The Science and Technology of Materials in Automotive Engines Part 8 pptx

The monolithic crankshaft uses the assembled connecting rod, while the assembled crankshaft uses the monolithic connecting rod Crankshaft type Con-rod type Engine Monolithic Assembly Mul

to 8.0% and an age-hardening effect is given by adding 3% Cu The alloy hasgood castability as well as high strength at intermediate temperature range.The tensile strength is 111–176 MPa in the as-cast state and 218–299 MPaafter T6 heat treatment.

The cylinder head receives a great amount of heat from the cylinderblock, so dimensional stability is required over a long period of time Thermalgrowth can result in microstructural change, which decreases long-termdimensional stability It occurs particularly in certain aluminum alloys atelevated temperatures T7 heat treatment is generally carried out to restrictthermal distortion (growth) of the alloy during operation T7 heat treatment(overaged) provides a more dimensionally stable microstructure than T4(naturally aged) or T6 (peak aged), and can reduce microstructural changes.11The strength changes with the grain size of castings, and generally, thethinner the casting, the higher the strength The intermediate temperaturestrength of AC4B is sufficient, while the corrosion resistance is a little lowdue to the included Cu

by turning an engine with an electric motor through the drive shaft.Measurements are used to optimize design Comprehensive quality control

is very important for all aspects of valve spring manufacture

1 Spring steels occur in two types (i) The spring property results from heat treatment after shaping (ii) The spring is shaped from pre-heat-treated steel The latter occurs

as piano wires, for which cold working gives the spring property, and oil-tempered wires, for which the spring property results from quenching and tempering Piano wire has a microstructure of strained pearlite, while the oil-tempered wire has one of tempered martensite Piano wire is likely to remain difficult to curl and hard to produce with a sufficiently thick diameter.

Table 7.2 Chemical composition of cylinder head material (%)

JIS Si Fe Cu Mn Mg Zn Ni Ti Pb Sn Cr Al Hard- Heat

ness treatment

AC4B 8.0 1.0 3.0 0.5 0.5 1.0 0.4 0.2 0.2 0.1 0.2 Balance 75 HB T6

2 Chuo Spring Co., Ltd., Corporate Catalogue, (2003) (in Japanese).

3 Ibaraki N R&D Kobe steel engineering report, 50(2000)27 (in Japanese).

4 Chuo Spring Co., Ltd., Corporate Catalogue, (1997) (in Japanese).

5 Takamura N., in Japan Society for Spring Research homepage, http://wwwsoc.nii.ac.jp, (2003) (in Japanese).

6 The spring limit value is defined as a limit stress after repeated deflection Springs

do not experience plastic deformation if used within the prescribed value alloy springs use low-temperature annealing to raise the spring limit value H Yamagata

Copper-and O Izumi: Nippon Kinzoku Gakkaishi, 44 (1990) 982 (in Japanese) Cold working

(including secondary working such as drawing or bending) is likely to cause stress corrosion cracking due to high residual stress The season cracking in brass is well known Low-temperature annealing is effective as a countermeasure.

7 Shot peening can introduce higher residual stress, as the original hardness of the worked piece is higher It also prevents heat checking of casting molds (hot die steel) Shot peening technology resulted from research by GM.

8 Suto H., Zanryuouryokuto Yugami, Tokyo, Uchida Roukakuho Publishing, (1988),

98 (in Japanese).

9 Metals Handbook 8th ed, vol 1, Ohio, ASM, (1961) 163.

10 The applied stress changes the spacing of crystal lattice planes X-ray diffraction techniques can count the direction and quantity of the principal stress through measuring changes in spacing.

