite. Upper bainite presents different morphologies, depending on local Ni and Cu amounts. The increase of sintering temperature leads to fewer morphologies of upper bainite, due to greater amount of diffusion processes. The lower bainite has always a typical morphology, whatever process parameters.

KEYWORDS:

• Bainite
• cooling speed
• microstructure
• PM steels
• sintering

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes on contributors

G. Filippo Bocchini is involved in PM since 55 years, with activities in industry and at various universities. He published nearly 400 papers, on different topics.

Maria Rosa Pinasco, recently retired, has been full professor of metallurgy at Genoa University, with main interests on heat treatments of metal alloys.

Maria Giuseppina Ienco, recently retired, has been associate professor of metallurgy at Genoa University, with main interests on microstructure evolution on cooling of metal alloys.

Gabriella Pellati, graduated at Genoa University, made her PhD on sinterhardening.

Elisa Pastore, graduated at Genoa University, made her PhD on changes occurring on cooling of sintered steels.

ORCID

Maria Rosa Pinasco http://orcid.org/0000-0002-5719-5676