Quality control testing of ferritic steels
Many of our most trusted structures and parts are made from high strength steels. High strength steels can be found in bridges, building I-beam supports, automobile and airplane parts, to name a few items. They undergo a heat treating process to change the material properties. This treatment allows some materials to have high weight bearing qualities or elastic type strength etc. To guarantee not only the performances but also safety of the final products, quality control testing is a must for the steel industry.
Part of the standard quality control testing involves both physical testing and visual examination by either light microscopy or Scanning Electron Microscopy (SEM) as shown in Figure 1. A comprehensive quality control program should also include X-ray diffraction as a complimentary technique. Unlike the physical and visual testing, the diffraction technique can determine the crystal structure changes quantitatively that can only be estimated by visual examination. There are a few different crystal structures of steel such as martensite and austenite, and the ratio of those can change the properties of steel.
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Figure 1: SEM image of pearlite (lamella layers of ferrite and cementite) |
During the heat treating process of ferritic steels, the temperature is raised until the material converts to almost a pure austenite or face-centered cubic crystal form. The temperature is held for a period of time and then allowed to cool at a very controlled rate. During a slow cooling segment, small amounts of carbon already present in the steel matrix begins to migrate away from the austenite grains, allowing the ferrite or body-centered cubic to replace the austenite. If the cooling rate is slow enough and properly controlled, martensite becomes the major phase at the end of the process. If the cooling rate is not properly controlled or uneven across the sample, austenite can remain even at the end of the process. This is called retained austenite and X-ray diffraction technique is often used to quantify it.
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Figure 2: Sample mounted in MiniFlex II for measurements |
The MiniFlex II benchtop X-ray diffraction system is routinely used for quality control measurements. The MiniFlex II can provide retained austenite, martensite, and cementite quantities on a wide variety of sample sizes. It can measure a wide variety of solid samples from large plates and metal disks, to ingots, coupons and bolts (figures 2 and 3).
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Figure 3: Bolt sample mounted in MiniFlex II holder for austenite / martensite measurements |
This system can use either a standard copper X-ray tube or cobalt depending on the versatility required on the system. A monochromator can be added to the copper system to reduce the background level that is high for iron containing samples. Examples of two small parts with various concentrations of retained austenite are shown in figures 4 and 5. In the analytical software (Retained Austenite Analysis Software), any combination of martensite and austenite peaks can be used In the examples below, (200) and the (220) for austenite (gamma iron) and (200) and (211) for martensite (alpha iron) were used.
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Figure 4: Bolt sample B results show 9% Retained austenite |
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Figure 5: Bolt sample A results show 4.7% retained austenite. |
Tags: cement, cement hydration, karst, calcium sulfate, calcium sulfate dihydrate, cement setting, cement retardent, anhdrite, gypsum, calibrations, quantitation, weight percent, ICP, XRF, XRD, X-ray diffraction, and MiniFlex