The Role of Alloying Elements on the Fabricability of Austenitic Stainless Steel

John C. Tverberg, P.E., Metals and Materials Consulting Engineers, Mukwonago, Wisconsin

How many times have fabrication problems developed when a new coil or a new heat of steel is put in production? The problems can be tearing, cracking, scratching, poorer weld penetration, poor electropolished surface or a host of other problems. The usual procedure to determine the source of the problem is a hardness test, tensile test, and metallographic cross section and to review the mill test reports. Sometimes the source of the problem is spotted, but most often nothing out of the ordinary is found. In these cases the problem lies in the composition of the steel even when the alloy is within the specified composition of the steel.

In this paper we will limit our discussion to the austenitic stainless steels, although many of the comments will apply to the other types as well. Many of the troublesome elements are not controlled, so it is essential that they be called out in either a customer specification or on the purchase order. Do not assume the alloy you buy is made to the mid range of each element. Since 1988 the steel mills have instituted a program called "alloy shaving" that uses the minimum alloying elements that will just prevent hot shortness and cold rolling cracking.

Alloy Design
Austenitic stainless steels are designed to give corrosion resistance in many environments, resistance to hydrogen and 885º F (475º C) embrittlement, good strength, good ductility and low hardness. In its simplest form stainless steel is iron with 12% minimum chromium. This is what makes stainless steel rust resistant and allows the passive film to develop. Stainless steel exists in three metallurgical conditions depending on composition and heat treatment: ferritic, martensitic and austenitic. These names refer to the crystallographic structure: ferrite is body-centered cubic, austenite is face-centered cubic and martensite is a distorted tetragonal which is the distorted face-centered cubic structure being changed into a body-centered structure.

Pure iron is body-centered cubic, existing from absolute zero to its melting point. As certain elements are added a "gamma loop" or austenite is created. These elements are carbon, chromium, nickel, manganese, tungsten, molybdenum, silicon, vanadium and silicon. Of these elements nickel, manganese, chromium and carbon have the ability to extend the gamma loop the farthest. It is the combination of nickel and chromium that allows the austenitic stainless steel to be face-centered cubic from absolute zero to the melting point. It is this gamma loop that differentiates the ferritic alloys from the martensitic alloys. Martensitic stainless steels have a narrow chromium range, 14 – 18%, and must contain carbon, because only in this range can pure austenite be formed on heating, thus obtaining martensite on quenching. Ferritic alloys are either below 14% or above 18%. Varying the alloying elements can modify the range. The most common method is to keep the carbon content low.

The most common austenitic stainless steels are based on the 18-8 composition, 18% Cr 8% Ni. If the steel has more than 8% nickel it is austenitic, if it has less nickel then it becomes duplex, that is austenite with ferrite islands. At 5% nickel the structure is about 50% austenite, 50% ferrite, and below 3% it becomes all ferrite. Therefore 8% nickel is the basis for the lowest cost austenitic stainless steel.

When alloying elements are added to the steel, they may take one of the positions in the basic crystal. These are called substitutional alloys and the alloy remains single phase. Other elements are small enough they can fit between the atoms and are called interstitial elements and the alloy remains single phase. Other elements will combine to form their own unique crystals and form certain phases. Still others act as dirt in the alloy and are called inclusions.