Daniel A. DeAntonio
Staff Specialist, Bar Product Engineering
Peter T. Thompson
Carpenter Technology Corp.
Aneed for durable components and maintenance-free operation is pushing materials used in automobiles to their mechanical and physical limits. This is especially true for specialty alloys in today's hotter-running engines, sensors, solenoids, computers, and controls. Such applications need strong metals that are nearly impervious to heat and corrosion, with special electrical or magnetic properties. Additionally, advances such as camless valve trains, and continuously variable-transmissions will demand much of alloys. Steel makers continue to keep pace with a lot of new ideas.
Stainless steels are about 50% stronger than lowcarbon steels. Their high strength-to-weight ratio, along with corrosion resistance, gives designers more options for machined and cold-formed parts. More than 60 standard and special stainless steels can cost effectively match even the most-demanding requirements. Stainless steels are corrosion resistant surfaceto-core. They offer an alternative for coated and plated parts that eventually chip, peel, or crack to expose the vulnerable alloy underneath.
The Carpenter Selectaloy system is a simplified method that helps designers select the best stainless steel based on corrosion and strength. The Selectaloy method uses 11 basic grades that are representative of certain types of stainless steels and heat-resisting alloys. Corrosion resistance increases vertically in the Selectaloy diagram, while mechanical properties (or strength) increase from left to right.
Type-304 stainless is the most widely used stainless grade and often serves as a good basis. But Type-316 stainless is a better starting point for parts needing more corrosion resistance. Next comes 20Cb-3 stainless for even higher resistance. In contrast, Type-430 and 405 stainless serve as bases for decreasing resistance. Reading the Selectaloy diagram from left to right across (starting at the lowest level of corrosion resistance), strength rises progressively from Type-405 to Types-410, 420, and 440-C stainless steels.
Custom-450, 431, and 455 stainless reside in the middle of the diagram to show how their corrosion resistance and strength compares with that of alloys along the vertical and horizontal axes.
Thus the first step is to choose a stainless alloy based on corrosion and strength criteria. The next parameter for consideration is part fabrication, i.e., will it be machined, headed, welded, or heat treated. These processes may affect properties essential to the application and influence alloy selection.
For example, Type-304 and 410 stainless steels come in alloy modifications that improve machining or coldheading properties. The modified versions retain corrosion resistance and mechanical properties comparable to those of the basic grade. For a machined part, however, Type-304 stainless is a problem. But four other alloy variations boost machinability — Project 70+ stainless Type 304, Type-303Se stainless, Type-303 stainless, and Project 70+ stainless Type-303, in that order.
There are similar choices for applications needing Type-304 stainless with better cold headability. Type-305 stainless followed by Type-302HQ-FM stainless, and finally Carpenter No. 10 stainless provide progressively easier cold heading.
The same concept applies to Type-410 stainless. Several modifications will offer progressively better machinability (Type-416 stainless, Project 70+ stainless Type 416, and Type-5-F stainless).
Typical applications for stainless steels include engine and fuel-handling components such as fuel-injector bodies, needles, armatures, pole pieces, and inlet tubes; piston rings and separators; transmission components; and intake/exhaust valves. In exhaust systems and pollution control, they also serve in hardware for catalytic converters, exhaust hangers, manifold bolts and pins, EGR components, and oxygen-sensor parts.
Stainless alloys also commonly serve in instrumentation, controls for shafts and power-window components, and in cruise control. Stainless parts for safety applications include air-bag inflater components, antilock braking solenoids, and brake-hose couplings. A host of functional and decorative applications such as windshield-wiper arms, headlight body screws, mirror cables, antenna base and cable fittings, antenna, fasteners, speedometer meter pins, and decorative nuts also demand stainless.
SOFT MAGNETIC ALLOYS
It's no secret that electronic controls get integrated into numerous automotive functions. This trend sparked the development of soft magnetic alloys. These alloys help optimize the output and response of electromechanical components such as cores, armatures, solenoid switches, and relays. Properties that are important to these soft magnetic materials include:
High saturation induction makes possible the development of strong magnetic fields that let solenoids and fuel injectors work with a minimum of energy input.
High permeability readily induces high magnetism, a property that helps shrink component size while bolstering efficiency.
Low coercive force promotes rapid magnetization and demagnetization, both essential attributes for opening and closing valves and injectors.
High electrical resistivity boosts electromagnetic efficiency by reducing energy losses associated with eddy current formation.
Good design requires that magnetic properties be consistent both over time and over the volume of the magnet. These properties are controlled by alloy metallurgical structure developed through heat-treatment controls and by consistent alloy chemistry managed by control of residual elements.
Electromagnetic components generally get annealed after fabrication to obtain desired magnetic properties. Annealing relieves residual stresses, recrystallizes the grain structure, and removes impurities such as carbon, oxygen, nitrogen, and sulfur.
Three basic families of soft magnetic alloys offer various combinations of magnetic and mechanical properties that handle most automotive applications. Dc normal induction curves are a common way of showing the relationship between flux density (kG) and magnetic-field strength (Oe) for representative soft alloys.
Electrical irons are relatively pure, low-carbon irons for magnetic solenoids that activate electrical controls as well as for magnetic circuit cores and relays. Premium quality, vacuum-melted core-irons are stabilized with vanadium. This helps maintain magnetic properties over time and promotes uniformity.Silicon irons are made by adding silicon to low-carbon iron, which increases hardness and electrical resistivity. Silicon Core Iron A, one of several alloys in this group, has magnetic properties like those of electrical iron. However, its electrical resistivity is 25
mQcm, compared with 13
mQcm for electrical iron. A free-machining (FM) version of the same alloy, Silicon Core Iron A-FM, boosts machinability with nearly identical magnetic properties. Silicon Core Iron B, with electrical resistivity of 40
mQcmserves in applications requiring low hysteresis loss, high permeability, low residual magnetism, and freedom from magnetic aging. An FM version, Silicon Core Iron B-FM, is also available. Silicon Core Iron C has the highest electrical resistivity (58
mQcmand offers maximum initial permeability, minimum hysteresis loss, low residual magnetism, and negligible magnetic aging.
