Structural Steel used for Steel Parts

Structural Steel should comply with all the requirements of:

 

Structural Steel should be of  standard quality steel grade Fe 410 and have minimum following requirements,

Yield Stress           ³   250 N/mm 2

Tensile Strength     ³   410 N/mm 2

Elongation             ³    23 %

The acceptable tolerance of the geometrical dimensions of the steel sections is limited to the ±       2.5%.

 

Fabrication of Steel Parts

· Straightening

All materials shall be straight and if necessary, before being worked shall be straightened and/or flattened by pressure, unless required to be of curvilinear form and shall be free from twists.

 

· Cutting

Cutting may be effected by shearing, cropping or sawing. Gas cutting by mechanically controlled torch is permitted for mild steel only. Gas cutting of high tensile steel is also permitted provided special care is taken to leave sufficient metal to be removed by machining so that all metal that has been hardened by flame is removed. No loads shall be transmitted into metal (except welded joints) through a gas cut surface.

 

· Holing and Punching

Holes for bolts or pins shall be drilled (not to be formed by gas cutting). Punching may be permitted for materials with thickness < 14mm, provided the holes are punched 3 mm less in diameter than the required size and reamed after punching to the full diameter. When holes are drilled or punched in one operation through two or more separable parts, these parts shall be separated after drilling or punching and the burrs removed. To obtain uniformity the use of templates and jigs is mandatory for holing.

 

· Assembly

The component parts shall be assembled in such a manner that they are neither twisted nor otherwise damaged and shall be so prepared that the specified cambers, if any, are provided.

 

· Bolting

Where necessary washers shall be tapered or otherwise suitably shaped to give the heads of nuts and bolts a satisfactory bearing. The threaded portion of each bolt shall project through the nut at least two threads. In all cases where the full bearing areas of the bolt is to be developed, the bolt shall be provided with a washer of sufficient thickness under the nut to avoid any threaded portion of the bolt being within the thickness of the parts bolted together. Threading of rods and holes shall be done by machines.

 

· Welding

Welding shall be in accordance with any of the following standards as appropriate, and shall be conducted by appropriately qualified welders:

 

If there are any new editions of Indian Standard those should be used as references. The use of templates and jigs is mandatory for welding of assembly. Following special requirement in conjunction with galvanization has to be made. Sections should have corners cropped to allow free flow of zinc during galvanization.

Avoid narrow gaps between plates. Overlapping surfaces and back to back angles and channels should be avoided. Where small areas are unavoidable edges should be sealed by a continuous pore free weld to prevent penetration of pickle acids.

Galvanization of Steel Parts

· Refer to "Specification for Hot Dip Galvanization of Steel Parts".

 

Reinforcement Steel

· Reinforcement steel should comply with all the requirements for:

· Steel Grade:   Fe 415, High Yield Strength Deformed Bars and have minimum following requirements,

          Yield Strength (0.2% proof stress)   ³   415 N/mm 2

 

Fixtures

· Refer to "Specification for Fixtures".

 

Specification for Hot Dip Galvanization of Steel Parts

1. Galvanization of Steel Parts should confirm to :

 

2. Galvanization Process

Galvanizing should follow the process as below :

 

· Preparatory Work

All welding slags should be removed from the steel parts to be galvanized. Acid traps should be avoided.

 

· Alkaline cleaning

       Rust, oil and other contaminants should be removed from the steel by preliminary treatment with Alkaline cleaning in Sodium Hydroxide Solution of 10 ~ 15% concentration.

The concentration of the solution should be monitored at reguler intervals and adjusted accordingly to have good cleaning effect.

 

· Rinsing

       After Alkaline cleaning, steel should be flushed with running water.

 

· Acid Bath

 The preliminary treated steel is then cleaned by Acid Bath in Hydrochloric Acid Solution of 50% concentration.

 During the operation, close control of acid content in the solution is necessory. It is done by testing for acid and iron contents at reguler intervals. The strength of the solution should be maintained by periodic addition of fresh concentrated acid maintaing P H value in range of 1 to 3 ( P H Meter reading). If the iron content in the solution is more than 100g/l, it should be changed by the new fresh solution. The iron cntent is measured by the specifoc gravity of the solution. Specific gravity of solution should not exceed 1.2 (Hydrometer reading), after which it should be changed by the new fresh solution.

 

· Rinsing

After Acid Bath steel should be flushed with running water.

 

· Pre-flux treatment

 Acid bathed steel is dipped in a solution of Zinc Ammonium Chloride (ZnCl 2 3NH 4 Cl) of 20 ~ 40% concentration. This solution is prepeared by mixing 45% of Zinc Chloride (Zncl 2 ) + 55% of Amonium Chloride (NH 4 Cl).

 The concentration of solution should be controled at reguler intervals. For this specific gravity of the solution is maintained in between 1.10 to 1.15 (Hydrometer reading) by adding required quantities of pre-flux chemicals.  

 

· Dry

  After pre-flux treatment, steel is dried for galvanization.

 

· Galvanization

  The treated steel is dipped into the molted Zinc (zinc bath). Temperature of the molted Zinc should be maintained within the range of 450 0 C to 470 0 C. Appropriate dipping time also should be maintained.

 

· Cooling

After the hot dipp, steel is cooled with water.

 

· Treatment of Threads : 

Extra zinc on threads, which is unsuitable for centrifuging should be removed with a rotating wire brush immediately after galvanizing before the coating sets or if it is not possible, with gas blow and cleaning with rotating brush if galvanization is already hardened.  Re-threading of the galvanized parts is not permitted.

 

3. Quality Checking of Galvanization

 

· Galvanizing Process

Check the galvanizing plant and its set up to be ensured that all the galvanizing procees is available and maintained.

 

· Quality of Zinc

       This can be checked by chemical testing of raw zinc used for the galvanization.  Zinc content should not be less than 98.5%.

 

· Quality of galvanization

By visual check the following :

- Galvanized steel should be free from rust and black spots, acid traps, welding slags.

- Galvanized coating should be free from ash, pimple and bulky deposition.

- Galvanized coating should be continuous, smooth and evenly distributed as far as possible.

- Zinc coat should be adherent.  This can be checked by hammering or scratching.

 

The Hangers (suspenders) of the cross beam of the SSTB Suspended bridge should pass the wrapping test. The test shall be done by wrapping the galvanized hangers/suspenders two round around the 26mm dia pipe or cable. There should not be any flake or crack so that any flakes of zinc can be removed by bare hand.

 

- No zinc paint or aluminum paint is applied to hide rust spots. This is very unhealthy practice, which frequently applied by the galvanizers.

- Zinc thickness can be measured by Deltascope.  In case of trail bridge steel parts, zinc coat should not be less than 80 µm .