One of the features that characterize stainless steels is a minimum 10.5% chromium content as the principal alloying element. Four major categories of wrought stainless steel, based on metallurgical structure, are austenitic, ferritic, martensitic, and precipitation hardening. Cast stainless-steel grades are generally designated as either heat resistant or corrosion resistant.
Austenitic wrought stainless steel are classified in three groups:
- The AISI 200 series (alloys of iron-chromium-nickel-manganese).
- The AISI 300 series (alloys of iron-chromium-nickel).
- Nitrogen-strengthened alloys.
Carbon content is usually low (0.15% or less), and the alloys contain a minimum of 16% chromium with sufficient nickel and manganese to provide an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy.
Nitrogen-strengthened austenitic stainless steels are alloys of chromium-manganese-nitrogen; some grades also contain nickel. Yield strengths of these alloys (annealed) are typically 50% higher than those of the nonnitrogen-bearing grades. They are nonmagnetic and most remain so, even after severe cold working.
Like carbon, nitrogen increases the strength of a steel. But unlike carbon, nitrogen does not combine significantly with chromium in a stainless steel. This combination, which forms chromium carbide, reduces the strength and corrosion resistance of an alloy.
Until recently, metallurgists had difficulty adding controlled amounts of nitrogen to an alloy. The development of the argon-oxygen decarburization (AOD) method has made possible strength levels formerly unattainable in conventional annealed stainless alloys.
Austenitic stainless steels are generally used where corrosion resistance and toughness are primary requirements. Typical applications include shafts, pumps, fasteners, and piping in seawater and equipment for processing chemicals, food, and dairy products.
Ferritic wrought alloys (the AISI 400 series) contain from 10.5 to 27% chromium. In addition, the use of argon-oxygen decarburization and vacuum-induction melting has produced several new ferritic grades including 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni. Low in carbon content, but generally higher in chromium than the martensitic grades, these steels cannot be hardened by heat treating and are only moderately hardened by cold working. Ferritic stainless steels are magnetic and retain their basic microstructure up to the melting point if sufficient Cr and Mo are present. In the annealed condition, strength of these grades is approximately 50% higher than that of carbon steels.
Ferritic stainless steels are typically used where moderate corrosion resistance is required and where toughness is not a major need. They are also used where chloride stress-corrosion cracking may be a problem because they have high resistance to this type of corrosion failure. In heavy sections, achieving sufficient toughness is difficult with the higher-alloyed ferritic grades. Typical applications include automotive trim and exhaust systems and heat-transfer equipment for the chemical and petrochemical industries.
Martensitic steels are also in the AISI 400 series. These wrought, higher-carbon steels contain from 11.5 to 18% chromium and may have small quantities of additional alloying elements. They are magnetic, can be hardened by heat treatment, and have high strength and moderate toughness in the hardened-and-tempered condition. Forming should be done in the annealed condition. Martensitic stainless steels are less resistant to corrosion than the austenitic or ferritic grades. Two types of martensitic steels -- 416 and 420F -- have been developed specifically for good machinability.
Martensitic stainless steels are used where strength and/or hardness are of primary concern and where the environment is relatively mild from a corrosive standpoint. These alloys are typically used for bearings, molds, cutlery, medical instruments, aircraft structural parts, and turbine components. Type 420 is used increasingly for molds for plastics and for industrial components requiring hardness and corrosion resistance.
Precipitation-hardening stainless steels develop very high strength through a low-temperature heat treatment that does not significantly distort precision parts. Compositions of most precipitation-hardening stainless steels are balanced to produce hardening by an aging treatment that precipitates hard, intermetallic compounds and simultaneously tempers the martensite. The beginning microstructure of PH alloys is austenite or martensite. The austenitic alloys must be thermally treated to transform austenite to martensite before precipitation hardening can be accomplished.
These alloys are used where high strength, moderate corrosion resistance, and good fabricability are required. Typical applications include shafting, high-pressure pumps, aircraft components, high-temper springs, and fasteners.
Cast stainless steels usually have corresponding wrought grades that have similar compositions and properties. However, there are small but important differences in composition between cast and wrought grades. Stainless-steel castings should be specified by the designations established by the ACI (Alloy Casting Institute), and not by the designation of similar wrought alloys.
