Cracked Casting Lecture 8

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Bangladesh University of Engineering and Technologyg y g g gy

MME6203AdvancedTopicsinFoundryEngineering

Lecture 8Lecture8

Casting Defects5. Linear Contraction in Casting 3

A.K.M.B.RashidProfessor,DepartmentofMMEBUET,Dhaka

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Todays Topics

z Cold cracking Crack initiation Crack initiation Crack growth

z Residual Stress Casting stress Quenching stress Stress relief

B. Rashid, DMME, BUET Lec #8: Casting defects cold crack & residual stress Page 03 of 20

Cold Cracking Cold cracking is a general term used to emphasize the

different nature of the failure from that of hot tearing.

Hot tearing implies a failure occurring at temperatures above the solidus, while cold cracking occurs at temperatures below solidus (thus, it can be rather worm).

While tear is a ragged failure in a weak material, a crack is more straight and smooth, and occurs in strong materials.z Stress required to nucleate and propagate crack is more significantz Stress required to nucleate and propagate crack is more significantz Stress was less significant in hot tearing; strain was more important.

Occasionally a failure appears to fall somewhere between the tear and crack.


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z The driving force for nucleation and growth of a cold crack is stress.

z Depending on the temperature at which the crack is formed, it may or may not be oxidized.

z Crack may either be transgrannular or intergrannulardepending on the relative strength of grain and grain boundaries.

The colour of the crack is a useful guide to when it is formed; an uncolouredmetallic surface will indicate that the crack occurred at a temperature near to room temperature; the normal temper colours ranging from light straw to yellow to blue to brown indicate greater expousure to time at temperature.


Crack Initiation

z Cracks start from stress raisers.z A stress raiser can be an abrupt change of section of the z A stress raiser can be an abrupt change of section of the

casting (this is well-known to design engineers); but in any case these do not likely to cause an increase in stress by much more than a factor of 2.

z More severe stresses are raised by sharper features, such as oxide skins and folds. This are already constitute a as oxide skins and folds. This are already constitute a kind of crack and are cast into place at the time of filling of the mould.

More dangerous because they occupy a large portion of the section of casting, and are undetectable.


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z Oxide films are probably the most important initiators of cracks in light alloys

Castings made using good running systems are usually not sensitive to problems of cracking.

z During welding, stresses in the weld are extremely high due to constraints of the surrounding solid metal (which is at room temperature, and very strong)

Cracks may start even from a micropore (which is spherical !) due to very hi h d i i fhigh driving force.


Crack Growth

z As casting cools, stress-relaxation processes become As casting cools, stress relaxation processes become progressively slower, and eventually stop altogether.

More elastic stress can be built up, which can speed up the process of crack growth Segregation of impurities to grain boundaries (for example, hydrogen, sulphur and phosphorous in steel) and formation of low-melting compounds (such as FeS, MnS, different carbides, AlN) help transgranular growth of crackcrack.


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Residual Stressz Stresses accumulated inside the casting during contraction

to room temperature may have been released by the casting failing by slow tearing or sudden cracking.g g y g g

z If the casting survived the catastrophic failure modes, these stresses are retained inside the casting. Is there any wrong about that ?

z There are reports that casting flew into pieces with a bang when being machined or even when simply standing on when being machined, or even when simply standing on the floor.

We are unaware that the casting may be on the brink of catastrophic failure, because, of course, the problem is invisible; the casting looks perfect. It is the last thermal treatment, and the rate of cooling from the final temperature, which is important so far as residual stresses are concerned.

z Overall casting stresses can be calculated as:

Casting Stress

= E = ET


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z Degree of ramming (variable mould hardness) or different levels of moisture content (variable mould strength) have no impact on the residual stresses of casting.

z Residual stresses are also independent of casting length (L).

z Casting temperature increases residual stresses.

z Reducing the time of casting in mould (stripping casting in mould (stripping time) reduces residual stresses in casting.

