Identify source of cracking

[amyh33]

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!

 

[Metalguy]

Sounds like you have untempered martensite from the laser hardening (unless you temper afterwards). That is a crack waiting to happen.

 

[amyh33]

Thank you Metalguy. Another question, if I may...

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?

 

[shooter45]

To answer for Metalguy - I would do the temper immediately after laser hardening.

 

[Metalguy]

Agree with Shooter45, temper as soon as possible. Without knowing what kind of steel you have it's tough to recommend a temperature, but for some tool steels even a low temp. of around 400 deg. F is OK. Time should be about 1 hr. per inch of thickness, at temp. If a tool steel or other high-alloy steel, consider a double temper to help get rid of retained austenite. Cryo treatment seems to help in this regard.

 

[Frederick]

It sounds to me as if you are creating tensile residual stresses in your surface during either grinding or heat treating or both. Grinding burn is probably the most predominent cause of tool wear, as it is very easy to induce very high temperatures under a grinding wheel. Be advised that it is not necessary to turn the surface blue to have grinding burn. Try etching the surface before and after grinding and heat treating to see which (if either) is the culprit.


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?

 

[amyh33]

Thank you all for your inputs. To answer some questions posed on further details:

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?

 

[Metalguy]

Double tempering is simply performing the tempering step twice. The reason is that many alloy steels have "retained austenite" in the hardened zone, and it is generally undesireable because it is quite soft. The other problem with it is that it can transform sometime later, and it will transform to untempered martensite. While tempered mart. is strong and not ordinarily brittle, untempered mart. is usually avoided at all cost. However, the second temper can safely be performed later-in fact, the later the better before the parts are shipped/used.

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.

 

[unclesyd]

Why did you change materials? Addition of nickel is most helpful in heat treating, especially case hardening, of an alloy steel. It allows a greater latitude in a heat treating procedures, time, temperature, and quench. It also enhances nearly all the physical properties, fatigue resistance, toughness, plasticity, etc. It most important in any air-hardening steel.

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.