Fatigue and Manganese Sulfide Stringers 3

[HJM]

A big, forged crankshaft was metallographically inspected. On the polished and etched surface near a notch radius at the tap, segregations of manganese sulfide (MnS) were found. The largest inclusions were between 100µm and 300µm wide. The grain size was irregular, indicating that the forging reduction had been low, at least locally.
The consequences of a fatigue failure would be very severe, so a high safety factor should be applied.


There was some concern on the impact of the observed segregations on the fatigue life. During service life, the component is exposed to at least 1 Gigacycle, so also small cracks could eventually grow and lead to failure.

I’m slightly confused after looking at the literature:

1) In the ASM Handbook, Volume 1, Properties and Selection: Irons, Steels, and High Performance Alloys, I found:
“From the minor elements, phosphorus and sulfur are the most common and are always present in the composition. They can be as high as 0.15% for low-quality iron and are considerably less for high-quality iron, such as ductile iron or compacted graphite iron. The effect of sulfur must be balanced by the effect of manganese. Without manganese in the iron, undesired iron sulfide (FeS) will form at grain boundaries. If the sulfur content is balanced by manganese, manganese sulfide (MnS) will form, ***which is harmless*** because it is distributed within the grains. The optimum ratio between manganese and sulfur for an FeS-free structure and maximum amount of ferrite is:
% Mn = 1.7(% S) + 0.15”

2) In the ASM Handbook, Volume 19, Fatigue and Fracture, it was confirmed that cracks larger than about 0.1mm can be treated with LEFM, and therefore are dangerous. (see e.g. Kitagawa-Takahashi-diagram).

3) After the forging process, the MnS precipitates are not distributed homogenously, therefore the potential for fatigue cracking seems to be is increased, especially since we must assume that the local stress (3D) also could act locally in the “worst” direction.

4) Till now, I did not find ***relevant experimental*** data on MnS and fatigue life.

Questions
********
- Is the statement of the ASM-HB valid for fatigue, too?
- Are there some experimental data on the influence of MnS on the fatigue performance, especially at high numbers of cycles?
- Shouldn’t any defect of a size beyond the Kitagawa-Takahashi-limit be treated as LEFM-cracks, or does it depend of the type of inclusions, too?

I would be very glad to hear comments on my questions!



Customers specification
-----------------------------
Heat treatment:
Forging reduction: min. 3.0
Normalized
Stress relieved.

Mechanical Data required:
Yield Strength min. 340 MPa
U.T.S: 590-740MPa
Ductility: 20% parallel, 14% perpendicular
Impact test (ISO-U-Notch): min. 20J/15J

Composition
C = max. 0.50%
Si = max. 0.40%
P = max. 0.035%,
S = max. 0.030%
Mn = 0.40% to 1.40%

whereas according to Equation 1 only 0.20% Manganese is needed.

 

[Metalguy]

MnS inclusions can and frequently are the initiation sites of fatigue cracks.

I think the part in your para. 1 about them being harmless applies to hot-working. FeS causes hot cracking (hot short).

Keep in mind that any non-metallic inclusion, such as MnS, in effect IS a tiny crack already. Lowering the S is the way to go-look at bearing steels.

 

[kenvlach]

HJM,
I am not qualified to comment on the fatigue model, but I can comment on the steel quality aspects (composition, MnS size and distribution). References are given at the end.

1) First, the composition does not correspond to any alloy that I recognize. This is not in itself a material problem. Here are comparable alloys, all of which I believe are capable of meeting your mechanical requirements:

comparable “Carbon steel & High-manganese carbon steels"
Applicable to semifinished products for forging, hot-rolled and cold-finished bars, wire rods, and seamless tubing
Designation Cast or heat chemical ranges and limits, %
UNSNo. SAE-AISINo. C Mn P max S max
G15410 1541 0.36-0.44 1.35-1.65 0.040 0.050
G10440 1044 0.43-0.50 0.30-0.60 0.040 0.050
G10450 1045 0.43-0.50 0.60-0.90 0.040 0.050
G10460 1046 0.43-0.50 0.70-1.00 0.040 0.050
G15480 1548 0.44-0.52 1.10-1.40 0.040 0.050
G10490 1049 0.46-0.53 0.60-0.90 0.040 0.050"
While silicon varies, I’d expect 0.2-0.3 or 0.20-0.35%.

