Unusual macrostructure of casting 4

[coreman73]

I am doing a failure analysis of a Spanner wrench. Specifically, the hook component has fractured and was returned by the customer. There is no information available explaining how the tool was being used when it failed.

The hook was cast from AISI 4140 grade steel modified to have a Silicon content of 0.70 wt%. Heat treating was carried out to achieve the required range of 37-45 HRC.

Test results so far:
1. chemical composition meets requirements of 4140 modified steel
2. microhardness testing showed surface hardness ~44-45 HRC to a depth of 0.003" before consistently indicating ~46-47 HRC all the way through to the core
3. dendritic microstructure that is highly inhomogeneous
4. no suspicious internal porosity was detected near the fracture site

I have attached some photos that should better explain what I'm looking at. My question is related to the macrostructure of this hook. I have never seen something quite like this and am wondering what might have caused it and if it's possible this could have contributed to failure. From what I can tell, it definitely did (see Fig. 2b) since the "dividing line" takes place directly at one side of fracture location.

I want to mention that measuring microhardness in the "light" and "dark" etched areas showed no difference. The microstructure also appears basically the same at 500x.

I would appreciate any advice regarding the macrostructure along with what might have contributed to failure (besides high bulk hardness).

 

[swall]

Are these investment cast? If so, the lack of control over thermal gradients that is typical of the process could account for the large dendrites at the fracture zone.

 

[coreman73]

swall,

Yes, these are investment cast. I have worked with quite a few investment casted samples before and have never come across such appearance.

Could the larger dendrites at the fracture zone acted as a stress riser contributing to failure?

 

[blacksmith37]

It sure smells like a casting. I formerly worked at an investment foundry that made very similar parts. We perfomed a "carbon restoration" cycle in an appropriate atmosphere and this gave a "Normalize" HT prior to the Q&T cycle. I recall a much finer grain. And the only failures were at porosites.
Or: I think the better toughness of a finer grain would have helped; also uniform hardness from carbon restoration and careful finish of the radius at the pin would help.

 

[metengr]

I would appreciate any advice regarding the macrostructure along with what might have contributed to failure (besides high bulk hardness).

Agree with the above posts, and the fracture surface looks ductile in appearance, probably a tensile overload event.

 

[arunmrao]

Unlike ,sand castings,investment cast parts pose a problem in maintaining proper thermal gradients during solidification. Normally such parts are then given a normalizing treatment to reduce segregation and remove inhomogeneities,while refining grain size. There are no porosities seen in the picture. In extreme cases, I have seen some of the badly produced pieces breaking when accidentally dropped.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[coreman73]

I really appreciate the input to all who posted.