4330 Modified (3.40 Ni) Charpy Results Failing - Need Heat Treat Advice 2

[Macdost]

Greetings,

We are using a 4330 modified;
C - 0.32
Mn - .60
Si - .26
Ni - 3.40
Cr - 1.20
Mo - .48
V - .118

We need transverse -40f impacts @ 34J average no lower than 27J
We cannot meet these requirements after many different heat treatments.

Does any one have experience with this grade mod and heat treating?
All I could find was an investigation by US Army that concluded;
Austenize @ 1500f, i hour, oil quench and double temper at 400f (2 hr. AC + 2 HR AC)! However their Ni was half ours (1.80)

Thank you, pulling our hair out!

 

[Macdost]

@redpicker et el - Might you know when the carbide precipitation stops - at what temperature for the subject material?

 

[redpicker]

The carbide precipitation occurs with the decomposition of the austenite. If the austenite decomposition occurs at higher temperatures and very slowly, then the carbides that form will more likely to be the complex M6C23 carbides and are more likely to be located at the prior austenite grain boundaries. The question is not really one of what temperature does the decomposition stop, but how long does it take to complete.

Take a look at the the Isothermal diagrams at the following link, Page 32.

http://www.nickelinstitute.org/~/Me...formationDiagramsofNickelAlloySteels_389_.pdf

While you are not dealing with isothermal transformation, the diagram indicates that above 900F, the austenite transformation is very sluggish. Transformation can take hours to complete (heck, even start!) at these temperatures.

If you can cool quickly to below 1200F or so, you run little risk of hard spots or cracking issues (since little transformation will have taken place). One way to accomplish this is with fan cooling and monitoring surface temperature. If you are dealing with large sections, you can cool until the surface is black (check with a non-contact pyrometer) and allow to air cool past that.

Once below 1200F, the assumption is that the cooling rate to room temperature will be faster than the austenite decomposition rate, so that the carbides will form in the 600F-800F range, which are likely to be well dispersed and more of the Fe3C type of carbide. In general, the faster the carbides form on austenite decomposition the faster they will dissolve on austenitization.

rp

 

[MagBen]

If your carbide is MC, this primary carbide is more likely from ingot during solidification, rather than from secondary precipitation . Heat treeatment becomes very difficult to break it down.

When you got astm 2 grain size, your may focus more on decreasing grain. What is your final size of bar after forging? if there were limited reduction at forging, the center of ingot could get no deformation, and no much of recrystallination occurred, leading to big grains. course grain structure can be refined by re-designing forging parameters, and/or by thermal cycling between room temperature and normalization.

 

[weldstan]

Correction to previous post - Fine grain melt with 0.02% to 0.04% aluminum addition.

 

[Maui]

Where exactly are you cutting the samples from in the forgings? Are they taken from the surface, the center, or the mid-radius position? Are they longitudinal or transverse samples? What is the physical size of the ingot that you start with at the beginning of the manufacturing process? And what are the physical dimensions of the forging from which you cut your test samples?

Maui

 

[Macdost]

We cut a piece off of the end of the block ~10" x 10"
They are taken from the surface
We do both longitudinal & transverse
Ingot is 58" x 103"
Forging is saw cut to ~22 x 27 it is from there that we take the samples

 

[Macdost]

@redpicker et el. Referring back to my first post where I summarize the chemistry - I have learned that others forging this grade have different levels of Mn , Si, cr & Mo AND they are getting the mechanicals we have had difficulty achieving;

Mn - Our Avg. 0.57 - others at (1)~0.265 & (2)0.80!

Si - Our Avg. 0.25 - others (1)0.075 & (2) 0.26

cr - Our Avg. 1.16 - others (1) 1.475 & (2) 1.14

Mo - Our Avg. 0.47 - others (1) 0.57 & (2) 0.48

As we are almost identical to second "other" (2) except Mn & First "other" (1) we are off on all, other direction in Mn this is confusing me.

Questions; (a) we are in the middle more or less on Mn, both "others" are making the mechanicals - why?

 

[metengr]

....and what is the grain size of the "others" you mentioned above? I think the focus on chemistry is only one aspect of your issue, and might be a small one at that. As mentioned in other posts, grain size plays a significant role in notch toughness behavior.

 

[Macdost]

I do not know others grain sizing. Unfortunately determining grain size is a little like putting cart before horse. We obviously know that our best results with stated chemistry in my first post, has grain size 7.5.

We are trying to determine how to move forward with new steel requisitioning, what is our best chemical composition to achieve above mechanicals..........

 

[redpicker]

What other properties are trying to meet, Tensile, yield, and hardness?

Are the "others" you are comparing too using the same section size?

