Low impact testing values for A694 F60 forgings 7

[sayee1]

This is just a brainstorming for possible reasons for low values on A 694 F60 forging @ -40 Degrees C.
1. Testing variance and improper root radius for notch.
2. Temperature, holding time and methodology for Quenching & Tempering.
3. Chemistry ???
4. The freedom of cross section of impact specimen (especially in plane of the notch) from material defects

Please add to the list...I know that the data is grossly inadequate.

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
If it moves, train it...if it doesn't move, calibrate it...if it isn't written down, it never happened!

 

[unclesyd]

Not with your particular material but here's some of the problems that caused us grief.

Right off check the testing apparatus or procedure as one testing machine or operator ate our lunch with bad values. Though not directly involved, I remember the conversations were mostly about the machine. All test that I've ran it has been my opinion that any problems would give higher(better) values than lower.

We also had problems at times with the proper machining of the notch(root radius)as you state if the material was close the acceptable values. I think the notch effect is underrated on the more brittle materials as we could get values 20%-50% higher than the normally accepted values once we perfected the sample preparation. This happened when we eliminated the possibility of a mechanical or metallurgical notch in the radius.

Another problem was that coupons were tested at much lower temperature than specified due to being cooled in the wrong box. The boxes were actually labeled wrong and no one
evidently looked at the temperature indication.

I think your statement #4 is very important as your material is a forging. I certainly would do a metallurgical investigation of the fracture surface/area.

 

[grampi1]

If relatively thick, Q&T may not be effective due to limits on depth of hardenability.

Should contain at least 0.01 of either Nb or Ti for grain size control, or -40C impacts will be problematic.

Reduction ration should be at least 8 to 1

A few micrographs of the grain structure would be helpful

 

[TVP]

In addition to notch radius, surface finish can have an effect on impact values as well.

 

[sayee1]

To add more to the initial post, there were three sample materials of 1/4th,1/2 and 3/4th the actual forging thickness with the same processing as the original forging. When the impact specimens from these three samples were tested, the 3/4t samples showed low impacts while the 1/2t and 1/4t showed higher values. So Grampi1's post on Q&T could be a valid reason. I am trying to get hold of the impact specimens. Will try and post photographs if I can lay my hands on them. Meanwhile if the hammer/pendulum strikes the impact specimen not perpendicaular but at an angle, what would the effect be on the impact values?
TVP
When you talk of surface finish, you mean in the notch or the impact specimen surface? What should it be and what are the normal aberrations observed? I know that shadowgraphs are required for the notch, but how frequent I do not remeber.

My biggest enemy w.r.t impact testing is the Broach used for the notching, becuse in the parts of the world I work in, they are pretty expensive and the laboratories keep on using the same ole broach for years giving highly erratic impact energy readings.

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
If it moves, train it...if it doesn't move, calibrate it...if it isn't written down, it never happened!

 

[kenvlach]

The Charpy test requirements, including finish, are given in ASTM E23-02a Standard Test Methods for Notched Bar Impact Testing of Metallic Materials.

An older copy (I’m at home) gives the surface finish requirements as:
2 micron (63 microinch RMS) on the notched surface and opposite face,
4 micron (125 microinch RMS) on the 2 other faces.
The root radius of the V-notch is 0.25 mm (0.010 inch) – very critical, as unclesyd mentioned.
Greater roughnesses reduce the energy absorbed.
As to the hammer hitting at an angle, my recollection is that is very dangerous – won’t even guess at the effect on energy.

The results of your different samples seem odd w.r.t to Q & T, but make sense in terms of greater reduction inducing greater grain refinement (grampi1’s idea).

What are the hardnesses of the test specimens?

 

[sayee1]

Kenvlach
Could you elaborate on "The results of your different samples seem odd w.r.t to Q & T, but make sense in terms of greater reduction inducing greater grain refinement (grampi1’s idea)."

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
If it moves, train it...if it doesn't move, calibrate it...if it isn't written down, it never happened!

