It doesn't matter how the bonds are obtained by Social Security or the Fed -- no matter what it affects interest rates.
If Social Security weren't buying government bonds (ok, not technically treasuries), that same debt would be issued as Treasuries or similar. Greater supply of treasuries means higher interest rates.
If the Fed weren't buying treasuries in the open market, demand would decline and the market interest rate would go up. Primary dealer bids are influenced by the market rate for treasuries, so the interest rate on new debt would go up.
If Social Security weren't buying government bonds (ok, not technically treasuries), that same debt would be issued as Treasuries or similar. Greater supply of treasuries means higher interest rates.
If the Fed weren't buying treasuries in the open market, demand would decline and the market interest rate would go up. Primary dealer bids are influenced by the market rate for treasuries, so the interest rate on new debt would go up.