Chapter - 2: Steel & It’s Alloys

CHAPTER - 2

STEEL & IT’S ALLOYS 14 MARKS

STEELS o Steels

are the alloys of iron & carbon in which carbon content is between 0.008 to 0.2 %. o Classification – Amount of carbon o Amount of alloying elements o Method of mfg o Depth of hardening o

STEELS o Amount

of carbon –

Low Carbon Steels o Medium Carbon Steels o High Carbon Steels o

o

Amount of alloying elements – High Alloy Steels o Low Alloy Steels o

STEELS o

Method of Manufacturing – o o o

o

o

Basic Open Hearth Furnace Acid Open Hearth Furnace Electric Furnace Basic Oxygen Furnace

Depth of Hardening – o o o

Non Hardenable Shallow Hardening Deep Hardening

ALLOY STEELS Alloy steels contain other elements like like Ni, Mn, Cr, W, Mo, V etc. o They are superior to plain carbon steels for o

o o o

o o o

More strength, hardness & toughness Better resistance to wear & abrasion High hardenability Less tendency to warping & cracking High corrosion & oxidation resistance Better machinability at high hardness

EFFECTS OF ALLOYING ELEMENTS Hardening / Strengthening of solid solution. o Increase in hardness o Formation of intermediate compounds o Shifting of critical temperature o Lowering critical cooling rate o Change in volume during transformation o Oxidation & corrosion resistance may increase o

EFFECT OF ALLOYING ELEMENTS o Solid

solution Hardening -

strengthening

/

EFFECT OF ALLOYING ELEMENTS o

Solid solution strengthening / Hardening – Most of the alloying elements are soluble in ferrite and form solid solution when added to steel. o Solid solutions are harder and stronger than pure metals and hence these elements increase the strength and hardness of steel. o Elements like P, Si, Mn are more effective as solid solution strengtheners. o

EFFECT OF ALLOYING ELEMENTS o Formation

of carbides –

Some alloying elements combine with carbon in steel and form respective carbides. o These alloy carbides are hard & increase wear & abrasion resistance of steel. o They also increase the resistance to softening at elevated temperatures. o Chromium & Vanadium carbides have maximum hardness. o

EFFECT OF ALLOYING ELEMENTS o Formation

of intermediate compounds

– Some of the elements form intermediate compounds with iron. o These phases increase the brittleness of steels and hence their presence is undesirable. o Ni, Si, Al, Zr, V o

EFFECT OF ALLOYING ELEMENTS o Shifting

of critical temperature and eutectoid carbon – o Elements

which are austenite stabilizers like Ni and Mn lower the eutectoid temp (A1) while the elements which are ferrite stabilizers raise this temperature.

EFFECT OF ALLOYING ELEMENTS o Lowering

of critical cooling rate –

Most of the alloying elements (except Co) shift the IT diagram to right side, thus decreasing the cooling rate. o This effect is very useful for increasing the hardenability of steel. o Mn, Mo, Cr, Ni o

TOOL STEELS o Refer

to a variety of Carbon & Alloy steels that are particularly well suited to be made into tools. o A typical composition contains – – 1% o P – 0.04% o Mn – 0.3 % o S – 0.04 % oC

o Si

– 0.25 %

TOOL STEELS o These

are classified & designated as per American Iron & Steels Institute (AISI) o They are classified as o

Cold Work Tool Steels o Water Hardening

Steels (W-Series) o Oil Hardening Steels (O-Series) o Air Hardening Steels (A-Series) o High Carbon Hi Chromium Steels (D-Series)

o Hot Work Tool

Steels

TOOL STEELS o

o

High Speed Tool Steels Special Purpose Tool Steels

COLD WORK TOOL STEELS Used for cold working of metals o Some of these contain no or very low amount of alloying elements and hence are less expensive. o Depending upon their hardening characteristics they are subclassified as – o

Water Hardening Steels (W-Series) o Oil Hardening Steels (O-Series) o Air Hardening Steels (A-Series) o High Carbon High Chromium Steels (D-Series) o

WATER HARDENING STEELS Essentially plain carbon steels with high carbon content (0.4% to 1.4%) o Because of poor hardenability they are water quenched o Best general purpose steels and are cheap. o Few limitations are o

