They are not true equivalents
ASTM A106 is seamless high-temperature pressure pipe; IS 1239 is ERW utility pipe. They cannot be substituted without engineer-written design revision.
ASTM A106 and IS 1239 are often compared because both can be carbon-steel pipe, but they answer different engineering questions. ASTM A106 is a seamless carbon-steel pressure-pipe standard for high-temperature service in refineries, power plants and process piping. IS 1239 is an Indian welded steel tube standard for water, sewage, gas and general utility lines in light, medium and heavy classes.
The standards therefore differ in manufacturing route, pressure intent, temperature intent and documentation. A106 normally means seamless pipe, schedule wall, Grade A/B/C, heat-wise MTC and possible IBR paperwork. IS 1239 normally means ERW pipe, nominal-bore class, black or galvanized finish and commodity utility service.
When substitution is safe and when it is not
Using ASTM A106 in place of IS 1239 is usually over-specification for utility water or structural pipe. The pipe may perform, but cost, delivery and fitting compatibility can become worse. A106 dimensions are schedule-based, while IS 1239 is NB class-based; the buyer must verify OD, wall, end finish and fittings before accepting a substitute.
Using IS 1239 in place of ASTM A106 is normally unsafe for high-temperature pressure service and unacceptable for IBR inspection. If the drawing says A106 Grade B or SA106 Grade B, do not downgrade to IS 1239 to save money unless the design engineer revises the specification in writing.
Buyer checks before issuing the PO
Treat the standard as part of the design basis, not a buying preference. ASTM A106 RFQs should state grade, schedule or wall, end finish, heat treatment, MTC type and whether IBR Form III-C is required. IS 1239 RFQs should state class, finish, length, threading and whether galvanizing is required. Without those fields, suppliers may quote different products against the same line item.
If a contractor proposes a cross-standard substitution, ask for a dimensional comparison, pressure-code acceptance and written engineer approval. Similar nominal size does not prove equivalent wall, metallurgy, allowable stress or inspection acceptance, and the apparent saving can disappear when fittings, rework, site welding changes, certificate mismatch, inspector queries, commissioning hold points, extra testing, downtime or delayed approval are counted.
Advantages and trade-offs
Decision guidance: Use A106 for steam, refinery and IBR boiler service where temperature and pressure dominate. Use IS 1239 for water, gas utility and structural pipe where cost and speed dominate.