In order to gain high strength, fine grain size, stronger anti-corrosion property, and especially low permeability, the material 022Cr19Ni10N was chosen to manufacture the Rod Travel Housing Forging (RTHF) for Control Rod Drive Mechanism (CRDM). But, cracks were found in some forgings failing to meet the requirements of ultrasonic testing (UT). The causes of the forging cracks of this austenitic stainless steel forging were investigated by means of metallography, scanning electron microscopy (SEM) and other experimental methodology. The results indicated that the second δ-ferrite phase leads to the forging cracks between γ-δ interface during the low temperature forging process, and finally leads to the forging failure.
It’s found that the cracks are distributing along the stripe δ-ferrite, and almost distributing in the same area as the large size δ-ferrite by metallography & SEM microstructure observation. The δ-ferrite is firstly found in the electroslag ingot, and in which, the distribution and size is different from the case to the core. The largest size δ-ferrite is around the core area, and this characteristic passes on to the final forging microstructure, although the shape, quantity & distribution of the δ-ferrite changed during the manufacturing process. Most forging cracks were found around the core area of the forging by UT examination. In the final forging process, when the forging temperature drops to 750∼850°C, the δ-ferrite have been forged to stripe shape and hundreds-micron size while the plasticity of the austenite reduce. What’s more, there are large hot plasticity differences between the δ-ferrite and the austenite, so the forging cracks initiate between γ-δ interface and extend to the area around to be a long crack in the low temperature forging process.
In order to avoid the forging cracks in the Rod Travel Housing Forging, it’s necessary to reduce the content of δ-ferrite or improve the final forging temperature. Improving the final forging temperature, to guarantee the plasticity of the δ-ferrite and austenite, is another process to reduce the cracks. But while the temperature improves, the grain size grows rapidly, and may form mixed structure. So the most effective mean to reduce the content of δ-ferrite is to redesign the chemical components, mainly by increasing the nitrogen content from 0.06 (wt, %) to 0.12(wt, %), which makes the low temperature forging process for fine grain size possible. In the high-nitrogen-content forging, the δ-ferrite distributed sporadically and no δ-ferrite strip is found. By increasing the austenite forming elements (especially nitrogen), the cracks during low temperature forging process are avoided. What’s more, owning to the optimization of chemical compositions and manufacturing processes, the Rod Travel Housing Forging got fine grain size, low relative permeability, and good comprehensive mechanical properties with the ultimate tensile strength up to 570MPa.
Low-temperature magnetoresistance in polycrystalline manganites: connectivity versus grain size