Large Crankshaft Repair 1

[redpicker]

This is a large crankshaft, about 14 inch main journals. A single throw of about 18 inches (or would that be twice that, 36 inches?). About 15 feet long. It drives a very large mechanical press forge. The problem is that it has developed cracks in the throws and one of the key-ways failed and it peeled about an inch of material off the OD in that region. Examination of the material that peeled off showed that it had had a previous weld repair on the OD and key-way that failed. Hardness testing in the HAZ of the repair revealed a hardness of 41 HRC.

The plan is to build-up the missing material and arc-gouge to remove existing cracks and weld repair. The repair facility did not plan on any stress relief after welding, but on seeing the hard HAZ, we recommended they post-heat. They recommended a 650 deg F.

I ran a chemistry on the material that peeled off
C .36
Mn .67
P .028
S .038
Si .27
Ni 1.80
Cr .76
Mo .38

I believe it was manufacured in the USA in the 1950's.

The material that peeled off had a hardness of around 22 to 24 HRC.

The plan is to repair with an E70 flux-cored wire. I recommended a 1200 deg F Stress relief (after seeing the base material chemistry).

Is this the best weld metal to use? There is about an inch or two that will have to be built-up, then re-machined. Someone suggested a "D2" wire, but I am unfamilar with this. I would expect the weld would need to be 20 to 24 HRC after post heat.

Thanks for any input.

 

[metengr]

redpicker;
Very interesting. What is the chemical composition of the crankshaft (away from the failed region - I presume that the chemistry you have reported on the peeled section could be weld metal, in lieu of actual base metal)? Also, what is the hardness of the crankshaft away from the previous weld repair?

The answers to these two questions will help to select the appropriate weld filler metal (because you do have several options), preheat and post weld heat treatment requirements.

In addition, have you performed any NDT (wet fluorescent MT) to assure all defects have been accounted for on the crankshaft?

 

[Pressed]

If it were me, first thing after it is removed from the press is to make a detailed print of the shaft. Then just get a new one made. Unless your seriously strapped for time, you will be better off in the long run to replace it. Cracks never go away on press cranks.

Companies that forge/machine shafts that size are not real busy, currently. You could also find a machine shop that would do the reverse engineering and machining. Enprotech and Press are two shops that come to mind.

 

[andyenergy]

Local repairs on something like this will tend to introduce bending. If it doesn't bend during local welding it might bend during stress relief. It might also bend during machining to remove the damaged layer in a circumferential band if it was hot spotted.

Several options as metengr says - proceed with caution and have the thing properly welding engineered if that is the route you take. Otherwise think of getting a new one then you wont be worrying about it every its operating...

 

[metengr]

Just a follow-up, and something to think about regarding weld repair;

Assuming that the reported chemistry is indeed the crankshaft forging base metal, this would imply a material close to AISI Type 4340 alloy steel. This is a high hardenability alloy steel that needs to be treated with extreme caution for welding. Frankly, I would not arc gouge unless it is done with a preheat of 450 deg F. You are better off either machining the previous weld repair and cracks or local grinding.

I checked the carbon equivalent based on the reported chemistry, this value is close to 0.82. This indicates a high preheat is required to avoid delayed cracking in the base metal HAZ, and to reduce the hardness of the HAZ during welding and before PWHT.

The presence of cracks in the corners of the crankshaft sends up a red flag that this is fatigue crack propagation. Considering what you are up against, I would agree that a replacement crankshaft is a wise decision. Weld repair can be performed to get this thing back into service. However, long term solution is replacement.

To deal with weld repair, this will require very controlled conditions of preheat and bead placement to keep distortion to a minimum. The preheat for these repair(s) will be 450 deg F minimum. I would maintain preheat for at least 30 minutes at temperature before welding.

Selection of filler metal for this repair E71T-12.

A local weld repair area should be balanced with a second weld area of the same size, 180 degrees opposite on the crankshaft outer diameter to attempt to balance residual stresses from welding. Another option is to machine a band 360 degrees around the entire circumference of the defect containing areas and weld restore around the entire circumference.

