Capacitive discharge welder - Uncertainties charging with a ...

Is this a "spot welder"?
You are discharging a 1 farad cap into your work--correct?
Single pulse?
Rep rate, even if manual?

For 10V, ε = CV²/2 = 50 Joules
For 20V, ε = 200J
Sounds plausible

What kind of peak current are you expecting?
What is the capacitor spec? ESR? --post link

Your circuit as shown has some problems, but we should be able to help.

First, start with a clean slate and draw the simplified discharge circuit--the SCR triggering has problems because as it starts to turn on, it starves its gate signal as the cathode voltage rises--this may not damage the SCR, but is a poor practice. The gate drive is far too weak--for something like this, you should shoot for a 1A, 5uS gate signal pulse.

I recommend that you ground the cathode of the SCR and place your electrodes between the cap anode and the SCR anode. Is this acceptable with you?

We will worry about the charging issues later--you can manually charge the cap and test the discharge concept first.