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I'm going to agree with Abhiman, Forging Materials can give a higher tensile strength, they're most recommended to use in most of the engineering procedures and overall structure analysis.

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I think for a given material, say 15C8, casting material will give a higher tensile strength than forging. Even though there is a complete recrystallization of molecules, there is more residual stress induced in forgings. Moreover, if there is no proper control on the process, there is a chance of formation of cracks along the parting/folding line which may hamper the mechanical properties of the forging for a same material. However, in case of castings, as we are pouring the molten metal, the solidification process matters more in this case. Also the type of grains (i.e the micro structure)also determine the tensile strength of the material. For example, the columnar grain structure will give less strength, whereas the more finer grain size will give you more tensile strength. This may require normalising and tempering process, but in either case, I feel that Castings have more tensile strength properties when compared to forgings.

Please comment.

Thank you

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Forging, of course, wherever is possible.

Tensile strength is withstanding the maximum stress before failure. It depends on structure of material. The finer it is, the stronger it will be.

Forging is mostly a hot process. So, the crystals tends to come as closer as possible with each other. This improves tensile strength.

If there occurs a crack, the material already failed. So, we are taking the best case possible.

Casting is a relatively easy n cheap process.

But the crystals don't form a finer structure. Cooling temperature varies for different sections. So, they are unable to form a fine structure.

An e.g.

Pistons of larger size (>150mm dia.) are preferred to be made by forging. As they have to withstand larger stresses.

So, as far as I'm concerned, forging is preferred over casting for better tensile strength. But, making and maintaining dies makes it more complicated. So, casting is easier in manufacturing.

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A steel forging will display a higher tensile strength than an iron casting per unit cross-sectional area.

The process of forging aligns the grain microstructure in a longitudinal direction and gives the forging directional strength.

Castings, on the other hand have no directional strength due to the solidification process of the molten metal. In addition, sand castings particularly are prone to porosity and exhibit little or no elastic and plastic deformation. While cast iron does have great wear resistance, it has low ductility and is also very brittle.

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Hi,

Well, I think one question remains: how?!  I'll try to answer according to what I studied for a presentation, maybe it's not accurate or even wrong, since it was some years ago.

As said, the forging proccess results in compressive residual stress in the surface. When under tensile strength the sum of the residual and applied stress willl be lower. Well, I'm not sure now, but I think cracks will in most cases begin in the surface, and incresing the resistance there will result in an overall increase of resistance. Anyway it will "hold" the yield of the material a little bit longer.

TL;DR: residual stress in the surface inhibits early yield and/or crack formation.

I sure welcome all critics and suggestions, I am also curious about the science behind it.

Kind regards.

_____________________________

Further reading: http://www.tms.org/superalloys/10.7449/2004/Superalloys_2004_315_322.pdf

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The answer is in the previous posts. It all boils down to the microstructure and the actual metal grains. In casting, the molten metal flows into the form to give it shape, but the grains, under no stress, form uniform shapes as the part cools to solid metal. In forging, the metal is heated to a plastic state, then hammered into shape. Under stress induced by the hammer blows, the grains for a flow pattern as the part stretches and forms. This flow pattern imparts higher tensile strength along the axis. It's been a while since I've studied this, but I believe the increase in tensile strength is nearly 40%-60% based on the same grade of metal. One real good example is bolts. Almost all bolt threads are roll-forged, but threads in casted parts are typically cut with a tap. When the threads fail, it is always the cut threads on the part that fail, almost never the threads forged on the bolt. Google 'steel forge flow' and look at images to see examples.

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This paper on compairing fatigue performance of forged steel an ductile cast iron make clear these differences :

  • Forged parts had a 26% higher tensile strength than the cast parts. This means you can have stronger shackles at a lower part weight.
  • Forged parts have a 37% higher fatigue strength resulting in a factor of six longer fatigue life. This means that a forged shackle is going to last longer.
  • Cast iron only has 66% of the yield strength of forged steel. Yield strength is an indicator of what load a shackle will hold before starting to deform.
  • The forged parts had a 58% reduction in area when pulled to failure. The cast parts only had a 6% reduction in area. That means there would be much greater deformation before failure in a forged part.

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The answer is the forging because of the fact that material flow can be confined and dispersed uniformly throughout the structure in forging which gives better tensile strength and fatigue properties compare to casting.Also, residual porosities in the component that reduces the tensile strength can be welded shut by forging.

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forged parts gives high strength because making a parts under compression action and take a different kind of heat treatment. but casting parts totally different the molten materials poring under gravity and its gives very less compared to forged. and its applications outer casing purpose. and holding purposes..

and micro structures are based on different kind of heat treatment

  

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