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Showing posts with label Alloys Technical. Show all posts
Showing posts with label Alloys Technical. Show all posts

Induction Furnaces | Channel Type and Coreless Induction Furnaces - Advantages and Disadvantages

An Induction Furnace uses induction to heat a metal to its melting point which is based on the theory of Electro Magnetic Induction. Depending on their frequency (50 Hz - 250 kHz) these can be divided to three types:

  1. High Frequency
  2. Medium Frequency
  3. Low Frequency
Their capacities range from less than 1kg to 100MT, which are used for re-melting of iron & steel (steel scrap), copper, aluminium, precious metals and alloys. Even most modern foundries use this type of furnaces and now more iron foundries are replacing Cupolas with Induction Furnace to melt cast iron as the former emit lots of dust & other pollutants. The Steel making via Induction Furnace route has certain advantages & disadvantages:

Advantages of Induction Furnace

  1. It has no electrodes and electric arcs which allow productions of steel & alloys low in carbon and occluded gases without any quality problem.
  2. Low melting losses & alloying elements.
  3. High power efficiency, therefore, cost-effective.
  4. Precise control of the operating parameters.
Disadvantages of Induction Furnace

  1. Refining in Induction Furnace is not as intensive or effective as in Electric Arc Furnace (EAF).
  2. Life of Refractory lining is low as compared to EAF.
  3. Removal of S & P is limited, so selection of charges with less impurity is required.
Induction Furnaces are generally of two types:

  1. Channel type induction furnace
  2. Coreless type induction furnace - Channel Type Induction Furnace

Channel Type Induction Furnace
These furnaces basically consist of a vessel to which one or more inductors are attached. The inductor is actually a transformer whereby the secondary winding is formed with the help of a loop of liquid metal confined in a closed refractory channel. In the furnace the energy is transformed from the power system at line frequency through a power supply to the inductor and converted into heat. One advantage of this type of furnace is that the vessel or upper case can be built in any practical size & shape to suit the application, but the disadvantage are like -
a. Power input limitation per inductor.
b. Necessity to maintain a liquid heel in the furnace always to avoid problems related to operational parameters and refractories.
For the above reasons Channel type Induction furnaces are treated as a receiver or holding vessel for homogenization of liquid metal with limited capability of melting.  - Coreless Induction Furnace

Coreless Type Induction Furnace
These furnaces are designed like a cylindrical crucible surrounded by a power coil in which energy is supplied either directly from the network (line frequency) or through a frequency converter. The magnetic field generated by the coil carries the energy to the charge.

Related Articles

Difference between Stainless Steel, Carbon Steel, and Alloy Steel


The basic difference between stainless steel, conventional alloy steel and carbon steel is that Stainless Steel contains a very high percentage of chromium (11 – 26 percent) and nickel (3.5 – 22 percent).
Through varying chromium content and by addition, substitution of other alloying elements like nickel, molybdenum, copper, titanium, aluminium, silicon, niobium, sulfur, selenium etc., resistance to corrosion, oxidation, abrasion, hardness and a variety of other distinct properties are either created or enhanced.
On the other hand, steel that contains carbon up to about 1.7 percent as an essential alloying constituent and has properties and structure made up mostly of the element carbon is better known as Carbon Steel. Most of the steel produced in the world is carbon steel. Unprotected carbon steel rusts when exposed to air or moisture but stainless steel is almost immune to rusting and ordinary corrosion.

