Grain size control in carburizing steels
Aaltio, Henna-Riikka (2014)
Aaltio, Henna-Riikka
2014
Materiaalitekniikan koulutusohjelma
Teknisten tieteiden tiedekunta - Faculty of Engineering Sciences
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Hyväksymispäivämäärä
2014-10-08
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201501291015
https://urn.fi/URN:NBN:fi:tty-201501291015
Tiivistelmä
As carburizing is a time-consuming process, the objective has been to shorten the proc-ess by increasing the carburizing temperature. Although economic advantage can be achieved by high temperature carburizing, the process isn’t that widespread due to the problem of grain growth which start to occur at higher temperatures. Conventional case-hardening steels show grain coarsening and abnormal grain growth at temperatures higher than 950 °C. To avoid this problem, micro alloying elements such as niobium and titanium are commonly added to form nanometer size precipitates to pin the grain boundaries and prevent the austenite grain growth at high process temperatures.
However, abnormal grain growth may still occur even with microalloy additions applied as the pinning precipitates start to dissolve into austenite. In order to demonstrate the conditions which lead to grain growth, an overview of previous studies of the same field and grain growth models have been observed. Size and volume fraction of generating precipitates, which have an influence on pinning force, are examined and heat treatment history in the whole process have been taken into account, as it has an significant impor-tance in high temperature carburizing.
Based on references new alloying concepts and heat treatment parameters were defined. Aim of the experimental study was to find an optimal alloying concept with micro alloy additions which prevents the grain growth up to higher than normal carburizing tem-peratures, even up to 1050 °C.
However, abnormal grain growth may still occur even with microalloy additions applied as the pinning precipitates start to dissolve into austenite. In order to demonstrate the conditions which lead to grain growth, an overview of previous studies of the same field and grain growth models have been observed. Size and volume fraction of generating precipitates, which have an influence on pinning force, are examined and heat treatment history in the whole process have been taken into account, as it has an significant impor-tance in high temperature carburizing.
Based on references new alloying concepts and heat treatment parameters were defined. Aim of the experimental study was to find an optimal alloying concept with micro alloy additions which prevents the grain growth up to higher than normal carburizing tem-peratures, even up to 1050 °C.