11 Boileau J.M., et al., SAE Paper 2003-01-0822.

The crankshaft converts reciprocative motion to rotational motion It containscounter weights to smoothen the engine revolutions There are two types ofcrankshaft, the monolithic type (Fig 8.1), used for multi-cylinder engines,and the assembled type (Fig 8.2) fabricated from separate elements, which

is mainly used for motorcycles The type of crankshaft determines what kind

of connecting rods are used, and the possible combinations of crankshaftsand connecting rods and their applications are listed in Table 8.1

Crankpin Oil hole

8.1 Monolithic crankshaft for a four–stroke engine The fueling holes are for lubrication.

cast iron, which has high strength (see Appendix D) Fuel-efficient enginesrequire a high power-to-displacement ratio, which has increased the use offorged crankshafts The proportions of the materials used for crankshafts incar engines in 2003 were estimated to be, cast iron 25%, toughened (quenchedand high-temperature tempered) or normalized steel 20%, and micro-alloyedsteel 55% Table 8.2 shows the chemical compositions of steel crankshafts.

8.2.1 The monolithic crankshaft

Figure 8.1 shows a forged crankshaft for a four-stroke engine Thecounterweight attached to the shaft balances the weight of the connecting

8.2 An assembly type crankshaft for a single-cylinder motorcycle A connecting rod, a needle bearing and crankshaft bearings are already assembled.

Table 8.1 Combination of crankshafts with connecting rods The

monolithic crankshaft uses the assembled connecting rod, while the assembled crankshaft uses the monolithic connecting rod

Crankshaft type Con-rod type Engine

Monolithic Assembly Multi-cylinder four-stroke car

engine, outboard marine engines

Assembly Monolithic Single- or twin-cylinder

four-stroke engine, two-four-stroke engine

rod (con-rod) and piston, to smooth revolutions The con-rod rides on thecrankpin via a plain bearing The main bearing of the crankcase supports themain journal of the crankshaft.

The deep grooves in monolithic crankshafts are obtained by hot forging(Table 8.1) Carbon steels such as JIS-S45C, S50C or S55C with normalizing

or toughening are used Cr-Mo steel (typically, JIS-SCM435) and Mn steelare used to increase the strength An alternative method using micro-alloyedsteel containing V is becoming more common, as it is cheaper and does notrequire additional quench-hardening

The intricate shape of the crankshaft requires a great deal of machining

It is common for about 0.1% lead or sulfur to be added to the base steel toimprove machinability,1 to make what is known as free-cutting steel Figure8.3 shows the microstructure of S50C-based leaded free-cutting steel afternormalized heat treatment Figure 8.4 is a sulfured steel with annealing.Included lead or MnS particles significantly function as a chip breaker and

a solid lubricant and increase machinability

Mass-produced sulfured steel is the oldest free-cutting steel The sulfur isdistributed homogeneously in the steel as MnS inclusions, which elongateaccording to the direction of rolling As a consequence, elongation and impactstrength in the direction transverse to rolling are weak The machinability ofthis steel is proportional to the amount of sulfur it contains Steel for highstrength applications needs to contain less than 0.12% sulfur Leaded free-cutting steel has isotropic properties in comparison with sulfured steel and isused for parts requiring high strength The disadvantage of this steel is lowfatigue life under rolling contact conditions Crankshafts are normalized orquench-tempered after machining To increase fatigue strength, inductionhardening, nitrocarburizing and deep rolling are frequently employed

Table 8.2 Chemical compositions of crankshaft materials(%) JIS-S45C, S50C and S55C are plain carbon steel In general, these are used in normalized state JIS- SCM415, 420 and 435 are Cr-Mo steel, which are usually used in a quench- hardened state The inside portion of a thick rod is unlikely to harden with

quenching because of the slow cooling rate Steels containing increased Cr and

Mo can harden the deep inside portion of a thick rod

40 µ m

8.4 Normalized microstructure of S50C sulfured free-cutting steel containing 0.06% sulfur The MnS is elongated like thin sheets in the pearlite matrix Chips break at the position of MnS or lead during machining, so that the chip does not tangle around the cutting tool.

8.3 Normalized microstructure of S50C leaded free-cutting steel containing 0.2% lead Globular lead particles of a few µ m disperse, while the matrix microstructure is not so clear due to weak etching.