Carbon steels are the base metals widely used in manufacturing in nearly every industry, including aerospace, aircraft, automotive, chemical, defense, and precision. Carbon steel’s strength is due to its crystalline structure. Groups of iron and carbon atoms are arranged in a lattice, with the carbon atoms preventing the iron atoms from slipping over each other, which imparts the steel more rigidity.
The addition of an alloy such as titanium or manganese strengthens this structure by adding different atomic sizes to the lattice. This reinforces steel's rigidity by further impeding molecular movement when the metal is subjected to stresses. There are four types of carbon steel based on the amount of carbon present in the alloy. - image
Stainless Steel Pipes, Board, Kitchen
Low Carbon Steel – Composition of 0.05%-0.29% carbon and up to 0.4% manganese. They are the most common form of steel commonly known as mild steel, a relatively low-cost material, easy to shape (malleable). Low carbon steels provide material properties that are acceptable for many applications.
Medium Carbon Steel – Composition of 0.29%-0.54% carbon, with 0.60%-1.65% manganese. Medium carbon steel is ductile, strong and has good wear resistance. Because of the above properties, they find use in forging, heavy industries, and automotive components.
High Carbon Steel – Composition of 0.55%-0.99% carbon, with 0.30%-0.90% manganese. It is very strong and holds shape memory well, making it ideal for springs and high-strength wires.
Ultra High or Very High Carbon Steel - Composition of 1%-2.1% carbon. Its high carbon content makes it an extremely strong material. Due to its brittleness, this grade requires special handling. However, ultra high carbon steels can be tempered to great hardness and are used for specialized products such as knives, axles etc. Tighter carbon content control for more consistent heat treatment. Steels with carbon content above 2% are considered to be cast iron.
Stainless Steel utilities
Alloy Steel is an iron based mixture containing manganese greater than 1.65%, silicon over 0.5%, copper above 0.6%, or other minimum quantities of different alloying elements such as chromium, nickel, molybdenum, or tungsten are present, each of which imparts different properties to alloy steel. Alloy Steels are made by combining elements during the smelting process when the iron is still molten. Chromium is added in smaller amounts (0.5-2%) to increase hardenability and larger amounts (4-18%) to increase corrosion resistance. Molybdenum is added in amounts of 0.25-0.40% to increase the strength of the steel. Nickel is added in smaller amounts (2-5%) to increase toughness and in larger amounts (12-20%) to increase corrosion resistance. Silicon is added to steel in smaller amounts (0.2-0.7%) to increase strength and in larger amounts (>2%) to improve its magnetic properties while addition of Sulfur or Lead is done to increase the weldability. 
Stainless steel is an alloy developed in the early 1900’s after metallurgists discovered that chromium added iron alloys displayed superior corrosion resistance to carbon steel alloys. The first products using stainless steel were produced in 1908 and the first patents were granted in 1912.
Stainless Steel is a highly durable alloy containing the following major ingredients:
  • 10 - 30 per cent of chromium by mass giving excellent corrosion and oxidation resistance to it.
  • Rest about more than 50 percent iron.
The corrosion resistance that is unique to stainless steel is the result of a transparent passive film of chromium oxide forms on the surface of the steel and protects it from oxidation. Higher chromium levels increase the corrosion resistance of the steel, but it creases the brittleness of the metal, making it hard to work with. There are different categories and types stainless steels. To know more please refer to -

