Retained Austenite (RA) in steel
What is Austenite? Austenite is a solid solution of mostly iron and carbon which is stable only within a particular range of composition and temperature, and is non-magnetic. It has a face-centered cubic crystal structure. Austenite only forms when an iron-based alloy is heated above about 750°C but not above about 1450°C . Austenite keeps its form at room temperature when special alloying elements have been added to the iron-based alloy (eg Austenitic Stainless Steel).
Austenite was named in honor Sir William Chandler Roberts-Austen, who created the first iron-carbon phase diagram in the 19th century.
On cooling below 700°C it is completely transformed into ferrite which is magnetic and cementite to form the eutectoid pearlite, together with free ferrite or free cementite, depending on whether the carbon content is less or greater than 0.87 percent respectively.
Austenite that does not transform to martensite upon quenching is called retained austenite (RA). Thus, retained austenite occurs when steel is not quenched to the Mf , or Martensite finish, temperature; which is, low enough to form 100% martensite. Because the Mf is below room temperature in alloys containing more than 0.30% carbon, significant amounts of untransformed, or retained austenite, may be present, intermingled with martensite at room temperature. The amount of retained austenite is a function of the carbon content, nickel and manganese content, quenchant temperature and subsequent thermal and/or mechanical treatments. Depending on the steel chemistry and specific heat treatment, the retained austenite level in the case can vary from over 50% of the structure to nearly zero. While large amounts of retained austenite (>15%) can be detected and estimated by optical microscopy, specialized equipment and techniques, such as x-ray diffraction methods, are required to accurately measure the amount of retained austenite to as low as 0.5%.
Why is RA problematic? The very characteristics that give retained austenite many of its unique properties are those responsible for significant problems in most applications. We know that austenite is the normal phase of steel at high temperatures, but not at room temperature. Because retained austenite exists outside of its normal temperature range, it is metastable — when given the opportunity, it will change or transform from austenite into martensite or bainite. In addition, a volume change (increase) accompanies this transformation and induces a great deal of internal stress in a component, often manifesting itself as cracks.
How does RA behave? Martensite is hard, strong and brittle while austenite is soft and tough. In some instances, when combined, the mixture of austenite and martensite creates a composite material that has some of the benefits of each, while compensating for the shortcomings of both. For any given application, mechanical properties are affected by a high percentage of retained austenite content. Retained austenite will transform to martensite if the temperature drops significantly below the lowest temperature to which it was quenched, or at the room temperature itself if the job is subjected to high levels of mechanical stress. Martensite, a body centered tetragonal crystal structure, has a larger volume than the face centered cubic austenite that it replaces. Where transformation occurs, there will be a localized 4–5% increase in the volume of the microstructure at room temperature and a resulting dimensional change in the geometry of the component. If great enough, this dimensional change could lead to growth and in severe instances, crack initiation.
How is the percentage of RA reduced? Tempering is one method used to transform retained austenite. A key is to hold for an adequate amount of time at temperature. Multiple tempers are often performed to ensure the maximum amount of retained austenite has been transformed. Other popular methods include cold treatment at — 120ºF (-85ºC) or cryogenic treatment to — 320ºF (-195ºC). It is well documented that as the temperature is lowered the degree of transformation increases.
Martensitic transformation, also known as incomplete transformation, does not consume the parent phase 100% due to carbon enrichment to the martensite lath/plate boundaries. Such regions would then have an entirely different and lower Ms temperature as compared to the already formed martensite. Theoretically speaking cooling to even lower temperatures does not eliminate the retained austenite altogether, and the incomplete nature of the reaction persist.
How to check amount of RA? The strength, durability, and dimensional stability of steel components are dependent upon the quantity of RA and excess amount may affect the reliability of the component in service. Retained austenite content is critical to these properties. Materials testing can quantify the amount of retained austenite present in a steel component. There are three main methods for determining the presence of retained austenite in steel — a> Metallography; b> Magnetic Induction; c> X-ray Diffraction
Metallography — Metallography is a very simple method of examining a steel sample for the presence of retained austenite. In this method, the sample micrograph is compared to a series of reference micrographs. Alternatively, the use of image analysis can be used. This method is accurate when higher amounts of retained austenite are present. It is very difficult to determine very low quantities of retained austenite because of the difficulty distinguishing the retained austenite. Surface preparation is critical.
Magnetic Induction — The magnetic measurement of retained austenite is a relatively fast and simple method of quantitative measurement of retained austenite. This technique measures the fraction of magnetic phases present in the sample. The non-magnetic phases present are retained austenite. This method was developed to measure the amount of ferrite in stainless steel welds but has been expanded to measure retained austenite.
X-ray Diffraction — Since austenite (face centered cubic, FCC) has a different structure than ferrite (body centered cubic, BCC) or martensite (body centered tetragonal, BCT), the X-ray diffraction pattern will be different. There will be peaks in the diffraction pattern that correspond to FCC peaks. The amount of retained austenite present is determined by examining the peak intensity from each phase. Retained austenite measurement by X-ray diffraction method is standardized and quantitative. It is non-destructive and measurements can be performed at any point in the manufacturing process.
Indirect way to have some guess regarding RA — This is somewhat useful as the standard measurement process of RA is somewhat expensive and skilled activity. But there is a way to indirectly judge the process effectiveness regarding the indicative amount of RA in a steel component. We can use Electromagnetic Induction Paint Thickness Gauges to judge the RA as this gauge measure the change in magnetic flux density at the surface through a magnetic probe when placed on the steel surface.
As this gauge differentiate the magnetic and non magnetic through change in flux density —thus the value shown is an indication of non magnetic portion under the purview of that probe and in steel, the non magnetic is the RA. In the heat treatment process of steel, we can have this data after the part being quenched and tempered or even after sub-zero treatment. By co-relating process wise data along with performance we can build up internal benchmarks for the process.