Service temperature provides the basis for a distinction between heat-resistant and corrosion-resistant cast grades. The C series of ACI grades designates the corrosion-resistant steels; the H series designates the heat-resistant steels, which can be used for structural applications at service temperatures between 1,200 and 2,200°F. Carbon and nickel contents of the H-series alloys are considerably higher than those of the C series. H-series steels are not immune to corrosion, but they corrode slowly -- even when exposed to fuel-combustion products or atmospheres prepared for carburizing and nitriding. C-series grades are used in valve, pumps, and fittings. H-series grades are used for furnace parts and turbine components.
Galling and wear are failure modes that require special attention with stainless steels because these materials serve in many harsh environments. They often operate, for example, at high temperatures, in food-contact applications, and where access is limited. Such restrictions prevent the use of lubricants, leading to metal-to-metal contact -- a condition that promotes galling and accelerated wear.
In a sliding-wear situation, a galling failure mode occurs first, followed by dimensional loss due to wear, which is, in turn, usually followed by corrosion. Galling is a severe form of adhesive wear that shows up as torn areas of the metal surface. Galling can be minimized by decreasing contact stresses or by the use of protective surface layers such as lubricants (where acceptable), weld overlays, platings, and nitrided or carburized surface treatments.
Test results from stainless-steel couples (table) indicate the relatively poor galling resistance of austenitic grades and even alloy 17-4 PH, despite its high hardness. Among the standard grades, only AISI 416 and 440C performed well. Good to excellent galling resistance was demonstrated by Armco's Nitronic 32 and 60 alloys (the latter were developed specifically for antigalling service).
Recent research findings prove that adding silicon to a high-manganese, nitrogen-strengthened austenitic stainless alloy produces a wear-resistant stainless steel. Wear and corrosion resistance are still considered unavoidable trade-offs in stainless, but the new formula promises to resist both conditions.
Beating corrosion is the number one reason for choosing stainless. But in cases where parts are difficult to lubricate, most stainless steels cannot resist wear. Under high loads and insufficient lubrication, stainless often sports a type of surface damage known as galling. In critical parts, galling can lead to seizure or freezing, which can shut down machinery.
Designers typically get around galling by using cast alloys or by applying a cobalt facing to stainless parts. Either way, the fixes can be expensive and may pose new problems that accompany the hard-facing process. These include maintaining uniform facing thickness and ensuring proper adhesion between facing and substrate. A new stainless formula aims to sidestep these difficulties by offering an alternative to expensive wear-resistant materials.
In search of a cost-effective alternative, researchers at Carpenter Technology, Reading, Pa., looked at elemental effects of silicon, manganese, and nickel on galling resistance of nitrogen-strengthened, austenitic stainless steels. Results of an initial test program determined that silicon was a catalyst for galling resistance, while nickel and manganese were not.
The silicon levels in a recently developed gall-resistant stainless alloy are between 3 and 4%. Silicon levels must remain lower than 5% to maintain the proper metallurgical structure. In addition, too much silicon decreases nitrogen solubility. To maintain strength, higher amounts of costly nickel would need to be added.
Researchers can now define optimum composition limits for a gall-resistant stainless steel. To prove the new steel's validity, properties such as galling, wear, and corrosion are evaluated and compared with commercially available stainless steels. Four alloys, a gall-resistant austenitic alloy called Gall-Tough, another austenitic alloys with higher nickel and manganese content (16Cr-8Ni-4Si-8Mn), and Types 304 and 430 stainless steels are included in the comparison.
Results show the galling threshold for gall-resistant stainless is over 15 times higher than that of conventional stainless steels. In addition, gall-resistant stainless withstands more than twice the stress without galling compared to the 16Cr-8Ni-4Si-8Mn alloy. Yet, the new formula sacrifices only a slight amount of corrosion resistance.
For strength and hardness, both gall-resistant stainless and the 16Cr-8Ni-4Si-8Mn alloy beat Types 304 and 430 alloys. The new alloy also shows a uniquely high ultimate tensile strength, possibly due to martensite formation during tensile testing. Ductility for all four alloys is excellent. These findings indicate that gall-resistant alloys can economically bridge the gap between corrosion, galling, and metal-to-metal wear resistance.