“Silicon is on of the principal deoxidizers used in steelmaking. The amount of this element in a steel, which is not always noted in the chemical composition specifications, depends on the deoxidation practice specified for the product. Rimmed and capped steels contain minimal silicon, usually less than 0.05%. Fully killed steels usually contain 0.15 to 0.30% silicon for deoxidation; if other deoxidants are used, the amount of silicon in the steel may be reduced.”

From the extremely wide Mn range and the [Si] limit of 0.40 wt%, I think it likely that the steelmaker has great latitude in using recycled material, and that the [Si] limit allows for deoxidation of melts from 0.3% and 0.35 % [Si] scrap. These limits also allow for sloppy steelmaking.

2) Second, you wondered whether the Mn is too high since it is more than sufficient for S tie-up as MnS. (Although we don’t know the actual content of this alloy). It is common to use excess Mn to ensure S tie-up. E.g., in free-maching steels, where the numbers are more significant, Mn:S ratios of 5:1 to 12:1 are common.

Also, the higher Mn content in your case may be there for better hardenability and to improve hot working, although the unusually wide composition range suggests that this is not the case (0.40% limit is too low).
For high manganese carbon steels (typically, 1.10-1.4 % Mn), the carbon level is chosen according to the strength desired, and the manganese content for this carbon level determines the microstructure for given cooling conditions

“Manganese is normally present in all commercial steels. It is important in the manufacture of steel because it deoxidizes the melt and facilitates hot working of the steel by reducing the susceptibility to hot shortness. Manganese also combines with sulfur to form manganese sulfide stringers, which improve the machinability of steel. It contributes to strength and hardness, but to a lesser degree than does carbon; the amount of increase depends on the carbon content. Manganese has a strong effect on increasing the hardenability of a steel.
Manganese has less of a tendency toward macrosegregation than any of the common elements. Steels with more than 0.60% Mn cannot be readily rimmed. Manganese is beneficial to surface quality in all carbon ranges (with the exception of extremely low-carbon rimmed steels.”

3) Re MnS sizes and distribution. I think a 300 micron inclusion is excessive, even allowing for elongation by x3 during forging. In my opinion, this is a sign of poor quality control during casting, i.e., slow solidification with excessive segregation. You are far better off with a greater number of dispersed, fine precipitates, as would happen if MnS precipitated from solid solution rather than from the melt.

I’m not sure what you mean by “After the forging process, the MnS precipitates are not distributed homogenously.”
If the distribution was homogeneous to begin with, the shape of the MnS inclusions will change (elongate, etc.) in the same manner as the adjoining steel grains [some MnS break-up is expected, possibly little for x3 deformation]. The distribution may be changing in an absolute sense, but not relative to the grain of the steel.
Perhaps, you mean that the non-uniform MnS distribution was only discovered after forging? If so, indicates segregation during solidification.

4) I think your company needs to buy better billet for forging, as your present steel seems inadequate. Below are some qualities usually specified (in addition to composition, of course):

“Carbon steels
Semifinished for forging
· Forging quality
o Special hardenability
o Special internal soundness
o Nonmetallic inclusion requirement
o Special surface”

 

[kenvlach]

References for above post:
[ASM] Metals Handbook Desk Edition, 2nd Edition (online),
‘The Making, Shaping and Treating of Steel,’ 9th Edn., U.S. Steel (1971):
chap. 41, “Carbon Steels” pp. 1117-1128,
chap. 51, “Machinability…” pp. 1275-1294
and MatWeb (online).

 

[TVP]

HJM,

Metalguy is correct-- the statement from ASM HANDBOOK Volume 1 is referring to the effect of MnS on hot rolling, not on fatigue strength. There is a considerable amount of information on MnS in ASM HANDBOOK Volume 19, which you apparently have, so I would suggest reviewing it.

One question for you-- Do you really mean that the width of the inclusions is between 100µm and 300µm? That is entirely too large for the width, which should be smaller than the nominal grain size (on the order of 1µm not 100µm). If you really are seeing inclusions that wide, then you have a significant problem with your steel. If fatigue strength is a concern, then I highly recommend you reduce the allowable sulfur content to something more like 0.010 max.

 

[Metalguy]

Hope I get a star!<g>

 

[HJM]

Thank you all for your fast and most clarifying help!
What I indeed had overlooked, is the fact that MnS is regarded to be harmless in CAST materials, which is much easier to understand.

