Carbon, LIBS, and The Difference Between L+H Steels

In this article we will discuss analysing carbon in three of the four main types of steel alloys; carbon steel, low alloy steel, stainless steel – but not in tool steel at this time.

Carbon steel is an alloy of iron and carbon. Low alloy steel includes carbon and small additions of other alloying elements such as chromium, manganese, molybdenum, etc. up to maximum of 5% total added alloying content. Stainless steel includes carbon and higher amounts of key elements such as chromium, nickel, and molybdenum with a wide range of concentrations.

The World Steel Association states that there are more than 3,500 different grades of steel. There are hundreds of different grades of stainless steel on the market. Each of these unique formulations of stainless steel offer some degree of corrosion resistance above and beyond that of plain carbon steel and low alloy steel.

Carbon is a key element in all these three types of steel. It is added at various levels from 0.005% to 1.2% depending upon the qualities needed for specific uses and applications. In carbon steels the element carbon is typically the only addition, while in low alloy and stainless steels other elements are added in a range of concentrations to impart various properties as needed for the intended use.

Some of the qualities that carbon imparts, depending on amount added, are: degree of weldability, hardness, corrosion resistance, tensile strength, ductility and many more physical properties depending on the combination of other elements. The lack of correct levels of carbon can result in alloy creep and stress rupture, weld decay, Intergranular corrosion and hydrogen stress cracking. Carbon acts as a hardening agent, preventing dislocations in the iron atom crystal lattice from sliding past one another. Although increasing carbon content improves hardness, it also increases brittleness and reduces weldability (above 0.25% C). When the carbon content increases, yield point and tensile strength increase, but the plasticity/ductility is reduced. High carbon content also reduces the air corrosion resistance of steel; in field environments rusting may result.

In welding, carbon equivalent (CE) calculations are used to predict heat affected zone (HAZ). By understanding any differences in chemistry through carbon equivalency calculation, it can be determined if the properties of two materials being joined together via a filler metal component are compatible for the process.  If the components are too dissimilar or if carbon equivalent (CE) approaches a higher, undesirable value then special precautions may be needed prior to and during welding such as prescriptive heat treatment, use of low hydrogen electrodes, controlling heat input.  Many of these guidelines are published in NACE (formerly the National Association of Corrosion Engineers) standards NACE MR0175/ISO 15156 and NACE MR0103/ISO 17945 intended for offshore, petrochemical and natural gas applications where carbon steels in the presence of hydrogen sulphide (H2S, sour service) are susceptible to sulphide stress cracking (SSC) or hydrogen stress cracking (HSC).

Difference between L+H stainless steels

There are hundreds of different grades of stainless steel on the market. Each of these unique formulations of stainless steel offer some degree of corrosion resistance above and beyond that of plain steel.

The 300 series of austenitic stainless steels are the most appropriate for critical industrial applications involving the need for the high corrosion resistant quality of this category of stainless steels. This series is an iron-based, low-carbon alloy that owes its high-corrosive resistance to chromium. The basic structure of 300-series austenitic stainless steel is 18% chromium, 8% nickel alloy, and 0.10% carbon; it is commonly known as 18/8 steel.

The 300 series designation contains many different compositions of alloy steel (303, 304, 305, 316, 321, 347, etc.) but the common factors among them are that carbon content is generally held to a maximum of 0.08%. The most common 300 series in use today are the 304 and 316 grades characterised by the basic 18-8 Cr-Ni chemistry, 304 grade, but with slightly more Ni and Mo added to upgrade to 316 grade.

The corrosion resistance of austenitic stainless steel comes from the chromium oxide protective layer formed on the metal surface. If the material is heated to a temperature between 450 ° C and 900 ° C, the structure of the material changes and chromium carbide forms along the edges of the crystal. Thus, a chromium oxide protective layer cannot be formed at the edge of the crystal, resulting in a decrease in corrosion resistance. This type of corrosion is called “intergranular corrosion.”

Low carbon 300 series stainless steel or “L grade”, e.g., 304L, 316L, were developed to combat this corrosion. Since the carbon content is reduced, chromium carbide is not produced and intergranular corrosion is not generated. Low-carbon stainless steel therefore has a proven resistance to most hostile chemical compounds and is used when the application requires maximum levels of resistance to corrosion and contamination.

In grade 304 stainless, the maximum carbon content is set at 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 minimises sensitisation, 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 304 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 304 part.

So, if 304L is weaker than standard 304 stainless steel, why would anyone want to use it? The answer is that the 304L alloy’s lower carbon content helps minimise/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 304 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. When stronger corrosion resistance is needed, other alloys, such as grade 316 stainless steel, are usually considered as an alternative.

How To Do Field Verification

As can be gleaned from the foregoing, carbon is a critical element to verify. The determination of carbon is therefore essential for a comprehensive analysis of grade and for assuring safe operation over time. The need for techniques and strategies to monitor and verify the carbon content, along with other characteristic alloying elements, therefore becomes a paramount consideration for all intents and purposes where steel or stainless steel is used.

Consequences of using the wrong grade of steel for the intended application can be anywhere from initial rejection by the buyer to entirely unsuitable for the application to the worst case: premature failure of a life-or-death, mission critical component. Many examples of such fatal incidents have been documented and widely publicised in the chemical and petrochemical industries. Another major challenge for the engineer who has specified an L grade is that much of the 300 series metal or fabricated parts purchased these days is dual certified, e.g., 316/316L or 304/304L. So, between lost or incorrect MTR’s (Material Test Reports) and dual certifications it is all the more critical to have a fast and convenient way to determine the actual carbon content before accepting or installing a possible wrong grade that will not survive the intended application!

Many approaches to this need for testing for carbon and other key elements have been in use over the years, including laboratory chemical and analytical techniques, heavy field mobile OES (Optical Emission Spectroscopy), luggable “portable” OES, the ubiquitous handheld XRF (X-Ray Fluorescence) spectroscopy and more recently handheld LIBS (Laser Induced Breakdown Spectroscopy). Many companies make a very good business of providing field analytical testing services using the three main analytical testing technologies, OES, XRF and LIBS. Although XRF, in the handheld pistol form, is the fastest and most convenient of the three technologies it has a drawback in that it is not capable of measuring carbon. Until the new handheld LIBS came along recently, testing of carbon in the field required OES analysers which doubled or more the investment in instrumentation while adding cumbersome size, heavy weight and rigorous analytical skills to operate. All that has changed with the introduction of handheld LIBS analysers that do both the carbon and other key alloying elements in steels.

Handheld LIBS: The Anytime, Anywhere Solution

With the recent introduction of miniaturised, high-power lasers we now have the ability to move LIBS from a “lab only” technique to use in the field. This has opened the door to fast and convenient field analytical determination of carbon in steels. Now, with often only one analyser instead of two, most steel testing and verification can be done on-site, quickly and efficiently. The handheld LIBS analyser is the latest and most advanced example of this ground-breaking, sea-change in on-site, and in-situ alloy testing. As these handheld LIBS analysers become more widely used, they will transform the way people work and the way alloy analysis is accomplished in the industrial workplace.

Source: Thermo Fisher Scientific – Analysing Metals