Stainless steels are a class of chromium-containing steels widely used for their corrosion resistance in aqueous environments and for service at elevated temperatures. Stainless steels are distinguished from other steels by the enhanced corrosion and oxidation resistance created by chromium additions. Chromium imparts passivity of ferrous alloys when present in amounts of more than about 11% particularly if conditions are strongly oxidizing. Consequently, steels with more than 10 or 12% Cr are sometimes defined as stainless steels.
Stainless steel castings are usually classified as either corrosion-resistant castings or heat-resistant. However this line of demarcation in terms of application is not always distinct, particularly for steel castings used in the range from 450 to 650oC. The usual distinction between heat-resistant and corrosion-resistant cast steels is based on carbon content.
In general, the cast and wrought stainless steels possess equivalent resistance to corrosive media and they are frequently used in conjunction with each other. One significant difference between the cast and wrought stainless steels is in the microstructure of cast austenitic stainless steels. There is usually small amount of ferrite present in austenitic stainless steel castings, in contrast to the single-phase austenitic structure of the wrought alloys.
The presence of ferrite in the castings is desirable for facilitating weld repair, but ferrite also increases resistance to stress-corrosion cracking. The principal reasons for this resistance are apparently:
- Silicon added for fluidity gives added benefit from the standpoint of stress-corrosion cracking.
- Sand castings are usually tumbled or sandblasted to remove molding sand and scale, this probably tends to put the surface in compression
Wrought and cast stainless steels may also differ in mechanical properties, magnetic properties, and chemical content. Because of the possible existence of large dendritic grains, intergranular phases, and alloy segregation, typical mechanical properties of cast stainless steels may vary more and generally are inferior to those of any wrought structure.
Cast stainless steels are most of ten specified on the basis of composition using the designation system of the High Alloy Product Group of the Steel Founders Society of America (the Alloy Casting Institute). The first letter of the designation indicates whether the alloy is intended primarily for liquid corrosion service (C) or high temperature service (H). The second letter denotes the nominal chromium-nickel type of the alloy. As nickel content increases, the second letter of the designation is changed from A to Z. The numeral or numerals following the first two letters indicate maximum carbon content (percentage x 100) of the alloy. Finally, if further alloying elements are present, these are indicated by the addition of one or more letters as a suffix.
Corrosion-Resistant Steel Castings. These steel castings for liquid corrosion service are often classified on the basis of composition, although it should be recognized that classification by composition often involves microstructural distinction.
Alloys are grouped as:
- Chromium steels
- Chromium-nickel steels, in which chromium is the predominant alloying element
- Nickel-chromium steels, in which nickel is the predominant alloying element.
The service ability of cast corrosion-resistant steels depends greatly on the absence of carbon, and especially precipitated carbides, in the alloy microstructure. Therefore, cast corrosion resistant alloys are generally low in carbon (usually lower than 0,20% and sometimes lower than 0,03%).
All cast corrosion-resistant steels contain more than 11% chromium, and most contain from 1 to 30% nickel (a few have less than 1% Ni).
In general, the addition of nickel to iron-chromium alloys improves ductility and imparts strength. An increase in nickel content increases resistance to corrosion by neutral chloride solutions and weakly oxidizing acids.
The addition of molybdenum increases resistance to pitting attack by chloride solutions. It also extends the range of passivity in solutions of low oxidizing characteristics.
The addition of copper to duplex (ferrite in austenite) nickel-chromium alloys produces alloys that can be precipitation hardened to higher strength and hardness. The addition of copper to single-phase austenitic alloys greatly improves their resistance to corrosion by sulfuric acid. In all iron-chromium-nickel stainless alloys, resistance to corrosion by environments that cause intergranular attack can be improved by lowering the carbon content.
Compositions of Heat-Resistant Steel Castings. Castings are classified as heat resistant if they are capable of sustained operation while exposed, either continuously or intermittently, to operating temperatures that result in metal temperatures in excess of 650oC. Heat-resistant steel castings resemble high-alloy corrosion-resistant steels except for their higher carbon content, which imparts greater strength at elevated temperature.
The three principal categories of this type cast steels, based on composition are:
- Iron-chromium alloys
- Iron-chromium-nickel alloys
- Iron-nickel-chromium alloys
In the cast stainless steels structures may be austenitic, ferritic, martensitic, or ferric-austenitic (duplex). The structure of a particular grade is primarily determined by composition. Chromium, molybdenum, and silicon promote the formation of ferrite (magnetic), while carbon, nickel, nitrogen, and manganese favor the formation of austenite (non-magnetic).
Chromium (a ferrite and martensite promoter), nickel, and carbon (austenite promoters) are particularly important in determining microstructure. In general, straight chromium grades of high-alloy cast steel are either martensitic or ferritic, the chromium-nickel grades are either duplex or austenitic, and the nickel-chromium steels are fully austenitic.
Cast austenitic alloys usually have from 5 to 20% ferrite distributed in discontinuous pools throughout the matrix, the percent of ferrite depending on the nickel, chromium, and carbon contents. The presence of ferrite in austenite may be beneficial or detrimental, depending on the application.
Ferrite can be beneficial in terms of weldability because fully austenitic stainless steels are susceptible to a weldability problem known as hot cracking, or microfissuring. The intergranular cracking occurs in the weld deposit and/or in the weld heat-affected zone and can be avoided if the composition of the filler metal is controlled to produce about 4% ferrite in the austenitic weld deposit. Duplex CF grade alloy castings are immune to this problem.
The presence of ferrite in duplex CF alloys improves the resistance to stress-corrosion cracking (SCC) and generally to intergranular attack. In the case of SCC, the presence of ferrite pools in the austenite matrix is thought to block or make more difficult the propagation of cracks. In the case of intergranular corrosion, ferrite is helpful in sensitized castings because it promotes the preferential precipitation of carbides in the ferrite phase rather than at the austenite grain boundaries, where they would increase susceptibility to intergranular attack.
The presence of ferrite also places additional grain boundaries in the austenite matrix, and there is evidence that intergranular attack is arrested at austenite-ferrite boundaries. It is important to note, however, that not all studies have shown ferrite to be unconditionally beneficial to the general corrosion resistance of cast stainless steels. Some solutions attack the austenite phase in heat-treated alloys, whereas others attack the ferrite.
Ferrite can be detrimental in some applications. One concern may be the reduced toughness from ferrite, although this is not a major concern, given the extremely high toughness of the austenite matrix. A much greater concern is for applications that require exposure to elevated temperatures, usually 315oC and higher, where the metallurgical changes associated with the ferrite can be severe and detrimental. In application requiring that these steels be heated in the range from 425 to 650oC, carbide precipitation occurs at the edges of the ferrite pools in preference to the austenite grain boundaries.