Question

What will happen to steel 4340 in a long term consideration upon creep and fatigue? What happen to it microstructure when undergo quenched and tempered?

Answer 1

Ans. When a steel 4340 in a long term considered upon creep and fatigue there will be progressive deformation in material.

Creep is defined as the slow and progressive deformation in the materials subjected to constant stress and at temperature approximately equal to the 0.4 times melting point temperature.

Creep strength is defined as the highest stress that a material can withstand for a specifies length of the time without exceeding a specified amount of strain at a particular temperature.

Hence during primary creep region strain Hardening effect dominant over the softening effect.

In steady state creep these effect balance each other while in teriary stage of creep the softening effect dominates strain Hardening.

Steel 4340 when subjected to quenching after heating it to above its recrystallization temperature microstructure changes to martensite which is body centered Tetragonal (BCT).

During this gamma austenite which has FCC structure is converted into BCT martensite configuration which is hardest form of carbon phase.

Quenching of steel result in residual stresses and quench crack formation. Also there will be retained austenite formation. Which may results in distortion of shape.

Chances of stress corrosion increases

To avoid such problems we generally prefer tempering of quenched steel.

Tempering relieve residual stresses and increases ductility and hence toughness of material increases. Tempered martensite formation takes place.

Tempered steel is corrosion resistance and mechanical properties are improved.

High temperature tempering of steel 4340 result in a slight grain growth in martensite hence strength and hardness will decrease.