Study On Effect Of Ta Content On The Mechanical Properties Of CLAM Steel

The China Low Activation Martensitic (CLAM) steel has the low activation, better void swelling resistance, excellent mechanical and thermal physical properties and mature technology and has been considered as the primary candidate structural material for Chinese ITER Test Blanket Moudle (TBM). As one of the key alloying elements in CLAM steel, Ta plays an important role on the property. Based on theory and experiment, the influences of the Ta content on tensile and impact properties of the CLAM steel were studied. Moreover, the fracture toughness was studied through experiment and simulation and the atomic distribution characteristics of Ta-rich precipitations were studied by Atom Probe Chromatography (APT). These results could provide supports for the further research of optimal composition design of tantalum content for CLAM steel.Firstly, the influences of Ta content on the tensile and impact properties of the CLAM steel were studied. Four25kg ingots with the Ta composition of0.027wt%,0.078wt%,0.15wt%and0.18wt%, respectively, were prepared and named as HEAT1009A-D. From the results of tensile and Charpy V-notch impact tests, it could be seen that, the Cr-rich M23C6carbide strengthening is considered to be the main possible reasons for the HEAT1009A with the highest strength; the lowest ductile-brittle transition temperature (DBTT) and higher upper-shelf energy (USE) of HEAT1009B may be explained by the finer grain size and finer Ta-rich MX particles; the finest grain size and highest content of undissolved Ta-rich MX precipitations with lager size are considered to be the possible reasons for the higher USE and the lowest DBTT of HEAT1009D; the grain became more fine-grained with the increase of the tantalum content as the coarse TaC could pin the austenite grain boundaries and keep grains fine during solid solution at the higher temperature, but when the tantalum content was higher than0.15%, the grain size showed no differences; the simulation by solid solubility product and diffusion dynamics also proved the above analysis. The fracture toughness assessment method was studied in order to further study the effect of Ta content on the fracture toughness of the CLAM steel. The fracture toughness (JIC) of CLAM steel was tested at room temperature through the compact tension specimen, the result is417.9kJ/m2, which is similar to the JLF-1at same experimental conditions. The micro structural observation of the fracture surface shows that the fracture mode is a typical ductile fracture. Meanwhile, the fracture toughness is also calculated on the basis of the fractal dimension and the calculated result is454.6kJ/m2, which is consistent well with the experimental result.Secondly, the evolvement of the Ta-rich precipitate in the CLAM steel was studied. Two25kg ingots were prepared with tantalum nominal composition of0.28and0.84wt%and named as HEAT1102A/B, respectively. With1473K high temperature austenitizing treatment for1or2hours, it is found that, the TaC particles were completely dissolve in HEAT1102A, but were not in HEAT1102B; The δ-ferrite was found in those specimens. These fitted with the results from the simulation with Thermo-calc, solid solubility product and diffusion dynamics. With1033K tempering after high temperature austenitizing treatment, the precipitates gradually coarsened at grain boundary with the extension of time. The precipitates mainly were Cr-rich particle for HEAT1102A and were Ta-rich particle for HEAT1102B. The precipitation of Cr would be slow down with the Ta added. In addition, the hardness of all the materials reduced with the extension of time, the hardness of HEAT1102B has reached a stable value after tempering1.5hours, while the hardness of HEAT1102A was still no stable trend after tempering500hours. The atomic distribution characteristics of CLAM steel were studied by Three-Dimension Atom Probe (3DAP), the results was showed as follows:in the Ta-rich precipitates area, Ta, V and C had a larger strength of segregation tendency and the amount of atom were more; In the region of interface, Cr, Ta, Si and C had a larger strength of segregation tendency, but the amount of atom were less.