Chromium-iron magnetic stainless steels compared to other soft magnetic alloys provide good corrosion resistance for devices exposed to weather, fuel, or other corrosive environments. Although these alloys have adequate magnetic properties for core applications, they allow higher core losses and provide lower saturation and permeability than silicon irons.Type-430F Solenoid Quality stainless steel has the best magnetic properties and lowest residual magnetism of all stainless grades. This has let it serve in corrosive applications for years. A second version, Type-430FR Solenoid Quality stainless steel gives improved wear resistance, higher resistivity (76
mand increased hardness
Chrome-Core is a family of alloys that provide a combination of corrosion resistance, magnetic properties, cost economy, and fabrication ease. They withstand corrosive fuels containing ethanol and methanol, and the contaminants sometimes associated with them.
The group consists of four alloys containing different levels of chromium, with a nonfree-machining companion version of each available for applications that need no machining. The sulfur addition to improve machinability of each FM variation has minimal effect on the alloys' magnetic properties.
Chrome Core 8-FM alloy, containing 8% chromium, and Chrome Core 12-FM alloy, containing 12% chromium, don't have the substantial decline in saturation induction associated with the 18% chromiumferriticstainless steels. They are options for magnetic components that need more corrosion resistance than pure iron, low-carbon steel, and silicon-iron. Chrome Core 12-FM alloy displays corrosion resistance similar to or approaching that of Type-430F/430 FR Solenoid Quality stainless when exposed to CM 85A fuel, with and without aeration.Chrome Core 8-FM alloy has the highest saturation flux density (1.8 Tesla) of all Chrome-Core alloys. It also has the lowest (49.2
mQcmelectrical resistivity. The flux density of the Chrome-Core alloys at both 8 and 12% chromium levels approaches that of Electrical Iron and Silicon Core Iron B-FM at magnetic field strengths greater than about 800 A/m. They also have the highest maximum permeability (3,100) in the Chrome-Core alloy group.
Chrome Core 13-FM alloy, containing 13% chromium, was developed for electromechanical devices that need peak magnetic properties in a stainless alloy. This alloy's combination of magnetic properties and corrosion resistance make it a fit for numerous stringent automotive applications. Key alloy compositional changes boost electrical resistivity and lower coercivity while maintaining good corrosion resistance and stable ferrite composition.
Chrome Core 18-FM, with 18% chromium has corrosion resistance superior to that of Type-430 FR Solenoid Quality stainless steel but retains comparable magnetic properties. And in some environments, Chrome Core 18-FM alloy exhibits corrosion resistance similar to Type-316 stainless steel.
The escalating temperatures found in high-performance engines and exhaust systems are fostering a need for hightemperature alloys. Candidates include grades commonly found in aerospace applications. These alloys maintain high strength and corrosion resistance while operating above 540°C (1,000°F).
Two in this group are low-nickel alloys and serve in valve applications and system components. Designated as 21-2N and 21-4N valve steels these alloys show good strength and temperature resistance to 760°C (1,400°F).
But higher nickel alloys are typically the choice to provide increased strength or temperature resistance. For example, a 70% nickel alloy called Pyromet Alloy 80A (UNS N07080) and Pyromet Alloy 751 work well in exhaust valves and other components that operate long term in 820°C (1,500°F) conditions. Both alloys resist high temperatures, oxidation, and fatigue. Another alloy called Pyromet Alloy 31V (UNS N07032) offers comparable strength and corrosion resistance to 820°C. But it also bears up well to sulfidation from sulfur-containing fuels used in some diesel and Third World engines.
For users demanding even higher strength, Pyromet Alloy 718 (UNS N07718) and Waspaloy (UNS N07001) are candidates. These are mainstay materials for aircraft gas-turbine engine parts that must maintain high strength at elevated temperatures. Typically, Pyromet Alloy 718 withstands 700°C (1,300°F) and Waspaloy is useful to 870°C (1,600°F).
In recent years, intermediate nickel alloys have seen more use because of their ability to balance high-temperature strength with high value. For example, NCF 3015 alloy (UNS S566315) with only 30% nickel has been used in high-temperature applications that were originally reserved only for the 70% nickel grades. This alloy serves in valves and other parts that must withstand oxidation and service temperatures to 760°C (1,400°F). Also in the intermediate nickel category is Pyromet Alloy A-286 (UNS K66286). It serves in manifold bolts thanks to its high ductility in notched sections and in other parts that need high strength and corrosion resistance to 700°C.
As critical fasteners and other parts demand stronger materials, designers can consider a few specialty alloys known for their ultrahigh strength.
- AerMet 100 Alloy — has an ultimate tensile strength (UTS) of 1,965 MPa (285 kpsi) and excellent fracture toughness.
- AerMet 310 Alloy — has exceptional ductility and toughness coupled with a 2,137 MPa (310 kpsi) UTS.
- MP35N — has a UTS approaching 2,067 MPa (300 kpsi)
- NiMark Alloy 300 — can attain a yield strength of over 1,862 MPa (270 kpsi).
These alloys have been used in applications ranging from racing components to parts for the new continuous variable transmissions.