tensile tests 1

[etch]

I have had a customer on asking about why his tensile samples do not meet the requirements laid down in the standard. The metal used is okay, but im looking for a plausible explanation as to why it does not meet the requirements.

Im thinking along the lines that, as the sample was cut from the casting and not taken from a seperately cast test bar, then the properties will be lower, due to large sectional thickness ( the spec is for properties taken from a seperately cast test bar). Also it could be due tot he location of the sample, i.e near a feeder or near a singular defect.

 

[TVP]

Mechanical properties are determined by the microstructure of the material. Microstructure depends on chemical composition and processing conditions. If the processing conditions yield different microstructures in the casting where the tensile sample is obtained and in the separately-cast test specimen, then obviously the mechanical properties will be different. Significant issues to address with respect to castings include:

a. grain structure - grain size, morphology of second phases, etc.
b. porosity - both macroporosity (entrapped gas) and microporosity (solidification shrinkage)
c. other defects - inclusions, hot tears, cracks, etc.
d. heat treatment - section size differences can affect quench rate, etc.

I would recommend you have a metallurgical evaluation performed on the tensile specimens. Evaluate them for the items I mentioned above. This should be part of the regular inspection and testing of any quality control system used during the manufacturing process.

 

[CoryPad]

If the specification is for a separately cast test specimen, but properties were determined from a specimen removed from an actual casting, then the reasons for lower the lower properties include:

1) thicker specimen from casting results in lower cooling rate, which results in larger grain/dendrite size

2) actual casting has discontinuous shape, which causes irregular flow, which causes increased porosity

Both of these factors adversely affect mechanical properties.

 

[Hush]

I went through this argument from your customer's standpoint with valve body castings. I was told by the foundry that results from the TC are typically 10% to 20% higher than those from the product for the reasons CoryPad gives. The larger the product cross section the larger the deviation.

You can reduce the deviation by specifying larger test coupons but the only method that will give you fair results is taking test bars from sample product. On new designs we now will cast one or more qualifying samples and cut them up for testing. This way we have actual results from the cross sectional areas we're concerned about to compare against results from the TC.

 

[mcguire]

The Metals Handbook, desk edition 1985 has good discussion and data on this topic for gray iron, page 5.5 and cast steel page 4.47. Your customer may be more convinced by this book than by us.

 

[bruv]

Around 4 years ago, all Norsok specifications were re-written. These are Norwegian oil industry specs, originally aimed for North Sea duty, covering a large range of materials and manufacturing techniques. I was led to believe that part of the re-write was because of the very problem mentioned above - that material was failing in service due to inadequate heat treatment of thick sections, although test results would appear to have shown otherwise.

Part of a typical (casting) spec now reads:

Samples for mechanical testing shall realistically reflect the properties in the actual components. Thickness of the test block shall be equal to the thickness of the actual components up to a maximum thickness of 100 mm. For flanged components the largest flange thickness shall apply.

Test specimens shall be cut from the 1/4 T location from the surface where T is the thickness of the test block.
Test block shall be integrally cast or gated onto the castings and shall not be removed from the castings until after the final quality heat treatment.

Previously there had been no requirement for integrally cast, matching thickness test pieces, just testing in accordance with the requirements of the material spec in question, typically ASTM.

 

[etch]

THanks again for your comments,


We currently sand cast components, which we have to cut integral test pieces out of, now these generally show far higher elongation than is recorded from the corresponding separetley cast test piece. THis is due to the far thinner section that the test piece is removed from.

Its hard to try and explain this to a non material /metallurgical person, that what is listed in the test book, for a separately cast test piece is not what you might get in a actual casting, due to the reasons listed above.

Thanks again for the help.

 

[greybert]

When taking samples by cutting from a casting, is there any standard that recommends minumum number of samples or an averaging technique, since I see large variations from sample to sample.

 

[jetmaker]

Question... why would anyone want test samples from a specimen that is not representative of the actual part you are working with? I've worked with some large landing gear forgings, and we required test coupons to be taken from each forging. These samples had to meet minimum values determined by us... based on our requirements. Additionally, one forging was sacrificed... cut up.. and had specimens tested in all the high stress regions. This way we were able to ensure that our analysis allowables were conservative to the actual ones. This method was also used for all castings.

Thanks,

jetmaker

 

[Hush]

greybert,

You want to get your specimens from within the 1/4 T envelope. I.E. the specimen should lie within 1/4 of the thickness from the midpoint or centre axis of the thickest cross section. Since it's a casting, orientation won't matter.

 

[arunmrao]

I am reproducing a paragraph from Steel Castings Handbook 6th Edition published by ASM Intl and SFSA pp18-9 to 18-10 for ready reference.

" Section Size Mass Effects,

Mass effects are common to steels,whether rolled,forged or cast,because the cooling rates during the heat treating operation varies with section size,and because the microstructure components,grain size and non metallic inclusions increase in size from surface to centerThese changes are illustrated in Figures-----.

Mass effects are metallurgical in nature, distinct from the effect of discontinuities.An example of how the mass component lowers strength properties for wrought AISI 8630 and for AISI 8650 steel plate is shown in Figure----. Properties are plotted for1/4T location halfway between surface and center of plate. Comparison of Figures -- indicates that toughness is proportional to strength only in a limited way and that a major loss in toughness may occur in heavier sections.

The section size or mass effect,is of particular importance to steel castings because the mechanical properties are typically assessed from test bars machined from standardized test coupons which have fixed dimensions and are cast separately from or attached to the castings(Figure ---). To remove test bars from the casting is impractical because removal of material for testing would destroy the usefulness of the component or require costly weld repairs to replace the material for testing purposes.

It cannot be routinely expected that test specimens removed from a casting will exhibit the same properties as test specimens machined from the standard test coupon designs for which minimum properties are established in specifications. The mass effect discussed above,i.e., the differences in cooling rates between that of test coupons and of the part being produced,is the fundamental reason for this situation. Several specifications provide for the mass effect by permitting the testing of coupons which are larger than the basic keel block in Figure -- and whose cooling is therefore more representative of that experienced by the part being produced. Among these specifications are ASTM specifications E208, A356 and A757."

 

[greybert]

My problem with samples being cut from the castings themselves is that the casting walls are barely able to allow .125"DIA samples and that , for samples this small , we expect large variations in elongation. Is there a recommended averaging technique over some minimum number of samples?