What Is ASTM A403?

ASTM A403 is the material specification for wrought austenitic stainless steel buttweld pipe fittings. It covers the grades most commonly used in corrosive, high-temperature, and hygienic piping systems: WP304, WP304L, WP316, WP316L, WP321, and WP347, among others. These fittings are dimensioned per ASME B16.9 (standard wall) or MSS SP-43 (Schedule 5S/10S for lighter wall).

Key Grades

Grade	UNS	Cr (%)	Ni (%)	Mo (%)	Min Yield (MPa)	Min Tensile (MPa)	Matching Pipe
WP304	S30400	18-20	8-11	-	205	515	A312 TP304
WP304L	S30403	18-20	8-12	-	170	485	A312 TP304L
WP316	S31600	16-18	10-14	2-3	205	515	A312 TP316
WP316L	S31603	16-18	10-14	2-3	170	485	A312 TP316L
WP321	S32100	17-19	9-12	-	205	515	A312 TP321
WP347	S34700	17-19	9-13	-	205	515	A312 TP347

CR vs CW Classes

ASTM A403 classifies fittings into two classes based on NDE requirements:

Class	NDE Requirement	Application
CR (Critically examined)	100% radiographic examination of welds	Critical process piping, high-pressure, lethal service
CW (Commercially examined)	Spot radiography or no RT required	General service, utilities

Most oil and gas piping specifications call for Class CR fittings in process service. Class CW is acceptable for utility and non-critical applications.

When to Use Each Grade

WP304/304L: General corrosion resistance. The “L” (low carbon, max 0.030%) variant prevents sensitization during welding, avoiding intergranular corrosion. WP304L is the default for most welded stainless piping.

WP316/316L: Superior corrosion resistance, especially against chlorides and acidic media. The molybdenum content (2-3%) provides pitting resistance. Standard choice for offshore, chemical processing, and pharmaceutical applications.

WP321/WP347: Stabilized grades (titanium in 321, niobium in 347) for high-temperature service above 425°C where sensitization during service is a concern. Used in furnace tubes, superheater piping, and high-temperature process lines.

Material Matching

Fitting grades must match the pipe material. A piping system in A312 TP316L pipe requires A403 WP316L fittings. Mismatching (e.g., WP304L fittings on TP316L pipe) creates a weak link in corrosion resistance and may violate the piping material specification.

For carbon steel BW fittings, see ASTM A234. For duplex stainless steel fittings, the applicable standard is ASTM A815. For nickel alloy fittings, ASTM B366 applies.

Testing

ASTM A403 requires:

• Solution annealing heat treatment (1040-1120°C, water quench)
• Tensile and hardness testing
• Intergranular corrosion test per ASTM A262 (Practice A or E, as specified)
• Mill test certificate per EN 10204 Type 3.1

Read the full guide to pipe fittings