Grades and Series of Stainless Steels - Compositions and Uses

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In continuation of our previous article on stainless steel Categories and Types of Stainless Steels: Compositions and Properties, the key points that will be discussed in short here are (this is the 2nd and last Part of this article):
  • Types of Stainless Steels
  • Series of Stainless Steel
  • Stainless steel compositions
  • Uses of different Series of Stainless Steels, etc.
Martensitic Stainless Steel (Compositions, Properties)
This category is identified by their Martensitic microstructure in hardened condition.   
Martensitic Stainless Steel contain –
  • Chromium: 11 – 18%
  • Nickel: 4 – 22%
  • Carbon: around 1%
  • Manganese: 1 – 1.5%
  • Silicon: 1 – 2%
Due to higher carbon to chromium content ratio, Martensitic Stainless Steel has an exclusive property that it can be hardened by heat treatment. They are usually less resistant to corrosion than other categories of stainless steels. Hardness, ductility and ability to hold an edge are some important characteristics of Martensitic Stainless Steel.
Martensitic Stainless Steel can be cold worked and machined without much difficulty, especially with low carbon content. Martensitic Stainless Steel possesses good toughness and can be hot-worked easily.
Duplex Stainless Steel (Compositions, Properties)
Duplex Stainless Steel has chemical composition and balanced (Duplex) microstructure of approximately equivalent fractions of ferrite and austenitite phases in annealed condition. Duplex Stainless Steel can be characterized by high chromium (i.e. 19 – 32%) and molybdenum (up to 5%) and lower nickel contents than Austenitic Stainless Steel. Addition of nitrogen causes formation of interstitial solid solution which promotes structural hardening and eventually, an increase in yield strength. Duplex Stainless Steel has about twice the yield and tensile strength compared to those of Austenitic Stainless Steel. The toughness of Duplex Stainless Steel grade is superior to that of Ferritic Stainless Steel grade but inferior to Austenitic Stainless Steel. The two-phased-microstructure of Duplex Stainless Steel promotes high resistance to pitting and also stress corrosion cracking.       
Precipitation Hardening Stainless Steel (Compositions, Properties)
Precipitation Hardening Stainless Steel contains 15 – 18% chromium and is a kind of chromium-nickel alloys.
In annealed condition, they may be either austenitic or Martensitic. They develop high strength with heat treatment. Precipitation hardening Martensitic stainless steel has corrosion resistance comparable to austenitic stainless steel but can be precipitation hardened to even higher strengths than other martensitic grade stainless steels.        

STAINLESS STEELS SERIES (Stainless Steel Grades)       
Stainless Steels are classified into three series:
  1. 200 series Stainless Steels
  2. 300 series Stainless Steels
  3. 400 series Stainless Steels
The following table shows the chemical compositions of the above three Series of Stainless Steels:
Chemical Compositions of 200, 300 and 400 Series of Stainless Steel
Chemical Composition
(in percentage)
200 series Stainless Steel
300 series Stainless Steel
400 series Stainless Steel
5.5 – 10.0
2.0 (max)
1.5 (max)
1.0 – 6.0
6.0 – 22.0
1.5 (max)
15.5 – 19.0
16.0 – 26.0
10.5 – 19.0
Iron & Other Minerals

The major uses or applications of 200, 300 and 400 Series Stainless Steels
200 Series Stainless Steels
The 200 series stainless steel is mainly used in making utensils, households including kitchen appliances, architecture and decorative items, furniture, tubes, pipes, automobiles, railways and transport industries.
300 Series Stainless Steels
The 300 series stainless steels have got all uses mentioned under 200 series besides, in refineries, petrochemicals, diary farm industries. They are also used in high temperature instruments such as in electric furnaces, pharmaceutical appliances, nuclear appliances, power plants, railway coaches, automobiles etc.
400 Series Stainless Steels
The 400 series stainless steels have some special uses such as in coinage, auto-exhausts, razor blades, power packing in petrochemicals etc. Besides, these are also used in consumer durable and transport industries.      

Also Read

Categories and Types of Stainless Steels: Compositions and Properties

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The article is about an alloy, a material without which perhaps it won’t be possible to pass a single day in our life. Yes, we are talking about stainless steel and as the title suggests the key points that will be described in short here (this is the 1st Part of this article):
  • What is stainless steel (alloy steel);
  • Metallurgical classification of stainless steels into various types and categories;
  • Stainless steel composition
  • Important properties of the various categories of stainless steel, and their specific uses  
Types and Categories of Stainless steel

