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Categories and Types of Stainless Steels: Compositions and Properties

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04-Aug-2020
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)
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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)
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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. 

Kyanite - Properties and Indian Occurrences

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25-July-2020

Kyanite which is an aluminium silicate with chemical formula Al2SiO5, belong to the Sillimanite group of minerals comprising Sillimanite, Kyanite, Andalusite, Dumortierite and Topaz. Kyanite is an important raw material for high alumina refractories. Especially, Raw Kyanite is extensively used for making high alumina insulation Refractory Bricks.



Apart from refractory industry kyanite particularly its blue variety, is also used as gem stone. The kyanite gem stone is believed to possess certain metaphysical properties with its ability to keep the mind calm and anxiety under control. The name Kyanite was derived from the Greek word Kyanos which means blue.

Mineralogy of Kyanite
https://www.industry.guru/2020/07/kyanite-properties-and-indian-occurrences.html - image of Kyanite lumps
Raw Kyanite (Lumps)
Kyanite is found as subhedral and tabular to elongated, thin, bladed crystals having blue or light-green colour in the form of crystalline aggregates in schists, gneisses, granite pegmatite and occasionally in eclogites. The crystal system is Triclinic; optically kyanite is colourless and feebly pleochroic from pale-blue to colourless with one set of perfect cleavage, first and second order interference colour (yellow, grey and blue). The distinguishing features of kyanite are its higher refractive index than those of Sillimanite and Andalusite while birefringence is lower. The oblique extinction angle up to 32O together with the biaxial interference, negative optic sign, and large optic axial angle are also distinctive for kyanite. Its hardness varies from 4 to 7 (Moh’s scale) and specific gravity is around 3.6 - 3.7.

Indian Occurrences
In the Indian subcontinent very good gem quality kyanite is found Nepal.
Kyanite is formed at medium temperatures and high pressures in a regionally metamorphosed sequence of rocks and is found associated with minerals like - muscovite, quartz, garnet, staurolite and rutile. Kyanite is also found as detrital mineral. For the Use of Kyanite in Refractory Industry the Directorate General of Technical Development (DGTD) has recommended the following specification:


Grade-I
Grade-II
Grade-III
Al2O3 (min)
58%
54%
46-48%
Fe2O3 (max)
1.5%
1.5%
2%
PCE (min)
37 (Orton)
37 (Orton)

  
Recoverable reserves of medium to high grade kyanite in India and the current trend of production-utilization causes serious concern because of dwindling availability of this mineral in India. During 1960’s the hard, massive, lumpy variety of kyanite with Alumina content more than 61% and Iron content around 0.8% of Lapsa Buru mines in Kharswan (Bihar) was the largest deposit in the world. Today the source has dried up. Only poorer quality is now available which cannot be used as such. Deposits of kyanite available in a few other places some of which are being mined and supplied at present are -

Chemical Compositions of Indian Kyanite deposits

SiO2
Al2O3
Fe2O3
TiO2
CaO
MgO
Na2O + K2O
LOI
Lapsa Buru (Bihar)
34.8
61.1
0.5 -1.3
--
0.2 -0.3
0.2
--
0.5
Singhbhum (Jharkhand)
46.5
45.97
0.5
1.5
--
--
--
1.1
Kudineerkati (Karnataka)
40.32
58.15
1.7
Tr.
0.52
--
--
1.32
Sulia (Karnataka)
32.8
61
1.85
2.7
--
--
--
1.65
Purulia
(West Bengal)
38.8
46.65
2
Tr.
--
--
--
1.65
Khammam (A.P.)
51.92
27
11.25
--
--
--
--
--







HeidelbergCement India - Lackluster Q1 performance due to covid lockdown restrictions

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22-July-2020
CEMENT INDUSTRY NEWS | INDUSTRY GURU
HeidelbergCement India Ltd on Wednesday reported a 38.07 per cent decline in net profit at ₹ 48.94 crore for the first quarter ended June 2020.
The company had reported a net profit of ₹ 79.03 crore in the April-June 2020 period of the preceding fiscal, HeidelbergCement India said in a BSE filing.
Total revenue from operations declined 30.80 per cent to ₹ 407.70 crore from ₹ 589.23 crore in the corresponding quarter of 2019-20.

