welding CA6NM

[Neweld]

Customer wants to fill cavatation in a CA6NM casting with an austenitic stainless steel filler metal to avoid preheat abd post weld heat treatment costs and time.

The question is, what will happen?

 

[21121956]

Hello everybody:

The ASTM CA-6NM is a martensitic stainless steel (generally CrNi 13.4).

The process of damage repair by welding of this material, can introduce stresses and deformations (warping) of the repaired areas with no good results for the components under rehabilitation.

The martensitic steels are tempered on air so; it conduces to limitations of the weldability that can result in flaws and cracks of the welded areas.

An especial technique for welding martensitic steels is the "buttering" which consists in the application of an overlay of austenitic steel (normally wire or stick E309 or E309L).

For the areas subject to minimum or medium stress, where the pitting is around 10% of the base material thickness; when the buttering has been accomplished, one can proceed to build up overlays with austenitic steel wire or stick (normally E308) as if the repaired area was originally of austenitic steel.

For that areas under high stress and where the pitting is greater than 10% of the base material thickness, it can be appropriate to weld (after the "buttering") with a martensitic wire or stick steel (E410 NiMo for instances) because this filler material has very similar physical and chemical properties than that of the ASTM CA-6NM stainless steel.

When this process is selected, preheating and post weld heat treatment are required of the piece under reparation.




El que no puede andar, se sienta.

 

[stanweld]

Type 309 welding filler metals are often used to make repairs of small flaws in CA6NM casting uncovered during machining - especially after near to final machining.

 

[EdStainless]

It really depends on the state of the existing material. If it is not a real high carbon and has been heat treated to a lower strength level then all that is needed may be to re-temper the part after welding.

If you can weld a layer of 309 and then reheat treat, you can then go back and weld on to the 309 without additional heat treatment.
This is usually not a real good idea.

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Plymouth Tube

 

[Neweld]

Big Thanks to 21121956, stanweld and EdStainless!

Customer is still adamant, NO PWHT! This thing is monstrous and significant heat induced distortion would be a disaster.

It is a four blade impeller, each blade the size of a single-car garage door with the cavatation area the size of a dinner plate roughly in the center of the convex side of each blade. The thickness in this area is guessed to be about 1/2 inch.

Preparation to bright shiny metal with a carbide tool bit and a "butter" layer of 1/8 inch dia. 309L SMAW electrode applied at a 200°F preheat using stringer bead technique should get us going.

We can do interpass (not interlayer) peening manually or pneumatically hoping to provide local interpass stress relief. The heat input from the following pass should take care of any workhardening due to peening. We are just trying to avoid accumulated stresses due to weld shrinkage and subsequent distortion or cracking.

Once the "butter" pass is down we can fill with 1/8 inch dia. 309L stick electrode to within two passes of the suface using the regimen described above while maintaining a local interpass temperature range of 300°F - 500°F.

We plan on using 1/8 inch dia. 410 Ni Mo-15 SMAW electrode to complete the remaining filler passes and cap while using the accepted "temper bead" technique. In so doing we should wind up with something close to parent metal surface performance in the eroded areas.

We can rotate welding from one impeller to the next to minimize heat buildup and still keep the job moving ahead.

Please critique...

neweld

 

[metengr]

I would be against using 309 for this repair. The issue is with the CA-6NM base material heat affected zone associated with the depth of repair locations. Not to be funny but you can use coat hanger weld rod and still not effect the base metal heat affected zone hardness. Using 309 for the initial weld layers and go to 410 Ni-Mo would not be wise.

What I would do is use a low preheat of 250 deg F. This is not that difficult to do with several rosebud torches. Use 410 Ni-Mo and with stringer beads with sufficient overlap use the temper bead approach and allow the weld region to cool to below 212 deg F. This is important to allow fresh martensite to form and become tempered during subsequent weld passes. The 410 Ni-Mo will match the CA 6NM in service and is very ductile material. Make sure you blend grind the repair areas and perform a Liquid Penetrant examination to ensure no defects in the weld region.

 

[blacksmith37]

Scanning ASTM STP 756 (a seminar on SS castings in 1980; Melilli editor), it appears you have a problem. Several presentations evaluated welding on CA6NM : they nearly all used matching filler metal ,BUT all went with some PWHT ( although a few "as welded" hardnesses are listed).
Could someone suggest only grinding out the cavitation pitting and not welding? This has been done on the tips of ship propellers, at least as a temporary fix.

