Effect of torch cutting steel for reuse 1

[kylesito]

Working on a lifting attachment for a client who is lifting large steel assemblies.

The base plate of the attachment will need to be welded on to an assembly, used to lift the assembly, and then cut off for reuse on subsequent assemblies.

The client has asked "if we torch cut off the base plate, what effect will this have on the strength of the steel in subsequent uses?" I'm at a loss with my experience to know what, if any, effect torch cutting steel would have. My instinct tells me it will likely make the base plate more brittle as time passes, but how to quantify this into a replacement schedule is beyond me.

Any thoughts?




PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[gtaw]

What type of steel?

Best regards - Al

 

[EdStainless]

Provided that it is a lower strength structural steel then it should be OK.
If the alloy is rich enough that it will air harden then you could have issues.
If possible I suggest a test, weld and cut and weld a number of times, and then check the strength and toughness.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[kylesito]

A36 if we can get it to work. If needed, the base plate part of the attachment (the part to see the repeated loading) can be specified as whatever.

PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[metengr]

If the plate material is ASTM A 36 no issues related to changes in bulk mechanical properties from torch cutting other than removing the heat affected material from the torch cut surface for re-use. Because it is used for lifting, I would perform a surface NDT (wet fluorescent MT) of the cut edges after removal of the heat affected material.

 

[racookpe1978]

This is one of those "black iron" "Do it cheap and simple" solutions that actually IS better with very low tech method and the lowest grade steel safely available.

Go with A36, but be certain you are actually getting A36!

You don't an unknown Chinese-melted copper-filled and stainless-mixed "zoo" of cast crap from melted automobiles, batteries, and seat covers.

 

[kylesito]

OK, really good suggestions here.

Metengr - In your opinion is there a specified distance we could call for as far as removal goes that would likely take care of the heat affected zone? I think the MT test is a good idea but would want to avoid trimming the steel, conducting the test, trimming steel, etc. I do plan on recommending the test though before the lug would be certified for reuse.

Racookpe - Interesting point here. So the lower grade steels actually work better for this? What about steel allows this to happen? (I spec the steel all the time but admittedly don't get much into the chemistry of it!) Also good point about the certified steel. This client buys steel by the trainload and we have asked before for certification sheets which aren't usually available. We typically have made them buy steel we are designing 'special order' so that we can get the certs on it.






PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[metengr]

1/8" to no more than 3/16".

 

[tbuelna]

I'd recommend having a chat with the engineers that prepared the analysis for the lifting lug welds, and see what they have to say regarding the matter. Most likely, as metengr noted, they'll want to see the material within the HAZ of the torch cut removed before re-use.

 

[kylesito]

Tbuelna - that's easy. We are them!

PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[gtaw]

ASTM A36 is a low carbon steel that has very low hardenability when cooled rapidly from temperatures in the austenitizing temperature range. This type of steel has been used for welded construction many years. The typical manner of preparing the edges for welding is to torch cut the bevel. The extent of post cutting cleaning it is simply remove the oxidized surface. Repeated thermal cycling should not be detrimental to the base metal if the surface is ground to remove the oxidized material to bright metal.

Chemistry is important when considering the affects of heating and cooling steels from the austenitizing temperature range. The cooling rate (for a given chemistry) from the austenitizing temperatures determines whether the material will become hardened or not. With the low carbon content of A36, there is little likelihood it will get hard enough to be of any concern.

The oxidized surface that results from the cutting process is detrimental to the welding operation. It should be removed down to bright metal to ensure the welder has a fighting chance of depositing a good weld. Inadvertent notches should be faired by grinding, again, just to provide the welder with a suitable surface for welding.

Thick plates should be preheated to the same temperature as welding, i.e., if the applicable welding standard requires preheat prior to welding, the same preheat can be used prior to torch cutting.

More often than not, the removal of the previous weld will remove any heat affected zone associated with the initial weld. The HAZ that results for the torch cutting operation will be not more detrimental than the HAZ from welding. It may extend slightly deeper into the base metal, but if the welding doesn't produce any problems, neither should the torch cutting.





Best regards - Al

 

[dvd]

One suggestion would be to make a tapped pad which is permanently welded to the structure and that the hoisting plate bolts to. The hoisting plate is unbolted and a new tapped pad is welded onto the next structure. The amount of time saved in removing the hoisting plate with scarf cutting by torch may pay for the steel pad which is abandoned in place.

 

[racookpe1978]

Let me phrase the problem 9the question ?) a bit differently - perhaps that way it will be more clear.


A36 "ordinary" steel is adequately tough, adequately flexible, adequately strong, easily weldable, and adequate,y available. Right?

So, you really don't need to change it, to do anything different UNLESS "qdequate" is "not good enough" for the particular job you are trying to do. Right?

So, you need a non-rusting steel -> You make a more expensive stainless steel.
You need a higher strength steel -> You "make" an exotic, expensive, carefully-treated and heat-treated high carbon-moly mix with very carefully controlled PWHT. Works. Right?

You need 80,000 psi steel for submarine hulls. you invent HY-80.

You "need" low-weight alloys for aerospace -> You invent expensive mixtures and alloys.

But -IF - (big "IF" there!) - you do not need the exotic crystal pattern for the "ordinary/average/weldable/"just plain steel" that will be used and torched and welded and re-torched until no longer useable, do you really need an exotic alloy with exotic crystals and complex PWHT?

If "ordinary" steel will meet your strength and weight criteria, use it. just plan on the (very slightly higher) weight, and the specific weight required.

 

[kylesito]

DVD - that was our initial solution. The problem though is that this presents a QC issue for the client and would like need to be removed after each use anyway. So this idea was nixed.

Racook - good thoughts. In this case, and knowing this client, simple and economical is the right approach over exotic. I think ordinary steel here is the right approach.

PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi