The Constructor

Stainless Steel Reinforcing Bars – Types and Properties

Sadanandam Anupoju

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What are Stainless Steel Reinforcing bars?

Stainless steel reinforcing bars are low carbon steel alloy bars and contains minimum 12% of chromium and 8 to 12% of nickel in it. Stainless steel bars are highly corrosive resistant and they can be cast, pressed and machined.

Types of Stainless Steel Reinforcing Bars

Based on the microstructure, stainless steels can be divided into four types which are:
  1. Martensitic Stainless Steel
  2. Ferritic Stainless Steel
  3. Austenitic Stainless Steel
  4. Austenitic-Ferritic or Duplex Stainless Steel

Fig 1: Stainless Steel Reinforcing Bars

Among the four types, only Austenitic and Duplex stainless steel bars are used as reinforcing bars in concrete structures.

1. Austenitic Stainless Steel

Austenitic stainless steel contain minimum 7 % of nickel which gives ductility to the steel, large scale of service temperature, nonmagnetic nature and also good workability while welding. This type of steel is mostly used stainless steel especially as reinforcement bars, industrial piping, and architectural facades etc. It is good against corrosion.

Fig 2: Stainless Steel Reinforcement Mesh

Austenitic Stainless steels are classified into different grades in different countries based on the proportions of main elements. Which are as follows

Table 1: Classification of Austenitic Steel according to International Standards

Steel Type                  Europe USA Britain Sweden
EN 10088-1 (DIN) France AISI BS SS
        Austenitic 1.4301 X5CrNi 18-10 304 314S11/314S15 2332
1.4401 X5CrNiMo 17-12-2 316 316S33 2347
1.4429 X2CrNiMoN 17-13-3 316LN 316S63 2375
1.4436 X5CrNiMo 17-12-2 316  - 2343
1.4571 X6CrNiMoTi 17-12-2 316Ti  - 2350
  The French designation given in above table represented as

2. Austenitic-Ferritic or Duplex Stainless Steel

Austenitic-Ferritic steel has combined properties of austenitic and ferritic stainless steel hence it is also called as duplex stainless steel. It contains less amount of nickel than austenitic. It provides good strength as well as ductility.

Fig 3: Stainless Steel Bars of Different Sizes

It is very good resistant against corrosion. Duplex steels are widely used in paper, pulp, petrochemical industries etc. Duplex steel are also classified according proportion of main elements with different grade in different countries.

Table 2: Classification of Duplex Steel according to International Standards

Steel Type Europe USA Britain Sweden
EN 10088-1 (DIN) France AISI BS SS
Ferritic – Austenitic (lean duplex type   1.4362   X2CrNiMo 23-4   2304   -   -
Ferritic – Austenitic (duplex type)   1.4462 X2CrNiMoN 22-5-3   2205   318   2377
 

Composition of Stainless Steel Reinforcing Bars

Stainless steel bars majorly contain chromium and nickel as the second major compound. They also contain some other elements like molybdenum, nitrogen and very less amount of carbon. Composition of different grades of stainless steels is expressed in below table. The grades specified by AISI are taken into consideration.

Table 3: Composition of Stainless Steel Types

Type of Steel Grade (AISI) Chromium (%) Nickel (%) Molybdenum (%) Nitrogen (%) Carbon (%)(max)
  Austenitic 304 18 8 0.08
316 17 12 2.5 0.08
316LN 17 12 2.5 0.13 0.03
Duplex 2304 23 4 0.3 0.1 0.03
2205 22 5 3 0.14 0.03

Functions of Ingredients of Stainless Steel Bars

Chromium, the major element of stainless steel gives protection against corrosion by forming an oxide film on the steel surface. This film is adherent, rough and invisible.  Whenever the bar is attacked chemically this film prevents the bar by releasing small amounts of oxygen. Increase in chromium content will increase the chances of formation of oxide film under critical conditions. Nickel improves the properties like ductility and formability of stainless steel. Other elements like Molybdenum, nitrogen are also improves the corrosion resistance along with chromium.

Fig 4: Stainless Steel Reinforcement for Foundation

Properties of Stainless Steel Reinforcing Bars

The Properties of stainless Steel reinforcing Bars are
  1. Corrosion Resistance
  2. High Strength
  3. Higher ductility
  4. Cryogenic Resistance
  5. Fire Resistance
  6. Magnetic Response

1.Corrosion Resistance

The main property of stainless steel is the corrosion resistance.  As explained earlier the corrosion resistance is obtained by the oxide layer formed by chromium content of steel.

2. High Strength

The tensile strength stainless steel bars is much more than mild steel bars. In stainless steels, duplex steel bars are stronger in tension than austenitic steel bars.

3. Higher ductility

Ductility of stainless steel is due to nickel content. Higher the nickel content higher the ductility. Austenitic steel has more ductility than duplex steel.

4. Cryogenic Resistance

At low temperatures, Austenitic Stainless steel bars have high tensile strength with excellent toughness. Other types like martensitic and ferritic are not suitable in cryogenic areas.

5. Fire Resistance

Steel generally has good fire resistance and when it comes to stainless steel, even at high temperatures it cannot lost its strength.

6. Magnetic Response

Except austenitic steel, all other types are able to attract to the magnet. Higher quantity of nickel eliminates the magnetic response of stainless steel.

Table 4: Comparison of Properties of Austenitic and Duplex Stainless Steels

    Property                                     Type of Stainless Steel
Austenitic Duplex
Yield Strength (Mpa) 205 -500 330-560
Tensile Strength (Mpa) 550-860 570-1000
Elongation percentage 30-60 25-40
Magnetic No Yes
 

Advantages of Stainless Steel Reinforcing Bars

Following are the advantages of Stainless steel reinforcing Bars.

Fig 5: Belt Parkway Bridge, NY, Constructed using SS Rebar

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