Identify source of cracking
Identify source of cracking
(OP)
The application is a laser hardened steel corrugated roll that is cracking and peeling a small layer of material at the top of the flute - slightly off center. Locations of failures do not appear to follow any geometric pattern, but each crack is identical. The failures are occurring during the chrome plating process in the plating bath. Cracks appear to start across the top of the flute perpendicular to roll length (and perpendicular to grinding direction). After a depth of approximately 1-2 mm, the crack bifurcates to travel in the direction of the roll length. It is evident that the material fracture is occurring early in the plating process as the fracture surfaces are chrome plated. It is curious that all the failures are the same configuration and path. some are worse than others and all seem to be oriented off center. To explain: If you consider the profile of a flute being a side profile of a hill, the material is peeling off on the "down-side" of the hill consistently- just slightly off center if the hill top is center.
At first hydrogen embrittlement was suspected, but since the cracks are propagating in the plating bath instead of afterwards and also the consistent configuration and dimensions of the crack growth makes me believe it is related somehow to processing.
Flutes are rough ground, laser hardened to Rc 58-60, then finish ground prior to plating. The initial cracks perpendicular to the flute length (and grinding direction) are typical of grinding cracks - but typically, these blunt in the gullets rather than split to travel along the flute length.
Any guidance on clues to look for in the microstructures and fracture surfaces would be appreciated!
At first hydrogen embrittlement was suspected, but since the cracks are propagating in the plating bath instead of afterwards and also the consistent configuration and dimensions of the crack growth makes me believe it is related somehow to processing.
Flutes are rough ground, laser hardened to Rc 58-60, then finish ground prior to plating. The initial cracks perpendicular to the flute length (and grinding direction) are typical of grinding cracks - but typically, these blunt in the gullets rather than split to travel along the flute length.
Any guidance on clues to look for in the microstructures and fracture surfaces would be appreciated!





RE: Identify source of cracking
RE: Identify source of cracking
If the surface is rough ground to dimension, then laser hardened, then finished ground to final dimension - at what point should the tempering be done? Before or after finish grinding?
Also, any recommendations on temp/time to relieve the untempered martensite?
RE: Identify source of cracking
RE: Identify source of cracking
RE: Identify source of cracking
Grinding burn is created by excessive heat in the area just under the grinding wheel. My research indicates that there is no coolant getting in there. It is concievable that you get a similar problem with laser hardening, although I don't know enough about your process to have an opinion. Question: Why use laser hardening?
RE: Identify source of cracking
Steel composition is
.51% C
1.01% Cr
.93% Mn
.23% Si
.23% Mo
.09% Cu
.013% V
.012% P
.001% S
Steel is forged then heat treated 8 hrs at 850 F prior to intial grinding.
I am not sure of the history/reasoning behind using laser hardening. Competitors in this industry use induction hardening to achieve same hardness level. Good results were achieved with laser hardening until a recent shift to eliminate about 2% nickel in the base steel composition. No other processing parameters were changed with this composition change; which I think is a contributing factor in this failure.
Metalguy - what would be involved in a "double temper" - before and after finish grinding? or is that a temperature cycle. Can you provide more insight on the cryo comment too?
RE: Identify source of cracking
The double temper is to temper any of the new mart. that was produced during/after cooling down from the 1st temper. Tool steels are commonly treated this way. Cryo treatments work by forcing even more retained aust. (if any) to transform, and must be followed by another temper (low temps. tend to force the transformation).
But if your parts are big, cryo treatment is likely to cost a lot-it's used mostly on small items.
RE: Identify source of cracking
The heat treatment you mentioned is somewhat unusual, time at temperature. What temperature are they quenching from?
I've seen incipient cracking in base materials that caused similar problems when heat treating and plating. In most cases a change to a higher nickel alloy resolved the problem.
I think you went the wrong direction in material selection.