I am a little confused on your section size. You mention a 50" x 103" ingot, a 10" x 10" block, then saw cutting the forging to 22 x 27. 22 x 27 inches? How do you saw cut 10" x 10" to 22" x 27"? It just doesn't make any sense. You say the test specimens are taken from the surface? The centerline of the test specimens cannot be on the surface, it has to be at least 1/2 the thickness of the test specimen below the surface. I just don't understand what you are trying to do.

 

[Macdost]

We need transverse -40f impacts @ 34J average no lower than 27J.

"Others" are using same ingot as us. The ingot is large but we are getting the reduction as we are doing other grades of same size ingot 58" x 103"

The finished block is saw cut from the drawn out forging. The blocks are ~43"l , 23"w, 26"h - sorry for confusion on that

Test specimens are taken out of a "Quarter T" configuration. Cut 91/2" off block, flip that section, cut 11" in, cut and turn 8", take coupons

 

[redpicker]

From your post on Dec 19, 2014

Tensile Min;
Longitudinal - 160 ksi (1103 mpa)
Transverse - 160 ksi (1103 mpa)

Yield 0.2% Offset Min;
Long. - 145 ksi (1000 mpa)
Trans. - 145 ksi (1000 mpa)

Elongation in 2" Min;
Long. - 14%
Trans. - 10%

Reduction in Area, Min;
Long. - 38%
Trans. - 25%

This should be 35-40 HRC which should use a tempering temperature of 1100-1125F. What heat treat cycle are you using? A thickness of 23" is very large to to be trying to quench, are you sure your quench is effective?

 

[Macdost]

Our best results had hardness of 321-341 Bhn

Best result heat treat cycle;

Normalize - 1775° - 16 hrs. @ temperature
Brine quench to below 800°
Quench – 1600° - 16 hrs @ temperature
Brine Quench to below 250° (brine 45°)
1st temper 1100° 18 hrs. @ temperature
Put in water to below 100°
2nd temper 1080° 22 hours @ temperature
Put in water to below 100°

My question pertains more to ordering new steel and the different chemistry "others" are using particularly as it pertains to Mn........

 

[redpicker]

I don't think chemistry is your issue. Specifically, the difference between 0.26 Mn and 0.60 Mn is not that great in this alloy, particularly when S is kept below 0.005. I am assuming you are dealing with remelt ingots (either VAR or VIM-VAR). The higher Mn will increase the hardenability, which is perhaps why the heat with the lower Mn has higher Cr and Mo. I think you are dealing with a process problem, not a chemistry problem. Have you taken a look at the microstructure?

What is the surface condition of the parts like? As-forged, including forging scale? Have you taken an as-quenched hardness? Is it low (below 401 BHN)?

If so, you might want to mill the surfaces to clean metal. That will improve the effectiveness of the quench. Be sure to break the corners to 0.75-1.0" x 45 deg (you could put a 0.75" Radius on them, but that is a bit fancy) to reducing cracking tendency. Yea, with 0.32 C you shouldn't have a cracking problem, but when you're dealing with 3-1/2 tons of steel, a quench crack can ruin your day.

 

[Macdost]

We have looked at microstructure to a degree, more precise grain size. Old heat treat that was failing miserably ~2.5, new heat treat with good results 6.5-7.5.

The good result as quenched hardness's have been 451-460.

Of the three chemistries, do you see one that stands out as better for this situation?

I will add information as I get it

 

[Macdost]

Our Ingots are not re-melt VAR or VIM-VAR.

Some of the parts have been milled prior to quench, did not have influence on mechanicals (same result as hot top or spout)

 

[TVP]

Ok, I think I finally understand this issue now. You are ordering an ingot with a custom composition, 4330V modified with higher Ni, with no reference to a standard industry specification for 4330, e.g. SAE AMS 6411, SAE AMS 6427, ASTM A646, etc. The initial samples had some issue with thermomechanical processing and/or heat treatment that produced a very low grain size, but subsequent processing has improved the grain size (PAGS 6.5-7.5) and hence the mechanical properties at low temperatures. Now, you want to define the chemical composition so that you will have a final specification. Here is my suggestion:

1. Use one of the standard specifications as a guide. If this will be conventional air melt + vacuum degas, not a consumable electrode process (VAR, ESR, etc.), then reference SAE AMS 6427 or ASTM A646.
2. Here is the composition specified in ASTM A646 for 4330 Mod.: C = 0.28-0.33, Mn = 0.75-1.00, Si = 0.20-0.35, S = 0.012 max, P = 0.015 max, Cr = 0.70-0.95, Mo = 0.35-0.50, V = 0.05-0.10, Ni = 1.65-2.00, Cu = 0.35 max.
3. Specify that the prior austenite grain size must be fine. Typical is for ASTM #5 or finer, but you may want to specify #6 or finer based on your data/test results.
4. Review the specs for information related to steel processing, inspection requirements, etc. Are you specifying requirements for macroetching of ingot or subsequent worked products (billet, bar, etc.)? What about NDT for surface or sub-surface defects?