The first thing that i would check is that laser thermometer thing. Check it next to a temp stick. I had a situation a couple of weeks ago welding 6061-t6 Al. Another welder had one of those laser things and gave it to me. Well, long story short, it ends up that the battery in it was 4 yrs. old. The thing was off. Way off. I knew it didn't take that Al. that long to get to 150*.
Next, i would preheat to your lower limit mentioned above, not 500*.
It also looks like you are not getting adequate gas coverage, or your base metal has surface contaminants, or both.
I would also cool it slower, rather than in still air. Wipe your filler clean also. And make sure that when the welder is preheating, that he's not using a sooty flame.
I had a really in depth post typed up for You and managed to loose it. I will give You a greatly abreviated version now, and You can call Me at (610) 326-0113 if You want more details.
Some questions for You:
1) What is the material this has to cut?
2) How many cycles does this need to perform?
The MG 710 rod is not a good choice for the structural part of the weld. It is used to deposit the hard working surface ONLY.
We used 9018 for the structural part of tool steel repairs, it might work out better for You.
This is a poor design, structurally joining tool steel by welding is not really a good method.
A better alternative is to machine from a single piece of A2, if this really needs to be made from tool steel.
Alternatively, You could make the part from mild steel & weld it with simple and time proven methods. Then You could use the MG 710 rods to put a tool steel cutting edge where You need it. Pre & post weld heat treat is still mandatory, but You avoid heat treating the entire part.
If You heat treat the entire part it will warp/distort quite a bit due to it's shape & size, in spite of being made from A2.
You could flame harden the edges and then draw at 400F for 60-62 RC or 700F for 57-59 RC, the softer temper gives greater toughness with some reduction in wear resistance. If You need more strength/toughness, there are other alloys that will better suit Your needs.