TMT Bar Manufacturing and Testing Procedures Notes

Objective: Focus on manufacturing and testing procedures for TMT BARS.
Manufacturer Principles: Precision manufacturing, rigorous testing, and quality assurance under TEJUKAYA (Established 1888).

Raw Materials: Main inputs include IRON ORE, COAL, and SCRAP.
Mill Types: - Three-high mill utilizing box passes. - Horizontal and vertical rolls with diamond, square, and oval-to-round passes.

HEMATITE ( $Fe_2O_3$ ): $60-70\%$ iron content; non-magnetic oxide ore; primary ore for steel making in India (Odisha, Chhattisgarh, Jharkhand, Karnataka).
MAGNETITE ( $Fe_3O_4$ ): $60-72\%$ iron content; strongly magnetic; used for high-grade steel and pellets.
LIMONITE ( $Fe_2O_3.nH_2O$ ): $40-55\%$ iron content; high moisture and low strength output.
SIDERITE ( $FeCO_3$ ): $45-48\%$ iron content; requires roasting to remove $CO_2$ .

Primary Crushers: Jaw crushers used for initial reduction of raw hematite chunks.
Secondary Crushers: Cone crushers used to achieve uniform particle size.
Grinding Mills: Ball mills grind ore into fine powder (up to 200 mesh or nanometer size).

Continuous Casting: Molten steel solidifies into slabs or billets.
Billet Heating: Heated in reheating furnaces to $1200-1250^{\circ}C$ to ensure plasticity for hot rolling.
Hot Rolling: Heated billets shaped into bars or sections.
Typical Billet Specs: Cross-sections from $100 \times 100\,mm$ to $300 \times 300\,mm$ ; lengths of $6\,m$ to $18\,m$ .

Three Essential Steps: 1. Quenching: Rapid water cooling of austenitized steel transforms it into martensite. 2. Self Tempering: Heat from the core tempers the martensite layer, reducing brittleness. 3. Atmospheric Cooling: Natural cooling stabilizes microstructure.
Microstructure Zones: - Harder Rim: Tempered Martensite (Hardness and strength). - Tougher Mid-radius: Ferrite + Pearlite (Stress resistance). - Softer Core: Acicular Ferrite + Pearlite (Ductility and stress absorption).
Phases: - Pearlite: Lamellar mixture of ferrite ( $\alpha-Fe$ ) and cementite ( $Fe_3C$ ). - Ferrite: Soft, ductile, magnetic phase ( $BCC$ structure).

Carbon (C): $0.2\%$ to $0.3\%$ . Increases strength but reduces ductility.
Manganese (Mn): $0.5\%$ to $1.0\%$ . Enhances yield strength and wear resistance.
Silicon (Si): $0.1\%$ to $0.5\%$ . Acts as a deoxidizing agent.
Sulfur (S) and Phosphorus (P): Must be kept below $0.05\%$ to prevent brittleness and sudden failure.

Corrosion Resistance Alloys: Chromium ( $Cr$ ), Nickel ( $Ni$ ), and Copper ( $Cu$ ).
Analysis: Optical Emission Spectrometer (SPECTROMAXX) used on polished $50-60\,mm$ rebar samples.
IS Standards: Tables for Fe 415, Fe 500, Fe 550, and Fe 600 grades define maximum constituent percentages (e.g., Carbon Max $0.30\%$ , Sulphur Max $0.040\% - 0.060\%$ ).

Tensile Strength: Maximum stress before breaking measured by Universal Testing Machine (UTM).
Yield Strength: Stress level at which permanent deformation begins (enhanced by Carbon, Manganese, and Vanadium).
Tensile Test Process: Specimen prepared to gauge length ( $5 \times \text{diameter}$ or min 200 mm).
Formulas: - Cross-sectional area ( $A$ ): $\frac{\pi d^2}{4}$ - Tensile Strength ( $\sigma_t$ ): $\frac{P_{max}}{A}$ - Percentage Elongation: $\frac{L_f - L_o}{L_o} \times 100$

Bend Test (IS 1786): Specimens bent $180^{\circ}$ around a mandrel; acceptance requires zero visible cracks.
Re-bend Test: 1. Bend to $135^{\circ}$ . 2. Submerge in boiling water ( $100 \pm 5^{\circ}C$ ) for 30 minutes. 3. Re-bend to $157.5^{\circ}$ in the opposite direction.
Gauge Length Rule: $L_o = 5.65 \sqrt{A}$ .
Mandrel Diameter ( $D_m$ ): Determined by $n \times \phi$ (e.g., $n=3$ to $6$ depending on grade).

Stacking: Use wooden sleepers/concrete blocks to maintain height of 150-300 mm from the ground. Maximum stack height is $1.0 - 1.5\,m$ .
Transport: Avoid overloading; protect with tarpaulin; secure bundles with steel straps to prevent shifting.
Key Properties: Strength ( $S$ ), Ductility ( $D$ ), Toughness ( $T$ ), Corrosion resistance ( $C$ ), Weldability ( $W$ ), Thermal resistance ( $T$ ), Fatigue resistance ( $F$ ), and Bond strength (Ribbed surface).

Epoxy Coated (FBE): electrostatic application of epoxy powder; thickness 70-150 microns.
Galvanized (Zinc Coated): Hot-dip process at ~450°C providing barrier and cathodic protection.
Dual Coated: Combination of Zinc layer (60-100 microns) and Epoxy layer (70-100 microns).
Polymer/PU Coated: thickness 100-500 microns; high chemical resistance.
CRS (Corrosion Resistant Steel): Inbuilt protection using alloys like Copper ( $Cu$ ), Chromium ( $Cr$ ), and Nickel ( $Ni$ ) throughout the entire cross-section (not a coating).

Column Fixing: Involves checking BBS (Bar Bending Schedule), marking layouts, fixing vertical bars with stirrups (135° hook), and staggering laps (50% bars at one level).
Beam Fixing: Fix bottom bars first, then top bars and stirrups; maintain clear cover (25-40 mm); avoid laps in high-stress zones.

Primary Steel: Made from Iron Ore via Blast Furnace (BF) / Basic Oxygen Furnace (BOF); involves high energy and $CO_2$ emissions.
Secondary Steel: Made from Scrap via Electric Arc Furnace (EAF) / Induction Furnace (IF); lower cost and energy consumption; widely used for TMT bars in India.

Main Raw Materials: Iron Ore, Sponge Iron (DRI), and Pig Iron.
Scrap Processing: Magnetic separation and water jet cleaning.
Flux Materials: - Lime ( $CaO$ ) and Dolomite: Remove acidic impurities like $SiO_2$ and $P_2O_5$ . - Fluorspar ( $CaF_2$ ): Improves slag fluidity and lowers melting temperature. - Bauxite ( $Al_2O_3$ ): Stabilizes slag and protects furnace refractory. - Quartz ( $SiO_2$ ): Controls slag basicity.
Refining Reactions: - $CaO + SiO_2 \rightarrow CaSiO_3$ - $3CaO + P_2O_5 \rightarrow Ca_3(PO_4)_2$ - $CaO + FeS \rightarrow CaS + FeO$
Argon Stirring: Inert gas blown at 10-20 Nm³/hr/ton to remove dissolved gases and inclusions.