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Old 03-25-2016, 02:50 AM   #11
bazmak
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I have machined another piece of rebar from nom 30mm to 7/8" a/f hexagon
My thoughts are as follows
The far east rebar is likely to be poor quality but the western world
has better quality control and certification,maybe I am just lucky however
1- My samples in my opinion are high grade high tensile steel
2- The steel machined better than hot rolled mild steel with carbide tooling
3- The finish was excerlent,far superior to hot rolled
4- Running a file over to clean up the burrs proved it was high tensile
as old school engs know it files and sings and the file slips a little
I would highly recommend others to give it a try.I have made sections
not readily available in my neck of the woods and will make a hex nut
next to see how it performs with drilling and tapping using HSS


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Old 03-25-2016, 02:08 PM   #12
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All the rebar I have ever used machined like annealed bubble gum, maybe there is a difference in the product that we get here.


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Old 03-25-2016, 04:06 PM   #13
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I think it is like the sash weights that I use, from the UK, great, but from what I have been told, from the US, crap.

All you can do is try some from your part of the world and see what the outcome is.

John
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Old 03-25-2016, 10:49 PM   #14
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I think there are basically 2 types of reinforcing bar
Rebar for main structural encasement/reinforcement,good quality with
certification in 3 grades of high quality. Then we have cheap reinforcing bar
and mesh which is classed as secondary reinforcing used to prevent
minor cracking of slabs.I gleaned that fron Wikipedia with a quick browse
but did not want to spend too m
uch time and effort.For those who try it and get good stuff then take my word its well worth the effort
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Old 03-26-2016, 03:00 PM   #15
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I believe the "good stuff" is more or less EN36B - annealed - heat treated it is much the same as cap screws.

Thanks to the wonders of cut and past - here is something I wrote years ago on steel wire & rod manufacture which may explain why there is such a range.

According to SKF bearings, typical "cheap" continuous cast steels contains 10^11 defects per ton - think about that - that's 100000 defects in every gram of steel.

These are mostly microscopic consisting of cracks, voids, bubbles, broken grain boundaries and inclusions of mostly refractory materials and Manganese Sulphide.

Manganese (amongst other things) is added to steel to turn soluable Iron Sulphide (bad news) into insoluable Manganese Sulphide (which mostly floats to the top) to extract the Sulphur from the steel as it has large deleterious effects.

This 10^11 defects per ton is typical for bottom poured continuous casting (with little quality control).

Since these defects are more or less the same size and distribution they will be reduced in size (but not overall % or distribution) by subsequent rolling and drawing operations.

So the smaller the diameter of the original pour relative to the finished wire size the worse the quality - but of course starting with a large strand and reducing it greatly involves a lot more cost and effort.

Timken Bearings USA erected the world’s largest strand caster (280mm diameter strands) to be ultimately rolled and drawn down to wire for the production of balls and rollers in order to get the size of defect down whilst maintaining the cost effectiveness of continuous casting.
In addition they used off-bottom pouring with induction heating / stirring with Argon purging and tight quality / cleanliness controls.

This would get the defects down to 10^9 to 10^10 defects per ton from the pour with a very fine defect size after being brought down from 280mm to (say) 10mm.

In order to improve quality further one has to go to ingot casting - again off-bottom pouring producing better results than the more common top pour - in addition the ingots themselves can either be top or bottom poured.
This can get the defects down to 10^8 to 10^9 defects per ton.

Also the ingot has to be dressed before further processing - removing the top and bottom of the ingot where the "heavy" and "light" inclusions have migrated to - obviously the temptation to not dress or under-dress is obvious.
Also the time it takes for the liquid metal to solidify is also a factor (in ingot casting) allowing the light / heavy inclusions to migrate - but like they say "time is money".

To get even better results the next step requires taking your dressed ingot and Electroslag Refining it - this consists of welding the ingot to an electrode and using it rather like an arc welding electrode to burn it into a further ingot crucible via a flux or slag specifically formulated to add or remove materials shown to be present / absent (by analysis) in the original ingot (specific to each and every ingot processed).

This is typically how most high end tool steels are produced and can get the defect levels down to 10^6 to 10^8 defects per ton - plus these defects are generally smaller.

With each improvement in cleanliness there is also an increase in mechanical properties of about 20% with each order of magnitude reduction in defects.

The highest tensile strength steels are about 120 ton / in^2 but winding wire cables can be 300 ton / in^2 and piano wire as high as 480 ton / in^2 (some of this is attributable to the strength of thin strands to do with energy levels about defects - a string of iron crystals has a theoretical tensile strength of about 1400 ton / in^2 { ±21GPa } but this cannot be achieved in large diameters due to defects and discontinuities in the material.
1 micrometre iron fibres reach 900 ton / in^2 )

So there is an awful lot of scope in cost and quality from one end of the spectrum to the other and you will find steel makers notoriously tight lipped on the subject.

I realised while talking to suppliers about my quality problems that I needed to understand the steelmaking process if ever I was going to get to the bottom of certain quality issues - once I knew what they were doing wrong and forced them to change, my problems went away by specifying ASTM cleanliness "4" - surface condition "A".
When I raised this issue with a German manufacturer they were puzzled - but then made this statement "In Germany we have five steel makers, four of them are sh1t !"
They had solved their quality problems by selecting a quality supplier rather than by specification. If you buy steel from SKF it will always be of a high quality - they don't make crap - but of course it costs more.

