History
The accumulated efforts of several individuals as far back as 1821 lead to the discovery of stainless steel in 1913 in Sheffield, England.
Harry Brearley was experimenting with steels for gun barrels and cutlery when he noticed that material did not rust which was attributed to the metal having its chromium content above a critical minimum threshold level. Below this level (see Stainless Steel Chemistry below), the metal rusts.
stainless steel Chemistry
Stainless steels are iron-based alloys which ASTM A941 (2010b) defines as containing a minimum chromium content of 10.5 percent, and a maximum carbon content of less than 1.20 percent.
The commonly used types of rebar have about twice the minimum chromium content. The reinforcing bars commonly used today have much lower carbon content at 0.03 percent, making them virtually “carbon free”, and less prone to corrosion.
Besides chromium, typical alloying elements are nickel, molybdenum, manganese and nitrogen which together make up almost 35% of the total chemistry by weight.
This chemistry makes stainless steel rebar the superior corrosion resistant steel product available.
What is Stainless Steel Reinforcement?
Stainless steel rebar is a round bar used to reinforce concrete structures mainly in applications where the service conditions are corrosive in nature due mainly to chloride attack or from other chemicals.
It also has application where the service conditions require low magnetic permeability, or where extremely low service temperatures prevail (LNG), and finally in design applications requiring extensive ductility while maintaining load bearing capacity (seismic).
Click to the file “Properties of Stainless Steel Rebar.docx”Unique Chemistry:
Unique properties, Superior performance
It gets its top corrosion resistance rating on the basis of having almost twice the stated minimum amount of chromium (10.5%) required to qualify and provide superior performance attributed to stainless steel.
At these levels of chromium above the minimum, a robust, stable, transparent, passive and adherent surface film of chromium oxide spontaneously forms and regenerates to protect the surface of the metal from corrosive attack.
It is this barrier that allows stainless steel to prevent the galvanic coupling, or the formation of a corrosion cell, with other less passive steel materials in the concrete. This is fundamentally how structures with stainless steel gain durability and extended service life without major repair.
Below the stated minimum level of 10.5% of chromium corrosion sets in rapidly.
[Note to file : Consider adding a print screen of AASHTO M334 with the Chromium content of MMFX vs Stainless]ASTM A955 – The Stainless Standard
The governing manufacturing guide for SSR is the international industry standard ASTM A955/A955M – Standard Specification for Deformed and Plain Stainless Steel Bars for Concrete Reinforcement. The standard is available through ASTM.
There are many similarities in the content of this Standard compared to conventional rebar standards. The bar sizes and the bend diameters are the same and it is a high strength product with Grade 80 steel available.
The elongations of stainless are much higher compared to regular steel. Also, there are two distinct metallurgical microstructures – austenitic steel and duplex (austenitic-ferritic).
The standard A955 includes the following information not provided in other rebar standards :
A list of the seven commonly available types of stainless steel rebar and their chemistry composition,
An “on board” mandatory corrosion resistance test (Note : No other rebar standard commits to an embedded corrosion resistance test,
A magnetic permeability test method,
Supplementary requirements relating to max/min limits on yield for seismic applications,
Mandatory pickling after rolling to remove a thin layer of metal to provide a clean metallic and passivated surface.
NO OTHER STANDARD MATCHES THESE FEATURES
Product Manufacturing
The main raw material used in the production of stainless steel is recycled steel scrap – up to 90% depending on the mill and market conditions – with the balance made up of various virgin alloys.
The rebar production process consists of :
melting the scrap material and the various alloys in an electric arc furnace,
removing impurities and reducing carbon in an argon oxygen decarborization vessel,
casting of ingots,
hot rolling to different ribbed or smooth bar diameters, and
finishing the bars which includes
the removal of mill scale usually by mechanical means, and
pickling which is the removal of a thin layer of metal to provide a clean metallic and passivated surface.
This last step of “finishing” is a requirement of A955 and ensures the highest level of corrosion resistance of the bar material. These bars are shipped and received by customers in a fully passivated state.
Buy America build america
In the United States, in order for public authorities to obtain federal funding for infrastructure projects, both the production of the ingots used to manufacture the stainless steel reinforcement material and the process of rolling of the bars must have been performed in the U.S.A.
The stainless steel rebar materials available from the members of SSIG guarantee that the rebar meets these BABA requirements.
Accessories : Couplers, Tie wire
Stainless steel couplers and tie wire are readily available from the stainless steel rebar fabricators. Click here will take the reader to the Resources Tab, “Material Technical Data Sheets” section, then to “Coupler Data Sheets” sub-section, in the file “5.0-Resources_SSIG Website_Rev1.docx”
Stainless steel Fabrication Standard
FABRICATION PROCESS
Fabrication of stainless steel reinforcing bars uses the same processes as for uncoated carbon reinforcing steel bars. Though contamination of stainless steel with black carbon can affect product performance, the impact is not considered detrimental.
Nevertheless, in the interest of maintaining superior product performance, the industry in concert with the CRSI has developed an ANSI standard to eliminate the possibility of black carbon contamination of the stainless steel during the various fabrication processes. The ANSI CRSI IPG4.1 document is titled “Standard Practice for Stainless Steel Reinforcing Bar Fabrication Facilities”.
A link to a summary of the ANSI standard is here. [See file “5.6-SSIG Website, Resources, Standards.docx and go to the 5th bullet, Stainless Reinforcement Fabrication Standard.]
The primary provisions of the Standard are as follows :
the establishment of key criteria regarding the acceptance and rejection of material throughout the fabrication process,
to avoid contamination, SSR is to be fabricated in a segregated and dedicated facility unless adequate cleaning or contamination isolation procedures are in use, and
all contact points of tools, machinery and equipment used during storage, fabrication, handling or shipping will be with materials that do not impart
any carbon steel contamination to the stainless product,
nor have they been previously used on
carbon steel,
low alloy steel, such as MMFX products,
other non-stainless ferritic materials.