Failure analysis of die case (strange surface layer?) 6

[coreman73]

I was wondering if I might be able to get some input from you guys regarding a failure I'm currently working on. I am investigating a die case that has fractured into two pieces. I've attached a .pdf of what I have so far. I've included some short background information as well that will explain a few things (too long to type up here).

Just to summarize my findings to this point:
1. steel chemistry meets spec for H13 hot work tool steel
2. bulk hardness is approximately 49.3 HRC, which is slightly above spec of 46-48 HRC
3. microhardness showed a decarburized layer of roughly 0.003" along ONLY the threaded area of inside diameter
4. failure mode is fatigue with propagation occurring directly through the bottom land of final thread on inside diameter
5. unknown surface layer (looks like some sort of scale/oxide) that has hardness of ~80 HRB found ONLY along the entire threaded area (all other surfaces are ~47-49 HRC)
6. microstructure and grain size are as expected - no issues here
7. no surface damage or deformation anywhere in the vicinity of fracture site (threads included)
8. due to ratchet/beach marks on fracture surface, this defect was already present prior to the die assembly being reworked (explained more in attachment)

I realize this is a lot of information but I would really appreciate some input as I'm a bit stumped right now. The only root cause I can come up with is that failure was initiated due to this strange surface layer along threads, which served as stress riser with the final thread being the weakest site hence failure initiating and propagating here. What could this unknown surface layer be and where might it have come from (manufacturing defect)??

Thank you in advance.

 

[coreman73]

TVP,

The independent lab said the following about their EDS:
"The EDS system we have is made by IXRF and is installed in our SEM. The lightest element we can detect is carbon."

 

[unclesyd]

Just made a call to verify a point that may add little or nothing to the failure investigation.
I called and checked with a real machinist and a fellow who cuts single point threads for a living. The still use high Sulfur cutting oil with taps and dies and I confirmed that it is also used on single point threading for the tougher alloys. This could be your source for the high Sulfur., improperly cleaned parts prior to heat treating.

Maui

Appreciate the information. I haven't seen the initial propagation angle in numerous failures when had in some tool steel pistons with configuration similar to the component depicted in the OP. Our failures were the result of essentially two problems very bad metal and even worse heat treatment and machining and if that wasn't enough, the threaded rod was jacked against the bottom of the bolt hole. These pistons were also subject to thermal cycling.
Here is a link to to an excellant paper "Martensite and Retained Austenite"by "George F. Vander Voort" that brings up another point that is often overlooked in the function of retained austenite and untempered martensite in tool steels. This was a was a real problem in our case with H13 and to a lesser degree with H11, D2, and A2, as we cycled the heavily loaded parts quite often. This one reason I commented about looking at the area as posted above.

Addenda:
When I started work many years ago the company had a man in charge of the heat treating department whose last name was Voort.

http://vacaero.com/Metallography-wi...-Voort/martensite-and-retained-austenite.html

 

[mpoulter]

From my understanding, EDS is unable to give very accurate composition data for low atomic number elements, even if they can be detected. Due to the low ED response, the software tends to overestimate how much of the element is actually present. If I'm remembering that correctly, that's probably why the lab omitted the Oxygen results in the first place. You'll notice that they also omitted the Carbon content from their analysis.

 

[Maui]

Unclesyd, it is rare that you see Stage I fatigue markings in any failure analysis, and I'm not surprised that you haven't run across them before. I appreciate the link for the paper. I'll read it.

EDS has its limitations (as do all testing procedures), and reliable values for oxygen and carbon levels can be difficult to establish using this technique. And due to the manner of scale formation, there will likely be a gradient in the carbon, oxygen, and chromium levels as the EDS scans are taken from the outside surface toward the interior of the scale layer.

Coreman73, you're welcome. The oxidation that was detected on the surface of the threads appears to be high temperature oxidation that was formed during the austenitizing step of the heat treatment cycle. It is very unlikely that this surface oxide is responsible for causing the failure that you observed. The initiation and propagation of the fatigue crack more likely stems from an improperly tempered/undertempered microstructure in combination with the stress concentrations provided by the roots of the screw threads. This is the reason why I suggested taking a closer look at the tempered microstructure.

Maui

 

[gearcutter]

This is a very educational post. Thanks to all participating. I've learned a lot.

On a side note; if case hardening of the threads is not required, it is possible for the threads to be machined post heat treatment.
IMO, unless the pitch of the thread is sufficiently large, I don’t think its sound design practice to carburize case harden any thread.
If your report is to include future recommendations; I would suggest a different way of manufacture to the supplier.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[coreman73]

I just wanted to thank everyone that contributed to this thread. I really appreciate it and learned quite a bit. Since the IGO is an obvious manufacturing defect that absolutely should not be there and actually could negatively affect die case performance, I went with that as the "most likely" root cause. At the very least, it gives the manufacturer something to improve upon.

I did also include that it was very possible that tempering was insufficient based on hardness and structural appearance so will see what the manufacturer can do about that as well. Maui, I was not able to obtain all the information necessary from them regarding HT specifics so couldn't really pursue this avenue further. At least it was documented and can be evaulated later if necessary.

Thanks again to all!

 

[unclesyd]

You might want to look at the literature for H13 and check what temperature will give you the specified hardness and temper at that temperature as tool steel generally closely follow the tempering curve. That will tell you if the manufacturer was sloppy in their QC. It will not tell you any thing about the Austenitizing and Quench which can be messed up very easily.


There has been mention that some retained Austenite could be helpful in some cases. This is true unless you have a very high precision tool steel component with tight tolerance as this part will undergo dimensional growth and take up your tolerances. We have had this happen at different times on every tool steel we use. The old adage was to temper as many times as needed to stabilize you component. In the case of most tool steels you only convert 90% of the Austenite on the first temper and 90% of the remainder each time you temper again. When I first started work the only way to convert the Austenite to an acceptable level was repeat the tempering cycle until the the require level of Austenite was reached. One now has the luxury of a sub-cooling treatment to eliminate the endless tempering cycles as long as you temper at the higher temperature at lest twice. Even if the tool steel isn't required to have the precision you still need to temper at least twice.

 

[Guest102023]

Have looked at the EDS report, and have one question: are the results representative of the entire rectangular region highlighted? Because if so, it will not offer many clues to the composition of the high-temperature corrosion product (which it undoubtedly is). You need to home in on the non-metallic areas.

I have done similar EDS investigations of high temperature corrosion using GR Petrology in Calgary (I have no business connections with them).

I believe unclesyd has suggested a very plausible explanation for the condition, especially if there are no other suspects in sight.