Cast versus forged :: From Purely corrosion point of View 5

[bmoorthy]

The material that are usually encountered in Oil, Gas and refinery industry, considering only the corrosion aspect, it is my view that casting is a superior product form as compared to forging, I do not know how i arrived at this conclusion, but i hold this idea for very long.

I just looked up some of the standards, code and specifications, while one can infer this from certain indicators like the limits of % S given for Forging could be more stringent in comparison with casting etc, but i could not get an explicit statements in any of these.

My questions are::

1) Is Cast superior product form as compared to forged purely from corrosion point of view (Assuming that casting is NDE tested and meets the requirement) and the shape and application of casting and forging are the same.

2) If in a highly corrosive line a one installs one cast valve and one forged valve and allow the line to operate, assuming that both the cast valve and forged valve are from same valve manufacturer and the valve manufacturer has foundry to make casting and also has forging shop to make forging and all the components other than the body and bonnet and covers are identical and only the change in product form are in the body, bonnet and cover (one is cast and another is forged) and casting in the cast valve has under gone additional NDE (RT / UT) and has passed the NDE, if a failure were to occur in the valve due to corrosion, it is my view the fist item to fail in corrosion is forged valve and not cast valve. Am i right?

 

[micalbrch]

A forged stainless steel part is in general more corrosion resistant than a casted part. The reason is the more fine-grained homogeneous microstructure, less inclusions and less liquations.

 

[EdStainless]

I will not comment on carbon steels, but with SS and Ni alloys the forged product will usually outperform the cast one by a large margin in corrosion resistance.
Even with highly modified casting chemistries and good annealing practice it is almost impossible to get a uniform composition and microstructure in the cast parts. The small local variations in chemistry play a role in the failure of the passive film and initiation of corrosion.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[metengr]

Forged material, in general, will have optimum corrosion resistance in comparison to cast material. The rationale is cast material can have inherent internal defects, microsegration, microporosity that would otherwise be eliminated from subsequent hot working to produce a final wrought product form. Even if the highest quality casting could be made, at best it may compare with a forged product but not exceed it. The only exception I have seen is powered metal/HIP parts that behave and have corrosion resistance over cast/ wrought material.

 

[SJones]

As an example, take a look at NORSOK M-630 wherein corrosion testing of 625 is only required for cast material.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[Kenvat]

bmoorthy
It looks to me that you are situated somewhere in Middle Eastern Country. Few years ago i was there and people (especialy piping engineers) were obsessed with forged balls for ball valves, forged body for even 24" valves and when i probed, i came to know that the piping engineers were not conversant on the product forms.

On another occasion when one vendor wrote A479/A182-316 in the cross section drawing the piping engineer cut of A479.


I realized that most of such preference towards forging comes out of ignorence and fear of unknown rather than genuine logical basis.

They were not aware the 2" forged bar can be classified as A 479 or A 182 (Dual certified). If one tells the piping engineer thatthe valve supplier is using 3" A 479 Bar to make valve, he rejects, if i tell him that he made the valve out of 3" A 182 bar, he accepts (with out knowing that A 182 allows use of A 479 and machne from that and still call it A 182).

Also if the valve data sheet shows A182 F 316 in the material of construction and if the valve vendor offeres A 351 CF8M they did not accept for manual ball valve, where as in the same line sitting next to the manual valve there was SDV, there CF3M was used. When i probed, i found out the Manual ball valve data sheet was handled by Piping group and the SDV was handled by Instrument group

It so happened that the Feed contractor had given data sheet where A 182 was written piping valves and the feed contractor had simply writted SS 316 in the feed data sheet for Shut Down valves. The feed specificaiton for SDV was more instrument intensive and focus on piping portion (valve portion) was scanty. When probed further i realized the Feed was done by US based Design company, who also has EPC interests and when the feed was done both the end user and feed personnel who made the feed specification and data sheets were both instrument engineers.