I was especially glad to read the detailed comments by kenvlach on the composition and his assessment of the (poor) quality if the material used by our customer, I fully agree with his conclusions.

I was well aware that in fracture mechanics, any brittle defect is to be treated as a crack or a precrack, and we did find MnS-stringers up to 0.3mm wide (aspect about 3:1). The customer was not too alarmed, since this was a single observation and the customer’s engineers were using the popular formula for critical crack size:

dKth = ds*sqr(pi*a(crit)), which points to a reasonable value for dKth.
(ds = 160MPa at R=0, acrit=0.0003m, so dKth = 5MPa·m^0.5, which sound reasonable).
And finally also the manufacturer was happy with this interpretation.
(Personally I don’t like the formula too much since it only serves as a rule of thumb, taken into account that we normally see a multiaxial stress state leading to mixed mode cracking at R-values different from 0 or -1. Some engineers use the results sometimes directly in a calculation of strength). Here comes the problem of predicting the growth of near threshold cracks at high numbers of cycles.

I’m glad to learn that the MnS often is observed to be the initiation site of fatigue cracks, as I suspected, but still have no experimental data from very high cycle fatige.
-> Are there some data from either laboratory tests or from failure reports demonstrating this for high numbers of cracks?

True, the problem could easily be solved by setting sulfur down to 0.01%, but some people put the optimum nearer the minimum of cost than to the maximum safety!
A practical reason for allowing defects as large as 0.3mm is the fact that the customer agreed the min. detectable defect size to 1mm, so there seemed to be no point in asking for a pure steel. But this is a historical reason; he could still keep the detectable defect size and in parallel use a cleaner steel.
We certainly will recommend (also based on your helpful comments) our customer to do that together with some improvements of the design.

 

[Metalguy]

I had to perform a failure analysis some years ago on a big diesel con rod. Engine was 7,000+ HP, V20, and used a master/slave rod arrangement. Machinist had bored the hole for the slave-rod pin in the master rod a bit oversize. Rather than scrap it, the manuf. decided to use a soft iron plating to restore the size. They violated their own procedures by plating right into an oil hole (stress-conc.)

2 days before Xmas, nearing the end of our 100 hr. full load test, the fatigue crack which started at the oil hole neared the edges of the rod-had cracked nearly 90% of the way thru that area of the rod before it let go.

Shrapnel all over the room-both rods, both pistons and a bolt-on counterweight went thru the side. My job-figure out why. Didn't take long after I got a call from their metallurgist telling me what happened! They had saved maybe $15k, cost us millions.

Lesson-even a &quot;soft&quot; plating can start fatigue cracks, if the plating has huge columnar grains which look like a forest of trees standing on the surface.

 

[kenvlach]

Metalguy,
That's pretty interesting.
Did the soft plating deform under load, allowing 'play' in the joint? (and maybe, work hardened the iron?)

 

[Metalguy]

No. It apparently was the columnar grain structure which allowed stresses to concentrate thru the plating thickness (by cracking between grains), and the cracks kept right on going into the base metal-even tho it took a huge fatigue crack-nearly 6&quot; long (thickness of rod measured parallel to crankshaft axis)) or so-to finally reach the point of overload fracture.

This one was REAL interesting. When the parts exited the crankcase they severed control-air lines and the air-intake shut. The engine kept right on running, although at ~ half-speed (300 rpm) and no load. Kept running for 20 minutes. Fire dept. came and sprayed water mist/foam all over the holes in the crankcase, and it finally quit. The large counterweight which got sheared off by one of the rods on the way out kept things reasonably balanced without the 2 rods. The crank rod-journal had a wound a few mm deep, but we were able to save the crank (reground in place) and metal-stitched the crankcase back together-with enough internal steel to make it stronger than new.

This rod problem ended up causing many other problems in various parts of the diesel-generator, including thrown windings on some gen. poles from the sudden slow-down and desychronization from the electrical grid. Also had other problems which weren't found until much later. One of the problems involved another &quot;saved&quot; main journal-saved because a different machinist had machined it a little too small, so they plasma-sprayed it back to size. Ran OK until the rod failure, but on the long awaited restart it ran too hot with a new bearing. I ended up lying on my back in a few inches of oil as the mechanics rotated the crank (slowly, using air-motor) so I could look at the whole journal. Con rod JUST cleared my chest as it went around-not my idea of fun, but it paid the bills!