Strength at elevated temperatures is poor o Distortion during hardening is more which leads to warpage and cracking o They are shallow hardening type steels o

OIL HARDENING STEELS (OHNS) These steels contain small amount of alloying elements such as W, Mn, Cr etc o Hardenability is better than W-series & hence are oil quenched. o Not so expensive and are used in blanking & forming dies, shear blades, guages etc. o The distortion during hardening is less and hence are known as “Oil Hardened Non Shrinkage” Steels. o

AIR HARDENING STEELS These steels have high hardenability due the addition of various alloying elements such as W, Mn, Cr etc in sufficient amounts. o The total amount of alloying elements exceeds 5%. o Have less distortion during hardening, high wear resistance and good depth of hardening. o They are used for slitting dies, drawing dies, intricate die shapes, guages and punches. o

HIGH CARBON HIGH CHROMIUM STEELS (HCHC) Have high hardenability and hence can be oil or air quenched. o Distortion during hardening is less. o They contain Carbonabove 1.5% and in some cases even it goes above 2% with Cr of about 12% and other elements in small amount. o This increases the hardness and wear resistance but are difficult to machine. o They maintain sufficient hardness upto about 500ºC. o

FREE CUTTING STEELS o Contain

Sulphur and Phosphorous o Have high Sulphur content than any other steel. o Carbon – 0.1 – 0.45 % and Sulphur – 0.08 – 0.3 %. o Used in rapid machining. o Now a days Lead is used instead of Sulphur as it improves machinability without loss of toughness.

HIGH CARBON HIGH CHROMIUM STEELS (HCHC) o

Since Oil hardening, Air hardening and HCHC steels show less distortion during hardening they are called nondeforming and non-shrinkage steels.

HOT WORK TOOL STEELS Mainly used for hot working of metals such as for drawing, forming, extruding etc. o They have good toughness, hardness, wear resistance at elevated temperatures. o They are with low to high alloy steels with relatively less carbon and are classified in three categories depending on the principal alloying elements – o

o Chromium type o Tungsten

type o Molybdenum type

HOT WORK TOOL STEELS o

Cr type steel contains – – 7 % Cr with small amount of one or more of other elements. And Carbon content of about 0.350.55%.

o3

o

W type steel contains – o 9 – 18 % W, 2-12 % Cr with Carbon content of about 0.3-0.5%.

o

Mo type steel contains – – 20 % of alloying elements and slightly high Carbon content of about 0.55-0.65%.

o 14

HIGH SPEED TOOL STEELS o These

steels maintain high hardness upto a tem of about 550°c & hence can be used for cutting of metals at high speeds. o They also have high wear resistance & cutting ability. o HSS are divided depending upon principal alloying elements as o Tungsten High Speed Steels o

Molybdenum Steels

STAINLESS STEEL o These

steels have high corrosion resistance and hence they do not corrode in most of usual environmental conditions. o Due to this they are called as ‘Stainless Steels’. o The high corrosion resistance is due to the presence of chromium in these steels.

STAINLESS STEEL o When

Cr is added to steel, it first combines with carbon and forms complex chromium carbides and the remaining Cr goes in solid solution form. o Higher the chromium in solid solution form and lesser the amount of carbides, the corrosion resistance is more.

STAINLESS STEEL o In

addition to Cr, many other elements such as Ni, Mn, Mo, Ti etc are added to improve certain peoperties. o Various types of steels have one or more of the following properties in addition to their high corrosion resistance – High ductility & formability o High resistance to scaling & oxidation at elevated temp. o

STAINLESS STEEL o

o o

Good machinability Good weldability Excellent surface finish & appearance.

STAINLESS STEEL - TYPES o Group A – o This group contains plain Fe-Cr alloys and the amount of Cr in the solid solution is less than 13%. o These steels show austenitic phase at high temp and hence can be hardened by martensitic transformation. o Due to this they are called as martensitic stainless steel. o They are hard, wear resistant & magnetic o They are used for springs, ball bearings, valves, razors, cutting tools, cutlery items etc.

