I have tried heating pipe for 15-20 seconds but still no melting of solder material.
That solder will melt like warm peanut butter when the right temp is reached.Never force it.The solder or brazing material will do the work for you. Good luck.
What you are lacking is flux and enough direct heat. Clean area to be soldered by lightly sanding or wire brushing to remove old oxidization. Preassemble. Apply lite coating of flux. Assemble. Heat fitting that solder will 'draw' into while observing the outer fitting 'take' the heat and change reflection. Flux is boiling at this stage and about boiled away. Lightly touch solder to joint to see if it has reached melting point while still heating. When solder will melt freely touch joint in several points of the compass using about one-half inch of solder roughly as you take heat off. Joint should remain still until solder hardens.
The pipe has to be water free ,and clean[steel wool,wire brush] you need flux to , heat pipe with torch until solder starts to melt [to the pipe] it should start to sweat
