Stellite Hardfacing - When is PWHT Req'd? 2

[tc7]

I've been looking over many of our WPS for hardfacing our valve seats, discs, plugs, etc. All are steam service applications. Typically we'll apply Stellite #6 on a chrome alloy base, GTAW process, ~.040" thk. PWHT has always been specified. However, I have recently found WPS's in file expressly for repairing cracks/indications of Stellite seats and these do not require PWHT.
Question #1 - Why is PWHT required on new work and not on a Stellite repair?
Question #2 - I have also noticed that the WPS for Stellite repair calls for a preheat of 700 deg F & interpass temp of ~800 deg F whereas the WPS for new work is only calling for preheat of 400 deg F & interpass temps of ~600 deg F. Does this make sense and explaination is appreciated.

Thanks if you can help.
T

 

[metengr]

We have many valves in high temperature steam service that have been installed using Stellite 6 with repairs to existing Stellite 6, using Stellite 21.

For low alloy steel valve parts, the preheat and post weld heat treatment requirements are dictated by the base metal composition. In some applications, we have used Inconel as a butter layer and deposited Stellite 6 on top of this butter layer to reduce susceptibility to cracking, during and after hardfacing. I have also used slightly increased preheat, and tight stringer beads to deposit Stellite 6 directly on top of low alloy steel.

Here are my responses to your questions, based on experience;

Question #1 - Why is PWHT required on new work and not on a Stellite repair?

PWHT should only be specified if welding is performed directly on base material that requires PWHT. PWHT is specified by most Codes and Standards to reduce susceptibility to cracking and to restore ductility and toughness in the base metal heat affected zone.

In my opinion, trying to weld repair an existing Stellite overlay is risky at best. I have seen maintenance personnel chasing their tails in trying to weld repair cracked Stellite seats that have been in service for years. The best repair approach is to remove the Stellite 6 by machining and start over.

Question #2 - I have also noticed that the WPS for Stellite repair calls for a preheat of 700 deg F & interpass temp of ~800 deg F whereas the WPS for new work is only calling for preheat of 400 deg F & interpass temps of ~600 deg F. Does this make sense and explanation is appreciated.

No, this makes no sense. Again, weld repairs to Stellite 6 hardfacing should not really be performed. If one has to perform weld repairs to an existing Stellite 6 hardfacing, your best approach is to use Stellite 21. This is a lower hardness version of Stellite 6 and is more forgiving (less crack sensitive).

 

[EdStainless]

I will second Metengr. We fought this issue for a long time on some mechanical parts we were making. We ended up with:
-standard preheat and interpass for the base alloy (usually 400F/600F)
-625 base layer overlay
-PWHT
-Stellite 21 overlay

We have issues with Stellite 6 (our parts were not very rigid) and so we stayed with the slightly softer material.
If we needed to repair we didn't remove all of the hardfacing, just all of the damaged areas. If we had to grind deaper than the 625 layer we pitched the parts.

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Rust never sleeps
Neither should your protection

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[tc7]

Thanks Gents,
So now if I wanted to follow your ideas about buttering with Inconel or 625 (is inconnel = 625 ??) do I need to have that expressly spelled out in the WPS? Meaning requalifying my revised WPS?? I think I know what your answer will be but please advise.

Also for Ed, you mentioned that you would attempt repair if the damage was NOT deeper than the 625 layer, and then you listed the PWHT.....would your PWHT be exactly same for repair as it would for complete new hardcoat?

Thankyou again.

 

[EdStainless]

Inconel is a trade name for a family of Ni based alloys.
625 is a grade name. If it is made by Special Metals it is called "Inconel 625".

We didn't PWHT after the hardfacing, only after the 625 overlay procedure. This will depend on your base alloy, you may be able to get away with this.
We did not re-PWHT after repair, becuase we never went deeper than our 625 interface layer. We were working on rather thin parts.

Metengr, your comments?

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[metengr]

Agreed, Ed.

do I need to have that expressly spelled out in the WPS? Meaning requalifying my revised WPS?? I think I know what your answer will be but please advise.

Yes.

 

[tc7]

One last question, if you don't mind: how are you sure that heat from the overlay or cladding repair (even if you haven't excavated the defects below the 625 layer) did not affect the base metal temper so as not to worry about PWHT?

 

[metengr]

This is a good question. This is why you qualify a weld procedure with the buttering, perform PWHT and then deposit the Stellite weld overlay to assure you have adequate ductility and strength in the base metal.

Normally, the heat affected zone from GTAW can extend 1/8" past the fusion zone. So, depending on the thickness of the butter layer, you can determine how deep you can go before the heat from welding affects the base metal properties. If you excavate into the butter layer, weld repairs will adversely effect the base metal properties possibly forcing you into another PWHT. The objective is to make sure you don't excavate into the butter layer.

 

[EdStainless]

In alloys with low hardenability even rather thin butter layers will suffice. And of course, if you are dealing with thin hardfacing layers it minimized the risks also.

It should be straight forward to overlay a set of flat coupons and see which process works best for you. Simple bend tests or impacts will tell you everything that you need to know before you actually start to qualify a procedure.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[tc7]

I spoke with our lead welders today who were familiar with the Stellite 6 repairs. One recalled the only way he could accomplish a sound weld which didn't crack afterwards was by turning up the preheat and interpass to the 700 & 800 deg F. They also have tried the Stellite 21 on repairs with NO success and didn't even want to discuss it as an option!

Can you guys take a swag as to what role the the high preheat and interpass temps may have had in the success of their Stellite 6 repairs?

Thanks.

 

[metengr]

tc7;
Stellite 6 has little ductility, meaning it can crack easily under tensile stresses (thermal or mechanical). So, increasing the preheat and interpass will result in more heat to the substrate and hopefully less thermal stress (or gradients).