How to read the markings on a pipe fitting

A fitting that arrives on site is only as trustworthy as its markings. They tell you who made it, from what steel, to which standard and, through the heat number, from which batch of metal. Learning to read them takes ten minutes and prevents most receiving-inspection disputes.

Why markings matter

Piping codes require material to be identifiable. If a fitting cannot be traced back to a test certificate, an inspector can reject it, and the cost of rework or replacement falls on the project. Markings are the physical link between the fitting in your hand and the paperwork in your file.

What you will normally find

The general marking system for valves, fittings, flanges and unions is described in MSS SP-25. Individual product standards such as ASME B16.9 and B16.11 add their own requirements. Typical markings are listed below.

• Manufacturer name or trademark
• Material grade, for example WPB, WP316L, F304 or A105
• Heat number or a heat code that maps to one
• Nominal size, and for reducers and tees both sizes
• Schedule or wall thickness, for example STD, XS, 40, 80S
• Dimensional standard, for example B16.9 or B16.11
• Pressure class for forged fittings, for example 3000 or 6000

Space on a small fitting is limited, so only the essentials may be stamped. The rest is then covered by the packing marking and the certificate.

Reading a typical marking

Take a buttweld elbow marked with the manufacturer, 6 x STD, WPB, B16.9 and a heat number. It reads as a 6 inch nominal size, standard wall, carbon steel to ASTM A234 grade WPB, made to ASME B16.9. For stainless fittings you may see a suffix after the grade. The letters S and W indicate seamless or welded construction, so WP316L-S is a seamless 316L fitting.

Grade versus specification

The grade designation identifies the chemistry and strength. A234 is the specification for carbon and alloy steel buttweld fittings, with WPB being the common carbon steel grade. A403 is the specification for austenitic stainless steel fittings with grades such as WP304, WP304L, WP316 and WP316L. A105 is the carbon steel forging specification used for forged fittings and flanges, and A182 covers forged alloy and stainless steel grades such as F304 and F316.

The heat number

A heat is one melt of steel. Every fitting made from that melt carries the same heat number, and the mill or manufacturer lists its chemical analysis and mechanical test results on the certificate. When the number on the fitting matches the number on the certificate, you can prove what the steel is. When it does not, you cannot.

A receiving inspection checklist

Positive material identification

Where the service is critical, or where traceability is in doubt, positive material identification (PMI) is used. A handheld X-ray fluorescence analyser reads the alloy content on the surface, which confirms, for example, that a fitting marked 316L really contains molybdenum. PMI checks chemistry only. It does not replace the certificate or the heat-number match.

Practical tips

• Keep fittings in their original packing until issue so markings are not scratched
• Photograph the marking on a sample from each heat during receiving
• Segregate carbon and stainless steel to avoid contamination and mix-ups
• Transfer the heat number to the spool drawing when fittings are fabricated into spools

Fortune Fittings Private Limited can supply fittings with markings and certificates you can verify. Ask us for test certificates when you request a quote.