100 µ m

8.2.2 The assembled crankshaft

Figure 8.2 shows an assembled crankshaft from a motorcycle, including theconnecting rod and crankpin The crankpin is precisely ground and force-fitted into the crankshaft body The disassembled state is shown in Fig 8.5.The appropriate fitting allowance and surface roughness give sufficient torque

To raise the torque, knurling, induction hardening or carburizing is oftencarried out around the hole

8.5 Disassembled crankshaft with the other web removed to show the big end.

Counterweight Connecting rod

Needle roller bearings (see Chapter 9) run on the surface of the crankpin.The high Hertzian stress caused by the rolling contact leads to fatigue failure

at the pin surface Therefore, a carburized Cr-Mo steel JIS-SCM415 or SCM420(described below) is used A bearing steel with a higher carbon content mayalso be used (SUJ2; see Chapter 9)

8.3 Rigidity

Monolithic crankshafts appear to have a high rigidity However, the crankshaft

is simultaneously subjected to bending and torsion when revolving Underthese conditions, it tends to wriggle like an eel,3 and failure can occur as aresult of fatigue The main bearing clearance can be as small as 70 µm, butunder these circumstances, the crankshaft deflects fully within the clearancewhile revolving The trend towards reducing crankshaft weight means thatthe main bearing portion supporting the crankshaft is less rigid This weakenedmain bearing cannot support the crankshaft sufficiently, which creates asevere fatigue situation

The crankshaft is subjected to two types of stress, static and dynamic.Combustion pressure, inertial forces of the piston and con-rod, bearing loadand drive torque all cause static stress The vibration causes dynamic stress

If it occurs at the resonating frequency, the deformation will be very highand will instantly rupture the crankshaft In order to achieve good acceleration,the crankshaft must have high static and high dynamic rigidity as well as lowweight

Modern engines are designed with size and weight reduction in mind Ashort and small crankshaft makes the engine compact and then allows othercomponents such as bearings and pins to be designed and built smaller,providing an overall reduction in system weight and associated cost savings.While a cast iron crankshaft is less expensive, the lower rigidity of castiron may allow abnormal vibrations to occur, in particular resonance, which

is likely to appear at lower rotational velocities when the rigidity of thecrankshaft is low At the design stage, this can be avoided by increasing thecrankpin diameter However, raising rigidity in this way increases weight.Alternatively, an increase in rigidity of more than 10% can normally begained by using steel instead of cast iron Steel crankshafts have betterpotential to reduce noise levels and harshness over the entire engine revolutionrange, and careful design can make their use possible

8.4.1 Deformation stress

The intricate shape of the crankshaft can be formed through hot forgingusing steel dies In a red-heat state, steel behaves like a starch syrup and isextremely soft, so it molds easily to the shape of the forging die Figure 8.64compares the deformation stress of a steel at two strain rates The stressrequired for deformation is low at high temperatures and hot forging takesadvantage of this soft state By contrast, deformation at low temperaturesrequires high stress, and the applied strain makes steel hard (known as workhardening, see Fig 8.7) Deformation increases the dislocation density in the

steel (see Appendix G), which causes hardening The crystal grains of steelhave equiaxed shapes after the annealing and prior to deformation, but theystretch heavily after deformation (Fig 8.7) Forging at low temperature (coldforging) cannot shape the deep grooves necessary for crankshafts and the diecannot withstand the load because work hardening dramatically increasesthe required load.

8.4.2 Recrystallization and recovery

Metals strained at low temperature undergo changes when heated Figure 8.8illustrates the hardness changes caused by heating Hardness does not changewhen the temperature is low, but rapidly decreases above temperature T1.Changes in hardness are accompanied by microstructural change caused byrecrystallization

Heavy deformation at low temperature leaves the metal hardened and themicrostructure changed, as shown in Fig 8.7 Recrystallization creates newcrystal grains in the strained matrix, which eliminates strain in themicrostructures and causes softening (Fig 8.8) The hexagonal pattern (grainboundary) indicates that the metal has recrystallized and that new crystalshave been generated Recrystallization substantially decreases dislocation

8.6 Influence of temperature and strain rate on the strength of carbon steel S35C Dynamic strain ageing causes the peak around the intermediate temperatures from 400 to 700 ° C The characteristic temperature range used for each forging process (cold, semi-hot or hot) is indicated Steels recrystallize above 700 ° C The forging above this temperature is referred to as hot forging At elevated

temperatures, the deformation speed significantly influences the deformation stress In general, the higher the speed (strain rate), the more the curve shape shifts to the higher temperature range The normal forging machine gives stroke speeds of 0.1 to 1/s by the strain rate value.

density Each metal has a specific minimum temperature (T1) at whichrecrystallization takes place.