I ramble on about rolling and wire drawing - but we don't need to go into that here.

So you can see why there is a huge range in quality....

Regards,
Ken
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Old 03-26-2016, 10:44 PM   #16
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Many thanks Ken,You have simplified and described in laymans terms
what a lot of us would be happy to know but I for one was not prepared
to spend a lot of time and effort investigating.I would greatly appreciate any further input
from you for our/my education.Perhaps you could list varios steels and varios
equivalents with a ball park figure in metric and imperial of their range of
high tensile strength etc.
The wiki general description of rebar is (tempered steel ) which I believe has a slightly higher carbon content and low alloy type of mild steel that can be hardened
to various degrees of heat treatment. Regards Barry
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Old 03-27-2016, 01:46 AM   #17
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It would seem the SKFs methods of high quality steel is the modern
equivalent of the ancient japanese art of swordmaking of a 1000yrs ago
Then they did not know much about metallurgy where iron has nom 5% carbon
and soft steel has nom 0.2% carbon with high carbon steel having nom 2.5%
They would produce carbon steel of varios grades in a clay pot to reduce the
remove the carbon from the iron (like a Bessemer converter)
The softer steel was used as the core of the sword and wrapped in lathers of higher quality higher carbon steel.Using nothing but a hammer,anvil and forge
the material was heated ,beaten,wrapped or folded and heated beaten again
for as many as 20/30 times. Many methods were a closely guarded secret
and stories go that the very swords were made for royalty by adding
meteorite pieces (nickel alloys) and diamond dust (carbon).It was all trial and error and experience but the steel was on a par with SKF
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Old 03-27-2016, 04:29 AM   #18
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Quote:
Originally Posted by bazmak View Post
China and India supply cheap matls and like everything else are cheap and poor quality.However for the western world there are stds for rebar and specifications and certifications,so to my mind good quality as would be required for building bridges etc.I googled and got plenty of info.There are 3 grades 30, to 50 tensile strength so some rebar must be good quality

in order for structural engineers to design reinforced concrete beams etc

Therefore to my mind not all bad.I seem to be lucky and have good stuff not foreign.Will keep you posted

Some years back there was a steel mill in a near by town setup to do rebar. Rebar in this case was reprocessed scrap. Having worked with rebar I can attest to it variable quality. It is certainly possible for various grades to exist but the stuff I've come across isn't consistent. Actually it is a lot like structural steel, which can be highly variable even in the same beam. Inclusions, hard spots and other quality issues are a reality in structural steels.
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Old 03-28-2016, 08:49 AM   #19
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I've always been fascinated by stories of steel making and sword making.
I suspect, Barry, that you were once very near a place called Shotley Bridge on the River Derwent that flows into the Tyne.

There was certainly swordmaking carried out by German craftsman originally from Solingen. Some of the swords were stamped with a fox as one of the families was called Fuchs. The legend is that some of the swords were so good that they could be coiled and stored in a gentleman's top hat.

Suffice to say steel making continued in Consett which is only a mile or so from Shotley Bridge. This were my father, his two brothers were trained as blacksmith/farriers when my grandfather moved from Shildon where Timothy Hackworth had his railway engine works.
Laughingly, Consett is where the other Atkinson lived. I'm referring to Mr Bean or Blackadder. Family connection? You tell me.

Cheers

Norman
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Old 03-28-2016, 02:33 PM   #20
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Steelmaking By Early Blacksmiths.

Cast Iron has a very large Carbon content – this is not some sort of accidental contamination but is essential as a Eutectic in its production.

Cast Iron melts at about 980°C and early clay refractories could not withstand the 1100 plus degrees needed to melt iron – nor would our forefathers have had the means to create such temperatures.

Nowadays we remove the carbon by blowing oxygen through the melt in the Bessemer process to produce Iron and steel.

Blacksmiths forged steel by taking a cast iron rod which was progressively pounded flat and folded over (whilst red hot) – this caused the ferroferric “millscale” formed to bring oxygen into the steel which combined with the carbon to give off carbon dioxide.

By repeating this process over and over the cast iron would eventually become a high carbon steel – that is why you would see a blacksmith quench the hot iron in water and strike a file over it – obviously while it is still cast iron you can remove material but once it goes glass hard and the file rings off the material we now have a high carbon steel.

If you kept up the process you would get lower and lower carbon content and eventually you would end up with “wrought” iron – the forefather of modern mild steel.
You could also go too far and end up with iron that was “overwrought”, thoroughly mild and shot through with unreduced mill scale.

Swordsmiths would produce two types of steel and further work them together to produce a blade that was made up of layers of very hard but brittle steel and layers of very tough steel.
Clearly this took a great deal of experience to accomplish – they had no idea of the chemistry going on.

Before the Bessemer process cast iron would be “puddled” in an open hearth kept heated by the exhaust from the blast furnace. Skilled workers known as “puddlers” would rake in millscale and rust (collected by urchins and apprentices throughout the mill) and the cast iron would become “puddled iron” which – as its melting point increased as the Carbon was removed – became increasingly toffee like and difficult to work.



Regards,
Ken


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