S235JR vs S355JR Structural steels 3

[Algirdas13]

Hi everyone,

I have a question regarding the S235JR and S355JR structural steels. I have noticed that when manufacturers evaluate structures' prices there are basically no difference in price between S235JR and S355JR steels. For this reason the question arises why to choose S235JR steel rather than S355JR when the price is the same? At the moment I know that the S355JR has higher point of yield and ultimate strength, but also contains more carbon, which makes welding more complicated.
Perhaps someone can share your insights when the question arises choosing between S235JR and S355JR steels, on which factors do you rely on when choosing from those two materials?

Thank you.

 

[HTURKAK]

When stiffness,buckling, ductility , fatigue requirements govern ,it is better to choose S235JR,
When strength ,min. self weight governs S355JR

Pls have a look to the following doc.
...


He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[Algirdas13]

HTURKAK, thank you for your reply and attached document, I really appreciate that!
To be honest in the attached document I wasn't able to find an explanation why S235JR is better choice for stiffness, buckling and fatigue requirements. As I understand both steel grades S235JR and S355JR has the same Young's modulus which is the main parameter for the stiffness (only referring to the material properties). Also, regarding the buckling, since the S355JR has a higher yield point, doesn't that make the material more resistant to the buckling? Furthermore, considering the fatigue requirements, from the information that I'm able to find the S355JR steel has a higher fatigue limit value comparing it to the S235JR steel. Maybe you could help me to understand what I'm missing out regarding the stiffness, fatigue and the buckling criteria? I would be very grateful for that.

 

[HTURKAK]

Pls carefully read the doc. again . The short answers to your questions ,
- If stiffness is a requirement rather than strength , ( Stiffness K= EI/L , , is a function of E and I nothing to do with yield strength )
- If buckling is a concern ( Simple Euler's buckling for rods Pcr=( Π**2 )* EI / Leff**2 still you do not see yield strength , and code specified buckling also minor affected ) Another case buckling of cylindrical shells σcl=0.6 Et/ r for µ=0.3 still you can not see yield or max tensile str. of the steel but E , thickness governs )
- If fatigue is a concern , grade S235JR more ductile than S355JR
Good luck..
...

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[Algirdas13]

HTURKAK, thank you for the answer. I really want to fully understand the situation, for this reason please find some insights below and correct me if I'm understanding something wrong.

- If stiffness is a requirement rather than strength , ( Stiffness K= EI/L , , is a function of E and I nothing to do with yield strength )

For the stiffness I was referring to Young's modulus which is the E. According to EN1993-1-1 §3.2.6 both S235 and S355 has the same Young's modulus value - 210 GPa. For this reason I don't see any differences in stiffness comparing these two materials.

- If buckling is a concern ( Simple Euler's buckling for rods Pcr=( Π**2 )* EI / Leff**2 still you do not see yield strength , and code specified buckling also minor affected ) Another case buckling of cylindrical shells σcl=0.6 Et/ r for µ=0.3 still you can not see yield or max tensile str. of the steel but E , thickness governs )

You are right, mostly in an ideal situations for columns buckling depends only on Young's modulus (looking into material properties). For this reason there is still no difference between S235 and S355. The reason I mentioned yielding was that in non standard structures the local buckling can occur. There are a lot of situations where local buckling is provoked in shell structures because of the yielding in stress concentrations and stress redistribution. As an example this is common situation for silos structures. Or other example can be lateral buckling of beam that flange has yielded.

- If fatigue is a concern , grade S235JR more ductile than S355JR

As I understand ductility mostly plays a role in an situation when the yielding is reached. The fatigue can occur bellow the yielding point. If S355 has a higher point of fatigue limit than S235, how the S235 steel can be better choice than S355? There may be even a situations when by selecting S235 steel structure will enter high cycle fatigue when in the same situation the S355 will not even reach the fatigue limit barrier.

 

[SWComposites]

What is your specific application for these steels?
Is your structure welded? If so, a material with better weldability is usually selected.
Do you have significant fatigue loads?

 

[Algirdas13]

Hi SWComposites, thank you for joining the thread!

Currently I'm looking for the answer generally for the industrial equipment. Some of the equipment has to encounter fatigue loads, and some don't.

All the time I thought that S235 steel is cheaper than S355 steel. Recently I got the news that those materials cost pretty much the same. During analysis of designs of other manufacturers equipment I see still plenty of S235 used in structures. For this reason I'm wondering why there are still so many structures designed using S235 steel, when it seems that S355 has the better mechanical properties and costs the same.