Under most circumstances you can replace cast components with forged components without fear. Under most circumstancesw cast components are cheaper than forged items (notwithstanding the NDE requirements in cast and repairs and rejections in casting)

For critical applications you need to analyze and select, when you make such selection you need to perform cost benefit analysis.

 

[Kenvat]

This one is reply to your original post

Q1) The material that are usually encountered in Oil, Gas and refinery industry, considering only the corrosion aspect, it is my view that casting is a superior product form as compared to forging, I do not know how i arrived at this conclusion, but i hold this idea for very long.



Reply to Q1 ) You probably read clause A 8 (pasted below) in NACE MR 0175/ISO 15156 and made your inference since NACE does not pose any restrictions on % S or % P for cast product form or you must have compared the chemistry between A 182 F 316 and A 351 CF8M and must have observed that Cast product form has richer chemistry (Cr, Ni) and must have arrived at your conclusion.

Probably that is why Edstainless would not comment on CS

From NACE
The probability of HIC/SWC is influenced by steel chemistry and manufacturing route. The level of sulfur in the steel is of particular importance, typical maximum acceptable levels for flat-rolled and seamless products are 0,003 % mass fraction and 0,01 % mass fraction, respectively. Conventional forgings with sulfur levels less
than 0,025 % mass fraction, and castings, are not normally considered sensitive to HIC or SOHIC.

 

[bmoorthy]

Ken

Are you saying that for CS (atleast from HIC prespective) cast is a better bet as compared with forged??

You got it right, it was for valve.

Also i noticed that in A 182 thereare supplementary requirements referring to A262 (IGC) where as A 351 or A 703 does not refer to IGC test either in mandatory or supplementary tests.



bmoorthy

 

[ThomasBTN]

Just from corrosion point of view::
Carbon steel Casting are superior to Carbon Steel Forging.

In higher alloys, it depends on the alloy, it varies. It also depends on which type of corrosion is being checked/addressed

From the chemical composition prespective:: It is always possible to obtain a unique composition to meet a unique requirement from a variety of foundries.

The presence of controlled amounts of ferrite in certain stainless steels leads to increased corrosion resistance, higher crack resistance and better weldability. Ferrite occurs normally in most cast stainless steels, with the ferrite level controllable to produce the desired combination of characteristics. However, ferrite impairs hot working properties and is normally not present in forged components.

The important class of work-hardenable steels also are not forgeable. Work-hardenable steels are generally high-manganese (approximately 13% Mn) alloys that become harder the more they are worked. Thus, they are ideal for dipper teeth, compactor feet and other earth-moving and excavation applications.



General :: There has been casting bashing over decades, both casting and forging processes have their advantages

If you need complicated shapes, go for investment casting, by investment casting you can get intricate shapes.

For any alloy, any material, casting has more homogeneous property. Forging has directional property.

If you extract 3 specimens, in 3 random directions from a cast valve body and test it as per ASTM A 370 and tabulate it against 3 tests from similar forged valve body, you will see the difference for your self. The physical property of casting will not be directional.

A 5 minute read from steel founder's society of America, may help dispel some of the misconceptions,

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

 

[Kenvat]

Tom

The first post was " Purely corrosion point of View" you have brought in anisotrophy angle into it. There is no evidence of anisotrphy contributing to corrosion resistance.

Although i agree that some of the alloys are not forgable, and when is faced with such situation, casting is the saver. But i have had many manufacturers complain that a particulr SDSS casting always fails, and that it is not castable. It is utter non sense, they just did not design their pattern and did not adpot correct foundry practice.


Casting buyers need to work closely with foundries at the design stage in order to insure that the design is able to take advantage of directional solidification. The poor quality image of castings is often the result of the buyer not understanding this process. The casting buyer must also understand that there are limitations to relying solely on NDT to verify quality. Quality is best enhanced by using tools such as solidification modeling at the design stage to insure the production of a high-quality product.

