Do We Need Hipped TIG Rods? How About Polished Rods?

Date 06-01-2007 10:36

A manufacturers data sheet for hardfacing TIG rods offer a "hipped" or "densified" version which is described as "rods with removed centerline porosity". Our sales rep could not add anymore information than what was published in the data sheet. I did a web search, but could find nothing on hipped rods - is this a new innovative product? or is it a gimmick that nobody needs? Can anyone please advise with knowledge on hipped rods - will they result in less porosity or cracking in the hardfacing application? My interest is in use for new and replacement steam valve seats. Typically we use 1/8 & 5/32 rods.

The same manufacturer offers "lanished" rods which was described to me as polished rods where the surface oxide and bamboo texture is removed from each rod. This got me thinking that oxides on the bare rod could introduce contaminants to the application that might affect final application quality and could be worth the extra cost. I am just guessing on this but very anxious for ideas of you all. Thankyou.

By Lawrence (*****)

Date 06-01-2007 11:20

Interesting claim Tom,

I am curious to know how one would "Hip" (Hot Isostatic Press) a piece of bare wire......

I guess maybe they could cast each individual piece of filler???? sounds spendy.

I've worked with plenty of "Hipped" inconel 718 on PW200 and PW400 compressor cases and we requred "MC grade" filler wire. (metellurgically controlled)

I would also like to see an explaination of the difference between densification and Hipping.

Good thread. I hope somebody knows something.

By Tommyjoking (****)

Date 06-01-2007 11:39 Edited 06-01-2007 11:57

gotta love the internet! omg there is a patent (applied) for this....

A filler-metal weld rod of a filler-metal composition is prepared by providing a mass of metallic powders, mixing the metallic powders with a temporary thermoplastic binder to form an injection-moldable mixture, and thereafter injection molding the injection-moldable mixture at an injection-molding temperature above the thermoplastic temperature of the thermoplastic binder to form an injection-molded rod. Any excess thermoplastic binder is removed from the injection-molded rod, and the injection-molded rod is thereafter sintered to form a filler-metal weld rod, with the temporary thermoplastic binder removed in the step of sintering.

I don't think that is actually the same thing as "Hipping"

Here is the link if you want to read it all: http://www.freepatentsonline.com/20060024190.html

shows how little I know...I always figured most fillers where "drawn" not cast.

By DaveBoyer (*****)

Date 06-02-2007 03:00

This process is called "Metal Injection Molding", but I didn't know it was used for welding rod. The process can make resonably acurate parts that require little to no machining with a wide range of mechanical properties. Whooda thunkit ?

By PhilThomas (**)

Date 06-01-2007 11:35

With the exception of aerospace rods (and I exclude this only because I don't feel sufficiently involved in that area to provide advice), there are very few times when you need anything but the standard cast rod for hardfacing with cobalt.

During continuous casting of the rods, the rod stops momentarily about every 1" as it passes through the casting die. This is what creates the "bamboo look" - also called the witness marks. In between these marks, there can be a small centerline shrinkage void.

For the valve seat applications, there is no need to spend extra money on HIP rods nor on ground rods. These valves are hardfaced all the time with the standard rods. The oxide layer on the outside will melt along with the rod during application. The old military spec for these rods required them to be ground finish. This was probably due to the older glass casting method for the rods which had an unfortunate tendency to leave glass embedded in the surface of the rods (this method is best described as sucking molten cobalt into a glass straw, letting it cool, and then then breaking the glass tube away). Other than parting you from a couple dollars per pound of your hard-earned money, ground finishes on continuous cast rod has no benefit.

The small shrinkage void is also not a problem in valve seat overlays. The nature of welding these alloys (which aren't very fluid at the melting point) will make you pause long enough to let the puddle catch up to the torch. Just be sure to focus on the leading edge of the puddle to ensure it doesn't have any voids before moving along and you'll be fine.

The one area of traditional hardfacing that has always called for the HIP rods (or densified) is saw tipping.