When recrystallized metal is annealed further at a higher temperatureabove T2 (Fig 8.8), the recrystallized grains grow Below T1, recrystallizationdoes not take place, and a rearrangement of dislocations along with a decrease

in density occurs, resulting in slight softening This is referred to as recovery.Plastic working carried out above the recrystallization temperature T1 isgenerally called hot working The temperature at which recrystallizationoccurs is different for each metal, the recrystallization temperature of steel isaround 700 °C

Hot forging of steel is carried out at the red heat state, above 700 °C (Fig.8.6) During hot forging, steel goes through recrystallization and recovery aswell as strains These softening processes remove the accumulated strain andthus the steel does not harden (Fig 8.7), making shaping easy Therecrystallization and recovery that take place during hot working are referred

to as dynamic recrystallization5 and dynamic recovery, respectively Theseprocesses eliminate work hardening despite the heavy deformation produced

Work hardening

Work softening

Strain Initial state

8.7 Temperature dependence of the stress-strain curve The three curves correspond to the high, intermediate and low deformation temperature from the bottom The illustrations indicate crystal grain shapes An annealed microstructure containing equiaxed grains is on the left circle Deformation changes it to the grain shapes shown in the right circles At high temperature, dynamic recovery and dynamic recrystallization take place, which soften steel The microstructure after deformation shows equiaxed grains when recrystallization takes place By contrast, the large deformation at low temperature makes grains elongated shapes Metal hardens with increasing strain and softening does not take place The hardening is called work

hardening or strain hardening.

by forging, giving malleability Recovery and recrystallization that take place

in cold-worked metal during annealing are different, and are called staticrecovery and static recrystallization, respectively

8.4.3 Hot forging

Figure 8.9 illustrates the die forging process for a monolithic crankshaft.6The steel bar is first sheared into a billet to adjust the weight Induction orgas heating heats the billet to around 1,000 °C, using rapid induction heating,which causes less decarburization or oxidation

Rough forging distributes the material thickness along the axis Shaping

by a forging roll and bending are then carried out simultaneously Die forgingthen forms the intricate shape, and finish forging adjusts dimensional accuracy.Burr shearing removes the flash from the shaped material, and the shapedmaterial is then straightened to remove the bend These processes are carriedout at redheat and the shaped material is machined after cooling

deformation (at the top left) are fully extended The heating of the deformed metal above the recrystallization temperature (T1) causes recrystallization, which removes the deformed microstructure and generates recrystallized grains (at the center) Below T1,

microstructure does not change apparently, but dislocations in the metal rearrange The heating (annealing) at higher temperatures (above T2) grows the recrystallized grains (on the right) The grain growth (coarsening) decreases hardness The smaller the resultant grain size, the higher the hardness (strength) Hence, overheating during annealing should be avoided.

A water-soluble lubricant is splashed on the die surface to cool it Therapid heating and cooling of the die shorten its life, to around 10,000 shots

or less Hot forging can shape intricate forms, but cannot give high dimensionalaccuracy because oxide scale accumulates on the surface, so the shapedmaterial must be milled to give the required shape

Crankshafts need high-strength materials, but these generally have lowforgeability or machinability, and as a result are costly to use Cr-Mo steel

Distributing volume along width direction

Cross-section shaping

Cross-section finishing

Trimming flash

8.9 Hot forging process for a four-stroke crankshaft The sheared billet experiences six processes from initial rough forging to final straightening In the rough shaping stage, the forging roll shapes the billet into the stepped shaft The fiber flow (described later) is schematically illustrated in the figure of bending process The shaped material is finally straightened if it is distorted.