FRACTURE TOUGHNESS AS A CONTROLLING TOOL AGAINST PITTING . 2

[ibf]

Gents:

Would fracture toughness be an appropriated mechanical testing for comparison between two forms of steel alloy (casting vs. forging - both made out of 4340) in order to determine resistance to pitting ? I am talking about gearing application where both the casting and the forging versions are assumed to have the same hardness , supposedly same microstructure and theoretically, the same degree of cleanliness (inclusions). Please, understand pitting in this case, as a fatigue type of failure. F.Y.I , I have already compared Charpy - V test values at ambient temperature for both versions an came up with 40 ft-lbs for the forging (liquid quenched & tempered) and 15 ft-lbs for the casting version (normalized & tempered) !!

Thank you.

 

[metengr]

ibf;
I will try to tackle this answer. In general terms, fracture toughness is a measure of a materials ability to resist fracture from a pre-existing crack. What you are measuring in a Charpy test is the energy required to fracture a specimen. The difference in fracture toughness between a forging and casting has more to do with grain structure. Castings typically exhibit lower mechanical properties in comparison to wrought products because of several possible reasons;
- macro or micro-segragation
- and lack of any directional grain structure or grain flow lines.

In wrought products, the directionality in grain structure from rolling or forging operations will result in improvements to material properties. However, this improvement is highly directional (e.g., one has to be aware of material property differences parallel with the grains and transverse to the grains) versus a casting.

In your specific situation you are trying to evaluate susceptibility to spalling or contact fatigue between two materials in a gear application. The mechanism for crack initiation and propagation is not related to fracture toughness, but the ability of a material to absorb locally high surface stresses without generating low cycle fatigue cracks.

 

[ibf]

Understood "metengr". That having been said, and by knowing that anisotropy of mechanical properties is a fact , which test would you recommend in order to highlight the greater likelihood of a casting to pitting when compared against a forged / rolled component (gear rim) ? Maybe we do not need to develop any testing just because castings will always present micro discontinuities inherent to the manufacturing process. Do you agree or is there such a test ?

Thanks.

 

[Carburize]

Ivo - interesting question!
I don't think fracture toughness is the property that will differentiate for many of the reasons metengr highlights. The key would be to identify the features in both structures which are the initiation sites for the pits and then work from there. In wrought product I would suspect that the initiation sites would be non-metallic inclusions whereas in the cast product they could be larger crack-like discontinuities.

 

[ibf]

Carburize: Fortunately, nowadays we can source both some good castings as well as some good forged & rolled plates. However, as I have mentioned before in other threads of this forum, open gears for the mining industry is an extremely agressive environment and contact fatigue has already been the root cause for tooth breakage. Tooth breakage failure modes are:

- Static overload.
- Sudden Fracture.
- Fatigue failure.

To make a long story short, I was just trying to find out what would be a reasonable / reliable test able to differentiate(under similar circunstances:same hardness range , same alloy , similar cleanliness of metal but different form though) how prone would I be to failure due to the inherent characteristics of each manufacturing process - castings ( with the usual microporosities) vs. wrought (with the usual directionality in grain structure as mentioned by metengr).

 

[Carburize]

Ivo

If you have any contacts at Timken or SKF they may know of work that has been done in this area on bearings

 

[ibf]

I want to thank you all for the inputs. Yes, I will try Timken & S.K.F.

 

[arunmrao]

ibf,
If you have more information on this please share it us. This is an issue which easily passes the blame on one or the other manufacturer when premature failure occurs or there is an early wear out( as in the case of cast sprockets.) I have personally stopped processing cast gears and sprockets.

Steel castings Handbook by ASM p 2-33 states that "Cast steel is exceptionally wellsuited for cut-tooth gears. Unlike wrought steel products,it does not possess "lines of flow". In cases where such lines run parallel to he periphery,potential cleavage planes extend transversely across the teeth and are focal points of possible tooth breakage."

I will be eagerly waiting to know more on this.

 

[ibf]

Sure, I will be glad to share my findings with this forum. Hopefully, we will be able to come up with something useful and interesting not only for gear designers. Likewise, I would appreciate if any of you find further info on that matter.

Thank you all.

 

[TVP]

metengr,

I respectfully disagree with your statement that "The mechanism for crack initiation and propagation is not related to fracture toughness, but the ability of a material to absorb locally high surface stresses without generating low cycle fatigue cracks". I understand that you were explaining the reasons why the Charpy impact data were so different between the forging and the casting, but fracture toughness is definitely related to crack initiation, whether it is high stress/high plastic strain (low cycle fatigue) or low stress/low plastic strain (high cycle fatigue). In fact, pitting resistance of gears is often related to the presence of inclusions. One reference on the subject is "The Role of Near-Surface Inclusions in the Pitting of Gears" by T. M. Clarke, et al, Tribology Transactions Vol. 28, no. 1, pp. 111-116, Jan. 1985.

ibf,

A fracture mechanics approach may be beneficial for your projecct, especially if you measure fatigue crack propagation in addition to quasi-static fracture toughness testing. Prof. Glodez from the University of Maribor has done some analytical modeling of pitting on gears using fracture mechanics, and Blake and Cheng from Vanderbilt also used stress intensity calculations for developing a life prediction model of gear pitting (including the effect of surface roughness and inclusions). You can find these references by searching Cambridge Scientific Abstracts or Elsevier's Scirus.

