recovering the amount from the US party

Without any formal agreement it is difficult to recover the money, unless, it is proved that the so called sub standard goods are not returned but utilized by the US company.

You  file suit for recovery if there is no arbitration clause, serve summon and prove your claim and get decree, afterwards

file execution, that may be bit technical, I will guide you about that aftrewards.  

Based on purchase order, you can file a civil suit for recorvery of amount in Indian Court. Generally summon be issued to foreign pary though embassy.

there breach of many sections under import and export schedules and law - unless you have Inward Remittance Certificate or Letter of Credit you cannot expoert materials, even if you have exported is surprising.

Records are not properly done. Especially when a trade between different countries, proper records are necessary. It will be difficult task to recover your money even after getting a decree but  may be possible with the help of experts.