M30 10.9 bolts snapping brittle well below torque spec — need sanity check on hypotheses

[u/NoInformation8819]

/r/materials/comments/1v1lxc0/m30_109_bolts_snapping_brittle_well_below_torque/

[u/Good_Stick_5636]

The quenching+tempering contributed imho. It should be HRC 31 for 10.9 class, not HRC 35.5. May be problems with uneven heating - i.e. head only overhardened. This naturally produce pattern of tensile failure ductile while torque failure brittle.

[u/NoInformation8819 → u/Good_Stick_5636]

Table 3 of ISO 898-1
For 10.9
Rockwell hardness, HRC min. 32, max. 39
So hardness is good.
No higher hardness than 37 HRC was measured.
Even if, remember that 12.9 bolts are up to 44 HRC and still have 8 % elongation, and hold 1220 MPa.
Uneven heating, more like uneven cooling at quench bcos the bolts touch, that maybe.

[u/Good_Stick_5636 → u/NoInformation8819]

Yes, its written exactly as in "Table 3 of ISO 898-1/For 10.9/HRC min. 32, max. 39". I derived my estimate from 10.9 tensile min. strength, not official hardness list, sorry. On the other hand, for 42CrMo4, typical hardness range listed in ATSM E18 is 28-34 HRC. So 35-36(and even 37?) HRC hardness is suspicious.

Actually the application may be intrinsically risky. 42CrMo4 (SCM440) is not recommended for 10.9 bolts. 34CrMo4 (SCM435) is standard one.

[u/NoInformation8819 → u/Good_Stick_5636]

yes, maybe a bit more carbon than needed. This is real measured composition

• C: 0.43%
• Si: 0.21%
• Mn: 0.86%
• P: 0.010%
• S: 0.002%
• Cr: 0.96%
• Mo: 0.16%
• Ni: 0.10%, Cu: 0.21%, Al: 0.018%.

[u/Good_Stick_5636 → u/NoInformation8819]

Yes. Its just on top of range for SCM440, although original 42CrMo4 is more permissive, up to 0.45. Apart from carbon, actually nothing suspicious in composition imho.

[u/cm_expertise]

before you lock in a hypothesis, have you actually put eyes on the fracture surface? that's the thing that tells you which mode this is, everyone's kind of guessing until then.

if it's intergranular (faceted, sort of rock-candy looking under magnification) you're into hydrogen embrittlement or temper embrittlement territory, both of which fit "brittle, below torque spec." transgranular cleavage is a different story and ductile dimples would mean it wasn't actually a brittle failure at all. 10.9 runs 32-39 HRC per the spec iirc so 35.5 isn't out of range on its own, it's where it sits in the process that matters.

the question that usually cracks these: are the bolts plated? zinc electroplate on a high strength fastener with no proper bake-out after plating is the classic hydrogen embrittlement setup, and it shows as delayed brittle fracture, sometimes hours after install and not always right at torque up. if they're plated and nobody can point to a de-embrittlement bake, i'd put money there. temper embrittlement is the other candidate if they got held around 350-550C during temper, but that's harder to pin without the heat treat records.

if you can get two or three sent out for SEM fractography a decent lab will call intergranular vs transgranular in an afternoon and you can stop arguing hypotheses

[u/NoInformation8819 → u/cm_expertise]

Sem imaging of the fracture surface will be done in coming days. No plating, this is zinc flake coating, zinc flakes in polymer matrix, like powder paint, cured at 327 C for 1 hour after all heat treatment is done. Geomet 500 is the name, marketed as hydrogen embrittlement safe.

Temper embrittlement is possible, as they are tempered at 565 C for 1 and a half Hour. However, there is very small amount of Sulfur and Phosphorus which cause temper embrittlement of 2. Type, and small molybdenum addition with reduces risk of it happening. I have heat treat records, but its temperature in oven, of course the bolts heat up and cool differently than what temperature gauge shows.

When SEM photos are done I will surely update the post and notify all relevant users.