…
Crankshafts materials should be readily shaped, machined and heat-treated, and have adequate strength, toughness, hardness, and high fatigue strength. The crankshaft are manufactured from steel either by forging or casting. The main bearing and connecting rod bearing liners are made of babbitt, a tin and lead alloy. Forged crankshafts are stronger than the cast crankshafts, but are more expensive. Forged crankshafts are made from SAE 1045 or similar type steel. Forging makes a very dense, tough shaft with a grain running parallel to the principal stress direction. Crankshafts are cast in steel, modular iron or malleable iron. The major advantage of the casting process is that crankshaft material and machining costs are reduced because the crankshaft may be made close to the required shape and size including counterweights. Cast crankshafts can handle loads from all directions as the metal grain structure is uniform and random throughout. Counterweights on cast crankshafts are slightly larger than counterweights on a forged crankshafts because the cast metal is less dense and therefore somewhat lighter.
Generally automobile crankshafts were forged in past to have all the desirable properties. However, with the evolution of the nodular cast irons and improvements in foundry techniques, cast crankshafts are now preferred for moderate loads. Only for heavy duty applications forged shafts are favoured. The selection of crankshaft materials and heat treatments for various applications are as follows.
(i) Manganese-molybdenum Steel.
This is a relatively cheap forging steel and is used for moderate-duty petrol-engine crankshafts. This alloy has the composition of 0.38% carbon, 1.5% manganese, 0.3% molybdenum, and rest iron. The steel is heat-treated by quenching in oil from a temperature of 1123 K, followed by tempering at 973 K, which produces a surface hardness of about 250 Brinell number. With this surface hardness the shaft is suitable for both tin-aluminium and lead-copper plated bearings.
(ii) 1%-Chromium-molybdenum Steel.
This forging steel is used for medium-to heavy-duty petrol- and diesel-engine crankshafts. The composition of this alloy is 0.4% carbon, 1.2% chromium, 0.3% molybdenum, and rest iron. The steel is heat-treated by quenching in oil from a temperature of 1123 K and then tempering at 953 K. This produces a surface hardness of about 280 Brinell number. For the use of harder bearings, the journals can be flame or induction surface-hardened to 480 Brinell number. For very heavy duty applications, a nitriding process can produce the surface to 700 diamond pyramid number (DPN). These journal surfaces are suitable for all tin-aluminium and bronze plated bearings.
(iii) 2.5%-Nickel-chromium-molybdenum Steel.
This steel is opted for heavy-duty diesel-engine applications. The composition of this alloy is 0.31% carbon, 2.5% nickel, 0.65% chromium, 0.55% molybdenum, and rest iron. The steel is initially heat-treated by quenching in oil from a temperature of 1003 K and then tempered at a suitable temperature not exceeding 933 K. This produces a surface hardness in the region of 300 Brinell number. This steel is slightly more expensive than manganese-molybdenum and chromium-molybdenum steels, but has improved mechanical properties.
(iv) 3%-Chromium-molybdenum or 1.5%-Chromium-aluminium-modybdenum Steel.
These forged steels are used for diesel-engine crankshafts suitable for bearing of hard high fatigue-strength materials. The alloying compositions are 0.15% carbon, 3% chromium, and 0.5% molybdenum or 0.3% carbon, 1.5% chromium, 1.1% aluminium, and 0.2% molybdenum. Initial heat treatment for both steels is oil quenching and tempering at 1193 K and 883 K or 1163 K and 963 K respectively for the two steels. The shafts are case-hardened by nitriding, so that nitrogen is absorbed into their surface layers. If the nitriding is carried out well in the journal fillets, the fatigue strength of these shafts is increased by at least 30% compared to induction and flame-surface-hardened shafts. The 3%-chromium steel has a relatively tough surface and hardness of 800 to 900 DPN. On the other hand the 1.5%-chromium steel casing tends to be slightly more brittle but has an increased hardness, of the order of 1050 to 1100 DPN.
(v) Nodular Cast Irons.
