{"id":11626,"date":"2022-04-12T12:01:06","date_gmt":"2022-04-12T12:01:06","guid":{"rendered":"https:\/\/www.thermofisher.com\/blog\/metals\/?p=11626"},"modified":"2026-02-03T13:27:53","modified_gmt":"2026-02-03T13:27:53","slug":"analyzing-304-and-321-steel-for-pmi-on-pipelines","status":"publish","type":"post","link":"https:\/\/www.thermofisher.com\/blog\/metals\/analyzing-304-and-321-steel-for-pmi-on-pipelines\/","title":{"rendered":"Analyzing 304 and 321 Steel for PMI on Piping Systems"},"content":{"rendered":"

\"imageI spent a day at an inspection company that delivers\u00a0Positive Material Identification (PMI) services to refineries. We focused on analyzing and sorting 304L & 321L<\/a> from 304H and 321H.* PMI inspection companies know that the\u00a0work of analyzing pipes and welds and determining their metal composition and carbon equivalency is crucial.<\/p>\n

As we noted in a\u00a0previous article,<\/a>\u00a0the Low carbon 300 series stainless steel (SS) or \u201cL\u00a0grade\u201d, e.g., 304L, was developed to combat\u00a0\u201cintergranular corrosion.\u201d\u00a0Low-carbon stainless steel has a proven resistance to most hostile chemical compounds and is used when the application requires maximum levels of resistance to corrosion and contamination – like in refineries.<\/p>\n

Carbon works as a hardening agent. Increasing carbon content increases hardness and strength but increases brittleness.\u00a0The metal\u2019s properties such as weldability and heat or corrosion resistance depend on the carbon content.<\/p>\n

304 Stainless<\/strong><\/p>\n

In grade 304H, a chromium nickel stainless steel, carbon content is between 0.04 and 0.1% (typically 0.08%), whereas grade 304L stainless steel\u00a0has a maximum carbon content of 0.03%. The \u201cL\u201d grades are used to provide extra corrosion resistance after welding.\u00a0\u00a0High carbon\u00a0or\u00a0\u201cH\u201d\u00a0grades\u00a0are used for higher strength.\u00a0L-grade stainless steels are typically used for parts which cannot be annealed after fabrication by welding. The low carbon minimizes sensitization, or chromium depletion at the grain boundaries of the material which would otherwise reduce its corrosion resistance.<\/p>\n

Grade 304L has a slight, but noticeable, reduction in key mechanical performance characteristics compared to grade 304H stainless steel.\u00a0This means that if you had two stainless steel parts and both parts had the exact same design, thickness, and construction, the part made from 304L would be structurally weaker than the standard 304H part at high temperature (500 to 800o<\/sup>C or 932-1472F).<\/p>\n

The 304L alloy\u2019s lower carbon content helps minimize\/eliminate carbide precipitation during the welding process. This allows 304L stainless steel to be used in the \u201cas-welded\u201d state, even in severe corrosive environments.\u00a0If the standard 304H stainless steel were used in the same way, it would degrade much faster at the weld joints than 304L. Using 304L eliminates the need to anneal weld joints prior to using the completed metal form\u2014saving time, effort, and money.<\/p>\n

321 stainless\u00a0<\/strong><\/p>\n

321 stainless steel belongs to the same 300 stainless steel series, and\u00a0has very similar chemistry to 304 stainless steel with the addition of Titanium at 5 x carbon% value (max 0.7%).\u00a0The titanium acts as a stabilizer and makes it more resistant to chromium carbide formation.<\/p>\n

Titanium stabilized steel is the material of choice in applications with working temperatures in the 400 – 900o<\/sup>C as it has improved stress fracture performance and high-temperature creep resistance, and its stress mechanical properties are superior to 304 stainless steel. Industrial applications such as chemical, coal and petroleum industries can operate at high temperature and would prefer 321 over 304.<\/p>\n

304 is widely used in low temperature industry applications like furniture decoration, food, and medical industries.<\/p>\n

When welding stainless at high temperature, Cr combines with carbon and precipitates chrome carbides at the grain boundaries, significantly reducing corrosion resistance in the heat affected zone (HAZ).<\/p>\n

At high operating temperature this reduced corrosion resistance leads enhanced corrosion and working life in the HAZ, and is also more susceptible to stress fractures.<\/p>\n

There are two ways to minimize chrome carbide precipitation, reducing the carbon content e.g. using 304L instead of 304H, or more effectively adding Ti at 5 times the carbon content (0.7% max) to create 321. The carbon is more attracted to the Ti during welding and therefore it leaves the chromium alone and minimizes chromium\u00a0carbides. Ti stabilizes the alloy.<\/p>\n

Niobium (Nb) is an alternative stainless steel stabilizer and is used in 347 SS.<\/p>\n

Carbon Equivalent<\/strong><\/p>\n

Carbon Equivalent can be measured and calculated directly using\u00a0LIBS technology<\/a>.\u00a0 Laser Induced Breakdown Spectroscopy (LIBS) is an analytical technique used to determine the elemental composition of materials.<\/p>\n

A\u00a0handheld LIBS analyzer<\/a>\u00a0fires a pulsed laser at the sample vaporizing the material to form a plasma on the surface with ~200 pulses per reading.\u00a0 Excited electrons return to ground state in atoms and ions, emitting light which is collected by the onboard spectrometers. The instrument\u2019s software and calibrations compare the wavelengths and intensity of spectral lines to quantify the concentrations of elements, and using a prescribed formula via a pseudo element feature, enables automatic calculation of carbon equivalency.<\/p>\n

The carbon equivalent (CE) concept converts the material composition into something useful for the evaluation of the weldability (and hardness) of the material and thus predict its behavior.<\/p>\n

LIBS and PMI<\/strong><\/p>\n

LIBS is an important technology used in the oil and gas industry for positive material identification (PMI) of piping, pressure vessels, valves, pumps, and finished welds, or to grade unknown materials to regain traceability and verification. \u00a0PMI is utilized for quality control and safety compliance, and is an integral part of both production and asset integrity management across many industries including oil and gas, power, chemical, pharmaceutical, nuclear, aerospace and fabrication. PMI is mandatory in many of these industries.<\/p>\n

As a reminder, changing the amount of carbon in these products can change the properties of the steel, making it more or less strong, hard, ductile or malleable.\u00a0 Fatal accidents and injuries, as well as leaks, premature pipe replacements, loss of property, and unplanned outages at refineries, chemical plants and gas processing facilities often can be traced back to equipment failures due to faulty or counterfeit metal building components or because piping is made from material that does not meet specifications.<\/p>\n

It was a good day at the inspection company.\u00a0 Who knows?\u2026 By analyzing these metals and alloys, we may have helped prevent a serious incident.<\/p>\n

\u00a0<\/em>*Editor’s Note:\u00a0 The analysis was done using a\u00a0<\/em>Thermo Scientific\u00a0Niton Apollo Handheld LIBS Analyzer<\/em><\/a><\/p>\n

Additional Resources:<\/em><\/p>\n