Investment Cast vs Forged 1

[CR100]

We have an application where a rod end needs to be designed. A grade 8 3/8 bolt will thread into this rod end. I have calculated the loads (axial through the bolt) and determined that I will need to use a 4130 type of material.

The question I have is that I am unable to find any comparisons of forged material and investment cast material for fatigue life. Of course everyone knows that the forging will be stronger, but a number of machining process will have to be performed on it (higher cost). Versus the investment cast piece would be as cast.

How does one choose on the process, I have requested information on the material from the investment cast company, all they have is YS, TS and Elongation%.

I have request similar information from the forging companies and they have pointed me to MATWEB.

I am looking for the proof in numbers that castings are more brittle than forgings.

 

[MikeHalloran]

Forgings are not necessarily stronger or more ductile than castings.

What they are, is much less likely to have internal flaws that will fail in service, since the forge hammer will probably open any pre-existing flaws and make them apparent before the part undergoes any further processing.





Mike Halloran
Pembroke Pines, FL, USA

 

[metman]

CR100,

From your post it appears that you would like to go with forging but need some ammunition for persuasion. But maybe I am reading too much between the lines.

If Matweb will give you YS,TS,EL%, you can plot stress/strain curves and compare the area under the curves. Or maybe the curves are given. More area more toughness -- less area more brittle. However this does not tell the complete story if impact is involved you need CVN or comparable values.

What is the environment? Temperature? Atmosphere? vibration?

 

[metengr]

Why not conduct a search using Google and sort the information yourself? The following information is available;

http://www.esi-group.com/products/casting/Etips/e-tips/eTip16.pdf

http://www.forging.org/fierf/pdf/Fatigue.pdf

http://pt.rexnord.com/customer_support/technical_information/HMS_paperdbl_col.asp

http://www.sfsa.org/sfsa/pubs/cvf/ecs.php

 

[metman]

Metengr,
Obviously I am not the OP but thanks because just for personal interest and edification I am reading the last two links.

The first two links do not work for me therfore I need computer help so I will go to tech-forums.net. and/or tek-tips.com.

 

[CR100]

The piece is a linkage that travels up and down on a transport vehicle. Forging is the way to go as the grain structure would of the forged piece would lend itself to the application.

Although the cast piece is strong enough its really comes down to the elongation before break of the cast piece 15% or the forging 25%. Although this should really only come into play if the piece is not strong enough or the load exceeds what it was designed for.

Thanks for the links, Metengr my numerous google and bing searches only come up with 1 of the 4 being common.

 

[swall]

A lot of this depends on configuration and section size of the part. You can expect more variability in a casting, in my experience, unless you get carried away and purchase castings subject to radiographic inspection. The soundness in an investment casting can vary, as there is no conrolling the thermal gradients during the solidification process. This can lead to unsound areas, and depending on where you are machining, you could end up removing good surface material and exposing material that is less sound.
What sort of heat treat procedure are you planning?

 

[mewhg]

Investment casting in my experience have a fair amount of subsurface defects (small voids, slag inclusions). If the part is highly loaded and in a critical application (aircraft, vehicle) forging is the better choice.

 

[Wrenchbender]

When making any comparisons, such as forged vs cast, you may want to keep the following in mind. A variable that typically has a noticeable effect on impact strength and fatigue life (not so much on tensile strength) is the secondary refining process used at the steel mill. The highest numbers come from vacuum arc remelted, or similar. VAR has the least amount of sub/microscopic junk that nucleates microcracks. (Also, not to insult your intel, don’t neglect the rod bolts.)

Somewhat off the topic - we are investigating the use of a cast plus HIP process to replace a forging. Initial results look promising, but too early to be conclusive.