V-Notch Impact Anomaly

[DLB-X7]

Hi Everyone,

I am hoping someone can help me understand the below - metallurgically and on a nanoscience stand point -

Material 1020 - ASTM A105
Normalized from the "as forged condition"
Normalize Temperature 1650F
Surface contact Thermocouple - 45 Minutes at temperature.
Preheated furnace - Furnace with ramp 100 F per hour from 1600F to 1650F (Below 1600F no ramp)
Cooling, Enclosed air chamber with fan for controlled atmosphere cooling
Furnace certified AMS2750 Latest revision Class 5
Furnace Atmosphere Nitrogen

ASTM - A370,E23 V-Notch Impact Testing
@-50F
Test 1 Ft/lbs 54,20,10 Lateral (in.) .053, .025, .014 Shear% 70, 90, 100
Test 2 180 deg from 1st Test, around 30" in dia. Ft/lbs 49,66,16 Lateral (in.) .049, .063, .018 Shear% 80, 70, 100

As you can see the values are anything but consistent

Standard size Tensile ASTM E8 - Tensile 71.7 KSI, Yield 44.0 KSI, Elong% 38, RA% 67

Material is machined to size - Thickest cross section 1.5 inches with no odd geometry.

Normally I conduct thermal processing at 1 hour per inch of thickness. Material is usually oversized to allow for oxidation and decarburization then machined after thermal processing. I have reduced soak time to minimize the depth of decarburization on the material surface.

Being 1.5 inches - Surface temperature of the material at 1650 F for 45 minutes - I am not really comprehending how the core is not at least at or above the critical A3 temperature for this material. Homogeneous Austenite for low carbon steel should take a short amount of minutes for the final disappearance of Carbon concentration Gradients around 1550 F and above that temperature within seconds "with composition in consideration". Prior microstructure is pearlite with fairly fine but, mixed grain size being that the material was in the "as forged condition" still air cooled to ambient.

We all understand how grains are formed during heating and how the FCC structure of "austenite" is formed - A1 and A3 and what there roll is in the carbon phase diagram and how grains are formed during a still air cooling along with the formation of pearlite, Fe-Fe3C and Ferrite microstructure.

I have studied steels in an atomic level perspective, with many studies on theories of pearlite, in-depth martensite transformations along with lattice structures and crystal defects for many years along with the heat treatment of many variety of steels each there own thermal processing. I deal with ASTM, API and critical 20E, 20F specifications. I am looking for a scientific theory along with hopefully a referenced, thesis, theory, book or law that would shed some light on why this anomaly in the impact testing is occurring.

The remedy for my situation is hopefully just increase soak time and test again however, what I am looking for is an in-depth perspective on what exactly may be happening or direction to literature that may be relevant. I can not visualize what is happening "I am probably just missing something simple that I am not thinking of however, I have not had any luck on finding in-depth information on what might be happening within the material and during the fracture.

If Soak was not long enough for full Austenitization
Microstructure was Pearlite and ferrite
Partial Austenitization
Inter-critical Spheroidlizing
Austenitizing grains cooled to form pearlite

I am thinking associated microstructures should show relatively high absorption of energy before fracture. Both Impact test specimens are taken at the same depth in the material.

Equalization of surface and core temperature is not really the information I am trying to find, it is what we want and need for the quality of the product however, nucleation and the growth of grains whithin a material does not happen at any one exact set temperature. Atomic movement happens throughout the entire range of temperatures - Composition matters, Temperature and Time matters.

The material processed and question is strictly academic and not for Production.

Thanks in advance for any input on this topic.

 

Clarifications and comments:

1) 'Material 1020 - ASTM A105' -- These are different steels; which are you testing? However they are similar in composition and therefore in response to heat treatment.
2) '54,20,10 Lateral (in.) .053, .025, .014 Shear% 70, 90, 100' -- The absorbed energy does not jibe with the % shear, even if the results are in different sequence.
3) Other mechanicals are good, consistent with a normalized low carbon steel.
4) 45 minutes @ T = 1.5" is plenty to thoroughly soak the coupon.
5) Spheroidization should be unlikely, but if it does happen it is easily identifiable by metallographic examination.

All the scientific factors you cite are important, but at the end of the day we normalize a steel of given composition and thickness according with accepted industry practice. The objective is to heat at the right time and temperature to attain full destruction of Fe3C, but not hot/long enough to permit excessive austenite grain growth. If in doubt reference the Fe3C-Fe phase diagram.


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

What does the structure look like?
Compare a high and a low.
My guess is non-uniform structure, either from forge itself or the slow cooling.

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P.E. Metallurgy, consulting work welcomed

 

[DLB-X7]

Reply to ironic metallurgist

1) Material is 1022 - Good catch
2) I will have a double check of the reported results.
3) I agree
4) I agree
5) I agree to the unlikely and metallography examination - The structure would have ductility and display a high energy absorption.

Yes, Time at Temperature are important factors.

Test results are the determining factor whether a heat treatment was successful, or adjustments need to be made.

I need to have an understanding on the possible factors that are causing a variation in the impact testing that are relevant in the current situation.

Theories on the subject matter will do.

Reply to EdStainless

I have not looked at the structure yet.

I will look into the forging and cooling aspects -

Thanks for the replies.

 

[LittleWheels]

I can't comment sensibly with regard to metallurgy but be careful with impact test results that you have not witnessed. Well-designed tests tend to produce a 'false fail' if any part of the test procedure is improperly carried out. Impact tests tend to produce a 'false pass' in those situations.

 

[weldstan]

I think you have done incredibly well with HT of 1022. I would have not expected such high absorbed energy at -50F and would have expected the lowest values to predominate. Do look at the microstructure of all specimens. What was the percent Mn and Mn:C ratio? Finally is the Lab NIST certified for impact testing.

 

LittleWheels,

How is my witnessing a test going to prevent catastrophic incompetence by the testing lab?



"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

Did you look at the notched before they were tested?
How were the notches measured? Optical comparator I hope.
If these were broached then you need to be concerned about the surfaces.
Gouges or tears can have a huge effect on results.
I have been known to use a lab that grinds the notches (they also make the ref bars for NIST).

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P.E. Metallurgy, consulting work welcomed

 

[LittleWheels]

I have witnessed the sampling and sample preparation and testing of confirmatory samples when I doubted some anomalous test results. It was a good thing to do, in some instances, and a waste of time in others. YMMV

 

[DLB-X7]

Reply to weldstan

C .1983
Mn .968
P .013
S .0026
Si .246
Cu .011
Ni .019
Cr .032
Mo .006
V .002

C.E. .369
Mn:C 4.88

Reply to All

The Lab has A2LA Accreditation
Yes, The samples are examined with a comparator
I was not able to examine the specimens prior to fracture.
I agree "sample prep is a key factor" All samples are prepared in compliance to ASTM standards or Better. I am getting the samples for further evaluation and will look over them once I have them in hand.

Thanks to everyone so far with their input.

 

[weldstan]

Again, I think you got the best results that could be expected with this material. There is no evidence of fine grain melting, yet ther had to be. I assume the Al content simply wasn't reported or tested. The low S and P certainly helped a lot.

 

[EdStainless]

You get fine grain steel with any of the following criteria:

A total aluminum content of 0.020 to 0.050%.
An acid soluble aluminum content of 0.015 to 0.050%.
A vanadium content of 0.02 to 0.08%.
A columbium content of 0.02 to 0.05%.


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P.E. Metallurgy, consulting work welcomed