Q235 vs Q345 Strength?

[R1formetoo]

Hello. I'm new here and I'm hoping you guys can help me out a bit. My name is Ian and I'm getting into the business of importing Chinese 2 post car lifts into the U.S. and though I have an engineering degree I'm mostly a business man. So here's my issue. Attached you'll find a photo of a lifting foot on a car lift. I imported this lift for testing and when I tested the lift to it's max capacity I noticed that the foot deflected downward enough to make the powder coating pop off the side of the foot. While it did not exhibit permanent deformation it did deflect enough under the load to cause suspicion. So, I have asked the factory in China to propose a way to strengthen this piece. Their reply was to replace the existing Q235 steel with Q345, which they claim is nearly twice as strong. My problem is that I cannot find anything on the web showing the mechanical properties of these two Chinese steels to verify that this is indeed a good decision. Can anyone help with firm information on these two steels? I thank all those in advance who can and do help.

Ian Gilmore

 

[bones206]

Q235 material is in accordance with PRC standard GB 700 and Q345 is in accordance with GB 1591. I have translated copies of these standards but they may be out of date now.

According to my documents:

The min. yield point of Q235 is 235 MPa for thicknesses < 17mm, 225 MPa for 16-40mm, 215 MPa for 40-60mm, 205 MPa for 60-100mm, 195 MPa for 100-150mm, and 185 MPa for >150mm. The tensile strength is 375-500 MPa.

The min. yield point of Q345 is 345 MPa for thicknesses < 17mm, 325 MPa for 16-35mm, 295 MPa for 35-50mm, and 275 MPa for 50-100mm. Tensile strength 470-630 MPa.

 

[R1formetoo]

Excellent, so this means the Q345 is a ballpark average of around 50% stronger. I'm ok with that. The Q235 did the job so I can live with this Q345 then. Thanks!

Ian

 

[sme75]

Ian,

If deflection is the source of your problem, a higher strength steel will not fix it. You will need to add stiffness to the piece to limit the deflection. It could also be just a bad paint job.

 

[R1formetoo]

I don't really know that I agree sme75, in the past when I've had similar deflection problems switching to a higher strength steel has solved the problem without additional bracing.

Ian

 

[Elabbasi]

The Q345 is stronger as bones206 noted. That means that it has a higher yield stress and higher tensile strength. Its stiffness (Young's modulus) should be about the same as the Q235 however.

Since you mentioned that there is no permanent deformation at the maximum load then the elastic strains might be what's causing the powder coating to pop off, and these will be the same for the 2 steels.

Also, the location of the peel is not where I expect the highest stresses and strains to be located, so it may be something else! Paint peels off for several reasons, and stress/strain is only one of them.

Nagi Elabbasi
Veryst Engineering

 

[BAretired]

How does the Modulus of Elasticity compare for the two steels? Probably about the same.

The paint did not peel off in a high stress area, so bad paint job sounds more likely.

BA

 

[R1formetoo]

I too am now very curious how the two modulus' compare.

Ian

 

[dhengr]

Ian:
I agree with BA’s last post, and would add that if paint were going to be cracking and chipping due to high stresses it would be at the welds joining the plate (foot) to the steel tube section. That welding and area is the area to watch. There really isn’t any appreciable stress at the point the paint bubble failed. If you loaded the rubber pad on its near edge in the photo, over the paint bubble, rather than fairly uniformly over its whole area, you could get some torsional stresses where the paint failed. But, I doubt that they would be a likely failure mechanism.

 

[R1formetoo]

I follow you dhengr. I drew attention to the powder coating popping off because this happened right as the lift attempted to lift this heavy load at that moment. I still wish someone knew the two modulus of elasticities of the two metals.

Ian

 

[bones206]

I don't think the Young's modulus will differ much from that of other mild steels (29,000-30,000 ksi). There are also different 4 grades of Q235 and 5 grades of Q345 which have varying carbon content.

The PRC Steel Design Code GB 50009 uses a value of E = 206,000 MPa (29,877,774 psi).

Perhaps the steel mill in China could provide you with some more specific material data.

 

[dcarr82775]

If (and that is a big if) the paint spall is stress related, it implies that the round black foot was loaded eccentrically and there are larger strains in the steel at the sides of the hole for the foot.

 

[hokie66]

Just another voice...the modulus of elasticity will be the same. If there was no permanent set, the deflection was elastic and will be the same with the different steel. Perhaps powder coating, at least this process, is not the right system to use.