Because of that I'm looking for the answers so that maybe I don't completely understand something about these two steels mechanical properties. Or I'm missing something out, that I don't take into account that is important for choosing between these two steels.

As far as I know the S235 is slightly better for welding than S355 but the difference is pretty much insignificant. Even the electrodes used for welding S235 and S355 are the same.

 

[kingnero]

As far as I know the S235 is slightly better for welding than S355 but the difference is pretty much insignificant.

Incorrect, and the difference gets bigger the thicker the steel. When you weld S355 12mm (1/2") thick, you can end up (depending on the actual chemistry) with non-acceptable hardnesses that will fail your procedure qualification. Preheating is a costly process step (time-consuming, extra clamping, ...), and will be necessary in almost all of the cases upwards of 12 mm. I'd try to avoid S355 whenever it's not strictly necessary.

 

[Algirdas13]

Incorrect, and the difference gets bigger the thicker the steel. When you weld S355 12mm (1/2") thick, you can end up (depending on the actual chemistry) with non-acceptable hardnesses that will fail your procedure qualification. Preheating is a costly process step (time-consuming, extra clamping, ...), and will be necessary in almost all of the cases upwards of 12 mm. I'd try to avoid S355 whenever it's not strictly necessary.

kingnero, thank you so much, that makes a lot of sense! Could you recommend some resources where I can find information on preheating requirements for S235 and S355 steels? I would like to have a better understanding on which thicknesses the difference starts to take the impact for choosing between S235 and S355.

 

[kingnero]

Any metallurgy or welding engineering course will get you started. But this isn't something you can just pick up by watching some youtube vids or reading some online pdfs.
There is however a very informative standard EN 1011-2 that gives guidelines on how to weld ferritic carbon steel. Amongst others, you'll find there how to determine/calculate the necessary preheat depending on the case you're in, however this standard will not tell you the "WHY" or the reasons for doing this.
Get some MTRs of the metals you're working with, so you have some real life examples and you'll be able to figure out the influence of the different factors that will decide on the preheat temp.

 

[Algirdas13]

kingnero, thank you very much! It seems that this topic is really more complicated than just reading some online pdfs. Just to be sure, do I understand correctly that the MRTs stand for the Material Test Reports?

 

[Klitor]

Kingnero,
It is quite general statement. Doubt anyone would let me use s355 if preheat was neccessary for 12mm elements.
Preheat woes (also quite generaly..just a structural engineer...have IWE guys in house) start at about 30mm

 

[kingnero]

@ Algirdas, Yes, MTR is Material (or Mill) Test Report.

@ Klitor, I have on multiple occasions encountered microstructural changes that are not acceptable on thicknesses as low as 10 mm.
I am interested in learning your HI and hardness values for welds on S355 on any thickness inbetween 12 and 30 mm (whatever data you have available). If you only start to preheat from 30 mm and up, I don't really want to know your in-house decision process. I also doubt any notified body will deliver a WPQR for your welding methodology, as I have my doubts whether it will pass all quality requirements.

 

[cherish]

Hi,

There's an important factor that is the yield stress factor epsilon, that will need to involve the yield strength of the steel. Then use that for classification of the cross section.

Below pictures is examples of the same tube size but one with S355 and one with S235, they are classified as different classes. Though one can be utilized with plastic capacity, or maybe the posibility of forming a plastic hinge, local buckling is important for some situations with section classification.

Engineering is a journey of thousand miles begins with a single step

 

[Gforce1]

As discussed above S235 would be used primarily for ductility or cost reasons (at least historically).

For primary structural steelwork, at least in the UK, many section sizes are not available in S235. Indeed in recent years S355 has all but replaced S275.

 

[Klitor]

@kingnero,

What I implied is that noone will make a fuss or even bat an eye if you choose s355 over s235 welding-wise (at least on heavy/industrial projects I work on)

Heck, as Gforce one also said, its even hard to find some profiles in s235...most vendors will substitute you s355 for the same price...

Further discusions about pqr,wbs,no-bo,charpy etc. would be out of my ballpark..as i said .. only a structural engineer on a structural engineering subforum.

 

[kingnero]

In mainland Europe, S235 and S275 are used interchangeably. S355 cannot be used when S235/S275 is prescribed.
Welding S355 also requires other certifications/qualifications than welding of S235/S275. Probably your heavy/industrial fabricators have the necessary knowledge for welding S355 (and they will most likely apply the correct preheat).
True though that some (closed) sections (certainly square and rectangular hollow sections) are much more available in the higher qualities.
I'm quite surprised to hear that S355 is used as a common substitute for S235. I wonder if that's a common practice, I hope others chime in too.