EFFECT OF QUENCHING, FOLLOWED BY TEMPERING, ON

HARDNESS

Abstract:

Heat treatment is an important tool to achieve desired properties in steel. In this experiment

effect of quenching followed by tempering is determined. The six samples of AISI 1045 steel

were used. Tempering is done at two different temperatures. Muffle furnace was used to achieve

the required temperature.

Objective:

To study the effect of Quenching followed by Tempering on Hardness.

Apparatus:

Samples of AISI 1045 steel

Muffle Furnace

Oil for Quenching

Tongs

Brinell Hardness testing Machine

Related Theory:

Introduction:

Heat treating is a group of industrial and metalworking processes used to alter the physical, and

sometimes chemical, properties of a material. Heat treatment involves the use of heating or

chilling, normally to extreme temperatures, to achieve a desired result such as hardening or

softening of a material. Heat treatment techniques include annealing, case

hardening, precipitation strengthening, tempering and quenching. It is noteworthy that while the

term heat treatment applies only to processes where the heating and cooling are done for the

specific purpose of altering properties intentionally, heating and cooling often occur incidentally

during other manufacturing processes such as hot forming or welding. The important principles

of heat treatment are as follows:

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Phase transformations during heating Effects of cooling rates on structural changes during cooling Effect of crbon content and alloying addition

Quenching:

QUENCHING refers to the process of rapidly cooling metal parts from the austenitizing or

solution treating temperature, typically from within the range of 815 to 870 °C (1500 to 1600 °F)

for steel. Stainless and high-alloy steels may be quenched to minimize the presence of grain

boundary carbides or to improve the ferrite distribution but most steels including carbon, low-

alloy, and tool steels, are quenched to produce controlled amounts of martensite in the

microstructure. Successful hardening usually means achieving the required microstructure,

hardness, strength, or toughness while minimizing residual stress, distortion, and the possibility

of cracking.

The selection of a quenchant medium depends on the hardenability of the particular alloy, the

section thickness and shape involved, and the cooling rates needed to achieve the desired

microstructure. The most common quenchant media are either liquids or gases. The liquid

quenchants commonly used include:

Oil that may contain a variety of additives

Water

Aqueous polymer solutions

Water that may contain salt or caustic additives

Quenching Process:

The rate of heat extraction by a quenching medium and the way it is used substantially affects

quenchant performance. Variations in quenching practices have resulted in the assignment of

specific names to some quenching techniques:

Direct quenching

Time quenching

Selective quenching

Spray quenching

Fog quenching

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Interrupted quenching

Factors Affecting Heat Transfer Rate.

The rate of heat transfer from a part being quenched may be affected by oxidation of the surface.

This can either increase or decrease the heat transfer rate, depending on the thickness of the

oxide developed. The effect of irregular configuration on heat flow from a gear to the quenching

area is illustrated in Fig. 2. High temperatures persist near the surface at the roots of the teeth

where large vapor bubbles are trapped. If the gear were induction or flame heated, and thus had a

uniformly thin heated layer conforming to the contour, quenching would progress more rapidly

and uniformly because heat also would flow simultaneously to the cold metal underlying the

heated exterior and the quenchant.

Fig. 1 Comparison of cooling rates and temperature gradients as workpieces pass into and through martensite transformation range for a conventional quenching and tempering process and for interrupted quenching processes. (a) Conventional quenching and tempering processes that use oil, water, or polymer quenchants.

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Fig. 2 Temperature gradients and other factors affecting the edgewise quenching of a gear in a quiescent volatile liquid. A, flow of heat from hot core of gear. Temperature and flow rate vary with time; B, vapor blanket stage still exists due to large source of heat and poor agitation; C,

trapped vapor bubbles condensing slowly; D, vapor bubbles escaping and condensing

Tempering Of Steel

Tempering is a process in which previously hardened or normalized steel is usually heated to a

temperature below the lower critical temperature and cooled at a suitable rate, primarily to

increase ductility and toughness, but also to increase the grain size of the matrix. Steels are

tempered by reheating after hardening to obtain specific values of mechanical properties and also

to relieve quenching stresses and to ensure dimensional stability.

Tempering usually follows quenching from above the upper critical temperature; however,

tempering is also used to relieve the stresses and reduce the hardness developed during welding

and to relieve stresses induced by forming and machining.

Principal Variables:

Variables associated with tempering that affect the microstructure and the mechanical properties

of tempered steel include:

Tempering temperature

Time at temperature

Cooling rate from the tempering temperature

Composition of the steel, including carbon content, alloy content, and residual elements

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In a steel quenched to a microstructure consisting essentially of martensite, the iron lattice is

strained by the carbon atoms, producing the high hardness of quenched steels. Upon heating, the

carbon atoms diffuse and react in a series of distinct steps that eventually form Fe3C or an alloy

carbide in a ferrite matrix of gradually decreasing stress level. The properties of the tempered

steel are primarily determined by the size, shape, composition, and distribution of the carbides

that form, with a relatively minor contribution from solid-solution hardening of the ferrite. These

changes in microstructure usually decrease hardness, tensile strength, and yield strength but

increase ductility and toughness. Under certain conditions, hardness may remain unaffected by

tempering or may even be increased as a result of it. For example, tempering a hardened steel at

very low tempering temperatures may cause no change in hardness but may achieve a desired

increase in yield strength. Also, those alloy steels that contain one or more of the carbide forming

elements (chromium, molybdenum, vanadium, and tungsten) are capable of secondary

hardening; that is, they may become somewhat harder as a result of tempering. Temperature and

time are interdependent variables in the tempering process. Within limits, lowering temperature

and increasing time can usually produce the same result as raising temperature and decreasing

time. However, minor temperature changes have a far greater effect than minor time changes in

typical tempering operations. This is discussed in more detail in the section "Tempering Time."

