SFWIW, if you truly need an 18 ft open span beam, consider the
following:

1) Build an "I" beam construction consisting of 20 ft long 2x6 top and
bottom flange with a 2x12 web.

2) Since the web is basically dead weight and contributes "Zip"
towards strength, lighten it along the lines of a bar truss design for
a flat roof industrial building.

Start with a 2x12x10 ft timber and cut it into 5 pcs, 2x12x24" long.

Locate a 2x12 at each end of the 2x6x20 pieces, then space 30" to the
next 2x12x24 piece, then repeat.

What you end up with is a 20 ft long structure with 5-24" wood pieces
and 4-30" open spaces alternately spaced.

Assemble with some 3" galvanized deck screws with hex heads if you can
find them.

Drill pilot holes for the deck screws.

You end up with a beam that is "strong like bull" yet light enough
that two people can lift it into place.

3) Attachments.

I would use loops around the beam rather than drilling holes thru the
beam for bolts for the swing chains.

Have fun.

Lew