Aneealing of Steel

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7/29/2019 Aneealing of Steel

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MATERIAL SCIENCE LAB IIIRD

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DEPARTMENT OF MECHANICAL ENGINEERING

EXPERIMENTNO:

OBJECT: To study of hardening of steel and effect of quenching medium on hardness.

THEORY: Hardening of steel

Hardening of steel is obtained by a suitable quench within or above the critical range. The

temperatures are the same as those given for full annealing. The soaking time in air furnaces

should be 1,2 min for each mm of cross-section or 0,6 min in salt or lead baths. Unevenheating, overheating and excessive scaling should be avoided. The quenching is necessary to

suppress the normal breakdown of austenite into ferrite and cementite, and to cause a partial

decomposition at such a low temperature to produce martensite.to obtain this, steel requires

a critical cooling velocity, which is greatly reduced by the presence of alloying elements,

which therefore cause hardening with mild quenching. The quenching is necessary to supress

the normal breakdown of austenite into ferrite and cementite, and to cause a partial

decomposition at such a low temperature to produce martensite. To obtain this ,steel requires

a critical cooling velocity ,which is greatly reduced by the presence of alloying elements,

which therefore cause hardening with mild quenching(e.g. oil and hardening steels). Steels

wit less than o,3% carbon cannot be hardened effectively,while the maximum effect is

obtained at about 0,7% due to an increased tendency to retain austenite in high carbon steels.


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Fig.1. Variation of hardness of martensite and bainite with carbon content

Water is one of the most efficient quenching media where maximum hardness is

required,but it is liable to cause distortion and cracking of the article.Where hardness can be

sacrificed,whale,cotton seed and mineral are used.These tend to oxidise and form sludge

with consequent lowering of efficiency.The quenching velocity of oil is much less than

water.Ferrite and troostite are formed even in small sections.Intermidiate rates between

water and oil can be obtained with water containing 10-30% Ucon,a substance with an

inverse solubility which therefore deposits on the object to slow the rate of cooling.to

minimise distortion,long cylindrical object should be quenched vertically,flat sections

edgesways and thick sections should enter the bath first.To prevent steam bubbles forming

soft spots ,a water quenching bath should be agitated.

Full hardened and tempered steels develop the best combinations of strength and non-

ductility.


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Tampering & toughening

The martensite of quenched tool steel is exceedingly brittle and highly stressed.Consequently

cracking and distortion of the object are liable to occur after quenching.Retained ausentite is

unstable and as it changes dimensions may after,e.g.dies may alter0,012 mm. It is

necessary,therefore,to warm the steel below the critical range in order to relieve stresses and

to allow the arrested reaction of cementite precipitaions to take place.This is known as

tempering.

150-250C.The object is heated in an oil bath, immediately after quenching, to preventrelative cracking, to relieve internal stress and to decompose austentite without muchsoftening.

200-450C. Used to toughen the steel at the expense of hardness. Brinell hardnessnumber is 350-450.

450-700C. The precipitated cementite coalesces into larger masses and the steelbecomes softer. The structure is known as sorbote, which at the higher temperatures

becomes coarsely spheroidised. It itches more slowly than troostite and has a brinell

hardness number 220-350. Sorbite is generally found in heat-treated constructional steels,

such as axels, shafts, crankshafts subjected to dynamic stresses. A treatment of quenching

and tempering in this temperature range is frequently referred to as toughening, and it

produces an increase in ratio of the elastic limit to the ultimate tensile strength.

The reactions in tempering occur slowly. Reaction time as well as temperature of heating

is important. Tempering is carried to an extend under pyrometric control in oil, salt (e.g

equal parts of sodium and potassium nitrates for 200C-600C) or lead baths and also in

furnaces in which the air is circulated by fans. After the tempering, the objects may be

cooled either rapidly or slowly, except for steels susceptible to temper britellness.

Temper colours formed on a cleaned surface still used occasionally as a guide totemperature. They exists due to the interference effects of thin films of oxides formed

during tempering , and they act similarly to oil films on water.

Alloys such as stainless steel forms thinner films than do carbon steels for a given

temperature and hence produce a colour lower in the series. For example, pale straw

corresponds to 300C, instead of 230C(table 1)


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Temper colour Temperature C Objects

Pale straw 230 Planning and slotting tools

Dark straw 240 Milling cutters and drills

Brown 250 Taps, shear blades for metals

Brownish-purple 260 Punches, cups, snaps, twist drills,reamers

Purple 270 Press tools, axes

Dark purple 280 Cold chisels, sets for steel

Blue 300 Saws for wood, springs

Blue 450-650 Toughening for constructional steels

For turning , planning , shaping tools and chisels , only the cutting parts need hardening. This is

frequently carried out in engineering works by heating the tool to 730C, followed by quenching

and cutting end vertically. When the cutting end gets cold, it is cleaned with stone and the heat

from the shank of the tool is allowed to temper the cutting edge to the correct colour . Then the

whole tool is quenched. Oxidation can be reduced by coating the tool with charcoal and oil.

Changes during tempering

the principles underlying the tempering of quenched steels have a close similarity to those of

precipitation hardening. The overlapping changes, which occur when high carbon martenstite is

tempered , are shown in fig.2 and as follows:-

Stage 1: 50-200C. martenstie breaks down to a transition precipitate known as c-carbide(Fe2,4C) across twins and a low cabon martenstie which results in slight dispersion

hardening , decrease in volume and electrical resistance.

