The use of high-temperature electric furnace process technology for the 18–8 stainless steel sensitized effects

Our experiment focuses on exploring the effects of the carbide precipitation on the mechanical properties and strain shape of type 18-8 within the sensitizing temperatures (475-850°C) in the hope of providing a helpful reference to the stainless steel industry when producing the stainless steel manufactures. We first heat up some specimens of 18-8(304) stainless steel individually in a high-temperature electric furnace to specific temperatures within the temperature range which the sensitizing treatment will be made easily, lasting for different periods of time at each temperature point and then we compare the toughness of each specimen. In addition, other specimens of 304 stainless steel are heated to four chosen sensitizing temperatures for 24 hours, observing the metallograph and sensitized phenomena for at individual temperature. To be well-rounded, we will also use the specimens of 304L for comparison purpose at the same time. Next, we proceed to the tension test, then the martensite transformation test with different strain ratios as well as the analysis of SEM & EDS to help understand well the effects of the sensitizing treatment on the stain shape. The results show as follows: Type 304 stainless steel (including 304 & 304L) dose produce carbide precipitation around the grain boundaries in the range of the sensitizing temperatures (475-850°C), which makes the number of Cr elements reduce in both sides of the grain boundaries and results in the chrome-deficiency areas. After tensile strain, the number of the slip lines in grains of type 304 stainless steel increases between 10%-40% strain ratios and the quantity of the grain transformation in all directions raises up accordingly. The strain shape becomes “branched shape” clearly when the strain goes greater. When the specimen is strained again in the strain ratio of 50%, they are finally broken since the stress reaches the maximum. With the raise of the strain ratio af- - ter tension, the quantity of the marten site of type 304 also increases. Both have a linear dependence on each other. The mechanical properties of 304L are usually not as good as those of 304 due to its low-carbon content. From the SEM observation, that there are definite precipitations around the grain boundaries, the higher the sensitizing temperatures go and the longer the temperatures endure, the more and definite the precipitation is. By means of the EDS element analysis, it is observed that what is found the most around the grain boundaries are Fe, Cr, and Ni elements; especially, Cr chemical composition is found in A, B & C areas beside the grain boundaries and the peak of Cr is pretty high.