Steel quality

[LambtonParticipant @lambton]

[LambtonParticipant @lambton]

Leaving politics totally aside there are legitimate concerns about the quality of some imported steel products. Rebar is a particular problem because in use it is totally encased in concrete so its condition cannot be monitored in service. There is a lot of inferior rebar washing around the world and regardless or where it is manufactured or its cost it should be subjected to the most rigorous testing to establish its chemical make-up and mechanical properties before it is supplied to customers, with inferior batches returned to its originator.

There could be similar problems with structural steel sections even though theses can usually be inspected in situ to detect fatigue cracking and other well-known defects. An example recently was the failure of part of the decking on the 50 year old Fourth Road Bridge. Last week a bridge over the M25 had to have emergency repairs following routine monitoring.

It is a fact of basic economics that low priced commodities will always find a ready market with higher priced ones being shunned. This is understandable and acceptable so long as the quality is maintained and is provable.

The astronaut Alan Shepard famously said “ It's a very sobering feeling to be up in space and realize that one's safety factor was determined by the lowest bidder on a government contract”.

Eric

[MWParticipant @mw27036]

Hi Lambton,

I'm no rebar expert but i think alot of it comes down to either the metallurgy or the hardening and heat treatment processes, to begin with, it's possible on cheaper grades that the wrong grade of steel is being used altogether,

secondly it's also the hardening process, which can be incredibly difficult to get right, because the tempering needs to be very precise over a large batch, too little and the rebar will remain too soft, too much and the rebar will be too hard and remain very brittle with no ductility.

Theres also the cooling process after tempering,i'd imagine in the interests of saving money the cooling process be forgone and lead to worse quality steel, when dealing with metals this hard, sometimes the cooldown needs to be controlled over time to ensure a good temper, otherwise, if left to cool naturally, the metal can receive an "air quench" and shock the metal.

They probably also rely on the fact (with huge building projects in places like UAE) that this stuff isn't going to be hanging around for very long and they'll be able to shift it before anyone notices, and who cares once it's inside the concrete? thats what they think like i guess anyway.

Michael W

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Walters on 04/04/2016 09:24:47:

… and who cares once it's inside the concrete?

.

Quote of the week

MichaelG.

[MWParticipant @mw27036]

Stop it, Michael G! you'll work the building trades experts up into a frenzy.

Michael W

[Clive HartlandParticipant @clivehartland94829]

A lot of Rebar assembly has shapes which the rebar is bent into the right form so any brittleness will show up straight away. Then again a lot of rebar is done away from site in a factory set up so that is a further check on quality.

[Douglas JohnstonParticipant @douglasjohnston98463]

I often wonder about the quality of all the dumped Chinese steel coming into this country. I hope this is not another scandal which will manifest itself when buildings start falling down. In my own neck of the woods the new Forth road bridge is being constructed with Chinese steel…..oh dear

Doug

[WindyParticipant @windy30762]

Posted by Michael Gilligan on 04/04/2016 09:47:33:

Posted by Michael Walters on 04/04/2016 09:24:47:

… and who cares once it's inside the concrete?

.

Like some of the dodgy car body repairs a coat of paint covers a multitude of sins

MichaelG.

[mechman48Participant @mechman48]

IIRC There's a clip on a YouTube link from a previous thread here that showed some rebar 'Made in Iran' that snapped in two when a length was hit against a concrete block on a construction site, maybe someone can remember where it is …

George.

[mechman48Participant @mechman48]

Found it….

**LINK**

George.

[Ian S CParticipant @iansc]

The quality of re bar has been bought up here in NZ with the rebuild of Christchurch city after the earth quakes 5 years ago.

The only heat treatment I can imagine re bar going through is annealing, it's got to be able to be bent, and some times to quite a small radius. More of the new buildings are being built with steel frames, rather than the reinforced concrete of the old ones.

Ian S C

[Anna 1Participant @anna1]

Maybe I am looking through rose tinted specs, but I am already aware of the variable machining quality of steel even in the same piece of material, I do not recall many years ago having this problem. Unfortunately for the most part we have no way of knowing the original source of the material. As an example,of the problem I cannot see how the compositon of the recycled material can be properly controlled when the recycled material is coming from all and anywhere.

kind regards

Anna

[John ReeseParticipant @johnreese12848]

I think the real problem is mis-labeling by the mill or vendor. Most users of material rely on the mill certifications as to physical properties and chemical composition. When dealing with off-shore materials I would want to see physical and chemical tests by an independent laboratory.

[JAParticipant @ja]

Posted by Anna 1 on 04/04/2016 12:24:22:

Maybe I am looking through rose tinted specs, but I am already aware of the variable machining quality of steel even in the same piece of material, I do not recall many years ago having this problem. Unfortunately for the most part we have no way of knowing the original source of the material. As an example,of the problem I cannot see how the compositon of the recycled material can be properly controlled when the recycled material is coming from all and anywhere.

kind regards

Anna

High quality high spec steels is made from steel scrap. Steel scrap of known providence and quality is remelted in an arc furnace or such like and a sample taken there and then. This is immediately analysed and alloying metals are added to the melt if required. This steel scrap does not come from junked cars etc but is usually sourced within the steel works, the cut off ends of strip, bar and such like. The whole system is called a rising scrap process: The higher the quality of steel, the higher the quality of scrap. This steel will have come a long way from a blast furnace and basic oxygen converter.

Rubbish steel scrap is added to the melt in a iron to steel converter to cool it down between blows. The reactions that convert iron to steel are very exothermic.

