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A minimum of about 10.5% chromium is usually required to passivate the surface and to classify a material as stainless steel. So long as this oxide layer is stable and continuous, the metal substrate is well protected from corrosion. It should be noted that nickel, while used in many grades of stainless steel, is not contained in all grades of stainless steel.
Since about the mid-1990s, plain carbon and low alloy steels have been replaced by stainless steel as the primary material for exhaust systems downstream of the exhaust manifold or turbocharger. This transition has taken place because of market demands for extended warranties, and because of demands mandated by emission standards. Technologies to meet increasingly stringent emission standards can raise exhaust temperatures which makes the task of meeting strength and durability requirements especially challenging. Emission standards also require that exhaust systems are designed in a manner that facilitates leak-free assembly, installation and operation for the full useful life of the vehicle.
From the early part of the 21st century, commodities including many of the alloying elements used in stainless steel, have experienced wide and rapid price fluctuations (see Figure 4 for example). In some cases, costs for these materials have increased by up to ~1000%. In response, stainless steel producers have added an alloy surcharge that can be adjusted as needed to account for these price variations. Many producers have focused considerable research efforts on reducing the sensitivity of the finished product’s price to these alloy surcharges.
Emission control systems such as actively regenerated diesel particulate filters (DPFs) and urea selective catalytic reduction (SCR) have also created new demands on material properties. Active DPF regeneration can produce exhaust temperatures as high as 800°C in parts of the exhaust system that would otherwise operate at much lower temperatures. Also, some commonly used stainless steels, such as type 304, have been found to corrode after exposure to urea decomposition products in high temperature environments.
Common alloying elements found in stainless steel and their effects are summarized in Table 1 [ASM 2001][Arlt 2007].
Table 1
Common alloying elements found in stainless steelElementEffectChromium
Essential in forming the passive film. Oxidation resistance increases at Cr levels above 10.5%
High Cr content can adversely affect mechanical properties, fabricability and weldability
Nickel
Stabilize the austenitic structure to enhance mechanical properties and fabrication characteristics
Resistance to stress-corrosion cracking is poorest at approximately 8 to 10% Ni but is restored at levels of about 30% Ni
Can minimize spalling of surface oxide during temperature cycling
Manganese
In moderate quantities and when nickel is present, performs many of the functions attributed to nickel
Interacts with sulfur to form manganese sulfides which can degrade corrosion resistance
Can improve the adhesion of oxide film and improve oxidation resistance
Molybdenum
In combination with chromium is very effective for passive film stabilization in the presence of chlorides
Especially effective in increasing resistance to the initiation of pitting and crevice corrosion
Carbon
Provides strength in the high-temperature applications
Detrimental to corrosion resistance through reaction with chromium to form chromium carbides
Detrimental to toughness in ferritic grades
Nitrogen
Enhances pitting resistance in austenitic grades
Detrimental to the mechanical properties of the ferritic grades
Aluminum
Enhance high-temperature oxidation resistance
Niobium and Titanium
Stabilizers used to preferentially combine with carbon and nitrogen to reduce the formation of chromium carbides and nitrides. This reduces the possibility of intergranular corrosion.
Titanium oxide may adversely affect brazability
Copper
Provide corrosion resistance to sulfuric acid
Improve formability
Silicon
Provides high-temperature oxidation resistance
Provides resistance to stress corrosion cracking and to corrosion by oxidizing acids
Of the varieties of stainless steel available, two that are important exhaust system materials are:
Ferritic stainless steel, and
Austenitic stainless steel.
Ferrous alloys can contain three important grain structures: ferrite, austenite and martensite. In ferrite, the iron atoms form a body centered cubic (BCC or α-iron) structure with iron atoms at each corner of a cube and one in the center of the cube, Figure 1. Austenite is a face centered cubic (FCC or γ-iron) structure with iron atoms at each corner and on the center of each face of a cube. Interstitial holes in the FCC structure allow austenite to accommodate a greater number of carbon atoms, up to 2.11% by weight, than the BCC structure of ferrite which can accommodate up to 0.0218% carbon. Austenite can be transformed into martensite, a very hard and brittle grain structure that is not normally used in exhaust piping systems.
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