 

[metengr]

As a follow-up, you really need to qualify a welding procedure using the above technique. Worst case scenario is that if the weld repair is not done properly, you will have either cracks surrounding the weld repair region within the CA-6NM base material causing the weld repair to break out in service OR you crack and liberate a section of the impeller in service. Not good scenarios.

 

[Neweld]

Thanks blacksmith37 and metengr!!

We have gotten by the PWHT issue BUT they are balking at the thought of qualifying a repair procedure...

Years ago I was doing a lot of die welding and am familiar with the welding of mostly high carbon and/or high carbon equivalency alloys re martensite transformantion (Mt). I should have reviewed my notes on the welding of type 410 SS. It just never occured to me... duh...

The "temper bead" technique as key.

metengr, two more questions. Is there a hold time at 212°F to allow for transformation of martensite? I assume this temp is a max, is there a less-than range here or can we just let it get down there and then manually preheat to 250°F and proceed?

Thanks again,

neweld

 

[metengr]

neweld;

metengr, two more questions. Is there a hold time at 212°F to allow for transformation of martensite?

After the initial application of preheat you weld the first pass and allow the temperature of the weld region to drop to below 212 deg F before you weld the second pass. You can eliminate the preheat requirement after welding has started. However, if you stop for the night and continue the next day, use the preheat again before welding.

I assume this temp is a max, is there a less-than range here or can we just let it get down there and then manually preheat to 250°F and proceed?

The preheat requirement I provided is a minimum and is to be used only prior to welding. Once you begin welding and continue no need for preheat, just allow the weld pass to cool to 212 deg F.

My comment about using a welding procedure is so that you understand and demonstrate proper implementation of the temper bead method. This is a technique that requires proper skill level and demonstration by the welder.

 

[Guest102023]

First off, base metal:
Hardness and hardenability of Ca-6NM is quite unique among the martensitic SSs. It is EXTREMELY sensitive to carbon content (you can't ever get carbon that is 'too low' in this alloy). So it would be useful to take a chemistry (in the repair area, since it is a large, i.e., non-homogeneous casting).

Filler metals:
Austenitic SS is perfectly acceptable and commonly done in the field, but it will not eliminate the need for preheat. If you do plan to PWHT, you should use use matching filler (generically, 410NiMo). If not, 308 is preferred over 309 due to superior cavitation resistance. There is a specialty alloy developed by R. Simoneau and others at IREQ which is marketed by Stoody as Hydroloy HQ913 (an expen$ive magic bullet!). Remember that differential alloys will set up a galvanic couple that could enhance cavitation in future.

Preheat:
That will depend on the hydrogen potential of the process. 200F for a low-H process, 250-300F for higher. Heat a wide area (more for mitigation of distortion), and use post-heat of 300F minimum for 2 hours minimum. Strictly follow low moisture practice. NEVER exceed 500F interpass.

Technique:
As mentioned by metengr, a temper bead technique, even a crude one, is beneficial for the HAZ, but also for the weld metal if 410NiMo is used (typically, with matching filler, the weld metal is at greatest risk for hydrogen cracking). The fundamental priciple is: every bead deposited directly onto the CA-6NM base metal should be subsequently tempered by another bead in the following layer. Hold weaving and bead/1st layer thickness to a minimum. SMAW electrode diameter 5/32" max; FCAW/GMAW wire diameter 0.045" max.

Distortion:
The extent of welding described here is more likely to cause permanent distortion than a properly done PWHT. (What is the section thickness?) Austenitic fillers will be slightly worse than martensitic in this respect due to their higher thermal expansion and greater yield strength at high temperature. As always, keep overwelding to an absolute minimum.

Concerning the 212F question: there is no problem getting essentially full martensite transformation at this temperature. Therefore it is not necessary to cool down to room temperature before further heat treatment (unlike P-91, with which this alloy otherwise has several metallurgical parallels).

Neweld:
Whatever you learned about 410SS probably does not apply to CA-6NM. Especially (and crucially) the maximum interpass temperature. Which is why the choice of '410NiMo' as the designation for filler metals was an unfortunate one.


I cut my teeth fighting with heavy-section CA-6NM, until I learned to work with it and finally to love and appreciate it