Plate Cracking during torch cutting

[tm0284]

A low alloy carbon steel plate was torch cut which resulted in cracking. This plate should of had a ferrite / pearlite structure and was not heat treated in any fashion. The plate passed ultrasound testing. What would cause this cracking to occur? The largest crack extended about 4 inches from the cut area. When looking at a polished cross section under microscopy I believe I might be seeing untempered martensite (but not being an expert on microscopy I cannot say this for certain).

What could cause this cracking to occur? And if it is martensite I am seeing under the microscope, how could it form there?

 

[redpicker]

I am a little confused about the specifics in your post.

First, is the plate a low-alloy steel plate or a carbon steel plate. Carbon steel can be difficult to crack during torch-cutting unless you are dealing with high carbon contents. Low-alloy steel covers a lot of ground; some low-alloy steels are very sensitive to cracking, others not so much. So, if you could clarify the chemical composition of the steel a bit more, it would help our ability to help you.

Once the chemical composition is nailed down, we need to know the heat treat condition. Depending on composition, the plate would need to be in the as-rolled, normalized, normalized and tempered, or hot rolled - stress relived condition. Without knowing the chemical composition, I cannot advise you which heat treat condition would be best for your application.

Finally, the issue of preheat and post-cutting cooling and/or stress relief would need to be addressed. This will depend on the composition as well as the geometry (thickness, width, length).

 

[tbuelna]

Just for my own understanding, is it common to go through the trouble of performing an ultrasound inspection of steel plate and then torch cut it? If the quality of the cut plate is important enough to warrant ultrasonic inspection, why not use a more benign process like water jet instead?

 

[tm0284]

Grade is A572-65. Carbon content around 0.1 and carbon equivalent of about 0.5. It was in the as-rolled condition, no heat treatment or stress relieve of any kind performed on the plate. Thickness of plate is 5 inches.

 

[tm0284]

Any ideas?

 

[metengr]

Consider preheat of 250 deg F to reduce susceptibility to cracking.

 

[kingnero]

you have a large thickness (5"), which aids in rapid cooling.
Ceq. of 0.5 is on the high side.

Both these facts support the suggestion by metengr.


Being regularly involved in welding and cutting, I must say I haven't seen this before (4" cracking after thermal cutting).
The cracking is most likely hydrogen induced. Is there an external source of hydrogen present (humid plate, cutting oil, coolant, grease, paint, rust, ...)?

Could someone enlighten me whether the process of the acetylene and oxygen reaction would offer enough hydrogen to enable HIC ? This interests me, as I have no idea whether this could be a factor or not.

------------------
EDIT:
The values given by Arunmrao (see below) yield a Ceq of 0.54 (IIW formula).
The higher C% also make the cracking more likely.

 

[arunmrao]

Grade is A572-65. Carbon content around 0.1 and carbon equivalent of about 0.5. It was in the as-rolled condition, no heat treatment or stress relieve of any kind performed on the plate. Thickness of plate is 5 inches.

I am confused pleas from the above statement, that the steel has C of 0.1% with Ceq of 0.5. My reference search provides the results given below. Kindly help clarify.





A 572-65
Carbon, C 0.26 % varies with plate thickness
Iron, Fe 98 %
Manganese, Mn 1.65 % varies with plate thickness
Phosphorous, P <= 0.040 %
Silicon, Si <= 0.40 %
Sulfur, S <= 0.050


"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[gtaw]

There is no mention of the orientation of the cracks.

Generally, it is good practice to use the same preheat that is recommended for welding when cutting heavy plate and structural sections.


Best regards - Al

 

[racookpe1978]

How long and what orientation of the cracks?
Laminar indications inside the plate that were already present?
Many inches of cracks, or just a few?
How fast were you cutting?

 

[tm0284]

I am not sure the cutting speed or conditions. Two small squares, aprox 10' x 10' was cut from the original full size plate. One had crack extending from the corner towards the center of the plate about 4inches. The other square had smaller cracks, less than 1" long, in 3 of the 4 corners. Crack direction was more transverse than longitudinal, but traveled in both directions.

No lamination present in the plate.

As for the question regarding the chemistry, 0.26 is the max carbon allowed I believe. Ni, Cu, Mn, C, Mo, V all play a part in the CE calculation. It is quite possible for a plate to have around 0.10% carbon and carbon equiv of around 0.4 to 0.5. Looking at the chemistry, the exact CE is 0.42 and 0.12 Carbon.

As for the HIC cracking, I haven't considered it as it normally doesn't occur under these conditions as far as I am aware.

I haven't ever seen this happen before which is why I am open to suggestions as to the possible cause.

 

[redpicker]

Quoting metengr for emphasis

Consider preheat of 250 deg F to reduce susceptibility to cracking.

With a thickness of 5" and a 10" x 10" square, you are likely building residual tensile stresses, particularly at the corners. The preheat will slow the transformation resulting from the flame-cutting and reduce these stresses. You may have other mechanisms active (hydrogen cracking, effect of prior microstructure, inclusion morphology, etc...) but a preheat is your best bet at inexpensive and an effective solution.

 

[tm0284]

Just to clarify, the cut was 10foot by 10foot, not 10" by 10".

Do you think the residual stress issue still applies?

 

[kingnero]

Yes