Should You Trust the Stainless Steel Mill Test Report?
Stainless steels have become essential to our everyday lives. They are produced as sheets, plates, bars, wire, and tubing to be fabricated into every imaginable type of material and equipment. This includes use in chemical and petrochemical industries, power generation, food production, architecture, construction, transportation, paper mills, water treatment, heavy industry and even the appliances we use every day to cook, wash clothes, or keep foods cold. Without stainless steels our life would be much poorer indeed.
Stainless steel is tough, ductile, corrosion resistant and versatile – that is, when it’s made correctly – and the right grade is used for the intended application. However, with 150 or more distinct grades of stainless steel in use, made up of different percentages of alloying metals and all looking exactly alike, how can one be sure that the right quality is used for the intended application? This is where analytical tools for testing and verification come in.
Getting the ‘recipe’ wrong for those variations may mean not meeting customer requirements – which could in turn cause corrosion, stress fractures or even rupture of the stainless steel. The wrong grade can have consequences as simple as economic loss, or as crucial as loss of life. Explosions in chemical and petrochemical plants are vivid reminders of the dangers of using incorrect alloys. Therefore, most industries strive to ensure product quality by verifying raw materials used during the production process.
Carbon is always present in stainless steel and is considered a key element to analyse. Depending on the grade of stainless steel, carbon content can be between 0.005% to 1.2%. Carbon has a major effect on corrosion resistance, hardness and weldability. In certain stainless steels, a high carbon content is undesirable, especially for welding due to the threat of carbide precipitation. In all categories except the hardened stainless category, the carbon level is kept quite low, as indicated, to facilitate welding and to help prevent corrosion. However, for high strength and hardness the carbon level is deliberately increased. For all of these reasons carbon is a critical element to verify. The determination of carbon is therefore essential for a comprehensive verification of grade and safe operation over time.
Among other material verification methods, the Mill Test Report (MTR) is utilised to validate goods. An MTR is a quality assurance document sent from the mills to their customers highlighting the physical and chemical properties of a material. These certificates describe the product’s alloy, temper, thickness, width, finish, and more importantly indicates a metal product’s compliance with international standards such the American National Standards Institute (ANSI) and the American Society of Mechanical Engineers (ASME).
However, in certain cases, the MTR can be unreliable or incorrect due to mismarking (a mix-up in the labelling process) or other reasons. The American Galvanizers Association cited an incident where an MTR misrepresented the silicon level of the steel because the methods used contained some systematic errors that decreased the accuracy of the report. (An example given was that only one unit was taken for analysis from a large population of steel and it did not necessarily represent all of the product.)
It is best practice to perform a second validation of the MTR to verify goods. Catching the issue at the forefront of inspection – using elemental analysis – avoids the costly problem of determining goods have been developed out of specification after adding value during the fabrication process. Oftentimes, the resulting occurrence must be scrapped entirely.
Material verification doesn’t stop at incoming inspection. Supervisors and QC managers must ensure that the correct materials are used throughout the production process. Once again, best practice requires elemental analysis – often for regulatory compliance. Testing may follow the part, assembly, or equipment right through to a final validation before shipping. When received by the customer, an incoming inspection is generally performed again. And so, it goes, continuing to be inspected along the path to the final use.
As we noted above, Carbon (C), is an important alloying element. To date, analysing carbon has been a challenge. The most popular material verification method, handheld x-ray fluorescence (XRF), has been incapable of detecting carbon content. Optical Emission Spectroscopy (OES) can detect carbon, but is only available in large, bulky carts, making it difficult to apply in awkward field environments (ladders, catwalks, ditches, tight spaces, etc.).
The latest technology, a handheld LIBS analyser, utilises Laser Induced Breakdown Spectroscopy (LIBS) to perform inspection of materials during the fabrication process.
Some LIBS instruments can measure low levels of carbon in L grade stainless steel of around 100-300 ppm (parts per million). This sensitivity enables the instrument to differentiate the levels of carbon in 304L with ~300 ppm or ~0.03% carbon, from the ~0.06% level in a 304 grade (for example). Other examples of the need to test for carbon include 316 vs. 316L/ 316H and 317 vs. 317L.
LIBS technology can confirm the chemistry or replace erroneous chemistry on MTRs, eliminate risks associated with welding incompatible alloys, using the wrong grade in a critical application and can even differentiate between very close alloy grades.
So I guess the best piece of advice when it comes to steel production is to not totally trust the MTR, and to use LIBS to verify.
Analysis of Stainless Steel Testing – Application Note
Analysis of Carbon Equivalents in Steel Components – Application Note
Carbon Equivalency in Welded Steel Components with LIBS – Blog article