Welding CA6NM

Date 04-11-2005 16:47

Bill,

I would stay with ther 410NiMo rod with a 400-500 F preheat, especially if no post weld tempering will be done. You could use a tempering bead technique to put the HAZ in a little better condition than as-welded.

By Bill Mc (**)

Date 04-11-2005 16:53

I am not sure the self lubricated bronze bushings in the near vicinity will survive the 500 deg. preheat. If they fail from the heating....I am in deep kaka.

By MBSims

Date 04-12-2005 01:40

Shallow kaka is not bad, but it's good to avoid the deep stuff. Depending on what type of bronze this is, you may get away with 300-400 F. I think this may be similar to what you have:

http://www.matweb.com/search/SpecificMaterial.asp?bassnum=MCUDCO

If so, you should be ok below 599 F, but it's good to have some room for error and to allow for heatup during welding. In any case, you'll want to check the bushing dimensions before and after welding, and clean them up as needed if they do change. May be helpful to put a piece of solid stock inside the bushings to maintain dimensional control during the repair. Distortion due to weld shrinkage may turn out to be a bigger factor than preheat temperature if the crack is very near the bushings. If it's that critical, it may be best to have the repair performed by a shop capable of replacing the bushings or plan on having it sent out after welding if it does distort too much.

By Bill Mc (**)

Date 04-12-2005 03:18

It is hard to explain in this limited space. There is no way to take a bushing out, or measure a bushing, without major dis-assembly.

Option 1: Fix it as installed the unit
Option 2: Take the unit apart, and take the failed gate out and either replace it, or effect a shop weld repair.

Customer would like to wait to do option 2 in the summer when the water is lower since it requires extensive amount of down time to do. Due to the approaching high water soon this is not an option due to time to complete.

Investigating possibility of option #1 now. We cannot start option #1 and fail...and be forced to do option #2 before summer. (If that happens ...where's that forum for posting resumes?).

I checked the carbon content of the gate = .06%-.07%. Isn't the purpose of the pre-heat for crack prevention rather than HAZ hardness?

(and thanks for the suggestion on the temper bead Marty)

By MBSims

Date 04-12-2005 04:11

What style of gate is this?

By Bill Mc (**)

Date 04-12-2005 21:38

a Wicket gate

By - Date 04-12-2005 20:16

As MBSims said, 410NiMo is the preferred filler metal for welding CA-6NM. but lacks good toughness in the "as welded" condition. A 309 is often used to weld martensitics to themselves and to other types of stainless steel to provide good toughness in the "as welded" condition. Without a PWHT, you must remember that the untempered HAZ will be hard and brrittle regardless of the filler metal used.

CM

By Bill Mc (**)

Date 04-13-2005 01:33

Right now the base metal is 36-38Rc

So are you recommending 309 "as welded"?

By MBSims

Date 04-13-2005 01:52

The HAZ isn't going to know whether you used 309 or 410NiMo. I think going with 309 with a temper bead technique is your best choice to get them through to the next teardown. There won't be a thermal expansion mismatch to deal with for this application and the 309 will have more ductility to accomodate weld shrinkage than the 410NiMo (40% elongation vs. 15%).

36-38 HRC is quite high for this material, it should be around 27 HRC. I suspect it may not have been heat treated properly at the time of manufacture.

I've been waiting for GRoberts to weigh in on this one. Where are you?

By Bill Mc (**)

Date 04-13-2005 15:16

I think you are right about the high hardness...and probably an OEM processing problem. I am also thinking that is the reason for the cracking failure. I can also find local areas in the low 40's to as high as 44Rc. The other gates without any cracks do not have that high of hardness.

By GRoberts (***)

Date 04-14-2005 02:15

I'd been reading this thread, but never when I had time to respond. Thanks for thinking of me. I don't think the responses so far have been too far off though.

The part in question does seem on the hard side for a wicket gate, but CA6NM can be tempered at lower temperatures than normal to reach quite high strengths. Most of the wicket gates I have experience with were ASTM A487 Gr. CA6NM Class A, which is 110KSI minimum tensile strength- much softer than the 36-38HRC. Also, if the carbon content of the gate is realy .07%, the material would actually be out of specification, as the maximum carbon for CA6NM is .06% (hence the "6" in the material designation.)

Back to the rapiar though. We have welded thousands of pounds of 410NiMo filler meatal on CA6NM (sometimes per month), but have never done it without PWHT. (Because all of our welding is in the shop, so there is never any reason you wouldn't PWHT.) I would have to agree with the responses so far though in several aspects regarding filler metal. E410NiMo will not have very good thoughness without PWHT, so E309, I think, is the better choice for that reason- as long as the lower strenght is not a concern. It will also help the base metal as well. Since it is lower strength, and more ductile, it will not place as much stress on the brittle HAZ that will be formed during welding. The additinal beneifit, is that since 309 is austinitic, it will not diffuse near as much hydrogen into the HAZ, and thus reduce the risk of underbead cracking. The temper bead suggestion is also a good one.

As for preheat, about half of the CA6NM welded at our shop is done without preheat, believe it or not. The material is fairly tough, and it does not normally cause a problem. The other half is for the US military, and they specify a 200F preheat. In your case, I would reccomend at least a 200F preheat for a couple of reasons. Without PWHT, any extra preheat will help reduce the residual stresses on the weld, and with a through crack on a wicket gate, you are probably taking about a fairly thick section. Also it may help reduce the hardness of the HAZ sligtly.

