Delta ferrite tools

And how is the amount of ferrite determined? But first, what is ferrite? This may be formed upon solidification from the molten metal delta ferrite or by transformation from austenite or sigma phase on cooling in the solid state alpha ferrite.

It is important to note that both phases are Alloyed, that is, they contain high levels of chromium, nickel, molybdenum, etc. Note: Many common materials also exist in different phases. Water may be either a liquid, a solid ice , or a gas steam.

Carbon may be either graphite or diamond. Silicon dioxide beach sand may be quartz, cristobalite, or tridymite. Low-Ferrite Austenitic Stainless Steel Foundries are often called upon to produce special grades of stainless steel. The principal factors which determine the amount of ferrite are chemical composition, casting section thickness and heat treatment cycle.

Composition — ASTM Specification A explains how the average amount of ferrite in the castings poured from a heat of molten Stainless steel may be estimated from the chemical composition.

The ferrite promoting elements are Cr, Mo, Si, and Cb. The austenite promoting elements are Ni, C, Mn, and N.

Of course, each of the elements must also be held within the limits established for the specific grade being produced. Section thickness — The ferrite content will vary from one location to another in a given casting. In general, a thicker section will have a higher ferrite than a thinner section in the same casting. It should also be noted that the matching filler metals used to join these alloys will also have higher ferrite contents.

This is a precaution against solidification cracking during welding. Return to Blog List. Click diagram for larger version The DeLong diagram is also a predictor of the amount of ferrite likely to be present in a weld deposit made from the wrought product.

Our Company. North America Asia Europe. United States P. All Rights Reserved. Several etchants revealed the carbides well. The sharpness and uniformity of the outlining was much better than for the delta ferrite. One stringer is in the upper right corner of the image.

The other large precipitates are sulfide inclusions. The grain and twin boundaries have been nearly fully covered by the carbide that precipitated in the post-welding heat treatment.

Besides the intergranular carbide there is some intragranular carbide. The large black round particles in Figure 5 are slag particles. The smaller, dark particles in the austenitic matrix are carbides. Figure 6 shows the base metal and the small delta ferrite stringers in the base metal are clearly visible.

Figure 7 shows the sigma phase in the weldment, colored orange brown and revealed sharply. Figure 8 shows the base metal.

Note the regions marked with the arrows that contain some larger carbides. Because neither sigma nor delta ferrite were revealed in the as-welded or aged specimens with room temperature aging, it is assumed that these particles are carbides. Figure 9 shows the carbides revealed in the weldment. This is useful as many other etchants also revealed the sigma, which makes it harder to see the carbides in the weldment. Figure 10 shows the sigma using NaOH in the formulation.

When KOH was used, the sigma was colored in similar manner after 30 s but with an additional 30 s etch, the color was largely removed. They are generally considered to be equivalent. This shows well-revealed carbides in the aged base metal. Figure 12 shows that ammonium persulfate outlined sigma quite nicely, but did not color it. Note that the outlining is much sharper and more uniform than it was for the delta ferrite Figure3d. The final example is the use of concentrated ammonium hydroxide electrolytically.

Results were no better than for other more easily used reagents and, because of the strong pungent odor of ammonium hydroxide and the irritation of the hands, eyes, nose and throat, its use is not recommended. In this use, the sigma was not revealed, although in prior use, it was different specimen. The study revealed that a variety of reagents can be used to reveal constituents in stainless steels. They are not useful for identification purposes.

At temperatures near the boiling point, the standard formulation colored delta ferrite non-uniformly and sigma uniformly. But there was no color difference. Ammonium persulfate is excellent for detecting sensitization as it reveals the carbides very clearly.

It outlined both delta ferrite and sigma. Again, the delta ferrite was not sharply outlined but the sigma was. Ammonium hydroxide is not recommended for use as it did nothing better than any other etch and it is very annoying to use. The microstructural constitution of stainless steels is quite complex, and exposure to high operating temperatures adds to the complexity as a variety of phases can occur.

In addition to the matrix phases of ferrite, austenite and martensite, as well as duplex austenite-ferrite and less commonly ferrite-martensite, there are numerous possible minor constituents. In the carbide family, M 23 C6 face-centered cubic and M 7 C 3 hexagonal carbides are the most common, but M 6 C face-centered cubic and MC face-centered cubic carbides are also observed in certain alloys.

Much has been published regarding selective etching techniques to differentiate sigma from ferrite, or delta ferrite, and for the various carbides, but little has been published regarding chi and Laves.