 

[ibf]

EXCELLENT TVP ! I WILL DO IT IMMEDIATELY.

THANK YOU.

 

[metengr]

TVP;
Point well taken. It is sometimes difficult to "discuss" thru email a complex subject like this. I think I became distracted in responding to the issue of wrought versus cast products in relation to Charpy impact versus KIc, which would be useful in terms of crack propagation from a pre-existing flaw (inclusion or casting defect, sub surface).

You get a star.

 

[TVP]

Thank you very much for the star...

I think we all understand the difficulty, which is why it is so nice to have such a good group of people that contribute frequently, that can elucidate other points, provide complimentary data, experience, etc. Perhaps I am biased, but I think that this forum, Metal and Metallurgy engineering, is by far the best one here at Eng-Tips. Maybe I'll lose my stars for saying so....

 

[CdotS]

ibf

You have said in your original posting the following:
"I have already compared Charpy - V test values at ambient temperature for both versions an came up with 40 ft-lbs for the forging (liquid quenched & tempered) and 15 ft-lbs for the casting version (normalized & tempered) !!"

1. Did you do Charpy tests for wrought and casting under the same heat treatment condition - (1) Queched+tempered and (2) normalized?
2. what is "normalized and tempered"? Why did you do tempering after normalizing?
3. If you have frequent failures, you may want to consider suitable surface treatments such as carburizing or similar which will prolong its fatigue life. The best test would be to test for contact fatigue will be do a contact fatigue tests of individual teeth of the actual gear. It uses a rod that repeatedly applies laod to the tooth in a machine such as Instron (in a fatigue mode). This has yielded useful results for carburized gears for automotive applications.
4. Pitting resistance depends on stress, speed, lubrication and surface conditions. Contact stress at the local contacting points is a function of applied load, geometry of the tooth of the gear, geometry of the countergear, lubrication, etc.
5. Castings always have cast structure i.e. dendrtitic grains. They have soldification shrinkages as well. Inclusions of sand and other impurities are difficult to avoid. You may want to consider forged gears where the grain flow is optimized for gear loading. This is different from machining a gear out of a wrought bar where the grain lines are cut, making the teeth weaker, leading to tooth breakage.
Hope this is helpful.

 

[ibf]

Hello Cdots; here you go:

1.& 2. The foundry cannot liquid quench our gears (4 segmented gears - 38 ft in diameter). Instead, they have increased molly (4340 modified)in order to get martensite upon normalizing & tempering at the controlling section of the gear (1.2 times the tooth height). F.Y.I : face width is ~ 1,000 mm and rim thickness is ~ 200 mm. The Charpy samples for the forged plates were removed from the three - dimensional center of the plate and for the casting from a standard cast-on coupon as per ASTM. As you can see , even with a coupon coming from a more unfavorable location, the coupon of the forging gave us a much higher Charpy figure than the one coming from a casting (by the way , the coupon coming from the forging was checked on the "Z" through - thickness direction.

3. Carburizing is not possible for such a size of a casting or forging - each segment is ~ 50 metric tons in weight and 7,000 mm long.

4. & 5. Agree and understand ; however, as mentioned before I need to go an "extra mile" in order to suffice the matter (sizes of these gears are kind of unique).

Thank you very much for your comments and inputs though.

 

[diamondjim]

As well as looking at the best material
are you using long and short addendum
gears to increase the arc of recess which
would help also to reduce the pitting?
What is the material of the mating pinion
and how is it manufactured, heat treated,
etc.? Isn't pitting a form of micro welding
between the parts due to high contact
stresses? If so, some research indicates
having different hardness levels of the two
parts helps reduce this. I assume the higher
nickel steels also help to reduce pitting.
Is the pitting concentrated near the pitch
line?

 

[ibf]

Pinions are forged 17CrNiMo6 - Nr, carburized + Q & T to 58/60 Rc (pinion shaft receives normalization heat treatment only). Pinion teeth are also shot peened to 61/62 Rc.Gear teeth are 310 / 350 BHN (regardless whether casting or rolled & forged plates).

 

[CdotS]

Ibf, I did not know the size of the gear you are dealing with - 38 ft in diameter! Wow!. Now I get the picture.

 

[ibf]

Isn`t it CHALLENGING ?

I am already contacting the names suggested by TVP and will keep you guys posted on my findings (if any ??).

 

[TVP]

lbf,

Pitting fatigue is also highly dependent upon surface hardness and surface roughness. There have been a number of studies performed to document this, several of which are included below:

http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2000/TM-2000-210044.html

http://amptiac.alionscience.com/pdf/AMPQ7_1ART01.pdf

http://www.scirus.com/srsapp/search?q=JSAE+Review+gear+pitting+fatigue&ds=jnl&ds=web&g=s&t=all