Stainless Steel is a highly durable alloy of iron containing about 10 – 30 per cent chromium, 0.3 – 1.0 percent carbon and some other metals as minor constituents, possessing excellent oxidation, corrosion and fatigue resistance properties. Stainless steel has also an aesthetic appeal, excellent lusture, low wear, high strength and durability.
CLASSIFICATIONS – What are the different grades of stainless steel
Stainless Steels are usually grouped into 5 metallurgical categories (grades):
  1. Austenitic Stainless Steels
  2. Ferritic Stainless Steels
  3. Martensitic Stainless Steels
  4. Duplex Stainless Steels
  5. Precipitation hardening Stainless Steels
Austenitic Stainless Steels (Compositions, Properties)
Industry Guru - representative image
These steels have an austenitic crystal structure formed through the use of austenitizing elements like nickel, manganese and nitrogen. These steels have austenitic structure from room temperature to temperature below melting range and hence, can not be hardened by heat treatment.
Austenitic Stainless Steels possess the highest corrosion resistance of all Stainless Steels and also have the greatest strength and resistance at high temperatures. These steels are capable of retaining ductility at temperatures as low as zero degree.
Compositions of Austenitic Stainless Steels:
  • Chromium: 16 – 26%
  • Nickel: 4 – 22%
  • Carbon: 0.03 – 0.25%
  • Manganese: 2 – 10%
  • Silicon: 1 – 2%
The purpose of adding nickel in austenitic grades is to increase density, co-efficient of thermal expansion and also corrosion resistance. Austenitic Stainless Steels have superior impact strength and toughness as compared to those of Ferritic Stainless Steels. The addition of manganese (Manganese Nickel Austenitic Stainless Steels) helps in achieving even higher strength than only nickel bearing grades over a wide range of temperatures. The yield strength also increases by nearly 40 percent while offering greater resistance to stress corrosion cracking carbide precipitation and pitting.     
Ferritic Stainless Steels (Compositions, Properties)
Industry Guru - representative image
Ferritic Stainless Steels are basically chromium based alloys and are generally ferritic at all temperatures although some of the grades exhibit austenitic structure at high temperatures and can transform to Martensite.
Compositions of Ferritic Stainless Steels:
  • Chromium: 11 – 26%
  • Carbon: 0.08 – 20%
  • Manganese: 1 – 1.5%
  • Silicon: 1 – 2%
In annealed condition, these steels show fully ferritic structure at room temperature. Ferritic Stainless Steels are not strengthened by heat treatment. In annealed condition, Ferritic Stainless Steels develop maximum softness, ductility and corrosion resistance.
Ferritic Stainless Steels are ferro-magnetic, ductile and malleable. But at elevated temperatures mechanical properties of Ferritic Stainless Steels are relatively inferior to the Austenitic Stainless Steels. Ferritic stainless steels are more corrosion resistance than Martensitic Stainless Steels. 

The GRAVITEL process for Electro Galvanizing of steel strip: Technique, Control, Advantages and Applications

