ISO 898 Phosphorus Layer Prohibition 1

[swall]

I am reading a hydrogen embrittlement research paper put out by Republic Steel (it is a reprint of a Kobe paper) and there is a section on phosphate wire rod coatings causing embrittlement in heat treated bolts made from said rod. There is a photomicrograph purporting to show a 12 micron phosphorus rich layer on the surface of the sample. And, the paper cites ISO 898-1-1988 as prohibiting a white phosphorus enriched layer on heat treated bolts.Anyone ever heard of this?

 

[CoryPad]

No, I have never heard of this. ISO 898-1 only has requirements for surface hardness (maximum value and maximum difference between surface and core). I just checked the most recent edition (1999) today.


Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[swall]

Thanks Cory. Figured that you'd know if anyone did.

 

[TVP]

Steve,

CoryPad must have overlooked it, because there is a footnote h) that appears in Table 2 of ISO 898-1 that reads as follows:

h A metallographically detectable white phosphorous enriched layer is not permitted for property class 12.9 on surface subjected to tensile stress.

I have read a couple of different papers that discuss this issue of incomplete phosphate coating removal leading to a white layer and possible embrittlement. One such reference is "Reduction of Hydrogen Embrittlement in High-Strength Precision Fasteners" by Rainer Rink in the February 2000 issue of Fastener Technology International. The following is an excerpt:

"...if any phosphate or sulfur containing lubricant remains on the surfaces of these fasteners, they diffuse into the steel and form delta ferrite, which in turn increases the hydrogen pickup and causes embrittlement. In view of this, the pre-washing before heat treatment is very important."

 

[CoryPad]

Yes, I did overlook it. I have heard of white layer embrittlement, but didn't know it was in ISO 898-1. Since automotive companies (and SAE specifications derived from ISO 898-1) don't use Property Class 12.9, I haven't needed that footnote. Sorry for the error.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[swall]

Thanks TVP and Cory.Very interesting stuff.My interest is not 12.9 fasteners, but phosphated Cr-Si wire rod coiled into springs and subsequently heat treated into the low HRC 50's.A new potential wire rod supplier brought this subject to our attention. I have not seen this white layer on any of our springs made from phosphated coated and lubed wire, but hypothesize that perhaps it is because our (proprietary) heat treat process is very rapid. No time for the phosphorous to diffuse.Another possibililty is that after the springs are shot peened, the white layer is oblitered by the shot peen surface deformation and simply is not detectable.To satisfy my own curiosity, I may do some heat treat experiments on some of the rod, but using an atmosphere furnace with a one hour austenitizing cycle.

 

[unclesyd]

I think we are getting we are expanding the boundary too much. A phosphate coating generally is an iron phosphate either with or without modification with another metal. I think what they are talking about is drag out from the clean the cleaning baths prior to heat treating. A lot of the are polyphosphate based and if the sodium variety they are extremely hard to rinse.
I have encountered this problem twice since I retired on continuos heat treating lines for fasteners, lines where the furnace belts come straight our of the rinse tank to a dryer then to a continuos furnace. If your cleaning and rinsing isn’t set up correctly the problem will manifest itself in the lost or breaking of the high dollar furnace chain.

Nearly all wire and small bar that we used in house had phosphate coating for lubrication when we were cold forming it. A phosphate coating is either used as the lubricant or surface modification to improve the adhesion of additional lubricants to improve the formability and save wear and tear on the tooling.
I have never seen a white phosphorous coating on the samples that I have crossed sectioned and analyzed.

CoryPad the only white layer I've seen on bolting is from decarb. The other white layer is from gaseous nitriding which I don't thing will be seen on bolting. One of these lines made fasteners for the auto industry. They were breaking so many belts that they didn’t get many bolts into the pipeline. Like I stated I never seen this problem affect bolts.