massive carbides in 17-7PH stainless steel 1

[metldude]

A detailed failure analysis of a 0.063" thick by 0.7" wide 17-7PH (Condition TH1050) flat torsion spring revealed two contributing factors: (1) scattered massive TiC particles that fractured, probably during cold reduction, leaving splits in the adjacent sound material; and (2) nearly continuous linear bands of ferrite, especially at the mid-thickness.
So my question: Is there any existing specification for 17-7PH that limits or even addresses the number or frequency or sizes of carbides?

 

[TVP]

I am not aware of any consensus standard (SAE, ASTM, ISO, etc.) that addresses the number or frequency or size of carbides in a precipitation hardening grade of stainless steel such as 17-7 (Type 631). This type of detail is usually agreed upon in a customer standard that is unique to the application. For example, large spring vendor Torsion Spring Co. has their own internal standard that generally references SAE, DIN, etc. standards, but then specifically addresses microstructure requirements, variation in tensile properties in a coil, etc. You will need to work with your material vendor in order to address this issue.

 

[EdStainless]

In my working with strip material, this is not a common situation. Was this material re-heattreated?
As you will see in another post, ferrite in 17-7 is an ongoing issue. The carbides are a different story. The make me suspicios of the thermomechanical history of this material.

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[unclesyd]

Tagging on to EdStainless'spost we had a incident where some 17/7 parts inadvertently cycled 24/7 for 2+ months in a pyrolysis furnace. The temperature cycles as followes; charged at RT heated to 670F held for 10-12 hrs then raised to 900F-925F for 6-8 hrs. then back to RT.
Though we use a similar furnace to clean PH parts the time at temperature per heat cycle in this furnace was 3+ times as long as a normal cleaning cycle.
When the parts were recovered they were put back into routine service. Two of the four components failed, threads stripping, during the initial assembly. The other two components were loss in the system. There is a possibility the two others might be 17/4 or 455.

Metallurgical examination of one failed component revealed very large carbides. The failure mode was quasi-cleavage originated from some surface breaking carbides. There were some other carbides with incipient cracking.

 

[EdStainless]

Knowing the kinetics, I would guess that there is some residual primary MC in every piece of 17-7. Long term exposure at 900F would sure be enough for these to grow.

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

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

 

[mighoser]

metldude,

Do you know which mill the material came from? We are doing an ongoing failure analysis investigation for .032" 17-7 in the TH1050 condition. We had high carbon content on one such failure. Anything else you care to share? We are dealing with a high hardness issue as well (parts measured as high as HRC 47).

 

[Matlsguy]

metldude,

Since Ti is an excellent carbide former I would guess that every 17-7 spring will have TiC carbides. If the TiC carbides were round and not elongated through the grain boundaries you shouldn't have abnormal mechanical issues. Remember its better that you form TiC carbides rather than Cr23C6 because we all know what that leads to. The continuous linear bands of ferrite are what troubles me are the delta ferrite stringers you are seeing. These phases form during solidification due to segregation of alloying elements such as chrome, which extends the delta ferrite field to lower temperatures. This phase is known to be problem because its britte and forms elongated phases. heat treatment can help dissolve this phase.

Regards

Matlsguy