The Heat Treatment of Steel

TEMPERING OF STEEL 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


• Increase toughness,


• Increase the grain size of the matrix.

Steels are tempered by reheating after hardening to obtain

• Specific values of mechanical properties


• To relieve quenching stresses


• 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 a tempered steel include:

· Tempering temperature

· Cooling rate from the tempering temperature

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

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 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,


• Decrease tensile strength,


• Decrease yield strength


• Increase ductility


• Increase 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.

OBJECTIVE:

To study the effect of Tempering Time on Hardness of given steel samples

APPARATUS:

3 samples of steel of grade 1045, hack saw, grinder for rough grinding, polishing paper(0 and 2), three muffle furnaces, and bucket of water.

PROCEDURE:

• First we took a rode of 1045 steel.


• Cut it into small cylindrical samples by using hack saw.


• Then we grind specimens to remove hack saw gouges


• we perform rough polishing by using 0 and 2 number paper.


• We place all samples into muffle furnace and heat it at 880C


• 30 minute soaking time is given and then quenched into water.


• Furnaces is operating at 600 c.


• We provide 45 min to get austenetizing temperature.


• We place samples in furnace to temper and provide soaking time 90,180,270 and 360min.


• Then after that we cool them slowly in air.


• Again polish them and check their hardness.

TEMPERING TIME

The diffusion of carbon and alloying elements necessary for the formation of carbides is temperature and time dependent. The changes in hardness are approximately linear over a large portion of the time range when the time is presented on a logarithmic scale.

Rapid changes in room-temperature hardness occur at the start of tempering in times less than 10 s. Less rapid, but still large, changes in hardness occur in times from 1 to 10 min, and smaller changes occur in times from 1 to 2 h. For consistency and less dependency on variations in time, components generally are tempered for 1 to 2 h. The levels of hardness produced by very short tempering cycles.