Variations in Energy Absorbed Values Between Charpy Specimens

[BMet53]

These may be too general questions but I will give it a go.

What would be considered the normal spread of values obtained between the 3 Charpy specimens in a test?

Does the expected variation depend upon the overall level of toughness eg 10J vs 100J?

To define a little further, the case in point deals with a 410 type stainless steel quenched and tempered to approx 235 BHN hardness, 105 ksi YS and 115 ksi UTS. There is a longitudinal charpy requirement of 20J min at RT.

There appears no issue obtaining the requirement. Results exceed 100J. But what has been observed (and has raised questions) on a number of tests is that 2 of the 3 specimens are closer together (10-20J difference) and one specimen is marked different, usually lower by an order of magnitude from 20-50J.

The specimens are extracted from a 4" long samples cut from a tubular forging whose cross section dimensions as heat treated were 8"OD X 4"ID. The longitudinal specimens were extracted beside each other in an "arc" at approximately mid-wall.

Any comments would be greatly appreciated.

Thank you.

 

[metengr]

What would be considered the normal spread of values obtained between the 3 Charpy specimens in a test?

CVN impact testing is qualitative test method to compare fracture behavior in notched samples as a function of service temperature. As such, the spread in CVN values can be significant (this is a relative term depending on product form - casting versus rolled versus forged material), which is the reason for 3 test samples per temperature. Another reason to eliminate one data point if the spread is significant (20% or 40% or 50%), caused by discontinuities related to inclusions, texturing or defects, etc.

Does the expected variation depend upon the overall level of toughness eg 10J vs 100J?

No.

 

[EdStainless]

One factor that needs to be examined is the uniformity of the bars.
When it comes to sample prep you only get what you pay for.
When I am doing work (especially with low ductility materials) I will only use bars that have been ground, all exterior surfaces and the notches. The notches are examined on an optical comparator and then under a steroscope. If there is a deviation in dimensions or a visible line in a notch that bar is tossed out.

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P.E. Metallurgy, Plymouth Tube

 

[weldstan]

While great spreads are more likely in welds and castings, they can be considerable in wrought products as well due to prep as per Ed, through-thickness locations, microstructure at notch location, etc. (see metengr).

 

[BMet53]

Thank you all for your valued comments.

With respect to the importance of specimen "quality" discussed by Ed Stainless, during this exercise some debate has arose as to the merit of using ground notches vs broached notches.

One school of thought is that notches manufactured by grinding should have a better surface finishes with less "notches" on the notch surface. The thinking is that the smoother the notch, the less variation from specimen-to-specimen should be observed.

However, a couple of the commercial test labs we have been using say they are only required by ASTM E23 to broach notches as long as they meet notch dimensional tolerances and that grinding doesn't necessarily improve results.

Although we understand the importance of the notch geometry, tolerances as per Appendix X1 of ASTM E23, we could not find any dissertation on the effects of notch finish. Section 7.2.2 of E23 only states that "Notches shall be smooth machined but polishing has proven generally unnecessary".

Would anyone have comments or experience with regard to the specific topic of notch surface finish quality?

Thanks again, everyone!

 

[metengr]

Typically, in my past life our internal test specification called for ground notches. We had used two test labs that routinely performed this activity.

 

[blacksmith37]

My experience (Q & T low alloy ) is that variations in the materials causes variation in test results. You will find a variation in stastical "sigma" ranges from mill to mill.

 

[FennLane]

I did some work some years ago on a very tough Reactor Steel and we looked in some detail at the influence of the notch in terms of angle, acuity and depth.

In these very tough steels there was very little variation in CVN.

We deliberately used notches produced by grinding, broaching, milling and using a shaper - which was really poor.

We varied notch depth from 1.75 to 2.25mm and notch angle by 10 degrees (except for

The CVN of the material was around 300CVN and we used a Wolpert 750J hammer for this testing.

I am sure that more brittle materials would be significantly more affected.

At 100J he material is relatively tough and within sensible limits I would think that notch preparation would not be so critical to see a 50% variation.

What is the FATT of 410 Stainless and could you be between upper and lower shelf values due heat treatment variation?

 

[Maui]

Look at the fracture surfaces of your test specimens to see how they differ. This type of test will almost always find the largest and the most densely concentrated non-metallic inclusions.

 

[LEB1]

What process was the 410 Stainless produced by?
Is this a repeating occurrence or a singular one?
If singular it is possible the difference can be attributed to the presence of intergranular carbides, in the notched area which is very difficult to determine prior to testing but can be assessed on the fractured sample. We carried out an investigation in one such instance and the lower result Charpy sample have evidence of Chromium Carbides at the grain boundaries at the notch base, whereas there was little or no evidence of these on the higher samples.

The homogeneity of 410 stainless should provide consistent values however in our experience historically variances of 20-30J is not unheard of indeed we have 27 instances which fall into this category from 200 sets in the last six months.

I agree with earlier comments that sample preparation is paramount, however for this material a ground notch or a milled notch appears suitable.

 

[Guest102023]

I could contribute if I could make sense of the question. Sorry.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[DeleteriousPhases]

Ensuring notch uniformity - whatever the method, is very important. We noticed nonsensical data often associated with poor notching technique, machining and machine finishing produces the best data.