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The GRAVITEL Process – Introduction
The GRAVITEL or Gravity for Electroplating process was developed by Maschinenfabrik Andritz (Ruthner Division) in Vienna, Austria at the beginning of 1980. The first large line was started in September’85 at Voestalpine in Linz, Austria. At present there are about ten high capacity electro galvanizing lines (EGL) around the world using GRAVITEL system successfully. It is estimated that by 2010, the installed capacity of GRAVITEL will exceed 4 million tons (MT) per year. The GRAVITEL process is carried out in a vertical cell, where the strip is running at a narrow distance between two movable anode boxes. The anodes are made of titanium sheet coated with conductive iridium oxide. The electrolyte then flows into the gap between anode and strip and is accelerated by gravity up to a speed of 5 meters/sec. High electrolyte flow enables high capacity electroplating at a current density up to 180 amp per square decimeter. 
GRAVITEL cell image
 Fig: GRAVITEL cell
Gravitel Process of Electro Galvanizing: Electrolyte Flow mechanism image
Fig: GRAVITEL - Electrolyte flow controlled by Frequency controlled Pump
The vertical GRAVITEL cell has high flexibility according to the width, shape of the steel strip and the plating mode i.e. one or two side coating. The variation of the strip width is adjusted by the electrolyte flow in the anode boxes, which is set by frequency controlled pumps. The GRAVITEL process is further characterized by even coating distribution; no edge masks are required since no edge over coating occurs. Each cell has 2 pair of anodes which can be engaged or disengaged hydraulically. In the engaged position the anode gap can be easily varied according to the waviness of the strip. The narrow gap between the anodes and the strip results in low ohmic loss in the electrolyte which leads to low energy consumption. While coating only one side, the electrolyte flow to the inner anode boxes is stopped and the inner anode boxes are moved away from the strip. Such operations are performed automatically and sequentially as the weld seam moves through the plating cells without interrupting the production or causing any strip loss. The plating current is transferred from the rectifier to the steel strip via the conductor roll. The contacting surface of the conductor roll is made of high duty alloys. In order to get the highest surface quality of the coated strip each conductor roll is rinsed in a tray and polished by an oscillating scraper. A closed loop cooling circuit is provided from inside to cool the conductor rolls.
Refractory Lining | Steel Technology | Jobs : Gravitel Process graphics
Fig: GRAVITEL- Automatic Anode adjustment for Strip Waviness
Zinc Coating (Galvanizing)
The GRAVITEL process uses a sulfate electrolyte in which zinc is plated at the cathode (the strip). The electrochemical reaction that takes place is –
Zn2+ + 2e → Zn
At the anode acid ions and oxygen are formed –
H2O → 2H+ + ½ O2 + 2e
The cathode current efficiency is high around 97%, the remainder being hydrogen evolution reaction. For each equivalent of zinc plated, exactly one equivalent of acid is generated, which can in turn be used to dissolve one equivalent of zinc in the dissolving station, according to the following reaction –
Zn + 2H+ → Zn2+ + H2  
The hydrogen evolved as above is diluted by air to a safe level well below the lower explosion limit of hydrogen-air mixture. The hydrogen level in the off-gas of the dissolving station is constantly monitored by a hydrogen detector. From the above equations it can be seen that maintaining the proper bath chemistry is also easy because the plating-dissolving process is actually self-regulating. Thus the GRAVITEL process of electro galvanizing is more advantageous than the systems with soluble anodes where the anodic dissolution efficiency is generally higher than cathode plating efficiency, which leads to a surplus of metals in the electrolyte. This is balanced by the partial use of insoluble anodes or (especially in the case of chloride electrolytes) by bleed-off of considerable quantities of electrolyte.

