Melting point of granite is about 1215–1260 °C, wrought iron about 1483-1593°C
0.22 celsius heat units would be about equal to 100 calories
So say if 0.22 celsius heat units basically equaled 0.22°C in temperature change (because I just glossed over this and have no idea if it actually is true), room temperature would be about 21°C so melting granite would require about 5631cal and wrought iron about 7145cal. On average we burn 100cal by walking leisurely for 18 minutes, which is 1080 seconds. Would mean walking 6082363 seconds for granite and 7717090 seconds for wrought iron.
Heat in MD depreciates at 1heat/second so 6,082,363 heat to hit the melting point of a granite prison gate and 7,717,090 heat for a wrought iron prison gate. And to hold it there for a while so more depending on how long it takes for it all to melt.
8-10 million heat seems like a decent ballpark