JA

Edited By JA on 04/04/2016 12:48:50

[Anna 1Participant @anna1]

Thank you JA. thats very interesting and helpful, Is there anyway we as model engineers can source reliable good quality steels, or for that matter non ferrous materials. I have recently rejected phosphor bronze that was full of inclusions.

kind regards

Anna

[Sam Longley 1Participant @samlongley1]

Concerning rebar. I always understood that when one applies tension to reinforcing steel bar, after a certain point it begins to stretch easily (relatively), then as more load is applied it starts to resist tension to a greater level than at the point where it originally began to "fail". At that point it is rebar. It is nothing to do with heat treatment & I always thought ( possibly incorrectly) that it was cold rolled, hence the surface finish, where the shape is rolled in as part of the stretching process. The rolling causes the "seam" each side of the bar

It is 49 years since I did "theory of structures" & "structural design" at college, so memory may be a little faded.

[John McNamaraParticipant @johnmcnamara74883]

Hi Sam My memory needed jogging too.

If you Google the following 3 searches….

steel stress strain graph images
steel creep graph images
steel deflection graph images

You will see various representations of the stress strain graph for Steel. While strain hardening does occur it is past the point of no return, there is a permanent change in the dimension of the steel part.

Up to the yield strength knee in the graph the material under load will return to its original size, obeying Hooks law.

Long term there is also creep, particularly if the load is near the yield point. Normally in a design a safety factor is applied depending on the use. Creep will be less with lighter loading factors. Where human life is involved a factor of 2 even 3 or more is common.

A steel part may pass the tests for a specific load and still not be satisfactory if it bends (Deflects) more than a particular design can tolerate. For high accuracy machinery Deflection is the main concern of the designer. A lathe bed is a good example, clearly there is no risk of it failing due to the load applied by the cutting tool and saddle. The designer made it massive to reduce deflection.

Regards
John

[J HancockParticipant @jhancock95746]

Not forgetting the propensity for 'steel' to try and turn itself back into its original state occupying

near on thirty-two times the volume as 'steel'.

That's why all the conrete columns burst apart, having been made of the wrong concrete too.

[Sam Longley 1Participant @samlongley1]

To be honest – at 69 years of age- with the boat about to be launched for the sailing season, I could not care less whether it stretches,deflects, shrinks, swells or just goes rusty.

[LambtonParticipant @lambton]

"That's why all the concrete columns burst apart, having been made of the wrong concrete too."

It is not inevitable that all concrete columns bust in fact very few do nowadays. This only happens when the alkali conditions inherent in properly made concrete fail and no longer protect the rebar from corrosion when water enters the structure through minute cracks. Freezing cause the water to expand and so the cracks enlarge and let in more water. The concrete/steel composite can resist these conditions so long as the concrete remains alkaline. Once the alkali conditions no longer prevail the rebar will rust which can spell disaster as rust occupies a greater volume than the parent steel thus putting a "busting" force on the surrounding concrete which causes major damage that will allow gross ingress of water. Also rusted steel suffers from corrosion fatigue which means it can fail at much lower loads than un-corroded steel.

Fatigue failure caused by repeated reversal of loads on a structure is often the failure mode of civil engineering structures even when first class materials and construction methods have been used e.g. the recent problems with the old Fourth road bridge.

The problem with very cheap imported steel is uncertainty of its composition, origin, mechanical properties etc. which could make the inherent known potential failure modes in engineering structures much more to occur.

If steel is transported half way around the world and sold for half the price of British made steel it is wise to be very, very cautious about its suitability for any construction project.

It is not a question of politics or even economics but one of caveat emptor.

Eric

[Anna 1Participant @anna1]

I was having a codgitate in the workshop regarding this steel quality thingy,and it occured to me, why don't the British manufacturers put a continuous manufacturers mark along the bar to indicate grade and quality as is often done on electrical cable and copper plumbing tube.

Kind regards

Anna

[KWILParticipant @kwil]

Just noticed that Sheffield Forgemasters have seen off the Koreans for a 550 ton cast frame Contract for a USA company. It will take weeks to cool in the casting pit to stay as good British Steel

Edited By KWIL on 04/04/2016 16:22:54

[Neil WyattModerator @neilwyatt]

Posted by Sam Longley 1 on 04/04/2016 15:35:21:

To be honest – at 69 years of age- with the boat about to be launched for the sailing season, I could not care less whether it stretches,deflects, shrinks, swells or just goes rusty.

I trust the boat does not have concrete hull…

Neil

[Dinosaur EngineerParticipant @dinosaurengineer]

Rebar steel "quality" can depend on the correct hot forming temperatures and hot forming reductions as well as the steel composition. The steel in service should never exceed the proof stress and a suitable high safety factor should be applied. There are many other factors involved and I wonder if all these factors are suitably applied at the design stage and are adequately monitored in the building stage.

One accident can wipe out all the cost saving of inferior cheap steel but we can't expect Politicians to understand that can we ? To allow accountants to make decisions on steel quality is the ultimate folly.

+ 1 to Lambton's comments..

[Neil WyattModerator @neilwyatt]

Posted by Lambton on 04/04/2016 15:44:12:

"That's why all the concrete columns burst apart, having been made of the wrong concrete too."

It is not inevitable that all concrete columns bust in fact very few do nowadays.

Some ten or more years ago much of the raised section of the M6 though Birmingham (including Spaghetti) needed extensive work to replace patches of concrete which has started spalling off. The cost was many millions and the traffic jams legendary..

Neil

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