| Summary: | 碩士 === 義守大學 === 材料科學與工程學系 === 89 === The purpose of this study is to discuss the influence of sampling positions within billet, alloy, element, soaking time, and reduction ratio on the d-ferrite content and morphology in stainless steel 23Cr-14Ni-2Mn as well as its high temperature mechanical properties. Results of this study can help the understanding of the high temperature mechanical properties of 23Cr-14Ni-2Mn stainless steels.
The experiments in this research showed, d-ferrite content in billet center is higher than in billet corner, so that the billet center has higher Creq/Nieq ratio according to composition analysis. By heat treatment of 23Cr-14Ni-2Mn stainless steels during different soaking time, the d-ferrite concentrated in early stage. Then d-ferrite transformed to austenite with the increasing of soaking time. The results from tests by different reduction ratios, δ-ferrite content was decreased with increasing reduction ratio if the observation was parallel to rolling direction. But there was no obvious variation in δ-ferrite content if observation direction was vertical to rolling direction.
The high temperature tensile tests revealed that Y309L stainless steel has better high temperature ductility than others, except its embrittlement at 1000℃. Y309B, Y309L1 and, Y309LSI stainless steels have good hot working capability in the temperature range between 1100℃-1200℃.
The high temperature tensile tests at different soaking time showed, that Y309L1 stainless steel has better high temperature ductility and higherδ-ferrite content in early stage. After 30 min soaking its high temperature ductility was bad and had least δ-ferrite content; the high temperature ductility was improved with theδ-ferrite content increased. There was no obvious variation in high temperature ductility of Y309B stainless steel at different soaking time because its δ-ferrite content did not change a lot.
High temperature ductility and high temperature tensile strength of 23Cr-14Ni-2Mn stainless steels were increased with increasing reduction ratio. Reasons for this could be microstructure of test materials became denser and grains refined. This induced less segregation in grain boundaries and decreased stress concentration on crack tip. The high temperature mechanical properties were not improved much in higher reduction ratio because grain refining is not obvious.
The relation between Creq/Nieq ratio, solidification morphology, soaking time, reduction ratio, δ-ferrite content and hot working properties were discussed systematically. The knowledge gained from this research can help the understanding of hot workability and improve the quality of austenitic stainless steels. It can also provide the reference resources for the machinery factory. They can be used in the preventing from hot cracking happened during hot rolling and the improvement of the product quality.
|
|---|
