KARNA STAYED UNHARMED WHILE REMOVING HIS ARMOUR AND EARINGS

Karna sacrifices his Kavach and Kundal to Indra & Indra blesses him with the weapon Shakti

We all recognize the courageous warrior Karna, who was born to Kunti and Surya god. But deserted by her, he was adopted by Radha and Adhiratha; the charioteer of Dhritarashtra. Karna was born with divine earrings and armour breastplate blessed by the sun god or Surya Dev. He used to be well recognized for giving charity, specifically to Brahmins. I had already referred to in my other posts about him assisting Duryodhana in his evil plan. I here additionally want to let you all be aware of his sacrifice too. It was difficult for Pandavas to defeat the powerful warrior Karna.

Yudhishthira was once afraid due to the fact of the divine armour of Karna. Indra being the father of Arjuna, was distressed about Pandavas too. Knowing about donating the behaviour of Karna, Indra decided to beg for the armour and earrings of Karna. Sun god became conscious of Indra's Intention, out of affection towards the son, he disguised himself as a Brahmin and appeared in the dream of Karna. 

He warned his son about Indra's plan and advised him, " Karna! Do not provide your armour and earrings to Indra Dev. As it will not only minimize your life span but also make it effortless for Arjuna to kill you then." Sun god next came to his actual form. Karna respectfully answered sun god that, "I can by no means deny Brahmins for something they asked for. Even I would sacrifice my own life for any Brahmin. In order to preserve my dignity I will sacrifice my armour and earrings to Indra." 

Also, he expressed his self-assurance in defeating Arjuna with his archery skills. Seeing Karna being cussed for his charitable nature, the sun god counselled him to ask for the divine weapon Shakti from Indra in return. Karna agreed to him and was ready for Indra to come.

Later Indra reached close to Karna in the form of a Brahmin. Karna out of hospitality welcomed him. Brahmin instructed him that he wished to have his Kavach and Kundal only. Karna requested him to ask for any other things aside from his Kavach and Kundal. But Brahmin insisted he was solely for the same. 

Then Karna instructed Indra Dev, "I already knew that you would come here. My Kavach and Kundal are made from nectar which provides me immortality. People will later make fun of you for making me do this." Then while remembering the words of the sun god, Karna requested him for a boon in return.

Indra agreed to provide him with any weapons other than his Vajra; the thunderbolt weapon. Since Karna had to lose immortality due to the fact of getting rid of the attached armour and earrings from his body, he requested Indra to provide his Shakti weapon at that time. Indra blessed him with his divine weapon Shakti and informed him that With such an effective weapon he would be in a position to kill anyone mighty warrior easily. Indra even then blessed him that whilst detaching the armour from his body, Karna would not have any wounds on his physique or feel any pain and baring these also he would appear wonderful just like his father sun god. Without any hesitation, Karna gave him his armour and earrings.

Ref: Mahabharata book

Read about why Karna was asked by Lord Krishna to become the king..