These cast irons are also known as speroidal-graphite irons or ductile irons. These grey cast irons have 3 to 4% carbon and 1.8 to 2.8% silicon, and graphite nodules are dispersed in a pearlite matrix instead of the formation off fake graphite. To achieve this structure about 0.02% residual cerium or 0.05% residual niagnesium or even both is added to the melt due to which the sulphur is removed and many small spheroids in the as-cast material are formed. The surface hardness of as-cast nodular iron is greater than for steel of similar strength, their respective hardnesses being 250 to 300 and 200 to 250 Brinell number. The flame or induction hardening can produce a surface with Brinell numbers of 550 to 580, and also a form of nitriding can be applied if necessary.
Nodular cast iron has the advantageous properties of grey cast iron (that is, low melting point, good fluidity and castability, excellent machinability, and wear resistance) as well as the mechanical properties of steel (that is relatively high strength, hardness, toughness, workability, and harden ability). Now-a-days a large number of crankshafts for both petrol and diesel engines are made from nodular cast iron in preference to the more expensive forged expensive forged steel. To support the slightly inferior toughness and fatigue strength of these cast irons, larger sections and the maximum number of main journals are used.
Heat Treatment.
(a) Flame and Induction Surface-hardening.
These are the surface hardening methods for steel having 0.3 to 0.5% carbons without the use of special compounds or gases. The basic principle is to rapidly apply heat to the surface followed by only water quenching. As it is heated locally instead of heating the entire mass, the hardening is greatly reduced and distortion of the journal is avoided.
Flame hardening is carried out by oxyacetylene flame at the surface layer temperature between 993 and 1173 K. The surface temperature depends on the carbon content equivalent of the different alloying elements in the steel. The heating process is followed by a water-jet quenching operation. Since the actual period for heating and cooling is critical, it is predetermined and is mostly automatically controlled.
Induction hardening is carried out by inducting heat electrically into the surface to be hardened. This case eliminates the danger of either overheating or burning the surface of the metal as with a flame hardening. An induction coil surrounds the journal and carries a high-frequency current. This induces circulating eddy currents in the journal surface thereby raising of its temperature and heat is mostly confined to the outer surface of the journals. In this process the higher the frequency of the current, the closer the heat is to the skin. The current is automatically switched off when the required temperature is attained and the surface is simultaneously quenched by water jet, which passes through holes in the induction block.
(b) Nitriding Surface-hardening Process.
In this process the journals are heated to 773 K for a predetermined time in an ammonia gas atmosphere, so that the nitrogen in the gas is absorbed into the surface layer. The alloying elements such as chromium, aluminium, and molybdenum, present in the steel, from hard nitrides. Aluminium nitrides form an intensely hard shallow case. Chromium nitrides diffuse to a greater depth than aluminium nitrides. The molybdenum increases hardenability, gives grain refinement, and improves the toughness of the core.
This process can use directly the journals ground to their final size as there is no quenching after nitriding thereby avoiding distortion unlike other surface-hardening processes. The slow rate of penetration of the surface makes the cost of the process high for example, it takes 20 hours to produce a case depth of about 0.2 mm.
(c) Carbonitriding Surface-hardening Process.
Tufftride’ is the best-know salt-bath carbonitriding process. The crankshaft is immersed in a bath of molten salts at a temperature of about 853 Kfor a relatively short cycle time of two to three hours. In the process both carbon and nitrogen dissociate from the salts and diffuse into the surface. Since nitrogen is more soluble than carbon in iron, it diffuses further into the material. Hard iron carbides and tough iron nitrides are formed on the surface thereby resistance to wear, galling (surface peeling), seizure, and corrosion are greatly increased.
Depending on the steel used, this outer layer is 6 to 16 jam deep with hardness varying from 400 to 1200 DPN. Underneath this outer layer, the excess nitrogen goes into solid solution with the iron due to which it is strengthened. This inner diffusion zone forms a barrier which prevents spreading of cracks leading to fatigue failure.
This surface-hardening treatment, also known as soft FLYWHEEL nitriding, is becoming increasingly popular for both steels the cast irons, and is expected to replace other more expensive processes for the components using plain carbon steels requiring surface hardness and corrosion resistance. This process is much quicker and cheaper and produces similar properties to nitriding, but the depth of hardness is normally less, which can be a problem if the shaft is to be reground.