GRAVITEL process of Electro Galvanizing
Process speed
Up to 180 m/minute
Current efficiency
Electrolyte velocity
Up to 5 m/sec
Electrolyte flow / cell
Up to 1000 m3/h
Current density
Up to 180 A/dm3
Insoluble Ti anodes with IrO2 coating
Anode – strip gap
8 mm
GRAVITEL: Process steps
To improve the flatness of the strip first it is passed through a tension leveler. This is required to run the anodes in the GRAVITEL cells in “narrow position” which enables an energy saving production and assures best coating distribution over the strip width. The strip then enters the chemical degreasing section where oil, grease and other similar particles present, if any, on the strip are removed from the surface in order to prepare it for subsequent coating. The cleaning is done by spraying hot alkaline solution onto both sides of the strip. An additional cleaning is also done by means of mechanical brushing of the strip. Further, a vertical electrolyte degreasing cell ensures that all the particles have been removed from the surface of the strips. The process is based on the central conduction system, i.e. there is no conductor rolls used as the current is applied directly to the strip via the electrolyte and also discharged likewise. When a strip is passing through a pair of cathodes, the strip surface will be polarized as anode and vice-versa. Hydrogen and oxygen are evolved respectively, on the strip surface resulting in a high cleaning effect. The degreasing section is followed by a counter current 3 -stage rinsing section to remove remaining cleaner solution from the strip. The first rinsing stage is executed as scrub brush machine for additional mechanical cleaning of the strip surface. Thereafter the cleaned strip proceeds to the pickling section where the strip surface is activated for the subsequent plating. This section consists of a vertical cell of the flooded type with dilute sulfuric acid as pickling agent. A 3 – stage rinsing section assures a clean strip for the galvanic process in the GRAVITEL cells. The actual plating takes place in a number of GRAVITEL cells as above.      
Electrolyte Circulation System
All GRAVITEL cells are connected to a big electrolyte circulation tank. Besides, there are various other circulation systems. The enrichment of the electrolyte with zinc proceeds in a conical tank filled with zinc granules – the zinc dissolving section. The electrolyte is brought upwards through the granules and overflows the dissolving tank into the settling tank where trapped hydrogen can escape from the liquid. The rate at which zinc dissolves depends on many factors such as electrolyte temperature, sulfuric acid content and its rate of flow through dissolving tank. The pump which controls the electrolyte flow to the dissolving station is equipped with variable speed drive in order to adapt the dissolving intensity to the actual zinc content of the electrolyte determined by total current consumption in the plating cells and by means of electrolyte analysis. The liberated hydrogen content in the exhausted air is monitored to avoid any danger. If the hydrogen level reaches the first limit the speed of the dissolving pump is reduced to the minimum. Then the dissolving station is drained and flushed with pure water to stop the hydrogen evolution. To separate the accumulated water from the electrolyte – mainly from the spraying pipes at the conductor rolls – a flash evaporator is installed which consists of two stages and operates at a low absolute pressure. The electrolyte heating and cooling is integrated in the evaporator control. Here the joule’s heat from the plating process is used for the evaporation, simultaneously cooling the electrolyte. The achieved distillate quality allows reusing the water as ‘demineralised’ water in the rinsing sections of the line. Thus the zinc electrolyte is never exchanged and is operating in a closed loop with zero effluents.     
After the plating process is finished, the strip is cleaned in a 4-stage rinsing section. The first stage can be operated as acidic rinse where the acid concentration is adjusted by a conductivity control loop; the three following stages are operated as cascade rinsing.
GRAVITEL: Process Control and Post Treatment
The process solution section is fully automatically controlled by Andritz Line Master (ALM-EGL). Based on primary coil data and the requested coating weight the coating algorithm calculates the set-point for process speed and rectifier current for each cell continuously in order to maximize the output of the electro galvanizing lines. A closed loop process control corrects the calculation based on the online measured zinc dissolving pump.
There are various possibilities of post treatment which can be done according to the customer requirement and application. The following post treatments are possible in an electro galvanizing line –
  1. Phosphatizing
  2. Anti-fingerprint coating
  3. Passivation
  4. Pre-lubrication coating
  5. Oiling
In case of phosphatizing the layer is applied by spraying. The surface of the strip must be activated for the subsequent phosphatizing. The phosphate layer serves as a lubricant for deep drawing and a basis for later applied paint. The phosphatizing solution is circulated through a bag filter and is heated up by a heat-exchanger which is operated with hot water. This operation is followed by a 3-stage rinsing section.
Antifingerment, passivation or pre-lubrication coatings are typically applied in a roll coater. In a roll-coater, the top and bottom side of the galvanized strip are coated with the respective liquid chemical. In order to achieve the required coating thickness the applicator and the pick-up rolls are driven by speed controlled motors and adjusted automatically. The roll coater is followed by a strip drier and an air-cooling section. In the exit portion of electro galvanizing line, an electrostatic oiler allows an application of protective oil on the finished (coated) strip.
Gravitel Process: Advantages and Applications
With more than ten operating plants and more under construction, the GRAVITEL system is the most widely used electro galvanizing process worldwide. The key benefits are highest surface quality, even coating distribution, high capacity electroplating and high flexibility in terms of strip width adjustment and single or double sided coating, energy- and zinc- saving operation with minimum amount of downtime. GRAVITEL process has got applications for electro-galvanizing of automotive exposed sheet, household and office appliances like air conditioners, microwaves, washing machines, flat TVs, computers etc. Important players in steel industry from America, Asia (especially China), and Europe chose the GRAVITEL process from Andritz AG for their new electro galvanizing lines.