Figure 6.22 Residual stress in aluminium alloy and grey iron castings as a function of stripping time. Data from Dodd (1950) and IBF Technical Subcommittee (1949).

z Relative variation in cross section or rigidity in different members of casting can increase the residual stresses in casting

Increasing the dimension of centre bar twice the dimension of outer bars generates a stress over 200 MPa in the centre bar, which is enough to fracture the bar during cooling.

z In steel castings, larger sections may be forced cooled or even chilled to equalize their cooling rates with those of thinner members up to the eutectoid temperature; below thinner members up to the eutectoid temperature; below this temperature cooling should be slow, so that plastic relaxation through creep can occur.


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z Residual stresses are likely to be higher if the casting is quenched by water

Quenching Stress

z Heat flows in time t in a material having thermal diffusivity D:

X = (Dt)1/2

D = K / CP PK = thermal conductivity (for aluminium, 200 W/m/K)P = density (for aluminium, 2700 kg/m3)CP = specific heat capacity at constant pressure

(for alumiunium, 1000 J/kg/K approx)D = thermal diffusivity (for aluminium, 10-4 m2/s)


z Water quenching cools the bar form 500 C to 250 C in about 5 s.

z Below this temperature, stress will start to accumulate because of slow relaxation process

z In this 5 s, heat will travel about 20 mm (from equation) i.e., throughout the cross-section.

The casting will not accumulate any residual stress.


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z For a larger casting (e.g., cylinder block) distance to travel heat is greater (of the order of 100 mm).

Thus the time available is still only 5 s, so heat will travel only 20 mm. The 100 mm cylinder block will suffer extreme non-uniformity, and consequently generate high quenching stress.

z To avoid high quenching stress in cylinder block The casting needs to cool at rate at which it can equalize its temperature within tolerable limits.within tolerable limits. By using forced air cooling, approximately 100 s is now available, sufficient for the heat to diffuse the 100-mm distance (as the above equation will confirm).


z For steel castings, thermal diffusivity D is close to 10-5 m2/s. The average diffusion distance of heat is only 7 mm in 5 seconds, and 30 mm in 100 seconds. This increases the problems of casting thick steel samples without generating much quenching stresses. That is why a long annealing cycle is required for relieving internal stresses in steel.

z There are other intermediate cooling rate options are available. Cooling in water also generate vapour blankets during emersion of hot casting, which generates complex stress pattern in casting, and differ from casting to casting.

z Uses of high-boiling-point liquids (such as oil) are unacceptable due to their flammability hazard and smoke generation problems.


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z Uses of liquid polymer are nowadays become more popular because of their high viscosity, which resists boiling and provides an even cooling.

z Some polymers (e.g., polyalkylene glycol) have reverse temperature coefficient solubility, due to which polymers become less soluble at high temperature, become sticky like solid grease. Due to this, boiling is inhibited and uniform cooling is attained. At lower temperatures, the polymers become soluble again, and can be taken back into solutioncan be taken back into solution.

These polymers are highly efficient in reducing quenching stresses in those castings which are required to be quenched as a part of their heat treatment. Properties developed are also more reproducible.


z Natural ageing is the most common way of relieving internal stresses in grey iron casting.

Stress Relief

z Artificial heat treatment is todays the most reliable and efficient (although somewhat more energy intensive !) method reducing internal stresses.

The casting is heated to a temperature at which sufficient plastic flow can occur by creep to reduce the strain, and hence reduce the stress) Thi i d i d t t k l ithi bl ti f th d f This is designed to take place within a reasonable time, of the order of an hour or so It is important that stress is not re-introduced by cooling from the stress-relieving treatment.


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z The casting is often found to initiate crack during re-heating.

The casting is already has high internal stress. Inside the furnace, the surface is heated first and expands, before the centre becomes warm. Thus the centre will be placed under an additional tensile stress, the total being perhaps sufficient to exceed its tensile strength The problem is avoided by re-heating sufficiently slowly that the temperature in the centre is able to keep pace (within tolerable limits) with that at the outside.

z S h h f li fz Some other approaches of stress relief: Application of vibration Application of sub-resonant treatment


Cracked Casting Lecture 8

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