Below I have a Picture of a schematic I am working with right now (at least variants of), component specifics listed below. a working version that excludes R6/7 and has Q1/2 as a set of matching MOSFETs in parallel, and a non-working version with R6/7 that I cannot discover why it's not working.

The issues I am running into are as follows:
1. When turned on Q1 heats up but nothing else (L3 material). (12Vdc PSU) (MOSFETs are not breaking after tests just slowly building up to ~110F)
2. Q1 immediately fails when exposed my 36Vdc PSU on all version of the build so far.

I got this far on trial and error, and a few dozen online DIY guides, I have no formal training or experience with this type of engineering of any kind, only a very basic understanding of electricity and components, and a lot of safety equipment/precautions.

PSU: S-1200-12 (12Vdc 100A) * (S-1500-36 (36V 41.6A) available / Goal) + Battery Kill Switch 48V 200A (to prevent slow start for the circuit)
R1: 47KR 5W (R1 & LED Removed for now on all builds)
D1: 200V 81A (Automotive Diode) *
R2, R3: 1KR 5W *
R4, R5: 10KR 1/2W *
D2, D3: 1N5439B (12V, 5W) *
D4, D5: FR307 (1KV, 3A) *
R6, R7: 20R (Have a second Build [12 Vdc] without this set that works, but I've seen guides that recommend these be added)
Q1, Q2: IXFK180N25T (180A, 250V, <-200ns) (IRFB4115 Available) *, **
C (All): .33uF (330nF) 1200Vdc Capacitors (Count ranges from 4 to 8 for testing) *
L1, L2: Large inductors (3In rings with varying number of turns for testing different sets) Generally 60uH - 200uH 25A *
L3: Copper Tube Coil (1/4In round Tube, 5 turns, 2.5In ID, lead length 4 inches) *

- I have attempted to upgrade most components to be overrated for safety reasons and to prevent failure.
* = same on working and non-working builds
** = In some practice builds Q1 & Q2 are actually 2 sets of 2 matching MOSFETs in parallel (4 total 2 on each side). *

The goal is to heat a section of metal to a fairly constant ~900 Degrees Fahrenheit, the current working build achieves ~450. Hence my intention to upgrade to a 36Vdc 1500W PSU. I do not have access to one with a higher Wattage output at the moment, but that seems irrelevant if I cannot get the circuit to work with what I have available. Would I need a higher output PSU in the case of requiring a much higher output heat? Alternatively, would my working variant need:
- different capacitance / count / voltage rating?
- different inductance? (I believe rings are large enough to have not reached full saturation) (~60uH seems to be more effective so far, the 200uH heats up but much slower)

lastly, if I desired to add some level of control on the heating output, where in the circuit would I place the controlling components? I'm assuming:
- some value of Potentiometer / Variable Resistor between Vcc and L1/2?
- what value ranges would have meaningful impact on the output heat/frequency?

Any help would be much appreciated, I've been fiddling with this for nearly 4 months and seem to have hit a roadblock hence my reaching out for help, or paraphs my unfamiliarity with the terminology is also causing me strife in not knowing what to look up so if I have stated something wrong, please let me know so I can look for the appropriate research material.

 

Why does the thread title contain "induction coil" in the title? an induction furnace (if that's what you're referring to) uses AC not DC, so I'm not clear on what's meant to be going on here.

 

Induction Coil / Forge / Furnace / Foundry, they all have nearly the same style ZVS circuit, and I have looked into the variations hence where I'm currently at, I probably assumed the terms were mostly interchangeable given the similarities and it seems I was wrong.

The idea is to use a DC input to convert into a Controllable Square Waveform @ a specific frequency I'm told around 40kH - 100kH would be sufficient. If the input was AC, I would need to rectify it to DC before I could use it with this schematic. At least that's what my research and a few blown MOSFETs has shown me through trial and error, again unless I've done something horribly wrong. Some of the reference material I have found starts with AC but recommend using a Battery or DC + switch / slow start circuit before they finish.

 

Does this help at all?

D1: 200V 81A (Automotive Diode)
R1, R2: 1KR 5W
R3, R4: 10KR 1/2W
D2, D3: 1N5439B (12V, 5W)
D4, D5: FR307 (1KV, 3A)
Q1 - Q4: IXFK180N25T (180A, 250V, <-200ns) (IRFB4115 Available if better for goal)
C: .33uF (330nF) 1200Vdc
L1, L2: 60uH - 200uH >25A Inductors (85X55X20mm Toroidal Ferrite Core, 2 Strands of 14AWG magnet wire 4 - 12 turns)
L3: Copper Tube Coil (1/4In round Tube, 5 turns, 2.5In ID, lead length 4 inches) Air Core, insert metal for heating

Q1 and/or Q3 fails with 36V