I also found an additional link that sheds some light on the subject.
http://www.usbr.gov/power/data/fist/fist2_5/vol2-5.pdf

Good luck with your repair.

By - Date 04-14-2005 04:17

Without a subcritical PWHT temperature range, the untempered HAZ will be hard and brittle regardless of the filler metal employed. The martensitic transformation temperature ranges and air-hardening characteristics of these steels are sufficiently high that preheating at 600F (316C) or below has minimal effects on the hardness of the HAZ or weld metal.

CM

By GRoberts (***)

Date 04-15-2005 14:57

That's why I said slightly. Preheat to retard the cooling of the HAZ may aid in autotempering of some of the first martensite to form during cooling. (Tha last marteniste to form in this material is obviously at too low of a temperature to be autotempered) However, I will say that I have not seen any data on this material that says how much at what preheats, but some (preheat) is better than none.

By - Date 04-15-2005 21:05

The recommended preheat for Martensitic steels depend on the carbon content. However, when welding casting alloy CA-6NM using a 410NiMo, preheat is not normally required, but a subcritical PWHT is recommended. I realize that the author stated that no PWHT is possible, so that is why a 309 is probably his best bet. A recommended technique to help temper the HAZ is to use a high preheat and interpass temperature, and a very slow cooling rate through the martensitic transformation down to about 250F.
Under "normal" conditions, Martensitics steels with a carbon content <0.05 recommends a peheat of 250F minimum, with a PWHT "optional". A carbon content of 0.05-0.15 recommends a preheat of 400F minimum, with a PWHT "recommended". A carbon content of >0.15 recommends a preheat of 600F minimum, with a PWHT "necessary".

Chuck

By GRoberts (***)

Date 04-15-2005 22:56

Those guidelines seem good for most applications. The main exceptions are the nickel containing martensitics that are significantly tougher than the regular martensitics. The CA6NM- generally considered a "soft-martensitic" was one of the precursors to these "Super-martensitic" stainless steels that contain nickel, and are considerably tougher than the martensitics most people have experience with. The PWHT is definitely recommended for the CA6NM whenever possible, but the new super-martensitics are designed to be welded without PWHT. They are coming up with amazing things all the time!

By - Date 04-15-2005 23:40

I think we are pretty much saying the same thing here, and agree that a PWHT is employed "whenever possible", but when welding CA-6NM, and using a 410NiMo filler, Specifications of NACE (National Association of Corrosion Engineers) often require the harndess not to exceed 22 Rockwell C to avoid cracking in service. To meet this requirement, a double intercritical and subcritical [1250F and 1140F (677C and 616C)] temperiing is employed. Actually, the CA-6NM is the highest Ni containing Martensitic alloy with 3.5-4.5% Ni. Yep, technology is changing daily. Bottom line....309 would be my first choice when adhering to the conditions set forth by the author.

Chuck

By MBSims

Date 04-14-2005 04:47

I would venture neither the reduced strength of the 309 weld nor the HAZ microstructure will be an issue in the near term if the gate has been operating with a 20 in. long through-wall crack on a part that is 30 in. high. The 309 weld is light years better than no weld at all in this case, even if it is not the "optimum" fix.

By - Date 04-14-2005 12:11

Absolutely... Could not have been said better. Thanks..

Chuck

By - Date 04-14-2005 04:09

Again, based upon the scenario you have presented us, a straight grade 309 should be your filler of choice.

Chuck

By Bill Mc (**)

Date 04-14-2005 14:35

Gentlemen...
As always, your expert assistance is appreciated! Thank you.

By - Date 04-14-2005 14:54

All,
Knowledge not shared is knowledge wasted.

Chuck

By rfazio1951 Date 05-09-2006 16:04

Listen to GRoberts . . . some of the other advice is questionable.

Recommendations for preheat of 400 or 500°F apply to OTHER martensitic SS -- CA6NM is exceptional in several respects. The differences are mainly due to extreme suppression of the Ms and Mf temperatures by nickel addition. This gives CA6NM the unusual property of being air hardening to virtually unlimited thickness. A desirable feature of the casting alloy and the matching filler is the very narrow freezing range, especially relative to 410.

Excessive preheat and interpass will lead to massive untempered martensite structures that are vulnerable to hydrogen cracking. The weld metal (if 410NiMo is used) is usually the most vulnerable. The mechanical properties of this alloy are EXTREMELY sensitive to carbon content, and 0.07% carbon, as well as the reported hardness, is very high. The original casting could not have been correctly processed.

At the end of the day . . . weld with 309, since the repair is temporary. Stresses on a wicket gate are not extreme, and I suspect the original defect was related to the original manufacture; i.e., casting defects. If that is the case, there is little reason to subsequently remove the 309 deposit. Good luck.

By - Date 05-09-2006 21:26

"Recommendations of preheat of 400 or 500F apply to OTHER martensitics...CA6NM is exceptional in several respects." The CA6NM is listed a having a P# of 6. The ASME Boiler and Pressure Vessel Code recommends a MINIMUM of preheat of 400F for those materials listed as P-6 is Section IX. This includes the CA6NM. The primary, or one of the primary, reasons for using a 410NiMo filler metal to weld CA6NM is to produce weld deposits with no ferrite in the as-welded condition. To further get information on the required heating and cooling rates, one only has to refer to the Construction Code section of the ASME Boiler and Pressure Vessel Code Book.