The main difference between a steel casting and a forging is that the forging is mechanically worked after solidification. This mechanical working imparts directionality, or anisotropy, to the forging. Castings and forgings are both susceptible to manufacturing problems and misapplication by the buyer.
In general, a forging is best suited to simple configurations that can be easily worked in a die or other tooling. It is also suited to applications in which the principal applied stresses are the same as the direction of mechanical working. A casting is best suited to complex shapes, custom or tailored chemistries, and to applications that are subject to multi-axial stresses.


I think bmoorthy appear to be an EPC guy and is being troubled by some form of feed document where forging is listed and not casting in a drawing or data sheet or specificaiton and he wants to use casting for obvious commercial advantage not withstanding NDE costs and rejection costs involved in selection of casting.

Most of the feed writers or data sheet writers of valve or pump (or other product form intensive products), do not understand the significance of what they put in and how it affects the next in line user of such document. The person who writes the data sheet may not even have clue that his writing ASTM A 182 F 316 as a valve stem will start a debate on whether a bar of A 479 can be used for stem in place of forged bar.

I am yet to come accross a corrosion or a material selection philosophy, for a given application, where one product form is banned or one product form is given preference over the other (Except of course in case of Carbon Steel, where sometimes rolled products are not preferred and cast or forged products are insisted)

Shell, Total, Exxon, BASF, BP in their material selection do not give any preference.

If a data sheet specifis A 182 and you want to know whether you can use A 479 or a 351, then look into material selection document or your contract whether there is prohibition and look into NACE/ISO 15156, unless there is specific prohibition, there is no need to insist on a given product form, although for stem or a shaft, i would not go for cast product form.

 

[bmoorthy]

Yes i am an EPC guy. I realize that on most occasions corrosion is not the governing factor in specifying the product form (as it has been pointed out) also strength is not the governing factor. On many projects i have comes accross many product forms in for the same piping class and for same service cast product form is permitted for higher sizes. for example In on of project (Oil & gas) the piping data sheet clearly shows B564 UNS N06625 for sizes 4" and lower size valves and for higher sizes the dat sheet simply gives 625. Also i have never come accross forged centrifugal pump body. There are centrifugal pumps in the same piping class with cast body.

For smaller sizes, forged items are generally less expenive then cast items since some of the specifications call for NDE for cast products and corrosion tests for cast product form (and not mention those NDE for forged product form) if the valve assembler procures the items from external source.

On the other hand, if the valve manufacturer has forging shop and casting foundry (like some of the Italian vendors), for them cast valves comes out less expensive, since the rejects can go for remelt. Even in the previous case (where the valve manufacturer procures the valve components from external sources) rejects can go for re melt, but in that case the valve manufacturer has to suffer schedule problems and the rejects has to be sold as scrap. If the valve assembler (who does not have in house foundry of forged shop) orders radiographed casting components and also get the cast pieces hydro tested and then takes it for valve assembly then the rejects will be much lesser.

Thanks Ed, Tom, Metengr,Sjones, Ken et al for the valuable inputs and refining my original thought.

 

[rconner]

As you have probably gathered from many posts one should probably not in general assume a forging is better than a casting purely on the basis of corrosion. You may be interested additionally in some very basic sort of first principle corrosion mechanisms expressed e.g. under the heading "Stress Cells" at the site

http://www.efunda.com/materials/corrosion/corrosion_types.cfm,

and even much earlier in a seminal corrosion reference "Corrosion in Action" (written many years ago by gentlemen named La Que and/or Uhlig?) Basically all else being equal and for the reasons expressed, a part with more stress or manufactured residual stress as I think explained therein can or will in general corrode more readily or preferentially than one with no or lesser stress.

 

[bmoorthy]

SJones
Shell MESC SPE 77/302 calls for corrosion test for 825 and 625 forging and does not call for corrosion test for cast product form.
NORSOK and "Shell" vary in their veiw when it comes to corrosion of product forms .

 

[SJones]

That's a bit of an oversight by Shell then, since sub-subclause 3.5.4 of the 2005 version of SPE 77/302 did call for corrosion testing of both product forms.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.