 

[kenvlach]

Re comments on “three sample materials of 1/4th,1/2 and 3/4th the actual forging thickness with the same processing as the original forging.”

The effect of material thickness is typically greater on quenching than on tempering, so presumably the thicker 3/4t sample would be less hard and more ductile. Hence, a higher Charpy energy is expected than for ¼ & 1/2t samples, contrary to the results.

During forging, the thinner samples would undergo relatively more deformation, yielding a more refined grain structure after heat treatment. Grain refinement generally increases toughness, although I must admit to guessing about the material since I don’t have ASTM A694 at hand.

 

[sayee1]

Kenvlach
"The effect of material thickness is typically greater on quenching than on tempering, so presumably the thicker 3/4t sample would be less hard and more ductile. Hence, a higher Charpy energy is expected than for ¼ & 1/2t samples, contrary to the results."
With the thickness playing a more important role on quenching than on tempering, wouldn't it mean that for the same heat treatment a higher hardness gradient would exist in the thicker sample. Thus if so the case, hardness would vary more for a higher thickness across the cross section, but the average hardness itself is higher for a lower thickness. Please confirm my understanding.


Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
If it moves, train it...if it doesn't move, calibrate it...if it isn't written down, it never happened!

 

[kenvlach]

Agree. The sample of greater thermal mass would show a gradient (more so with mass, less with alloying), and the entire hardness profile, from surface to core, should be lower than the smaller samples.

What is the alloy and heat treatment? Will you be able to obtain hardness values? All other factors being constant, toughness should decrease as hardness increase.

 

[MOB1]

It is our experience that, assuming the impact specimens are prepared to standard requirements and that the temperature of test is accurate, the fall off of impact values towrads the centre of a large forging is due to deterioration in the micro structure of the material. That is, there is a change from a martensitic structure to a mixed structure of martensite/bainite or even ferrite/pearlite appearing. These mixed structures have inferior impact toughness.
The problem boils down to either a poor material choice for the thickness involved or a slack quench which has not produced the desired micro structure through the full section thickness.

 

[grampi1]

What is the thickest section size and the thickness of the 3/4T coupon?

If less than 2.5", its probably not inability to quench adeqately, suggesting inadequate deformation/hot working during forging. Over 2.5", various mfg's have propreitary methods of handling this question.

 

[sayee1]

Material : SA 765 Gr II
Chemistry
Min Max typical
(Specified) Actual
C 0.23 0.18
Mn 0.60 1.3 1.19
Si 0.15 0.35 0.24
P 0.02 0.009
S 0.0052 0.002
Cr 0.40 0.09
Ni 0.5 0.06
Mo 0.25 0.01
Al 0.05 0.031
Cu 0.35 0.11
V 0.05 0.005
Nb 0.005
CEQ 0.43 0.4107


Forging dimensions: 1170 OD X 520 ID x 1350 mm Length
Q fromm 900 Deg C & T@620 Deg C for min 4.5 hrs max 13.5 hrs(3x tempering)
Heating and cooling rate for tempering max 60 Deg C/hr
Rp-Yp Rm-Ts A-E Z-Ra Impact Hardness
N/mm2 N/mm2 % % J@-46 Deg C HB
Tang 3/4"xT 342 529 33.3 71.4 64-85-107 160-155-155
Tang 1/4TxT 331 508 35.8 72.8 13-8-11 155-150-155
Tang 1/2TxT 332 510 35.1 73.5 75-17-63 155-155-155
Tang 3/4"xT 353 542 33.4 71.3 15-30-48 155-150-155
Tang 1/4TxT 315 492 36.1 73.5 33-57-22 145-150-150
Tang 1/2TxT 317 505 37.4 73.1 9-13-13 150-150-150

Sorry, I posted in my initial mail as A694F60 based on the data I had, but it was actually SA 765 Gr II. ANyway it is also a Q&T steel so not much change in the discussion.
What I am not sure now is what does the manufacturer of the forging mean by 3/4"xT, 1/2TxT and 1/4TxT. I interpret this as the testting done at various thicknesses of the actual forging to check properties across the cross section, but some of my colleagues interpret it as samples of 3/4", 1/4T and 1/2T being put thru same working and HT as the original forging and then various tests conducted from these samples.
Difficult to interpret hardness-impact correlation but since impact specimens still preserved, I have asked for hardness on each impact specimen (if possible).....
Your interpretations please!

