Applications and Uses
- Chemical Industries
- Oil & Gas Industrires
- Power Plant Industries
- Shipbuilding Industries
- Fertilizer Industrires
- Petrochemical Industrires
- Sugar Industrires
- Cement Industrires
During the alloy process elements such as carbon are introduced to the metal. These added elements interrupt the flow of the individual grains, increasing strength. Thus, control of the metal crystal structure is a key element in successful heat treating.
A Metal can also exist in various phases: Ferrite, austenite and cementite. To better understand these phases, look at the Iron-Carbon Phase Diagram. The Y-axis (vertical) is a measurement of temperature while the X-Axis (Horizontal) is a measurement of the carbon content of the steel. The far left hand side of the X-axis represents the Ferrite phase of steel (low carbon content) while the far right hand. Side represents the cementite phase of steel (high carbon content), which is also known as iron carbide. The curved horizontal line that occurs just above 1333 ºF represents the austenite phase of steel.
When ferrite (low carbon steel) is at room temperature, it has a body-centered-cubic structure, which can only absorb a low amount of carbon. Because Ferrite can only absorb a very low amount of carbon at room temperature, the un-absorbed carbon separates out of the body-centered-cubic structure to form carbides which join together to create small packets of an extremely hard crystal structure within the ferrite called cementic. However, when ferrite is heated to a temperature above the transformation line( austenite line) the body-centered-cubic structure changes to a face-centered-cubic structure, thus allowing for absorption of the carbon into the crystal structure.
Once the steel enters the austenitic phase all of the cementite dissolves into austenite. If the steel is allowed to cool slowly, the carbon will separate out of the ferrite as a cubic-structure reverts from face-centered back to body-centered. The islands of cementite will reform within the ferrite, and the steel will have the same properties that it did before it was heated. However, when the steel is rapidly cooles, or quenched, in a quenching medium (such as oil, water or cold air) the carbon does not have time to exit the cubic structure of the ferrite and it becomes trapped within it. This leads to the information of martensitic; microstructure that produces the most sought after mechanical properties in steel fasteners.
During quenching, it is impossible to cool the specimen at a uniform rate throughout. The surface will always cool more rapidly than the interior of the specimen. Therefore, the austenite will transform over a range of temperatures, yielding a possible variation of microstructure and properties depending on the position within the material.
|Fasteners Produced From AISI 4140& 4142 Steel|
|Fastener||ASTM A193 B7||SAE J429 Gr. 8||ASTM A574 SHCS|
|Tensile strength||125,000 PSI min (2 1\2in and under)||150,000 PSI min||180,000
( through ½ in ) 170,000 PSI min
( above ½ in)
105,000 PSI min
(2 1/2in and under)
|130,000 PSI min||153,000 PSI min|
( through ½ in ) 135,000 PSI min
( above ½ in )
|Hardness||HRC 35 max.||HRC 33-39||HRC
( through ½ in ) HRC 37-45
( above ½ in )
The initial heat treating process is relatively the same for the entire three products. The parts are heat treated until fully austenitized and then are quenched and tempered in the oil. This tempering temperature dictates the final product. A lower tempering temperature will produce a harder and higher tensile strength part for these alloys steels. However, the lower tempering temperatures will also mean lower ductility, impact strength, and possibly lower fatigue life.