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Introduction
Whether the order is for seamless pipe for a petrochemical plant, flat bar for a food processing line or structural sections for an offshore platform, the material at the receiving bay must be exactly the same as ordered. Within that process of verification are two types of testing: chemical testing and mechanical testing. These two specialties are concerned with very different questions. Chemical testing provides information on the composition of the material, whereas mechanical testing provides information on how the material will react to service loading, temperature, and deformation. No one category is redundant with the other. A correct chemical analysis can be obtained from a piece of stainless steel, but it may not have the features necessary for the tensile strength specified by the relevant product specification. On the other hand a batch can meet all the mechanical requirements, but may contain an off specification carbon or sulphur content which may lead to intergranular corrosion in a welded assembly in a corrosive medium. Having a clear grasp of the purpose and scope of each type of testing, as well as that of their limitations will equip buyers better to make clear specifications, review MTCs with confidence and prevent costly material substitutions and in-service failures.
What Is Chemical Testing for Stainless Steel?
Chemical tests identify the chemical make-up of a stainless steel heat. They go to confirm that the alloy delivered is a 316L, as the level of carbon is at or below 0.03 percent, which defines the "L" variant and the limit of sensitisation during welding, for instance; that the level of chromium, nickel and molybdenum falls into the range set by the controlling standard, for example; and that the alloy delivered is 316L, as this is specified with the alloy grade.
Chemical analysis of the principal alloying elements tested include chromium, nickel, molybdenum, carbon, nitrogen, manganese, silicon, phosphorus and sulphur, but some of these elements are not tested in some grades and standards. Stainless steel is corrosion resistant because its passive oxidation layer consists of chromium which is present at least about 10.5% in all grades. Nickel stabilises the austenitic microstructure and enhances the toughness. Grades containing molybdenum, such as 316, 317 and duplex alloys, are more resistant to pitting and crevice corrosion in a chloride environment. Care must be taken to control the amount of carbon: in austenitic grades, carbon can precipitate as chromium carbides at the grain boundaries during welding and leave the surrounding matrix without sufficient chromium, thus providing a path for corrosion attack, called sensitization.
There are basically 3 different types of chemical analysis performed on stainless steel. The most common production method is optical emission spectrometry (OES), which uses a high energy spark or arc to excite atoms within a polished sample, and then measures the concentrations of the elements that gave rise to the light spectrum, with high speed and good precision, for most elements of interest. Another way is wet chemistry in the lab or X-ray fluorescence (XRF) analysis, especially for elements which OES instruments are not as reliable. The material usually is identified using XRF handheld gun on the job and is referred to as positive material identification (PMI). While PMI is quick and causes no damage to the test area, buyers should be mindful that the handheld instrument may not reliably detect or quantify all light elements based on the type of detector and calibration, for instance, carbon is not typically quantifiable by field-portable XRF. The PMI is thus complementary and not a substitute for a complete mill chemical analysis.
For flat-product orders, the same testing plan should identify whether the material is supplied as stainless steel sheet, stainless steel plate, or stainless steel coil because the applicable dimensions, tolerances, and sampling requirements can differ by product form.
What Is Mechanical Testing for Stainless Steel?
Mechanical Testing measures the responses of a stainless steel product to an applied load or controlled deformation. If a chemical analysis determines what a material is, mechanical testing will determine what a material will resist. The most basic mechanical properties measured are tensile strength and yield strength. Tensile strength is the highest level of stress (tension) a material can withstand before it will break; yield stress is the point at which a material moves from elastic to permanent plastic deformation. Both units are in megapascals or pounds per square inch, depending on which standard is being used. Elongation is the ductility of the material – the percentage of elongation is the amount that the material can be stretched before it breaks. Applications where forming, bending or drawing operations are necessary, structures that are required to absorb energy and not suddenly break when over loaded, require a high elongation value. Hardness testing typically measured by Brinell, Vickers or Rockwell is a rapid test of resistance to surface indentation and is occasionally used as a surrogate test for the heat treatment condition. The specific tests applied to tubular products are the flattening, flaring and bend tests, designed to evaluate the forming capability of the tube; that is, resistance to splitting along the weld seam or parent metal. If the product is expected to be used at low temperature, toughness is measured using impact testing at a temperature determined by the product specification, using either Charpy or Izod pendulum. The mechanical tests to be conducted are determined by the product form, the applicable product standard and any additional requirements set forth in the purchase order. Not all product types need all the tests. Seamless pipe for ASTM A312 is not necessarily subject to impact or flaring testing unless specifically requested by the customer or required by an application code, but must be tested to withstand tensile testing and a hydrostatic or non-destructive electric test. It is the purchaser's responsibility to check the tests necessary for the product to meet the requirements of the standard they are considering and to ensure that a compliant certificate does not imply compliance with all mechanical properties.
