Other properties of hot rolled steel
What is the hardening process?
Hardening is a kind of heat treatment process that can improve the plain-carbon steel properties in hardness and abrasive resistance. This process starting with steel is heated to the austenitizing temperature, held in furnace to acquire homogeneous austenitic structure and then quenched at such a rate that martensitic structure is produced or critical cooling rate. In order to obtain fully martensitic structure in steel, the related factors are recommended as following ;
1. Carbon content : The higher percentages of carbon in steel, the more occasion that martensitc structure is produced. Moreover,the alloying elements such as Nickel, Chromium and Molybdenum will also increase hardenability by decreasing the critical cooling rate.
2. Cooling rate : Cooling rate that required to provide martensite should not be lower than the critical cooling rate and also depends on severals factors as follwing ;
– The surface always cools faster than the center of the part. In addition, as the size of the part increases, the cooling rate at any location is slower. Consequently, the smaller part has more possibility to produce fully martensitic structure than the bigger one in the same condition.
– The different quenching media provide different cooling rate. For example, water and brine (water plus various percentages of Sodium choride or Calcium choride) provide a faster cooling rate than oil. In addition, agitation of the quenching media is one method which also increases the cooling rate
Reference: 1.Donald R. Askeland.,The Science and Engineering of Materials, 3rd editionn,PWS Publishing company (1994)
What is the heat treatment of steel and what is the purpos
Heat treatment of steel is a process to improve steel properties especially mechanical properties, by means of thermal process to obtain the appropriate properties for each application. Steels which have been undertaken this process will have the better mechanical properties than those as rolled such as increasing hardness and also abrasive,wear resistance.
What is a Hot-Dip Galvanizing?
The galvanizing process consists of three basic steps;
1. Surface preparation
1.1 Caustic cleaning: A hot alkali solution often is used to remove organic contaminants such as dirt, paint marking, grease and oil from the metal surface. Epoxies, vinyl, asphalt or welding slag must be removed by grit/ sand-blasting or other mechanical means.
1.2 Pickling: Scale or rust normally is removed by pickling in a dilute solution of hot sulfuric acid or ambient temperature hydrochloric acid.
1.3 Fluxing: Fluxing removes oxides and prevents further oxides from forming on surface of metal prior to galvanizing.
– Dry galvanizing process: The steel or iron is dipped or pre-fluxed in an aqueous solution of zinc ammonium chloride. Then material will be dried before immersing in molten zinc.
– Wet galvanizing process: A blanket of liquid zinc ammonium chloride is floated on top of the molten zinc. The steel or iron being galvanized passes through the flux on its way into the molten zinc.
In this step, the material is immersed in a bath (at least 98% pure molten zinc, maintaining at 449°C) until it reach bath temperature. The zinc metal reacts with the iron on the steel surface to form a zinc/iron inter-metallic alloy. The products are withdrawn from zinc bath; the excess zinc is removed by draining, vibrating and/or centrifuging and they are cooled in either water or ambient air immediately.
The two properties with closed scrutinizing are thickness and appearance of coating. The physical and laboratory tests may be performed to determine thickness, uniformity, adherence and appearance.
Reference: American Galvanizers Association, Hot-Dip Galvanizing for Corrosion Protection of Steel Products.
Shearing and Blanking
Shearing is the separation of metal by two blades moving. A narrow strip of the metal is severely plastically deformed to the point where it fractures at the surfaces in contact with the blades. The fracture then propagates inward to provide complete separation.
The clearance between the blades is an important variable in shearing operations. With the proper clearance the cracks that initiate at the edge of the blades will propagate through the metal and meet near the center of the thickness to provide a clean fracture surface.
Insufficient clearance will produce a ragged fracture and also will require more energy to shear the metal than when there is proper clearance. With the excessive clearance there is a greater distortion of the edge and more energy is required because more metal must plastically deform before it fractures.
Too large clearance, burr or sharp projections are likely to form on the sheared edge. A dull cutting edge also increases the tendency for the formation of burrs.
The height of burr increases with increasing clearance and increasing ductility of the metal.
Clearances generally range between 2-10% of the thickness of the sheet; the thicker the sheet the larger the clearance.
Neglecting friction, the force require to shear a metal sheet is the product of the length cut, the sheet thickness, and the shearing strength of the metal.
Blanking: the same concept of shearing
When the metal inside the contour is desired part: call blanking.