The company said its sales volume declined due to suspension of operations during covid lockdown period. "Volume decreased by 32 per cent, primarily driven by the suspension of operations in April 2020. The decrease in volume impacted revenue and profitability during the quarter," said HeidelbergCement India, a subsidiary of HeidelbergCement Group.
Cement sales volume during the quarter was 857 KT as against 1,258 KT earlier. Total expenses were at Rs 342.99 crore as against Rs 479.25 crore in Q1 FY20, down 28.43 per cent.
In an interview to ET Jamshed Naval Cooper, MD of HeidelbergCement India Ltd said, capacity utilization will be hovering between 55% and 60%. Today many of the cement companies will have a very high breakeven.
You cannot keep a cement processing plant shut continuously. We made a big mistake of having a lockdown in the cement plant, he said. However, he is hopeful that after Diwali (which falls in November this year), most migrant labourers will return and capacity utilisation of cement plants should touch 70%.


Use of Kyanite as Refractory Raw Material

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21-July-2020


Kyanite is one of the Alumino-Silicate group of minerals comprising Sillimanite, Kyanite, Andalusite, Dulmortierite, Topaz and Mullite all with similar chemical composition but different physical behavior and hence, uses. Kyanite, raw as well as calcined, have separate applications as refractory raw materials because of their distinct characters. 

Raw Kyanite

Industry Guru - image of raw kyanite
Raw Kyanite (Refractory Grade)
To be suitable for refractory raw material, kyanite should have very negligible amount of impurities which include free silica, alkali, iron oxides, calcium and magnesium. Kyanite, Sillimanite and Andalusite all these three minerals convert to Mullite and silica when they are calcined to temperatures between 1250OC and 1500OC. The inversion kyanite to mullite and silica (glass) begins at the periphery of grains and this rate of conversion depends the following:
>> Particle size
>> Firing temperature
>> Soaking time
>> Impurities present.
The heating of raw kyanite is accompanied with its volume expansion and a decrease in its specific gravity from 3.6 to 3.06, which takes place over a small range of temperature around up to 1350OC. 
Because of this property raw kyanite - 
  • Raw kyanite is extensively used for making high alumina insulation bricks, insulating mortars and castables.
  • Raw kyanite fines (pulverized) can be added to refractory mixes in different proportions as a measure to control or compensate the overall shrinkage due to other raw materials. 


Calcined Kyanite

Industry Guru - image of calcined kyanite lumps
Calcined Kyanite (lumps) to be used in making Refractories
In order to make it a volume stable refractory material, kyanite is pre-calcined at 1420OC to mullite and Cristobalite before use. Sometimes the lumps are very hard to crush after calcination. To avoid this, after calcination the kyanite lumps are, sometimes, quenched in water to make them crumble easily. Thereafter, it is ground, graded into various fractions as per requirement. These grains of calcined kyanite being volume stable are used with other raw materials for making refractory bricks and castables. The various refractory properties are:
P.C.E - above 1785OC or 35+ (Orton)
RUL (refractoriness under load) - 1750OC
Porosity = 24 - 25%
Properly calcined kyanite is a very good refractory raw material because of its high alumina percentage and low iron contents. As compared to other refractory raw materials kyanite can be sometimes, very handy for boosting alumina content and other refractory properties of the product at the same time maintaining its cost effectiveness.
Refractories made from calcined kyanite possess:
  • Higher modulus of rupture and creep resistance
  • Lower co-efficient of expansion
  • High thermal shock resistance
  • Increased durability, about three times that of ordinary bricks
  • Superior resistance to salt attacks and chemical corrosion
  • Very low co-efficient of spalling
Related Article: Kyanite - Properties and Indian Occurrences

South African cement industry worst affected due to corona virus lockdown restrictions

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19-July-2020
CEMENT INDUSTRY NEWS | INDUSTRY GURU
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Cement Plant
PPC head of inland business Bheki Mthembu, says the lack of large-scale construction projects in his country has left South African cement industry heavily dependent on residential construction. “Demand is less than the supply. Most of our cement goes to retailers and then local builders, but we still cater to larger companies when bulk deals are required. The lack of large-scale construction projects has left the industry heavily dependent on residential construction. Government needs to support us through infrastructure maintenance and other projects. We were already in survival mode; Covid-19 has almost been the final nail in the coffin,” said Mr. Bheki Mthembu.




South African cement industry at present are working at roughly 50% of the capacity utilisation level in June 2020 compared to that in June 2019 following the restart of production due to the relaxing of the corona virus lockdown to Level 3 from Level 4 on 1 June 2020. The construction sectors reduced by more than half since the 2010 FIFA World Cup when it supported 250,000 jobs. While the Level 5 government lockdown restrictions brought industry to a standstill in March, cement production restarted in May under Level 4.
However, construction was still prohibited and local stores had shortages of cement when they were allowed to open with restrictions being reduced to level 3 in June. The price of cement also has gone high sharply from ZAR80 (US$4.77) to ZAR120-150 (US$7.16-8.96) in June as cement producers have been facing tough situations in sourcing raw materials to produce cement.

 (Source: globalcement.com - edited)