Broken Steel Casting with crystalized structure -- Root Cause 1

[b_unruhly]

Hello,

We have a steel casting that has failed in a new way for me.

In the interior of the casting, there is a very apparant crystal structure. Much more dramatic than the "Rock Candy" fractures that I have studdied. it is also only on the interior of the cross section.

The part failed under tension, as it was being tourqued to spec. (40NM),

We are looking to identfy the root cause and corrective action.

Thank you,

 

[enginesrus]

At first glance that almost looks like inclusions. So why use cast steel for a fastener?

That part should be made from a solid bar not a casting.

 

[ShataCastings]

Firstly, we need to know Crystalline fracture firstly:

1. What is Crystalline fracture?
A crystalline fracture is a type of fracture that occurs in steel when it fails under load without exhibiting significant macroscopic deformation. The fracture surface is generally flat, with a bright gray color, a strong metallic luster, and noticeable crystalline particles. When rotated under strong light, the fracture surface glistens.

2. Is Crystalline fracture normal?
Steel with a crystalline fracture, compared to the same grade of steel with a fibrous fracture, has lower impact toughness at operating temperatures. Additionally, the coarser the grains in the fracture, the more pronounced the characteristics of brittle fracture in the steel.

For steel that is required to be processed into pearlitic microstructure, a crystalline fracture is considered normal. However, for steel that is meant to be processed into sorbitic (fine pearlitic) microstructure, it is regarded as abnormal. Some technical specifications stipulate that the area occupied by crystalline features must not exceed 50% of the total fracture area. If it does exceed this limit, the steel may only be used if the impact toughness at -40 degrees Celsius exceeds a specified value.

Improvement suggestion:
1. Material choose: Was the material be chosen with required mechanical properties?
2. Material inspection: Check the chemical composition, reduce impurity content; ask supplier to do mechanical composition & mechinical property inspection.
3. Modification treatment: Refining the crystalline microstructure of cast ingots is particularly significant for preventing intergranular cracking
4. Optimize casting process: Optimize the casting temperature, speed, cooling to reduce stress.
5. Heat treatment: Properly controlling the heating temperature and holding time

Question, what's the prcocess sequence for this part, casting--rough machining--heat treatment-- precision machining?

 

[b_unruhly]

At first glance that almost looks like inclusions. So why use cast steel for a fastener?

That part should be made from a solid bar not a casting.

The other half of this casting has some more complex geometry for the mating part on the other side.

This could have been designed as a press fit assembly, inclusive of a fastener, but was not.

 

[EdStainless]

Did they cast any test bars of similar size to these fasteners?
They cooled slowly resulting in a coarse grain size.
Combine this with the higher interstical content of cast material and your tensile strengths will be very low.
It really should be a forged stud that threads into the casting and then holds the fastener.

 

[mfgenggear]

Op
Wrong material for a fastener.
Should be wrought or forged.

Wrong mechanical and metalurgical properties.
Torque values vs yield and ultimate. Vs hardness.

 

[b_unruhly]

I keep trying to post replies here and it is not comming through. Fingers crossed this works. Thank you all for your replies

The client has produced this, as a casting for over a decade without issues. Understand bar or forged would be stronger--but there is a big difference in strength between this recent failure and previous production.

We investigated this with the foundry, and they are saying this is because the submitted sample does not have heat treatment.

I do know that makes a big difference in strength--but not sure we would have this kind of break?

What is strange is that the interior of the casting is crystalized but the outside is not.

That makes me think this could have more to do with chemistry (Rock candy fracture) or cooling--as the part cools from outside to inside.

They are doing new samples with heat treatment--we will check after that.

Process sequence-- Casting + Fettling. Machining is done by the client.

New process Sequence -- Casting + Fettling + Heat Treatment.

Material is spec'd as SS400. I'm having trouble finding chemistry for this or an equivelent casting grade to validate the chemistry identified by spectrometer

 

[mfgenggear]

OP
Let's start over.
Please advise what are the requirements.
Let's set a base point.
Material requirement, heat treat requirement. The physical and the elements of the casting.

Hardness required, ultimate and yield tensile required.
Now send out some of failed fasteners to a Matlab once
The requirements are established. The material condition physical yield tensile required for the amount of torque.
Run spectro analysis for the establishing type of material and elements. Tip if the hardness is 40 or higher HRc if it was process with acid etch with H2O
Or similar it will induce hydrogen embrittlement. Which is very brittle and the microstructure will appear glass like. Have it tested.

 

[mfgenggear]

OP
Start a fish chart , and as it is tested remove on the chart what it is not. Since the are small parts as said before running samples at the pour is essential for tensile testing, and Matlab test, for metallurgical.
Pull material certs, heat treat certs, pull and review the actual router traveler and review for irregularities. Not post heating/stressing relief harden material is what causes embrittlement, incorrect heat treat procedure, or not following
Specifications or time and temperature. It the little issues, like cold stabilize or temper as soon as it is quenched. If is required for.

 

[ShataCastings]

I keep trying to post replies here and it is not comming through. Fingers crossed this works. Thank you all for your replies

The client has produced this, as a casting for over a decade without issues. Understand bar or forged would be stronger--but there is a big difference in strength between this recent failure and previous production.

We investigated this with the foundry, and they are saying this is because the submitted sample does not have heat treatment.

I do know that makes a big difference in strength--but not sure we would have this kind of break?

What is strange is that the interior of the casting is crystalized but the outside is not.

That makes me think this could have more to do with chemistry (Rock candy fracture) or cooling--as the part cools from outside to inside.

They are doing new samples with heat treatment--we will check after that.

Process sequence-- Casting + Fettling. Machining is done by the client.

New process Sequence -- Casting + Fettling + Heat Treatment.

Material is spec'd as SS400. I'm having trouble finding chemistry for this or an equivelent casting grade to validate the chemistry identified by spectrometer

It'll be better to machie threads before heat treatment, to reduce stress while machining.

 

[SwinnyGG]

Heat treatment absolutely alters grain size.

Non-heat treated cast steel parts exhibiting very large grain sizes and very low tensile strength should not surprise anyone. The foundry is correct.

So, is this a case where they have been heat treating parts for 50 years and then stopped?

Or did a part sneak past a step in the process because of a process control failure?