Analyzing 304 and 321 Steel for PMI on Piping Systems

I spent a day at an inspection company that delivers Positive Material Identification (PMI) services to refineries. We focused on analyzing and sorting 304L & 321L from 304H and 321H.* PMI inspection companies know that the work of analyzing pipes and welds and determining their metal composition and carbon equivalency is crucial.

As we noted in a previous article, the Low carbon 300 series stainless steel (SS) or “L grade”, e.g., 304L, was developed to combat “intergranular corrosion.” Low-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.

Carbon works as a hardening agent. Increasing carbon content increases hardness and strength but increases brittleness. The metal’s properties such as weldability and heat or corrosion resistance depend on the carbon content.

304 Stainless

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 has a maximum carbon content of 0.03%. The “L” grades are used to provide extra corrosion resistance after welding. High carbon or “H” grades are used for higher strength. L-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.

Grade 304L has a slight, but noticeable, reduction in key mechanical performance characteristics compared to grade 304H stainless steel. This 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 800^oC or 932-1472F).

The 304L alloy’s lower carbon content helps minimize/eliminate carbide precipitation during the welding process. This allows 304L stainless steel to be used in the “as-welded” state, even in severe corrosive environments. If 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—saving time, effort, and money.

321 stainless

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

Titanium stabilized steel is the material of choice in applications with working temperatures in the 400 – 900^oC 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.

304 is widely used in low temperature industry applications like furniture decoration, food, and medical industries.

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).

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.

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 carbides. Ti stabilizes the alloy.

Niobium (Nb) is an alternative stainless steel stabilizer and is used in 347 SS.

Carbon Equivalent

Carbon Equivalent can be measured and calculated directly using LIBS technology. Laser Induced Breakdown Spectroscopy (LIBS) is an analytical technique used to determine the elemental composition of materials.

A handheld LIBS analyzer fires a pulsed laser at the sample vaporizing the material to form a plasma on the surface with ~200 pulses per reading. Excited electrons return to ground state in atoms and ions, emitting light which is collected by the onboard spectrometers. The instrument’s 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.

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.

LIBS and PMI

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. PMI 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.

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. 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.

It was a good day at the inspection company. Who knows?… By analyzing these metals and alloys, we may have helped prevent a serious incident.

*Editor’s Note: The analysis was done using a Thermo Scientific Niton Apollo Handheld LIBS Analyzer

Additional Resources: