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Thermal Stresses in Castings
 
Thermal Stresses in Castings



Are you Cryo’ing for more power?!

In the present age of production manufacturing, castings used to manufacture high performance automotive and power train parts exhibit a lot of thermal stress related cracking and breaking. Through the study of cryogenics research and with the integration of a cryogenics process into their manufacturing process, the thermal stresses placed on these high performance and high horsepower output castings can all be avoided by displacement of the stresses throughout the casting, instead of having it centralized in any particular area. “Cryogenic tempering” is not a surface treatment, but will provide a complete and thorough stress relief process. Through a cryo process, high performance engines and high performance parts, will exhibit increased tensile strength, increased engine horsepower and torque, and decreased downtime and breakage. Please read on about the extreme stress that is induced throughout the manufacturing, production and machining phases, and the thermal stresses that are introduced into the castings parts and how to get rid of this stress through a cryogenics processing.

Residual Stresses formed during castings

Foundries and production manufacturing facilities throughout the world have stories of castings that have splintered into pieces with a bang for no apparent reasons when being machined or even just sitting in wait on the floor. Most stories are dismissed as the part being “a lemon” and no second thought is put into them. As a matter of fact more than $50 million is attributed to thermal stress-generated casting defects a year (ASTM Handbook of Residual Stress and Deformation of Steel). As shown in many cryogenics researches, these defects can be essentially eliminated through a cryogenics process.

In a perfect world when a completely round symmetrical casting, with only one size cross section throughout the entire piece, is cooled at a uniform rate, there would be no thermal stresses placed on the internal matrix of the part. But unfortunately, large metal balls are not good for everything, including such things as high performance engine blocks. Add to the fact that production manufacturing is always looking for ways of speeding processes up, not all casting principles are followed correctly. Of course most parts are not directly ready to throw into a high performance engine just yet, so there is also the stress and tensions put onto the parts by machining as well.

Because of the fact that most – actually all casting parts save for spherical shapes –have different cross section thicknesses, the different cross sections cool at contrary rates. Of course the exterior of the object cools before the internal does as well. As the thinner sections cool and solidify, they begin to contract, the larger sections still being in a slightly molten stage will give way to this contraction. However, by the time the thicker sections begin to cool and contract, the thinner sections are already in a solid state with no room to give way to contraction of the larger sections. Naturally, this will not hold the larger sections back from completing their cooling effect. The contradiction of the two sections ends up leaving the material with significant residual stress. This is in addition to the thermal stress created through different cooling stages from external to within the object.

So what does that mean?

It means that on a molecular level there is an imbalance of proper mechanical structuring between the molecules creating, not only a weak spot, but a potential danger point. You can think of this stress spot as the weakest link in a chain, or a couple bricks missing in the wall, but no matter how you look at it, this stress point will be where the first signs of stress failure will begin to happen.

How do you remove these stress areas?

There are a couple different ways to remove the accumulated and residual stresses. Some, however, only remove the external stress, by bombarding the object with more stress, which increases tensile strength just below the surface of the part. But this is only a surface treatment and is more like treating a symptom than the underlying problem itself, albeit, most defects will generate from the surface and work their way in. Cryogenics research has shown that a cryogenic process – otherwise known as  cryogenic tempering – will properly relieve the underlying residual stresses by relaxing the molecules into a uniformly distributed pattern; creating a tighter bond between them. This in turn increases tensile strength prevents distortion in heated parts and closes the grain structure. Cryogenics however is not just a surface treatment and will relieve stress throughout the part.

And this is just relieving the residual stress. A cryogenics process does much more to help your ferrous metals than just a relief of stress. Follow this link to find out how cryo’ing your high performance parts will also help significantly improve wear resistance on our wear resistant solution page.

In a practical sense:

Automotives – high performance engines – when a motor is running, the block begins to build heat and starts to distort. But not only does the block distort, so do the affected high performance parts. The combination of the two throws the proper alignment out of balance, taking away from engine horsepower, torque and efficiency. Although the high performance parts were manufactured true and straight, they will not hold true when they are put under enough heat and stress to cause them to temporarily deform. All with existing built up residual stress; it is just a matter of putting too much torque on the parts before the “weakest link” will show through. Cryo the block and parts, and you remove the weakest link.

As mentioned in the wear solutions section of the Cryotron website, contrary to popular belief, it is the tempering that sets the hardness value of metals, so if done correctly, a cryogenics process will never change the hardness value. This is one of the reasons why “cryogenic tempering” is not a correct statement. There is no need to worry about brittleness; as proven in cryogenics research, your high performance parts will actually become LESS brittle after treatment. The reverse does not apply. In a cryogenic stress relief, there is no such thing as cryo hardening.

So lets talk numbers, what exactly can you expect?

There are many factors that can influence the outcome of a cryogenics process, such as composition of the metal, whether or not a heat treatment was performed on the part, manufacturing, machining, etc. But virtually all power train and engine parts respond to cryogenic treatment. Treated piston rings will seat better against treated cylinder walls reducing blow-by and increasing engine horsepower. High performance engine blocks will not distort when subjected to heat and vibration. Pistons and cylinder heads stand up to detonation better. Suspension springs, torsion bars, and sway bars show longer life and retain a better consistency after cryogenics process. Camshafts, bearings, timing gears, valves and valve springs, timing chains, gears, axles, ring and pinion, roller bearings, CV joints, and clutches……

The Deep Cryogenic Treatment process is also highly known for treating brake rotors, with life expectancies of at least twice as long!

Cryogenics research has shown that high performance engine tests have shown 10 times less wear after cryo!

Fuel efficiency, engine horsepower and torque are all increased with engine cryo treatments.

Less heat, less friction, increased tensile strength throughout the engine and power train, are all attributes of cryogenic technology.

Aluminum engine block will see less fatigue and cracking due to heat and over stressing.

Bottom Line:

Professional racers, amateur competitors, and high performance enthusiasts have found that cryogenic tempering saves money, reduces mechanical time, and gives them a substantial competitive edge. From an engine horsepower and torque gain of up to 10% to a reduction on internal friction and stresses - not just a surface treatment, but thorough release of stress - on your high performance engine and parts that can make them last 50% to 5 times longer! As the cost of quality parts continues to escalate, you will want them parts to perform and last as they were built to. The best way to protect your investment is to install the best product you can obtain for your dollar; cryogenics processing beats the competition, hands down.

So why are the manufacturers not cryo treating?

This is a two part answer; the first part is quite simple. Industries that are into production manufacturing make money by manufacturing. If they built a part that lasted 5 times longer, would they be able to charge 5 times more to make up for lost sales? It is highly unlikely that it would catch on too quickly. However, the second part to the question is: they are starting to…although; they are very few and far between. These manufacturers are starting to realize that the demand for quality is making a return. Cryotron, as manufacturers of cryogenic processors, are starting to see a trend take place. We were once asked “what will be the advantage of cryogenically treating if all the competition is doing it?” Our reply is quite simple; “If that is the case, would you want to be the manufacturer that DOESN’T do it?!” There are always advantages to being the first to offer a high quality product.



   
 


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