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
If it moves, train it...if it doesn't move, calibrate it...if it isn't written down, it never happened!

 

[stanweld]

The thickness (325 mm or 12 3/4&quot of the SA-765 forging dictates that sampling be done per 6.2.3 Method 3; 6.2.3.1, the longitudinal axis of the specimen shall not be nearer than 3/4" to any heat treated surface and the midlength of the specimens shall be at least 1 1/2" from any second heat treated surface. Per 6.2.3.2 When for ASME Code usage, specimens shall be positioned t x 2t where t is the distance from the area of significant loading to the nearest heat-treated surface but not nearer the surface outlined in 6.2.3.1. Additional test specimens appear to have been provided at the 1/4T and 1/2T locations and possibly 3/4T locations (from the OD surface).

Note that sampling can be done on test forgings or prolongations of the production forgings. The 3/4T location would be closest to the ID of the surface and most likely had not been quenched as rapidly as the 1/4 T surface. Often times a vapor barrier is set up at the ID surface during quenching which retards the quench rate. I have seen Weidmenstaten structures in these locations that exhibited much lower impact toughness properties.

 

[MetalMickey]

Suggest you try an approach on a number of fronts:

1/ Review the forging regime to see if the pre-existing structure has been sufficiently removed (i.e. a minimum of a 5:1 reduction is a good guideline, from starting ingot to final piece (this includes upsetting and becking). This will be corroborated by 3/ below.

2/ Check the % shear (i.e. ductile / brittle percentage on the impact cleaved surface), refer to ASTM A370 for guidance. This should determine any change in the nature of the impact performance.

BTW is the issue isolated to a single impact result? (Possibly a defect or prep problem) or are all then results low for a particular location? (More likely to be a structural variation).

3/ A hardness and micro survey through the section. This should determine if there is a variation through the section. This is common in 'thick' Q&T sections.

If these point to variation in the sectional microstructure or worst still, non metallic inclusions, this would be the major contributing factor in the low impact result. The testing variations are valid, however in my experience they are much more difficult to prove.

As an observation, trying to justify a low impact result is even more difficult given its entirely arbitrary nature as a quality control check. As such the most effective course of action is to re-treat and re-test.

 

[sayee1]

BTW is the issue isolated to a single impact result? (Possibly a defect or prep problem) or are all then results low for a particular location? (More likely to be a structural variation).

Not isolated to a single impact result but low for a particular location.

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
If it moves, train it...if it doesn't move, calibrate it...if it isn't written down, it never happened!

 

[Cello]

I would also check the chemistry (LECO instrument for instance fits the purpose) on the broken specimen with low impact values. Carbon content could be sensibly higher than heat analysis (C=0.18) due to segregation and this has an extremely negative effect on low temperature impact energies. On these steel grades, when the Mn/C ratio drops far below 8-10 you may run into those problems. Indeed, looking at the forging shape and dimensions you supplied, I would suggest that in order to manufacture the forging a round ingot was used, and its typical segregation pattern can lead to high deviations especially of the carbon content from the heat value.

One second check I would make on the broken impact specimen is the grain size. Based on my experience, a good forging process would produce a through thickness recrystyllized grain structure capable of an ASTM 9-10 or even finer grain size on this kind of forging. If by chance you spot a 6-7 or even lower, this could explain the low impact. I understand that cooling rate is very important too, but believe me when I state that a fine recrystillized grain structure due to a proper and well set forging cycle, can assure deep seated(1/2TXT) impact values greater than 40J at –46°C even when a normalize heat treatment is adopted for a product of this size made in this steel grade.