Fatigue behavior of castings 1

[arunmrao]

Castings are commonly said having poor fatigue strength. This is true for cast test bars. Design rules however more favorable as corners and fillet radii are greater than for most other processes allowing sharper corner. I would like to have your feedback and experience on this matter.

Do you share my opinion or do you have a different opinion. Please share your experience!

The original credit for this post belongs to someone else,and with their permission I am submitting the same here.



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"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[mrfailure]

I do not agree. Castings can have substantial fatigue life and strength. Think, for example, of cast turbine blades. The situation will vary with individual circumstances, processing, operating conditions, etc.

Aaron Tanzer

http://www.lehightesting.com

 

[btrueblood]

Cast (vs. forged or hot-worked material) has greater possibility for defects (voids, segregation, rougher surface finishes) that affect fatigue life. A good foundry can help the designer eliminate defects in critical regions of the parts, but nobody can eliminate all of them, all of the time. We design castings to a lower design stress than equivalent forged materials. But we still use castings for many parts, because the part complexity, size, shape is just not suitable for other near-net technologies.

 

[metengr]

The statement in the original post is flawed . That is my opinion and having seen many castings which have survived under cyclical loading conditions is my basis for this opinion.

 

[btrueblood]

Metengr,

Disagree strongly that it's flawed, since the original statement is "...commonly said to have lower strength" - note it doesn't say "said by experts" . Unless you know a lot more commoners?

 

[JoeyV87]

There is a fair point raised that when designing castings, we tend to avoid sharp corners, which act as stress raisers. In addition to this, many cast components (especially investment cast ones) can be used in the as-cast condition, avoiding finishing operations which can create crack initiation points at the surface.

Typically, however, forged microstructures are just stronger and (as pointed out above) freer from defects, making their fatigue lives longer.

 

[arunmrao]

Whenever a choice has to be made for a part which undergoes cyclic stresses designer prefers machined or forged component over a cast one. This is because he has less confidence on a cast part due to its inherent limitations. Hence he applies a safety factor ranging from 4-5 for a casting,making the part heavier.

There is a thinking to reduce the factor to 2 or 3. Using the fracture mechanics route a critical flaw size is defined. However the existing NDT techniques are unable to recognize the flaws when they are 10mm (indicative) and below making the choice difficult.

Additionally say in case of steel casting the inhomogeneities are large, there is grain size variation due to differences in cooling rate across sections,segregation and improper response to heat treatment add to difficulties. Finally we have the residual stresses in the castings making the choice of steel casting for varying loads very difficult.

However, I have been making parts for hydro electric machines like runner blades,pelton wheel,buckets,guide vanes stay vanes,pulp refiner plates,slurry pump and process pump impellers shot blasting machine blades,etc as castings. These are profiles difficult to process by machining or forging,.

I feel,the process controls and integrity of steel castings need to be improved before they are accepted by designers.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[btrueblood]

Arun,

Steel castings are "accepted by designers" as they currently can be manufactured. Improving process controls and metal integrity can (and does) reduce safety factors for many parts. SF of 4-5 applies to castings used in pressure vessels per the current ASME code(s), with improving (smaller) SF being used when casting integrity (by process control and NDT) can be increased by known methods.

Personally, I think cast ductile iron is lovely stuff to design with.

 

[rconner]

[and apparently good enough for up to hot nuclear waste containers per

http://www.ductile.org/didata/Section2/2intro.htm

and

http://www.tes.bam.de/en/umschliessungen/behaelter_radioaktive_stoffe/behaelterpruefungen/index.htm

etc.]

 

[tbuelna]

arunmrao,

With fatigue, generally it's what you can show by analysis. The cast material fatigue allowable can vary based on your foundry, heat treat, and QA processes. Very high quality results can be obtained in castings with advanced techniques such as vacuum casting, squeeze casting, HIP-ing of castings, etc.

However, even NDT processes such as radiographic are not perfect at catching flaws. For example, it can quite difficult to detect a small fragment of investment shell in a casting using radiographic. So in this case you would need to perform your fracture analysis based on a minimum detectable flaw size.

In the aircraft world, structural casting strengths are almost always de-rated by a "casting factor". FAR part 25.621 describes the application of casting factors.

http://rgl.faa.gov/Regulatory_and_G...85256672005027f7!OpenDocument&ExpandSection=2

Hope that helps.
Terry

 

[arunmrao]

I agree that most of the general engineering castings might not require such attention. It is only when we desire to get into the higher levels of service requirements,the challenges are posed.

For aluminum and magnesium alloy castings,technology is well developed and factors like HIP provide a means of improving acceptance.

It is in the case of sand cast steel castings,which form the bulk of the requirements, my concerns or those by the OP become relevant.

Efforts are made to define the critical flaw size for a given service condition,but then identifying the flaw and ensuring that no flaw larger than the critical one exists becomes difficult.

There is research work going to improve grain refinement ,have better feeding systems, vacuum degassing or AOD techniques applied etc.



_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[tbuelna]

arunmrao,

What is the specific reason you have chosen sand casting for your steel parts? Does the part have a shape/feature that requires coring which can only be produced in sand? Does the production rate not economically justify using other casting techniques?

The reason I ask is that much better metallurgical and dimensional results can be achieved in steel using investment casting as opposed to sand casting, as long as the shape of your part permits. Sand casting typically has lower non-recurring costs than investment casting in production, with regards to tooling. But if all you need is a few parts, rapid prototype investment cast parts can be cost competitive with sand cast parts.

Finally, with regards to vacuum degassing of the metal, this is not really possible with sand casting processes, since the sand mold is not airtight.

Good luck.
Terry

 

[arunmrao]

tbuelna,

The castings that I deal with are fairly large from 50 kgs to 5000kgs,hence sand castings is the only route. Recently I was asked,if I knew someone familiar with 100 ton(single piece) casting and above. This is for a new foundry in the power sector.

Merits of investment cast parts are many and well appreciated. They find better conformance to drawings with good surface finish and are reliable too. Due to the size and weight limitation, they have limited application.

In the case of sand castings,a quick polystyrene pattern can be made and sample processed within 24 hours. (up to 5 tons single piece0

By vacuum degassing ,you could reduce the gases in the liquid metal(read hydrogen),but pouring is in open air,thereby losing some of the benefits. I used to degass by passing argon for a few alloys.

I have been working for spares in mining industries. Here the trend is to have ultra high strength cast steels and metal matrix composite to improve wear life of the parts replacing the traditional manganese steel parts. This reduces the down time of the machine and also cost of part per ton of crushed ore.

There is a lot of work to be done in this area,to improve reproducibility and reliability. Structure property correlation becomes very important.

( I shall get off my soap box thanks for reading through and sparing your valuable time. It is a Sunday morning here!!)

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[blacksmith37]

My $0.02 : It is easy to make a poor/mediocre casting that more-or-less does the job. It is more difficult to produce a wrought component with imperfections. The resulting general impression is that castings are not so good because so many of marginal quality are seen.
I dealt with API 6A wellhead components; When a casting is made by a "good" foundry with appropriate NDE and witnessing it is as reliable as a forging.