Fumio Hataya

Effect of titanium, niobium and zirconium on creep rupture strength and ductility of cobalt base superalloys

An attempt has been made to develop a cobalt base casting superalloy (30Cr-10Ni-7W-Co) having high creep rupture strength and ductility for first stage nozzles of gas turbines. In cobalt base superalloys, there was found to exist a close correlation between the creep rupture strength and MC type carbide forming elements such as Ti, Nb and Zr. In cobalt base alloys with 0.25 wt pct C, precipitation and coarsening of carbides can be reduced by addition of Ti, Nb and Zr. Therefore, by adding the optimum amount of Ti, Nb and Zr, precipitation of carbides in the alloy reaches such an amount as to give the highest creep rupture strength. Excess addition of Ti, Nb and Zr does not improve the creep rupture strength. By adding C, creep rupture strength of the cobalt alloy with Ti, Nb and Zr can be improved and becomes the highest at 0.40 wt pct. C. According to the experimental results, the creep rupture strength becomes the highest at a value of (Ti + Nb + Zr)/C (atomic ratio) of about 0.3. Contrary to the expectation, it was found in this experiment that the ductility in creep rupture tests increases with increasing carbon content up to 0.6 wt pct.

Effects of alloying elements on cavitation-erosion resistance of austenitic stainless steel deposited metals.

There are increaseing needs for the fabrication of hydraulic machineries which are used under the severe condition of cavitation. So, the anti-erosion weld deposits are usually overlaid on the position where the cavitation is likely to take place. However, there is large discrepancy that the surfacing weld deposits which has better weldability exhibit less erosion resistance. Increase in the hardness of the deposits is effective to improve its erosion resistance, while it spoils the weld-cracking avoidability of the deposits. In order to prevent the cracking of the deposits, it is necessary to increase its ductility. There should be a strong demand for the development of the surfacing weld deposits of the good combination of the erosion resistance and weldability.Investigations have been performed on the effects of the contents of carbon, manganese and nickel on the erosion loss and the elongation of the austenitic weld deposits containing 20% chromium and 7%cobalt. Results are as follows;(1) Erosion loss and its deviation increase as the nickel equivalent increases. It is necessary to limit the maximum of nickel equivalent less than 12.5% in order to assure erosion loss less than cobalt base alloy of low carbon content whose composition system is 0.06%C-19%Cr-9%Ni-15%W.(2) Elongation behaves likely to erosion loss. It is necessary to limit the minimum of nickel equivalent in order to assure the elongation more than 10%.(3) The erosion loss Cw (mg) and the elongation El(%) can be respectably estimated by the following experimental formulas. Cw=-64.6+115.60+10.1Mn+18.6Ni and El=-21.4+51.1C+2.5Mn+4.8Ni.(4) The weld deposits whose chemical composition system is 0.25%C-3%Mn-3%Ni-20%Cr-7%Co have good combination of erosion resistance and weldability.