For stainless steel bar or pipe procurement, buyers should match the mechanical test results to the ordered grade, size, heat-treatment condition, and governing product standard.
Chemical Testing vs Mechanical Testing: Key Differences
The key to take from any comparison of the two types of tests is that they cannot replace each other. While correct chemistry is imperative and essential, it is not enough to ensure that the mechanical property requirements are met. On the other hand, the mechanical properties obtained during the test are good indicators of performance characteristics in the test environment, but do not necessarily imply resistance to corrosion in a particular service environment, which depends primarily on composition. The main differences are summarised in the table below.
| Question | Chemical Testing | Mechanical Testing |
|---|---|---|
| What does it verify? | Alloy composition | Strength, ductility, and deformation behaviour |
| What can it confirm? | Grade identity and suitability for corrosive environments | Compliance with specified load-bearing property requirements |
| Typical methods | OES, laboratory analysis, PMI | Tensile, hardness, flattening, flaring, bend, impact |
| Can it replace the other? | No | No |
Destructive Testing, NDT and PMI: Avoiding Confusion
One other distinction that consumers often mistake is the difference between destructive testing and non-destructive testing (NDT) and PMI. The tensile test, flattening test and bend test are most of the mechanical tests and are destructive in nature. They need to have test pieces cut from the length of the sample that had the same heat and condition as the production item, and then be taken out of the sample and used for testing until they fail or reach a specific deformation, after which they are scrapped. The tests give statistically representative data if the sampling plan specified in the product standard is correctly followed, but the tests require material and make the tested specimens unusable for other tests. Non-destructive testing is any type of testing that identifies discontinuities in the product without affecting its intended function, such as ultrasonic testing (UT), radiographic testing (RT), liquid penetrant inspection (LPI), magnetic particle inspection (MPI) and eddy current testing (ECT). NDT can detect cracks, seams, laps, laminations or porosity that could compromise structural integrity or pressure-tightness during operation. It does not, however, give details of composition or mechanical properties. NDT and mechanical testing complement each other in different ways in terms of product integrity. PMI is in a different class altogether. It is used mainly as a verification or sorting tool to help verify at goods-in or at the point of fabrication that the grade marked is the actual alloy present. It is not meant to replace a complete mill chemical analysis and a PMI result should not be used to replace an MTC for formal compliance. The three techniques destructive mechanical testing, NDT, and PMI, provide differing information on various aspects of material quality and are best used in conjunction with one another.
Test Results on a Stainless Steel Mill Test Certificate
A mill test certificate is a written report that correlates a production heat and a lot or batch of material with the testing conducted during the metal manufacturing process. Being able to read an MTC is a skill that should be acquired by any buyer of stainless steel. The certificate should be signed by the manufacturer and include the manufacturer's location, a certificate number and a reference to the purchase order involved. In addition to these administrative areas, the buyer should verify that the heat number on the certificate is the same as that marked on the physical product as this is the most significant link between the document and the physical product. The certificate should include the product standard and edition to which the material has been made, the grade designation and the UNS designation, the product dimensions and tolerances, and the heat treatment condition (usually solution annealed and quenched for austenitic grades, thus dissolving carbide precipitates and restoring corrosion resistance after hot working). Chemical analysis section specifies the amount of each heat component, and the amount of each element must be compared to the limits in the applicable standard. The mechanical properties section lists the tensile and yield strength, elongation and hardness, if any, for comparison with the standard properties. In the event of NDT or hydrostatic tests, a certificate or a test report should confirm the result. The certificate should be signed or stamped by a recognized person certifying that the tests have been performed and that the sample meets the requirements of the indicated standard.
Inspection Certificates: EN 10204 3.1 vs 3.2
Metallic products have inspection documents that are defined by the European standard EN 10204. For stainless steel, process and pressure applications such as types 3.1 and 3.2 are the most frequently specified document types. A 3.1 certificate includes certain inspection results, actual chemical and mechanical test values, and is certified by a representative who is independent from the manufacturing department. The independence requirement bestows more weight on the 3.1 certificate than on a report from a works test. In addition to the manufacturer's authorised representative, the 3.2 certificate also includes a purchaser's representative, or a representative of a third party inspection body designated by the purchaser. Several major industry codes and engineering specifications enforce the 3.2 level, such as oil and gas, nuclear and pressure vessel construction, where independent third-party oversight of the manufacturing process is significant to contractual and regulatory requirements. The 3.2 certificate doesn't necessarily indicate that the actual material is of a higher quality than a 3.1 certificate; it reflects only the certificate level of the document, not necessarily the quality of the underlying material. The 3.2 certificate does offer additional independent engagement with the inspection process itself.