Stage 2. 205-305C. decomposition of retained austenite to bainite and decrease inhardness.

Stage 3.250-500C. conversion of aggregate of low carbon martensite and c-carbide intoferrite and cementite precipitated along twins, which gradually coarsens to give visible

particles and rapid softening ,Fig .3.

Stage 4.carbide changes in alloy steel at 400-700C. In steels containing one alloyingaddition, cementite forms first and the alloy diffuses to it. When sufficiently enriched the

Fe3C transforms to an alloy carbide. After further enrichment this carbide may be

superseded by another and this formation of transition carbides may be repeated several


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times before the equilibrium carbide forms. In the chromium steel, changes are :Fe3C

>Cr7C3>Cr23C6. In steels containing several carbide forming elements the reaction

are often more complex, and the carbides which decompose are not necessary followed

by carbides based on the same alloy elements. The transformation can also occur in

suitable gradual exchange of atoms without any appreciable hardening; or by resolution

of existing iron carbides and fresh nucleation of coherent carbide with considerable

hardening that counteracts the normal softening that occurs during tempering. In some

alloy steels, therefore, the hardness is maintained constant up to about 50000C or in some

cases it rises to a peak followed by a gradual drop due to breakdown of coherance of the

cabide particles. This agehardenning process is known as secondary hardening and it

enhances high temperature creep properties of steel ( e.g. steel E in fig.2). Chromium,for

a example, seems to stabalise the size of the cementite particles over a range 200-5000C.

vanadium and molybdenum form a fine dispersion of coherant precipitates (V4C3Mo2C)

in a ferrite matrix with conciderable hardening. When overr-ageing starts the V4C3 grows

in the grain boundaries and also forms a Widmansttten pattern of plates within the grain.

Figure 2. Tempering curves for 0.35% C steel and die steel


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a) As quenched. Laths with high density of dislocation.b) Tempered 300C. Widmanattten precipitation of carbides within laths.c) Tempered 500C. Recovery of dislocation structure into cells with laths.d) Tempered 600C. Recrystallisation cementite rebucleatedequioxed ferrite boundaries.e) High C twinned martensite.f) Tempered 100C. fine e-carbides across twins.g) Tempered 200C. coherant cementite along twins, c-carbides dissolve.h) Tempered 4000C. breakdown of twinned sstructure. Carbides grow and spheroidise.Figure 3. Low cardon lath martensites have a high Ms temperature and some tempering oftenoccurs on cooling, i.e. autotempering.

Quenching:-

The quenching and partitioning process has been developed to produce high strength steel.

After austenitisation and interrupted quinching and austanite transforms parrtly to martensite.

The remaining austantite is stablised by carbon partitioning from martensite. After final


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quench the tampered martasite gives a high strength level. The TRIP-assisted local strain

hardening assures satisfying dutility. The process desingh is limited by reactions competing

against the carbon partitioning, i.e. carbon precipitation and isothermal bainite transfomation.

For a given chemical composition a vast scope of adjustable mechanical properties are

investigated as a function of a process perameters. The corresponding micro structure is

characterised by light and electron microscopy and XRD measurements. Suplimentary

information on the process kinetics is obtained by dilatometry. Silicon shows more effective

in retarding cementite precipitation than aluminium. Retained austenite occurs in filmy

constitution between bcc laths as well as blocky grains.

Quenching:- It is the process of rapidly cooling the metal from the solution or austenitizing

treating temperature, typically from within the range of 815C 1100C (1500F to 2012F) for

steel. High-alloy and stainless steel may be quenched to minimise grain bour dary carbides or to

improve the ferrite distribution but most steel including carbon, low-alloy, and tool steel, are

quenched to produce controlled amounts of martensite in the micristructure. Successful

hardenind usually means achieving the required microstructure, hardness, or toughness while

minimizing residual stress, distortion, and the possibility of cracking.

The a quench medium is usually a liquid such as water and depends on the harden-ability of the

particular alloy, the section thickness and shape involved, and the cooling rates needed toachieve the desired microstructure. The most common quenchmedia are either liquid or gases.

The liquid quench commonly used include:

Oil that may contain a variety of additives Leceted liquid polymers Water that may contain salt or caustic additives


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Gas type quench medians may also be used, such as inert gases including helium, argon, and

nitrogen. These quench gasses are somrtimes used after austenitizing in a vaccum.

The ability of a quenchto harden steel depends on the cooling characteristics of the quenching

medium valadity and quantity. Quenching effectiveness is dependent on the steel chemical

composition , type of quench or the quenchuse conditions. The design of the quenching systemand the thoroughness with which the system is maintained also contribute to the success of the

process.

The reson of the quenching system process is to cool steel from the austenitizingtemperature

quickly enough to form the desire microstructural phases, sometimes bainite but mero often

martensite. The basic quench function is to control the rate of heat transfer from the surface of

the part being quenched.

Quenching process

The rate of heat extraction by a quenching medium and the way it is used substantially affectsquench performance. Variations in quenching partices have resulted in the assignment of specific

names ti some quenching techniques-


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Direct quenching Time quenching Selective quenching Spray quenching Fog quenching Interrupted quenching

Result:-

The study of hardening of steel and effect of quenching medium on hardness has been done.

Aneealing of Steel

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