Chapter 13 Cooling Rate and Hardenabilityof Steels Cooling Rate • Hardenability
The face-centered cubic crystal structure of austenite transforms by shear into the body-centered tetragonal cubic crystal structure of martensite.
The hardness of martensite is a function of the carbon content of the steel.
Lath martensite and plate martensite require the use of an electron microscope for complete resolution.
The morphology of upper bainite consists of a feathery structure, and that of lower bainite consists of a needle-shaped structure.
Hypoeutectoidsteels, eutectoid steel, and hypereutectoid steels are the three main groupings of steels identified on the iron-carbon diagram.
As the cooling rate increases, diffusion of carbon has less time to occur, which results in a slight change in the shape of the iron-carbon diagram. The eutectoid composition is shifted to the left for hypoeutectoid steels and to the right for hypereutectoid steels.
Isothermal transformation (I-T) diagrams typically exhibit three distinctive regions and the temperature and time boundaries for the transformation of austenite.
The microstructure of steel depends on the rate that it cools to the isothermal transformation temperature.
I-T diagrams for hypoeutectoid steels include a region for proeutectoid ferrite. I-T diagrams for hypereutectoid steels include a region for proeutectoid cementite.
The products of transformation are indicated along the bottom of C-T diagrams and on the right-hand side of I-T diagrams.
A 1080 steel develops higher surface hardness when quenched, but a 4140 steel has higher hardenability because it retains hardness across the section thickness.
The critical cooling rate is the slowest cooling rate that misses the nose of the I-T or C-T diagram.
The Jominy end-quench specimen is austenitized and quenched under standardized conditions.
On an end-quench hardenability curve, hardness is plotted against distance from the quenched end of the Jominy bar.
The ASTM graph paper used for plotting end-quench hardenability curves indicates the variation of cooling rate with distance from the quenched end of the Jominybar.
High-hardenability steels exhibit hardness that is maintained for greater distances from the quenched end of the Jominy bar than low-hardenability steels.
A hardenability band indicates the maximum and minimum hardenability boundaries for a given grade of steel.
Correlation of the end-quench hardenability curve with the matching C-T diagram enables the phases formed at different locations along the Jominy end-quench specimen to be predicted.
The most common criterion for hardenability on the end-quench hardenability curve is the point of inflection (50% martensite). Cooling rates at given distances from the quenched end of the Jominy bar can be correlated to the cooling rates at four different locations in the quenched specimen.
Severity of quench increases from air, to oil, to water, to brine. The amount of agitation of the quenching medium also increases the severity of quench.
Curves of Du/D versus HD are used for estimating the severity of quench (H) of the quenching medium.
The hardenability of various steels is rated using the ideal critical diameter (DI) values. The higher the DI, the greater the hardenability.
Ideal critical diameter (DI) is related to the actual critical diameter (D) by the severity of quench (H). For a perfect quench (H = ∞), DI and D are equal.
Like carbon, most alloying elements have a depressing effect on the Ms temperature.
Retained austenite is usually difficult to resolve in the optical microscope, but it is sometimes observed as white patches in a martensite structure.