Monitoring and controlling Surface Ferrite Content in Stainless Steels is vital. Here's why:
Steel comes in many forms and is chosen for its specific properties to meet the demands of the environment it is likely to encounter during its operational lifetime. In industrial applications, tanks and pipelines need to withstand high temperatures and pressures and often be resistant to harsh chemicals.
To make steel stainless or more correctly, corrosion resistant, iron is alloyed with at least 10% chromium and other metals such as, nickel, molybdenum, manganese and titanium, together with carbon and nitrogen. Chromium has an important characteristic of forming an oxide layer or passivation layer. It is extremely thin and not visible, so the metal retains its lustre, but it is also very hard and protects the metal from corrosion. Crucially it is self repairing in many conditions, so minor scratches are re-covered with the oxide, if oxygen is present in the environment.
Different steels and their crystalline structure
As well as specifying steels by their composition, they are also classified by another important characteristic, their crystalline structure. Austenitic stainless steels, containing high levels of chromium and nickel are tough and durable, but in some circumstances are prone to pitting due to chemical attack. If oxygen is not present, such as in chemical production or with high chloride levels, such as off-shore applications, the passivation layer cannot be reformed. In these circumstances a tiny surface pinhole can hide a large cavity, which if undetected can cause structural failure. Ferritic stainless steel, although highly corrosion resistant and often better able to withstand pitting, is, in some circumstances, less durable. The corrosion resistance of austenitic steels can be improved by the addition of higher levels of chromium and molybdenum and other elements but at great expense. A much cheaper option is duplex steel, which is a mixture, usually 50:50 or 60:40 of both. Duplex steels have higher strength and resistance to localised corrosion than austenitic steel, with the corrosion resistance of ferritic. The down side is that to maintain its properties it must be carefully handled, to avoid the structure having the weaker properties of each of the components prevailing. It is especially important to monitor and control the ferrite content, especially the surface ferrite content.
Considerations when welding
Stainless steels are often heat treated to improve their properties, but incorrect treatment can affect the surface ferrite content. Almost invariably pipes and tanks are welded and it is essential that the composition of the welds match the properties of the welded materials. When welding duplex stainless steels the composition of the welding rods and the temperatures reached can affect this balance, and lead to long term failure of the structure.
In joint welding and even more in surfacing, it is important to test the ferrite content of the weld, since ferrite influences the properties of a material in many ways. On the positive side it tends to prevent heat cracks in austenitic welded material because niobium, silicon, phosphorus and sulphur are highly soluble in it and are prevented from forming low-melting eutectics at the grain boundaries, which are therefore able to take up cooling stresses without producing cracks. It has a negative effect on the corrosion chemistry of the material, since when there is too much of it an unfavourable ferrite network develops and also the ferrite can decay under the influence of heat into austenite, carbides and possibly the sigma phase. The resulting loss of chromium and molybdenum from the austenite reduces the corrosion resistance of the material.
How ferrite content can be determined
The ferrite content can be determined by metallographic means, which is normally a laboratory procedure but can, if necessary, be performed on site. It involves examining the crystalline structure of a sample under a microscope. The limitations are that it is usually destructive and ferrite appears in many differing forms, which cannot always be recognised. It also only examines a small part of a structure.
X-ray diffraction can be used, but this is a laboratory technique and it can only analyse the sample a few microns below the surface and at proportions above 3% ferrite. Chemical analysis can be used if the composition is known, but heat treatments make this a very approximate method. Simple pull-off magnetic gauges are even less accurate and are compromised by sample curvature, wall thickness and are not usable for thin austenitic cladding on ferromagnetic substrates.
Electronic magnetic induction instruments such as Fischer's Feritescope range, however, provide a very practical solution. These instruments operate by generating an alternating magnetic field in the sample which is proportional to the ferrite content and which the instrument can detect. The instrument, using a non-destructive magnetic method, measures the relative permeability of a material in the alternating magnetic field of its probe. This provides a ferrite content reading, which is largely uninfluenced by various extrinsic parameters (lift-off, strain in specimen, curvature of specimen surface, thickness of welded material). The results of comparison measurements have confirmed that the values obtained are correct and these instruments are widely specified as essential on projects ranging from oil exploration, chemical production, utility and other processing plants exposed to heat, aggressive media and high pressure.
Fischer Instrumentation(GB) Ltd is based in Hampshire and is part of the Helmut-Fischer Group, Sindelfinden, Germany - innovative leaders in the field of coating thickness measurement, material analysis, microhardness testing, electrical conductivity and ferrite content measurement as well as for density and porosity testing. With over 50 years of continual research and development, the company is able to recommend the best solution for any application.
To make steel stainless or more correctly, corrosion resistant, iron is alloyed with at least 10% chromium and other metals such as, nickel, molybdenum, manganese and titanium, together with carbon and nitrogen. Chromium has an important characteristic of forming an oxide layer or passivation layer. It is extremely thin and not visible, so the metal retains its lustre, but it is also very hard and protects the metal from corrosion. Crucially it is self-repairing in many conditions, so minor scratches are re-covered with the oxide, if oxygen is present in the environment.