STAINLESS STEEL - TYPES o Group B – o These steels are also plain Fe-Cr alloys but the amount of Cr in the solid solution is more than 13%. o Cr is ferrite stabiliser i.e. it lowers Acm temp & raises A3 temp. At about 12.5 % Cr and no carbon, Acm & A3 temp merge into one another and Austenite phase disappears. o Hence thee steels show only ferrite from roo temp to high temp and are called as ferritic stainless steels.

STAINLESS STEEL - TYPES o Group B – o They can not be hardened by martensitic transformation due to the absence of austenite phase. o Due to the presence of high amount of Cr, their corrosion & oxidation resistance is high as compared to group A steels. o They are soft, ductile, malleable & magnetic in character. o They are widely used for vessels in chemical

& food industries, heat exchagers, juice carrying pipes in sugar indutries etc.

STAINLESS STEEL - TYPES o Group C – o This group includes those alloys which contain at least 24% of the total of Cr, Ni & Mn. o The amount of Cr in this steel is at least 18%. o Since Ni & Mn are austenite stabilisers, these steels are austenitic at room temperature and hence are called as austenitic stainless steels. o They are soft, ductile & non magnetic. o They are used in engine manifolds, utensils, wrist watches, sanitary fittings etc.

STAINLESS STEEL - TYPES o Precipitation Hardenable Stainless Steel –

They contain elements such as Mo, Ti, Al, Cu or N in addition to basic elements Cr & Ni. o Higher strength is developed due to precipitation of certain compound and they maintain high strength upto a temp of about 550°C. o They are used in aircraft and missile industries. o

SPECIFICATION OF STEELS •

Steels are classified on the basis of certain criteria like method of mfg, chemical composition, quality etc. • Majority of the specifications are based on chemical composition because the chemical compo throws light on the probable heat treatment that can be given and the resulting mechanical properties.

SPECIFICATION OF STEELS •

Designers and purchasers have to specify their requirement according to these specs / designations. • Therefore it is necessary to have some knowledge of these specs.

SPECIFICATION OF STEELS •

Indian Standard Designation System (ISI) • Part 1 covers the designation of steel based on letter symbols and part 2 covers the designation of steel based on numerals. • For the purpose of code designation, steels have been classified on the basis of mechanical properties and chemical composition.

SPECIFICATION OF STEELS •

Code designation on the basis of mechanical props is based on the Tesile Strength or Yield Strength. Symbol ‘Fe’ is used to designate minimum tensile strength and ‘Fe E’ to designate minimum yield strength in N/mm2. • Symbol ‘St’ is used when Tensile strength Is in Kg/mm2 and ‘St E’ when yield strength is in Kg/mm2.

SPECIFICATION OF STEELS •

Designation system of steels on the basis of chemical composition consists of a numerical figure indicating 100 times the average % of carbon content. • Letter ‘C’ is used for plain carbon steel and letter ‘T’ is used for tool steels. Letter ‘C’ or ‘T’ is followed by a figure indicating 10 times the average percent of Manganese content.

SPECIFICATION OF STEELS •

Alloy steels are designated In the symbolic form on the basis of their alloy content by first specifying the average content of carbon in hundredths of a percent, followed by the chemical symbols of significant elements in the descending order of percentage content.

SPECIFICATION OF STEELS Typical Examples – • C 20 •

•

•

25 C 5 – •

•

Average C content 0.25% & 0.5% Mn

15 Ni 13 Cr 1 Mo 12 •

•

Steel with average Carbon content 0.2%.

C : 0.15%, Ni : 1.3%, Cr : 1%, Mo : 0.12%

35 Mn 1 S 18 •

C : 0.35%, Mn : 1%, S : 0.18%

SPECIFICATION OF STEELS Typical Examples – • 20 Mn Cr 1 •

•

•

C : 0.2%, Mn 1%, Cr 1%

T 75 W 18 Cr 4 V 1 – •

Tool Steel with C : 0.75%, W : 18%, Cr :4%, V :1% •

T 35 Cr 5 Mo 1 V 30 •

Tool Steel with C : 0.35%, Cr : 5%, Mo :1%, V :0.3%

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