With few exceptions, tempering is done at temperatures between 175 and 705 °C (350 and 1300

°F) and for times from 30 min to 4 h.

Experimental Procedure:

Firstly, heating a muffle furnace up to a temperature 900ºC. This is the austenitizing

temperature.

After heating, we place the three samples in the muffle furnace. Firstly there was a drop

in temperature but we were waited for reaching the temperature again 900ºC.

The 1st two steel samples held at this temperature for 45 minutes for the formation of

homogeneous structure throughout its mass. After holding the sample for 45 minutes we

removed them and quenched them in oil.

The steel sample 2nd is heating to a temperature of 950ºC and held it at this temperature

for 20 minutes for the formation of homogeneous structure throughout its mass. After

holding the sample for 20 minutes we removed it and quenched in oil.

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The steel sample 3rd is heated to a temperature of 1000ºC and held it at this temperature

for 15 minutes for the formation of homogeneous structure throughout its mass. After

holding the sample for 15 minutes we removed and quenched in oil.

Air cooling we performed Brinell hardness test on the 3 samples and see that there is any

difference in hardness or not.

For Tempering:

Firstly we heat the muffle furnace to a temperature of 550ºC.

After heating the furnace, we place the three steel samples (900,950.1000ºc) in the

furnace for 35 minutes. After holding for 35 minutes we removed the samples and cooled

them in the air.

After Air cooling we performed Brinell hardness test on the 3 samples and see that there

is any difference in hardness or not.

Similarly, first we rise the temperature of furnace up to 650ºC.

After heating the furnace, we place the three steel samples (900,950.1000ºc) in the

furnace for 35 minutes after holding for 35 minutes we removed the samples and cooled

them in the air.

After Air cooling we performed Brinell hardness test on the 3 samples and see that there

is any difference in hardness or not.

Observation and calculations:

Quenching Data :

Sample 1 at 900o C Sample 2 at 950o C Sample 3 at 1000o CLoad (Kg) 3000 3000 3000

Diameter of indent(mm)

2.6 2.55 2.45

BHN 555 578 637HRc 55 56 59

Tempering at 650ºC:

Sample 1 at 900o C Sample 2 at 950o C Sample 3 at 1000o CLoad (Kg) 3000 3000 3000

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Diameter of Ball(mm)

10 10 10

Diameter of indent(mm)

3.6 3.52 3.57

BHN 285 289 290HRc 30 31 31

Tempering at 550ºC:

Sample 1 at 900o C Sample 2 at 950o C Sample 3 at 1000o CLoad (Kg) 3000 3000 3000

Diameter of Ball(mm)

10 10 10

Diameter of indent(mm)

3.35 3.37 3.20

BHN 331 332 363HRc 36 36 39

Graph:

1.

2.

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3.

4.

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Discussion:

The Hardness is decreases with the increase in tempering temperature. Tempering is necessary to

restore some ductility (i.e., toughness) to the rapidly quenched steel. On re-heating iron-carbon

martensite below 727oC, certain metastable carbides (not Fe3C) begin to form as the result of

diffusion of carbon out of the carbon-rich Martensite. A mixture of carbides and ferrite results.

Through this tempering the hardness of the steel begins to drop. If tempering is continued at a

sufficiently high temperature for a long enough period of time, the carbide will eventually be

completely converted to cementite (Fe3C). The equilibrium, final state structure would be

cementite particles in a ferrite matrix. The toughness of steel is also strongly dependent on the

environmental temperature, and can be very low at low temperature. Plain carbon steel shows a

greater decrease in hardness during tempering at temperatures above 300°C. therefore, they

cannot be used for high strength applications where temperature higher than 300°C are

generated. Resistance to softening of carbon steels can be improved by the addition of alloying

elements, such as chromium, nickel, molybdenum, vanadium, etc.Martensite is too brittle to

serve engineering purpose. Tempering is necessary to increase ductility and toughness of

martensite. Some hardness and strength is lost. Tempering consist on reheating martensitic steels

(solution supersaturated of carbon) to temperatures between 150-650ºC to force some carbide

precipitation. In 1st stage 80-160ºC α’is transformed into α”(low C martensite) + ε

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carbide(Fe2.3C). During 2nd stage 230-280ºC γ retained is converted into bainite. At 3rdstage

160-400ºC α”+ ε carbide transformed into α + Fe3C (tempered martensite) ,3rdstage (cont) 400-

700oCGrowth and spherodization of cementite and other carbides.

Reference:

ASM Hand Book, VOl 4, HEAT TREATING, Published in 1991, Quenching of Steel,

Tempering of Steel.

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