Determining the Fracture Mode: Brittle Fracture vs. Fatigue in a Pump Shaft

[Vahid.A]

Hello,
A broken pump shaft made of 1.4021 stainless steel failed at room temperature. The broken surface looks smooth and shiny, with some sparkles when tilted. Under the microscope, the cracks mostly followed the grain boundaries (intergranular fracture), which usually points to brittle fracture. We initially thought the material might have been weakened (e.g., due to bad heat treatment). But mechanical tests don’t back this up: the impact toughness test gave 32 Joules, which is higher than the required value for this steel.

Why it’s likely not fatigue:
No classic fatigue clues like beach marks, ratchet marks, or fine striations on the surface.
Fatigue cracks usually grow through the grains (transgranular), but here the cracks followed the grain boundaries. While rare, intergranular fatigue can happen in special cases (like high heat), but this failed at normal temperatures.

The confusion:
The grain-boundary cracking suggests brittleness, but the material’s impact toughness is good. What’s going on? Please Let me know your thoughts!

 

[mfgenggear]

OP
Please provide the AISI. Steel type
The heat treat procedure
Was there NDT prior to and after heat treat

I not experience with SEM
But am an with all mention above.
From the broken shafts the appears defects on the surface of the shaft , it appears to have discolored areas. Which could be visible cracks.

May or may not have been caused by heat treat.

 

[EdStainless]

So why aren't you calling those curved lines beach marks?
It sure looks like fatigue to me.

 

[mfgenggear]

Here is a great link

A Mechanic's Story: Basic Component Fatigue | Reliability

We are trying to create an intellectual curiosity within the front lines about 'making the call'...is it Fatigue, or some other fracture pattern?

reliability.com

 

[SJones]

There is an interesting striation at the two o’clock surface that should be investigated further. Depending on the fracture toughness (note: fracture toughness, not Charpy energy) it could be that a short fatigue crack created the conditions for a brittle fracture across the bulk of the cross section.

 

[Vahid.A]

OP
Please provide the AISI. Steel type
The heat treat procedure
Was there NDT prior to and after heat treat

I not experience with SEM
But am an with all mention above.
From the broken shafts the appears defects on the surface of the shaft , it appears to have discolored areas. Which could be visible cracks.

May or may not have been caused by heat treat.

Thank you for participating in this thread. Similar grade in ASTM would be ASTM A276-420. The heat treatment was QT800 (950 0C -1050 0C quenched by forced air and tempered at 650-750 0C) and was tested according to ASTM A388-11.

 

[Vahid.A]

So why aren't you calling those curved lines beach marks?
It sure looks like fatigue to me.

Thank you for the comment. When examined under light and tilted, the fracture surface exhibits a sparkling effect. This characteristic appears uncommon for fatigue-induced failures, as such fractures occasionally display such reflective properties. Additionally, the intergranular morphology of the fracture surface further contrasts with fatigue cracks, which are more typically transgranular. That said, I value your expertise and would appreciate your thoughts on this observation. If possible, could you suggest a reference or resource that addresses this distinction? I would greatly appreciate your guidance.

 

[Vahid.A]

There is an interesting striation at the two o’clock surface that should be investigated further. Depending on the fracture toughness (note: fracture toughness, not Charpy energy) it could be that a short fatigue crack created the conditions for a brittle fracture across the bulk of the cross section.

Thank you for participating in this thread. I assume this is the most probable scenario. I was unable to observe any striations in the SEM images. Could you please elaborate on your observation? Are you referring to the lines with the darker background in the photo (Broken Shaft.jpg) that resemble beach marks? Just want to confirm if I’m interpreting this correctly.