How Buyers Should Review Chemical and Mechanical Results
Reviewing an MTC is no more about seeing "meets requirements" somewhere on the document. First of all, a comprehensive review starts with the verification of the heat number with the physical marks of the product. The buyer then compares each chemical element value to the respective limit in the noted standard edition, bearing in mind that the compositional limits listed in different standard editions may be slightly different; the noted standard and edition given on the certificate must be the one that was ordered. It is also important to note that heat analysis is the analysis of the bulk melt while product analysis is taken from the product and may have different or slightly broader tolerances. To verify mechanical properties, the buyer is advised to consider the test units used, as well as to confirm the hardness result, if reported, is in the correct scale of the material and application. When elongation is reported, the gauge length used should be specified with the report because elongation is dependent upon the gauge length and a direct comparison between certificates is only valid if they are based on the same length convention. Results may also vary depending on the condition and orientation of tensile specimens, such as whether they are longitudinal or transverse to the rolling direction; some standards specify orientation for certain applications. Lastly, the buyer should verify the test certificate with the bundle tag or lot number of the material received to ensure that the certificate he/she is considering is the one for the same lot that was received, and not just the one with the same heat number.
What Testing Requirements Should Be Included in a Purchase Order?
The purchase order is a document that spells out the exact requirements of the buyer. Some specifications are ambiguous, and can lead to problems such as disputes, rejection of material and/or non-compliant material being delivered to service. At the very least a stainless steel purchase order must include the product standard and its edition, grade and designation UNS, the test to be performed by its name and the document type required for its delivery, EN 10204. If additional corrosion testing is required intergranular corrosion testing to ASTM A262, pitting resistance testing, or stress corrosion cracking testing it is to be specified as such because it is not part of the base product test and a mill will not perform such testing unless specifically requested. Likewise, the PMI, NDT method and acceptance criteria, hydrostatic test pressure, and third party inspection hold points all call for certain call-up in the order documentation. Unless the buyer has detailed project-specific requirements, acceptance criteria shall be identical to that of the specified standard; if they are more stringent, they must be detailed. In addition, the following requirements should be included with regard to marking the heat number, grade, standard, size and the manufacturer's mark on the product itself so that traceability can be maintained from the point of receipt until installed: The best way to ensure that the testing and documentation provided will meet the actual needs of the application is to specify all these requirements clearly at the time of order, instead of trying to negotiate these requirements retrospectively, after the material has been produced.
If final dimensions are part of the order, send the drawing and tolerance requirements before production. Jianglin can support custom cut stainless steel sheet and stainless steel laser cutting for project-specific parts.
How Jianglin Supports Testing-Compliant Stainless Steel Orders
Once the material grade, product form, dimensions, applicable standard, testing scope, inspection document, and marking requirements are defined, buyers can combine material supply with stainless steel CNC machining or custom stainless steel fabrication. Send the drawing or specification together with quantity and testing requirements to request a quote for your project.
Frequently Asked Questions
Can PMI replace a mill test certificate?
No. PMI is mainly a grade-verification or sorting tool. Buyers should still review the MTC and the chemical and mechanical results required by the applicable product standard and purchase order.
Which tests are required for a stainless steel product?
The required tests depend on the product form, grade, applicable standard, service conditions, and project specification. State each additional test and its acceptance criteria in the purchase order.
What is the difference between EN 10204 3.1 and 3.2?
A 3.1 certificate is validated by the manufacturer's authorised inspection representative independent of production. A 3.2 document adds validation by the purchaser's representative or an appointed third-party inspector.
What should buyers send when requesting a quote?
Send the product form, grade, standard and edition, dimensions, quantity, surface or processing requirements, required tests, certificate type, marking instructions, and any drawing or project specification.
Conclusion
The two pillars of stainless steel material assurance are chemical and mechanical testing. Chemical testing is used to confirm the identity of the grade and to provide the compositional information that is used to predict the performance of the corrosion. Mechanical testing verifies the product is as strong, ductile and tough as required for the loading it will take in service. The two sets of data are complementary and a buyer who has confidence in both is better prepared to make a confident purchase of stainless steel because he understands what each test measures, how the results will be presented on an MTC, what an EN 10204 3.1 or 3.2 certificate entails, and how to clearly specify requirements in a purchase order. Too, the small investment in understanding these testing fundamentals really pays off in terms of less receiving disputes, lower risk of in-service failure, and greater accountability of the supplier along the value chain.
Send your drawing or specification, material grade, dimensions, quantity, test requirements, and certificate type to request a quote for your project.