The post weld heat treatment should be performed in the vertical position to keep distortion to a minimum. The 1200 deg F PWHT is acceptable. The time at temperature will be dictated by the weld deposit thickness NOT the base metal thickness.

After PWHT, I would perform a wet fluorescent MT, machine and final NDT. To reduce susceptibility to fatigue crack initiation, I would consider shot peening all weld repaired areas on the crankshaft.

 

[andyenergy]

would concur with evrything that metengr states , bead overlap would be very important. Would machine the circumferential band as a preference and rotate the shaft while welding.

 

[redpicker]

OK, metengr, to answer your questions.

One, this is the largest piece of forging equipment we have; restoring it to service quickly is critical.

Yes, this is the chemistry of the base material, away from the weld. I was suprised to see the 4340 chemistry as I didn't expect that. Although not reported above, the aluminum content was 0.003 (V and Ti were also below 0.005); that is no strong deoxidizers, which makes me believe it is an open-hearth steel (along with the S and P levels).

I had done Rockwell hardness on the base material and got 22-24 HRC. I also performed a Brinell and got 255 HBW.

We regularly weld 4100 modified steels (modified for higher hardenability), so I appreciate the need for preheat. Unfortunately, the repair facility does not want to use more than 350 deg F. Since the amount of welding is so great, I suspect that will be OK. I hope so, anyway.

I didn't like the idea of air-arc to remove the cracks, but it was basically do that or nothing. They were supposed to grind and dye-pen after gouging. All NDT has been dye-penetrant.

Yes, we believe these to be fatigue cracks. I know the failed keyway is fatigue. The machine was sold to us about 10 years ago as "completely refurbushed" which aparently included weld repair of the crank. It is obvious that this repair did not include a stress relief. I suspect, but cannot verify, that the new cracks initiated in the HAZ of previous weld repairs.

They are using E70T-4 wire for the repair. They are about half-way done welding. They will machine after stress relief, so they don't think distortion will be that bad of a problem. This is all being done about 1000 miles away, so I am very limited as to what information I can get and even have even less ability to control how they do it.

Anyway, thanks for the input. Will the E70T-4 wire be OK? I guess the options are
1) just finish with the E70T-4,
2) stop using the E70T-4 and begin using E71T-12 over what is already there
3) remove all the E70T they have laid down and replace E71T-12

 

[metengr]

I love it, this is one of those critical jobs which has already started and questions are asked well into the job. Anyway, the E70T-4 filler will work. I had recommended the higher strength version to reduce the strength mismatch berwene the weld and base metal. Make sure you have all of the repair data (PWHT charts) and have a final NDT using wet fluorescent MT not PT performed by an ASNT qualified organization.

Get yourself a replacement forging.

 

[redpicker]

Thanks, again, metengr

I had already recommended the SR cycle and that we perform wet-mag (using our own Level II inspectors) on receipt before installation. In addition, I recommended they re-locate the key-way 180 degrees so that it is machined in original base material, not weld metal. Suprisingly, this had not been considered.

I also recommended getting started on having a new one made, but I doubt that recommendation will receive "management approval".

 

[GRoberts]

I am a little surprised that someone would use E70T-4 for a large repair like this. E70T-4 is a self-shielding electrode that is not known for its toughness. It may give the shop a high deposition rate, but I would think the integrity of the repair would be the most important factor. One manufacturer of the electrode doesn't even reccommend that the 5/64" or smaller electrode be used on steel greater than 1/2" thick. Others reccommend theirs for heavy equipment repair, but I don't think it is the best choice. If FCAW is used, E80T1-Ni1 or Ni2 is commonly used for large equipment repairs sucessfully. It has much higher toughness and can be low hydrogen. The other problem to be considered is PWHT. Since E70T-4 is normally used for field welding (due to self-shielded characteristics), I would be surprised if the manufacturers have any data regaring properties after PWHT. While PWHT may be necessary due to the HAZ of the base metal, the PWHT could severly embrittle the weld metal, or loose a lot of strength (or it may not, but little information is available on the subject). At the minimum I would do a test on the exact same (mfgr and brand name) E70T-4 electrode that the repair facility used to see how the electrode responds to PWHT.